The memory properties of cold-worked titanium rods in scoliosis constructs

Time series monitoring changes in titanium and stainless steel rod curvature kept at a constant temperature of 37 C as a function of time.

To assess the possibility of loss of curvature in titanium rods after scoliosis surgery.

Titanium rods have gained use in scoliosis surgery due to their excellent biocompatibility, while allowing medical personnel to obtain undistorted magnetic resonance imaging scans following surgery. However, the impression of several clinicians has been that when screw pullout and/or loss of sagittal balance occurs, it may be due to the rods losing some of their curvature.

Five 6-mm rods of differing compositions and lengths (titanium 300 and 100 mm, stainless steel 300 and 100 mm, prebent titanium 85 mm) were bent at room temperature with a 3-point rod bender, then placed in an incubator at 37 C. Digital photographs were taken every 2 weeks and analyzed to extract the radius of curvature of each rod.

The Ti rods had a significantly decreasing curvature with time. The prebent Ti and stainless steel rods did not exhibit significant change in curvature.

Titanium rods bent at room temperature and then exposed to body temperature over time tend to exhibit "metal memory"; they gradually revert to their original shape. This may result in loss of sagittal balance and/or proximal screw pullout.