COMPARISON OF BIOMECHANCIAL PERFORMANCE FOR THREE ARTIFICIAL DISCS IN LUMBAR SPINE

Abstract

Even though spinal fusion has been used as one of the common surgical techniques for degenerative lumbar pathologies, high stiffness in the fusion segment could generate clinical complications in the adjacent spinal segment. To avoid these limitations of fusion, the artificial discs have recently used to preserve the motion of the treated segment in lumbar spine surgery. However, there have been lacks of biomechanical information of the artificial discs to explain current clinical controversies such as long-term results of implant wear and excessive facet contact forces. In this study, we investigated the biomechanical performance for three artificial discs in the lumbar spinal segments by finite element analysis.

A three-dimensional finite element model of five spinal motion segments, from L1 to S, in intact lumbar spine was reconstructed from CT images. Finite element models of three artificial discs, semi-constrained and metal on polyethylene core type (ProDisc^® II, Spine Solutions Inc., USA; Type I), semi-constrained and metal on metal type (MaverickTM, Medtronic Sofamor Danek Inc., USA; Type II), and un-constrained and metal on polyethylene core type (SB ChariteTM III, Dupuy Spine Inc., Switzerland; Type III) were developed. Each artificial disc was inserted at L4–L5 segment, respectively. Upper and lower plates of artificial discs were attached on the L4 and L5 vertebrae. Some parts of ligaments and intervertebral disc in L4–L5 motion segment were removed to insert artificial discs. Nonlinear contact conditions were applied on facet joints in lumbar spine model and artificial discs. Bottom of sacrum was fixed on the ground and 5Nm of flexion and extension moments were applied on the superior plate of L1 with 400N of compressive load along follower load direction.

In extension, all three artificial disc models showed higher rotation ratio at the surgical levels, but lower rotations at the adjacent levels than those in the intact model. There was no big difference of the intersegmental rotations among the artificial disc models. For the comparison of the peak von-Mises stresses on the polyethylene core in flexion, 52.3 MPa in type I implant was higher than 20.1 MPa in Type III implant while the peak von-Mises stresses were similar, 25.3 MPa and 26.5 MPa in Type I and III, respectively in extension. The facet contact forces at the surgical level for the artificial disc models showed 140 to 160 N in extension whereas the facet contact force in the intact model was 60 N.

From the results of this study, we could investigate the biomechanical characteristics of three different artificial disc models. The relative rotation at the surgical level would be increases at the early outcome after total disk replacement. The semi-constrained type artificial disc could generate higher wear risk of the implant than unconstrained type. Also all types of artificial disc model have higher risk of facet joint arthrosis, and especially in the semi-constrained and metal on metal type. The results of the present study suggested that more careful care must be taken to choose surgical technique of total disc replacement surgery.