Locking metacarpophalangeal joint of the thumb.

It is well known that when the metacarpophalangeal joint in the thumb is forcefully hyperextended the volar plate tears, followed by dorsal dislocation, and when forcefully deviated laterally the collateral ligament on the ulnar or radial side tears. However, reports on the locking phenomenon of the thumb seem to be very few. The only two reports that the author is aware of are those by Kobayashi (1968) in which one case was introduced, and by Inoue (1969) who described eight cases with wedging of the sesamoid bone of the metacarpophalangeal joint of the thumb. Five of the cases were reduced manually, while four required surgery. However, they reported that the mechanism involved could not be ascertained. During recent years the author has experienced seven cases, six of which required surgery, and thus has been able to acquire some knowledge as to its mechanism. As shown in Table 1 all cases were males in their twenties with the exception of one boy ten years old. The cause of injury in the majority was forced hyperextension of the metacarpophalangeal joint of the thumb sustained during sporting activity. Case 1. A twenty-seven year old male experienced pain and disturbance of flexion following hyperextension of the metacarpophalangeal joint of the right thumb sustained during volley ball practice. As X-ray examination by a local doctor failed to demonstrate dislocation, wet packs were applied. However, as the condition failed to improve, he visited our clinic one week after injury. The findings at time of initial examination are shown in Fig. 1 (a). There was slight swelling in the metacarpophalangeal joint area, and while this joint was slightly hyperextended, the interphalangeal joint was in slightly flexed position. There was slight ulnar deviation of the metacarpophalangeal joint and movement of the thumb was strongly inhibited by pain. Fig. 1 (c) shows the X-ray findings. There is hyperextension and slight dorsal subluxation of the metacarpophalangeal joint, and the sesamoid bone can be seen wedged in the space on the volar side of the joint. Surgery was performed under axillary block anaesthesia after manual reduction proved unsuccessful. First, an incision was made on the radial side of the metacarpophalangeal joint, followed by an incision between the volar plate and the accessory ligament along the radial edge of the sesamoid bone. An incision of the joint capsule produced a small flow of haemorrhagic synovial fluid. The joint cavity was opened carefully so as to study the state of the sesamoid bone, and when it was pulled in the volar direction, it became released from its locked position and smooth joint movement was restored. The state is as shown in Fig. 1 (d). It was noted that the sesamoid bone was wedged between the osteophyte projecting f rom the radiovolar side of the metacarpal head and the