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6.  Musculoskeletal System

Chapter Editors: Hilkka Riihimäki and Eira Viikari-Juntura

 


 

Table of Contents

Tables and Figures

Overview
Hilkka Riihimäki

Muscles
Gisela Sjøgaard

Tendons
Thomas J. Armstrong

Bones and Joints
David Hamerman

Intervertebral Discs
Sally Roberts and Jill P.G. Urban

Low-back Region
Hilkka Riihimäki

Thoracic Spine Region
Jarl-Erik Michelsson

Neck
Åsa Kilbom

Shoulder
Mats Hagberg

Elbow
Eira Viikari-Juntura

Forearm, Wrist and Hand
Eira Viikari-Juntura

Hip and Knee
Eva Vingård

Leg, Ankle and Foot
Jarl-Erik Michelsson

Other Diseases
Marjatta Leirisalo-Repo

Tables

Click a link below to view table in article context.

  1. Structure-function of joint components
  2. Prevalence of back disorders, in Finns over 30 years
  3. Reducing the risks for low-back pain at work
  4. Classification-low-back disorders (Quebec Task Force)
  5. Permissible motions for head in prolonged driving
  6. Incidence of epicondylitis in various populations
  7. Incidence of tenosynovitis/peritendinitis
  8. Primary osteoarthrosis of the hip in Malmö, Sweden
  9. Guidelines for the treatment of rheumatoid arthritis
  10. Infections known to trigger reactive arthritis

Figures

Point to a thumbnail to see figure caption, click to see figure in article context.

MUS050F1MUS050F2MUS050F3MUS040F1MUS020F1MUS020F2MUS020F3MUS020F4MUS020F5MUS130F1MUS130F2MUS130F3MUS080F1MUS080F4MUS080F5MUS090F1MUS090F2MUS090F3MUS090F4MUS110F1MUS140F1MUS170F1MUS170T1MUS170T2

 


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Wednesday, 16 February 2011 20:28

Overview

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Musculoskeletal disorders are among the most important occupational health problems in both developed and developing countries. These disorders affect the quality of life of most people during their lifetime. The annual cost of musculoskeletal disorders is great. In the Nordic countries, for example, it is estimated to vary from 2.7 to 5.2 % of the gross national product (Hansen 1993; Hansen and Jensen 1993). The proportion of all musculo-skeletal diseases that are attributable to work is thought to be approximately 30%. Thus, much is to be gained by prevention of work-related musculoskeletal disorders. To accomplish this goal, a good understanding is needed of the healthy musculoskeletal system, musculoskeletal diseases and the risk factors for musculo-skeletal disorders.

Most musculoskeletal diseases cause local ache or pain and restriction of motion that may hinder normal performance at work or in other everyday tasks. Nearly all musculoskeletal diseases are work-related in the sense that physical activity can aggravate or provoke symptoms even if the diseases were not directly caused by work. In most cases, it is not possible to point to one causal factor for musculoskeletal diseases. Conditions caused solely by accidental injuries are an exception; in most cases several factors play a role. For many of the musculoskeletal diseases, mechanical load at work and leisure is an important causal factor. Sudden overload, or repetitive or sustained loading can injure various tissues of the musculoskeletal system. On the other hand, too low a level of activity can lead to deterioration of the condition of muscles, tendons, ligaments, cartilage and even bones. Keeping these tissues in good condition requires appropriate use of the musculoskeletal system.

The musculoskeletal system essentially consists of similar tissues in different parts of the body, which provide a panorama of diseases. The muscles are the most common site of pain. In the lower back the intervertebral discs are common problem tissues. In the neck and the upper limbs, tendon and nerve disorders are common, while in the lower limbs, osteoarthritis is the most important pathological condition.

In order to understand these bodily differences, it is necessary to comprehend basic anatomical and physiological features of the musculoskeletal system and to learn the molecular biology of various tissues, the source of nutrition and the factors affecting normal function. The biomechanical properties of various tissues are also fundamental. It is necessary to understand both the physiology of normal function of the tissues, and pathophysi- ology—that is, what goes wrong. These aspects are described in the first articles for intervertebral discs, bones and joints, tendons, muscles and nerves. In the articles which follow, musculoskeletal disorders are described for the different anatomical regions. Symptoms and signs of the most important diseases are outlined and the occurrence of the disorders in populations is described. Current understanding, based on epidemiological research, of both work-and person- related risk factors is presented. For many disorders there are quite convincing data on work-related risk factors, but, for the time being, only limited data are available on exposure effect relationships between the risk factors and the disorders. Such data are needed in order to set guidelines to design safer work.

Despite the lack of quantitative knowledge, directions for prevention can be proposed. The primary approach to prevention of work-related musculoskeletal disorders is redesign of work in order to optimize the workload and make it compatible with the physical and mental performance capacity of the workers. It is also important to encourage workers to keep fit through regular physical exercise.

Not all musculoskeletal diseases described in this chapter have a causal relationship to work. It is, however, important for occupational health and safety personnel to be aware of such diseases and consider workload also in relation to them. Fitting the work to the performance capacity of the worker will help him or her to work successfully and healthfully.

 

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Wednesday, 16 February 2011 20:51

Muscles

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Physical activity may increase muscle strength and working capa-city through changes such as growth in muscle volume and increased metabolic capacity. Different activity patterns cause a variety of biochemical and morphological adaptations in the muscles. In general, a tissue must be active to remain capable of living. Inactivity causes atrophy, especially in muscle tissue. Sports medicine and scientific investigations have shown that various training regimes can produce very specific muscular changes. Strength training, which places strong forces on the muscles, increases the number of contractile filaments (myofibrils) and the volume of the sarcoplasmic reticulum (see figure 1). High-intensity exercise increases muscular enzyme activity. The fractions of glycolytic and oxidative enzymes are closely related to the work intensity. In addition, prolonged intense exercise increases the capillary density.

Figure 1. A diagrammatic representation of the major components of a muscle cell involved in excitation-contraction coupling as well as the site for ATP production, the mitochondrion.

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Sometimes, too much exercise can induce muscle soreness, a phenomenon well known to everyone who has demanded muscular performance beyond his or her capacity. When a muscle is overused, first deteriorating processes set in, which are followed by reparative processes. If sufficient time for repair is allowed, the muscle tissue may end up with increased capacities. Prolonged overuse with insufficient time for repair, on the other hand, causes fatigue and impairs muscle performance. Such prolonged overuse may induce chronic degenerative changes in the muscles.

Other aspects of muscle use and misuse include the motor control patterns for various work tasks, which depend on force level, rate of force development, type of contraction, duration and the precision of the muscle task (Sjøgaard et al. 1995). Individual muscle fibres are “recruited” for these tasks, and some recruitment patterns may induce a high load on individual motor units even when the load on the muscle as a whole is small. Extensive recruitment of a particular motor unit will inevitably induce fatigue; and occupational muscle pain and injury may follow and could easily be related to the fatigue caused by insufficient muscle blood flow and intramuscular biochemical changes due to this high demand (Edwards 1988). High muscle tissue pressures may also impede muscle blood flow, which can reduce the ability of essential chemicals to reach the muscles, as well as the ability of the blood to remove waste products; this can cause energy crises in the muscles. Exercise can induce calcium to accumulate, and free radical formation may also promote degenerative processes such as the breakdown of muscle membrane and the impairment of normal metabolism (mitochondrial energy turnover) (figure 2). These processes may ultimately lead to degenerative changes in the muscle tissue itself. Fibres with marked degenerative characteristics have been found more frequently in muscle biopsies from patients with work-related chronic muscle pain (myalgia) than in normal subjects. Interestingly, the degenerated muscle fibres thus identified are “slow twitch fibres”, which connect with low-threshold motor nerves. These are the nerves normally recruited at low sustained forces, not high force related tasks. The perception of fatigue and pain may play an important role in preventing muscle injury. Protective mechanisms induce the muscles to relax and recover in order to regain strength (Sjøgaard 1990). If such biofeedback from the peripheral tissues is ignored, the fatigue and pain may eventually result in chronic pain.

Figure 2. A blow-up of the muscle membrane and structures inside the muscle in figure 2. The chain of events in the pathogenesis of calcium () induced damage in muscle cells is illustrated

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Sometimes, after frequent overuse, various normal cellular chemical substances may not only cause pain themselves but may increase the response of muscular receptors to other stimuli, thereby lowering the threshold of activation (Mense 1993). The nerves which carry the signals from the muscles to the brain (sensory afferents) may thus be sensitized over time, which means that a given dose of substances which cause pain elicit a stronger excitation response. That is, the threshold of activation is reduced and smaller exposures may cause larger responses. Interestingly, the cells which normally serve as pain receptors (nociceptors) in uninjured tissue are silent, but these nerves may also develop ongoing pain activity which can persist even after the cause of the pain has terminated. This effect may explain chronic states of pain which are present after the initial injury has healed. When pain persists after healing, the original morphological changes in the soft tissues may be difficult to identify, even if the primary or initial cause of the pain is located in these peripheral tissues. Thus, the real “cause” of the pain may be impossible to trace.

Risk Factors and Preventive Strategies

Work-related risk factors of muscle disorders include repetition, force, static load, posture, precision, visual demand and vibration. Inappropriate work/rest cycles may be a potential risk factor for musculoskeletal disorders if sufficient recovery periods are not allowed before the next working period, thus never affording enough time for physiological rest. Environmental, sociocultural or personal factors may also play a role. Musculoskeletal disorders are multifactorial, and, in general, simple cause-effect relationships are difficult to detect. It is, however, important to document the extent to which occupational factors can be causally related to the disorders, since, only in the case of causality, the elimination or minimization of the exposure will help prevent the disorders. Of course, different preventive strategies must be implemented depending on the type of work task. In the case of high-intensity work the aim is to reduce force and work intensity, while for monotonous repetitive work it is more important to induce variation in the work. In short, the aim is optimization of the exposure.

Occupational Diseases

Work-related muscle pain is reported most frequently in the neck and shoulder area, the forearm and the low back. Although it is a major cause of sick-leave there is much confusion with regard to classifying the pain and specifying diagnostic criteria. Common terms which are used are given in three categories (see figure 3).

Figure 3. Classification of muscle diseases.

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When muscular pain is assumed to be work-related, it can be classified into one of the following disorders:

  • Occupational cervicobrachial disorders (OCD)
  • Repetition strain injury (RSI)
  • Cumulative trauma disorders (CTD)
  • Overuse (injury) syndrome
  • Work-related neck and upper-limb disorders.

 

The taxonomy of the work-related neck and upper-limb disorders clearly demonstrates that the aetiology includes external mechanical loads, which may well occur in the work place. Besides disorders in the muscle tissue itself, this category includes also disorders in other soft tissues of the musculoskeletal system. Of note is, that the diagnostic criteria may not allow to identify the location of the disorder specifically to one of these soft tissues. In fact it is likely that morphological changes at the musculo-tendinous junctions are related to the perception of muscle pain. This advocates the term fibromyalgia to be used among local muscle disorders. (See figure 3)

Unfortunately, different terms are used for essentially the same medical condition. In recent years, the international scientific community has focused increasingly on classification and diagnostic criteria for musculoskeletal disorders. A distinction is made between generalized and local or regional pain (Yunus 1993). Fibromyalgia syndrome is a generalized pain condition but is not considered to be work related. On the other hand, localized pain disorders are likely to be associated with specific work tasks. Myofascial pain syndrome, tension neck and rotator cuff syndrome are localized pain disorders that can be considered as work-related diseases.

