Cervical Disc Prosthesis Polyethylene Wear Following The ISO Cervical Test

Introduction: Cervical total disc replacement (TDR) procedures are an option for treatment of cervical spinal disc degeneration. The goal of TDR replacement is to replace the painful and degenerated TDR with a new bearing unit to enable pain free motion. A young and active patient population demands a proven durable replacement bearing surface to accomplish this goal. The wear of that surface may be dictated by design considerations such as bearing surface material or designed constraints. The Prodisc-C device is an example of a semi-constrained metal-onpolyethylene (PE) cervical TDR replacement based on past highly successful total joint replacement models. PE wear debris should be well characterized during cervical TDR replacement design. Currently, cervical TDR replacement wear data exists for several design and material combinations [2]. It is important to document the in-vitro wear performance for cervical TDR replacement designs, and to verify this work through patient follow-up and finally retrieval analyses. An ISO input parameter set for simulation of cervical TDRs exists [3]. Current information on PE wear in cervical TDR replacements based on ISO inputs has not yet been published. The purpose of this investigation was to assess the in vitro wear performance of an articulating, semi-constrained, metal on polyethylene cervical TDR replacement when experiencing ISO cervical load and motion conditions. Materials and Methods: Two temperature controlled, eight station spine simulators (Endolab, Germany), designed according to ISO/DIS 18192, were mounted with 4 Prodisc-C (Synthes,USA) cervical artificial discs per simulator. The Prodisc-C articulation surface is a CoCr alloy/UHMWPE couple that represents a semi-constrained design. The implants were fixed with polyurethane, and care was taken to assure proper center of rotation alignment. Two specimens served as loaded soak controls. The ISO cervical wear test inputs a “cross-shear” motion profile. The magnitudes and frequencies of the ISO loading and motion profiles experienced by the implants on this simulator are shown in Fig 1. The cross-shear motion pattern is based upon literature values for physiologically based loads and motions in the cervical spine [3].