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Getting a grasp on Carpal Tunnel Syndrome.

Getting a Grasp On Carpal Tunnel Syndrome

The impact of repetitive motion injuries in the workplace is increasingly being recognized by management due to soaring workers' compensation costs and expensive damages awarded plaintiffs in civil court cases. Costs to the worker in pain and suffering and to the employer in lost productivity and low morale are also high.

Carpal Tunnel Syndrome (CTS), caused by the compression of the median nerve as it passes through the carpal tunnel in the wrist, is one condition that is spurring these high medical expenses and claims costs. The compression occurs when the size of the tunnel is reduced by wrist deviation or swelling of the finger flexor tendons which also pass through this area. According to an article by researchers T. Armstrong and D.B. Chaffin in the Journal of Occupational Medicine, "Symptoms include pain, numbness, tingling, clumsiness and lack of sweating in parts of the hand innervated by the median nerve.... Because symptoms are frequently most acute during bed rest, the term 'nocturnal numbness' is commonly used in reference to Carpal Tunnel Syndrome."

CTS is cumulative and can lead to permanent hand disabilities, so early diagnosis and treatment are recommended. A test for CTS can be made by pinching the forefinger and middle finger to the thumb while holding the wrist flexed. The test is positive if pain develops within 12 minutes. Immediate treatment involves stopping the offending activity. Indeed, one study found that symptoms improved in 18 workers who changed jobs.

Repetitive motion is not the only cause of these injuries. Secondary factors may, in fact, better predict injury. These factors fall into two categories as viewed by the employer: controllable or job-related and uncontrollable or worker-related. Job-related factors can be addressed using administrative and engineering controls, including job rotation, job enrichment, methods changes, tool redesign, task elimination, work pace and rest control, work-station design and supervisor and employee training programs. Worker-related factors are different for each employee and uncontrollable because the risks will not change unless an employer selects a different employee. Federal law precludes most of this selection process, but by knowing the high risk factors, management may be able to institute additional safeguards in particular cases.

Controlling the Syndrome

Wrist deviation leads to changes in pressure within the carpal tunnel. In one study, pressure was measured on the median nerve in the wrist. Bending the wrist forward 90 degrees (acute volar flexion) resulted in a significant increase in presure. Bending the wrist backward 90 degrees (hyperextension) yielded three times as great an increase in pressure. Turning the wrist sideways toward the little finger (ulnar deviation) also contributes to this effect, according to Armstrong and Chaffin. They conclude that finger movement with wrist flexion increases the risk of CTS. In fact, the preponderance of evidence supports recommendations for workers to keep their wrists straight while rotating the forearm and hand.

The mechanics leading to this increase in pressure during wrist deviation are complex. Intuitively, the size of the tunnel would constrict, like a hose, when it is bent 90 degrees. The structure of the tunnel and other components within it also play a part. For example, in passive (unloaded) flexion and extension, the ante brachial facia, which runs parallel to the median nerve, compresses the median nerve. In addition, the tendons passing through the tunnel have been shown to behave biomechanically, like belts wrapped around a pulley, and can therefore be quantitatively analyzed. The force on these tendons has been linked to compression of the median nerve. Compounding the problem is the fact that repeated compression of the tendons in the tunnel can lead to inflammation of the synovial tissues surrounding them, thus further compressing the median nerve. Studying Armstrong's biomechanical model yields a better understanding of the interrelationships of the factors affecting CTS.

Force Proportional to Load

The force the tendon exerts in the tunnel (over the "pulley"), F1, is proportional to the load on the tendon, Ft, and the radius of the anatomical "pulley," r. Thus, the equation is F1 = Ft/r. The load on the tendon is the force applied at the finger; this is a measure of grip force, which involves grasping an object with the whole hand. Measured F1 to be four times the grip force in a pinch grip, which involves compressing an object between the finger(s) and thumb, and three times the force in a grasp (power grip), the radius is a function of wrist dimensions and could be a significant factor in the high occurrence of CTS in women. For women with small wrists, F1 is 14 percent greater for flexion and 25 percent greater for extension than it is for men with large wrists, according to Armstrong's biomechanical model.

