Biomechanical effect of constraint in lumbar total disc replacement: a study with finite element analysis

Biomechanical effect of implantation of an artificial disc (AD) was investigated using the nonlinear three-dimensional finite element model of L4-L5. The SB CHARITE and the Prodisc were chosen as the representative prosthesis of unconstrained and constrained ADs (UADs and CADs) and compared with the intact human intervertebral disc.

To investigate the effect of implantation of an AD to spinal functional unit and to evaluate the difference between the unconstrained and constrained models.

Intact osteoligamentous L4-L5 finite element model was created with 1-mm computed tomography scan of a cadaveric spine and known material property of each element. Two models implanted with ADs, unconstrained or constrained model, were also developed. The implanted model predictions were compared with that of the intact. Range of motion, force on the spinal ligaments, force on the facet joint, stress on the vertebral body and vertebral endplate with flexion/extension, lateral bending, and axial rotation under 400 N compressive preload were compared among the models.

The implanted models showed increased range of motion in flexion/extension, lateral bending, and axial rotation compared with that of the intact. Under 6-Nm moment, the range of motion were 140%, 170%, and 200% of intact in the UAD model and 133%, 137%, and 138% in the CAD model to each direction of loading. The forces on each ligament were different among the models with various loading conditions. Force on the facet, stress on the vertebral body and vertebral endplate were much larger in implanted model, especially in the CAD model.

By the result of this study it is obvious that implanted segment with AD has large range of motion and suffers from increased loading to surrounding bone and ligaments. The UAD has larger range of motion but exert less loading to the implanted segment than the CAD. It seems that the mobile center of rotation of the UAD has the ability to lessen the facet contact force and stress on the vertebral body.