Environmental Medicine

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Environmentally Induced Disorders of the Musculoskeletal System Mohamad Parniapour, PhD, * Margareta Nordin, PhD, t Mary Louise Skovron, PhD,+ Victor H. Frankel, MD, PhD§

Disorders of the musculoskeletal system are among the most common disabling medical conditions in terms of morbidity and economic cost to society. Data from the US Health and Nutrition Examination Survey (HANES I) of 1971-1975 indicated that 47.8 per cent of adults examined by the physician had either a history of chronic musculoskeletal symptoms or a musculoskeletal abnormality.20 Back trouble was the most frequent problem, followed by knee, hip, finger, and shoulder problems. It is important to note that among those persons reporting a history of musculoskeletal symptoms, those who have some disability tend to be older, nonwhite, of lower education and income, and widowed, separated, or divorced. Persons who believe that their symptoms are secondary to trauma report being disabled by their impairment more than those who do not believe injury or accident caused their impairment. 2o Yelin and associates 58 showed that persons with musculoskeletal conditions had high rates of work disability relative to those with other chronic conditions, primarily because of their age, level of co-morbidity, and the interaction of physical limitations imposed by their illness and job requirements. Signs and symptoms provide poor prediction of musculoskeletal conditions that result in work disability. The American Academy of Orthopaedic Surgeons estimated that about 23 million people have musculoskeletal impairments. In this study, musFrom New York University; and the Hospital for Joint Diseases Orthopaedic Institute, New York, J\iew York *Research Assistant Professor, Program of Occupational Biomechanics; and Assistant Director, Occupational and Industrial Orthopaedic Center tProgram Director and Associate Professor, Program of Occupational Biomechanics; and Director, Occupational and Industrial Orthopaedic Center :j:Assistant Professor, Environmental Medicine; and Senior Epidemiologist §Professor of Orthopaedic Surgery; and President and Chairman, Orthopaedic Department

Medical Clinics of North America-Vo!' 74, No. 2, March 1990

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culoskeletal disorders ranked second in frequency among causes of visits to physicians, third in frequency among causes of hospital utilization, third in frequency among reasons for surgical procedures, and third among disease categories in frequency of acute conditions. The total annual costs of musculoskeletal conditions were estimated to exceed $65 billion in 1984. 25 In 1978, a national survey by the Social Security Administration estimated that 16.5 per cent of the population in the United States-21 million adults-were disabled. 53 Furthermore, musculoskeletal conditions are the leading causes of disability in people in their working years.31 In the following pages, low back pain, osteoporosis with associated fractures, osteoarthrosis, and cumulative trauma disorders are examined, along with fundamental biomechanical concepts.

LOW BACK PAIN The diagnosis of low back pain remains a problem. It has been reported that 88 per cent of the time, the physician is unable to verify a pathoanatomic cause for back complaints. The onset of low back pain is usually gradual and is seldom related to an accident or unusual activity. Most (60 to 80 per cent) of the population is afflicted with low back pain during their lifetimes; half improve within a week, and 70 per cent get better within a month. About 7 per cent still report pain after 6 months, and these persons account for 80 per cent of total back pain costs. 7, 30 Several etiologies have been suggested: intervertebral disk disease, facet joint injury, endplate injuries, intraspinal or supraspinal ligamentous injury, paraspinal muscles injury, and microcompression or macrocompression fracture of a vertebral body. 36 One of the shortcomings of many of the epidemiologic studies of low back pain is the lack of differentiation among subgroups within the population. Current empiric evidence shows that low back pain sufferers are heterogeneous in their responses to psychological inventories. 9 It should be noted that surgery is warranted in only a small percentage of cases and that much of the conservative therapies in use await scientific proof of efficacy.28 It is believed that the first episode of low back pain is hard to predict and consequently difficult to prevent. The current consensus is that the prevention of the chronicity oflow back pain is much more cost effective than prevention of acute low back pain. 4 , 7 A combined personnel program and a back program significantly reduced the number of back injuries at a geriatric hospital. The personnel program was the more effective and was intended to reduce the duration of wage-loss claims through an effort to increase the communication among the claimants, their physicians, the worker's compensation board, and the hospital. The back program was designed to provide intensive feedback training,'56 Functional restoration and counseling guided by objective measurement of physical function has shown success for patients with chronic low back pain. After 2 years, 87 per cent of program graduates and 41 per cent of a nonrandom comparison group were found to be working. 37 Risk factors for low back injury are described as being either individual

