Patellar Articular Overlap on MRI Is a Simple Alternative to Conventional Measurements of Patellar Height

BACKGROUND Patella alta describes an abnormally high-riding patella in relationship to the femur and has been shown to correlate with patellofemoral pain, instability, chondromalacia, and arthrosis. Conventional measurements of patella alta involve multiple measurements and are often not defined on cross-sectional imaging as related to radiographs. HYPOTHESIS Patellar articular overlap on sagittal magnetic resonance imaging (MRI) will correlate well with conventional measurements of patella alta as measured by a standardized technique defined by our group. STUDY DESIGN Cohort study (diagnosis); Level of evidence, 2. METHODS MRIs of 239 knees were reviewed by 3 attending surgeons with practices focusing on patellofemoral disease, as well as 2 sports medicine fellows and 1 musculoskeletal radiologist. Measurements included articular overlap, percentage of articular coverage, Caton-Deschamps index, Blackburne-Peel index, and modified Insall-Salvati index. RESULTS Interrater reliability was high for Caton-Deschamps, Blackburne-Peel, and modified Insall-Salvati indices (intraclass correlation coefficient [ICC], 0.877, 0.828, and 0.787, respectively). Articular overlap and percentage articular coverage correlated well with each other (ICC, 0.961; P < .001) and with the Caton-Deschamps (overlap r = -0.271, P < .001; coverage r = -0.131, P = .037) and Blackburne-Peel (overlap r = 0.343, P < .001; coverage r = -0.238, P < .001) indices. Articular overlap and percentage coverage failed to correlate with the modified Insall-Salvati index (overlap r = -0.117, P = .091; coverage r = 0.007, P = .918). CONCLUSION Patellar articular overlap and percentage of patellar articular coverage show promise as a simpler alternative to conventional, ratio-based measurements of patellar height. Future studies are needed to evaluate the range of normal and the relationship to our traditionally used measurements.