Determining Edge of Failure of a Spinal Wear Bearing

Mechanical test samples are often characterized using time-varying loads, either through slow monotonic changes in quasi-static testing, or through more rapid cyclic changes in dynamic testing. Spinal implant static and fatigue strengths are typically evaluated using such protocols, including such as standards ASTM F1717, ASTM F2077, and ASTM F2346. In contrast, wear tests such as ASTM F2423 and ASTM F2624 and ISO 18192-1, apply a single set of loading parameters over millions of cycles. These parameters are selected based on known, but limited, physiologic data. Wear tests however may be very sensitive to input parameters. Moreover, variations in surgical technique, patient anatomy, and pathology greatly alter the loading environment on the device. Wear tests ideally would evaluate the device under both the expected duty cycle as well as under varying test parameters.