A biomechanical comparison of headless tapered variable pitch and AO cortical bone screws for fixation of a simulated lateral condylar fracture in equine third metacarpal bones

Objective-To compare drilling, tapping, and screw-insertion torque, force, and time for the 4.5-mm AO and 6.5-mm Acutrak plus" (AP) bone screws, and t o compare the mechanical shear strength and stiffness of a simulated comple te lateral condylar fracture of the equine third metacarpal bone (MC3) stab ilized with either an AO or AP screw. Study Design-In vitro biomechanical assessment of screw-insertion variables , and shear failure tests of a bone-screw-stabilized simulated lateral cond ylar fracture. Sample Population-Eight pairs of cadaveric equine MC3s. Methods-Metacarpi were placed in a fixture and centered on a biaxial load c ell in a materials-testing system to measure torque, compressive force, and time for drilling, tapping, and screw insertion. Standardized simulated la teral condylar fractures were stabilized by either an AO or AP screw and te sted in shear until failure. A paired t test was used to assess differences between screws, with signifcance set at P < .05. Results-insertion and mec hanical shear testing variables were comparable for AO and AP insertion equ ipment and screws. Conclusion-The 6.5-mm tapered AP screw can be inserted in equine third meta carpal condyles and is mechanically comparable with the 4.5-mm AO screw for fixation of a simulated lateral condylar fracture. Clinical Relevance-Considering the comparable mechanical behavior, the pote ntial for less-persistent soft-tissue irritation with the headless design, and the ability to achieve interfragmentary compression by inserting the sc rew in one hole drilled perpendicular to the fracture plane, the 6.5-mm tap ered AP screw may be an attractive alternative for repair of incomplete lat eral condylar fractures in horses. (C) Copyright 2001 by The American Colle ge of Veterinary Surgeons.