Magnitude of the problem

BUREAU OF LABOR STATISTICS DATA

An injury or illness is work related if an event or exposure in the work environment either caused or contributed to the resulting condition or significantly aggravated a preexisting condition.⁴ The Bureau of Labor Statistics (BLS) annually reports on the number of workplace injuries, illnesses, and fatal injuries in the United States. In addition to collecting private sector data, since 2008 the BLS began reporting injury and illness data on public sector workers in state and local governments (e.g., police and fire fighters).

MSDs are the most common type of occupational condition reported on the BLS survey, typically representing almost a third of all BLS-reported injuries and illnesses.⁵ In 2014, the BLS estimated that 365 580 cases of MSDs occurred for an incidence rate of 33.8 cases per 10 000 full-time workers, a rate that is trending downward since 2011 (Figure 2.1). In 2014, workers who sustained an MSD required a median of 13 days to recuperate before returning to work, compared to 10 days in previous years.⁵ This finding suggests that while MSDs are trending down, the cases may be becoming more severe. Carpal tunnel syndrome is probably the most well-known MSD, but sprains, strains, and tears are the most common diagnosis.⁶ The BLS reports the MSD rate is higher among males. In 2014 the MSD incident rate was 37.5 per 10 000 full-time workers, compared to 29.7 per 10 000 among female workers, a trend that has persisted over the past decade.⁵ The 45–54-year-old age group has the highest reported rate (40.4 per 10 000), followed by the 35–44-year-old age group (36.2 per 10 000) in 2014, again a trend that has persisted over the past decade⁵ (Figure 2.2). It should be noted that the BLS data significantly underestimates the true number of these conditions.^7,8

Occupations at risk

Case reports have given rise to a number of disorders named for the patient’s occupation (Table 2.1).

These disorders are not unique to their occupations. In 2014, the BLS reported nursing assistants had the highest MSD rates, followed by emergency medical technicians/paramedics, fire fighters, and refuse/recyclable material collectors.⁵ The National Health Interview Survey described cases of self-reported carpal tunnel syndrome to be highest among mail/message distributors (prevalence 3.2%), health assessment and treatment occupations (2.7%), and construction trades (2.5%).¹⁸ The Wisconsin workers’ compensation program reported wrist injury to be highest among dental hygienists, data entry keyers, and hand-grinding and polishing occupations.¹⁹ Although the various occupations have different rates of MSD, the BLS and workers’ compensation data point to almost all occupations reporting at least one case of work-related MSD.

Industries at risk

According to the BLS data, the transportation and warehousing industry had the highest number and rate of MSDs followed by the healthcare and social assistance industry (Figure 2.3). Work-place evaluations have also identified a high prevalence of UE MSD in the animal-slaughtering and processing industries (beef, pork, poultry, and fish).^20–23 Workers in the poultry industry with an astonishingly high prevalence (34%) were found to have carpal tunnel syndrome using self-reported hand and wrist symptoms, hand diagrams, and nerve conduction studies (median mononeuropathy) to define carpal tunnel syndrome.²⁰

In summary, despite their numerous limitations, BLS and workers’ compensation data are sufficient to confirm that the UE MSD problem is large and that rates significantly differ between industries and occupations signifying that workplace factors are important risk factors.

Epidemiology

One of the main purposes of epidemiologic studies is to identify factors that are associated (positively or negatively) with the development or recurrence of adverse medical conditions. No single epidemiologic study determines causality. Rather, results from epidemiologic studies can contribute to the evidence of causality. Over the past decade, several publications have reviewed the medical and ergonomic literature to determine whether scientific evidence supports a relationship between workplace physical factors and MSDs. The most comprehensive review was completed by Bernard et al. at the National Institute for Occupational Safety and Health (NIOSH).²⁴ This review focused on disorders that affected the neck (tension neck syndrome), upper extremities (shoulder tendinitis, epicondylitis, CTS, hand–wrist tendinitis, and hand–arm vibration syndrome), and the lower back (work-related low back pain). A database search strategy initially identified 2000 studies, but laboratory, biomechanical, clinical treatment, and other nonepidemiologic studies were excluded, leaving 600 for systematic review. The review process consisted of three steps.

The first step gave the increased emphasis, or weight, to studies that had high participation rates (>70%), physical examinations, blinded assessment of health and exposure, and objective exposure assessment. The second step assessed for any other selection bias and any uncontrolled potential confounders. The final step summarized studies with regard to strength of the associations, consistency in the associations, temporal associations, and exposure–response (dose–response) relationships.

