Role of the finger flexors in rheumatoid deformities of the metacarpophalangeal joints

It is proposed that rheumatoid deformity of the central three metacarpophalangeal joints is caused primarily by the flexor tendons acting on diseased joint restraints. During pinch and grasp the tendons bend volarly and ulnarly at the tunnel mouth; the resultant pulley forces damage the supporting collateral ligaments, especially on the radial side. The flexor tendons and proximal phalanges can then displace volarly and ulnarly, and the fingers deviate ulnarly. In the fifth digit, ulnar deviation apparently results from the incompletely opposed pull of the abductor digiti quinti.