What Are Characteristic MRI Findings After Lateral Patellar Dislocation? Magnetic Resonance Imaging Characteristics of the Medial Patellofemoral Ligament Lesion in Acute Lateral Patellar Dislocations Considering Trochlear Dysplasia, Patella Alta, and Tibial Tuberosity–Trochlear Groove Distance

Background: There are a number of different factors that are thought to play a role in patellar instability, including injuries to the medial patellofemoral ligament (MPFL), trochlear dysplasia, patella alta, and an increased tibial tuberosity-trochlear groove (TT-TG) distance. Objective: To use MRI to assess injury patterns to the MPFL, trochlear dysplasia, patellar height, and TT-TG distance in patients suffering an acute lateral patellar dislocation (LPD), and to determine how these factors interact. Design: Diagnostic case-control study. Level of Evidence IV. Methods: MRIs were assessed for radiographic evidence of an acute LPD. Medical records of 73 patients were reviewed to ensure that they had an acute patellar dislocation. Patients' MRIs were compared with those of 73 ageand gender-matched controls who had evidence of prior patellar dislocation or medial collateral ligament injury. MPFL disruption was evaluated on axial images for the femoral region, midsubstance, and patellar insertion. Patellar height was assessed using the Insall-Salvati ratio on sagittal images. Trochlear dysplasia was assessed on axial images. In addition, the TT-TG ratio was assessed. Results: The authors found that 72 patients (98.6%) with a patellar dislocation had an injury to the MPFL. There were different sites of injury to the MPFL: femoral attachment, 50.0%; midsubstance, 13.9%; patellar insertion, 13.9%; combination insertion and patella, 22.2%; and combination MPFL and midsubstance, 4.2%. The authors looked at the subgroups of MPFL injury sites for statistical analysis. There was a complete MPFL rupture in 51.4% of cases. Complete tears were localized more often at the femoral insertion site. There was not a statistically significant correlation between complete and partial tears with respect to trochlear dysplasia, TT-TG distance, and trochlear asymmetry. The mean Insall-Salvati ratio was significantly higher in the study group than in the control group (1.21 vs 1.04). In the control group, 69.9% of patients had normal trochlear anatomy; in the study group, 74.0% had some form of trochlear dysplasia. TT-TG distance in the study group was statistically significantly higher than in the control group (12.75 vs 10.66). The authors found no difference between first-time and recurrent dislocators with respect to the parameters studied. Conclusions: Patellar dislocation and MPFL injury depend on a variety of factors. Reviewer's Comments: This very detailed article shows that there is an interplay between patellar dislocation and a variety of other factors, including patella alta, trochlear dysplasia, and TT-TG distance. This raises the question of whether isolated MPFL reconstruction will optimally prevent recurrent instability. There may have been a selection bias in the study given its retrospective nature. It is not clear if all patients with a patellar dislocation seen at this clinic underwent MRI. (Reviewer-Nathaniel P. Cohen, MD).