Three-dimensional finite element analysis of translaminar lag screw designs for L5 bilateral spondylolysis

This finite element study aimed to quantitatively compare the biomechanical properties of translaminar lag screws with different diameters and structural designs for the surgical repair of L5 bilateral spondylolysis, to provide evidence for individualized screw selection.

An intact model of L4-S1 segment (Model A) and a L5 bilateral spondylolysis model (Model B) were established using Mimics, Geomagic, and SolidWorks. Additional models were constructed incorporating translaminar lag screws of varying diameters: 4.5 mm (solid, Model C1; cannulated, Model C2), 4.0 mm (solid, Model D1; cannulated, Model D2), and 3.5 mm (solid, Model E1; cannulated, Model E2). Following model validation, a 400 N axial load and a 10 N m moment were applied to simulate five conditions. Analyzed parameters included range of motion (ROM), maximum displacement, and maximum von Mises stress in the disc, pars interarticularis, and fixation screws.

Compared to the intact model (A), the spondylolysis model (B) showed significant instability. All fixation models (C1-E2) effectively restored ROM to intact levels, regardless of screw diameter or structural design. Larger-diameter (4.5 mm) and solid designs demonstrated slightly better control of model displacement. Increasing diameter significantly reduced stress concentrations within the pars interarticularis and fixation screws. For a given diameter, cannulated screws sustained higher stress than solid screws. The 3.5 mm cannulated screw exhibited the highest screw stress, approaching the yield strength of titanium alloy under rotational loading.

All translaminar lag screw designs can effectively restore stability in L5 spondylolysis. While different diameters and structural designs have a minor impact on stability restoration, they significantly affect implant stress. In this specific patient-derived model, the 4.5 mm solid screw demonstrated the most favorable biomechanical profile, with the lowest stress on the implant and effective control of graft displacement. These findings suggest a potential mechanical advantage for 4.5 mm solid screws; however, confirmation through multi-specimen studies or clinical trials is required before broader clinical recommendations can be made.