The relationship of grip strength to CTS involves the force it applies to the tendons in the tunnel. Therefore, to alleviate pressure on the tendons, grip strength requirements must be controlled. Pinch grips are particularly suspect because the grip strength comes primarily from the fingers and the tendons in the tunnel.

In a paper titled "Carpal Tunnel Syndrome: Contribution of Flexor Tendons," researchers argue that "Repetitive pinch (in the commonly used configuration of the three-jawed chuck), carried out while the wrist is in some degree of flexion, may contribute significantly to the syndrome.... A frequent complaint of patients with idiopathic Carpal Tunnel Syndrome is that their symptoms are made worse by such activities."

Grip strength can be evaluated to see if a task can be accomplished. The chuck uses three fingers, instead of two with the pulp pinch, and distributes the force over more tendons. The lateral pinch applies force between the thumb and index finger and uses muscles other than those in the tunnel. In a study by Armstrong and Chaffin documenting workers who sew seat covers, the "diseased" group used the pinch grip more frequently than the control group.

Gloves also affect the force on the tendons. According to Armstrong, grip strength can be reduced 30 percent or more by wearing gloves. Additional force is required to hold an object because of the reduced gripping ability of the glove. In addition, reduced sensory feedback causes an employee to exert more force than necessary. In both cases, the force is greater with gloves than without them. However, since the effect is exacerbated by poorly fitting gloves, as little of the hand should be protected as possible. Alternatives include cutting off portions of the glove, using gauze tape on fingers rather than wearing gloves or eliminating the need for hand protection.

Work Load Is a Factor

The work load and force of exertion contribute to wrist stress. Biomechanically, the level of exertion can be evaluated similarly to grip strength in view of the fact that hand symptoms increase with physcial stress. According to causation studies conducted in part by Armstrong, occurrence rates are 5 percent for light work, 8 percent for moderate work and 11 percent for heavy work. Ways of decreasing occurrence rates include reducing the amount of weight handled, changing the shape or size of objects to ease handling, gripping at the center of gravity or using two hands. Also, a heavy work load or pace program linked to an incentive system may overtax the body's ability to recover. A National Institute of Occupational Safety and Health study in 1984 found a higher incidence of vibration syndrome among incentive workers than hourly employees. The syndrome results when vibrations are transmitted through the nerves in the hands. In two different studies, an increase in activity has been associated with the onset of symptoms in 43 percent and 63 percent of cases, respectively. Overtime, increased work load and job changes were cited as reasons for the changes in employee work levels. In another study,26 percent of the women and 12 percent of the men experienced hand or wrist injuries in highly forceful, repetitive jobs, whereas none of the men and only 3 percent of the women incurred injury in jobs requiring little force or repetition.

Not-So-Good Vibrations

Vibrations are transmitted to the hands in most manual-powered operations that require the use of impact tools, hand-held power tools, benchmount grinders and buffers or steering wheels. The use of these instruments results in Raynaud's Phenomenon, commonly known as "White Finger," which is caused by the disturbance of blood flow to hands and fingers. Vibrations also contribute to cumulative trauma disorders (CTD) through Tonic Vibration Reflex (TVR), a gradually increasing, involuntary muscle contraction. As it relates to CTS, this contraction results in an uncontrolled increase in grip pressure, requiring more force than necessary to hold an object. TVR can be voluntarily controlled through feedback, although it is almost impossible to accomplish this in a production environment.

In many cases, using a hand tool is the same as doubling work load. In a study of 30 workers with CTS, researchers found the odds of vibration being present in an employee's job at a ratio of 7-to-1, while repetitive motion was figured at only 2.1-to-1. In two separate studies, which surveyed 3,466 workers, vibration was found to double the likelihood of CTD symptoms.

