• C Rahm, K Ito, and J Auer.
• Veterinary Surgery Clinic, University of Zürich, Switzerland.
• Vet Surg. 2000 Nov 1; 29 (6): 564-71.

ObjectiveTo compare acute fixation stability and insertion effort of cortex bone screws with and without a shaft inserted in lag fashion in equine metacarpal (metatarsal, MC(T)III) bone.MethodsScrew types with independent variables of screw diameter (4.5 or 5.5 mm) and shaft type (without shaft, with 20-mm shaft, or with 25-mm shaft) were studied. Bone specimens cut from distal equine MC(T)III condyles were used. After screw insertion in lag fashion into 2 bone blocks with an instrumented device, shear tests were conducted in a mechanical testing machine. Outcome variables of peak insertion torque, insertion energy, stiffness. yield strength, and displacement at 3 kN of load were compared.ResultsThe effects of screw design were substantial. Screws with shaft were 30% to 40% stiffer and 60% to 70% stronger than screws without shaft. Screws with shaft could tolerate 80 to 95 kg more force than screws without shaft before yielding. At 3 kN load, the displacement with screws with shaft was 55% to 60% of that with screws without shaft. Screws with a long shaft tended to perform better than those with a short shaft. There was no difference in the shear stiffness, shear yield strength, or shear displacement between the 2 screw diameters. Although larger diameter screws required more insertion effort, and screws with a short shaft required the most insertion energy, these differences were small.ConclusionsCortex screws with a long shaft of 4.5- or 5.5-mm diameter provide better stability in equine MC(T)III condyle bone with less insertion effort compared with those with a short shaft or no shaft.Clinical SignificanceCortex bone screws with a shaft inserted in lag fashion should be considered for the fixation of equine MC(T)III condylar fractures.

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