IN VITRO MECHANICAL EVALUATION OF SPINAL FIXATION ROD CONNECTORS

ABSTRACT Objective Evaluate and compare the mechanical resistance and the fatigue behavior associated with the use of three different modalities of vertebral fixation system rod connectors through in vitro pre-clinical mechanical tests developed specifically for this application (linear, lateral with square connector and lateral with oblique connector). Methods Cobalt chromium rods 5.5 mm in diameter were used and coupled with three types of connectors: a) side rod with oblique connector, b) side rod with square connector, and c) rod and linear connectors. Quasi-static mechanical four-point bending and fatigue tests were performed. The variables measured were (I) the bending moment at the yield limit, (II) the displacement at the yield limit, (III) the rigidity of the system in flexion and (IV) the number of cycles until system failure. Results The linear system presented the greatest force and the greatest moment at the yield limit, as well as the greatest stiffness equivalent to bending. All specimens with square and oblique connectors endured 2.5 million cycles in the minimum and maximum conditions of applied moment. The specimens with linear connector endured 2.5 million cycles with fractions of 40.14% of the bending moment at the yield limit, but failed with levels of 60.17% and 80.27%. Conclusions Systems with linear connectors showed greater mechanical resistance when compared to systems with square and oblique connectors. All systems supported cyclic loads that mimic in vivo demands. Level of evidence V; In vitro research.

摘要

研究目的：评估并对比三种不同脊柱固定系统杆连接器的机械强度与疲劳性能，采用专为该应用开发的体外临床前力学测试方案，测试类型涵盖直连式、方形侧向连接器式与斜向侧向连接器式。

研究方法：使用直径5.5mm的钴铬合金杆，搭配三类连接器：a) 斜向侧向连接器杆；b) 方形侧向连接器杆；c) 直连式杆与连接器。开展准静态四点弯曲力学测试与疲劳测试，记录的测试变量包括：(I) 屈服极限弯矩、(II) 屈服极限位移、(III) 系统弯曲刚度，以及(IV) 系统失效前的循环加载次数。

研究结果：直连式系统在屈服极限处的载荷与弯矩均为最高，同时弯曲等效刚度也最大。所有搭载方形与斜向连接器的试件，在所施加弯矩的最小与最大工况下，均完成了250万次循环加载。搭载直连式连接器的试件，在屈服极限弯矩的40.14%载荷下可完成250万次循环，但在60.17%与80.27%该弯矩载荷下发生失效。

研究结论：相较于搭载方形与斜向连接器的系统，直连式连接器系统具备更优异的机械强度。所有测试系统均可承受模拟体内生理需求的循环载荷。证据等级为V级；属于体外研究。