Patellar Tendon-Lateral Trochlear Ridge Distance Is More Specific Than Tibial Tubercle-Trochlear Groove for Patellofemoral Instability.

PURPOSE: To assess the reliability, predictability, and discriminative capacity of 3 anatomic measurements, tibial tubercle-trochlear groove (TT-TG), tibial tubercle-posterior cruciate ligament, and patellar tendon-lateral trochlear ridge (PT-LTR), where PT-LTR may offer a complementary approach to assessing recurrent patellofemoral instability (PFI) risk. METHODS: Patients diagnosed with PFI before primary patellofemoral stabilization surgery at a single institution between 2022 and 2024 were included. A control group without PFI was propensity matched 1:1 for age, sex, and laterality. Magnetic resonance images were evaluated to measure TT-TG, tibial tubercle-posterior cruciate ligament, and PT-LTR. A priori power analysis determined the cohort size. Statistical analyses compared differences between the PFI and control groups with Student t-tests, receiver operating characteristic analyses, and area under the curve with resultant sensitivity, specificity, and an optimal cutoff. RESULTS: Patients included 50 knees with PFI (28 women, 22 men; mean age 18 ± 5 years). The control group included 50 knees (27 women, 23 men; mean age 18 ± 4 years). Excellent inter- and intra-rater reliability (intraclass correlation coefficient > 0.7) was shown for TT-TG and PT-LTR in the PFI and control groups. The distances for TT-TG, tibial tubercle-posterior cruciate ligament, and PT-LTR were significant predictors (P ≤ .025) of PFI. The average PT-LTR distance difference for the PFI and control groups was 7.0 mm (P < .001). PT-LTR showed an area under the curve of 0.80 and, when ≥4.6 mm, was 66% sensitive and 92% specific for PFI. CONCLUSIONS: The PT-LTR distance is a reliable and discriminative metric for identifying PFI in both pediatric and adult populations. A PT-LTR cutoff value of 4.6 mm showed diagnostic discrimination and substantially high specificity for recurrent PFI, indicating strong ability to identify patients with instability. LEVEL OF EVIDENCE: Level III, retrospective case control study.