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The Effects of Rod Contouring on Spinal Construct Fatigue Strength

Lindsey, Colleen, MS*†; Deviren, Vedat, MD*; Xu, Zheng, BS*; Yeh, Ru-Fang, PhD; Puttlitz, Christian M., PhD§

doi: 10.1097/01.brs.0000224177.97846.00
Biomechanics
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Study Design. 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.

Summary of Background Data. 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.

Conclusion. Intraoperative rod contouring using a French Bender significantly reduces the fatigue life of titanium spinal constructs.

An in vitro corpectomy (ASTM F-1717) model was used to investigate the differences in fatigue resistance between straight rods and rods contoured using a French Bender. Contouring significantly reduced the fatigue life of titanium constructs compared with stainless steel constructs. There were also strong trends in the differences between constructs that had contoured and straight titanium rods.

From the Departments of *Orthopaedic Surgery and †Department of Mechanical Engineering, University of California, Berkeley, CA; University of California, San Francisco, CA; ‡Epidemiology & Biostatistics, University of California, San Francisco, CA; and §Department of Mechanical Engineering, Colorado State University, Fort Collins, CO.

Acknowledgment date: March 24, 2005. First revision date: June 13, 2005. Second revision date: August 3, 2005. Acceptance date: September 16, 2005.

Supported by Stryker Spine.

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

Corporate/Industry funds were received in support of this work. Although one or more of the authors(s) has/have received or will receive benefits for personal or professional use from a commercial party related directly or indirectly to the subject of this manuscript, benefits will be directed solely to a research fund, foundation, educational institution, or other nonprofit organization which the author(s) has/have been associated.

Address correspondence and reprint requests to Christian M. Puttlitz, PhD, Orthopaedic Bioengineering Research Laboratory, Department of Mechanical Engineering, 1374 Campus Delivery, CO State University, Fort Collins, CO 80523-1374; E-mail: puttlitz@engr.colostate.edu

© 2006 Lippincott Williams & Wilkins, Inc.