Bernard et al. concluded that a substantial body of credible epidemiologic research provides strong evidence of an association between MSDs and work-related physical factors. This is particularly true when there are high levels of exposure or exposure to more than one physical factor (e.g., repetition and forceful exertions). The strength of the associations for specific physical stressors varies from insufficient to strong (Table 2.2). The consistently positive findings from a large number of cross-sectional studies, strengthened by the available prospective studies, provide strong evidence for an increased risk of work-related MSDs for the neck, elbow, and hand–wrist for jobs that require high repetition, high force, awkward postures, and vibration. This conclusion was supported by subsequent prospective studies and reviews by the National Academy of Sciences and the Institute of Medicine.^16,25–27

Survey methods

Employee or supervisor interviews, employee diaries, and employee-completed questionnaires are useful because of their low cost, rapid availability, and, for some, the ability to obtain historical data about previous exposures.^28–32 One of the most commonly used survey tools is the so-called Borg, or rating of perceived exertion, scale.^33,34 A 15-point scale (6–20) was created to reflect the linear relationship between physical workload and heart rate divided by 10 (e.g., a heart rate of 60 beats/minute corresponds to 6 on the scale). The scale is presented to the subject before the start of a job or job task with anchors of “no exertion at all = (6)” to “maximal exertion = (20).”³⁴ The subject is then asked to rate his or her exertion level after completing the job and/or job task. A 10-point Borg scale was also created to account for large muscle group exertion, rather than heart rate or total body exertion.^34,35

The accuracy of self-assessment surveys has been questioned because of the potential for the worker to either underreport or overreport exposures. For example, highly motivated subjects might underestimate their exertion, while unmotivated subjects might overestimate their exertion.³⁶ This potential problem has led many to utilize observational checklists (described below).

Observational methods

Observational methods, such as observational checklists, are commonly employed to objectively assess the workplace for physical stressors. Some checklists can be used by healthcare providers with limited expertise,^37–41 others require some training,^29,42,43 while others require a considerable amount of experience and training.^44–47Table 2.3 provides the reader with a simple checklist for healthcare providers with limited expertise. For healthcare providers or others with some training, the hand activity level (HAL) could be utilized. It is described in more detail in the Exposure Guidelines section”

Measuring workers

If the above checklist suggests that physical stressors exist in the workplace, quantitative measurement of those risk factors should be considered. However, quantitative measurement of ergonomic hazards can require the use of specialized equipment and training and expertise in its use and interpretation of the results.²⁹

Methods used to generate quantitative information on physical stressors include electrogoniometers (dynamic measurements of posture), accelerometers, and imaging techniques (electronic and laser optical recordings). Two devices that may be useful outside of research settings are spring scales or gauges to estimate force requirements and simple goniometers to measure static postures. Both of these tools have been used successfully in workplaces due to their simplicity.

Internal forces can be measured using surface electromyography (EMG), but currently available equipment is expensive, and its use requires training and expertise to perform and interpret. Video and imaging systems as a means to measure posture have been used primarily in the laboratory setting where the camera’s line of sight is perpendicular to the planes of the measured body segments. But given the dynamic nature of most job activities, their use in the workplace seems limited unless multiple cameras can be used from a variety of viewing angles. Goniometer use for measuring static postures is well established, but few jobs require continuous static postures. Electrogoniometers can measure dynamic postures, but their accuracy and associated analytic methods are not well established.

American Conference of Governmental Industrial Hygienists (ACGIH)

The ACGIH provides guidelines for industrial hygienists to use while making decisions regarding safe levels of exposure to various hazards in the workplace. The organization issued a guideline known as the “hand activity level” (HAL) based on the hand, wrist, and forearm exposure to repetition and peak normal force for mono-task jobs.⁴⁸ Mono-task jobs are defined as jobs that repeatedly perform a similar set of motions or exertions for ≥4 hours per day.

