Introduction: Patellar height abnormalities have been associated with patellar instability, which is common among adolescents and young adults. Pre-injury patellar height is often unknown in patients with a patellar instability episode. The purpose of this study was to determine whether patellar height of one knee could effectively estimate patellar height of the contralateral knee in control patients between the ages of 13 and 25 years. Methods: A retrospective chart review was conducted to identify 40 patients who obtained bilateral knee radiographs with no prior surgical intervention or diagnosis of injury to the extensor mechanism. CatonDeschamps Index (CDI) was measured by three different examiners on two separate occasions. Inter- and intraobserver intraclass correlation coefficients (ICCs) were calculated, and mean side-to-side difference was calculated between left and right CDI measurements. Additionally, left and right means were compared, and 95% confidence intervals were calculated. Results: Intra-observer reliability of ICCs were good to excellent among all examiners (0.92, 0.91, and 0.86), and inter-observer reliability was good at 0.85. Mean CDI side-to-side difference was 0.02 (95% confidence interval 0.05, -0.01). Average CDI ratios were left knee CDI of 1.10 and right knee CDI of 1.12. Conclusions: Good to excellent inter- and intra-observer reliability was demonstrated. There was no difference found between left and right knee CDI measurements. These results confirm that patellar height measurements in one knee can reliably estimate the patellar height in the other knee in patients between the ages of 13 and 25

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