Patellofemoral instability in children: Correlation between risk factors, injury patterns, and severity of cartilage damage

OBJECTIVE. The purpose of this study was to compare MRI findings between groups with and without patellofemoral instability and to correlate the MRI findings with the severity of patellar cartilage damage. MATERIALS AND METHODS. Fifty-three children with patellofemoral instability and 53 age- and sex-matched children without patellofemoral instability (15.9 ± 2.4 years) were included. Knee MRI with T2-weighted mapping was performed. On MR images, femoral trochlear dysplasia, patellofemoral malalignment, medial retinaculum injury, and bone marrow edema were documented. The degree of patellar cartilage damage was evaluated on MR images by use of a morphologic grading scale (0-4) and on T2 maps with mean T2 values at the medial, central, and lateral facets. MRI findings were compared between the two groups. In cases of patellofemoral instability, MRI findings were correlated with the severity of cartilage damage at each region. RESULTS. Trochlear structure and alignment were significantly different between the two groups (Wilcoxon p < 0.0001). In patellofemoral instability, a high-riding patella was associated with central patellar cartilage damage with a higher morphologic grade and T2 value (Spearman p < 0.05). The severity of medial retinacular injury and presence of bone marrow edema at either the medial patella or the lateral femoral condyle were associated with a higher grade of medial patellar cartilage damage (Wilcoxon p < 0.05). None of the other findings correlated with the severity of patellar cartilage damage. CONCLUSION. Patients with patellofemoral instability have significantly different trochlear structure and alignment than those who do not, and these differences are known risk factors for patellofemoral instability. However, the only risk factors or injury patterns that directly correlated with the severity of patellar cartilage damage were patella alta, medial stabilizer injury, and bone marrow edema.