Basic scientific considerations in total disc arthroplasty.

BACKGROUND CONTEXT: Total disc arthroplasty serves as the next frontier in the surgical management of intervertebral discogenic pathology.
PURPOSE: As we move from an era of interbody spinal arthrodesis to one in which segmental motion is preserved, this promising new technology offers increasing clinical and research challenges in the areas of spinal kinematics, histologic osseointegration at the prosthetic-bone interface and the effects of particulate wear debris. STUDY
DESIGN: The primary focus of this paper is to provide a methodologic basis to investigate the spinal kinematics, histologic osseointegration and particulate wear debris after total disc arthroplasty by using in vitro and in vivo models.
METHODS: Part I: Using an in vitro cadaveric model, multidirectional flexibility testing evaluated the functional unit kinematics under the following L4-L5 reconstruction conditions: 1) intact spine, 2) Charite disc prosthesis, 3) BAK cages, 4) BAK cages+ISOLA pedicle screw or rod fixation (anteroposterior). Part II: A total of 27 mature baboons (n=27, Papio cynocephalus) underwent L5-L6 total disk replacement procedures to investigate the biomechanical, histochemical and biologic ingrowth characteristics of two different lumbar disc prostheses (AcroFlex and Charite) for total disc arthroplasty. Functional spinal unit fusion status was assessed by using radiographic analysis, biomechanical testing, undecalcified histopathologic and histomorphometric analyses. Part III: Using a total of 50 New Zealand white rabbits, this investigation served to quantify the neural and systemic tissue histopathologic response, after epidural application of four different types of spinal instrumentation particulate wear debris: 1) sham (control) (n=10), 2) stainless steel 316LVM (n=10), 3) titanium alloy Ti-6AL-4V (n=10), 4) cobalt chrome alloy (n=10) and 5) ultrahigh molecular weight polyethylene (UHMWPE) (n=10).
RESULTS: In vitro multidirectional flexibility testing demonstrates the operative and adjacent level motion-preserving properties of total disc arthroplasty versus interbody arthrodesis cages and pedicle screw spinal instrumentation. To this end, disc replacement preserves the normal centrode or locus of intervertebral rotation at the operative and adjacent intervertebral spinal levels compared with conventional stabilization implants. On the basis of nonhuman primate modeling in the current studies, porous titanium interface surfaces afforded the greatest percentage of trabecular ingrowth at the prosthesis-end plate interface. In vivo segmental motion under multidirectional testing was preserved with the Charite device and slightly diminished with the AcroFlex implants. The porous ingrowth coverage at the bone-metal interface was more favorable for total disk replacement (range, 40% to 50%) compared with that reported for cementless total joint components in the appendicular skeleton (range, 10% to 30%). Direct epidural application of spinal instrumentation particulate wear debris elicits a chronic histiocytic reaction localized primarily within the epidural fibrous layers. Moreover, particles have the capacity to diffuse intrathecally, eliciting a macrophage and cytokine response within the epidural tissues, cerebrospinal fluid and spinal cord itself. Overall, on the basis of the postoperative time periods evaluated, no evidence was observed of an acute neural or systemic histopathologic response to the materials included in the current project.
CONCLUSION: The implementation of dynamic spinal stabilization systems for fusionless correction of spinal deformity, dynamic posterior stabilization and total disc arthroplasty necessitates improved understanding with regard to spinal kinematics, patterns and mechanisms of histologic osseointegration and the neurohistopathologic response to particulate wear debris. Collectively, the current studies provide a methodologic basis to comprehensively evaluate these three areas.