Inappropriate intrinsic muscle action in the rheumatoid hand.

The classic appearance of the rheumatoid hand with swollen and often dislocated metacarpophalangeal (MCP) joints, atrophic intrinsic muscle compartments, ulnar deviating fingers, and variously hyperextended or flexed and swollen interphalangeal joints is easily recognized. The explanation of the mechanisms involved in the pathogenesis of the intrinsic muscle atrophy (exclusive of the atrophy of the intrinsic muscles of the median nerve innervated thenar group in the carpal tunnel syndrome) has not been generally accepted. This, no doubt, has been in part attributable to the paradox of the apparent increased intrinsic muscle activity (intrinsic plus of Bunnell) seen in the rheumatoid hand associated with obvious gross interosseous atrophy and weakness of hand function (Bunnell, Doherty, and Curtis, 1948; Brewerton, 1957; Flatt, 1959). Boyes (1964) called attention to overactivity or 'spasm' of the intrinsic muscles. Bunnell showed that, as a consequence of this overactivity or intrinsic 'plus' activity occurring in spastic patients (pillroller hand) and as a result of electrical stimulation, the finger when maximally extended at the MCP joint would be unable to fully flex at the proximal interphalangeal (PIP) joint. This restriction of motion was attributable to traction of the 'spastic' or tight intrinsics on their attachments to the extensor hood (Bunnell, 1953). In the presence of abnormally mobile or diseased finger joints, this intrinsic 'plus' action, resulting from prolonged contraction or ischaemic or posttraumatic contracture, could lead not only to subluxation of the MCP joint but to hyperextension of the PIP joint as well (Boyes, 1964; Harris and Riordan, 1954; Flatt, 1968). Thus the stage is set for the well-known swan-neck deformity (Fig. 1). That this deformity is not exclusively found in rheumatoid arthritis is well illustrated in one of Bunnell's cases of an ischaemiainduced intrinsic 'plus' deformity resulting from a a Volkman's contracture (Flatt, 1959, 1968). Since Bunnell's identification of the intrinsic 'plus' deformity there has arisen some difficulty in the .:~