Patellar height measurements: Insall-Salvati ratio is most reliable method.

Purpose: Patella alta is a risk factor for patellofemoral pain and instability. Several measurement methods and imaging modalities are in use to measure patellar height. The first aim of this study was to determine the intra- and interrater reliability of different patellar height measurement methods on conventional radiography (CR), CT and MRI. The second aim was to examine the applicability of patellar height measurement methods originally designed for CR on CT and MRI.

Methods: Forty-eight patients who were treated for patellar instability were included. All patients had undergone a pre-operative conventional radiograph, CT scan and MRI. Five methods for measuring patellar height were performed on radiographs, CT and MRI by four observers. For each measurement, the intra- and interrater reliability was determined by calculating the intra-class correlation coefficient (ICC). A Bland-Altman analysis was performed for measurements with an ICC ≥ 0.70.

Results: The Insall-Salvati (IS) ratio was the only measurement that showed good intra- and inter-observer reliability on CR, CT and MRI. The intra- and inter-observer reliability of the patellotrochlear index (PTI) for MRI was good to excellent for all observers. The IS ratio showed a moderate to good reliability for comparison of all three imaging modalities with the best agreement between radiography and MRI. The other patellar height measurements showed only poor to moderate inter-method agreement.

Conclusion: In this study, the Insall-Salvati ratio shows better intra- and inter-observer reliability than the Blackburne-Peel ratio, the Caton-Deschamps ratio and the modified Insall-Salvati ratio on all imaging modalities. Radiography and CT seem to have better reliability than MRI. The patellotrochlear index, however, shows good inter- and intra-observer reliability on MRI. Only for the IS method was there acceptable agreement between CR and MRI. This means that the established Insall-Salvati normal values could be used for MRI as well. This study shows that the most reliable method to measure patella height is the Insall-Salvati ratio measured on conventional radiographs or the patellotrochlear index on MRI.

Level Of Evidence: Level II diagnostic.