Reliability of Patellar Height Measurement in Children and Adolescents on MRI

Imaging-based measurement indices of patellar height are essential for diagnosing patella alta in children and adolescents. The Patellotrochlear Index (PTI) measures the cartilaginous anatomy which is crucial for identifying pathologic changes at the patellofemoral joint, while the Insall-Salvati Index (ISI) and Caton-Deschamps Index (CDI) focus on the bony structures. We aimed to evaluate the reliability of PTI on MRI for measuring patellar height and its relationship with ISI and CDI in identifying patella alta in children and adolescents.

This study retrospectively assessed 197 patients with patellofemoral joint instability (PFJI) who underwent knee MRI scans between January 2021 and December 2023. An age-matched and sex-matched control group was used for comparison. Each group was further subdivided into 3 age groups. ISI, CDI, and PTI on MRI were independently measured by 2 different raters. The study calculated the inter-rater reliability using the intraclass correlation coefficient (ICC) and examined the correlations between the different metrics.

No significant differences in PTI were observed among the 3 age groups in both control and PFJI patients. The ICC reliability for PTI was demonstrated to be good to excellent in control patients and good in PFJI patients. There was a significant correlation between the ISI and the CDI (r=0.704, P<0.01) in the control patients and a moderately significant correlation between ISI and CDI (r=0.402, P<0.01) in the PFJI patients. However, no significant correlation was found between PTI and ISI (r=-0.073, P=0.056) or CDI (r=-0.069, P=0.076) in control patients, and no significant correlation existed between PTI and ISI (r=-0.115, P=0.208) or CDI (r=-0.023, P=0.805) in PFJI patients.

PTI demonstrates strong reliability and shows no significant correlation with either ISI or CDI, suggesting it may provide additional benefits for assessing patellar height on MRI in children and adolescents. Therefore, PTI should be measured in addition to ISI/CDI in children and adolescents with potentially abnormal patellar height.