Arthroscopic reduction and percutaneous fixation of fifth carpometacarpal fracture dislocations

Rationale

Arthroscopy of the first carpometacarpal (CMC) joint has become routine. The literature contains multiple reports of arthroscopic-guided reduction and percutaneous pin fixation of Bennett fractures involving the first CMC joint. The same techniques can be applied to fracture dislocations involving the fifth CMC joint. This is one situation where arthroscopy is especially beneficial because the articular fracture fragment is often volar and difficult to visualize and reduce from a dorsal approach.

Anatomy and pathomechanics

Nakamura et al. studied 80 cadaver arms and described the CMC joint detail. Two distinct dorsal ligaments were identified that attached to the dorsal aspect of the fifth metacarpal (MC). One of these extended from the ulnar base of the fifth MC to the hamate (fifth MC ulnar side base to hamate ligament) and the other from the radial base of the fifth metacarpal to the hamate and sometimes to the fourth metacarpal ulnar base (fourth MC ulnar side base to fifth MC radial side base ligament). An intermetacarpal ligament attached the radial base of the fifth metacarpal to the ulnar base of the fourth metacarpal. One volar ligament attached to the fifth MC base and extended either to the hook of the hamate or to the ulnar base of the fourth MC. There were no intraarticular ligaments except for one ligament that was located between the third and fourth MC and the capitate/hamate.

Dzwierzynski et al. also studied the intermetacarpal ligament anatomy. They noted that the alignment of the interosseous ligaments between the fourth and fifth metacarpals differed from the ligament alignment between the second and third and third and fourth metacarpals, which allows a greater degree of motion in the fifth CMC joint (approximately 25 degrees of flexion/extension) compared with the fourth CMC joint (approximately 15 degrees of flexion/extension). They also observed that when these metacarpals flex at the CMC joints, as in grasping, the dorsal interosseous ligament tightens and the anterior interosseous ligament relaxes. When the metacarpals extend at the CMC joints, the anterior ligament tightens and the posterior ligament relaxes, which retains a rigid interconnection between the bones.

An axial load to the fourth and fifth metacarpal heads secondary to a clenched fist blow is often cited as the most common mechanism of injury of a fracture dislocation of the fifth CM joint. In one clinical study, the authors postulated that flexion during impact results in a dorsal dislocation of the small finger MC base, dorsal CMC ligament disruption, and oftentimes a hamate dorsal rim fracture. Yoshida et al. attempted to reproduce the mechanism of injury in a cadaver study by dropping an 8 kg weight from various heights onto the fourth and fifth metacarpal heads in a specially designed jig. The hand was placed in the clenched fist position with the ring CMC joint in 20 degrees of flexion, the small CMC joint in 30 degrees of flexion, and the wrist in 20 degrees of extension. A dorsal hamate fracture occurred in 45% of the specimens, whereas a fracture of the volar aspect of the ring and small finger MC base was present in 40% and 20% of the specimens, respectively. The small metacarpal volar-based fracture fragment remained attached to the ring MC ulnar-side base–small MC radial-side base ligament.