Age-Based Patellofemoral Morphology in the Immature Knee

Purpose: Patellar instability (PI) is a common cause of anterior knee pain and disability in the pediatric population. The use of patellofemoral measurements on MRI provides a quantitative means for PI assessment and has now become an important diagnostic tool, but these techniques largely rely upon adult standards. Our goal is to describe morphologic trends in the skeletally immature knee and to predict the age at which adult norms can reliably be used in the evaluation of the pediatric knee. Methods: We retrospectively reviewed 144 normal knee MRIs in 133 skeletally immature patients that presented between 2002 and 2014. Patients were equally distributed by age and gender with ages ranging from 1-16. MRI exclusion criteria included: moderate to severe effusions, cartilaginous defects, patellofemoral abnormalities, ligamentous injury, neoplasms, infection, congenital disease, or arthritic changes. All 1 and 2 year olds were included due to lack of MRIs and only females younger than 15 were used to account for anticipated physical closure. All measurements used cartilaginous landmarks and results were stratified based on age and gender. Each measurement was charted in a linear regression model or analyzed with Student’s t test. Results: Each measurement can reliably be performed at all ages with good interand intra observer reliability. All MR measurements were graphically represented in a linear regression model and are shown to approach adult norms with increasing age. The age at which there is no statistical difference between our pediatric patients and the adult norms is shown as the “regression cutoff”. Further t-test analysis suggests a 2nd cutoff that serves as the age at which younger should not be compared to adult norms. Conclusion: The measurements commonly used to evaluate for patellar instability in the adult population are subject to considerable variation throughout skeletal maturation. Based on our analysis, children < 10 years of age should not be compared to adult standards. Conversely, children ≥ 10 appear to have reached near patellofemoral maturation and show consistent and progressive development of patellofemoral morphology with increasing age. Significance: The ability to predict morphologic abnormalities in the first decade of life may lend to earlier surgical intervention or realignment procedures. Surgical outcomes may be augmented by remaining patellofemoral growth and remodeling, especially in those patients under 10 years of age.