BASIC SCIENTIFIC CONSIDERATIONS IN TOTAL DISC ARTHROPLASTY

Abstract

Introduction Total disc arthroplasty serves as the next frontier in the surgical management of intervertebral discogenic pathology. 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. The primary focus of this paper is to provide a methodological basis to investigate the spinal kinematics, histologic osseointegration and particulate wear debris following total disc replacement arthroplasty 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) Charitè Disc Prosthesis; 3) BAK Cages; 4) BAK Cages + ISOLA pedicle screw/rod fixation (anteroposterior). Part II: A total of twenty-seven 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 replacement arthroplasty. Functional spinal unit fusion status was assessed using radiographic analysis, biomechanical testing, undecalcified histopathologic and histomorphometric analyses. Part III: Using a total of Fifty New Zealand White rabbits, the current study served to quantify the neural and systemic tissue histopathological response, following 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 multi-directional 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 to conventional stabilization implants. Based on non-human primate modeling in the current studies, porous titanium interface surfaces afforded the greatest percentage of trabecular in-growth at the prosthesis-end-plate interface. In-vivo segmental motion under multi-directional testing was preserved with the Charité 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 to 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 / cytokine response within the epidural tissues, cerebrospinal fluid and spinal cord itself. Overall, based on the post-operative time periods evaluated, there was no evidence of an acute neural or systemic histopathologic response to the materials included in the current project.

Conclusions The implementation of dynamic spinal stabilization systems for fusionless correction of spinal deformity, dynamic posterior stabilization and total disc replacement arthroplasty necessitates improved understanding with regard to spinal kinematics, patterns / mechanisms of histologic osseointegration and the neurohistopathologic response to particulate wear debris. Collectively, the current studies provide a methodological basis to comprehensively evaluate these three areas.