Effect of thread direction on rotational stability in lag-screw fixation of sacroiliac luxation: An ex vivo cadaveric study in small-breed dogs.

Objective: To assess the effect of screw thread direction on rotational resistance in canine sacroiliac (SI) luxation models using left- and right-handed screws.

Study design: Controlled laboratory study.

Sample population: Twenty-four adult canine pelves with proximal femora were examined.

Methods: Four groups (n = 6 each) were established: right-handed screw/right SI luxation (RhRSI), right-handed screw/left SI luxation (RhLSI), left-handed screw/left SI luxation (LhLSI), and left-handed screw/right SI luxation (LhRSI). Under fluoroscopy, 2.4 mm cortical screws were placed into the SI joint in a lag fashion. An acute failure test measured force and torque at yield and peak points, with the ilium and femur positioned at a 108° angle and displacement at 0.099 cm/s. Torque (N cm) was calculated from force (N) and the moment arm (cm).

Results: Differences in median torque were found at yield and peak points. RhRSI gave 50.08 N cm versus 16.01 N cm for RhLSI (p < .01), and LhLSI showed 39.42 N cm versus 19.93 N cm for LhRSI (p < .03). At peak, RhRSI recorded 67.55 N cm compared to 28.14 N cm for RhLSI (p < .01), and LhLSI reached 51.79 N cm versus 28.28 N cm for LhRSI (p < .05). All samples failed by rotation without screw breakage or fractures.

Conclusion: Right-handed screws provided greater rotational resistance in right-sided luxation, and left-handed screws in left-sided luxation, which demonstrated that screw thread direction influenced fixation stability in SI luxation.

Clinical significance: The findings suggest that selecting screw thread direction can enhance biomechanical stability in SI luxation repair, improving surgical outcomes for affected dogs.