[Rhizarthrosis and thenar muscles. A clinico-anatomic study]

Advanced osteoarthrosis of the first carpometacarpal joint is characterized by a subluxation (luxation) of the joint, by an adduction deformity and by hyperextension of the metacarpophalangeal joint of the thumb. This morphological and functional investigation aims to underline the importance of the intrinsic thumb muscles in this complex deformity of the thumb. The vector of the M. abductor pollicis brevis and the physiological cross-section of the thenar muscles were determined in eight osteoarthrotic and ten healthy hands. The superficial head of the M. flexor pollicis brevis and the M. abductor brevis were investigated electromyographically in patients as well as in healthy controls. There is disturbed function of the M. abductor pollicis brevis in cases of arthrosis, caused by a changed vector of the muscle. The physiological cross-section of the superficial head of the M. flexor pollicis brevis is diminished by about fifty per cent. The electromyographic records of patients lead to the conclusion that there is a particular loss of activity in the short flexor and abductor muscles. The muscular alterations taken together cause functional imbalance of the thenar muscles. The abductor muscle is not able to maintain the coordinating system that consists of two straight and one oblique muscle and the Z-deformity of the thumb results.