Objective: To determine the influence of interlocking screw threads on the biomechanical properties of repaired canine humeral condylar fractures. Study Design: Ex vivo biomechanical study. Sample Population: Thirty-six humeral condyles. Methods: Simulated fractures of the lateral aspect of the humeral condyle were stabilized by a 3.5 mm interlocking thread screw (ITS) or 3.5 mm buttress thread screw placed in lag (BTS-L) or positional fashion (BTS-P) and axially loaded at a walk, trot, 2-mm displacement, and failure cycles. Compact flute drill bits (CFBs) were used for ITS constructs and standard flute drill bits (SFB) for BTS constructs. The effects of bit type on drilling parameters and screw type on screw insertion properties and fragment stability were assessed. Results: CFB produced a 6°C greater temperature increase (p =.042) and required 20 N higher torque (p =.003) than SFB. Insertional torque was greater for ITS than BTS-P (p =.001) and BTS-L (p =.001). Condylar fragment rotation at failure was lower in ITS (lsmean ± SE, 8.3° ± 1.9°) than BTS-L constructs (14.5° ± 2.3°, p =.011). ITS resisted greater loads (1503 ± 105 N) than BTS-P (1189 ± 99 N, p =.038) but not BTS-L (1249 ± 123 N, p =.121) constructs. Conclusion: Biomechanical performance of constructs was improved with ITS rather than BTS fixation. Clinical Significance: ITS can be considered for stabilization of humeral condylar fractures in adult dogs.
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