As a collection, the research findings on the risk factors for carpal tunnel syndrome (CTS) could be described as an incomplete puzzle with mismatching pieces. One recent attempt to assemble it produced another piece that doesn’t seem to fit. It drew conclusions that could complicate the business of devising risk assessment tools and preventive measures for CTS, and could also be a gift for employers with pending CTS workers’ compensation claims. But the accumulated weight of prior studies could keep the claims and personal injury lawsuits alive.
The new study all but severed the link between occupational risk factors and CTS, an association that is institutionalized in workers’ compensation systems everywhere. David C Ring, MD, a surgeon and assistant professor of orthopaedic surgery at Massachusetts General Hospital in Boston, presented the results in March at the 74th Annual Meeting of the American Academy of Orthopaedic Surgeons. A team led by Dr. Ring evaluated data from 117 CTS studies, with scores based on Bradford-Hill criteria, to determine the strength of a cause-and-effect relationship between work and CTS.
They found that average scores for biological factors (genetics, race, age, etc.) were double those of occupational factors (occupation, repetitive hand use, vibration, etc). In addition, the average strength of causal association (odds ratio) was about three times as strong for biological factors as it was for occupational factors.
In an AAOS news release about the findings, Dr. Ring noted that the link between hand use and CTS is commonly accepted as true, but the scientific evidence suggests otherwise. “The link between hand use and carpal tunnel syndrome is overstated and may be inaccurate. In contrast, there is strong evidence for an inherent, genetic risk for CTS,” he explained.
Dr. Ring differentiates between carpal tunnel syndrome and repetitive strain injury, which he prefers to call idiopathic arm pain. In this condition, he says, there is pain “but no evidence of injury. It doesn’t involve the carpal tunnel.”
He noted that “this application of stringent science to theories of causation may affect claims of disability, workers compensation and personal injury.”
But it might not. The accumulation of research that appears to strengthen the link represents a significant counterweight to his case. It could be argued that a study published in the February 2007 issue of Human Factors: The Journal of the Human Factors and Ergonomics Society finds in favor of future claimants and plaintiffs. The researchers, led by Peter J. Keir at McMaster University in Canada, believe the set of guidelines they developed from their data can be used in the design of work and tools to protect workers. And they say the guidelines could also be used to identify tasks that may put workers at risk for developing or aggravating CTS.
The study was described in a news release as the first that systematically identifies how carpal tunnel pressure can be examined to establish limits on how much a wrist can be flexed before nerve damage sets in. The research team studied the pressure placed on the nerve in the carpal tunnel in various wrist postures in 37 men and women with no prior CTS symptoms. They concluded that “sustained wrist deviation from neutral at work” may be a CTS risk factor, and that injury is likely to occur if sustained pressure on the carpal tunnel reaches 30 mmHG.
Dr. Keir and his team faulted the methodology of studies that dismiss wrist posture as a risk factor, explaining it “may be attributable to the methods by which wrist posture was analyzed in those studies, especially if the studies used peak wrist posture rather than mean posture.”
The Keir et al study is particularly valuable for its detailed review of previous research and its comprehensive bibliography on CTS.
Recent research from the Mayo Clinic in Rochester could also be regarded as favoring future claimants and plaintiffs. Peter Amadio, M.D., a Mayo Clinic orthopedic surgeon who led the study, blames tissue abnormalities for CTS. The team’s findings appear in the November 2006 issue of Plastic and Reconstructive Surgery.
The study did not probe the cause of the shearing, Dr. Amadio explained, but does investigate the role of trauma or stress to the tissue lining from repeated finger movements. “Such shearing injuries could be the result of marked or repetitive differential motion of adjacent digits, and may support the hypothesis of a traumatic cause for carpal tunnel syndrome.” The researchers found that as the injury heals, the resulting scar tissue impedes the sliding motion of the tendon, compresses the median nerve, cuts off the nerve’s blood supply and eventually leads to the pressure buildup characteristic of CTS.
In a report published by Harvard University in 2005, experts reviewed many prior studies and their conclusions also buttress the work-CTS link. “Hands: Strategies for strong, pain-free hands,” cleared computer use and tapping of any blame for CTS, shared Dr. Ring’s believe that genes played a role, but adhered to the occupation-CTS link.
According to the researchers, even as much as seven hours a day of tapping on a computer keyboard won’t increase the risk. They observed that CTS is more common in assembly-line workers, particularly in manufacturing, sewing, cleaning or meatpacking. Genes account for about half the cases, they said. As contributing risk factors they listed overweight, pregnancy and diseases such as diabetes, rheumatoid arthritis, and lupus, and thyroid irregularities, connective tissue disorders and previous bone dislocation or fracture.
The Mayo Clinic’s general treatise on CTS, published on its web site, could be “meat” for lawyers and expert witnesses representing either side of a workers’ compensation or CTS personal injury case. It explains that CTS is generally accepted as a “new condition of the information technology age, born from long hours of computer keyboarding,” but has actually been around for much longer. “Evidence of people experiencing signs and symptoms of carpal tunnel syndrome occurs in medical records dating back to the beginning of the 20th century,” it notes.
The Mayo account blames pressure on the median nerve for CTS, adding that the pressure can stem from anything that reduces the space for it in the carpal tunnel. Causes might include anything from bone spurs to the most common cause, which is swelling or thickening of the lining and lubricating layer of the tendons in the carpal tunnel.
As contributing factors, the Mayo identifies gender, hereditary and physical characteristics, such as a narrow carpal tunnel, an array of medical conditions and medications that includes hormones.
The part of the Mayo account likely to be the most valuable to workers’ representatives is when it cites as risk factors repetitive use or injury and repetitive flexing and extending of the tendons in the hands and wrists
This article originally appeared in The Ergonomics Report™ on 2007-03-21.