Correlation Between Rod Fracture and Shear Stress: A Novel Parameter

Area of Science:

• Orthopedic Surgery
• Biomechanical Engineering
• Spinal Fusion Research

Background:

• Adult spinal deformity correction often involves spinal instrumentation.
• Rod fracture (RF) is a potential complication following spinal fusion surgery.
• Accurate identification of patients at risk for RF is crucial for surgical planning and patient outcomes.

Purpose of the Study:

• To evaluate the accuracy of a novel rod shear stress (RSS) parameter in predicting RF risk.
• To compare the predictive accuracy of RSS against its constituent variables, specifically the number of instrumented levels.

Main Methods:

• Retrospective review of adult spinal deformity patients (≥18 years) with ≥24 months follow-up.
• Calculation of RSS parameter (RSS=Nwd²) using patient weight (w), number of instrumented levels (N), and minimum rod diameter (d).
• Analysis using receiver operating characteristic (ROC) curves to determine area under the curve (AUC), sensitivity, specificity, and likelihood ratios (LRs).

Main Results:

• The RSS parameter demonstrated the highest discriminative accuracy with an AUC of 0.73 (±0.11).
• At a cutoff of 30.1, RSS achieved 71.4% sensitivity and 71.4% specificity (LR, 2.5).
• The number of instrumented levels showed a strong correlation with RFs, with an AUC of 0.65 (±0.12).

Conclusions:

• The RSS parameter, derived from readily available data, shows potential for predicting patient-specific RF risk after spinal fusion.
• The number of instrumented levels is also a significant predictor of RFs and is not substantially less accurate than RSS.
• Further prospective studies with larger sample sizes are recommended to definitively establish the superiority of RSS or other parameters for RF prediction.