The first step is selecting a job period that represents an average activity. Then observe (or videotape) the activity for several job cycles. The second step rates the HAL. This can be accomplished by two methods: (i) a trained observer using a validated rating scale based on exertion frequency, rest pauses, and speed of motion (Figure 2.4)^42,48 or (ii) calculated using information on the frequency of exertion and the work/recovery ratio (Table 2.4).⁴⁸ The third step identifies forceful exertions and postures by (i) observer ratings, (ii) worker ratings, (iii) biomechanical analysis, or (iv) instrumentation. Since the latter two methods (biomechanical analysis or instrumentation) require considerable expertise and equipment, the following discussion will focus on observer and worker ratings. Observer ratings of force utilize the same Latko et al. scale described earlier (Figure 2.4).⁴² Factors that the observer should consider include the weight, shape, and friction of the work object, posture, glove fit and friction, mechanical assists, torque specification of power tools, quality control, and equipment maintenance. Worker ratings utilize the same Borg scale (1–10) described earlier.^34,35 Suppose, for example, a male worker rates his job’s grip strength requirements as four (somewhat strong). To normalize this force, we measure the worker’s grip strength (300 newtons (N)) and compare this to the average male strength (500 N). Therefore, the normalized peak force = 4 × 300 N/500 N = 2.4. The precision of both the observer and worker ratings is improved by having multiple observers/workers rate the same job.

The HAL and the normalized force estimates can now be plotted and compared to the TLV® (Figure 2.5). Employees performing job tasks above the solid top line will be at significant risk of acquiring a UE MSD, and specific control measures should be utilized so that the force/repetition for a given level of hand activity is below this line. The dotted lower line represents an “action limit,” the point at which general controls, including surveillance (discussed below), are recommended. The TLV® does not specifically account for awkward or extreme postures, contact stresses, low temperatures, and vibration; therefore, professional judgment is needed to account for these additional stressors. If any of these stressors are present on jobs, the TLV® and the action limit will be lower.

Others

Since ACGIH TLV® does not account for all potential physical stressors, the reader is encouraged to review other proposed UE exposure guidelines, such as the strain index proposed by Moore and Garg^45,49 and the Rapid Upper Limb Assessment (RULA) proposed by McAtamney and Corlett.⁴⁶

Posture is an important potential physical stressor.²⁹ Posture can be defined as the position of a part of the body relative to an adjacent part, as measured by the angle of the connecting joint. Standard posture definitions (neutral and nonneutral) and normal ranges of motion have been developed by the American Academy of Orthopaedic Surgeons.⁵⁰ Postural stress develops as a joint reaches its maximal deviation; therefore, postures should be maintained as close to neutral as possible. In addition to postures at the extreme end of a joint’s range, tasks that require finger-pinching postures have been associated with UE musculoskeletal disorders. Kodak has proposed the following posture guidelines⁵¹:

• Keep the work surface height low enough to permit employees to work with their elbows at their sides and wrists near their neutral position.
• Keep reaches within 20 inches in front of the work surface so that the elbow is not fully extended when forces are applied.
• Keep motions within 20–30° of the wrist’s neutral point.
• Avoid operations that require more than 90° of rotation around the wrist.
• Avoid gripping requirements in repetitive operations that spread the fingers and thumb apart more than 2.5 inches. Cylindrical grips should not exceed 2 inches in diameter, with 1.5 inches being the preferable size.

Federal Ergonomic Standard

In 2000, OSHA developed and issued an ergonomic standard. In 2001, Congress, under the Congressional Review Act, passed a resolution of disapproval, thereby eliminating the standard. In its place, OSHA has issued industry-specific guidelines.⁵² OSHA has developed ergonomic guidelines to prevent MSDs for meatpacking plants (1993), retail grocery stores (2004), shipyards (2008), nursing homes (2009), foundries (2012), and poultry processing (2013).⁵² In addition, OSHA continues to inspect and, if appropriate, cite companies for ergonomic hazards under its general duty clause.

California Ergonomic Standard

In 1993, the California State Legislature required its Occupational Safety and Health Standards Board to develop “standards for ergonomics in the workplace designed to minimize instances of injury from repetitive motion.”⁵³ Subsequent legal challenges shaped its content, coverage, and start date. The standard, adopted in 1999, applies to a job, process, or operation where a repetitive motion injury (RMI) has occurred to more than one employee under the following conditions:

1. A licensed physician objectively identified and diagnosed the RMI.
2. The RMI was work related (≥50% caused by a repetitive job, process, or operation).
3. The employees with RMIs were performing a job process or operation of identical work activity (performing same repetitive motion task).
4. The employee reported the RMI to the employer in the last 12 months.

