C2 pedicle lag screws with different thread designs for the fixation of axis ring fractures: a finite-element analysis

English Keywords:Axis ring fracture  C2 pedicle lag screw  Finite element  Double thread  Single thread

English Abstract:

  【Abstract】 Objectives: To compare the biomechanical characteristics of C2 pedicle lag screws of single-thread, double-thread, and full-thread in the fixation of axis ring fractures, and to provide reference for clinical selection of different types of pedicle lag screws. Methods: The thin-section CT scan data of the C0-C3 segments of a healthy 28-year-old male adult volunteer were obtained to establish the original model of 3D hexahedral mesh finite element of normal C0-C3 segment. According to the previous classification of C2 pedicle fracture of axis ring fracture, three C2 pedicle fracture models of A, B and C were established on the original model by weakening the strength of mesh unit. The C2-3 segment was simulatedly fixed with single-threaded, double-threaded and full-threaded screw models, respectively. A 50N vertically downward pressure was applied to the occipital condyle of the model to simulate the head gravity, and a 1.5N·m torque was applied to the model to load the four physiological loads of flexion, extension, lateral flexion and rotation. The displacement and stress distribution of the fracture end nodes of each fixed model under different working conditions were compared. Results: In fracture model A under all motion conditions, the maximum displacement of the node path of single-threaded screw was smaller than that of the double-threaded screw and full-threaded screw; In model B, single-threaded screw and double-threaded screw were smaller than full-threaded screw in the maximum displacement of node path under all motion conditions, while single-threaded screw was smaller than double-threaded screw in the maximum displacement of node path under flexion and extion, and greater than double-threaded screw under lateral bending and rotation conditions; In model C, the maximum displacement of the single-threaded screw in each direction of the node path was all greater than that of the double-threaded screw and the full-threaded screw. In terms of extreme stress value, in fracture model A, single-threaded screw was the smallest, full-threaded screw was greater under flexion and extension conditions and smaller under lateral bending and rotation conditions than double-threaded screw; in model B, single-threaded screw was the smallest except the rotation condition; in model C, single-threaded screw was the smallest and full-threaded screw was the largest. The stress of the single-threaded screw was mainly concentrated near the fracture end and the contact area between the nail cap and the surface of vertebral body, and the extreme stress was at the screw end of the fracture end; The stress of double-threaded screw and full-threaded screw was mainly concentrated in the head end thread and tail end thread, the stress of the contact area between the nail cap and the posterior surface of vertebral body was small, and the extreme stress value was at the junction of the tail end thread and the cortical bone. Conclusions: The biomechanical effect of single-threaded screw fixation of the posterior wall of vertebral body and the side of superior articular process of type B fracture and type C fracture is better. Double-threaded screw fixation of type A fracture and the pedicle fracture side of type B fracture is better in biomechanical effect; The biomechanical properties of full-threaded screw are not as good as single-threaded and double-threaded screws in fixing all fracture types.