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Computational and Experimental Fatigue Analysis of Contoured Spinal Rods
Agnese Piovesan, Francesca Berti, Tomaso Villa1
1Laboratory of Biological Structure Mechanics,Department of Chemistry, Materials andChemical Engineering "Giulio Natta",Politecnico di Milano,Piazza Leonardo da Vinci 32,Milan 20133, Italy.
Journal of Biomechanical Engineering
|February 8, 2019
Summary
Residual stresses significantly impact spinal rod fatigue. Lordotic configurations experience higher stress and earlier failure, while kyphotic configurations show improved fatigue life.
Area of Science:
- Spinal surgery
- Biomechanical engineering
- Materials science
Background:
- Posterior fixation with contoured rods is a standard treatment for spinal deformities.
- Rod contouring introduces residual stresses, affecting mechanical performance.
- Understanding fatigue behavior under different configurations is crucial for patient safety.
Purpose of the Study:
- To develop and validate a numerical framework predicting fatigue behavior of contoured spinal rods.
- To assess the influence of residual stresses on fatigue life in lordotic and kyphotic configurations.
- To compare numerical predictions with experimental fatigue data.
Main Methods:
- Finite element models (FEM) for static contouring simulations.
- Subsequent cyclical loading simulations to predict fatigue behavior.
- Comparison of predicted stress distributions with experimental fatigue test results.
Main Results:
- Lordotic configuration showed increased stress and earlier failure (2.4 x 10^4 cycles) compared to straight rods (1.9 x 10^5 cycles).
- Kyphotic configuration exhibited stresses below the limit curve, achieving run-out at 10^6 cycles.
- Microscopy confirmed agreement between numerical predictions and experimental fatigue crack locations.
- Contouring technique (UC or FB) did not significantly affect fatigue outcomes.
Conclusions:
- Residual stresses play a critical role in altering mean stress and influencing fatigue failure in spinal rods.
- The higher failure rate in lordotic rods can be attributed to the superposition of tensile cyclic load and residual stresses.
- The developed numerical framework provides a validated tool for predicting spinal rod fatigue behavior.
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