Pins and Needles

bones of the arm

You rely on your wrists for hundreds of motions, from a quick flick to cut carrots or hands on hips to make a point, to something more exaggerated, even strenuous. Your wrists flex to knead dough, push a lawnmower uphill, type a report. You don't even think about it—until something goes wrong, until the numbness, tingling, and pain are so bad that you wake up in the middle of the night, your hands so weak that uncapping the pill bottle is a struggle.

The median nerve in your wrist tells the brain what you touch and directs your hands to react. Damage it, and you may suffer from carpal tunnel syndrome (CTS). Heecheon You, a doctoral candidate in industrial and manufacturing engineering at Penn State, is looking at the "human" factor in CTS—why some people are plagued by this disorder while others, performing the same tasks, are not. The carpal tunnel, he explains, consists of the median nerve, the carpal bones, assorted tendons, and the transverse carpal ligament. When the wrist is flexed, the tunnel bends, like a drinking straw, pinching whatever lies inside it. Just as a bent straw restricts the passage of liquid, a flexed carpal tunnel impacts the median nerve, whose signals determine movement and feeling. As the motion is repeated, the nerve can be pinched and rubbed, eventually removing the insulating layer called the myelin sheath, and creating a feeling of pins and needles in the thumb and first three fingers.

This numbness and tingling often indicates the presence of CTS, which is both common and costly. Hundreds of thousands of workers have reported the weakened grasp symptomatic of CTS. Rehabilitation, including surgery, can cost more than $10,000; full recuperation can take up to two years, during which time sufferers may have to depend on others to button shirts or handle scissors, even to write grocery lists.

"CTS comes from improper workplace design that causes awkward postures, with excessive force applied to the finger-tips or the base of the palm," explains You. It is associated with repetitive work involving the wrist, often including vibration from power tools and lifting heavy objects. So, why is You concentrating on the people themselves instead of on the environment in which they work? "Workers who are female, older, obese, overstressed, and inactive are more likely to get carpal tunnel syndrome," says You. "I am trying to determine how each risk factor contributes to the development of the problem." Then You plans to create a risk assessment model for workplace designers. Special adapters that cushion the wrist and keep it flat, like foam padding on an elevated mouse pad or an ergonomic keyboard, have been developed for office workers. With knowledge of who is more susceptible to CTS, You explains, designers can adapt other workspaces to avoid putting workers' wrists in painful positions.

You has differentiated between major symptoms associated with CTS—numbness, tingling (paresthesia), and nocturnal awakenings—and minor symptoms—pain, clumsiness, and weakness—that are associated with lesser problems like tendonitis. To examine the biological significance of these symptoms, You used electro-diagnostic tests, which precisely monitor damage to nerve conduction. In an upcoming study, You will ask 100 participants, half with no wrist problems, the other half with CTS, to fill out questionnaires to measure the severity of their symptoms and to identify their personal characteristics, perceptions at work, and work conditions. He will contrast these two groups, hoping to pinpoint risk factors, then develop a model explaining the relationship between symptom severity, each individual's susceptibility, and his or her work practices—whether they hit the top of a stapler with a fist, for instance, or press it down with the base of the palm. The results of this study, he hopes, will provide a hands-down predictor for CTS.

Heecheon You is a Ph.D. candidate in the department of industrial and manufacturing engineering in the College of Engineering, 207 Hammond Bldg., University Park, PA 16802; 814-863-2740; hxy120@psu.edu. His advisers are Andris Freivalds, Ph.D., professor of industrial and manufacturing engineering in the College of Engineering, 207 Hammond Bldg.; 863-2361; axf@psu.edu; and Zachary Simmons, M.D., associate professor of medicine in the College of Medicine, the Milton S. Hershey Medical Center, 500 University Dr., Hershey, PA 17033; 717-531-8692; zsimmons@med.hmc.psghs.edu.

Last Updated January 01, 1999