Titanium versus stainless steel for anterior spinal fusions: an analysis of rod stress as a predictor of rod breakage during physiologic loading in a bovine model.

Study Design: In vitro biomechanical evaluation of rod stress during physiologic loading of anterior scoliosis instrumentation.

Objectives: To determine effects of material properties and rod diameter on rod stresses in anterior scoliosis instrumentation.

Summary Of Background Data: Relationships between instrumentation dimensions, materials, and potential rod failure in anterior scoliosis instrumentation remain unclear.

Methods: Eighteen immature bovine spines were randomized to 3 groups: 1) 4.0-mm stainless steel, 2) 5.0-mm stainless steel, and 3) 4.75-mm titanium alloy. Spines underwent physiologic tests in flexion-extension, lateral bending, and torsion. Rod surface strains were converted to rod stress and normalized to each material's yield stress. Construct stiffness and the normalized rod stresses were compared with a one-way ANOVA (P < 0.05).

Results: The 4.0-mm steel and 4.75-mm titanium construct stiffness was similar across all tests. The 5.0-mm steel system was significantly stiffer than 4.0-mm steel (lateral bending/torsion) and 4.75-mm titanium (torsion/flexion) constructs. Rod surface stress was significantly lower for the 4.75-mm titanium rod compared with 4.0-mm and 5.0-mm steel rods for all tests.

Conclusions: The percentage of yield stress was lowest for the 4.75-mm Ti rod for all tests due to titanium's greater yield stress. This suggests the 4.75-mm rod has a lower fatigue failure risk than either steel construct.