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or workplace. Individual risk factors that have been implicated include age, sex, anthropometry, musculoskeletal abnormalities, muscle strength and endurance, smoking, physical fitness, psychological factors, and previous attacks of low back pain. Workplace risk f:1Ctors include work shift; heavy work; lifting, bending, and twisting; constrained posture; slipping; prolonged sitting; truck driving and whole-body vibration; the potential to influence the work situation; and overtime. 4 . 48 In a recent review of the retrospective and prospective studies of low back pain, it was concluded that an individual's physical factors have little influence on the risk of experiencing future acute low back pain. In a prospective study of risk factors for industrial low back pain among 3020 aircraft manufacturing employees, the aforementioned conclusion was confirmed. 4 A higher risk for first time occurrences was suggested in men with greater flexibility and lower isometric endurance. 6 Chaffin and Park 15 showed an association between industrial back pain and insufficient strength relative to job demand. Cady and associates lO had shown a negative connection in firefighters between work-related back incident reports and overall fitness. The Malma longitudinal study; indicated that job dissatisfaction had the strongest independent correlation with locomotor discomfort. The independent variables in discriminating between subjects with and without back pain were, in men, body weight, job satisfaction, and occupational work load, and, in women, job satisfaction. Vallfors 54 observed that absenteeism and disability from low back pain are more likely when there is less availability of a lighter job during convalescence from the pain episode, when the patient perceives that the job is too heavy, and the working environment is unpleasant and noisy. Dissatisfaction with job status; monotonous, repetitive tasks; and the patient's report of fatigue at the end of the working day are associated with greater absenteeism and disability. . The control of back pain has been approached by reducing the probability of the initial episode (initial control), reducing the length of disability when it does occur (secondary control or effective case management), and reducing the chance of recurrence (tertiary control). A combination of the job design (ergonomics), job placement (worker selection), and education (training approaches) has shown success in controlling low back pain in diverse industries. 51 These three approaches are not equally effective: redesigning "the pathological tasks and workplaces" is far more effective than training in safe lifting techniques. 42

OSTEOARTHRITlS Osteoarthritis is the most common joint disease, affecting 12.1 per cent of adults in the United States. 20 Estimates of prevalence and incidence depend on whether the cases are defined according to radiographic changes or clinical signs and symptoms. The clinical manifestations are primarily joint pain and stiffness with accompanying loss of function. The radiographs are graded on the basis of the presence of osteophytes and joint space narrowing, subchondral cysts, and bony remodeling. One problem is that

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not all patients who show radiographic evidence of osteoarthritis have symptoms or disability. The risk factors most strongly associated with osteoarthritis are age, gender, ethnic and racial background, occupational history, repetitive physical activity, trauma, abnormal biomechanics, genetic predisposition, and obesity. Obesity appears to contribute to osteoarthritis of the knee joints, and not to affect other joints. The involvement of the small joints of the hand in cotton pickers and weavers, of elbows and knees in coal miners, and of the metatarsophalangeal joints in ballet dancers points to occupational or repeated physical loading as an important risk factor. 30 Peyron's review of epidemiologic studies of osteoarthritis extensively cites the studies that link mechanical loading and repetitive movement to osteoarthritis. Anthropologic investigations of ancient populations have shown the high incidence of osteoarthritis of the larger joints in comparatively young subjects-elbow and shoulder-because of their lifestyle. 4:3 The Malma longitudinal study showed subjects with moderate or heavy work loads had more locomotor complaints, particularly in the back, hips, and knees. Women with Heberden's nodes more often had a moderate physical workload. 3 Arthritis is defined as joint deterioration from inflammatory causes and osteoarthrosis as joint deterioration from mechanical causes. 44 • 45 Osteoarthrosis is considered an imbalance between mechanical stresses on the joint and the ability of the tissues of the joint to withstand those stresses. It is believed that cartilage fibrillation need not be progressive, and that articular damage does not necessarily lead to osteoarthrosis. In addition, the integrity of the articular cartilage depends on the mechanical properties of its bony bed. Changes in the relative density and architecture of the underlying bone can have a profound effect on both the initiation and progression of cartilage damage. One mechanism of initiation may be a steep stiffness gradient in the subchondral bone. Repeated failure of musculoskeletal peak dynamic force attenuation can cause stiffening of the subchondral bone, leading to changes in joint conformation, the creation of areas with high stress concentration, and a rise in the energy absorbed by the cartilage. In this model, it is assumed that to prevent the initiation or progression of the osteoarthrosis one needs to absorb the incoming shock to the musculoskeletal system with viscoelastic shoe inserts. The experimental data implicate loading rate as a significant factor in osteoarthrosis. Lowering the loading rate, even at higher loads, may spare joints from deterioration. 44. 45 ..37 The properties of the articular cartilage, which functions as a wearresistant, nearly frictionless, load-bearing surface in diarthrodial joints, depends on its composition, physiochemical properties, and ultrastructural and molecular organization. Proteoglycan loss, increased tissue hydration, loss of compressive stiffness, and disruption of the collagen fibrillar network are associated with cartilage degradation. In normal cartilage, the chondrocytes are able to maintain a balance of synthesis and degradation. 40 Others believe that a breakdown in the lubrication mechanism in the joint leads to osteoarthritis. Chemical changes in the synovial fluid or the cartilage may prevent proper nourishment to the cartilage, which is avascular.22 The role of metabolic activity and the nutritional routes of

