Jordy van Sambeeck

Patellar height measurements: Insall-Salvati ratio is most reliable method 2 37 Cut-off points were used to classify the ICC’s for intra- and inter-observer and inter-method reliability, but an ICC remains a qualitative measure. Regarding inter-observer and inter-method reliability it was hypothesized that an ICC of 0.70 could also be high enough to ensure good reliability. To quantify this, a Bland Altman analysis was performed for all measurements with an ICC of 0.70 or higher. Results are shown in table 4. The results of Bland Altman analyses for inter-method reliability of the IS measurement are not displayed in a table but were 0.084 (± 0.182) for CR versus CT, 0.059 (± 0.122) for CR versus MRI and 0.101 (± 0.209) for CT versus MRI. Discussion The most important finding of the present study was that the inter- and intra-rater reliability was good for the Insall-Salvati (IS) ratio on all imaging modalities and for the patellotrochlear index (PTI). Smith et al. [33] researched intra observer reliability for different patellar height measurements on CR and found the reliability of CD to be better than BP and IS. This is in contrast to the current study where the IS method had the best intra- observer reliability compared to CD, BP and MIS. This was not only the case for CR but for CT and MRI as well. Smith et al. [33] also propose that the intra-observer reliability of a measurement method may be related to experience, which is what this study showed as well. The ICC’s for intra-observer reliability of the different measurements conducted by the medical student were generally lower than those of the orthopedic resident, the orthopedic surgeon and the radiologist. Barnett et al. [3] in their study of different patellar height measurements on MRI found a good intra observer reliability for IS, BP, CD and PTI on MRI. The current study also showed good intra observer reliability for IS and PTI on MRI, however BP, CD and MIS showed poor reliability. Different observers may choose a different sagittal slice on different occasions which leads to a decreased intra- and interobserver reliability. Most authors report the use of the mid-sagittal slice to perform measurements [3, 23, 24], but in patellar instability patients this is rarely the slice with the maximal length of the patellar bone, tendon or cartilage. Due to the fact that the patella is often lateralized, this will lead to different interpretations of which slice is the most accurate to perform measurements, when for example on one slice the cartilage is thickest and on another the patellar length is highest. So differences in interpretation of cartilage thickness, patellar length and 3D configuration might give rise to different measurement results. Having more experience with these measurements will increase uniformity of the observer and as a consequence increase the intra-observer reliability. The results of the medical student, later excluded from the results, confirm that experience is needed for adequate measurements; these measurements cannot be done reliably by unschooled personnel.