The biomechanical effects of angulated boxer's fractures

Many clinical studies have demonstrated that in the conservative care of bo xer's fractures (casting, with or without reduction), between 20 degrees an d 70 degrees of dorsal angulation is acceptable. This biomechanical study c haracterized how boxer's fracture angulation affects the ability of the int rinsic muscles to initiate grip. The flexor digiti minimi and third volar i nterosseous were modeled in this study. Muscular excursion of the intrinsic s were modeled with a cable and sheath device. Metacarpophalangeal joint fl exion per unit excursion curves were collected when the metacarpal neck was at a fracture angle of 0 degrees,15 degrees, 30 degrees, 45 degrees, 60 de grees, and 75 degrees. As fracture angles increased, the distance between t he origin and insertion of the flexor digiti minimi decreased, creating sho rtening, or slack, of the modeled muscle. Slack was defined as excursion ge nerating no joint flexion. Muscle shortening data were integrated with Jaco bsen's muscle fiber length data and Elftmann's fiber length/tension relatio nship to estimate how the fracture angle affects the initiation of metacarp ophalangeal joint flexion and the strength of grip. According to this model 's data, fracture angles of up to 30 degrees are compatible with nearly nor mal mechanics. A 30 degrees angulation is associated with a flexor digiti m inimi 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. (I Hand Surg 1999;24A: 835-844. Copyright (C) 1999 by the American Society for Surgery of the Hand.).