 

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Wednesday, 16 February 2011 21:22

Tendons

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The deformation that occurs as force is applied and removed is called “elastic” deformation. The deformation that occurs after force application or removal is called “viscous” deformation. Because tissues of the body exhibit both elastic and viscous properties, they are called “viscoelastic”. If the recovery time between successive exertions is not long enough for a given force and duration, the recovery will not be complete and the tendon will be stretched further with each successive exertion. Goldstein et al. (1987) found that when finger flexor tendons were subjected to 8 seconds (s) physiological loads and 2 s rest, the accumulated viscous strain after 500 cycles was equal to the elastic strain. When the tendons were subjected to 2 s work and 8 s rest, the accumulated viscous strain after 500 cycles was negligible. Critical recovery times for given work-rest profiles have not yet been determined.

Tendons can be characterized as composite structures with parallel bundles of collagen fibres arranged in a gelatinous matrix of mucopolysaccharide. Tensile forces on the ends of the tendon cause unfolding of corrugations and straightening of the collagen strands. Additional loads cause stretching of the straightened strands. Consequently, the tendon gets stiffer as it gets longer. Compressive forces perpendicular to the long axis of the tendon cause the collagen strands to be forced closer together, and result in a flattening of the tendon. Shear forces on the side of the tendon cause displacement of the collagen strands closest to the surface with respect to those farthest away, and gives the side view of the tendon a skewed look.

Tendons as Structures

Forces are transmitted through tendons to maintain static and dynamic balance for specified work requirements. Contracting muscles tend to rotate the joints in one direction while the weight of the body and of work objects tends to rotate them in the other. Exact determination of these tendon forces is not possible because there are multiple muscles and tendons acting about each joint structure; however, it can be shown that the muscle forces acting on the tendons are much greater than the weight or reaction forces of work objects.

The forces exerted by contracting muscles are called tensile forces because they stretch the tendon. Tensile forces can be demonstrated by pulling on the ends of a rubber band. Tendons also are subjected to compressive and shear forces and to fluid pressures, which are illustrated in Figure 4 for the finger flexor tendons in the wrist.

Figure 1. Schematic diagram of tendon stretched around an anatomical surface or pulley and the corresponding tensile forces (Ft), compressive forces (Fc), friction forces (Ff) and hydrostatic or fluid pressure (Pf).

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Exertion of the fingers to grasp or manipulate work objects requires the contraction of muscles in the forearm and hand. As the muscles contract, they pull on the ends of their respective tendons, which pass through the centre and circumference of the wrist. If the wrist is not held in a position so that the tendons are perfectly straight, they will press against adjacent structures. The finger flexor tendons press against the bones and ligaments inside the carpal tunnel. These tendons can be seen to protrude under the skin toward the palm during forceful pinching with a flexed wrist. Similarly, the extensor and abductor tendons can be seen to protrude on the back and side of the wrist when it is extended with outstretched fingers.

Friction or shear forces are caused by dynamic exertions in which the tendons rub against adjacent anatomical surfaces. These forces act on and parallel to the surface of the tendon. Friction forces can be felt by simultaneously pressing and sliding the hand against a flat surface. The sliding of tendons over an adjacent anatomical surface is analogous to a belt sliding around a pulley.

Fluid pressure is caused by exertions or postures that displace fluid out of the spaces around the tendons. Studies of carpal canal pressure show that wrist contact with external surfaces                                                                                                                           and certain postures produce pressures high enough to impair                                                                                                                       circulation and threaten tissue viability (Lundborg 1988).

Contraction of a muscle produces an immediate stretching of its tendon. Tendons join muscles together. If the exertion is sustained, the tendon will continue to stretch. Relaxation of the muscle will result in a rapid recovery of the tendon followed by a slowed recovery. If the initial stretching was within certain limits, the tendon will recover to its initial unloaded length (Fung 1972).

Tendons as Living Tissues

The strength of tendons belies the delicacy of the underlying physiological mechanisms by which they are nourished and heal. Interspersed within the tendon matrix are living cells, nerve endings and blood vessels. Nerve endings provide information to the central nervous system for motor control and warning of acute overload. Blood vessels play an important role in the nourishment of some areas of the tendon. Some areas of tendons are avascular and rely on diffusion from fluid secreted by synovial linings of outer tendon sheaths (Gelberman et al. 1987). Synovial fluid also lubricates movements of the tendons. Synovial sheaths are found at locations where tendons come into contact with adjacent anatomical surfaces.

Excessive elastic or viscous deformation of the tendon can damage these tissues and impair their ability to heal. It is hypothesized that deformation may impede or arrest circulation and nourishment of tendons (Hagberg 1982; Viikari-Juntura 1984; Armstrong et al. 1993). Without adequate circulation, cell viability will be impaired and the tendon’s capacity to heal will be reduced. Tendon deformation can lead to small tears that further contribute to cell damage and inflammation. If circulation is restored and the tendon is given adequate recovery time, the damaged tissues will heal (Gelberman et al. 1987; Daniel and Breidenbach 1982; Leadbetter 1989).

Tendon Disorders

It has been shown that tendon disorders occur in predictable patterns (Armstrong et al. 1993). Their locations occur in those parts of the body associated with high stress concentrations (e.g., in the tendons of the supraspinatus, the biceps, the extrinsic finger flexor and extensor muscles). Also, there is an association between the intensity of work and the prevalence of tendon disorders. This pattern also has been shown for amateur and professional athletes (Leadbetter 1989). The common factors in both workers and athletes are repetitive exertions and overloading of the muscle-tendon units.

Within certain limits, the injuries produced by mechanical loading will heal. The healing process is divided into three stages: inflammatory, proliferatory and remodelling (Gelberman et al. 1987; Daniel and Breidenbach 1982). The inflammatory stage is characterized by the presence of polymorphonuclear cell infilt- ration, capillary budding and exudation, and lasts for several days. The proliferatory stage is characterized by the proliferation of fibroblasts and randomly oriented collagen fibres between areas of the wound and adjacent tissues, and lasts for several weeks. The remodelling phase is characterized by the alignment of the collagen fibres along the direction of loading, and lasts for several months. If the tissues are re-injured before healing is complete, recovery may be delayed and the condition may worsen (Leadbetter 1989). Normally healing leads to a strengthening or adaptation of the tissue to mechanical stress.

The effects of repetitive loading are apparent in the forearm finger flexor tendons where they contact the inside walls of the carpal tunnel (Louis 1992; Armstrong et al. 1984). It has been shown that there is progressive thickening of the synovial tissue between the edges of the carpal tunnel and the centre where the contact stresses on the tendons are the greatest. Thickening of the tendons is accompanied by synovial hyperplasia and proliferation of connective tissue. Thickening of the tendon sheaths is a widely cited factor in compression of the median nerve inside the carpal tunnel. It can be argued that thickening of the synovial tissues is an adaptation of the tendons to mechanical trauma. Were it not for the secondary effect on the median nerve compression resulting in carpal tunnel syndrome, it might be considered a desirable outcome.

Until optimal tendon loading regimes are determined, employers should monitor workers for signs or symptoms of tendon disorders so that they can intervene with work modifications to prevent further injuries. Jobs should be inspected for conspicuous risk factors any time an upper limb problem is identified or suspected. Jobs also should be inspected any time there is a change in the work standard, procedure or tooling, to insure that risk factors are minimized.

 

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Wednesday, 16 February 2011 22:39

Bones and Joints

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Bone and cartilage are part of the specialized connective tissues that make up the skeletal system. Bone is a living tissue that replaces itself continuously. The hardness of bone is well suited to the task of providing mechanical support function, and the elasticity of cartilage, to the ability of joints to move. Both cartilage and bone consist of specialized cells that produce and regulate a matrix of material outside the cells. The matrix is abundant in collagens, proteoglycans and non-collagenous proteins. Minerals are present in bone matrix as well.

The external part of bone is called the cortex and is compact bone. The more spongy inner part (trabecular bone) is filled with blood-forming (haematopoietic) bone marrow. The inner and outer parts of the bone have different metabolic turnover rates, with important consequences for late life osteoporosis. Trabecular bone regenerates itself at a greater rate than compact bone, which is why osteoporosis is first seen in the vertebral bodies of the spine, which have large trabecular parts.

Bone in the skull and other selected sites forms directly by bone formation (intramembranous ossification) without passing through a cartilage intermediate phase. The long bones of the limbs develop from cartilage through a process known as endochondral ossification. This process is what leads to the normal growth of long bones, to the repair of fractures and, in late adult life, to the unique formation of new bone in a joint which has become osteoarthritic.

The osteoblast is a type of bone cell that is responsible for synthesis of the matrix components in bone: the distinct collagen (type I) and proteoglycans. Osteoblasts also synthesize other non-collagenous proteins of bone. Some of these proteins can be measured in serum to determine the rate of bone turnover.

The other distinct bone cell is called the osteoclast. The osteoclast is responsible for resorption of bone. Under normal circumstances, old bone tissue is resorbed while new bone tissue is generated. Bone is resorbed by production of enzymes that dissolve proteins. Bone turnover is called remodelling and is normally a balanced and coordinated process of resorption and formation. Remodelling is influenced by body hormones and by local growth factors.

Movable (diarthrodial) joints are formed where two bones fit together. Joint surfaces are designed for weight bearing, and to accommodate a range of motion. The joint is enclosed by a fibrous capsule, whose inner surface is a synovial membrane, which secretes synovial fluid. The joint surface is made of hyaline cartilage, beneath which is a backing of hard (subchondral) bone. Within the joint, ligaments, tendons and fibrocartilaginous structures (menisci in certain joints, such as the knee), provide stability and a close fit between joint surfaces. The specialized cells of these joint components synthesize and maintain the matrix macromolecules whose interactions are responsible for maintaining the tensile strength of ligaments and tendons, the loose connective tissue that supports the blood vessels and cellular elements of the synovial membrane, the viscous synovial fluid, the elasticity of hyaline cartilage, and the rigid strength of subchondral bone. These joint components are interdependent, and their relationships are shown in table 1.

Table 1. Structure-function relationships and inter-dependence of joint components.

Components

Structure

Functions

Ligaments and tendons

Dense, fibrous, connective tissue

Prevents over-extension of joints, provides stability and strength

Synovial membrane

Areolar, vascular and cellular

Secretes synovial fluid, dissolves (phagocytoses) particulate material in synovial fluid

Synovial fluid

Viscous fluid

Provides nutrients for cartilage injoints, lubricates cartilage during joint motion

Cartilage

Firm hyaline cartilage

Constitutes the joint surface, bears weight, responds elastically to compression

Tidemark

Calcified cartilage

Separates joint cartilage from underlying bone

Subchondral bone

Hard bone with marrow spaces

Provides backing for joint surface; marrow cavity provides nutrients to base of cartilage and is thesource of cells with potential fornew bone formation

Source: Hamerman and Taylor 1993.

Selected Diseases of Bones and Joints

Osteopenia is the general term used to describe reduced bone substance detected on x rays. Often asymptomatic in early stages, it may eventually manifest itself as weakening of bones. Most of the conditions listed below induce osteopenia, although the mechanisms by which this occurs differ. For example, excessive parathyroid hormone enhances bone resorption, while calcium and phosphate deficiency, which can arise from multiple causes and is often due to inadequate vitamin D, results in deficient mineralization. As people age, there is an imbalance between formation and resorption of bone. In women around the age of menopause, resorption often predominates, a condition called type I osteoporosis. In advanced age, resorption can again dominate and lead to type II osteoporosis. Type I osteoporosis usually affects vertebral bone loss and collapse, while hip fracture predominates in type II.

