The effect of fracture malunion at the mid-shaft of the metacarpal on the extrinsic muscle forces

Objective. To investigate the combined effect of dorsal angulation and shortening in metacarpal mid-shaft fracture malunions on the extrinsic flexion and extension muscle forces. Design. An experimental study on human cadavers. Background. Malunion of the metacarpal mid-shaft fractures are common sequelae and can result in angular deformities of the shaft, bone shortening, or as in most cases, a combination of both in several variations. The degree of the loss of function or efficiency depends on the extent of the malunion. This would also have a bearing as to whether surgical intervention is necessary. Methods. Experiments were performed on the second metacarpal in eight fresh normal cadaver hands. The extrinsic tendon forces to obtain full flexion and extension of the digit were measured at a fixed tendon excursion. The intact metacarpal was used as the experimental control. An oblique osteotomy was done on the mid-shaft of the metacarpal and combinations of bone shortening (0–5 mm) and dorsal angulation at the mid-shaft of the metacarpal (0–60 °) were simulated. The tendon forces were then measured again as a percentage of the control, given the same excursion. Results. The extension force was found to increase with increasing dorsal angulation and decrease with increasing bone shortening. While the flexion force decreased with increasing dorsal angulation and shortening. Dorsal angulation had a greater effect than bone shortening on the extension and flexion forces. Conclusion. Simulated malunion metacarpal fractures in human cadavers, has an effect on the efficiency of the measured extensor and flexor forces. In extension, dorsal angulation was found to increase the force required for the same amount of excursion. However, with added bone shortening, the extension force required decreased. This could be a compensatory mechanism in malunion metacarpal fractures. In flexion, a lower force was required with bone shortening and dorsal angulation for the same excursion.