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articular cartilage in the development of osteoarthritis has been investigated. 27 In adult human joints except phalanges, in addition to the synovial route, the subchondral route contributes to the nutritional supply. The number of vessels is greatest in the center of the joint, where the articular cartilage is the thickest and also bears the greatest load. The breakdown of this route could lead to degeneration of the cartilage. Subchondral vessels decrease with age and are sealed off by advancement of the subchondral plate toward the articular cartilage. 26, 27

OSTEOPOROSIS Osteoporosis may be defined as a reduction in bone mass that increases the susceptibility to fracture by even slight or trivial trauma, Osteoporosis predisposes to fracture of the hip, vertebrae, distal radius, humerus, and pelvis. Peak bone mass of both trabecular and cortical bone appears to be reached in the middle or late 30s, after which bone mass begins to decline. The cortical, and probably trabecular, bone loss accelerates in women during the decade after the menopause. :30 Fracture of the hip is associated with more deaths, disability, and medical costs than all other osteoporotic fractures combined. In 1983, the overall cost of osteoporosis was estimated to be $6.1 billion in the US. Among patients who were functionally independent and living at home at the time of fracture, 15 to 25 per cent are in long-term care institutions for at least 1 year after fracture, and 25 to 35 per cent are still dependent on other people or devices for mobility. A 12 to 20 per cent higher age- and sex-adjusted mortality rate is observed in persons who fracture hips. 19,30 Risk factors associated with osteoporosis are: female sex, white or Asiatic race, surgical or early menopause, positive family history, low calcium intake, hyperparathyroidism, and rheumatoid arthritis. Less surely established are sedentary lifestyle, nulliparity, alcohol abuse, cigarette smoking, and high caffeine, protein, and phosphate intake. 19,3.5 An increase in the age-adjusted incidence of fragility fracture has been shown by most epidemiologic studies. 41 From 1965 through 1983, the hip fracture rate in Goteborg increased by 109 per cent, only 20 per cent of the increase being explainable by the age factors in the population. 59 Comparative radiographic studies have shown that the average mass of both the trabecular and cortical bone has decreased in the proximal femur during the past 40 years. 50 Immobilization decreases bone mass; physical activity can prevent bone loss or even increase bone mass. These results point to the deterioration of the quality of the skeleton in successive generations. The incidence of hip fractures seems to be lower in rural environments than in urban populations. Urban pollution and the presence of such metals as aluminum are suggested environmental factors. Obrant and associates 41 explain the age~adjusted increase in hip fracture by the decrease in the physical activity level, nutritional habits, increased life expectancy at the expense of a less healthy state during the last years oflife, and the increased use of tobacco and alcohol. Treatment of osteoporosis is less effective after fracture. Thus, the

