Magnetic Resonance Imaging–Based Topographical Differences Between Control and Recurrent Patellofemoral Instability Patients

Background: Plain films and computed tomography (CT)–based imaging were the first to establish measurements that evaluated patellar instability. Limited research has shown the efficacy of magnetic resonance imaging (MRI) in evaluating these established measurements. Purpose: To identify morphological differences between normal knees and those with patellofemoral instability on MRI to determine what measurements are significant and how MRI-based means differ from historical means based on radiograph and CT imaging. Study Design: Case control study; Level of evidence, 3. Methods: Eighty-one controls and 40 patients with recurrent patellar instability between 2006 and 2010 were reviewed. The control patients had a history and an examination negative for patellofemoral symptoms. Patients with patellar instability had a history of at least 2 frank patellofemoral joint dislocations (PFJDs). The MRI images were obtained on the nonweightbearing knee in full extension. Measurements of patellar tilt, trochlear morphologic characteristics, and tibial tuberosity–trochlear groove (TTTG) distance were evaluated on axial slices, and patellar height was measured on sagittal images. Trochlear shape was assessed at the proximal and distal trochlea. Results: All measurements of patellar tilt (mean ± SD) were found to be significantly different between the 2 groups. For patellar height, the Insall-Salvati ratio (control, 1.08 ± 0.02; PFJD, 1.26 ± 0.03) and Caton-Deschamps ratio (control, 1.13 ± 0.02; PFJD, 1.29 ± 0.03) proved to be significantly different. Trochlear morphologic characteristics had numerous measurements prove to be significantly different proximally and distally. These included classic measurements such as sulcus angle (control, 148.48° ± 0.94°; PFJD, 165.57° ± 2.65°) and lateral trochlear inclination (control, 21.27° ± 0.66°; PFJD, 13.31° ± 1.36°) proximally and less established measurements such as the ratio of external (lateral) trochlea to internal (medial) trochlea (control, 1.51 ± 0.05; PFJD, 2.11 ± 0.17), a measurement of facet asymmetry. Conclusion: The MRI-based patellar tilt measures proved to be an excellent group of measurements for delineating between controls and those with instability. Patella alta ratios, such as Insall-Salvati and Caton-Deschamps, demonstrated a statistically significant difference between normal and recurrent dislocators. Trochlear measurements proved significantly different at the proximal and distal trochlea. Our findings demonstrate that MRI is appropriate to help discern recognized pathologic abnormalities that characterize patellofemoral instability.