Computer Use & Cause Carpal Tunnel Syndrome? Is there a connection?

by Stephen Bowden, Chartered Ergonomist for Morgan Maxwell

The brief answer is no!

In a medical paper published in 2002 Falkiner and Myers concluded that except in the case of work that involves very cold temperatures (possibly in conjunction with load and repetition) such as butchery, work is less likely than demographics (such as age or sex for example) and disease-related variables (such as diabetes for example) to cause Carpal Tunnel Syndrome (CTS). Furthermore a Systematic Review of the medical literature published in 2008 by Thomsen and colleagues concluded there is insufficient epidemiological evidence that that work in the form of computer work causes Carpal Tunnel Syndrome.

Also authorities such as the Mayo Clinic point out that several medical studies have evaluated whether there is an association between computer use and Carpal Tunnel Syndrome and it states that “there has not been enough quality and consistent evidence to support extensive computer use as a risk factor for Carpal Tunnel Syndrome, although it may cause a different form of hand pain.”

So why, in the face of the medical evidence has computer work and in particular the use of the computer mouse been cited by computer equipment manufacturers as a cause of Carpal Tunnel Syndrome?

A brief reminder of what Carpal Tunnel Syndrome is will help to answer this question:
Carpal Tunnel Syndrome is found in the wrist, and occurs when a nerve, called the median nerve, that controls sensation and hand movement becomes compressed. It is known medically as an entrapment neuropathy. When the median nerve is compressed the symptoms of Carpal Tunnel Syndrome, which can affect all parts of the hand, will manifest themselves, but tend to involve the thumb index finger and middle finger. So If you feel a tingling sensation, numbness and pain in these fingers, and half of your ring finger, but not your little (pinky) finger, this could be indicative of Carpal Tunnel Syndrome. Feeling to the little finger is provided by different nerves.

Sensations usually appear at night, where they can develop gradually and can become much worse as the evening goes on. The symptoms often affect both hands eventually, and can sometimes extend beyond the area in which the median nerve passes, resulting in arm ache.

Sometime ago computer mouse manufacturers discovered that there had been some research carried out which investigated the compressive forces around the median nerve at the wrist during certain positions and movements of the hand involving pressing and pinching tasks carried out by the fingers.

The research carried out by Keir and his colleagues which was published in 1998 and showed that the forces produced during fingertip loading such as a pinch grip between the index finger and thumb or pressing a finger downwards could produce compression forces on the median nerve sufficient to damage it and therefore cause the symptoms of Carpal Tunnel Syndrome. The computer manufacturers took the forces at face value failing to realise that the forces used in computer mouse use do not cause the pressure threshold to be reached within the wrist to cause injury to the median nerve and erroneously embarked on the false premise of selling products designed to eliminate use of the pinch grip.

The threshold of force required to produce sufficient compression around the median nerve to cause it damage is 510 grams. The force to produce a click in most computer mice ranges between 50 and 70 grams which equates to 140 grams of pinch force i.e. 0.27 of the of the threshold force required to cause damage to the median nerve. The actual forces used by the hand during computer mouse activity do not therefore produce sufficient pressure to be a primary cause of median nerve damage at the wrist and the primary cause of damage occurs elsewhere often occurring from a combination of activities and health conditions such as:

• Health conditions which can reduce blood flow to the hands, such as diabetes, obesity, rheumatoid arthritis and gout (researchers at the Diabetes Institute, Eastern Virginia Medical School report in the journal Diabetes Care that entrapment neuropathies such as Carpal Tunnel Syndrome are prevalent in those with diabetes, affecting in 1 in 3 patients).
• Previous injuries or damage to the wrist, such as broken or dislocated bones.
• Pregnancy – Roughly 50 per cent of women develop carpal tunnel syndrome during pregnancy
• Family history of carpal tunnel syndrome
• Work that involves very cold temperatures (possibly in conjunction with load and repetition) such as butchery.

Summary

There is no need to avoid using a computer mouse that utilises the pinch grip for its operation. Indeed design concepts that researchers such as those recommended Balakrishnan & MacKenzie (1997) who pointed out that devices that exploit the thumb and index finger working in unison are likely to yield high performance.  Zhai and his colleagues also demonstrated that
well-designed pointing devices which rely on all parts of the human upper limb working in synergy can outperform devices which inappropriately depend on a particular limb segment such as the shoulder for their entire operation should now be the focus of computer mouse design leaving the fallacy of computer mouse usage as a cause of Carpal Tunnel Syndrome where it belongs in the past.

References
Zhai, S., Milgram, P., & Buxton, W; 1996; The influence of muscle groups on performance of multiple degree-of-freedom input. Proceedings of the CHI `96 Conference on Human Factors in Computing Systems, 308-315. New York: ACM.
Balakrishnan, R., & MacKenzie, I. S; 1997; Performance differences in the fingers, wrist, and forearm in computer input control. Proceedings of the CHI ’97 Conference on Human Factors in Computing Systems, pp. 303-310. New York: ACM.
Keir PJ, Bach JM, Rempel DM; 1998; Fingertip loading and carpal tunnel pressure: differences between a pinching and pressing task. J Orthop Res; 16 : 112-5.
Falkiner and Myers S; 2002; When exactly can carpal tunnel syndrome be considered work-related? ANZ J Surg. Mar;72(3):204-9.
Thomsen et al, 2008; Carpal tunnel syndrome and the use of computer mouse and keyboard: A systematic review;BMC Musculoskelet Disord. 2008; 9: 134.