Osteoarthritis (OA) is the principal chronic disorder of certain movable joints, and its incidence increases with age. By age 80, almost all people have enlarged joints on the fingers (Heberden’s nodes). This is usually of very limited clinical significance. The principal weight-bearing joints which are subject to osteoarthritis are the hip, knee, feet and facets of the spine. The shoulder, while it is not weight bearing, may also suffer from a variety of arthritic changes, including rotator cuff tear, subluxation of the humeral head and an effusion high in proteolytic enzymes—a clinical picture often called “Milwaukee Shoulder” and associated with substantial pain and limitation of motion. The main change in OA is primarily one of degradation of cartilage, but new bone formation called osteophytes is usually seen on x rays.

 

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Wednesday, 16 February 2011 22:41

Intervertebral Discs

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The intervertebral discs occupy about one-third of the spine. Since they not only provide the spinal column with flexibility but also transmit load, their mechanical behaviour has a great influence on the mechanics of the whole spine. A high proportion of cases of low-back pain are associated with the disc, either directly through disc herniation, or indirectly, because degenerated discs place other spinal structures under abnormal stress. In this article, we review the structure and composition of the disc in relation to its mechanical function and discuss changes to the disc in disease.

Anatomy

There are 24 intervertebral discs in the human spine, interspersed between the vertebral bodies. Together these make up the anterior (front) component of the spinal column, with the articulating facet joints and the transverse and spinous processes making up the posterior (rear) elements. The discs increase in size down the spine, to approximately 45 mm antero-posteriorly, 64 mm laterally and 11 mm height in the lower back region.

The disc is made of cartilage-like tissue and consists of three distinct regions, (see figure 1). The inner region (nucleus pulposus) is a gelatinous mass, particularly in the young person. The outer region of the disc (annulus fibrosus) is firm and banded. The fibres of the annulus are criss-crossed in an arrangement which allows it to withstand high bending and twisting loads. With increasing age the nucleus loses water, becomes firmer and the distinction between the two regions is less clear than early in life. The disc is separated from the bone by a thin layer of hyaline cartilage, the third region. In adulthood the cartilage endplate and the disc itself normally have no blood vessels of their own but rely on the blood supply of adjacent tissues, such as ligaments and vertebral body, to transport nutrients and remove waste products. Only the outer portion of the disc is innervated.

Figure 1. The relative proportions of the three main components of the normal adult human intervertebral disc and cartilage endplate.

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Composition

The disc, like other cartilage, consists mainly of a matrix of collagen fibres (which are embedded in a gel of proteoglycan) and of water. These together make up 90 to 95% of the total tissue mass, although the proportions vary with location within the disc and with age and degeneration. There are cells interspersed throughout the matrix that are responsible for synthesizing and maintaining the different components within it (figure 2). A review of the biochemistry of the disc can be found in Urban and Roberts 1994.

Figure 2. Schematic representation of disc structure, showing banded collagen fibres interspersed with numerous bottle-brush-like proteoglycan molecules and few cells.

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Proteoglycans: The major proteoglycan of the disc, aggrecan, is a large molecule consisting of a central protein core to which many glycosaminoglycans (repeating chains of disaccharides) are attached (see figure 3). These side chains have a high density of negative charges associated with them, thus making them attractive to water molecules (hydrophilic), a property described as swelling pressure. It is very important to the functioning of the disc.

 

 

 

 

 

Figure 3. Diagram of part of a disc proteoglycan aggregate. G1, G2 and G3 are globular, folded regions of the central core protein.

MUS020F3Huge aggregates of proteoglycans can form when individual molecules link onto a chain of another chemical, hyaluronic acid. The size of aggrecans varies (ranging in molecular weight from 300,000 to 7 million dalton) depending on how many molecules make up the aggregate. Other smaller types of proteoglycans have recently also been found in the disc and cartilage endplate—for example, decorin, biglycan, fibromodulin and lumican. Their function is generally unknown but fibromodulin and decorin may be involved in regulating collagen network formation.

Water: Water is the main constituent in the disc, making up 65 to 90% of the tissue volume, depending on age and region of the disc. There is a correlation between the quantity of proteoglycan and the water content of the matrix. The amount of water also varies depending on the load applied to the disc, hence water content differs night and day since load will be very different when sleeping. Water is important both to the mechanical functioning of the disc and for providing the medium for transport of dissolved substances within the matrix.

Collagen: Collagen is the main structural protein in the body, and consists of a family of at least 17 distinct proteins. All collagens have helical regions and are stabilized by a series of intra- and inter- molecular crosslinks, which make the molecules very strong in resisting mechanical stresses and enzymatic degradation. The length and shape of different types of collagen molecules and the proportion which is helical, vary. The disc is composed of several types of collagens, with the outer annulus being predominantly type I collagen, and the nucleus and cartilage endplate predominantly type II. Both types form fibrils which provide the structural framework of the disc. The fibrils of the nucleus are much finer (>> mm in diameter) than those of the annulus (0.1 to 0.2 mm in diameter). The disc cells are often surrounded by a capsule of some of the other types of collagen, such as type VI.

Cells: The intervertebral disc has a very low density of cells in comparison to other tissues. Although the density of cells is low, their continuing activity is vital for the health of the disc, as the cells produce macromolecules throughout life, to replace those which break down and are lost with the passage of time.

Function

The main function of the disc is mechanical. The disc transmits load along the spinal column and also allows the spine to bend and twist. The loads on the disc arise from body weight and muscular activity, and change with posture (see figure 4). During daily activities the disc is subject to complex loads. Extending or flexing the spine produces mainly tensile and compressive stresses on the disc, which increase in magnitude going down the spine, due to differences in body weight and geometry. Rotating the spine produces crosswise (shear) stresses.

Figure 4. Relative intradiscal pressures in different postures compared to the pressure in upright standing (100%).

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Discs are under pressure, which varies with posture from around 0.1 to 0.2 MPa at rest, to around 1.5 to 2.5 MPa while bending and lifting. The pressure is mainly due to water pressure across the nucleus and inner annulus in a normal disc. When load on the disc is increased, pressure is distributed evenly across the endplate and throughout the disc.

During loading the disc deforms and loses height. The endplate and annulus bulge, increasing the tension on these structures, and the pressure of the nucleus consequently rises. The degree of deformation of the disc depends on the rate at which it is loaded. The disc can deform considerably, compressing or extending by 30 to 60% during flexion and extension. Distances between adjacent spinal processes can increase by over 300%. If a load is re- moved within a few seconds, the disc quickly returns to its former state, but if the load is maintained, the disc continues to lose height. This “creep” results from the continuing deformation of the disc structures and also from fluid loss, because discs lose fluid as a result of the increased pressure. Between 10 and 25% of the disc’s fluid is slowly lost during daily activities, when the disc is under much greater pressures, and regained when lying down at rest. This loss of water can lead to a decrease in an individual’s height of 1 to 2 cm from morning to evening among dayworkers.

As the disc changes its composition because of ageing or degeneration, the response of the disc to mechanical loads also changes. With a loss of proteoglycan and thus water content, the nucleus can no longer respond as efficiently. This change results in uneven stresses across the endplate and the annulus fibres, and, in severe cases of degeneration, the inner fibres may bulge inward when the disc is loaded, which, in turn, may lead to abnormal stresses on other disc structures, eventually causing their failure. The rate of creep is also increased in degenerated discs, which thus lose height faster than normal discs under the same load. Narrowing of the disc space affects other spinal structures, such as muscles and ligaments, and, in particular, leads to an increase in pressure on the facet joints, which may be the cause of the degenerative changes seen in the facet joints of spines with abnormal discs.

Contribution of Major Components to Function

Proteoglycans

Disc function depends on maintaining equilibrium in which the water pressure of the disc is balanced by the disc swelling pressure. The swelling pressure depends on the concentration of ions attracted into the disc by the negatively charged proteoglycans, and thus depends directly on the concentration of proteoglycans. If the load on the disc is increased, water pressure rises and disturbs the equilibrium. To compensate, fluid seeps out of the disc, increasing proteoglycan concentration and disc osmotic pressure. Such fluid expression continues either until the balance is restored or the load on the disc is removed.

Proteoglycans affect fluid movement in other ways, as well. Because of their high concentration in the tissue, the spaces between chains are very small (0.003 to 0.004 mm). Fluid flow through such small pores is very slow, and thus even though there is a large pressure differential, the rate at which fluid is lost, and hence the rate of disc creep, is slow. However, since discs which have degenerated have lower proteoglycan concentrations, fluid can flow through the matrix faster. This may be why degenerated discs lose height more quickly than normal discs. The charge and high concentration of proteoglycans control the entry and movement of other dissolved substances into the disc. Small molecules (nutrients like glucose, oxygen) can easily enter the disc and move through the matrix. Electropositive chemicals and ions, such as Na+or Ca2+, have higher concentrations in the negatively charged disc than in the surrounding interstitial fluid. Large molecules, such as serum albumin or immunoglobulins, are too bulky to enter the disc, and are present only in very low concentrations. Proteoglycans may also affect cellular activity and metabolism. Small proteoglycans, such as biglycan, may bind growth factors and other mediators of cellular activity, releasing them when the matrix is degraded.

Water

Water is the major component of the disc and rigidity of the tissue is maintained by the hydrophilic properties of the proteoglycans. With initial loss of water, the disc becomes more flaccid and deformable as the collagen network relaxes. However, once the disc has lost a significant fraction of water, its mechanical properties change drastically; the tissue behaves more like a solid than a composite under load. Water also provides the medium through which nutrients and wastes are exchanged between the disc and the surrounding blood supply.

Collagen

The collagen network, which can support high tensile loads, provides a framework for the disc, and anchors it to the neighbouring vertebral bodies. The network is inflated by the water taken in by the proteoglycans; in turn the network restrains the proteoglycans and prevents them from escaping from the tissue. These three components together thus form a structure which is able to support high compressive loads.

The organization of the collagen fibrils provides the disc with its flexibility. The fibrils are arranged in layers, with the angle at which the fibrils of each layer run between the neighbouring vertebral bodies, alternating in direction. This highly specialized weave allows the disc to wedge extensively, thus allowing bending of the spine, even though collagen fibrils themselves can extend by only about 3%.

Metabolism

The cells of the disc produce both large molecules and enzymes which can break down matrix components. In a healthy disc, the rates of matrix production and breakdown are balanced. If the balance is upset, the composition of the disc must ultimately change. In growth, synthesis rates for new and replacement mol-ecules are higher than degradation rates, and matrix materials accumulate around the cells. With ageing and degeneration, the reverse occurs. Proteoglycans normally last for about two years. Collagen lasts for many more years. If the balance is disturbed, or if cellular activity falls, the proteoglycan content of the matrix eventually decreases, which affects the mechanical properties of the disc.

Disc cells also respond to changes in mechanical stress. Loading affects disc metabolism, although the mechanisms are not clear. At present it is impossible to predict which mechanical demands encourage a stable balance, and which may favour degradation over synthesis of matrix.

Supply of nutrients

Because the disc receives nutrients from the blood supply of the adjacent tissues, the nutrients such as oxygen and glucose must diffuse through the matrix to the cells in the centre of the disc. Cells may be as much as 7 to 8 mm from the nearest blood supply. Steep gradients develop. At the interface between the disc and the vertebral body, the concentration of oxygen is around 50%, while at the centre of the disc it is below 1%. Disc metabolism is mainly anaerobic. When oxygen falls below 5%, the disc increases production of lactate, a metabolic waste product. The lactate concentration in the centre of the nucleus may be six to eight times higher than that in the blood or interstitium (see figure 5).

