Biomechanical study of different kinds of Pedicle lag screw in posterior fixation for the fracture of the ring of axis： a finite-element analysis

English Keywords:Fracture of the ring of axis  Pedicle lag screw  Finite element

English Abstract:

  【Abstract】 Objectives: To evaluate the biomechanical stability of posterior fixation of fracture of the ring of axis(FRA) with semi-threaded and full-threaded pedicle lag screw using finite element analysis, and to provide theoretical reference for clinical selection of pedicle lag screw. Methods: A 28-year-old healthy male volunteer was scanned by 256-slice spiral CT from C0 to C3 with a slice thickness of 0.625mm. Using Mimics 20.0, 3-matic 13.0 and Ansys 16.0, a three-dimensional hexahedral mesh finite element model(FE/Intact) of C0-3 segments of normal cervical spine was established and validated. Three models of axis ring fracture (FE/Fracture A, B and C) were simulated and validated by weakening the strength of mesh element on the validated model. A semi-threaded screw and a full-threaded screw internal fixation model were established, and used to fix the FE/Fracture A, B, and C fracture models respectively. After boundary restraint, four kinds of physiological loads were applied: forward flexion, backward extension, lateral flexion and axial rotation. The three-dimensional angular range of motion(ROM) and the displacement of fracture end nodes under different working conditions were compared among these models. Results: The ROM of FE/Fracture A, B and C models increased significantly compared with the FE/Intact model under the same loading conditions. After the FRA was fixed by the semi-threaded screw, the C2-3 ROM of the FE/Fracture A model in the flexion, extension, lateral flexion and axial rotation were 2.9°, 2.2°, 2.0°, and 1.8°, which were 37.18%, 37.93%, 41.67%, and 50.00% prior to fixation; As for the full-thread screws, C2-3 ROM were 3.0°, 2.3°, 2.1°, and 1.9°, which were 38.46%, 39.66%, 43.75%, and 52.78% of the ROM prior to fixation. After the FRA was fixed by the semi-threaded screw, the C2-3 ROM of the FE/Fracture B model in the flexion, extension, lateral flexion and axial rotation were 2.9°, 2.2°, 2.0°, and 1.8°, which were 37.18%, 37.93%, 41.67%, and 50.00% prior to fixation; the full-thread screws C2-3 ROM were 3.0°, 2.3°, 2.1°, and 1.9°, being 38.46%, 39.66%, 43.75%, and 52.78% prior to fixation. The C2-3 ROM was 3.2°, 2.4°, 2.3°, 2.5° in each working condition of the model after the FE/Fracture B was fixed by semi-threaded screw. Compared with 8.1°, 6.2°, 5.0°, and 4.0° before fixation, they were respectively 39.51%, 38.71%, 46.00%, and 62.50%; the full-thread screws C2-3 ROM were 3.3°, 2.4°, 2.5° and 2.6°, being 40.74%, 40.32%, 48.00%, and 65.00% prior to fixation. The C2-3 ROM was 3.6°, 3.0°, 2.8°, 2.5° in each working condition of the model after in each working condition of the model after the FE/Fracture C was fixed by semi-threaded screw. Compared with 8.9°, 6.3°, 5.7°, and 4.6° before fixation, they were respectively 40.45%, 47.62%, 49.12%, and 54.35%; the full-thread screws C2-3 ROM were 3.7°, 3.3°, 2.9° and 2.7°, being 41.57%, 52.38%, 50.88%, and 58.70% prior to fixation. In the same fracture model, Wilcoxon′s signed-rank test was used to analyze and compare the node displacement after two kinds of screws were fixed. The difference between the two groups was statistically significant(P<0.05). The semi-threaded pedicle lag screw internal fixation system was superior to the full-screw screw in fixing the fracture of FRA and limiting the displacement of the fracture end node. Conclusions: Semi-threaded pedicle lag screw fixation for FRA provides greater biomechanical stability than full-thread screw. It can meet the normal physiological activities of the C2-3 segment and achieve the purpose of stabilizing the fracture end to reconstruct the stability of the upper cervical vertebra.