Normal Parameters Of Skeletally Immature Knees: Developmental Changes On Magnetic Resonance Imaging

OBJECTIVES: A child’s knee undergoes significant morphologic changes during development. Age-specific normal parameters have not been previously described. The purpose of this study was to delineate the changes that occur with growth utilizing magnetic resonance imaging (MRI). METHODS: A retrospective review of a radiologic database of MRI knees, ages 4 to 18 years, was undertaken using “normal” diagnosis codes, including: knee pain, plicae, septic arthritis, jumper’s knee, growing pains, benign neoplasm, and osteomyelitis. 19 osseous parameters were measured, plus their cartilaginous counterparts, if different. Spearman’s rho correlations were calculated between age and MRI measures. RESULTS: 132 MR images were reviewed. Significant correlation with age was observed in all but MPFL insertion on the sagittal view for the osseous measurements (p<0.05). Similarly, the cartilage measures all demonstrated significant correlation with age except MPFL insertion on the sagittal view, but also the Notch-Width index, and trochlear morphology. The magnitude of correlation was significantly different between the osseous and cartilage measures for all but 3 of the variables: osseous and cartilage Insall-Salvati ratios, and insertion of MPFL on the femur. Osseous patella height stabilizes by age 10 with values ranging from 0.9-1.2 until age 18. Cartilage patella height demonstrates a modest linear increase with age (rho=0.352). MPFL insertion on the sagittal view remains stable with age (rho=0.036, p>0.05) and the mean insertion remains below the growth plate (11.5 mm to 4.9 mm). On the coronal view, MPFL insertion modestly increases with age (rho=0.33, p<0.001) starting below the growth plate on average (4.3 mm to 2.6 mm), until age 7 when the mean insertion appears to be at or above the growth plate. CONCLUSION: Most of the studied parameters follow a clear trend of growth and change in size that correlates with maturity. However, the normal Insall-Salvati ratio that delineates patella alta does not appear to be within the normal range until age 10. Furthermore, the discrepancy in the literature concerning the location of the MPFL insertion on the femur in relation to the physis may be attributed to migration of the insertion site during maturation, with the MPFL insertion being below the physis until age 7 when it migrates to insert at or above the physis.