Objective: This study aimed to introduce a novel angular measurement—tibial tubercle–posterior cruciate ligament angle (TT–PCL angle)—to quantify tibial tubercle lateralization in patients with patellofemoral instability (PFI). Secondary objectives included assessing its reliability, diagnostic performance, and correlation with established linear measurements.
Materials and Methods: This retrospective case-control study included 43 patients with objective PFI and 100 controls. Two independent observers measured TT–PCL angle, TT–TG distance, TT–PCL distance, and their normalized variants on axial CT scans at two time points. Inter- and intraobserver reliability were assessed using intraclass correlation coefficients (ICCs). Correlations between measurements were analyzed, and diagnostic performance was evaluated using ROC curve analysis. Results: All variables were significantly higher in the PFI group than in controls (p < 0.001). The TT–PCL angle demonstrated excellent intra- and interobserver reliability (ICC > 0.90). A cutoff value of >20.25° yielded 65.1% sensitivity and 85.0% specificity for detecting pathological lateralization. The angle strongly correlated with TT–PCL and normalized TT–PCL distances, but not with TT–TG measures in the PFI group.
Conclusion: The TT–PCL angle is a reliable and practical tool for assessing tibial tubercle lateralization. Unlike conventional measurements, it is based solely on tibial landmarks and is unaffected by knee size or femoral anatomy. Its strong diagnostic performance and reproducibility support its potential use in routine clinical practice.