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

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.

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.

STUDY DESIGN: Case-control study; Level of evidence, 3.