Comparison of a novel pedicle subtraction osteotomy model using the traditional American Society of Testing and Materials standard for spinal biomechanics fatigue testing [RETRACTED]

Laboratory investigation

Jessica A. Tang The Taylor Collaboration, and University of California, San Francisco, California

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Object

There is currently an internationally recognized standard (F1717) provided by the American Society of Testing and Materials (ASTM) for fatigue testing of spinal fixation constructs, assuming relatively straight posterior rods between 2 vertebral segments. However, there is currently no standard that effectively describes the changes in forces and moments for procedures that induce more drastic curvatures, such as pedicle subtraction osteotomies (PSOs). In this study, the author proposes a modified version of the ASTM F1717 standard to compensate for changes in loading conditions for PSO constructs.

Methods

Twelve specimens were divided into 2 groups: 6 modeled after the original ASTM standard and 6 after the modified version. Three specimens each from the 2 groups had rods contoured to corresponding PSO angles of 20° and 60°, respectively. Specimens were cycled at 4 Hz at a 400 N/40 N or 700 N/70 N load ratio until failure was observed or run-out (testing cycle end point) was reached at 2,000,000 cycles. Cox proportional hazards regression was used to analyze the effect of rod curvature on the fatigue strength of the 2 different models.

Results

Results indicated that contouring rods from a PSO angle of 20° to 60° significantly increased the fatigue life of the screw-rod construct (hazard ratio [HR] 1.57, p = 0.0144) for the original model, but had the opposite effect of decreasing the fatigue life for the modified model (HR 0.64, p = 0.0144).

Conclusions

Because there is extensive data showing that contouring rods to more extreme angles significantly lowers their fatigue life, the modified ASTM model may be more accurate for simulating constructs that assume insignificant rod bending.

Abbreviations used in this paper:

ASTM = American Society of Testing and Materials; HR = hazard ratio; PSO = pedicle subtraction osteotomy.
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