A comparison of sagittal MRI and lateral radiography in determining the Insall-Salvati ratio and diagnosing patella alta in the pediatric knee

Background: The Insall-Salvati ratio is a technique for determining patellar height that relies on bony landmarks. Magnetic resonance imaging (MRI) and plain radiography are used interchangeably to assess the Insall-Salvati ratio in the pediatric population despite the lack of validity in the literature.

Objective: The purpose of this study was to investigate if the Insall-Salvati ratio and patella alta as determined on MRI are comparable to those determined on radiography in pediatric patients.

Materials and methods: We conducted a retrospective review of 49 pediatric patients (age range: 7.5-17.0 years) with unfused growth plates who underwent both knee MRI and lateral knee radiography. Measurements for calculating the Insall-Salvati ratio (the ratio of patella tendon length to patella length) were obtained by three observers. Data were analyzed using paired t-tests and Pearson's correlation. A reliability assessment and inter-method agreements were performed. Patella alta was defined as an Insall-Salvati ratio > 1.2. Additional cutoffs of Insall-Salvati ratios > 1.3 and > 1.4 were also analyzed.

Results: There was no statistically significant difference between Insall-Salvati ratio as determined on MRI (mean: 1.20) and radiographs (mean: 1.25; P > 0.05). There was a strong correlation between Insall-Salvati ratio as determined on MRI and radiographs (Pearson's r = 0.6) with moderate consistency (Cronbach's alpha = 0.78). There was a good level of agreement between the diagnosis of patella alta on MRI and radiographs when defined as an Insall-Salvati ratio greater than 1.2 and 1.3 (Cohen's kappa = 0.61).

Conclusion: The results demonstrate a strong association between Insall-Salvati ratio and patella alta derived from MRI and radiographs in children ages 7.5 years and older.