Figure 5. The main nutritional pathways to the inter- vertebral disc are by diffusion from the vasculature within the vertebral body (V), through the endplate (E) to the nucleus (N) or from the blood supply outside the annulus (A).

MUS020F5

A fall in the supply of nutrients is often suggested to be a major cause of disc degeneration. Endplate permeability of the disc decreases with age, which may impede nutrient transport into the disc and could lead to accumulation of wastes, such as lactate. In discs where nutrient transport has been reduced, oxygen concentrations in the disc centre can fall to very low levels. Here anaerobic metabolism, and consequently lactate production, increases, and the acidity in the disc centre may fall to as low as pH 6.4. Such low values of pH, as well as low oxygen tensions, reduce the rate of matrix synthesis, resulting in a fall in proteoglycan content. In addition, the cells themselves may not survive prolonged exposure to acid pH. A high percentage of dead cells have been found in human discs.

Degeneration of the disc leads to a loss of proteoglycan and a shift in its structure, disorganization of the collagen network and an ingrowth of blood vessels. There is the possibility that some of these changes could be reversed. The disc has been shown to have some capability of repair.

Diseases

Scoliosis: Scoliosis is a sideways bend of the spine, where both the intervertebral disc and vertebral bodies are wedged. It is usually associated with a twisting or rotation of the spine. Because of the manner in which the ribs are attached to the vertebrae this gives rise to a “rib hump”, visible when the affected individual bends forward. Scoliosis may be due to a congenital defect in the spine, such as a wedge-shaped hemi-vertebra, or it may arise secondary to a disorder such as neuromuscular dystrophy. However, in the majority of cases the cause is unknown and it is hence termed idiopathic scoliosis. Pain is rarely a problem in scoliosis and treatment is carried out, mainly to halt further development of the lateral curvature of the spine. (For details on clinical treatment of this and other spinal pathologies see Tidswell 1992.)

Spondylolisthesis: Spondylolisthesis is a forward, horizontal slip of one vertebra in relation to another. It may result from a fracture in the bridge of bone connecting the front to the posterior of the vertebra. Obviously the intervertebral disc between two such vertebrae is stretched and subjected to abnormal loads. The matrix of this disc, and to a lesser extent, adjacent discs, shows changes in composition typical of degeneration—loss of water and proteoglycan. This condition can be diagnosed by x ray.

Ruptured or prolapsed disc: Rupture of the posterior annulus is quite common in physically active young or middle-aged adults. It cannot be diagnosed by x ray unless a discogram is carried out, whereby radio-opaque material is injected into the centre of the disc. A tear can then be demonstrated by the tracking of the discogram fluid. Sometimes isolated and sequestered pieces of disc material can pass through this tear into the spinal canal. Irritation or pressure on the sciatic nerve causes intense pain and paraesthesia (sciatica) in the lower limb.

Degenerative disc disease: This is a term applied to an ill-defined group of patients who present with low-back pain. They may show changes in the x ray appearance, such as a decrease in disc height and possibly osteophyte formation at the rim of the vertebral bodies. This group of patients could represent the endstage of several pathological pathways. For example, untreated annular tears may eventually take on this form.

Spinal stenosis: The narrowing of the spinal canal that occurs in spinal stenosis causes mechanical compression of spinal nerve roots and its blood supply. As such it can lead to symptoms such as weakness, altered reflexes, pain or loss of feeling (paraesthesia), or sometimes have no symptoms. The narrowing of the canal can, in turn, be caused by various factors including protrusion of the intervertebral disc into the canal space, new bone formation in the facet joints (facet hypertrophy) and arthritis with inflammation of other soft connective tissues.

Interpretation of more recent imaging techniques in relation to disc pathology has not been completely established. For example, degenerated discs on magnetic resonance imaging (MRI) give an altered signal from that seen for “normal” discs. However, the correlation between a disc of “degenerate” appearance on MRI and clinical symptoms is poor, with 45% of MRI-degenerate discs being asymptomatic and 37% of patients with low-back pain having normal MRI of the spine.

Risk Factors

Loading

Load on the discs depends on posture. Intradiscal measurements show that the sitting position leads to pressures five times greater than those within the resting spine (see Figure 8). If external weights are lifted this can greatly increase the intradiscal pressure, especially if the weight is held away from the body. Obviously an increased load can lead to a rupture in discs that otherwise might remain intact.

Epidemiological investigations reviewed by Brinckmann and Pope (1990) agree in one respect: repetitive lifting or carrying of heavy objects or performing work in flexed or hyperextended posture represent risk factors for low-back problems. Similarly, certain sports, such as weight lifting, may be associated with a higher incidence of back pain than, for example, swimming. The mechanism is not clear, although the different loading patterns could be relevant.

Smoking

The nutrition of the disc is very precarious, requiring only a small reduction in the flow of nutrients to render it insufficient for the normal metabolism of the disc cells. Cigarette smoking can cause such a reduction because of its effect on the circulatory system outside the intervertebral disc. The transport of nutrients, such as oxygen, glucose or sulphate, into the disc is significantly reduced after only 20 to 30 minutes of smoking, which may explain the higher incidence of low-back pain in individuals who smoke compared to those who do not (Rydevik and Holm 1992).

Vibration

Epidemiological studies have shown that there is an increased incidence of low-back pain in individuals exposed to high levels of vibration. The spine is susceptible to damage at its natural frequencies, particularly from 5 to 10 Hz. Many vehicles excite vibrations at these frequencies. Studies reported by Brinckmann and Pope (1990) have shown a relationship between such vibrations and the incidence of low-back pain. Since vibration has been shown to affect the small blood vessels in other tissues, this may also be the mechanism for its effect on the spine.

 

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Wednesday, 16 February 2011 23:00

Low-Back Region

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Low-back pain is a common ailment in populations of working age. About 80% of people experience low-back pain during their lifetime, and it is one of the most important causes for short- and long-term disability in all occupational groups. Based on the aetiology, low-back pain can be classified into six groups: mechanical, infectious (e.g., tuberculosis), inflammatory (e.g., ankylosing spondylitis), metabolic (e.g., osteoporosis), neoplastic (e.g., cancer) and visceral (pain caused by diseases of the inner organs).

The low-back pain in most people has mechanical causes, which include lumbosacral sprain/strain, degenerative disc disease, spondylolisthesis, spinal stenosis and fracture. Here only mechanical low-back pain is considered. Mechanical low-back pain is also called regional low-back pain, which may be local pain or pain radiating to one or both legs (sciatica). It is characteristic for mechanical low-back pain to occur episodically, and in most cases the natural course is favourable. In about half of acute cases low-back pain subsides in two weeks, and in about 90% within two months. About every tenth case is estimated to become chronic, and it is this group of low-back pain patients that accounts for the major proportion of the costs due to low-back disorders.

Structure and Function

Due to upright posture the structure of the lower part of the human spine (lumbosacral spine) differs anatomically from that of most vertebrate animals. The upright posture also increases mechanical forces on the structures in the lumbosacral spine. Normally the lumbar spine has five vertebrae. The sacrum is rigid and the tail (coccyx) has no function in human beings as shown in figure 1.

Figure 1. The spine, its vertebrae and curvature.

MUS130F1

The vertebrae are bound together by intervertebral discs between the vertebral bodies, and by ligaments and muscles. These soft-tissue bindings make the spine flexible. Two adjacent vertebrae form a functional unit, as shown in figure 2. The vertebral bodies and the discs are the weight-bearing elements of the spine. The posterior parts of the vertebrae form the neural arch that protects the nerves in the spinal canal. The vertebral arches are attached to each other via facet joints (zygapophyseal joints) that determine the direction of motion. The vertebral arches are also bound together by numerous ligaments that determine the range of motion in the spine. The muscles that extend the trunk backward (extensors) are attached to the vertebral arches. Important attachment sites are three bony projections (two lateral and the spinal process) of the vertebral arches.                  

Figure 2. The basic functional unit of the spine.

MUS130F2

The spinal cord terminates at the level of the highest lumbar vertebrae (L1-L2). The lumbar spinal canal is filled by the extension of the spinal cord, cauda equina, which is composed of the spinal nerve roots. The nerve roots exit the spinal canal pairwise through intervertebral openings (foramina). A branch innervating the tissues in the back departs from each of the spinal nerve roots. There are nerve endings transmitting pain sensations (nociceptive endings) in muscles, ligaments and joints. In a healthy intervertebral disc there are no such nerve endings except for the outermost parts of the annulus. Yet, the disc is considered the most important source of low-back pain. Annular ruptures are known to be painful. As a sequel of disc degeneration a herniation of the semigelatinous inner part of the intervertebral disc, the nucleus, can occur into the spinal canal and lead to compression and/or inflammation of a spinal nerve along with symptoms and signs of sciatica, as shown in figure 3.

Figure 3. Herniation of the intervertebrai disc.

MUS130F3

Muscles are responsible for the stability and motion of the back. Back muscles bend the trunk backward (extension), and abdominal muscles bend it forward (flexion). Fatigue due to sustained or repetitive loading or sudden overexertion of muscles or ligaments can cause low-back pain, albeit the exact origin of such pain is difficult to localize. There is controversy about the role of soft tissue injuries in low-back disorders.

Low-Back Pain

Occurrence

The prevalence estimates of low-back pain vary depending on the definitions used in different surveys. The prevalence rates of low-back pain syndromes in the Finnish general population over 30 years of age are given in table 1. Three in four people have experienced low-back pain (and one in three, sciatic pain) during their lifetime. Every month one in five people suffers from low-back or sciatic pain, and at any point in time, one in six people has a clinically verifiable low-back pain syndrome. Sciatica or herniated intervertebral disc is less prevalent and afflicts 4% of the population. About half of those with a low-back pain syndrome have functional impairment, and the impairment is severe in 5%. Sciatica is more common among men than among women, but other low-back disorders are equally common. Low-back pain is relatively uncommon before the age of 20, but then there is a steady increase in the prevalence until the age of 65, after which there is a decline.

Table 1. Prevalence of back disorders in the Finnish population over 30 years of age, percentages.

 

Men+

Women+

Lifetime prevalence of back pain

76.3

73.3

Lifetime prevalence of sciatic pain

34.6

38.8

Five-year prevalence of sciatic pain having caused bedrest for at least two weeks

17.3

19.4

One-month prevalence of low-back or sciatic pain

19.4

23.3

Point prevalence of clinically verified:

   

Low-back pain syndrome

17.5

16.3

Sciatica or prolapsed disc*

5.1

3.7

+ age-adjusted
* p < 0.005
Source: Adapted from Heliövaara et al. 1993.

The prevalence of degenerative changes in the lumbar spine increases with increasing age. About half of 35- to 44-year-old men and nine out of ten men 65 years or older have radiographic signs of disc degeneration of the lumbar spine. Signs of severe disc degeneration are noted in 5 and 38%, respectively. Degener-ative changes are slightly more common in men than in women. People who have degenerative changes in the lumbar spine have low-back pain more frequently than those without, but degener-ative changes are also common among asymptomatic people. In magnetic resonance imaging (MRI), disc degeneration has been found in 6% of asymptomatic women 20 years or younger and in 79% of those 60 years or older.

In general, low-back pain is more common in blue-collar occupations than in white-collar occupations. In the United States, materials handlers, nurses’ aides and truck drivers have the highest rates of compensated back injuries.

Risk factors at work

Epidemiological studies have quite consistently found that low-back pain, sciatica or herniated intervertebral disc and degener-ative changes of the lumbar spine are associated with heavy physical work. Little is known, however, of the acceptable limits of physical load on the back.

