Quantitative Measurement of Thumb Adduction and Abduction

ABSTRACT

Quantitative studies of hand motor strength are limited by devices that measure grip and pinch strength. A complete quantitative analysis of pure thumb abduction and adduction motor strength in the normal population has not been performed. Isolated analysis of this motor function can be important in the pre- and postoperative evaluation as well as the long-term management of patients with disorders that can result in weakness of these specific muscles.

Volunteers (294 males and 306 females) with no history of upper extremity disorders or surgery between ages 10 and 70 years were tested. The study participants were stratified according to age and weight. Hand dominance and mid-range grip strength were measured. Thumb abduction and adduction were measured using the Thumb Abduction Adduction Measuring device, which consists of a slot to immobilize the palm and a pressure sensitive ring for the thumb. Force is measured through a 50-lb load cell, which is accurate to 0.1 lb. Values for abduction and adduction strength were recorded at 30º, 45º, and 60º of thumb abduction.

Males at the 45º right thumb position exhibited an average adbuction strength of 4.3±1.7 lbs and adduction strength of 17±4.5 lbs. For females, abduction was 2.3±0.9 lbs and adduction was 14.3±3.8 lbs at the 45° position. No statistical significance was noted in abduction or adduction strength between dominant and nondominant hands. At higher degrees of thumb abduction, abduction strength decreased whereas adduction strength increased. The ratio of adduction and abduction strength with age was poor for males and females. The correlation with weight was better and significant with grip strength.

The Thumb Abduction Adduction Measuring device was easy to use and required a small amount of time to complete. Studies that quantify the relative contribution of median and ulnar innervation in patients with carpal tunnel syndrome and ulnar neuropathy are underway. As this is the first device to specifically measure pure thumb abduction and adduction strength, studying the natural history of these disorders might allow determination of a strength loss threshold to guide when operative release may be indicated.