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prevention of bone loss and falls is the preferred approach. Those persons thought to be at greater risk-small, thin white or Asian women who have an early menopause-would be good candidates for preventive intervention. This consists of dietary and lifestyle alteration, primarily increases in calcium intake and exercise, and the judicious use of estrogens. 35 Programs to prevent falls through the elimination of hazards and adaptation of the environment to the normal deterioration of fimctional capacities such as losses in hearing, vision, speed, strength, postural control, and coordination are also important to the elderly population. Preventive efforts should be directed to enhancing peak bone mass in young people and maintaining the optimal level of physical activity (stress) in order to preserve not only the integrity of the bone but also the neuromuscular system. 35

CUMULATIVE TRAUMA DISORDER In a study of 1979 workers' compensation claims for wrist injuries, more than 6 per cent of these cases were related to inflammation or irritation of the joint, tendons, or muscles as well as to diseases of the nerves or peripheral ganglia. The average cost per claim was $618 in medical payment and $1026 for indemnity compensation. 29 The concept of cumulative trauma was introduced by Ramazinni, who ascribed the gradual development of these diseases to violent and irregular motions and unnatural postures of the body. 46 The list of occupational risk factors has increased to include repetitive motion, forceful motions, mechanical stresses, static or awkward postures, vibration, wearing gloves, and cold. The nerves, bursae, tendons, tendon sheaths, blood vessels, and muscles of the upper extremity are the most frequently reported sites of cumulative trauma disorders. Common examples of work-related disorders are carpal tunnel syndrome, de Quervain's disease, trigger finger, lateral epicondylitis (tennis elbow), rotator cuff tendinitis, and tension neck syndrome. In the modern industrial workplaces, repetitive motions of the hands, wrists, shoulders, and neck occur commonly. For example, a data entry operator may perform 20,000 key strokes per hour, a worker in a meat-processing plant may perform 12,000 cuts with a knife per day, and a worker on an assembly line may elevate the right shoulder above the acromion 7500 times per day. As much as 25 per cent of the work force in some jobs is afflicted with trauma disorders requiring medical attention. 21 Goldstein and coworkers 24 subjected profundus tendons of hand specimens to various regimens of force and frequency to determine the elastic and viscous stress-strain functions. When the recovery time between successive loads was not sufficient, tendons subjected to physiological loads showed significant creep. Viscoelastic tendon models may be used to identify the safe maximum exertion and total work time and the necessary rest allowance. Surgical treatment of the cumulative trauma disorders may be ineffective if the occupational causes are not controlled. Prevention strategy call be grouped into four categories: reduction of the exposure to occupational risk factors, conditioning processes that increase the tolerance of workers

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to the risk factors, organizational mechanisms to allow rest periods, and the preplacement procedures to identify people at high risk, which has been least successful. The most promising preventive approach has been the control of occupational risk factors by redesigning the tools or modifying the task or the workplace conditions. 2. 3 The research effort in designing proper ergonomic tools has not been successful despite the urgent need.

BIOMECHANICS Biomechanics is the study of the relation between forces and motions of the biologic system. The field of biomechanics can be divided into rigid and deformable body mechanics. The former is the study of the macroscopic motion of objects that are assumed not to deform. The latter studies the effects of applied forces on the induced changes in the size and shape of such objects as bone, ligaments, and tendons and internal distributions of stress and strain. Stress is defined as force per unit area. Normal stress is generated by the application of a force perpendicular to the surface, whereas shear stress is generated by a force parallel to the surface. Normal strain is produced by a normal stress and is defined as the ratio of change in length with respect to the original length. A shear strain generated on the top face of a cube will cause the front face to deform from a square into a parallelogram. Specimens can be loaded, and the amount of their deformation can be measured to characterize their force-deformation curves. Such curves are used to study structural behavior, whereas stress-strain curves represent the material property. Structural behavior depends on both geometry and material property. 11 Upon removal of the load, the original length is restored in the elastic deformation region. The material yields at a stress level known as yield strength. Stresses above yield strength will cause plastic deformation; eventually, fracture or failure will occur at ultimate tensile strength or fracture strength. The slope of the linear elastic region of the stress-strain curve has been defined as Young's modulus of elasticity. The enclosed area under the stress-strain curve quantifies the absorbed energy per volume. The elastic modulus, yield strength, and ultimate strength can be used for parametric comparison of different materials and for comparing the effects of such factors as immobilization, aging, and healing. 8 The factors that determine the deformation characteristics and fracture resistance of a bone are the direction, magnitude, and the rate of force application; the size and geometry of the bone; and the material properties of the tissue. In the longitudinally oriented cortical-bone specimen, the ultimate strength is greatest in compression and lowest in shear. II In the transversely oriented specimen, the ultimate strength in tension, compression, and shear is lower than in the longitudinally oriented specimen. The stress-strain characteristics of bone as viscoelastic material are dependent on the strain rate. Thus, the bone can resist rapidly applied forces much better than slowly applied forces. In normal activities, bone is subjected to strain rates on the order of 0.01 per second (i.e., a shortening of 1 per cent

