Derivation of Clinically Relevant Boundary Conditions Suitable for Evaluation of Chronic Impingement of Lumbar Total Disk Replacement: Application to Standard Development

Currently available standardized methods for evaluating the long-term wear of total disk replacements do not incorporate the effects of potential device impingement. Creation of a standard that incorporates device impingement is difficult without a thorough understanding of the associated biomechanical environment. Arbitrary modification of the currently available wear-test protocols to account for device impingement may add unnecessary cost, and potentially inaccurate, unrealistic results. Finite element models provide the ability to control variation and test for a wide range of parameters without the excessive time and monetary costs associated with cadaveric testing or wear simulations. However, careful validation and verification of these models is required in order to ensure predictability. Retrieved implants can be used to validate the clinical predictability of finite element models (FEMs). The objective of the current study was to quantify the ability of a previously developed FEM of the lumbar spine to predict polyethylene damage modes and impingement in actual clinical scenarios, and extract the loading and boundary conditions for implementation into a new lumbar TDR wear simulation standard. In order to achieve this objective, actual clinical scenarios, associated with retrieved implants, were modeled and simulated. We hypothesized that clinical damage modes, including both impingement and non-impingement scenarios, can be predicted using a FEM that incorporates case-specific clinical factors, anterior-posterior shear forces, coupled translations, and facet contact.