Pseudorotation of the Little Finger Metacarpal

INTRODUCTION

The little finger metacarpal is the most commonly injured bone in the hand (McKerrell et al., 1987). Reports of the incidence of rotational malalignment following these fractures vary from 0% to 25% (Ford et al., 1989; McKerrell et al., 1987; Royle, 1990), but most agree that rotational malalignment is an indication for reduction (Eichenholtz et al., 1961; Ford et al., 1989; Green and Butler, 1996; Holst-Nielsen, 1976), which must usually be stabilized operatively (Royle, 1990). Assessment of rotation is made by observing either the posture of the flexed finger, or the plane of the nailplate with the finger extended (Green and Butler, 1996; Royle, 1990; Stern, 1999). Stern (1999) noted the natural tendency of the little finger to override the ring finger and that a false appearance of internal malrotation may be caused by limitation of little finger metacarpophalangeal joint flexion. Following anatomical open reduction of fifth metacarpal shaft fractures, we have observed the persistent appearance of a supination deformity even with the finger fully flexed. Our hypothesis is that intermetacarpal swelling can cause internal rotation of the fully flexed little finger in the absence of a fracture. We have tested this hypothesis with an in vivo human experimental model.

SUBJECTS AND METHODS

The hypothesis tested was that soft-tissue swelling in the fourth intermetacarpal space can cause apparent rotational deformity in the absence of a fracture. Soft-tissue swelling was simulated by the infusion of normal saline into the fourth intermetacarpal space. The subjects were healthy male volunteers with no history of injury to the hand being studied. Initially, it was intended to perform the experiment on the hands of patients having a general anaesthetic for other reasons. However, as the procedure had not been performed before, it was not known to be harmless and could not be ethically conducted on patients’ normal hands. The study was thus conducted on three of the authors’ non-dominant hands.

Two milliliters of 1% lignocaine were introduced into the dorsum of the fourth intermetacarpal space at the level of the intermetacarpal ligament. Using a 19G needle, normal saline was then slowly injected into the fourth intermetacarpal space and web space. Each hand was assessed after 4 and 8 ml of fluid had been injected. Rotation of the little finger was assessed clinically and radiographically quantified. Clinical assessment involved observation with the fingers flexed and then extended. Firstly, we observed the distal phalangeal alignment with the metacarpophalangeal and proximal interphalangeal joints fully flexed and the distal interphalangeal joints extended (Fig 1). Secondly, we examined the nail plate alignment with the fingers extended (Fig 2).

Rotation was also quantified radiographically, according to the technique described by Green and Butler (1996): wire markers were aligned with the distal phalanges of each finger using fluoroscopy, prior to infusion (Fig 3). The alignment of the distal phalangeal metal markers was then assessed in the flexed position using the fluoroscope both before and after saline infusion (Fig 4). Hard copies of these images were obtained and lines were drawn along the axial markers proximally. The angle subtended by the index and little finger lines was measured before and after completion of each saline injection. Any change in this angle would represent digital rotation due to swelling. The minimum distance between the ring and little finger metacarpal heads was measured on each radiograph, and named the intermetacarpal distance (IMD). The radiographic axes of the ring and little metacarpals were marked on each radiograph, and the intermetacarpal angle (IMA) subtended by these axes was measured in each case. The intermetacarpal distance and the intermetacarpal angle were used to quantify little finger metacarpal abduction.

RESULTS

In all three subjects, the saline infusion caused internal rotation of the flexed little finger, as assessed clinically (Fig 1) and radiographically (Fig 4). However, the plane of the little fingernail plate was not altered by the injection (Fig 2). The saline infusion only caused small increases in the intermetacarpal distance and intermetacarpal angles (Table 1).

Following the injection, all the subjects experienced intermittent aching for at least 2 weeks. One of the subjects experienced cold intolerance for 1 month after the injection and had a palpable swelling in the tendon of extensor digiti minimi for 3 months. It was thus decided not to continue the experiment on the hands of anaesthetized patients.

DISCUSSION

Malrotation at a metacarpal fracture can only occur around the axis of the metacarpal, and will alter the alignment of the flexed finger, as well as the plane of the nail plate when the finger is extended. When the metacarpal is intact, passive internal rotation of the little finger is possible with the metacarpophalangeal joint fully flexed (Stern, 1993) and we suggest that the increased hydrostatic pressure in the fourth intermetacarpal space caused by the saline injection can produce this passive internal rotation. Stern contends that this passive rotation occurs at the carpometacarpal joint. The swelling we produced was not enough to cause rotation of the carpometacarpal joint with the finger extended. However, finger flexion may push the contents of the fourth intermetacarpal space dorsally, further increasing the pressure to cause carpometacarpal rotation. Abduction at the carpometacarpal joint could also account for an appearance of supination when the digit is flexed. In cadavers a mean of 13° of abduction has been found to occur at the little finger carpometacarpal joint (Batmanabane and Malathi, 1985), but we found only 1° or less of little finger metacarpal abduction occurring when significant pseudorotation was present, and suggest that an intact intermetacarpal ligament would restrict further abduction.

Our results suggest that most of the passive internal rotation of the flexed little finger occurs around an axis perpendicular to the plane of the palm. Since metacarpal abduction is minimal, the rotation must occur around the proximal phalangeal axis, which could occur only at the metacarpophalangeal joint. The collaterals of the metacarpophalangeal joint are tightest in full flexion, minimizing abduction and adduction, but some rotation is still possible (Simon et al., 1994). As we have excluded other causes of the pseudorotation through our experiment, we conclude that it occurs at the metacarpophalangeal joint, and is driven primarily by swelling in the web space.

Soft-tissue swelling in the fourth intermetacarpal space can cause an appearance of supination of the flexed little finger when the metacarpal is intact. Such an appearance should be interpreted with caution in the presence of a metacarpal fracture. True rotation at a metacarpal fracture will alter the plane of the nail when the finger is extended, and this is not caused by soft-tissue swelling.

In the setting of an acute little finger metacarpal neck fracture accompanied by swelling and an appearance of malrotation, one solution is to re-examine the patient after a week of treatment with anti-swelling measures. If the malrotation remains after the swelling has subsided, appropriate measures should then be taken.