BIOMECHANICAL COMPARISON OF THE HERBERT AND AO CORTICAL BONE SCREWS FOR COMPRESSION OF AN EQUINE 3RD CARPAL BONE DORSAL PLANE SLAB OSTEOTOMY

Objective-To assess feasibility of insertion of 4.5-mm Herbert cannula ted bone screws (HS) using fluoroscopic guidance and compare the mecha nical shear strength of these HS and 4.5-mm AO cortical bone screws (A O) for fixation of dorsal plane slab osteotomies in equine cadaver thi rd carpal bones (C3). Animals or Sample Population-Eight equine cadave rs. Methods-Bone mineral composition and density of contralateral C3 w ere confirmed to be equivalent using dual-energy x-ray absorptiometry. A standard 10-mm C3 slab osteotomy was reduced using HS or AO instrum entation under fluoroscopic guidance. Specimens were loaded in shear u ntil failure, using a materials testing apparatus. Results-HS and AO i nstrumentation allowed accurate reconstruction of the osteotomy, but t here was difficulty encountered seating the HS proximal self-tapping t hreads. There was no significant difference in maximal load to failure , stiffness, or mode of failure of constructs created with the HS and AO screws. Conclusions-Use of 4.5-mm HS for repair of C3 radial facet, dorsal plane slab fractures may result in a mechanically comparable f ixation to a repair using a 4.5-mm AO. Equine dorsal C3 may be too den se, however, to allow placement of the proximal self-tapping threads o f the HS without potentially excessive application of torque to the sc rew itself. Clinical Relevance-Dorsal plane, radial facet slab fractur es of the equine C3 are a significant clinical problem. Accurate recon struction and stabilization are necessary for return to athletic funct ion. (C) Copyright 1998 by The American College of Veterinary Surgeons .