Comparative US-MRI evaluation of the Insall–Salvati index

Purpose: To investigate whether the universally accepted range of normal patellar height ratio derived from MRI for the Insall–Salvati (IS) method could be similarly applied to ultrasound (US). Materials and methods: This study included 52 patients (age range 11–75 years) who underwent a bi-modality (US and MRI) examination, with a total of 60 knees evaluated. IS index (ratio of the patella tendon length to length of the patella) was acquired with both methods. Two operators, with different experiences of musculoskeletal imaging and blinded to the results of other investigators, separately performed the MRI and US measurements. Results: For the two operators, MRI reported a mean value of patellar height ratio of 1.10 ± 0.16 (mean ± standard deviation SD), while US a mean value of 1.17 ± 0.16 (mean ± SD). For comparable results, the small addition of 0.16 is needed for the measurements on US compared with MRI. Inter-observer agreements using intra-class correlation coefficient (ICC) was, respectively, 0.97 for MRI and 0.98 for US. The difference of mean values in patellar height ratios between MRI and US was not statistically significant (p = 0.15). The ICC between the two modalities was 0.94. Conclusion: According to our experience, IS index can be appropriately evaluated on US images, reducing the need of other imaging techniques.