To the editor,
We read with great interest the comprehensive finite element analysis by Jo et al. [
1] examining cement augmentation strategies in multilevel lumbar fusion. However, we respectfully raise several methodological considerations that may enhance the predictive accuracy and clinical applicability of such computational models. Specifically, reliance on peak von Mises stress (PVMS) as the primary metric for screw loosening prediction, and the reported L1 fracture risk findings, merit careful reconsideration in light of recent validation evidence.
Recent validation studies demonstrate that static PVMS correlates poorly with clinical screw loosening outcomes. The case-control study of Jiang et al. [
2] found that simple axial pullout force showed no significant difference between groups (p=0.122), while pullout force under craniocaudal cyclic loading achieved an area under the curve of 0.806. Fasser et al. [
3] explicitly concluded that “axial pullout force was not a reliable predictor of clinical PSL events,” emphasizing that models incorporating individualized cyclic joint loading showed substantially greater promise.
The fundamental limitation is that PVMS measures yield stress potential rather than the micromotion and interface mechanics that govern clinical loosening. Screw loosening is predominantly a fatigue phenomenon driven by cyclic loading and progressive interface degradation. Contact pressure distribution and micromotion assessment better capture these failure mechanisms by quantifying the biomechanical conditions at the bone-screw-cement interface where loosening initiates. The study’s finding that type 3 cement augmentation produced the highest L1 fracture risk contradicts established clinical and cadaveric evidence. Theologis and Burch [
4] found 0% revision for proximal junctional fracture with 2-level cement augmentation versus 19% without (p=0.02), while Ghobrial et al. [
5] reported 0% versus 13% (p=0.03). Most strikingly, the cadaveric study of Kebaish et al. [
6] demonstrated 17% fracture rate with cement augmentation versus 83% without—essentially the inverse relationship.
This discrepancy raises questions about whether specific modeling assumptions influenced the predicted stress distributions. Janssen et al. [
7] demonstrated that cement does not adhere to bone in the presence of blood, fat, and marrow, with interface strength deriving from mechanical interlock. Their validation showed that frictional contact modeling (μ=0.3) matched experimental stiffness closely, while bonded contact assumptions overestimated stiffness by more than twofold. If tied contact was assumed at the cement-bone interface, this could artificially create stress concentration patterns that overestimate fracture risk [
8].
U.S. Food and Drug Administration guidance and ASME V&V 40 standard emphasize that single-point stress metrics are mesh-dependent and may not represent actual failure mechanisms [
9]. Distributed contact pressure analysis provides physically measurable quantities that correlate more directly with interface failure modes and can be validated experimentally [
10]. Reporting pressure distributions (median, 90th, and 95th percentile values), contact area evolution, and regional concentrations would provide more robust predictions than single peak values.
To strengthen clinical applicability, we suggest: (1) incorporating cyclic loading protocols to better simulate physiological conditions and improve loosening prediction, (2) implementing validated frictional contact assumptions at cement-bone interfaces rather than tied contact, and (3) reporting contact pressure distributions alongside or instead of peak stresses for more clinically relevant predictions.
The work of Jo et al. [
1] addresses an important clinical question. By incorporating validated metrics such as contact pressure distribution and cyclic loading-based micromotion analysis, alongside careful attention to interface modeling, future studies can achieve greater predictive accuracy for guiding clinical decision-making. These methodological enhancements would strengthen the computational framework and align FEA predictions more closely with clinical evidence.
NOTES
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Conflict of Interest
The authors have nothing to disclose.
REFERENCES
- 1. Jo MY, Lee SJ, An JH, et al. Biomechanical impact of cement augmentation on pedicle screw fixation and adjacent segment disease in multilevel lumbar fusion: a finite element analysis. Neurospine 2025;22:763-73.
- 2. Jiang C, Ouyang H, Li Y, et al. Craniocaudal cyclic load improves risk assessment of lumbar pedicle screw loosening: finite element analysis based on computer tomography. Front Bioeng Biotechnol 2025;13:1542352.
- 3. Fasser MR, Gerber G, Passaplan C, et al. Computational model predicts risk of spinal screw loosening in patients. Eur Spine J 2022;31:2639-49.
- 4. Theologis AA, Burch S. Prevention of acute proximal junctional fractures after long thoracolumbar posterior fusions for adult spinal deformity using 2-level cement augmentation at the upper instrumented vertebra and the vertebra 1 level proximal to the upper instrumented vertebra. Spine (Phila Pa 1976) 2015;40:1516-26.
- 5. Ghobrial GM, Eichberg DG, Kolcun JPG, et al. Prophylactic vertebral cement augmentation at the uppermost instrumented vertebra and rostral adjacent vertebra for the prevention of proximal junctional kyphosis and failure following long-segment fusion for adult spinal deformity. Spine J 2017;17:1499-505.
- 6. Kebaish KM, Martin CT, O'Brien JR, et al. Use of vertebroplasty to prevent proximal junctional fractures in adult deformity surgery: a biomechanical cadaveric study. Spine J 2013;13:1897-903.
- 7. Janssen D, Mann KA, Verdonschot N. Finite element simulation of cement-bone interface micromechanics: a comparison to experimental results. J Orthop Res 2009;27:1312-8.
- 8. Yang K, Zhu X, Sun X, et al. Bone cement distribution patterns in vertebral augmentation for osteoporotic vertebral compression fractures: a systematic review. J Orthop Surg Res 2025;20:568.
- 9. Baumann AP, Graf T, Peck JH, et al. Assessing the use of finite element analysis for mechanical performance evaluation of intervertebral body fusion devices. JOR Spine 2021;4:e1137.
- 10. Kinzl M, Benneker LM, Boger A, et al. The effect of standard and low-modulus cement augmentation on the stiffness, strength, and endplate pressure distribution in vertebroplasty. Eur Spine J 2012;21:920-9.
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