A New Assessment of Patellar Instability Using Coronal Magnetic Resonance Images of the Patella Superimposed on the Femur and Its Clinical Utility

Objective This study aimed to evaluate the clinical utility of a new assessment for patellar subluxation using magnetic resonance (MR) images of the patella superimposed on the femur and to determine its correlation with the tibial tuberosity–trochlear groove (TT-TG) distance. Materials and Methods This retrospective study included 81 men and 44 women who underwent MR imaging at our institution. There were 19 cases of patellar dislocation. The control group consisted of 116 patients. An assessment of the distance of subluxation was carried out using coronal superimposed MR images. The results were correlated with the TT-TG distance calculated using the axial combined MR images. The sensitivity and specificity of various distances of subluxation were evaluated as diagnostic thresholds for patellar dislocation, where the clinical diagnosis was used as the standard of reference. Results The use of the distance of subluxation on coronal superimposed MR images showed discrete difference between patellar dislocation and controls. The mean distance of subluxation was approximately 3 times larger for patients with patellar dislocation compared with controls (P < 0.001). Likewise, the mean TT-TG distance was approximately 50% greater for patients with patellar dislocation compared with controls (P < 0.001). However, there was no correlation between the distance of subluxation and the TT-TG distance. The best diagnostic discrimination was achieved at the 7-mm threshold of subluxation. Conclusions The use of the distance of subluxation on coronal superimposed MR images can be a useful method to evaluate patellar dislocation. The best diagnostic discrimination was achieved at a threshold of 7 mm.