Influence of the geometry of a ball-and-socket intervertebral prosthesis at the cervical spine: a finite element study.

STUDY
DESIGN: A conceptual study of the influence of the geometry of an articulated disc prosthesis at the cervical level using a 3-dimensional nonlinear finite element model.
OBJECTIVE: To investigate how the mechanical behavior of the functional spinal unit is affected by the position of the center and the size of the radius of a ball-and-socket design at the cervical level. SUMMARY OF BACKGROUND DATA: Little is known about the changes in kinematics and internal efforts after intervertebral disc replacement at the cervical spine, specifically regarding the influence of the geometry of the articulated prosthesis.
METHODS: A ball-and-socket artificial disc was integrated in a validated 3-dimensional nonlinear finite element model of the cervical spine (posterior geometric center, large radius). The model was loaded in flexion, extension, lateral bending, and axial torsion. Two variant designs were investigated: anterior center and small radius. The intervertebral range of motion, the mean center of rotation, and the contact forces in the facet joints and in the bearing surface of the prosthesis were investigated.
RESULTS: The range of motion was similar with all prostheses. The posterior geometric center was associated with an adequate mean center of rotation in flexion/extension. The large radius of curvature was associated with the partial unloading of the facet joints and the redistribution of the constraints at the ball-and-socket interface.
CONCLUSION: Our data estimate the influence of the geometrical parameters of a ball-and-socket total disc replacement on kinematics and constraints at the cervical functional spinal unit. Under the experimental conditions, the facet forces were kept below their normal range in the case of the posterior center and a large radius.