If the above conditions are met, the employer is required to develop an ergonomic program with the following three components: worksite evaluation, control of workplace exposures, and employee training. The worksite evaluation requires that each job, process, or operation of identical work activities be evaluated for exposures causing RMIs. If these exposures are found, they must be corrected in a timely manner or, if not capable of being corrected, have the exposures minimized to the extent feasible. In addition, employees must receive training on the following:

• The employer’s ergonomic program
• The exposures which have been associated with RMIs
• The symptoms and consequences of injuries caused by repetitive motion
• The importance of reporting symptoms and injuries to the employer
• Methods used by the employer to minimize RMIs

Muscles

Muscle consists of muscle fibers (muscle cells), nerve elements (motor neurons, afferent neurons, receptors of different types), connective tissue, and blood vessels. Muscle fibers are classified into two types: Type I fibers, also known as slow-twitch or red muscle fibers, and Type II fibers, also known as fast-twitch or white muscle fibers. In muscle fibers, the smallest morphological contractile unit is the sarcomere, built of actin and myosin filaments. The smallest functional unit is the motor unit, which consists of a motor neuron cell and the muscle fibers that its branches supply. The muscles of the body are the generators of internal force that convert chemically stored energy into mechanical work. A muscle contracts its threadlike fibers, which shortens the length of the muscle, thereby generating a contractile force.

Myalgia is the medical term for the symptom of muscle pain. The most common type of myalgia, delayed-onset muscle soreness (DOMS), is a contraction-induced injury after vigorous or unaccustomed exercise. DOMS is a self-limiting condition that typically appears within the first 24 hours after exercise, peaks at 48–72 hours, and resolves within 1 week. Histologic and chemical changes are found in affected muscles, but these changes are not permanent and lead to a conditioning effect when occurring in a graduated manner.^54–56 Armstrong proposed the following theory for the pathogenesis of DOMS^54–56:

• High mechanical forces, particularly those associated with eccentric exertions, cause structural damage of the muscle fibers and associated connective tissue structures.
• This structural damage alters the sarcolemma’s permeability, producing a net influx of calcium into the cell. This calcium inhibits mitochondrial production of ATP and activates proteolytic enzymes that degrade Z-disks, troponin, and tropomyosin.
• The progressive degeneration of the sarcolemma is accompanied by diffusion of intracellular products into the interstitium and plasma; this attracts inflammatory cells that release lysosomal proteases, which further degrade the muscle proteins.
• Active phagocytosis and cellular necrosis lead to accumulation of histamine, kinin, and potassium, which stimulate regional nociceptors, resulting in the sensation of DOMS.

Eccentric contractions (muscle activation while the muscle is stretched), rather than isometric contractions, are felt to lead to DOMS.⁵⁷ Eccentric contractions can occur when muscles are exposed to either a single rapid stretch or a series of repetitive contractions.⁵⁴ Both models are consistent with DOMS requiring a temporary reduction in physical loading because of pain or discomfort. This is followed by a gradual increase in physical loading to stimulate healing and subsequent tissue-remodeling processes.

Muscle also undergoes a number of age-related changes, such as a 20% decrease in muscle mass, a 20% reduction in maximal isometric force, and a 35% decrease in the maximal rate of developing force and power.⁵⁸ This latter reduction is not due to differences in muscle recruitment strategies, but rather due to a change in the contractility of the muscle itself.⁵⁹ This translates into a marked decrease in the ability to sustain power over repeated contractions in older individuals. In addition, animal experiments have also demonstrated that older muscle damages more easily and heals more slowly.^60,61 These effects may help explain why older athletes seem to require greater rest intervals between training sessions and why workers in physically demanding jobs tend to change to less demanding jobs with age.⁶²

