Surface slide track mapping of implants for total disc arthroplasty

Total disc arthroplasty has recently become a potential alternative to spinal arthrodesis. Until recently, there has been no standardized method for evaluating the wear of an artificial disc and myriad testing conditions have been used. The American Society for Testing and Materials (ASTM) and International Organization of Standardization (ISO) recently published guidance documents for the wear assessment of intervertebral spinal disc prostheses; however, various kinematic profiles are suggested, leading to different wear paths between the articulating surfaces of the implants. Since the wear between materials is influenced by the type of relative motion, it is important to select test conditions that lead to clinically realistic results. The purpose of this study was to characterize the slide tracks generated by 7 test conditions allowed for by the ISO and ASTM guidance documents and in Euler sequences consistent with 4 commercially available spine wear simulators. The analysis was performed for a ball-in-socket articulation under both lumbar and cervical motion test conditions. Results were generated analytically using a mathematical algorithm and then validated experimentally. Four tests resulted in elliptical sliding tracks of similar geometries for both the lumbar and cervical conditions. Curvilinear and ribbon-shaped wear paths were generated for 3 tests. With the data normalized for implant diameter, the sliding distance was similar between the lumbar and cervical conditions allowed for in the ASTM guidance. This distance differed compared with the results for the ISO guidance document where the lengths of cervical slide tracks were twice those for the lumbar conditions. Slide tracks were also found to be insensitive to the type of simulator under all testing conditions.