Palmar Dislocation of the Metacarpophalangeal Joint of the Finger

CASE REPORT

A 52-year-old woman suffered an injury to her left hand when she was dragged with great force by a dog while holding the dog leash in her left hand. Dorsal dislocations of the proximal interphalangeal joints of the middle and little fingers, and a palmar dislocation of the metacarpophalangeal joint of the ring finger were diagnosed (Fig 1). The proximal interphalangeal joints were successfully reduced under local anaesthesia, but reduction of the metacarpophalangeal joint failed. She was referred to us, 6 days later and an open reduction was attempted. Through a dorsal incision, the extensor hood was incised radial to the extensor tendon. The base of the proximal phalanx was found palmarly dislocated, and the flexor tendon lay radial to the joint (Fig 2). Both of the collateral ligaments were torn, and the dorsal capsule was avulsed from the dorsal aspect of the base of the proximal phalanx. There was a small chondral defect on the dorsoulnar aspect of the metacarpal head. The metacarpophalangeal joint could be reduced manually, but it was unstable, and readily redislocated palmarly. A palmar incision was then made, revealing a rupture of the flexor tendon sheath and avulsion of the palmar plate from the base of the proximal phalanx (Fig 3). The base of the proximal phalanx protruded on the ulnar side of the flexor tendon into the subcutaneous layer. The radial collateral ligament was mainly avulsed from its distal, and the ulnar collateral ligament from its proximal, attachment. Both collateral ligaments were repaired with suture anchors (Micro Anchor, Mitek, Norwood, MA), stabilizing the joint (Fig 4). The extensor hood was repaired and the finger was immobilized with the metacarpophalangeal joint in 30° flexion in a dorsal splint for 2 weeks. Active range of motion exercises were then started. Eight months after surgery the metacarpophalangeal joint flexed to 70° and the extension was only limited by 10°. The patient experienced no pain or disability in any of her daily activities.

DISCUSSION

Palmar dislocation of the metacarpophalangeal joint of a finger is a rare injury. Since the first description by McLaughlin (1965), only 15 cases have been reported in English language literature (Table 1), and its pathogenesis remains uncertain (Hargarten and Hanel, 1992; Lam et al., 2000). Wood and Dobyns (1981) reported three cases, and reproduced the injury in five of ten cadaver specimens by applying hyper flexion and a proximal translational force to the proximal phalanx. They concluded that avulsion of the dorsal capsule from the metacarpal neck and its interposition into the metacarpophalangeal joint as a result of a hyper flexion injury were the essential pathologic processes in the injury. Renshaw and Louis (1973), however, stated that the mechanism of injury is forceful hyperextension of the metacarpophalangeal joint, followed by distal attachment avulsion of the palmar plate which is subsequently entrapped in the palmarly dislocated metacarpophalangeal joint. However, they could not reproduce either disruption of the palmar plate or the palmar dislocation in any of 16 cadaver specimens. Betz et al. (1982) suggested that a combination of active flexion and forceful hyperextension was important in producing a palmar dislocation, and stated that the lack of an active flexion component explained why it had not been possible to produce palmar dislocations in cadavers. Rotatory force was thought to be responsible for the ruptures of the collateral ligaments that occurred in four of the 15 previously reported cases (Betz et al., 1982; Mlsna et al., 1993; Moneim, 1983; Wood and Dobyns, 1981).

From the history obtained from our patient, the main cause of the injury in our case was probably a hyperextension force applied during strong active flexion of the digit. The chondral defect at the dorsoulnar aspect of the metacarpal head and the rupture of the palmar plate also suggest that a hyperextension injury was the mechanism of dislocation. The rupture of the collateral ligaments may indicate that forceful rotation occurred during the dislocation.

Reduction in our case was not difficult, but the joint easily dislocated palmarly until the collateral ligaments were repaired.

The rupture of the flexor tendon sheath as well as the ruptures of the palmar plate and the collateral ligaments may explain the palmar instability. If the tendon sheath is not intact, the flexor tendons run more palmarly than normal when the metacarpophalangeal joint is flexed and this makes their moment arm greater than that of the extensors. In an unstable metacarpophalangeal joint which has no support from the collateral ligaments, the palmar plate or the joint capsule, this imbalance of the flexor–extensor mechanism may make the joint unstable in the palmar direction. This is why maintenance of the reduced position was difficult in our case.

In cases where the metacarpophalangeal joint is stable after closed reduction, non- operative treatment yields the favourable functional results (Boland, 1984; Khuri and Fay, 1986; Takami et al., 1999). However, if the instability persists after closed reduction or reduction is impossible, one should not hesitate to carry out an open reduction. Wood and Dobyns (1981) reported a case whose metacarpophalangeal joint redislocated two times after closed reduction, and underwent an arthrodesis 9 months after the initial injury. The results of operative treatment more than 3 months after the injury are generally poor (Qui, 1992; Wood and Dobyns, 1981).

At surgery, if the capsule or the palmar plate are interposed in the metacarpophalangeal joint they should be extracted and the collateral ligaments should be repaired if they are torn. The suture anchor system was useful in our case for repair of the collateral ligaments.