Development and Validation of a Three-Axis OPG-Based Classification for Mandibular Third Molars: A Retrospective Study Assessing Inferior Alveolar Canal Relationship and Nerve Impingement Risk

Abstract

Background: Accurate preoperative assessment of the relationship between impacted mandibular third molars and the inferior alveolar canal (IAC) is critical to minimize the risk of inferior alveolar nerve (IAN) injury. Although orthopantomography (OPG) remains the first-line imaging modality worldwide, a standardized and integrative OPG-based classification system for comprehensive risk stratification is lacking.

Aim: To develop and validate a structured three-axis OPG-based classification system integrating root morphology, IAC positional relationship, and radiographic impingement risk indicators, and to evaluate its clinical applicability in predicting IAN injury risk.

Materials and Methods: This retrospective observational study analyzed 278 mandibular third molars using archived digital OPGs obtained between 2022 and 2025. A novel three-axis classification was applied: Axis 1—root morphology (Straight, Curved toward IAC, Deflected/Divergent); Axis 2—IAC positional relationship (Separate, Superimposed, Over Canal); Axis 3—impingement risk based on modified Rood and Shehab radiographic signs (Low, Moderate, High). Additional assessment included Winter’s angulation and Pell and Gregory classification. Inter-observer reliability was evaluated using Cohen’s kappa. Statistical analysis included chi-square tests and one-way ANOVA with post-hoc Tukey analysis.

Results: Inter-observer reliability was excellent (κ = 0.87). Curved and deflected/divergent roots demonstrated significantly higher impingement risk compared to straight roots (p < 0.001). Teeth categorized as Over Canal exhibited the smallest mean apex–canal distance (0.9 ± 0.3 mm) and the highest frequency of high-risk radiographic signs. The most common low-risk pattern was Straight–Separate–Low Risk, whereas Curved/Deflected–Over Canal–High Risk patterns showed the greatest potential for IAN impingement. Mesioangular angulation and Pell and Gregory Class II Level B/C impactions were significantly associated with increased risk.

Conclusion: The Three-Axis OPG-Based Classification is a reliable, reproducible, and clinically meaningful framework for assessing mandibular third molar–IAC relationships using panoramic radiography alone. It enhances preoperative risk stratification, supports selective CBCT referral, improves surgical planning, and aligns with radiation protection principles. The model also provides a foundation for future AI-assisted automated risk prediction.