Aplasia of the thenar eminence: a partial and distal form of radial ray dysplasia. 6 cases

Six cases of thenar muscle aplasia are reported. Aplasia of the abductor pollicis brevis and opponens pollicis muscles was noted in all cases. The flexor pollicis brevis muscle was inconstantly involved and the abductor pollicis muscle was normal. The aplasia was unilateral in five cases and bilateral in one. Bone dysplasia of the scaphoid, first metacarpal and thumb was associated with muscle aplasia. Arterial dysplasia was also observed in four cases. In all cases electrophysiological examination showed that the thenar branch of the median nerve was not involved. Electromyography showed no sign of denervation of the thenar muscles. Motor and sensory conduction velocities of the hand nerves were normal. Thenar aplasia is considered a partial and distal form of the radial ray defect caused by a focal embryonic necrosis before the differentiation stage.