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

Abstract

To assess the effect of screw thread direction on rotational resistance in canine sacroiliac (SI) luxation models using left- and right-handed screws. Controlled laboratory study. Twenty-four adult canine pelves with proximal femora were examined. 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). 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. 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. The findings suggest that selecting screw thread direction can enhance biomechanical stability in SI luxation repair, improving surgical outcomes for affected dogs.