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.
METHODS
In vitro biomechanical evaluation of rod stress during physiologic loading of anterior scoliosis instrumentation.


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


BACKGROUND
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.
Wedemeyer
Michelle
M
Department of Orthopedics, Children's Hospital, San Diego, San Diego, CA, USA.
Parent
Stefan
S
Mahar
Andrew
A
Odell
Tim
T
Swimmer
Troy
T
Newton
Peter
P
D000818:Animals; D002417:Cattle; D008422:Materials Testing / Q000379:methods / Q000592:standards*; D013123:Spinal Fusion / Q000295:instrumentation* / Q000379:methods / Q000592:standards*; D013193:Stainless Steel / Q000592:standards*; D013314:Stress, Mechanical; D014025:Titanium / Q000592:standards*; D016474:Weight-Bearing / Q000502:physiology
D013193:Stainless Steel; D014025:Titanium