Direct restoration failure after head and neck radiotherapy: prevalence and independent predictors from a multicenter mixed-effects analysis.

OBJECTIVES: To determine the prevalence of direct restoration failure in head and neck cancer (HNC) survivors following radiotherapy and identify independent patient-, treatment-, salivary-, and restoration-related predictors using mixed-effects logistic regression. METHODS: This multicenter cross-sectional study included 115 HNC survivors from three tertiary hospitals, contributing 423 direct restorations placed after radiotherapy. Restoration outcomes were assessed using the FDI World Dental Federation criteria. Demographic, oncologic, treatment, oral, salivary, and restoration characteristics were recorded. Salivary dysfunction was evaluated using unstimulated whole salivary flow (UWS), the Clinical Oral Dryness Score (CODS), and the Summated Xerostomia Inventory (SXI). Mixed-effects logistic regression with a patient-specific random intercept identified independent predictors of restoration failure. RESULTS: Of 423 restorations, 159 failed (37.6%). Secondary caries was the predominant failure mechanism (45.3%), followed by fracture (20.1%) and defective marginal adaptation (18.2%). Higher UWS independently reduced the odds of restoration failure (adjusted OR 0.59, 95% CI 0.42-0.83), whereas higher radiotherapy dose (adjusted OR 1.06 per Gy, 95% CI 1.00-1.13), glass-ionomer restorations (adjusted OR 2.40, 95% CI 1.21-4.77), restorations involving ≥3 surfaces (adjusted OR 3.83, 95% CI 1.56-9.42), restoration age >24 months (adjusted OR 3.55, 95% CI 1.90-6.62), and endodontically treated teeth (adjusted OR 1.86, 95% CI 1.05-3.30) independently increased failure risk. The final model showed good discrimination (AUC = 0.764). CONCLUSIONS: Direct restoration failure is common after head and neck radiotherapy and is independently influenced by radiation-related salivary dysfunction, radiotherapy exposure, and restoration characteristics. Objective salivary assessment may improve risk stratification and support individualized preventive and restorative care. Patients with impaired salivary function and multiple restoration-related risk factors may particularly benefit from individualized risk-based follow-up and preventive care. CLINICAL SIGNIFICANCE: Objective assessment of salivary gland function, together with restoration- and treatment-related factors, may improve risk stratification, restorative treatment planning, and individualized maintenance protocols for head and neck cancer survivors following radiotherapy.