Tendons

As a general rule, tendons transmit the contractile force generated by muscles to bone. Tendons are composed of collagen fibrils grouped into fibers that are collected together into fiber bundles that are united into fascicles.⁶³ A large number of fascicles form the tendon. The fiber bundles and fascicles are enclosed in thin films of loose connective tissue called the endotenon. This connective tissue contains blood vessels, lymphatic vessels, nerves, and elastic fibers and allows the fascicles to slide relative to one another. The whole tendon is wrapped in connective tissue called the epitenon. In some tendons, a further sheath, the paratenon, surrounds the tendon. The paratenon is merely a specialization of the areolar connective tissue through which many tendons run. A number of structures associated with tendons control and facilitate their movement. Where tendons wrap around bony pulleys or pass over joints, they are held in place by retaining ligaments (retinacula or fibrous sheaths that prevent bowstringing). Tendons glide beneath these retaining structures due to the lubrication provided by the synovial sheath.⁶⁴ In some regions, tendons are prevented from rubbing against adjacent structures by bursae. Although tendons generally have a good blood and nerve supply, regions of tendon subjected to friction, compression, or torsion are hypovascular or avascular. The general structure of tendons is modified in two regions: the sites where they attach to bone (enthesis) and the region where they are compressed against neighboring structures (around bony pulleys).⁶⁵ Fibrocartilage formation at the site of this compression loading is considered a normal/adaptive response. In summary, tendons have the capacity to change their structure and composition in response to mechanical stimulation. In most cases, this mechanical stress is beneficial and adaptive for maintaining cell activity and tissue function.

Peripheral nerves

Peripheral nerves carry signals to and from the central nervous system. A nerve fiber (neuron) consists of the nerve body, which is located in the anterior horn of the spinal cord (motor neuron) or in the dorsal root ganglia (sensory neuron), and a process extending into the periphery—the axon.⁶⁶ The axon is surrounded by Schwann cells. In myelinated fibers, a Schwann cell is wrapped around only one axon, in contrast to nonmyelinated fibers, where the Schwann cell wraps around several axons. Myelinated and nonmyelinated nerve fibers are organized in bundles, called fascicles, which are bound by supportive connective tissue, the perineurium. The bundles are usually organized in groups, held together by loose connective tissue called the epineurium. In between the nerve fibers and their basal membrane is the intrafascicular connective tissue—the endoneurium. The amount of connective tissue components varies between nerves and between various levels along the same nerve. The myelin insulation divides the axon into short, uninsulated regions (nodes of Ranvier) and longer, insulated regions (internodes). Conduction of nerve impulses proceeds by sequential activation of successive nodes without depolarization of the intervening internode (saltatory conduction).

Muscles

Myopathy is the medical term for measurable pathologic changes in a muscle, with or without symptoms. Myopathies can be due to a variety of congenital (e.g., muscular dystrophy) or acquired (e.g., inflammatory, metabolic, endocrine, or toxic) disorders. These diseases are not typically work related and will not be discussed further.

Muscle pain syndromes of unknown etiology can be classified into two categories: general and regional. General muscle pain involving all four quadrants of the body is called primary fibromyalgia. Primary fibromyalgia is not work related because, by definition, trauma-induced myalgia is excluded by the specific diagnostic criteria set by the American College of Rheumatology.⁶⁷ Regional muscle pain syndromes, not involving the whole body, often fall under the term myofascial syndrome. This has been defined as a painful condition of skeletal muscle characterized by the presence of one or more discrete areas (trigger points) that are tender when pressure is applied.⁶⁸ These muscle-related syndromes, a common example of which is tension neck syndrome, could be associated with work exposures. A variety of mechanisms have been proposed to account for this syndrome. A few are listed below.

Tendons

Physicians in sports medicine have suggested that tendon disorders fall into four main categories: paratendonitis, paratendonitis with tendinosis, tendinosis, and tendinitis.⁷⁶ These categories are based on clinical and histologic findings. It can be difficult to distinguish between these specific conditions on clinical evaluation alone. Because most conditions can be treated conservatively, histologic changes have been documented in only a subset of patients whose cases proceeded to surgery. Thus, many cases are defined simply as “tendinitis” based on history, examination, and impaired function.

Proposed mechanisms for work-related tendon disorders include⁷⁷(i) ischemia in hypovascular tissues, (ii) microinjuries incurred at a rate that exceeds repair potential, (iii) thermal denaturation, (iv) dysregulation of paratendon–tendon function, and (v) inflammatory processes secondary to some, or all, of these other factors.