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of the original length per second), whereas in trauma, strain rates of 10.0 per second (i. e., a shortening of 10 per cent of the original length per second) can be exceeded. J I, 47 In addition to a single traumatic episode, musculoskeletal tissues in these tests are commonly exposed to repetitive loading at stress levels lower than yield stress. Biomechanical studies of cyclical loading of bones have shown the poor fatigue resistance and the gradual failure that involves the progressive accumulation of diffuse microdamage. Fatigue damage in bone secondary to repeated mechanical loading has been implicated in the development of degenerative disorders, avascular necrosis, osteochondritis, senile femoral neck fractures, spondylolisthesis, pathologic fracture, fatigue fracture, and the failure of bone following orthopedic implant procedures. 11, 14 The strength of cancellous bone is approximately proportional to the square of the apparent bone density. Thus, reduction of apparent bone density, as in osteoporosis, will reduce the strength and fatigue resistance significantly. The disease processes can alter the mechanical response of the bones by changing the microstructure or material properties of bone tissue.l!, 14 Bone has been studied as a structure, as a material, and as a biologic system. The study of internal organizations of the bone during functional adaptation is aimed at understanding the process by which the structural and geometric properties of the bone evolve under given external conditions set by the environment. 49 One of the first mathematical analyses of a bone remodeling process has been reported by Cowin and coworkers.17, 18 They distinguished between internal and surface remodeling. The concept of internal remodeling is based on the assumption that the stiffness matrix depends on the volume fraction of the bone matrix material. The time derivative of this volume fraction was approximated by a nonlinear differential equation using the small strain theory of adaptive elasticity. Depending on the initial conditions, either stable or unstable equilibria can be reached that could simulate the clinical observation of osteoporosis and osteopetrosis. The model of surface bone remodeling assumes a control equation in which the speed of remodeling, the vector of growth velocity normal to the free surface of bone is proportional to the deviation of local strains from a reference value. Remodeling equilibrium is the set of conditions under which there is no net deposition or re sorption of the bone tissue. There is a range of strain values associated with remodeling equilibrium. Numerical simulation showed that if a thin-walled right circular cylinder capable of surface remodeling is subjected to an axial compressive load and a twisting torque, the effect of increasing the torque is the same as that of decreasing the axial compressive load. The mean radius of the cross section increases and the wall thins. 16 Carter and associates 13 presented a general theory for the mechanical regulation of biologic processes in the chondro-osseous skeleton that is based on the loading history of the tissue. The basic principles of the model are that the application of intermittent shear stress (or strain energy) to cartilage will accelerate, whereas intermittent compressive hydrostatic stress will retard or arrest, proliferation, maturation, degeneration, and

