Verification of ideal screw size, trajectory, and shape for single and double endplate penetrating screw trajectories using osteoporotic vertebral body models based on the finite element method.

Abstract

Study design: This is a finite element study.

Purpose: To identify optimal trajectory, screw size, and screw shape using the finite element method.

Overview of literature: Patients with diffuse idiopathic skeletal hyperostosis often develop spinal instability after fractures due to ankylosis and bone fragility. We developed single or double endplate penetrating screw trajectory (SEPST/DEPST) to improve fixation strength by penetrating the vertebral endplate. However, the optimal screw length, diameter, and shape remain unclear.

Methods: Finite element models of T12 and L1 were constructed from computed tomography images of osteoporotic patients. Three analyses were conducted: (1) the impact of various screw diameters with DEPST, (2) a comparison of fixation strength between short DEPST (S-DEPST), which penetrates the posterolateral endplate, and conventional DEPST (C-DEPST), and (3) a comparison between conventional cancellous thread screws (CTS) and endplate screws (ETS). Pullout strength (POS) was measured in all analyses. Vertebral motion angle (VMA) of the lower instrumented vertebra (LIV) was measured in analyses (2) and (3), and the four-directional load test (4DLT) was performed in analysis (2).

Results: Larger screw diameters with DEPST correlated with elevated POS. S-DEPST demonstrated significantly better fixation strength with a POS 1.46 times higher than C-DEPST and 2.5 times higher than traditional trajectories. S-DEPST also demonstrated higher fixation in all directions in 4DLT. However, no significant difference was observed in the VMA of LIV. ETS demonstrated slightly higher fixation than CTS, but the difference was not statistically significant.

Conclusions: Fixation strength improved with larger screw diameters in DEPST. S-DEPST provided additional fixation due to rim penetration. ETS may offer a higher fixation strength and warrants further validation.