Biomechanical evaluation of the kinematics of the cadaver lumbar spine following disc replacement with the prodisc-L prosthesis

Study Design. Biomechanical study of the ProDisc-L in a cadaveric model under pure moment loading. Objective. To determine the kinematic properties of a lumbar spine motion segment and the adjacent level following ProDisc-L disc replacement in the cadaveric spine. Summary of Background Data. Total disc replacement is intended to preserve native motion, in an attempt to prevent accelerated adjacent segment degeneration. The quality and quantity of the motion following TDR may have important consequences on the facet joints of the same motion segment, as well as the motion at the prosthetic component interface. Methods. Ten cadaveric lumbar spines were radiographed (L3-L5) and tested under pure moments (+10 Nm to -10 Nm) with an applied follower load (200 N). Load-deformation was tested in flexion/extension, lateral bending (LB), and axial rotation (AR). Range of Motion (ROM) data were recorded. Superior adjacent disc pressure (L3-L4) was measured using subminiature pressure transducers. The L4-L5 disc was subsequently instrumented with a ProDisc-L. Radiographs and biomechanical tests were repeated. Results. Disc replacement significantly reduced extension (ROM 2.2° ± 0.5° before and 1.2° ± 0.7° after instrumentation) (P = 0.001), but not flexion (ROM 5.6° ± 3.1° before and 6.2° ± 1.2° after) (P = 0.34). Combined flexion/extension motion was marginally reduced (P = 0.517). LB ROM (7.4° ± 2.0°) was marginally reduced (P = 0.072) following instrumentation (6.2° ± 2.5°), while ROM in AR (3.4° ± 1.1°) was significantly increased (4.4° ± 1.2°) (P = 0.001). Superior adjacent segment ROM was preserved.No significant differences in disc pressure were observed at the adjacent motion segment before (199 kPa at maximum flexion and 171 kPa at maximum extension) or after disc replacement (252 kPa and 208 kPa, respectively). Conclusion. In cadaveric spines, ROM of operated and adjacent motion segments was preserved following ProDisc-L insertion. Excision of the anterior anulus may increase laxity, which is taken up by the restoration of disc height and lordosis, at the cost of a moderate loss of flexion/extension motion. Adjacent segment kinematics were unaffected following TDR.