Shoulder disorders provide evidence for the ischemic theory. Work above one’s head can have two effects: compression (impingement) and reduced local blood flow. Impingement comes from the narrow space between the humeral head and the tight coracoacromial arch. As the arm is raised in abduction, the rotator cuff tendons and the insertions on the greater tuberosity are forced under the coracoacromial arch.⁷⁸ Reduced local blood flow occurs when the supraspinatus muscle is statically contracted, increasing the intramuscular pressure higher than the arterial pressure of the vessel traversing the supraspinatus muscle belly and supplying it with oxygen.⁷⁹ These work factors, in combination with the fact that the entheses of the three tendons comprising the rotator cuff are hypovascular, lead to tendon degeneration manifested by microruptures and calcium deposits. Once the tendons are degenerated, exertion may trigger an inflammatory response resulting in active tendinitis.⁸⁰

The pathogenesis and pathophysiology of lateral epicondylitis are less clear. Histologic evaluation of tennis elbow tendinosis identifies a noninflammatory response in the tendon. This histopathology reveals disorganized immature collagen formation in association with immature fibroblastic and vascular elements. This has been named angiofibroblastic tendinosis and is thought to be the result of an avascular degenerative process.⁸¹ This pathology is located at the enthesis of the extensor carpi radialis brevis (ECRB) tendon. Factors associated with its development include age, systemic factors, direct trauma, and repetitive overuse from sports, occupation, and performing arts. It is theorized that multiple repetitive eccentric loading of the ECRB results in tension loading, microruptures of the peritendon, and secondary anoxia and degenerative consequences.

Tendon sheaths

Tendons can become trapped in their synovial sheaths due to a narrowing of their fibro-osseous canal. Tendons passing through stenotic canals frequently have a nodular or fusiform swelling and can be covered with granulation tissue.⁸² Whether these tendon changes are a cause or an effect of the narrowing is unclear. If the narrowing occurs in the first dorsal compartment, the disorder is known as De Quervain’s tenosynovitis.⁸² If it occurs in the flexor digits or thumb (A1 pulley), it is known as trigger finger or trigger thumb.⁸³

While the term tenosynovitis implies inflammation of the tendon sheath, inflammatory cells are rarely found on histology. The lack of inflammatory cells could represent a sampling bias, since typically only chronic severe cases are biopsied, when the inflammatory process could have already run its course.⁸⁴ The fundamental pathologic change is hypertrophy and/or fibrocartilaginous metaplasia; however, recent studies suggest that the fibrocartilaginous metaplasia represents an adaptive response to compressive forces.⁶⁵

The pathogenesis of tendon entrapment disorders involves static compression, repeated compression, and acute trauma. The static compression model is based on clinicians’ observations that De Quervain’s tenosynovitis is related to repeated, prolonged, or unaccustomed exertions that involve the thumb in combination with nonneutral wrist or thumb postures. Tensile loading of the abductor pollicis longus or extensor pollicis brevis, in combination with their turning a corner at the extensor retinaculum, creates a compressive force. The retinaculum responds with functional hypertrophy or fibrocartilaginous metaplasia. The duration of compression is more important than the number (repetition) of compressions. The repeated compression theory relies on the same biomechanical argument, except that the number of episodes of loading (repetition) is more critical than the accumulated duration of loading.

Peripheral nerves

Although a number of peripheral nerve entrapment disorders exist, CTS is the most common and most studied and will be the only nerve entrapment disorder discussed in this chapter.

CTS is the entrapment of the median nerve within the carpal canal at the wrist. Three mechanisms have been suggested⁸⁵: (i) friction associated with repetitive tendon motions, leading to flexor tendon sheath irritation and swelling, (ii) repeated direct mechanical trauma to the median nerve by structures within the carpal tunnel, and (iii) prolonged elevated pressure within the carpal tunnel, leading to ischemia, tissue swelling, and epineural fibrosis. The last mechanism is supported by the following: (i) carpal tunnel pressure (CTP) is almost always higher in patients with CTS than in normal subjects⁸⁶; (ii) surgical decompression (carpal tunnel release surgery) seems to be effective at reducing the elevated CTP and improving symptoms⁸⁷; (iii) histologic studies of the flexor tendon sheaths biopsied during carpal tunnel release show edema and vascular changes consistent with long-standing ischemia⁸⁸; (iv) animal models of acute and chronic nerve compression show physiologic and histologic findings consistent with nerve ischemia⁸⁶; (v) in human studies, acute elevation of CTP results in acute nerve dysfunction, with the critical threshold varying according to the subject’s diastolic blood pressure⁸⁹; and (vi) human studies have found a dose–response relationship between CTP and wrist posture,⁹⁰ fingertip loading,⁹¹ and repetitive hand activity.^25–27,92 These findings are consistent with the static and dynamic biomechanical models of CTS.⁹³

