Spine rod straightening as a possible cause for revision

In a previous study, the authors examined the elastic and short-term anelastic springback of Ti6Al4V, CoCrMoC and A316L stainless steel spine rods to observe how the rods mechanically respond in OR contouring. In that study rods were 200 mm long and only the movement at the tip was recorded. The implication of that work was that rods will straighten in-vivo, however, in order for the mechanism of straightening to be determined, the movement of individual bends over time must first be elucidated. Spine rods used were, commercially pure titanium (CP Ti) a primarily α-phase; Ti-6Al-4V; α/β-phase titanium alloy from two different suppliers (denoted by, Ti-6Al-4V (L) and Ti-6Al-4V); β-phase titanium (TNTZ) and CoCrMoC. Following contouring the rods were aged unconstrained, in normal atmosphere or simulated body fluid (SBF) in a CO₂ incubator for up to 288 h. Elastic springback is significantly different between alloys with different microstructures. Both types of Ti6Al4V rods, while meeting the ASTM F136 industry standard, have significantly different properties, most importantly yield strength, flexural modulus, and springback. Environment showed no significant impact on anelasticity. The anelastic response of Ti6Al4V L sample, which has relatively more beta phase than the Ti6Al4V sample, follows the pure beta phase TNTZ in its extended time response. CoCrMoC and CP Ti have a very reduced anelastic response compared to the other alloys. This potentially can have unanticipated effects on the outcome of spine procedures, as the surgeon is reliant on the rods having similar properties to achieve a desired outcome.