Biomechanical Comparison of a Headless Compression Screw Fastener and AO Cortical Bone Screw for Fixation of a Simulated Equine Third Carpal Bone Slab Fracture.

Frontal plane slab fractures account for the majority of third carpal bone (C3) fractures in racing and performance horses. Recommended treatment is stabilization with a lagged AO cortical screw. Associated complications are fragment splitting, fragment spinning, and irritation of dorsal soft tissue structures. A novel, headless, cannulated screw with interlocking threads the Headless Compression Screw Fastener (HCSF) has been developed to resist multidirectional forces and bending moments; however, it has not been applied in the horse. Simulated C3 frontal plane slab fractures were created in nine paired carpi from equine cadaver limbs, fixed with either the HCSF or AO cortical bone screw, and loaded in shear to failure. The effect of screw type on stiffness, maximum load to failure, and yield load was assessed in separate linear mixed models. No significant (P< .05) difference between screw types was detected in terms of maximum load to failure (P= .084), stiffness (P= .26), or yield load (P= .088). Mode of failure was screw bending in all specimens. For some samples in both groups, failure was associated with the sagittal fracture at the screw-bone interface. The HCSF was successfully used to repair simulated third carpal bone fractures. The different head and thread pitches of the HCSF effectively compressed the fracture. The headless design eliminates the need for counter sinking. There was no significant difference in maximum load to failure, stiffness, nor yield load compared to the cortical screws. These results invite clinical application to be investigated.