Increased sagittal patellar height and reduced trochlear engagement on MRI are associated with recurrent patellar instability in skeletally immature knees: A comparative study.

PURPOSE: To apply the magnetic resonance imaging (MRI)-based concept of functional patella alta to skeletally immature patients and compare sagittal patellar height and trochlear engagement between paediatric knees with objective patellar instability (OPI) and controls without patellofemoral disorders. METHODS: This retrospective study included 85 skeletally immature knees (42 OPI and 43 controls) from 83 patients undergoing MRI between 2021 and 2025. OPI required at least two lateral patellar dislocations. Measurements included patellar height index (PHI), sagittal patellar cartilage length (SPCL), patello-tibial distance (PTD), patellar tendon length (PTL), sagittal trochlear cartilage length (STCL) and sagittal patellofemoral engagement (SPE). Three blinded observers measured every knee; their mean was analysed. Functional patella alta was defined using control-derived interquartile-range thresholds. Interobserver reliability used two-way random-effects, absolute-agreement intraclass correlation coefficients (ICCs). Group comparisons and logistic regression were performed. RESULTS: OPI knees had significantly higher PHI (1.178 ± 0.171 vs. 1.083 ± 0.132; p = 0.005) and lower SPE (0.354 ± 0.207 vs. 0.484 ± 0.200; p = 0.004), with greater SPCL, PTD and PTL and shorter STCL (all p < 0.05). Groups were unmatched for age, with older OPI patients (p < 0.001). Functional patella alta occurred in 16/85 knees (18.8%): 14/42 OPI (33.3%) and 2/43 controls (4.7%) and was strongly associated with OPI (odds ratio [OR] 10.25; 95% confidence interval [CI] 2.16-48.66; p = 0.003). Reliability was excellent for SPE (ICC 0.95), PTL (0.97) and PTD (0.94), good for SPCL (0.80) and moderate for PHI (0.72). CONCLUSION: Paediatric recurrent patellar instability is characterised by increased patellar height and reduced sagittal trochlear engagement on MRI. Functional patella alta applies to skeletally immature knees, but instability occurs at lower sagittal engagement values than in adults. These cohort-specific exploratory thresholds require external validation before clinical use. LEVEL OF EVIDENCE: Level III, retrospective comparative study.