Biomechanical influence of the surgical approaches, implant length and density in stabilizing ankylosing spondylitis cervical spine fracture

Abstract

Ankylosing spondylitis cervical spine fractures (ASCFs) are particularly unstable and need special consideration when selecting appropriate internal fixation technology. However, there is a lack of related biomechanical studies. This study aimed to investigate the biomechanical influence of the pattern, length, and density of instrumentation for the treatment of ASCF. Posterior, anterior, and various combined fixation approaches were constructed using the finite element model (FEM) to mimic the surgical treatment of ASCFs at C5/6. The rate of motion change (RMC) at the fractured level and the internal stress distribution (ISD) were observed. The results showed that longer segments of fixation and combined fixation approaches provided better stability and lowered the maximal stress. The RMC decreased more significantly when the length increased from 1 to 3 levels (302% decrease under flexion, 134% decrease under extension) than from 3 to 5 levels (22% decrease under flexion, 23% decrease under extension). Longer fixation seems to be more stable with the anterior/posterior approach alone, but 3-level posterior fixation may be the most cost-effective option. It is recommended to perform surgery with combined approaches, which provide the best stability. Long skipped-screwing posterior fixation is an alternative technique for use in ASCF patients.

Figure 1

Finite element model used for the simulations in this study. Color coding was used to distinguish the different parts of the model. Ligaments (ALL/PLL/CL) (green) and the outermost layer of the annulus are defined as sites of ossification to simulate the end stage of AS in the cervical spine.

Figure 2

AS cervical fracture model used in this study. The whole CL was removed, and 0.5 mm of the ALL and PLL was removed. (A) Fracture of CL. (B) Fracture of the outer layer of the annulus, ALL and PLL.

Figure 3

Lateral view of the thirteen implant configurations and schematic diagram of the implants.

Figure 4

Sagittal RoM change at the fractured segment (C5/6) in different implant configurations.

Figure 5

ISD of anterior screws. (A,B) Oblique view of von Mises stress distribution of the screws in A1 under flexion and A2 under extension. (C) MS of each anterior screw in different implant configurations.

Figure 6

ISD of anterior plates. (A,B) Oblique view of the von Mises stress distribution of the plates in A1 and A2 under flexion. (C,D) Oblique view of the von Mises stress distribution of the plates in A1 and A2 under extension. (E) MS of plates in different implant configurations.

Figure 7

ISD of posterior screws. (A,B) Oblique view of von Mises stress distribution of the screws in P1 and P2 under flexion. (C,D) Oblique view of the von Mises stress distribution of the screws in P3 and P3 skip under extension. (E) MS of screws in different implant configurations.

Figure 8

ISD of posterior rods. (A,B) Oblique view of the von Mises stress distribution of the rods in P1 and P2 under flexion. (C,D) Oblique view of the von Mises stress distribution of the rods in P3 and P3 skip under extension. (E) MS of rods in different implant configurations.