Medial patellofemoral ligament anatomy: is it a predisposing factor for lateral patellar dislocation?

Purpose Because the medial patellofemoral ligament (MPFL) is the primary restraint to lateral dislocation of the patella, we aimed, in this controlled study, to verify whether the MPFL with different measurements could be considered another predisposing factor for patellar dislocation. Methods A group of 100 consecutive individuals without the criteria for patellar dislocation (trochlear dysplasia, patella alta and lateral patellar tilt) was recruited as a control group and underwent magnetic resonance imaging (MRI) study and another group of 50 patients with patellar instability. Femoral condyles, interepicondylar distance, length and thickness of the MPFL were measured. Results In the control group, the MPFL was 38-60 mm long. Individuals with patellar instability who had no episode of patellar dislocation had a 4.11-mm longer ligament than controls (p = 0.032), while patients with instability with a previous history of dislocation had a 13.54-mm longer MPFL than controls (p < 0.001). Thickness of the MPFL at the patellar insertion was lower in individuals with patellar instability with a history of dislocation (p < 0.001). An instability coefficient (IC) less than 1.3 indicates that the MPFL is insufficient. Conclusions Individuals with patellar instability and previous patellar dislocation present with longer MPFL when compared to controls, and an associated IC less than 1.3 can be considered a predisposing factor for patellar dislocation. Treatment of patellar instability is a challenge, and it is difficult to identify what is the predisposing factor. This study has verified the measurements of the MPFL for the first time and presents values of thickness and length that can be considered as indications for surgical reconstruction. Level of evidence: III.