THE EFFECT OF FRACTURE MALUNION AT THE MIDSHAFT 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 extrinsi c flexion and extension muscle forces. Design. An experimental study o n human cadavers. Background. Malunion of the metacarpal mid-shaft fra ctures are common sequelae and can result in angular deformities of th e shaft, bone shortening, or as in most cases, a combination of both i n 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. Experiment s were performed on the second metacarpal in eight fresh normal cadave r hands. The extrinsic tendon forces to obtain full flexion and extens ion 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 meta carpal (0-60 degrees) were simulated. The tendon forces were then meas ured again as a percentage of the control, given the same excursion. R esults. The extension force was found to increase with increasing dors al angulation and decrease with increasing bone shortening, While the flexion force decreased with increasing dorsal angulation and shorteni ng. Dorsal angulation had a greater effect than bone shortening on the extension and flexion forces. Conclusion. Simulated malunion metacarp al fractures in human cadavers, has an effect on the efficiency of the measured extensor and flexor forces. In extension, dol sal angulation was found to increase the force required for the same amount of excur sion. However, with added bone shortening, the extension force require d decreased. This could be a compensatory mechanism in malunion metaca rpal fractures. In flexion, a lower force was required with bone short ening and dorsal angulation for the same excursion. (C) 1998 Elsevier Science Ltd. All rights reserved.