The effects of rod contouring on spinal construct fatigue strength.
METHODS
In vitro fatigue loading using a corpectomy model outfitted with posterior pedicle screw instrumentation.


OBJECTIVE
The purpose of this study was to detect differences in fatigue resistance of titanium and stainless steel spinal constructs that use rods contoured using a French Bender, and to compare differences in fatigue resistance of contoured and straight titanium rods.


BACKGROUND
Instrumentation failure is generally thought to be caused by fatigue or cyclic loading. Intraoperative contouring of the posterior rods is almost always required to match the native kyphotic (thoracic) or lordotic (cervical or lumbar) spinal curvature. How bending these rods affects their overall fatigue resistance is not well described. In addition, changes in fatigue resistance may be a function of material type.


METHODS
Spinal constructs were evaluated using the ASTM F1717-01 model. Two different titanium-based rods (Ti6AL4V and CpTi) and two different steel-based rods (Orthinox and 316L stainless steel) were evaluated in this study (n = 6 for each group). Rods were contoured at two points using a French Bender and were rigidly coupled to polyaxial pedicle screws within UHMWPE vertebral bodies. Constructs were cycled at a load ratio of 10 between a minimum and maximum loading regime of -250 N/-25 N and -700 N/-70 N at a frequency of 4 Hz. Estimated maximum nominal stresses at various points of interest in the spinal constructs were calculated using beam theory. Effects of the rod material, load, and stress on the number of cycles to failure were analyzed using Cox proportional hazards regression.


RESULTS
All of the spinal constructs with contoured CpTi rods and contoured Ti6Al4V rods failed at one of the bends in the rods. Almost all of the spinal constructs with straight CpTi rods and straight Ti6Al4V rods failed where the blocker screw fastens the rod to the coupler of the polyaxial screw head. Contoured titanium constructs demonstrated significantly lower fatigue life than contoured 316L constructs. Contouring tended to lower the fatigue life of both the Ti6Al4V and CpTi constructs.


CONCLUSIONS
Intraoperative rod contouring using a French Bender significantly reduces the fatigue life of titanium spinal constructs.
Lindsey
Colleen
C
Department of Orthopaedic Surgery, University of California, Berkeley, CA, USA.
Deviren
Vedat
V
Xu
Zheng
Z
Yeh
Ru-Fang
RF
Puttlitz
Christian M
CM
D001858:Bone Nails; D001863:Bone Screws; D004548:Elasticity; D006801:Humans; D066298:In Vitro Techniques; D008422:Materials Testing; D008953:Models, Anatomic; D008962:Models, Theoretical; D011475:Prosthesis Failure; D012306:Risk; D013123:Spinal Fusion / Q000295:instrumentation* / Q000379:methods; D013193:Stainless Steel; D013314:Stress, Mechanical; D014025:Titanium
D013193:Stainless Steel; D014025:Titanium