Two-Finger Typists at Risk for Carpal Tunnel Syndrome

ROCHESTER, Minn., Nov. 10 -- The time-honored "hunt-and- peck" approach to typing at a computer keyboard, using just the index fingers, may trigger a cascade of events that climaxes with carpal tunnel syndrome.

Two-finger typing is likely to cause a shearing injury to subsynovial tissue in the wrist, suggested Kai-Nan An, Ph.D., a professor of bioengineering at the Mayo Clinic, and colleagues in the November issue of Plastic and Reconstructive Surgery.

However, two-finger typists can reduce stress on the subsynovial tissue by curling the other fingers when pecking away with index fingers, Dr. An said. Extending the other digits can spell bad news.

The visceral synovial layer, which lies between the tissue and the tendon in the wrist, is comprised of a type of collagen that is particularly susceptible to microdamage, the investigators said.

Dr. An and his colleagues examined biopsy specimens from the hands of 11 patients with idiopathic carpal tunnel syndrome as well as tissue samples from two cadavers with a history of carpal tunnel and tissue from 14 cadaver controls using light and scanning electron microscope imaging.

Dr. An said that carpal tunnel has traditionally been approached from the end stage, when the medial nerve is compressed. "What is really exciting about our work is that we are identifying the origin of the problem at the earliest stage," he said.

By knowing where to look-the subsynovial connective tissue-it may be possible to screen for microdamage and intervene with either ergonomic training to prevent repeated damage "by using anti-fibrotic therapies that will prevent the injury from progressing to the fibrotic stage, which is the cause of pain and disability."

He said that he and his colleagues are working to establish ultrasound markers that could be used to scan for signs of early damage.

The subsynovial tissue consists of a number of layers, much like the layers in a puff pastry. These layers of fibrous bundles run parallel to the tendons and are connected to both the flexor tendons and the synovial membrane.

When fingers are moved, both the tendons and this subsynovial tissue are stretched, but some movements cause maximum stress, he said.

For example using index fingers to type while keeping the other digits extended, results in maximum stress on the tendons and tissue-enough stress to shear the subsynovial tissue from its moorings.

This was confirmed by analysis of tissue from both patients and cadavers with carpal tunnel syndrome which found that "closer to the tendon, the vertical interconnecting fibrils between fibrous layers were absent," Dr. An and colleagues wrote.

Moreover, damage caused by over-stretching caused the subsynovial tissue to thicken and become more fibrotic, he said.

If these findings are confirmed in other studies, "it may be possible to design hand or finger exercises to reduce the stress caused by stretching the tendons and subsynovial tissue," Dr. An said. The exercises would be warm-ups, similar to the type of exercises done by athletes before they workout.

But while this finding provides evidence of the histology behind carpal tunnel, Dr. An and colleagues found "no significant association between the severity of carpal tunnel syndrome and the changes in the subsynovial tissue observed with scanning electron microscopy," they wrote.

He maintained that the small sample size is the likely explanation for this limitation, he said.

The National Institutes of Health funded the study. The authors reported no financial conflicts.