La Barbera Luigi, Villa Tomaso
1 Laboratory of Biological Structure Mechanics, Department of Chemistry, Materials and Chemical Engineering "Giulio Natta," Politecnico di Milano, Milan, Italy.
2 IRCCS Galeazzi Orthopedic Institute, Milan, Italy.
Proc Inst Mech Eng H. 2017 Feb;231(2):176-185. doi: 10.1177/0954411916684365. Epub 2017 Jan 18.
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.
对脊柱后路稳定装置进行的机械可靠性测试是依据美国材料与试验协会的标准F1717开展的，该标准描述了如何在椎体切除模型中用类似椎骨的测试块组装植入物。最近的一项研究提议修订该标准，以描述解剖学上的最坏情况，而非目前采用的平均情况，并引入无支撑螺钉长度作为一个机械参数。本文研究了此类修订对一种已上市植入物耐久性的影响。实验疲劳测试表明，F1717标准的修订导致被测植入物的疲劳强度降低了320万次循环：这一数值与目前推荐的循环次数上限（500万次循环）相当。通过静态测试和应变片验证的数值分析支持了实验结果，并表明修订后植入物上的应力可能会增加，在螺钉上最多增加50%（最常见的失效模式），其中无支撑螺钉长度单独造成的应力增加高达40%。ASTM F1717标准的修订将确保受试植入物具有更高的安全性，可能适用于更广泛的患者群体。
