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

Noshchenko, Andriy PhD*; Patel, Vikas V. MA, MD*; Baldini, Todd MS; Yun, Lu MD; Lindley, Emily M. PhD*; Burger, Evalina L. MD*

doi: 10.1097/BRS.0b013e3181e6ce16
Biomechanics

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°C–25°C) without autoclaving before contouring; (2) preliminary autoclaving (1, 5, 10, 20 cycles) at 135.0°C ± 2°C before contouring followed by body temperature (37.2°C ± 2°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°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.

Hand contoured rods manufactured from commercially pure titanium used in spine surgery lose their curvature when sterilized and then maintained at human body temperature. The shapes of rods manufactured from titanium alloy (Ti-6Al-4V), and stainless steel were more stable. The curvature changes are likely due to thermomechanical properties.

*The Spine Center at the University of Colorado, Denver, CO

Bioengineering Division, Department of Orthopaedics, University of Colorado, Denver, CO.

Address correspondence and reprint requests to Evalina Burger, MD, Department of Orthopaedics, The Spine Center, University of Colorado Denver, 12631 E 17th Ave., Mail Stop B202, Aurora, CO 80045; E-mail: Evalina.burger@ucdenver.edu

Acknowledgment date: August 26, 2009. First revision date: February 23, 2010. Second revision date: April 6, 2010. Acceptance date: March 12, 2010.

The device(s)/drug(s) are FDA-approved or approved by corresponding national agency for this indication.

No funds were received in support of this work. No benefits in any form have been or will be received from a commercial party related directly or indirectly to the subject of this manuscript.

© 2011 Lippincott Williams & Wilkins, Inc.