Patellar Height Measurements on Radiograph and Magnetic Resonance Imaging in Patellar Instability and Control Patients

Insall-Salvati (IS) ratio, modified IS ratio (mIS), Caton-Deschamps (CD) index, and Blackburne-Peel (BP) index are patellar height measurements commonly made on lateral radiographs (XRs) in patients with patellofemoral instability. More recently, magnetic resonance imaging (MRI) has been used as an additional imaging modality to measure patellar height. However, different imaging modalities have not been previously compared for all of these measurements. First, to compare the patellar height measurements of IS, mIS, CD, and BP on XR and MRI in first-time lateral patellar dislocation (LPD) patients and a control population and, second, to evaluate for the effect of sex and skeletal maturity. All first-time LPD patients with both MRI and XR of the knee within 6 weeks of injury from 2008 to 2012 were included in the LPD group (n = 112). Patients without patellofemoral instability who underwent both MRI and XR of the knee served as controls (n = 129). Two independent observers measured patellar height. Mean values were compared among the radiographic modalities, sex, and skeletal maturity status. Interobserver reliability was assessed using intraclass correlation coefficients (ICCs). Patellar height in the LPD group measured greater on MRI than on XR for all four patellar height ratios (IS: 0.11; mIS: 0.07; CD: 0.18; BP: 0.08). In controls, MRI also demonstrated higher readings than XR on two indices (CD: 0.17; BP: 0.16). Skeletally immature patients measured higher than skeletally mature patients in the majority of the measurements by 0.12 to 0.20 on MRI and XR; the majority of the measurements showed no sex differences. ICC was excellent for IS on both MRI (ICC = 0.830) and XR (ICC = 0.849) and for CD on XR (ICC = 0.786). Patellar height measured greater on MRI than XR by approximately 0.10 across four patellar height measurement indices. Skeletally immature patients measured higher than skeletally mature on both MRI and XR, but sex did not have an effect. This is a retrospective cohort study and its level of evidence is 3.