Biomechanical evaluation of total disc replacement arthroplasty: an in vitro human cadaveric model.

STUDY
DESIGN: This in vitro biomechanical study was undertaken to quantify the multidirectional intervertebral kinematics following total disc replacement arthroplasty compared to conventional stabilization techniques.
OBJECTIVE: Using an in vitro human cadaveric model, the primary objective was to compare the multidirectional flexibility properties and map the center of intervertebral rotation of total disc arthroplasty versus conventional threaded fusion cages and cages augmented with transpedicular fixation for single-level spinal instrumentation. SUMMARY OF BACKGROUND DATA: The utilization of motion-preserving implants versus instrumentation systems, which stabilize the operative segments, necessitates improved understanding of their comparative biomechanical properties.
METHODS: A total of eight human cadaveric lumbosacral spines (L2 to sacrum) were utilized in this investigation and biomechanically evaluated under the following L4-L5 reconstruction conditions: 1) intact spine; 2) SB Charitè disc prosthesis; 3) BAK cages; and 4) BAK cages + ISOLA pedicle screw/rod fixation (anteroposterior). The superior (L3-L4) and inferior (L5-S1) intervertebral levels remained uninstrumented to quantify adjacent level properties. Multidirectional flexibility included pure, unconstrained moments (+/-8 Nm) in axial rotation, flexion-extension, and lateral bending, with quantification of the operative and adjacent level range of motion and neutral zone, which were normalized to the intact spine condition.
RESULTS: The SB Charitè prosthesis indicated an average percentage increase in axial rotation range of motion by 44% compared to the intact condition (P < 0.05), whereas the BAK and anteroposterior reconstructions decreased range of motion by 29% and 80%, respectively (P < 0.05). The SB Charitè was significantly different from BAK and combined anteroposterior reconstructions (P < 0.05). Flexion-extension indicated a minor increase in range of motion for the SB Charitè (3%) versus the intact disc (P > 0.05), whereas the BAK and anteroposterior stabilization groups resulted in significant decreases in range of motion (BAK = 57%, anteroposterior = 93%) (P < 0.05) when compared to the intact and SB Charitè conditions. Based on flexion-extension radiographs, the intervertebral centers of rotation were in the posterior one-third of the operative intervertebral disc only for the SB Charitè reconstruction and intact spine condition, with definitive evidence of physiologic intervertebral translation (intact 2.06 +/- 77 mm; SB III = 1.9 +/- 0.98 mm).
CONCLUSIONS: Total disc arthroplasty serves as the next frontier in the surgical management of discogenic spinal pathology. The SB Charitè restored motion to the level of the intact segment in flexion-extension and lateral bending and increased motion in axial rotation. The anterior annular resection necessary for device implantation and unconstrained design of the prosthesis account for this change in rotation. The normal lumbar flexion-extension axis of rotation is an ellipse rather than a single point. Only disc replacement rather than pedicle instrumentation or BAK interbody instrumentation preserves the kinematic properties and normal mapping of segmental motion at the operative and adjacent intervertebral disc levels.