Patella Alta and Trochlear Dysplasia as a Determinant of Patellar Tendon Lateralization and Inclination: A Cross-sectional MRI Study in Adolescents With Patellofemoral Instability

Abstract

Background: Patellofemoral instability (PFI) is traditionally assessed using tibial tubercle–trochlear groove (TT-TG) distance, patellar height indices, and trochlear morphology. However, soft tissue–based measures of patellar tendon orientation may provide complementary insight into patellar containment and extensor mechanism malalignment. Patellar tendon–lateral trochlear ridge distance (PT-LTR) has been proposed as a tendon-based marker of lateralization. The authors introduce the patellar tendon inclination angle (PTIA): a magnetic resonance imaging (MRI)–based parameter designed to quantify the obliquity of the patellar tendon relative to the femoral condyles. Purpose: To define and validate PTIA and to compare its performance with PT-LTR. Secondary objectives were to assess correlation of PTIA and PT-LTR with established morphologic risk factors for PFI and to evaluate their diagnostic accuracy for identifying PFI. Study Design: Case-control study; Level of evidence, 3. Methods: MRI studies of 284 knees (PFI, n = 142; controls, n = 142) were included in this analysis. PTIA was developed and measured by orthopaedic surgeons, and their agreement was calculated. PT-LTR, sulcus angle, patellar tilt, lateral patellofemoral angle, TT-TG distance, and Caton-Deschamps index (CDI) were measured for all knees as well. Correlations between PT-LTR/PTIA and other classic measurements were assessed to evaluate the relationship of these novel tendon-based parameters. Subgroup analyses examined the effect of abnormal TT-TG distance (>16 mm), patella alta (CDI ≥ 1.3), and trochlear dysplasia (sulcus angle ≥ 145°). Receiver operating characteristic curves were used to determine diagnostic accuracy. Results: Patients with PFI demonstrated significantly higher PT-LTR (mean ± SD, 9.76 ± 7.66 mm) and PTIA (22.76°± 11.22°) as compared with controls (P < .001). PT-LTR and PTIA were significantly positively correlated with patellar tilt, CDI, and sulcus angle. Patella alta significantly increased PT-LTR (P < .001). Receiver operating characteristic analysis demonstrated excellent diagnostic performance for PT-LTR (area under the curve, 0.903; optimal cutoff, 4 mm) and PTIA (area under the curve, 0.866; optimal cutoff, 16°). Conclusion: PT-LTR and PTIA are reliable MRI-based markers of PFI in adolescents. Although both correlate with established risk factors, including TT-TG distance, patella alta, and trochlear dysplasia, they remain informative even in the setting of normal TT-TG values. PTIA, introduced as a novel parameter, complements PT-LTR by quantifying tendon obliquity and providing additional information on patellar tendon alignment.