Evaluation on the effectiveness of standards for preclinical mechanical characterization of spinal fixators

Introduction: Posterior spinal fixators are subjected to many load cycles after implantation due to walking, and failure events are continuously reported. To avoid this issue and evaluate preclinically the mechanical reliability of fixators a vertebrectomy (ASTM F1717 standard) and a physiological anterior support (ISO 12189) models are available. The aim of the study is to assess the international standards for the preclinical evaluation of posterior spinal fixators and to propose improvements. Materials and methods: Several anatomical/biomechanical parameters useful to describe the anatomy of the functional spine units were considered. Their value depending on the spinal level was obtained from literature or from direct measurements on biplanar stereoradiographies. Numerical models describing experimental setups were used to study the contribution of each parameter on the stress on the implant. The worst case condition was also determined. Results: Vertebrectomy condition may guarantee high safety of the implant once implanted in an average patient from a physiological population. The worst-case combination of parameters demonstrates higher loads than those reached using the current standard (screw: +15 %; rod: +9 % at L1). The physiological condition may not be safe enough: despite the anterior support characteristics lay within the literature range of values, it may lead to a stress increase even beyond 350 %. Discussion and conclusions: The study investigates the influence of biomechanical parameters on the stress on the fixator. Standards revision according to the anatomical worst-case condition (L1 level) would guarantee a higher safety for a greater range of patient population. Ongoing experimental testing partially corroborates numerical results.