Low-back pain is related to frequent or heavy lifting, carrying, pulling and pushing. High tensile forces are directed to the muscles and ligaments, and high compression to the bones and joint surfaces. These forces can cause mechanical injuries to the vertebral bodies, intervertebral discs, ligaments and the posterior parts of the vertebrae. The injuries may be caused by sudden overloads or fatigue due to repetitive loading. Repeated microtrauma, which may even occur without being noticed, have been proposed as a cause for degeneration of the lumbar spine.

Low-back pain is also associated with frequent or prolonged twisting, bending or other non-neutral trunk postures. Motion is necessary for the nutrition of the intervertebral disc and static postures may impair the nutrition. In other soft tissues, fatigue can develop. Also prolonged sitting in one position (for instance, machine seamstresses or motor vehicle drivers) increases the risk of low-back pain.

Prolonged driving of motor vehicles has been found to increase the risk of low-back pain and sciatica or herniated disc. Drivers are exposed to whole-body vibration that has an adverse effect on disc nutrition. Also sudden impulses from rough roads, postural stress and materials handling by professional drivers may contribute to the risk.

An obvious cause for back injuries is direct trauma caused by an accident such as falling or slipping. In addition to the acute injuries, there is evidence that traumatic back injuries contribute substantially to the development of chronic low-back syndromes.

Low-back pain is associated with various psychosocial factors at work, such as monotonous work and working under time pressure, and poor social support from co-workers and superiors. The psychosocial factors affect reporting and recovery from low-back pain, but there is controversy about their aetiological role.

Individual risk factors

Height and overweight: Evidence for a relationship of low-back pain with body stature and overweight is contradictory. Evidence is, however, quite convincing for a relationship between sciatica or herniated disc and tallness. Tall people may have a nutritional disadvantage due to a greater disc volume, and they may also have ergonomic problems at the worksite.

Physical fitness: Study results on an association between physical fitness and low-back pain are inconsistent. Low-back pain is more common in people who have less strength than their job requires. In some studies poor aerobic capacity has not been found to predict future low-back pain or injury claims. The least fit people may have an increased overall risk for back injuries, but the most fit people may have the most expensive injuries. In one study, good back muscle endurance prevented first-time occurrence of low-back pain.

There is considerable variation in the mobility of the lumbar spine among people. People with acute and chronic low-back pain have reduced mobility, but in prospective studies mobility has not predicted the incidence of low-back pain.

Smoking: Several studies have shown that smoking is associated with an increase in the risk of low-back pain and herniated disc. Smoking also seems to enhance disc degeneration. In experimental studies, smoking has been found to impair the nutrition of the disc.

Structural factors: Congenital defects of the vertebrae as well as unequal leg length can cause abnormal loading in the spine. Such factors are, however, not considered very important in the caus-ation of low-back pain. Narrow spinal canal predisposes to nerve root compression and sciatica.

Psychological factors: Chronic low-back pain is associated with psychological factors (e.g., depression), but not all people who suffer from chronic low-back pain have psychological problems. A variety of methods have been used to differentiate low-back pain caused by psychological factors from low-back pain caused by physical factors, but the results have been contradictory. Mental stress symptoms are more common among people with low-back pain than among symptomless people, and mental stress even seems to predict the incidence of low-back pain in the future.

Prevention

The accumulated knowledge based on epidemiological studies on the risk factors is largely qualitative and thus can give only broad guidelines for the planning of preventive programmes. There are three principal approaches in prevention of work-related low-back disorders: ergonomic job design, education and training, and worker selection.

Job design

It is widely believed that the most effective means to prevent work-related low-back disorders is job design. An ergonomic intervention should address the following parameters (shown in table 2).

 

Table 2. Parameters which should be addressed in order to reduce the risks for low-back pain at work.

Parameter

Example

1.  Load

The weight of the object handled, the size of the object handled

2.  Object design

The shape, location and size of handles

3.  Lifting technique

The distance from the centre of gravity of the object and the worker, twisting motions

4.  Workplace layout

The spatial features of the task, such as carrying distance, range of motion, obstacles such as stairs

5.  Task design

Frequency and duration of the tasks

6.  Psychology

Job satisfaction, autonomy and control, expectations

7.  Environment

Temperature, humidity, noise, foot traction, whole-body vibration

8.  Work organization

Team work, incentives, shifts, job rotation, machine pacing, job security.

Source: Adapted from Halpern 1992.

 

Most ergonomic interventions modify the loads, the design of objects handled, lifting techniques, workplace layout and task design. The effectiveness of these measures in controlling the occurrence of low-back pain or medical costs has not been clearly demonstrated. It may be most efficient to reduce the peak loads. One suggested approach is to design a job so that it is within the physical capacity of a large percentage of the working population (Waters et al. 1993).  In static jobs restoration of motion can be achieved by restructuring the job, by job rotation or job enrichment.

Education and training

Workers should be trained to perform their work appropriately and safely. Education and training of workers in safe lifting have been widely implemented, but the results have not been convincing. There is general agreement that it is beneficial to keep the load close to the body and to avoid jerking and twisting, but as to the advantages of leg lift and back lift, the opinions of the experts are conflicting.

If mismatch between job demands and the strength of workers is detected and job redesign is not possible, a fitness training programme should be provided for the workers.

In prevention of disability due to low-back pain or chronicity, back school has proven effective in subacute cases, and general fitness training in subchronic cases.

Training needs to be extended also to management. Aspects of management training include early intervention, initial conservative treatment, patient follow-up, job placement and enforcement of safety rules. Active management programmes can significantly reduce long-term disability claims and accident rates.

Medical personnel should be trained in the benefits of early intervention, conservative treatment, patient follow-up and job placement techniques. The Quebec Task Force report on the management of activity-related spinal disorders and other clinical practice guidelines gives sound guidance for proper treatment. (Spitzer et al. 1987; AHCPR 1994.)

Worker selection

In general, pre-employment selection of workers is not considered an appropriate measure for prevention of work-related low-back pain. History of previous back trouble, radiographs of the lumbar spine, general strength and fitness testing—none of these has shown good enough sensitivity and specificity in identifying persons with an increased risk for future low-back trouble. The use of these measures in pre-employment screening can lead to undue discrimination against certain groups of workers. There are, however, some special occupational groups (e.g., fire-fighters and police officers) in which pre-employment screening can be considered appropriate.

Clinical characteristics

The exact origin of low-back pain often cannot be determined, which is reflected as difficulties in the classification of low-back disorders. To a great extent the classification relies on symptom characteristics supported by clinical examination or by imaging results. Basically, in clinical physical examination patients with sciatica caused by compression and/or inflammation of a spinal nerve root can be diagnosed. As to many other clinical entities, such as facet syndrome, fibrositis, muscular spasms, lumbar compartment syndrome or sacro-iliac syndrome, clinical verification has proven unreliable.

As an attempt to resolve the confusion the Quebec Task Force on Spinal Disorders carried out a comprehensive and critical literature review and ended up recommending the use of the classification for low-back pain patients shown in table 3.


Table 3. Classification of low-back disorders according to the Quebec Task Force on Spinal Disorders

1.         Pain

2.         Pain with radiation to lower limb proximally

3.         Pain with radiation to lower limb distally

4.         Pain with radiation to lower limb and neurological signs

5.         Presumptive compression of a spinal nerve root on a simple radiogram (i.e., spinal instability or fracture)

6.         Compression of a spinal nerve root confirmed by: Specific imaging techniques (computerized tomography,  

            myelography, or magnetic resonance imaging), Other diagnostic techniques (e.g., electromyography,

            venography)

7.         Spinal stenosis

8.         Postsurgical status, 1-6 weeks after intervention

9.         Postsurgical status, >6 weeks after intervention

9.1.      Asymptomatic

9.2.      Symptomatic

10.       Chronic pain syndrome

11.       Other diagnoses

For categories 1-4, additional classification is based on
(a) Duration of symptoms (< 7 days; 7 days-7 weeks; >7 weeks),
(b) Working status (working; idle, i.e., absent from work, unemployed or inactive).

Source: Spitzer et al. 1987.


 

For each category, appropriate treatment measures are given in the report, based on critical review of the literature.

Spondylolysis and spondylolisthesis

Spondylolysis means a defect in the vertebral arch (pars inter- articularis or isthmus), and spondylolisthesis denotes forward displacement of a vertebral body relative to the vertebra below. The derangement occurs most frequently at the fifth lumbar vertebra.

Spondylolisthesis can be caused by congenital abnormalities, by a fatigue fracture or an acute fracture, instability between two adjacent vertebrae due to degeneration, and by infectious or neo- plastic diseases.

The prevalence of spondylolysis and spondylolisthesis ranges from 3 to 7%, but in certain ethnic groups the prevalence is considerably higher (Lapps, 13%; Eskimos in Alaska, 25 to 45%; Ainus in Japan, 41%), which indicates a genetic predisposition. Spondylolysis is equally common in people with and without low-back pain, but people with spondylolisthesis are susceptible to recurrent low-back pain.

An acute traumatic spondylolisthesis can develop due to an accident at work. The prevalence is increased among athletes in certain athletic activities, such as American football, gymnastics, javelin throwing, judo and weight lifting, but there is no evidence that physical exertion at work would cause spondylolysis or spondylolisthesis.

Piriformis syndrome

Piriformis syndrome is an uncommon and controversial cause for sciatica characterized by symptoms and signs of sciatic nerve compression at the region of the piriformis muscle where it passes through the greater sciatic notch. No epidemiological data on the prevalence of this syndrome are available. The present knowledge is based on case reports and case series. Symptoms are aggravated by prolonged hip flexion, adduction and internal rotation. Recently piriformis muscle enlargement has been verified in some cases of piriformis syndrome by computed tomography and magnetic resonance imaging. The syndrome can result from an injury to the piriformis muscle.

 

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Wednesday, 16 February 2011 23:22

Thoracic Spine Region

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The most common symptoms and signs that occur in the upper region of the back and spine are pain, tenderness, weakness, stiffness and/or deformity in the back. Pain is much more frequent in the lower (lumbar) back and in the neck than in the upper trunk (thoracic back). Besides local symptoms, the thoracic disorders may cause pain that radiates to the lumbar region and the lower limbs, to the neck and shoulders, to the rib cage and to the abdomen.

Painful Soft-Tissue Disorders

The causes of thoracic back pain are multifactorial and often obscure. The symptoms in many cases arise from an overuse, an overstretching and/or usually mild ruptures of the soft tissues. There are, however, also many specific disorders that can lead to back pain, such as severe scoliosis (hunchback) or kyphosis of different aetiology, Morbus Sheuermann (osteochondritis of the thoracic spine, sometimes painful in adolescents but seldom in adults), and other deformities which may follow trauma or some neurologic and muscular diseases. Infection in the spine (spondylitis) is often localized to the thoracic region. Many kinds of microbes may cause spondylitis, such as tuberculosis. Thoracic back pain may occur in rheumatic diseases, especially in ankylosing spondylitis and in severe osteoporosis. Many other intraspinal, intrathoracal and intra-abdominal diseases, such as tumours, may also result in back symptoms. Generally, it is common that the pain may be felt in the thoracic spine (referred pain). Skeletal metastases of cancer from other sites are frequently localized to the thoracic spine; this is especially true of metastatic breast, kidney, lung and thyroid cancers. It is extremely rare for a thoracic disc to rupture, the incidence being 0.25 to 0.5% of all intervertebral disc ruptures.

