The function of the abductor pollicis longus muscle as a joint stabiliser.

The abductor pollicis longus and extensor carpi ulnaris muscles function as a dynamic collateral ligament of the wrist. There are indications of a comparable situation over the carpo-metacarpal joint of the thumb. From experiments with tendon movements, we found that in passive motions of the thumb only the tendons of the superficial division show an excursion, regardless of the position of the wrist joint. However, in motions of the wrist joint the tendons show an identical excursion over an equal distance. These are small in pronation and supination but much larger in the mid-position between them. On some movements of the thumb and the wrist, no tendon excursions can be measured. There are strong indications that the deep division, particularly, has a stabilising function on the basal joint of the thumb.