Diagnostic value of the patellar tendon–lateral trochlear ridge distance and the patellar tendon–trochlear groove angle in differentiating lateral patellar instability from malalignment‐induced patellofemoral pain

PURPOSE: To compare the patellar tendon-lateral trochlear ridge (PT-LTR) distance and patellar tendon-trochlear groove angle (PT-TGA) between patients with lateral patellar instability (LPI) and those with malalignment-induced patellofemoral pain (PFP), and to evaluate their differential diagnostic validity. METHODS: Sixty patients with LPI (mean age: 24.2 ± 7.1 years) and 60 patients with malalignment-induced PFP (mean age: 25.3 ± 6.2 years) were included. PT-LTR, PT-TGA, as defined by Dai et al. (PT-TGA1) and Hinckel et al. (PT-TGA2), and additional patellofemoral alignment parameters-including the tibial tubercle-trochlear groove (TT-TG) distance, tibial tubercle-posterior cruciate ligament (TT-PCL) distance, lateral trochlear inclination (LTI) angle, patellar height, valgus deviation, femoral and tibial torsion, knee rotation and leg axis-were measured. The unpaired t tests with Welch's correction, receiver operating characteristic (ROC) curve analysis via the Youden index, and parametric power analysis were utilized. RESULTS: PT-LTR, PT-TGA1 and PT-TGA2 values were significantly greater in LPI patients compared to those with PFP (PT-LTR: 8.8 ± 6.7 mm vs. 3.9 ± 5.3 mm; PT-TGA1: 29.1 ± 11.3° vs. 17.3 ± 8.9°; PT-TGA2: 35 ± 9.6° vs. 24.9 ± 8.5°; all p < 0.0001). ROC analysis revealed AUCs of 0.73 (PT-LTR), 0.79 (PT-TGA1) and 0.78 (PT-TGA2). PT-TGA1 demonstrated the highest sensitivity (85%), whereas PT-TGA2 showed the highest specificity (83%). Significant differences were also found for LTI (p < 0.0001), patellar height (p < 0.0001), TT-PCL (p < 0.0001) and valgus deviation (p = 0.004), but not for TT-TG, femur/tibia torsion, or knee rotation. Power analysis confirmed robust statistical validity (Z ≈ 5.60; power = 99.987%). CONCLUSIONS: While PT-LTR and PT-TGA measurements were significantly greater in patients with LPIs than in those with malalignment-induced PFP, only PT-TGA measurements demonstrated sufficient diagnostic accuracy to identify LPI. The results reinforce the conceptual advantage of angular over linear measurements of the extensor apparatus. LEVEL OF EVIDENCE: Level III, diagnostic cohort study.