Examination: At examination many intra- and extraspinal disorders causing symptoms in the thoracic back should always be kept in mind. The older the patient, the more frequent the back symptoms arising from primary tumours or metastases. A comprehensive interview and a careful examination are therefore very important. The purpose of the examination is to clarify the aetio-logy of the disease. The clinical examination should include ordinary procedures, such as inspection, palpation, testing of the muscle strength, the joint mobility, the neurological state and so on. In cases with prolonged and severe symptoms and signs, and when a specific disease is suspected by plain x ray, other radiography tests, such as MRI, CT, isotope imaging and ENMG can contribute to clarifying the aetiological diagnosis and to localizing the disorder process. Nowadays, MRI is usually the radiological method of choice in thoracic back pain.

Degenerative Thoracic Spine Disorders

All adults suffer spinal degenerative changes which progress with age. Most people do not have any symptoms from these changes, which are often found while investigating other diseases, and are usually without any clinical importance. Infrequently, the degenerative changes in the thoracic region lead to local and radiating symptoms—pain, tenderness, stiffness and neurological signs.

Narrowing of the spinal canal, spinal stenosis, may lead to compression of vascular and neurologic tissues resulting in local and/or radiating pain and neurologic deficiency. A thoracic disc prolapse seldom provokes symptoms. In many cases a radiologically detected disc prolapse is a side finding and does not provoke any symptoms.

The main signs of degenerative disorders of the thoracic spine are local tenderness, muscle spasm or weakness and locally decreased mobility of the spine. In some cases there may be neurological disturbances—muscle paresis, reflex and sensation deficiencies locally and/or distally of the affected tissues.

The prognosis in thoracic disc prolapse is usually good. The symptoms subside as in the lumbar and neck region within a few weeks.

Examination. A proper examination is essential especially in old persons in prolonged and severe pain and in paresis. Besides a detailed interview, there should be an adequate clinical examination, including inspection, palpation, testing of mobility, muscle strength and neurological state. Of the radiological examinations, plain radiography, CT and especially MRI are advantageous in evaluating the aetiological diagnosis and the localization of the pathological changes in the spine. ENMG and isotope imaging may contribute to the diagnosis. In the differential diagnosis laboratory tests may be valuable. In pure spinal disc prolapse and degenerative changes there are no specific abnormalities in the laboratory tests.

 

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Wednesday, 16 February 2011 23:23

Neck

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Pain and discomfort in the neck are some of the most common symptoms associated with work. They occur in heavy, manual work as well as in seated, sedentary work, and the symptoms often last for prolonged periods of time—in fact, in some cases, over the whole lifetime. It follows that disorders of the neck are difficult to cure once they have arisen, and therefore much emphasis should be put into primary prevention. There are three main reasons why neck disorders are common in working life:

  1. The load on the neck structures is maintained for prolonged periods of time, due to high visual demands of the job and to the need of stabilization of the neck-shoulder region in working with the arms.
  2. Psychologically demanding jobs with high demands on concentration and on quality and quantity of work output are common, and induce an increased activity in neck muscles. This tension increases further if the job in general is psychologically stressful, due to, for example, poor industrial relations, little influence on the organization of work and so on.
  3. The discs and joints of the neck are frequently the site of degenerative changes, which increase in prevalence with age. This reduces the capacity to withstand occupational workloads. It is also likely that the rate of degeneration increases as the result of physical demands of the job.

 

Anatomy and Biomechanics of the Neck

The musculoskeletal part of the neck consists of seven vertebral bodies, six intervertabral discs (consisting of cartilage), ligaments to hold these together and linking them to the skull and to the thoracic spine, and muscles surrounding the spine. Although each joint of the cervical spine has a very limited range of motion, the neck can be bent, extended, twisted and tilted with a relatively large range of motion (see table 1). In a normal upright posture and looking straight forward, the centre of gravity of the head and neck is actually situated in front of the centre of support, and therefore needs to be balanced by the dorsal muscles, that is, those situated behind the vertebral bodies. When the head is tilted forward more muscle force is needed to balance the head, and when forward tilt of the head is maintained for prolonged periods of time a substantial muscle fatigue can develop. In addition to muscle fatigue, tilting and bending the head leads to increased compression of the inter-vertebral discs, which may accelerate degenerative processes.

Table 1. Normal and permissible for prolonged driving range of motion (ROM) in degrees, for head.

 

Normal1

Permissible2 for prolonged driving

Lateral bend

45

Twist

60

0 – 15

Flexion

45

0 – 25

Extension

–45

0 – –5

1 American Academy of Orthopedic Surgeons 1988.
2 Hansson 1987

The muscles surrounding the neck are also active in arm work, in order to stabilize the shoulder/arm complex. The trapezius and several other muscles originate on the cervical spine and extend downwards/outwards to insert on the shoulder. These muscles are commonly the site of dysfunction and disorders, especially in static or repetitive work tasks where the arms are elevated and the vision is fixed.

The structures stabilizing the neck are very robust, which serves to protect the nervous tissue inside the spinal canal and the nerves emerging from the intervertebral openings and supplying the neck, upper extremity and upper part of the thorax. The intervertebral discs, the adjoining parts of the vertebral bodies and the facet joints of the intervertebral foramina are often the site of degenerative changes, which can exert pressure on the nerves and narrow their space. (See figure 1).

Figure 1. Schematic drawing of a cross-section of three of the lower cervical vertebral bodies (1) with intervertebral discs; (2) intervertebral foramina; (3) and nerve roots; (4) seen from the side.

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As mentioned in the introduction, symptoms like pain, ache and discomfort in the neck are very common. Depending on the criteria used and the method of investigation, the prevalence rates for neck disorders vary. If a postal enquiry or an interview focusing on musculoskeletal disorders is used, the prevalence of disorders is usually higher than in a thorough investigation also including a physical examination. Thus comparisons between groups should be made only when the same investigation technique has been employed. Figure 2 gives one-year prevalence figures for a representative sample of the Icelandic population who answered a postal enquiry, the so-called “Nordic” questionnaire on musculo- skeletal disorders (Kuorinka et al. 1987). Neck trouble (pain, ache or discomfort) was the third most common (38% average for the whole sample), after shoulder (43%), and low-back (56%) problems. Neck trouble among women was more common than among men, and there was an increase in prevalence up to age 25 to 30, when the rates stabilized; they again went down somewhat at age 50 to 55. In a representative sample of 200 men and women from Stockholm, aged 16 to 65 years, the 12-month prevalence was about 30% among the men and 60% among women. The experience of recent pain in the neck with a duration of at least one month, was found among 22% of a population sample in Gothenburg, Sweden—again rated third most common after shoulder and low-back pain.

Figure 2. Twelve-month prevalence of symptoms of neck trouble of a random sample of the Icelandic population (n=1000)

MUS080F3

Risk Factors at Work

Neck disorders are considerably more prevalent in certain occupational groups. Using the Nordic questionnaire (Kuorinka et al. 1987), Swedish occupational health services have compiled data from several occupations. The results indicate that the risk of neck trouble (pain, ache or discomfort) is very high among visual display unit (VDU) operators, sewing machine operators, seamstresses and electronic assembly workers, with a 12-month period prevalence greater than 60%. In addition, up to one-third of those who report disorders also state that the problems have an impact on their working lives, either causing them to take sick-leave, or necessitating a change of job or work tasks.

Epidemiological studies of neck and shoulder disorders have been reviewed, and the different studies have been pooled by type of exposure (repetitive work and work above shoulder level, respectively). Soft-tissue disorders of the neck, such as tension neck and other myalgias, were considerably increased in a number of occupational tasks like data entry, typing, scissors manufacturing, lamp assembly and film rolling.

Degenerative disorders of the intervertebral discs of the neck are more common among coal-miners, dentists and meat industry workers (Hagberg and Wegman 1987).

Posture

Prolonged flexion, extension, lateral bending and twisting of the neck induce muscle fatigue, and may lead to chronic muscle injuries and degenerative changes of the cervical spine. The muscle activity needed to counteract the weight of the head in forward flexion of the neck increases with the flexion angle, as shown in figure 3. Fatigue and pain are common in neck flexion if prolonged work is performed. When the head is tilted forward to the extreme of its range of motion, the main load is transferred from muscles to ligaments and joint capsules surrounding the cervical spine. It has been calculated that if the entire cervical spine is flexed maximally, the torque exerted by the head and neck on the disc between the seventh cervical and the first thoracic vertebral body is increased by a factor of 3.6. Such postures lead to pain within only about 15 minutes, and usually the posture has to be normalized within 15 to 60 minutes because of intense pain. Postures where the neck is bent forward for prolonged periods of time—several hours—are common in assembly work in industry, in VDT work and in packaging and inspection tasks where the work stations are poorly designed. Such postures are frequently caused by a compromise between the need to perform work with the hands, without elevating the arms, and the simultaneous need for visual control. For a review of the mechanisms leading from muscle fatigue to injury, see the accompanying article “Muscles”.

Figure 3. Percentage of maximal neck extension strength required at increasing neck inclination (flexion).

MUS080F5

Extension of the neck for prolonged periods, as in overhead work in the building industry, can be very tiring for the muscles in front of the cervical spine. Especially when carrying heavy protective equipment like safety helmets, the torque tilting the head backwards can be high.

 

 

 

 

 

 

 

 

 

Repetitive movements

Repetitive movements performed by the hands increase the demands on stabilization of the neck and shoulder region, and thereby increase the risk of neck complaints. Factors like high demands on speed and precision of movements, as well as high demands on force exerted by the hands, imply even larger demands on stabilization of the proximal body regions. Repetitive movements of the head are less common. Rapid and repeated changes between visual targets are usually accomplished through eye movements, unless the distance between the objects observed is fairly large. This may occur for example at large computerized work stations.

Vibration

Local vibration of the hands, such as working with drills and other vibrating hand-held machines, is transferred along the arm but the fraction transferred up to the shoulder-neck region is negligible. However, the holding of a vibrating tool may induce muscle contractions in the proximal shoulder-neck muscles in order to stabilize the hand and the tool, and may thereby exert a tiring effect on the neck. The mechanisms and the prevalence of such vibration-induced complaints are not well known.

Work organization

Work organization in this context is defined as the distribution of work tasks over time and between workers, the duration of work tasks, and the duration and distribution of rest periods and breaks. The duration of work and rest periods has a profound effect on tissue fatigue and recovery. Few specific studies on the effect of work organization on neck disorders have been performed. In a large epidemiological study in Sweden, it was found that VDU work exceeding four hours per day was associated with elevated rates of neck symptoms (Aronsson, Bergkvist and Almers 1992). These findings have subsequently been confirmed in other studies.

Psychological and social factors

Associations between psychological and social factors at work and disorders of the neck region have been demonstrated in several studies. Especially factors such as perceived psychological stress, poor control of work organization, poor relations with management and work mates and high demands on accuracy and speed of work have been highlighted. These factors have been associated with an increased risk (up to twofold) of disorders in cross-sectional studies. The mechanism is likely to be an increase of tension in the trapezius and other muscles surrounding the neck, as part of a general “stress” reaction. Since well-controlled longitudinal studies are scarce, it is still uncertain whether these factors are causal or aggravating. Moreover, poor psychological and social conditions often occur in jobs also characterized by prolonged awkward postures.

Individual factors

Individual characteristics like age, sex, muscle strength and endurance, physical fitness, body size, personality, intelligence, leisure time habits (physical activity, smoking, alcohol, diet) and previous musculoskeletal disorders have been discussed as factors which might modify the response to physical and psychosocial exposures. Age as a risk factor is discussed above and is illustrated in figure 2.

