Muscle moment arms in the first dorsal extensor compartment after radial malunion. A cadaver study.

BACKGROUND Functional loss is a common complication of the fractured distal part of the radius. The purpose of the present study was to determine if the moment arms of the first dorsal extensor compartment are altered by distal radial fracture malunion. We hypothesized that the moment arms of the abductor pollicis longus and extensor pollicis brevis are significantly affected by dorsal angulation, radial inclination, and radial shortening, the most common deformities accompanying distal radial malunion. METHODS Moment arms of the extensor pollicis brevis and abductor pollicis longus were estimated in twelve cadaver wrists with use of the tendon-displacement method, which involves calculating the moment arm as the derivative of tendon displacement with respect to joint angle. Tendon displacement was quantified in different wrist postures before and after a closing-wedge osteotomy simulating a complex malunion of an extra-articular radial fracture. RESULTS The simulated distal radial malunion resulted in a decrease in the wrist flexion moment arm for both the extensor pollicis brevis (p = 0.0003) and the abductor pollicis longus (p < 0.0001). The wrist flexion moment arms for the extensor pollicis brevis and abductor pollicis longus decreased by a mean (and standard deviation) of 114% +/- 75% and 77% +/- 50%, respectively, after the osteotomy. The wrist radial deviation moment arms for the extensor pollicis brevis and abductor pollicis longus increased by 16% +/- 26% (p = 0.071) and 28% +/- 44% (p = 0.043), respectively, after the osteotomy. Radiographs of the wrist that were made before and after the osteotomy indicated that radial tilt changed from 11.1 degrees of volar angulation to 14.8 degrees of dorsal angulation, radial inclination decreased from 21.8 degrees to 7.7 degrees, and radial height decreased from 11.6 to 4.4 mm. CONCLUSIONS Distal radial malunion alters the mechanical advantage of the muscles in the first dorsal extensor compartment.