Patellar height measurements: Insall–Salvati ratio is most reliable method

Abstract

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. ResultsThe 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.

Is the Clinician's Eye a Valid and Reproducible Tool for Diagnosing Patella Alta on a Lateral Knee Radiography?

“…CE's interobserver agreement was also moderate, higher than mIS but inferior to IS and C-D. A possible explanation for the moderate interobserver agreement is that one of our observers is a first-year orthopaedic resident, with far less experience than the other two observers. Verhulst et al 15 reported a similar variability of observers but eliminated the less experienced observer from the statistical analysis. Smith et al 14 assessed intraobserver reliability for different patellar height radiograph measurements and found the reliability of C-D to be better than Blackburn-Peel (BP) and IS.…”

“…Recent studies 2 , 17 describe good interobserver and intraobserver reproducibility for IS, BP, C-D, and patellar/trochlear index on MRI. Verhulst et al 15 recently measured patellar height on radiographs, CT, and MRI in 48 patients who were treated for patellar instability. This study, similar to the current study's results, shows that the most reproducibility method to measure patellar height is the IS ratio measured on conventional radiographs and the patella-trochlear index on MRI.…”

“…A possible limitation of the current study is the variable angles of flexion of the analyzed x-rays, which could lead to imperfect measurements. Most studies report this as a limitation, 2 , 4 , 6 , 9 , 15 , 16 but in current clinical practice, most routine lateral radiographs of the knee are performed within a range of knee flexion. For example, Seyahi et al 18 reported that the lateral knee radiographs vary in their institution between 20° and 45°.…”

Objective assessment of patellar maltracking with 3 T dynamic magnetic resonance imaging: feasibility of a robust and reliable measuring technique

“…Radiological parameters of anatomical risk factors were assessed on static transverse and sagittal MRI images. Patella height was determined using the Caton-Deschamps Index (CDI) on sagittal MRI images, with the relaxed knee placed in 20° of flexion, using a standard MRI knee coil 14,32 . www.nature.com/scientificreports/ The distance between the tibial tubercle and the trochlear groove (TT-TG) was measured in accordance with Schoettle et al 9 , with the knee placed in the same position.…”

The effect of patellar facet angle on patellofemoral alignment and arthritis progression in posterior-stabilized total knee arthroplasty without patellar resurfacing

“…An angle smaller than 180°means varus alignment and greater than 180°means valgus alignment [17]. The Insall-Salvati ratio is defined as the ratio of the length of the patellar tendon to the maximum length of the patella [18,19]. The patellar facet angle was evaluated on the Merchant view [9].…”