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ossification. Carter and his coworkers have tested their theory with the aid of the finite element models to simulate skeletal development, degenerative processes, tissue differentiation, and fracture healing. 13 Dynamic loading is more effective than static loading in bone remodeling. 34 Low-frequency oscillations that can be detected in mechanically loaded bone were proposed to have relevance in controlling the remodeling activity.39 The stress-induced remodeling is being studied at the cellular and the extracellular matrix level under controlled stress and strain conditions, with the following working hypothesis. The strain of extracellular matrix, through attachments between the matrix, cell membrane, and cytoskeleton, induces changes in cytoskeleton configuration, which cause the changes in levels of cyclic adenosine monophosphate (cAMP) that have been observed during stress-induced remodeling. 1 The proteoglycan within the bone tissue is oriented after short periods of loading. Although there was no difference in collagen birefringence in sections taken from loaded and nonloaded bones, there was a difference in proteoglycan birefringence induced by Alcian blue staining. This difference, which persisted for 24 hours and decayed by 48 hours, indicates a loading-related increase in the orderliness of proteoglycan orientation. 34 This phenomenon points to the ability of bone to store its loading history. Studies have shown that the cross-sections of load-bearing bones are increased by increased activity 55 and that the rate of the age-related loss from the lumbar vertebrae in normal women can be decreased by exercise. 32 On the other hand, bedrest;32 immobilization, weightlessness in space flight,52 and stress shielding by implants lead to bone loss. The aforementioned analytical models and the experimental animal and human studies all show that functional strains and load histories influence the remodeling activities, but much of the detailed mechanisms and process remain to be discovered. The stress-induced remodeling paradigm is less developed for other connective tissues. Immobilization significantly compromises the properties of ligaments and the bone-ligament-bone complex, the linear slope, and the ultimate load. Moreover, the energy-absorbing capabilities of the rabbit medial collateral ligament (MCL)-bone complex during tension decreases to approximately one third that of the contralateral control. The strength recovery is slow. It has also been shown that early motion enhances the healing and repair of ligaments after an injury. The current consensus is to discontinue surgical repair of class I and II MCL injuries and substitute early protected motion and standard programs of rehabilitation. 55 Injuries can be categorized with respect to the vectorial description of the applied forces (magnitude and direction), the rate of the force application, the quality of the tissues being stressed, and the biologic responses to injuries. If the forces and the impact energy far exceed the ultimate strength and energy absorption capability of the tissue, the quality of tissue prior to injury is no longer a determining factor. In such a situation, the most severe injuries, acute macrotrauma, will result. Microtrauma, on the other hand, can be a result of smaller abnormal forces applied to either normal or abnormal tissues (e.g., tissues with previous injuries or pathology). Microtrauma has a more insidious onset, more chronic symptoms and

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cumulative etiology, and less objective proof of specific tissue trauma. Injury mechanisms thus have a spectrum between the two extreme cases identified. 55 We have reviewed the current theory regarding the response of biologic systems to environmental stress and strain at the cell and tissue level. Medical intervention is not usually sought after the onset of impairment or injury at the tissue or cellular level; rather, pain, discomfort, or functional limitation causes the individual to seek medical help. Pain has been shown to be influenced by the cultural background, psychological factors, and situational circumstances. 38 While considering the whole person's response (strain) to environmental stresses, one needs to expand the concept of environmental stress to include organizational, psychosocial, and mental stresses, in particular for the chronic musculoskeletal diseases. It is argued that psychological and physical stressors have slightly different effects: psychological stressors impinge on the psychophysical system, whereas the physical stressors additionally stress the physical system and weaken certain body parts. Individual coping strategies may contribute to the development of the musculoskeletal complaints and can moderate the work and leisure stresses on the musculoskeletal complaints. 23

SUMMARY The human suffering and economic cost attributable to musculoskeletal disorders cannot be overemphasized. Biomechanical principles have been introduced to explain the microtrauma and macrotrauma as mechanisms of injury. Stress-induced remodeling is a useful paradigm in the study of environmentally induced disorders. The concept of environment should include psychosocial and organizational factors with respect to musculoskeletal complaints and chronic pain. Low back pain, osteoarthritis, osteoporosis and its associated fractures, and cumulative trauma disorders are the most critical and common musculoskeletal disorders. The risk factors for low back pain have been identified, and preventive measures for reducing the chronicity of low back pain have been discussed. Preventive approaches to control or limit the fractures associated with osteoporosis and loss of bone mass are preferred to simple management of the fracture of osteoporotic patients. It can be concluded that too little or too much stress can disturb the homeostatic or the equilibrium state of health and that much remains to be researched to quantify the optimal stress levels.

