Postural changes in thenar paralysis and their significance

A comparison of postures in 145 thumbs having paralysis of some, all, or none of the intrinsic muscles showed that the carpometacarpal joint was more hyperextended when only the extrinsic muscles were present. When the opponens and short flexor were also working hyperextension was less apparent. The metacarpophalangeal joint was flexed when only the extrinsic muscles were working and in less flexion when the opponens was also present. The joint was stable in flexion when the extrinsic muscles, opponens and short flexor were all present. The interphalangeal joint was flexed when only the extrinsic muscles were working, in less flexion when the opponens was also present, and stable in extension when the intrinsic muscles, opponens, and flexor pollicis brevis were all present. This study shows that the function of the opponens and short flexor muscles are both necessary to obtain the optimal balance of forces for thumb function.