Toward the definition of a new worst-case paradigm for the preclinical evaluation of posterior spine stabilization devices

Abstract

Mechanical reliability tests on posterior spine stabilization devices are based on standard F1717 by the American Society for Testing and Materials, which describes how to assemble the implant with vertebrae-like test blocks in a corpectomy model. A recent study proposed to revise the standard to describe the anatomical worst-case scenario, instead of the average one currently implemented, and introduce the unsupported screw length as a mechanical parameter. This article investigates the implications of such revisions on the endurance properties of an implant already on the market. Experimental fatigue tests demonstrate that the revision of F1717 standard leads to a reduction of 3.2 million cycles in the fatigue strength of the tested implant: this amount is comparable to the run-out number of cycles (5 million cycles) currently recommended. The numerical analysis, validated with static tests and strain gauges, supports the experimental findings and demonstrates that the stress on the implant may increase upon revision up to a 50% on the screw (most recurrent failure mode), with the unsupported screw length contributing alone up to 40%. The revision of ASTM F1717 standard would guarantee higher safety for the implant to test, potentially covering for a wider population of patients.