Eliminating vibration might mean doing away with motors which, in turn, would result in more manual effort and forceful exertions - in effect, trading one problem for another. Minimizing or dampening vibration associated with the use of power tools is a more constructive solution. In one study, rubberized handles reduced the transmission of vibration by two-thirds. Eliminating direct contact by using holding fixtures with dampers is another worthwhile consideration.

Another contributor to CTS is pressure exerted on the base of the palm. Pressure on the flexor retinaculum, which similar to the ante brachial fascia runs parallel to the median nerve, compresses the median nerve, a situation that occurs when using screwdrivers, knives or a jewelry beader. In addition, temperature contributes to CTS. Cooling the fingers leads to loss of sensory feedback and reduces manual dexterity. This contributes to an increase in grip and tendon forces in the carpal tunnel. In one clinical study, a drop in skin temperature from 24 degrees Celsius to 12 degrees Celsius resulted in an 80 percent reduction in productivity. One solution is, of course, to protect hands from heat and cold. The problem can also be controlled by altering such job factors as product, tools and environment. These factors act on the wrist by applying force to the tendons in the carpal tunnel or causing inflammation, both of which compress the median nerve.

Genders Benders

Employees can be viewed individually or in groups. However, the grouping mentioned most frequently regarding CTS is male vs. female with the typical CTS patient characterized as a woman at or near menopause. The group-related factor involved in this distinction is wrist size differences between men and women. The other factors, although predominantly occurring in women, are best viewed by reviewing individual health records.

Given the laws against sexual discrimination and the trouble identifying workers at risk through available pre-employment tests, worker-related factors should be viewed on an individual basis. Armstrong and other researchers found a 3-to-1 female-to-male ratio of reported cases of CTS. Other studies also show a higher incidence rate for females, leading to a belief that biomechanics and biology may explain this relationship. In Armstrong's biomechanical model, the radius of the "pulley," which is a factor of wrist size, is directly proportional to the force of the tendon. In two separate studies, researchers used computerized CAT scanners to show the relationship between a small carpal tunnel and idiopathic CTS.

According to researcher G. S. Phalen, "The fact that the majority of patients are women at or near menopause suggests that the soft tissues may be affected by hormonal changes." This theory was confirmed in a 1985 study. Menopause, gynecologic surgery, oral contraceptives and pregnancy affect hormonal balance. Menopause was found to be the most corrolated factor, but it is confounded by age. Gynecologic surgery was the second highest factor. Both menopause and surgery result in a reduction of estrogen. In the case of surgery, however, the drop in estrogen is sudden and dramatic. Pregnancy and oral contraceptives do not seem to lead to as drastic a hormonal change, although fluid retention during pregnancy does impact pressure within the tunnel.

Society may also have contributed to the current situation. In the past, women held more repetitive jobs, such as assembly worker, packer and data enterer, than men who worked as machinists, operators and supervisors. Non-discrimination laws are altering that situation, resulting in a "Work force more closely resembling the general population in respect to susceptibility to injury," according to Armstrong and B. Silverstein. Today, women also find themselves employed in white collar and supervisory jobs requiring forceful exertions.

Associated Diseases

Certain diseases associated with CTS often cause inflammation or reduce lubrication in the tunnel. In studies of 823 CTS cases, researchers found 16.8 percent also had arthritis, 10.8 percent had diabetes and 10.6 percent had other CTD such as trigger finger and tennis elbow. In addition, diabetes and arthritis increase the risk that a worker will suffer from CTS symptoms. Wrist trauma caused by a change in the functioning of the joint has also been linked to CTS. The wrist is fragile because it provides motion and flexibility and transmits force. An injury changes this interaction and can cause further damage. According to Armstrong," Median nerve injury can be produced by a blow to the wrist, a laceration, burn or other acute wrist trauma. These, in turn, can produce symptoms of CTS."

