Thermomechanical Effects of Spine Surgery Rods Composed of Different Metals and Alloys

Study Design. A basic science study monitoring changes in the curvature of hand contoured commercially pure titanium (CPTi), titanium-aluminum-vanadium alloy (Ti-6Al-4V), and stainless steel (SS) rods maintained at different temperature conditions. Objective. To quantify changes in rod-shape at temperatures representative of those used in clinical practice. Summary of Background Data. The shape of implanted rods can be displaced due to thermo-mechanical properties of the materials. Warmer temperatures likely initiate this effect. A study of shape loss characteristics of various rod implants may help eliminate undesirable outcomes caused by shape displacement. Methods. Three different types of rods (CPTi, SS, and Ti-6Al-4V) were hand contoured and then maintained in one of following temperature conditions for 35 days: (1) room temperature (20 degrees C-25 degrees C) without autoclaving before contouring; (2) preliminary autoclaving (1, 5, 10, 20 cycles) at 135.0 degrees C +/- 2 degrees C before contouring followed by body temperature (37.2 degrees C +/- 2 degrees C). Each rod was 5 mm in diameter and 200 mm long. The rods were mounted over graph paper in fixed positions and photographed to measure displacement of the tip as a function of the curvature. Results. Statistically significant shape loss of the rods manufactured from all the tested materials was found. The hand contoured CPTi rods displayed considerably higher loss of curvature over time than Ti-6Al-4V and SS rods at all tested temperature conditions. Preliminary autoclaving at 135 degrees C before contouring tended to amplify this effect, in particular 1 cycle of autoclaving. If the number of preliminary autoclaving cycles was higher (5-10), a tendency of decrease of shape loss effect was observed in Ti-6Al-4V and CPTi rods. Conclusion. The shape of the hand contoured CPTi rods was the least stable of the rods across all applied temperature conditions. The SS and Ti-6Al-4V rods were more stable than CPTi rods. Autoclaving before handcontouring tended to increase rods' shape loss.