Females usually report a higher prevalence of neck symptoms than males. The most likely explanation is that exposure to both physical and psychosocial risk factors is higher in women than among men, such as in work with VDUs, assembly of small components and machine sewing.

Studies of muscle groups other than those of the neck do not consistently indicate that a low static strength implies an elevated risk of development of disorders. No data are available concerning neck muscles. In a recent study of a random population of Stockholm, low endurance at neck extension was weakly associated with later development of neck disorders (Schüldt et al. 1993). Similar results have been reported for low-back disorders.

In a longitudinal study in Sweden, personality type was a risk factor for development of shoulder-neck disorders (Hägg, Suurküla and Kilbom 1990). Those employees who had a type A personality (e.g., were ambitious and impatient) developed more serious problems than others, and these associations were not related to individual productivity.

Little is known of the association between other individual characteristics and neck disorders.

Prevention

Work station design

The work station should be organized so that the head is not statically bent, extended or twisted beyond the limits given for the permissible range of motion given for prolonged driving in table 1. Now and then, movements that are within the limits for normal range of motion are acceptable, as well as the occasional movement to the individual extremes. Experimental studies have shown that the load of the neck muscles is lower with a slightly backward tilted trunk than with a straight upright posture, which in turn is better than a forward tilted trunk (Schüldt 1988).

The set-up of the workstation and the positioning of the work object requires a careful consideration and a trade-off between the demands for optimal head and shoulder-arm posture. Usually the work object is positioned somewhat below elbow height, which may however induce a high strain on the neck muscles (e.g., in assembly work). This requires individually adjustable work stations.

Visual strain will increase the tension of the neck muscles, and therefore attention should be given to the lighting and contrasts of the work station and to readability of information given on VDUs and on printed material. For VDU work the viewing distance should be optimized to about 45 to 50 cm and the viewing angle to 10 to 20 degrees. The vision of the worker should be optimized with the aid of glasses.

Work organization

In work with static loads on the neck, such as in assembly and data entry VDU work, frequent breaks should be introduced to provide recovery from fatigue. Recommendations to introduce one break of about 10 minutes per hour and to limit VDU work to a maximum of four hours per day have been issued in some localities. As pointed out above, the scientific basis for these recommendations with regard to the neck is relatively weak.

Clinical Characteristics and Treatmentof Neck Disorders

Painful soft-tissue disorders

Tension neck and other myalgias

The most common localization for neck tension and other myalgias is in the upper part of the trapezius muscle, but other muscles originating in the neck are often affected simultaneously. Symptoms are stiffness of the neck and ache at work and at rest. Frequently, excessive muscle fatigue is perceived, even during short-lasting and low-level periods of work. The muscles are tender, and often “tender points” can be found on palpation. Tension neck is common in jobs with prolonged static loads on neck and shoulders. Microscopic examination of the tissue has shown changes in the muscle morphology, but the mechanisms are incompletely understood and are likely to involve both the blood circulation and the nervous regulation.

Acute torticollis

This state of acute pain and stiffness of the neck can be provoked by sudden twisting of the head and extension of the opposite arm. Sometimes no provoking event can be identified. Acute torticollis is believed to be caused by strain and partial ruptures of the ligaments of the neck. Usually the pain and stiffness subsides within a week following rest, external support of the neck (collar) and muscle-relaxing medication.

Degenerative disorders

Acute disorder (disc herniation)

Degeneration of the cervical spine involves the discs, which lose some of their resistance to even mild stresses. Herniation of the disc with extrusion of its contents, or bulging of it, can compromise nervous tissue and blood vessels laterally and posteriorly to the disc. One acute degenerative disorder of the disc is compression of the nerve roots extending from the spinal cord and supplying the neck, arms and upper thorax. Depending on the level of compression (disc between second and third cervical vertebrae, third and fourth, and so on), acute sensory and motor symptoms arise from the regions supplied by the nerves. The investigation of acute symptoms of the neck and arms includes a thorough neurological examination in order to identify the level of a possible disc prolapse and plain x-ray examination, usually supplemented with CT scanning and MRI.

Chronic disorders (Cervical spondylosisand cervical syndrome)

Degeneration of the cervical spine involves narrowing of the disc, formation of new bone (so-called osteophytes) extending from the edges of the cervical vertebra, and thickening of the ligaments as in acute disorder. When osteophytes extend into the foramina, they may compress the nerve roots. Spondylosis is the term used for the radiological changes in the neck. Sometimes these changes are associated with chronic local symptoms. Radiological changes may be advanced without serious symptoms and vice versa. Symptoms are usually ache and pain in the neck, sometimes extending to the head and the shoulder region, and reduced mobility. Whenever nerve roots are compressed, the diagnosis cervical syndrome is used. Symptoms of cervical syndrome are ache and pain in the neck, reduced mobility of the neck, and sensory and motor symptoms from the side of the compressed nerve root. Symptoms like reduced sensitivity to touch, numbness, tingling and reduced strength are common in the hand and arm. Thus symptoms are similar to those arising from acute disc prolapse, but usually the onset is more gradual and the severity may fluctuate depending on the external workload. Both cervical spondylosis and cervical syndrome are common in the general population, particularly among aged persons. The risk of cervical spondylosis is elevated in occupational groups with a sustained, high biomechanical load on the neck structures, like coal-miners, dentists and meat industry workers.

Traumatic disorders (whiplash injuries)

In rear-end car accidents, the head (if not restricted by support from behind) is tilted backward at high speed and with great force. In less severe accidents only partial muscle ruptures may occur, whereas severe accidents may seriously damage the muscles and ligaments in front of the cervical spine and also damage nerve roots. The most serious cases occur when the cervical vertebrae are dislocated. Whiplash injuries need careful examination and treatment, as long-lasting symptoms such as headaches may persist if the injury is not cared for properly.

 

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Wednesday, 16 February 2011 23:32

Shoulder

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Disorders of the shoulder region are common problems in both the general and working population. As many as one-third of all women and one-quarter of all men report feeling pain in the neck and shoulder every day or every other day. It is estimated that the prevalence of shoulder tendinitis in the general population is about 2%. Among male and female workers in the United States, the prevalence of shoulder tendinitis has been estimated to be as high as 8% among those exposed to highly repetitive or high-force hand motions, compared to about 1% for those without this type of musculoskeletal stress.

Anatomy

The bones in the shoulder include the collarbone (clavicle), the shoulder blade (scapula) and the (shoulder) glenohumeral joint, as shown in figure 1. The collarbone is connected to the body by the sternoclavicular joint, and to the shoulder-blades by the acromioclavicular joint. The sternoclavicular joint is the sole connection between the upper extremity and the rest of the body. The shoulder blade has no direct connection of its own and thus the shoulder is dependent on muscles for being fixed to the trunk. The upper arm is connected to the shoulder blade by the glenohumeral joint.

Figure 1. Schematic view of the skeletal parts of the shoulder-girdle.

MUS090F1

The function of the shoulder is to provide a platform for the upper extremity, and for some of its muscles. Although the glenohumeral joint has a greater range of movement than, for example, the lower extremity in the hip, this flexibility has developed at the price of stability. While the hip joint has very strong ligaments, the ligaments in the glenohumeral joint are few and weak. In order to compensate for this comparative weakness, the glenohumeral joint is surrounded by shoulder muscles in the form of a cuff and is called the rotator cuff.

 

 

 

 

Biomechanics

The arm represents about 5% of the total body weight, and its centre of gravity is about midway down between the glenohumeral joint and the wrist. When the arm is raised and bent either away from or towards the body (abduction or flexion), a lever is created in which the distance from the centre of gravity increases, and hence the twisting force, and the loading torque, on the glenohumeral joint increases. The rate at which the torque increases, however, is not simply directly proportional to the angle at which the arm is bent, because the mathematical function which describes the biomechanical forces is not linear but is rather a sine function of the abduction angle. The torque will decrease only by about 10% if the flexion or abduction angle is decreased from 90 to 60 degrees. However, if the angle is decreased from 60 to 30 degrees, torque is reduced by as much as 50%.

The flexion strength in the glenohumeral joint is about 40 to 50 Nm for women and about 80 to 100 Nm for men. When the arm is held straight out (90-degree forward flexion) and no external load is placed on the arm—that is, the person is not holding anything or using the arm to exert a force—the static load is still about 15 to 20% of the maximal voluntary capacity (MVC) for women and about 10 to 15% MVC for men. If a tool weighing 1 kg is held in the hand with an arm extended, the corresponding load in the shoulder would be about 80% of the MVC for women, as illustrated in figure 2.

Figure 2. Female and male strength showing the results of holding a 1 kilogramme tool in the hand with the arm held straight at different angles of shoulder flexion.

MUS090F2

The most important muscles for abduction—or raising the arm away from the body to the side—are the deltoid muscle, the rotator cuff muscles and the long head of the biceps. The most important muscles for forward flexion—raising the arm away from the body to the front—are the anterior part of the deltoid muscle, the rotator cuff muscles, the coracobrachialis muscle and the short head of the biceps brachii muscle. Inward rotation is performed by the pectoralis major muscle, the subscapularis muscle, the anterior part of the deltoid muscle and by the latissimus dorsi muscle. Outward rotation is performed by the posterior part of the deltoid muscle, the infraspinatus muscle and the minor and major teres muscles.

Rotator cuff muscles are engaged in any movement of the glenohumeral joint, which is to say any movement of the arm. The rotator cuff muscles originate from the shoulder blade, and their tendons are arranged around the humerus in the form of a cuff, from which their name is derived. The four rotator cuff muscles are the supraspinate, the infraspinate, the teres minor and the subscapularis muscle. These muscles function as ligaments in the glenohumeral joint and also keep the humeral head against the shoulder-blade. A rupture of the rotator cuff (e.g., of the supraspinous tendon) will cause a reduction in abduction strength, particularly involving those positions where the arm is bent away from the body. When the function of the deltoid muscles is lost, the abduction strength can be reduced by as much as 50%, regardless of the angle at which the arm is being bent.

Any time there is forward flexion or abduction of the arm, a load will be placed on the system. Many motions will cause a twisting force, or torque, as well. Since the arm is connected to the shoulder blade by the glenohumeral joint, any load that is placed on this joint will be transferred to the shoulder-blade. The load in the glenohumeral joint, measured in % MVC, is almost directly proportional to the load placed on the muscle which fixes the shoulder blade into place, the upper trapezius.

Major Specific Work-Related Diseases

Rotator cuff disorders and biceps tendinitis

Tendinitis and tenosynovitis are inflammations of a tendon and the synovial membrane of a tendon sheath. The tendons to the rotator cuff muscles (supraspinatus, infraspinatus, subscapularis, and teres minor muscles) and the long head of the biceps brachii are common sites for inflammation in the shoulder. Large movements of the tendons are involved at these locations. During elevation, as the tendons pass to the shoulder joint and under the bony structure there (the coraco-acromial arch), they may be impinged upon, and inflammation may result. These disorders are sometimes termed impingement syndromes. Inflammation of a tendon may be part of a general inflammatory disease, such as in rheumatoid arthritis, but also may be caused by local inflammation which results from mechanical irritation and friction.

Shoulder joint and acromioclavicular joint osteoarthritis

Shoulder joint and acromioclavicular joint osteoarthritis, OA, are degenerative changes of cartilage and bone in the joints and intervertebral discs.