Age is not a reliable predictor of CTS, but it does yield some worthwhile statistics. Armstrong and Phalen studied subjects in their mid-50s, an age group that represents not only the average working age, but includes workers with the highest incidence of CTS. A number not available, but possibly better suited to drawing conclusions, is incidence percentage by age group. Time on the job is not a major factor in CTS cases. A study of wrist tissue density shows an increase at the point of flexion, but only in extreme cases would this result in CTS. Aging seems to indicate increasing risk due to accumulated motion, yet incidents of CTS decline after age 60. Is this the result of a smaller percentage of cases in the work force or possibly a decrease in physical efforts? Grip strength also decreases with age, dropping 15 percent after age 30, according to Armstrong. Assumptions can be made about age, but no direct correlation has been established.

A Rule-of-Thumb Ranking

CTS is certainly more than just a repetitive motion disease. The risks of developing symptoms are affected by a variety of factors which impact differently depending on the job, worker and interaction with other risks. Ranking the importance of each factor may not apply to all situations, but the following evaluation will generalize these risks. Job-related factors, by rule-of-thumb ranking, include vibration, wrist deviation, forceful exertions and work load, grip force and gloves, pressure on palm and temperature. They reflect not only the relative risk of damaging the wrist but the performance frequency of each factor. Their combined effects, such as pinch grip with a deviated wrist, also complicate attempts to develop a rigorous ranking system. The most useful application of this information might be to look for each of these conditions, especially where they occur together.

Worker-related factors, by rule-of-thumb ranking, include hormonal changes related to gynecologic surgery, menopause and pregnancy, diseases such as diabetes and arthritis, and wrist trauma. These factors were separated from the ranking because the two types of risks need to be treated differently. Controllable factors can be addressed by engineering methods. Individual workers are best monitored by supervisors and personnel managers through one-on-one decision making or company policy. These worker-related problems warrant their own priorities and must be given the same consideration as job-related factors because both provide opportunities to eliminate injuries and losses. Although this information does not provide obvious solutions - company policies, laws and employee relations are also elements of the final equation - screening for these conditions and sudden changes, such as surgery or injury, may provide reasons to justify instituting administrative controls. Education or direct supervision would spotlight these conditions and appropriate steps could then be taken. An employer might want to relocate a pregnant employee who works in a highly repetitive job or refuse an employee with a bruised wrist from working overtime in a job requiring the use of hand tools.

Gender considerations were intentionally omitted because of legal considerations. Hormonal changes are solely a female phenomenon and so, in a sense, gender is a factor. From the standpoint of grip strength or wrist size, engineering controls are recommended unless the person in question, male or female, is weaker than an average employee. If engineering controls are not available, some form of non-gender screening that tested for job requirements may be applied. The most practical approach to dealing with CTS and increasing workers' compensation costs involves a combination of training, engineering and administrative controls. The most pressing concern is engineering because there is no excuse for not improving work environments. Administration is more complex because costs, risks, safety and rights are all considerations. Training can increase the awareness of management, supervisors and workers to the risks associated with CTS.

The cumulative nature of CTS makes on-the-job controls inadequate because they affect only a small portion of workers' lives. Housework, hobbies and other daily functions contribute to injury. The trend in workers' compensation is to make the employer responsible for treatment if the job contributes to the problem. Therefore, education is invaluable because it carries over into all aspects of workers' lives. A potential lost-time injury at work may be avoided without any changes in the workplace by reducing risks off the job. CTS presents a complex challenge to the employer, but prevention is possible.

Peter A. Storti is operations support manager for Abbott Laboratories, a producer of medical diagnostic pharmaceuticals.
COPYRIGHT 1990 Risk Management Society Publishing, Inc.
No portion of this article can be reproduced without the express written permission from the copyright holder.
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Author:Storti, Peter A.
Publication:Risk Management
Date:Mar 1, 1990
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