VOLAR DISLOCATION OF THE FIFTH CARPOMETACARPAL JOINT.

Editor,

A 25-year-old right-handed housewife presented with severe left hand pain resulting from a fall from standing height. Tenderness and swelling was present over the ulnar side of the injured hand and the little finger appeared foreshortened. No neurological deficit was noted. Radiographs of the injured hand demonstrated a volar–ulnar dislocation of the 5th metacarpal base (Figure 1 panels a and b). Under general anaesthesia, closed reduction and percutaneous wire fixation restored congruence and stability to the dislocated 5th carpometacarpal (CMC) joint (Figure 2). Six weeks post-surgery the wires were removed and hand therapy initiated. Clinically, the patients left 5th CMC joint was stable and radiographs demonstrated joint congruency. The patient regained full function of her injured hand within 6 months.

Fig 1.

(panels A&B): Posteroanterior radiograph (a) demonstrating dislocation of the 5th CMC joint with loss of convergence of the metacarpal cascade lines (4 white lines – only 3 converge); true lateral radiograph (b) demonstrating anterior displacement of the 5th metacarpal base (white arrow).

Fig 2.

Intra-operative screening image demonstrating reduction and wire fixation of the 5th CMC joint with restoration of convergence of the metacarpal cascade lines (4 converging white lines).

CMC joint dislocations most commonly involve the 5th CMC joint and are usually dorsal. Isolated volar dislocation of the 5th CMC joint is a rare injury with sporadic cases reported in the literature. The injury is thought to result from a direct blow transmitted to the dorso-ulnar aspect of the 5th metacarpal base resulting in disruption of the supporting peri-articular soft tissues. The deep motor branch of the ulnar nerve lies volar to the 5th CMC joint as it courses around the hook of the hamate and is vulnerable to injury in volar dislocations. A careful neurological assessment of the injured hand is therefore essential.

CMC joint dislocations can be easily missed and failure to diagnose this injury may predispose the patient to pain and weakness of grip.1 Careful radiographic evaluation is paramount. Postero-anterior (PA), oblique and true lateral views of the injured hand should be obtained. Loss of convergence of the metacarpal cascade lines on the PA view is a key radiographic sign (see Figure 1a). The intermetacarpal angle, i.e. the angle between best-fit lines drawn down the medullary canals of the 2nd, 3rd and 5th metacarpals, is normal in volar dislocations and should not be used in isolation to exclude these injuries. If clinically suspicious, additional views should be obtained with the forearm rotated to identify any displacement of the 5th metacarpal base obscured by superimposition of the central metacarpals.

Closed reduction of the dislocated 5th CMC joint and cast immobilisation is an option, however due to the degree of soft tissue disruption the injured joint is often unstable, as in our case, and temporary percutaneous wire fixation is required to restore joint stability and facilitate soft tissue healing.[1,2,5] Open reduction may be required where there is soft tissue interposition preventing closed reduction of the dislocated joint or in cases of delayed diagnosis.

In summary, isolated volar dislocation of the 5th CMC joint is a rare injury. Careful analysis of the injury radiographs and further views of the injured hand can reduce the risk of a missed dislocation. Reduction and temporary wire stabilisation of the injured joint is recommended.