Stress analysis of interbody fusion--finite element modelling of intervertebral implant and vertebral body.
OBJECTIVE
To investigate stresses in cage type interbody fusion systems during compressive loading.Design. The study uses finite element methods to investigate predicted stresses. Previously published experimental material properties are used as inputs to the numerical simulation.


BACKGROUND
Interbody spinal fusion procedures using cage style intervertebral implants often cause subsidence failure of the vertebral endplate, resulting in potential pain and mechanical instability of the fusion system.


METHODS
Finite element models were developed to simulate compressive load transfer between interbody implants and adjacent vertebral body. The vertebral body was modelled using separate finite element mesh regions for cancellous core and cortical shell, with the meshes tied together at the core/shell interface. Coulomb friction was implemented to model the contact between implants and vertebral endplate.Results. Simulation results predicted endplate stresses of approximately 12 times the nominal contact pressure due to differing deformation stiffnesses of the implant and endplate structures. Reduction of the cancellous core elastic modulus to simulate severely osteoporotic bone resulted in endplate stresses up to three times higher than the values for an intact cancellous core.


CONCLUSIONS
In this study, finite element analysis was used to investigate the stresses in interbody fusion systems. Published vertebral loads corresponding to certain activities were shown to generate endplate stresses which approach and exceed the failure stress for cortical bone. Endplate stresses are strongly dependent on the modulus of the underlying cancellous core.


CONCLUSIONS
Endplate subsidence failure can potentially occur at the corners of existing cage-type interbody implants under physiological compressive loads. Matching material properties between cortical endplate and implant does not guarantee optimal contact conditions, and overall bending stiffness should be assessed.
Adam
Clayton
C
School of Mechanical, Manufacturing and Medical Engineering, Queensland University of Technology, GPO Box 2434, 2 George St., Brisbane, Q4001, Australia. c.adam@qut.edu.au
Pearcy
Mark
M
McCombe
Peter
P
D019245:Compressive Strength; D003198:Computer Simulation; D004548:Elasticity; D019544:Equipment Failure Analysis / Q000379:methods*; D020342:Finite Element Analysis; D017276:Friction; D006801:Humans; D016268:Internal Fixators; D008954:Models, Biological; D011475:Prosthesis Failure; D013123:Spinal Fusion / Q000295:instrumentation* / Q000379:methods; D013131:Spine / Q000503:physiopathology* / Q000601:surgery; D013314:Stress, Mechanical; D016474:Weight-Bearing