Psychosocial

Numerous studies have reported an association between psychosocial factors and UE MSD. Several models have been developed to explain these associations including the following:

• The balance theory of job design and stress⁹⁴
• The biopsychosocial model⁹⁵
• The ecological model⁹⁶
• The workstyle model⁹⁷

Figure 2.6 summarizes the salient features of these models.⁹⁸

The biologic mechanism by which psychosocial stress and psychological strain lead to UE MSD has not been fully elucidated, but research suggests that it may involve neuroendocrine, vascular, and immunological pathways. For example, perceived stress can trigger the “fight-or-flight” response stimulating the endocrine system (e.g., cortisol, adrenalin, and noradrenalin). Stress can also lead to increased muscle tension,⁹⁹ decreased blood supply in the extremities,¹⁰⁰ breakdown of muscle protein and its repair,¹⁰¹ and alteration of the inflammatory or immune response.¹⁰² All these short-term responses might increase the risk of UE MSDs.

Skeptics of the possible role between psychosocial factors/stress and UE MSD point out that support for the association comes from a plethora of cross-sectional studies. Due to inherent properties of their study design, cross-sectional studies have difficulty establishing the direction of the association (i.e., cause vs. effect). Longitudinal studies, on the other hand, can overcome this limitation. A review and meta-analysis of 54 longitudinal studies concluded that psychosocial factors are independent predictors for the onset of MSDs⁹⁸ and should play an important role in prevention and intervention programs.^103,104

Clinical evaluation

Like all conditions with a potential occupational etiology, the health history is a fundamental component of the evaluation.¹⁰⁵ The history should (i) characterize the symptoms, (ii) provide an employee description of work activities, and (iii) identify predisposing conditions or factors. In addition, the history should inquire about previous occupations and job tasks, symptoms during previous jobs, home responsibilities, home use of power tools, hobbies, similar symptoms among coworkers, and broader changes in the workplace (e.g., changes in equipment, bonus or incentive programs, layoffs, and training). This information should be documented in the employee’s medical record. If the employee description of work activities is unclear, or if further information is needed to understand employee job tasks and workplace conditions, this can be ascertained by visiting the workplace or viewing job tasks recorded on videotape. Simply reviewing a written description of job tasks may not provide an adequate understanding of the ergonomic stresses involved in the job.

After the history is taken and information obtained about workplace conditions and job tasks, the neck and UE should be examined.¹⁰⁶ A comprehensive examination would include inspection, palpation, assessment of the ranges of motion, evaluation of sensory and motor function, and applicable provocative maneuvers. Employees with underlying systemic disease (e.g., diabetes mellitus) may require a more complete examination involving other organ systems. Results of the examination findings, both positive and negative, should be documented in the employee’s medical record.

Clinical diagnosis

Using the information from the clinical evaluation, an assessment or diagnosis should be made. Diagnoses should be consistent with the International Classification of Diseases, Tenth Revision (ICD-10) (Table 2.5). Terms such as repetitive motion disorder (RMD), repetitive strain injury (RSI), overuse syndrome, and cumulative trauma disorder (CTD) may be useful for surveillance purposes or epidemiologic investigations, but they are not ICD-10 diagnoses and should not be used as individual medical diagnoses.

Once a diagnosis is made, an opinion is usually rendered regarding whether occupational factors caused, aggravated, or contributed to the condition. This tends to be the most difficult portion of the assessment. Unlike the classic occupational diseases, such as asbestosis or silicosis, most occupational MSDs do not have a pathognomonic finding or test specific to the exposure. Therefore, the importance of a thorough exposure assessment cannot be overemphasized.¹⁰⁷ In 1979, NIOSH published a guide for state agencies and physicians on the process of determining work relatedness of disease.¹⁰⁸ The process outlined in this guide, modified for MSD, is still relevant today:

• Has a disease condition been established by accepted clinical criteria?
• Does the literature support that the disease can result from the suspected agent?
• Has exposure to the agent been demonstrated?
• Has the exposure been of sufficient degree and duration to result in the diseased condition?
• Have nonoccupational factors been considered?
• Have other special circumstances been considered?

Clinical interventions

The goals for treatment are the elimination or reduction of symptoms and impairment and return to work under conditions that will not exacerbate the disorder. The expected duration of treatment, dates for follow-up evaluations, and time frames for improvement or resolution of symptoms should be specified at the initial evaluation and updated on subsequent evaluations.

