Biomechanical study between two varieties of axial screw fixation techniques: Analysed by finite elements

Background: The atlantoaxial vertebral model was established in order to compare the biomechanical properties of C2 pedicle and translaminar screws from the perspective of the screws themselves.

Methods: A finite element model of the screw-vertebrae was developed. The screw load-displacement ratios were analysed under up/down and left/right load conditions; the vertebral load-displacement ratios under flexion/posterior extension (FLX/EXT), left/right lateral bending (LLB/RLB), and left/right rotation (LAR/RAR) load conditions; the bone-screw interface stress values and screw load-displacement ratios under physiological load conditions; and the structural stress values of the screw-rod structure under front/back and left/right load conditions.

Results: The C2 pedicle screw group (C2PG) exhibited significantly greater load-displacement ratios than did the C2 translaminar screw group (C2TG). The vertebral load-displacement ratios were significantly greater in the C2PG than in the C2TG. Under physiological loading, the maximum stress of the cortical bone in the C2TG exceeded the threshold significantly, and the anteriormost part of the vertebrae exhibited much greater displacement in the C2TG than in the C2PG. In screw-rod stability studies, C2TG exhibited greater peak stress in the screw-rod structure under anterior-posterior loading.

Conclusions: This study is the first to analyse the biomechanical properties of two types of axial screws from the perspective of the screws themselves. In this study, C2 pedicle screws exhibited greater biomechanical stability from the perspective of the two screws themselves. From the perspective of a single screw-vertebrae model, this may account for the higher postoperative revision rate of axial translaminar screws than pedicle screws in atlantoaxial internal fixation.