Normal Parameters of the Skeletally Immature Knee: Developmental Changes on Magnetic Resonance Imaging

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 knee morphology changes that occur with maturation utilizing magnetic resonance imaging (MRI).

A retrospective review of knee MRIs, ages 4 to 18 years, was performed using "normal" diagnosis codes, including: knee pain, plicae, septic arthritis, jumper's knee, growing pains, benign neoplasm, and osteomyelitis. Seventeen osseous parameters were measured, plus their cartilaginous counterparts, if different. Spearman ρ correlations were calculated between age and MRI measures.

A total of 132 MR images were reviewed. Significant correlation with age was observed in all osseous measurements, except medial patellofemoral ligament (MPFL) insertion (P<0.05). Similarly, the cartilage measures demonstrated significant correlation with age, except the notch width index, MPFL insertion, and trochlear morphology. The magnitude of correlation was significantly different between the osseous and cartilage measures for all but 4 of the variables: anterior cruciate ligament (ACL) length and width, intercondylar width and tibial tubercle-trochlear groove (TT-TG) interval. Sex seems to affect ACL length, Insall-Salvati ratio, intercondylar width, and sulcus angle.The MPFL origin starts distal to the physis and moves proximal with age (ρ=0.33, P<0.001), when at age 7 years, it is located at or above the femoral physis. The mean ACL diameter grows from 6.5 to 9.8 mm. Cartilage sulcus angle remains consistently 150 degrees, and the TT-TG interval shifts from 6 to 12 mm with maturity.

The normative data presented here demonstrates that maturity correlates well with the morphologic changes seen in most of the studied parameters. These data not only aid in the identification of the abnormal findings by sex and age, but also provides normal sizes of the ACL and location of insertion for the MPFL which can benefit surgical planning.