Developmental Morphology in Childhood Patellar Instability: Age-dependent Differences on Magnetic Resonance Imaging

Background: Magnetic resonance imaging (MRI) has been used to define pathologic morphology of patellar instability primarily in adults, but few studies have evaluated skeletally immature patients. The purpose of this study was to delineate differences in morphologic parameters between normal children and children with patellar instability. Methods: A retrospective review of knee MRIs, ages 8 to 18 years, using a "normal" cohort without evidence of patella instability (normal group, NG) and a cohort with unstable patella (unstable group, UG). Thirty-four measurements were made on patellar characteristics, trochlear morphology, limb alignment, and the medial patellofemoral ligament (MPFL). Spearman's rho correlations were calculated between age and MRI measures. Results: There were 132 NG and 66 UG children that met criteria. The majority of parameters considered diagnostically relevant in adults, also seem to be statistically relevant for children. A new measure, MPFL insertion site to the physis distance, was also significantly different (NG: -1.8 +/- 3.5 vs. UG: 0 +/- 3.4 mm). More importantly, some measurements demonstrated both a difference between the 2 groups and an age correlation: osseous sulcus angle (NG: 133.4 +/- 12.4 vs. UG: 145.2 +/- 11.2 degrees, age correlation P = 0.01) and cartilaginous sulcus angle (NG: 138.2 +/- 9.5 vs. UG: 152.5 +/- 11.7 degrees, age correlation P = 0.03). Conclusions: Several end-stage adult parameters are significant as good predictors of patellar instability when seen on a child's MRI; but, some parameters, such as sulcus angle, are dependent on the age of the child. The most significant variation in regards to age is seen with sulcus angles. Moreover, there is a significant change in the location of the MPFL attachment on the femur as it relates to the physis with age, as well as stability. Level of Evidence: Level III-prognostic study, case-control study.