Cervical Artificial Disc Wear: The Influence of Surgical Placement
Relative motion at interacting implant surfaces generates wear debris over time leading to periprosthetic osteolysis and device failure. Factors related to implant design, patient habitus and surgical approach will impact the generation of wear debris and influence the clinical longevity of artificial spinal disc devices. Further, surgeons play an important role in selecting the appropriate implant size as well as its placement within the disc space to optimize soft tissue balance and alignment. Current wear testing standards for artificial discs do not account for the influence of anatomic structures or variations in disc placement. Dooris et al. suggested that anterior placement of the device led to increased facet joint loads in compression and extension. These findings suggest that if the implant is placed posteriorly within the disc, the spinal stiffness will be restored and facet loads will be maintained at pre-implantation levels. This handout describes the influence of neutral, anterior and posterior disc positioning on surface stresses and polymeric wear volumes using a finite element model and further seeks whether corroboration with clinical wear patterns exists.
Clinical Retrieval and Simulator Comparison of an Investigational Cervical Disc Replacement: An A Priori Requirement
The evolvement of cervical and lumbar disc replacement designs as alternatives to spinal fusion has resulted in a significant number of ongoing United States Food and Drug Administration (USFDA)-sponsored clinical trials. While these seek to establish “safety and effectiveness”, they are of limited in vivo duration and benefit from long-term benchtop comparison. To assure implant durability, mechanical and biological evaluation of these devices, particularly long-term fatigue behavior, is necessary. Both the ASTM and ISO have provided guidance in the evaluation of the performance of artificial spinal discs. These guides propose the biochemical environment, motions, and loading appropriate to simulate long-term use of prostheses employed in total disc arthroplasty. The parameters evaluated include wear measured by gravimetric weight loss, as well as, changes in the articular surface shape and roughness to the extent that these may influence function. This handout describes the wear characteristics of an articulating cervical disc replacement during approximately 80 years of simulated loading in an electro-mechanical multi-axial spinal disc simulator with comparison to clinical retrievals. These results are useful in demonstrating the safety and effectiveness of this device and also present a pre-clinical evaluation methodology for future disc replacement designs.
Evaluation of Total Disc Replacements with a Novel Multi-Axis Spine Simulator: An A Priori Requirement
The evolvement of cervical and lumbar disc replacement designs as alternatives to spinal fusion has resulted in a significant number of ongoing FDA-sponsored clinical trials. While these seek to establish “safety and effectiveness”, they are of limited in vivo duration. To assure implant durability, mechanical and biological evaluation/testing of these devices, particularly long-term fatigue characteristics, is necessary. The ASTM has expressed an interest in the performance of artificial spinal discs in Standard Guide for the Functional, Kinematic, and Wear Assessment of Total Disc Prostheses (F 2423-05). This guidance proposed the biochemical environment, motions, and loading appropriate to simulate long-term use of prostheses employed in total disc arthroplasty. The parameters evaluated include wear measured by gravimetric weight loss, as well as changes in the articular surface shape and roughness to the extent that these may influence function. This handout describes a novel, electro-mechanical, multi-axial spinal disc simulator whose operation supplements the above and establishes a long-term fatigue and particulate characteristics for both articulating and non-articulating (elastomeric) spinal implant devices. It enables performance comparison between devices, and fulfills the pre-clinical evaluations of every FDA-sponsored clinical device trial.
The Current State of Cervical and Lumbar Spinal Disc Arthroplasty
The growth of spinal implant and orthobiologic technologies over the last several years is increasing in tempo and fast approaching the US hip and knee markets in annual dollar sales. During this time, a number of start-up and established medical device manufacturers have focused increasing resources on solutions for spinal problems. The role of the orthopaedic and neurosurgeon in these enterprises as inventor, owner, and user has contributed to this march of progress. This handout describes a small (<1%), but increasingly visible, aspect of these advancing technologies, that of artificial disc replacement.