Disrupted medial parapatellar triangle as an MRI finding of chronic patellofemoral instability.

BackgroundAcute findings of patellar dislocation in magnetic resonance imaging (MRI) can diminish with time; therefore, additional findings may help diagnosis of chronic patellofemoral instability.PurposeTo define a previously undescribed MRI finding-medial parapatellar triangle (MPT)-and to examine the diagnostic value of the disruption of MPT in chronic patellofemoral instability.Material and MethodsOur study was performed on 24 knees that underwent medial patellofemoral ligament reconstruction for chronic patellofemoral instability and 48 knees of patients with similar age and sex as control group. MPT was defined as an acute-angled triangle delineated by patellar cartilage, medial retinaculum, and medial femoral condyle in the section passing through the upper third of the patella on axial proton-density weighted with fat saturation (PD fat-sat) MRI scans. Disruption of MPT was assessed by two authors blinded to the diagnosis. Inter- and intra-observer reliability was assessed using Cohen's κ test. Sensitivity, specificity, positive predictive value (PPV), and negative predictive value (NPV) were calculated for each observer and observation.ResultsIn the study group, the triangular structure was disrupted in 18 knees for the first observer's first observation, and 17 knees for the second observation; for the second observer, it was 15 knees and 17 knees, respectively. In the control group, the numbers were 2, 6, 1, and 3, respectively. Mean Cohen's κ statistic was 0.81 (range=0.73-0.87). Sensitivity, specificity, PPV, and NPV were 66.7%-75%, 87.5%-97.9%, 72.7%-94.4%, and 84%-88.5%. respectively.ConclusionDisruption of MPT is an additional finding that may help the diagnosis of chronic patellofemoral instability.