Effect of total disc replacement with ProDisc on intersegmental rotation of the lumbar spine.

Study Design: The mechanical behavior of the lumbar spine after insertion of a ProDisc prosthesis was studied using 3-dimensional nonlinear finite element models.
Objective: To determine how the mechanical behavior of the lumbar spine is affected by the implant position and height, as well as by removing different portions of the natural disc and resuturing the anterior longitudinal ligament (ALL).
Summary of Background Data: Little is known about how the affected and adjacent levels of the spine are influenced by the implant height and position or by the surgical procedure.
Methods: The artificial disc ProDisc was integrated in a validated 3-dimensional, nonlinear, finite element model of the lumbar spine. The model was loaded with the upper body weight and muscle forces to simulate standing, 30 degrees flexion, 15 degrees extension, and 6 degrees axial rotation. The disc position was varied by up to 2 mm in both an anterior and posterior direction. Three different disc heights were investigated, as well as the influence of removing different portions of the natural disc and resuturing the ALL.
Results: Implant position strongly influences intersegmental rotation for the loading cases of standing and flexion. A disc height 2 mm in excess of the normal disc space increases intersegmental rotation at implant level during standing and extension. The values for intersegmental rotation are closer to those for the intact spine when lateral portions of the anulus are not removed. Resuturing the ALL has a strong effect on the loading cases of extension and standing.
Conclusions: When implanting an artificial disc, great care should be taken in choosing the optimal height and correct position for the implant. Lateral portions of the anulus should be preserved whenever possible. A perfect reconstruction of the ALL would help to restore the biomechanics to normal.