Human abductor pollicis brevis muscle "divisions" and the nerve hila.

The interpretation of neuromuscular impairment may be aided by an analysis of functional anatomic factors. The anatomy of the abductor pollicis brevis (APB), a muscle used for skilled movements, was investigated in 19 embalmed hands, and the muscular divisions joining the common tendon at different sites were reproduced on clear vinyl sheets. The median nerve pathway to thenar muscles was followed to where it divides to the APB nerve and to the sites of the main terminal hila. Transverse (x) and longitudinal (y) muscle axes were established with the aid of landmarks to reproduce the nerve pathway on the skin surface. In the rather thick APB, three groups of six muscular heterogeneous divisions were regularly present. The dorsal aponeurotic expansion of the thumb receives the first group. The second group forms a continuous vertical line from the base to the body of the first phalanx. The outside site of its lateral tubercle takes the central tendon from the penniform third group. Hence, a reverse figure-seven distal insertion can be observed. The deepest medial (V2) and the most superficial lateral (V3) divisions had the highest mean diameters. The APB nerve fell between V2 and V2' and its line of projection supplied a guideline to establish an x axis at the proximal one-third of the muscle. The nerve hila plotted in relation to the x-y axes revealed a mode of location. The classical description of a thin APB muscle, made up of two bellies of parallel fibers seems incorrect. The APB nerve is not found on the deep aspect of the muscle as stated earlier, but within the muscle. The well-innervated muscular divisions point to the possibility of their individual use. The APB neuromuscular projection to the skin should allow more accurate fundamental EMG studies of the thumb and therefore provide a basis for more effective treatment in cases of impaired APB.