REFERENCES 1. Alexander H, Ricci JL, Grande DA, et al: Mechanisms of stress-induced remodelling. In Skalak R. Fox CF (eds): Tissue Engineering. New York, Alan R Liss, 1988, pp 189-

194 2. Armstrong TJ: Ergonomic and cumulative trauma disorders. Hand Clin 2:553-565, 1986

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3. Armstrong TJ, Radwin RG, Hausen DJ, et al: Repetitive trauma disorders: Job evaluation and design. Hum Factors 28:325-336, 1986 4. Bathe MC: The reliability of physical factors as predictors of the occurrence of back pain reports: A prospective study within industry [doctoral dissertation]. Gothenburg University, 1989 5. Bergenudd H: Talent, occupational and locomotor discomfort [doctoral dissertation]. Lund University, 1989 6. Biering-Sorensen F: Physical measurements as risk factors for low-back tronble over a one-year period: 1983 Volvo Award in Clinical Science. Spine 9:106-119, 1984 7. Bigos SJ, Battie MC: Acute care to prevent back disability: Ten years of progress. Clin Orthop 221:121-130, 1987 8. Black J: Orthopaedic Biomaterials in Researcb and Practice. New York, Churchill Livingstone, 1988 9. Bradley LA, Van der Heide LM: Pain-related correlates of MMPI profile subgroups among back pain patients. Health Psychol 3:157-174, 1984 10. Cady LD, Bischoff DP, O'Connel ER, et al: Strength and fitness and subsequent back injuries in firefighters. J Occup Med 21:269-272, 1979 11. Carter DR: Biomechanics of bone. In Nahum AM, Melvin J (eds): The Biomechanics of Trauma. Norwalk, CT, Appleton-Century-Crofts, 1985, pp 136-165 12. Carter DR: Mechanical loading history and skeletal biology. J Biomech 20:1095-1109, 1987 13. Carter DR, Fyhrie DP, Whalen RT: Trabecular bone density and loading history: Regulation of connective tissue biology by mechanical energy. J Biomech 20:785-794, 1987 14. Carter DR, Caler WE, Spengler DM, et al: Fatigue behaviour of adult cortical bonethe influence of mean strain and strain range. Acta Orthop Scand 52:481-490, 1981 15. Chaffin DB, Park KS: A longitudinal study of low back pain associated with occupational weight lifting factors. Am lnd Hyg Assoc J 34:513-525, 1973 16, Cowin SC: Bone remodelling of diaphyseal surfaces by torsional loads: Theoretical predictions. J Biomech 20:1111-1120, 1987 17. Cowin SC, Firoozbakhsh K: Bone remodelling of diaphyseal surfaces under constant load: Theoretical predictions. J Biomech 14:471-484, 1981 18. Cowin SC, Van Buskirk WC: Internal bone remodelling induced by a medullary pin. J Biomech 11:209-275, 1978 19. Cummings SR, Kelsey JL, Nevitt MC, et al: Epidemiology of osteoporotic fractures. Epidemiol Rev 7:78-208, 1985 20. Cunningham LS, Kelsey JL: Epidemiology of musculoskeletal impairments and associated disability. Am J Public Health 74:574-579, 1984 21. Fine LJ, Silverstein B: Work related disorders of the neck and upper extremity. In Levy BS, Wegman DM (eds): Occupational Health: Recognizing and Preventing Occupational Work-Related Disease. Boston, Little, Brown, 1988, pp 358-370 22. Freeman MAR: Adult Articular Cartilage. New York, Grune & Stratton, 1973 2,3. Frese M: Stress at work-Coping strategies and musculoskeletal complaints. In: WorkRelated Musculoskeletal Disorders: Proceedings of an International Symposium. Bonn, Verlag fur neue Wissenschaft, 1987, pp 121-139 24. Goldstein SA, Armstrong TJ, Chaffin DB, et al: Analysis of cumulative strain in tendons and tendon sheaths. J Biomech 20:1-6, 1987 25. Grazier KL, Holbrook TL, Kelsey JL, et al: The frequency of occurrence, impact and cost of musculoskeletal conditions in the United States. American Academy of Orthopaedic Surgeons, 1984 26. Greenwald AJ, Haynes DW: A pathway for nutrients from the medullary cavity to the articular cartilage of the femoral head. J Bone Joint Surg 51[B]:747-753, 1969 27. Haynes DW: Early aging nutritional changes at base of the articular cartilage. In Nelson CL, Dwyer AP (eds): The Aging Musculoskeletal System. Lexington, DC Heath and Company, 1984, pp 121-126 28. Heliovaara M: Epidemiology of sciatica and herniated lumbar intervertebral disc. Helsinki, Social Insurance Institution, 1988 29. Jenson R, Klein B, Sanderson L: Motion-related wrist disorders traced to industries, occupational group. Monthly Labor Review, September 1983, pp 13-16

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