BEYOND 'HARD BONE': QUANTITATIVE METHODOLOGIES FOR OBJECTIVE BONE MODEL SELECTION IN ORTHOPEDIC DEVICE VERIFICATION TESTING

Orthopedic device verification testing assumes that the mechanical properties of the tissues used in bench testing are representative of the in vivo mechanical loading the device will encounter during use. Descriptors of bone quality (soft, normal, hard) to be used in testing have been employed for more than 30 years without a standardized quantitative description and with a wide variation between laboratories and manufacturers. This article presents the scientific basis for this gap and a quantitative protocol for choosing an appropriate bone model according to IMI. A Medtronic/Active Life Scientific and University of Washington research collaboration develops a cross-model hardness profile referenced to clinically established sclerotic bone using the OsteoProbe device that measures resistance to penetration quantified as the Bone Material Strength index (BMSi) using a cross-model matrix of synthetic polyurethane foams, animal bone and fresh cadaveric human tissue. Results showed that commercially relevant surrogates labeled 'hard' span a large, inconsistent region of BMSi space and that worst-case model selection was quantitative, not based on inference from surrogate labels. Given the context of regulatory verification and validation used to characterize safety-critical orthopedic devices, this approach could be adopted as an industry standard. Adoption of this approach would materially improve the scientific rigor and clinical relevance of bench test evidence submitted in support of device market authorizations