Epidemiology

There is a high prevalence of shoulder tendinitis among welders and steel-platers, with rates of 18% and 16%, respectively. In one study which compared welders and steel-platers to male office workers, the welders and steel-platers were 11 to 13 times more likely to suffer from the disorder, as measured by the odds ratio. A similar odds ratio of 11 was found in a case-control study of male industrial workers who worked with their hands held at or about shoulder level. Automobile assemblers who suffered from acute shoulder pain and tendinitis were required to elevate their arms more frequently and for longer durations than were workers who did not have such job requirements.

Studies of industrial workers in the United States have shown there to be a prevalence of 7.8% of shoulder tendinitis and degenerative joint disease (shoulder) of cumulative trauma disorders (CTDs) among workers whose tasks involved exerting force or repetitive motions, or both, on the wrist and hands. In one study, female students performing repetitive shoulder flexion developed reversible shoulder tendinitis. They developed the condition when the flexion rate, over the course of one hour, was 15 forward flexions per minute and the angle of flexion was between 0 and 90 degrees. Boarding, folding and sewing workers suffered about twice as much shoulder tendinitis as did knitting workers. Among professional baseball pitchers, approximately 10% have experienced shoulder tendinitis. A survey of swimmers in Canadian swimming clubs found that 15% of the swimmers reported having significant shoulder disability, primarily due to impingement. The problem was particularly related to the butterfly and freestyle strokes. Tendinitis of the biceps brachii was found in 11% of the 84 best tennis players in the world.

Another study showed that shoulder joint osteoarthritis was more common in dentists than among farmers, but the ergonomic exposure related to shoulder joint OA has not been identified. An increased risk for acromioclavicular OA has been reported among construction workers. Heavy lifting and handling of heavy tools with hand-arm vibration have been suggested as the exposure related to acromioclavicular joint OA.

Mechanisms and Risk Factors of Disease

Pathophysiology of shoulder tendinitis

Tendon degeneration is often the predisposing factor for development of shoulder tendinitis. Such degeneration of the tendon can be caused by impairment of circulation to the tendon so that metabolism is disrupted. Mechanical stress may also be a cause. Cell death within the tendon, which forms debris and in which calcium may deposit, may be the initial form of degeneration. The tendons to the supraspinatus, the biceps brachii (long head) and the upper parts of the infraspinatus muscles have a zone in which there are no blood vessels (avascularity), and it is in this area that signs of degeneration, including cell death, calcium deposits and microscopic ruptures, are predominantly located. When blood circulation is impaired, such as through compression and static load on the shoulder tendons, then degeneration can be accelerated because normal body maintenance will not be functioning optimally.

Compression of the tendons occurs when the arm is elevated. A process that is often referred to as impingement involves forcing the tendons through the bony passageways of the shoulder, as illustrated in figure 3. Compression of the rotator cuff tendons (especially the supraspinatus tendon) results because the space between the humeral head and the tight coracoacromial arch is narrow. People who are suffering with long-term disability due to chronic bursitis or complete or partial tears of the rotator cuff tendons or biceps brachii usually also have impingement syndrome.

Figure 3. Impingement

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The circulation of blood to the tendon also depends on muscle tension. In the tendon, circulation will be inversely proportional to the tension. At very high tension levels, circulation may cease completely. Recent studies have shown that the intramuscular pressure in the supraspinous muscle can exceed 30 mm Hg at 30 degrees of forward flexion or abduction in the shoulder joint, as shown in figure 4. Impairment of blood circulation occurs at this pressure level. Since the major blood vessel supplying the supraspinous tendon runs through the supraspinous muscle, it is likely that the circulation of the tendon may even be disturbed at 30 degrees of forward flexion or abduction in the shoulder joint.

 

 

 

 

 

 

 

 

 

 

 

 

Figure 4. Raising the arm to different elevations and at different angles exerts different intramuscular pressures on the supraspinous muscle.

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Because of these biomechanical effects, it is not surprising to find a high risk of shoulder tendon injuries among those who are involved in activities that require static contractions of the supraspinatus muscle or repetitive shoulder forward flexions or abductions. Welders, steel-platers and sewers are among the occupational groups whose work involves static tension of these muscles. Assembly line workers in the automotive industry, painters, carpenters and athletes such as swimmers are other occupational groups in which repetitive shoulder joint movements are performed.

In the degenerated tendon, exertion may trigger an inflammatory response to the debris of dead cells, resulting in an active tendinitis. Also, infection (e.g., viral, urogenital) or systemic inflammation may predispose an individual to reactive tendinitis in the shoulder. One hypothesis is that an infection, which makes the immune system active, increases the possibility of a foreign body response to the degenerative structures in the tendon.

Pathogenesis of osteoarthrosis

The pathogenesis of osteoarthrosis, OA, is not known. Primary (idiopathic) OA is the most common diagnosis in absence of predisposing factors such as previous fractures. If predisposing factors exist, the OA is termed secondary. There are disputes between those who claim (primary) OA to be a metabolic or genetic disorder and those who claim that cumulative mechanical trauma also may play a part in the pathogenesis of primary OA. Microfractures due to sudden impact or repetitive impact loading may be one pathogenic mechanism for load-related OA.

Management and Prevention

In this section, non-medical management of shoulder disorders is considered. A change of workplace design or change of work task is necessary if the tendinitis is considered to be due to high local shoulder load. A history of shoulder tendinitis makes a worker doing repetitive or overhead work susceptible to a relapse of tendinitis. Loading of the osteoarthritic joint should be minimized by ergonomic optimization of work.

Primary prevention

Prevention of work-related musculoskeletal disorders in the shoulder can be achieved by improving work postures, motions, material handling and work organization, and eliminating external hazardous factors such as hand-arm vibration or whole body vibration. A methodology that may be advantageous in improving ergonomic working conditions is participatory ergonomics, taking a macro-ergonomic approach.

  • Work postures: Since compression of the shoulder tendons occurs at 30 degrees of arm elevation (abduction), work should be designed that allows the upper arm to be kept close to the trunk.
  • Motions: Repetitive arm elevations may trigger shoulder tendinitis, and work should be designed to avoid highly repetitive arm motions.
  • Material handling: Handling of tools or objects may cause severe loading on shoulder tendons and muscles. Hand-held tools and objects should be kept at the lowest weight feasible and should be used with supports to assist in lifting.
  • Work organization: Work organization should be designed to allow pauses and rests. Vacations, rotations and job enlargement are all techniques which may avoid repetitive loading of single muscles or structures.
  • External factors: Impact vibration and other impacts from power tools may cause strain on both tendons and joint structures, increasing risk of osteoarthrosis. Vibration levels of power tools should  be  minimized  and  impact  vibration  and  other  types of impact exposure avoided by using different types of support or  levers.  Whole-body  vibrations  may  cause  reflectory  contractions of shoulder muscles and increase the load on the shoulder.
  • Participatory ergonomics: This method involves workers themselves in defining the problems, and solutions, and in the evaluation of the solutions. Participatory ergonomics starts from a macro-ergonomic view, involving analysis of the whole production system. Results of this analysis might lead to large-scale changes in production methods that could increase health and safety as well as profit and productivity. The analysis could lead to smaller-scale changes as well, such as in workstation design.
  • Preplacement examinations: Current available information does not support the idea that preplacement screening is effective in reducing the occurrence of work-related shoulder disorders.
  • Medical control and surveillance: Surveillance of shoulder symptoms is readily carried out using standardized questionnaires and inspection walk-throughs of workplaces.

 

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Wednesday, 16 February 2011 23:43

Elbow

Written by

Epicondylitis

Epicondylitis is a painful condition which occurs at the elbow, where muscles which permit the wrist and fingers to move, meet the bone. When this painful condition occurs on the outside it is called tennis elbow (lateral epicondylitis). When it occurs on the inside of the elbow bend, it is called golfer’s elbow (medial epicondylitis). Tennis elbow is a fairly common disease in the general population, and in some studies high occurrence has been observed in some occupational groups with hand-intensive tasks (table 1); it is more common than medial epicondylitis.

Table 1. Incidence of epicondylitis in various populations.

Study population

Rate per 100

person-years

Reference

5,000 workers of diverse trades

1.5

Manz and Rausch 1965

15,000 subjects of a normal population

<1.0

Allander 1974

7,600 workers of diverse trades

0.6

Kivi 1982

102 male meatcutters

6.4

Kurppa et al. 1991

107 female sausage makers

11.3

Kurppa et al. 1991

118 female packers

7.0

Kurppa et al. 1991

141 men in non-strenuous jobs

0.9

Kurppa et al. 1991

197 women in non-strenuous jobs

1.1

Kurppa et al. 1991

 

Epicondylitis is thought to be caused by repetitive and forceful exertions of the wrist and fingers; controlled studies have, however, given contradictory results concerning the role of hand- intensive tasks in the development of the disease. Trauma can also play a role, and the proportion of cases occurring after trauma has ranged from 0 to 26% in different studies. Epicondylitis usually occurs in people aged 40 years and older. The disease is rare under the age of 30. Little is known of other individual risk factors. A common view about the pathology is that there is a tear at the insertion of the muscles. Symptoms of epicondylitis include pain, especially during exertion of the hand and wrist, and gripping with the elbow extended may be extremely painful.

There are various concepts of the pathogenesis of epicondylitis. The duration of epicondylitis is usually from some weeks to some months, after which there is usually complete recovery. Among workers with hand-intensive tasks the length of sick-leave due to epicondylitis has usually been about or slightly over two weeks.

Olecranon Bursitis

Olecranon bursitis is an inflammation of a liquid-filled sac on the dorsal side of the elbow (olecranon bursa). It may be caused by repeated mechanical trauma (traumatic or “student’s” bursitis). It may also be due to infection or associated with gout. There is local swelling and wavelike motion on palpation due to accumulation of fluid in the bursa. When there is raised skin temperature, an infectious process (septic bursitis) is suggested.

Osteoarthrosis

Osteoarthrosis or degenerative disease that results from a breakdown of cartilage in the elbow is rarely observed in people under the age of 60. However, an excess prevalence of osteoarthrosis has been found among some occupational groups whose work includes intensive use of hand tools or other heavy manual work, such as coal miners and road construction workers. Valid studies with no excessive risk in such occupations have also been reported, however. Elbow arthrosis has also been associated with vibration, but it is believed that osteoarthrosis of the elbow is not specific to vibration.

The symptoms include local pain, first during movement and later also during rest, and limitation of the range of motion. In the presence of loose bodies in the joint, locking of the joint may occur. Loss of the ability to extend the joint completely is especially disabling. Abnormalities seen on x rays include the growth of new bone tissue at the sites where ligaments and tendons meet the bone. Sometimes loose pieces of cartilage or bone can be seen. Damage to the joint cartilage may result in destruction of the underlying bone tissue and deformation of joint surfaces.

The prevention and treatment of elbow osteoarthrosis emphasize optimizing work load by improving tools and work methods to decrease the mechanical loads imposed on the upper limb, and minimizing exposure to vibration. Active and passive movement therapy may be used in order to minimize restrictions in the range of motion.

 

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Contents

Preface
Part I. The Body
Part II. Health Care
Part III. Management & Policy
Part IV. Tools and Approaches
Part V. Psychosocial and Organizational Factors
Part VI. General Hazards
Part VII. The Environment
Part VIII. Accidents and Safety Management
Part IX. Chemicals
Part X. Industries Based on Biological Resources
Part XI. Industries Based on Natural Resources
Part XII. Chemical Industries
Part XIII. Manufacturing Industries
Part XIV. Textile and Apparel Industries
Part XV. Transport Industries
Part XVI. Construction
Part XVII. Services and Trade
Part XVIII. Guides