The biomechanical effects of angulated boxer's fractures.

Many clinical studies have demonstrated that in the conservative care of boxer's fractures (casting, with or without reduction), between 20 degrees and 70 degrees of dorsal angulation is acceptable. This biomechanical study characterized how boxer's fracture angulation affects the ability of the intrinsic muscles to initiate grip. The flexor digiti minimi and third volar interosseous were modeled in this study. Muscular excursion of the intrinsics were modeled with a cable and sheath device. Metacarpophalangeal joint flexion per unit excursion curves were collected when the metacarpal neck was at a fracture angle of 0 degree, 15 degrees, 30 degrees, 45 degrees, 60 degrees, and 75 degrees . As fracture angles increased, the distance between the origin and insertion of the flexor digiti minimi decreased, creating shortening, or slack, of the modeled muscle. Slack was defined as excursion generating no joint flexion. Muscle shortening data were integrated with Jacobsen's muscle fiber length data and Elftmann's fiber length/tension relationship to estimate how the fracture angle affects the initiation of metacarpophalangeal joint flexion and the strength of grip. According to this model's data, fracture angles of up to 30 degrees are compatible with nearly normal mechanics. A 30 degrees angulation is associated with a flexor digiti minimi grip strength of 92% maximum and preserves 78% of the intact finger's range of motion. We therefore conclude that 30 degrees is the upper limit for acceptable final angulation.