Passive surveillance

Passive health surveillance systems utilize existing databases to identify high-risk industries, occupations, or tasks that are associated with disease or injury. Examples of these databases in occupational medicine include the OSHA 300 Logs, workers’ compensation records, medical department logs, clinical laboratory data, hospital discharge records, and accident reports. Jobs or tasks with an increased UE MSD injury rate can be targeted for further ergonomic and/or medical evaluation. In addition, jobs or departments can be followed over time, to monitor trends in these rates.

Although attractive because of their low cost, passive surveillance databases are sometimes developed for purposes other than surveillance and may have significant limitations.¹²⁶ These limitations include underreporting and exposure misclassification. For example, underreporting in the OSHA 300 Logs can occur for any of the following reasons: symptomatic employees not seeking medical care (“macho” attitude, ignorance that the condition could be work related, or fear of employer retaliation), restricted or no access to employee health facilities, or misunderstanding about when a case is to be recorded on the OSHA 300 Log.⁷ Exposure misclassification can occur when employees use a general term to describe their job title; for example, an employee in the poultry industry may report his or her job title as “cutter” in a plant with five distinct cutting positions. Each one of these cutting jobs may be associated with a different ergonomic hazard, and the identification of high-risk jobs requires specific knowledge of the employees’ cutting position.

Active surveillance

Active surveillance systems generate more accurate databases to identify high-risk positions. Direct symptom surveys are good examples of active disease surveillance tools in occupational medicine¹²⁷ and have been developed for musculoskeletal disorders.¹²⁸ Symptom surveys collect more accurate information and can serve a triage function if performed in a confidential manner. Active surveillance systems can also collect information on exposures. This information is typically established by plant personnel or nonmedical consultants; however, healthcare providers may be called upon to participate in a comprehensive ergonomic program. If exposure surveillance is a component of that program, the survey instrument in Tables 2.4 and Figures 2.4 and 2.5 could be used to establish an exposure surveillance database.

Engineering controls

Studies have shown that engineering interventions can reduce ergonomic hazards and can lead to a reduction in MSDs.^16,130,131 In some situations, the interventions are obvious and represent common sense solutions. On the other hand, worksites frequently require a more comprehensive approach to the control of ergonomic hazards. This comprehensive approach should address the following risk factors: repetition, force, posture, and vibration.

To reduce repetitiveness, the following interventions could be used: (i) enlarged work content, (ii) automation of some job tasks, (iii) uniform spreading of work across the work shift, and (iv) job restructuring. To reduce force or mechanical stressors, the following interventions should be considered: (i) decreasing the weight of tools, containers, and parts; (ii) optimizing the size, shape, and friction of handles; and (iii) using torque control devices. Reduction of awkward or extreme postures could be achieved by (i) locating the work more appropriately and (ii) selecting tool design and location based upon workstation characteristics. Engineering controls for the reduction of vibration are reviewed in Chapter 4.

In some instances, however, engineering controls are not currently available. Until engineering controls become available, other aspects of an ergonomic program—administrative and medical treatment controls—can be implemented.

Administrative controls

Administrative controls can be defined as work practices or training used to reduce employee exposure to ergonomic stressors. Examples of work practice controls include (i) more frequent and longer rest breaks,¹³²(ii) limiting overtime, (iii) varying work tasks or broadening job responsibilities, and (iv) periodic rotation of workers between stressful and less stressful jobs. Since job rotation exposes more workers to the more stressful job, it is suitable only where short-term performance of the stressful job poses no appreciable ergonomic hazard. Otherwise, other control methods must be utilized. Training programs range from fundamental instruction on the proper use of tools and materials to instruction on the use of protective devices. Improper work technique has been associated with the development of UE MSDs.^133,134 Finally, efforts to improve the workplace climate and culture with programs like participatory ergonomics are also considered a type of administrative control.^135,136

Medical treatment

The goal of medical treatment as a component of an ergonomic program is to provide prompt evaluation and treatment to limit the severity, disability, and costs associated with these disorders. It should also serve to initiate a reevaluation of the ergonomic stresses associated with the affected worker’s job and institution of appropriate control measures. Medical treatment in this sense is a secondary and tertiary prevention mechanism. Medical management programs are an important component of any successful ergonomic program.^137–139 However, they should always be used with engineering and administrative controls during the implementation of a complete ergonomic program.¹²⁹