Function of the MPFL
The MPFL acts as a passive restraint, helping to keep the patella properly aligned. When it is damaged, the patella is free to sublux or even dislocate to the outer side. When reconstructing the ligament, it is important that it is correctly placed, or else it will be under too much tension at certain positions within the movement range.
Origin and insertion of the medial patellofemoral ligament: a systematic review of anatomy. Aframian A, Smith TO, Tennent D, Cobb JP and Hing CB. Knee Surg Sports Traumatol Arthrosc. 2017; 25(12): 3755–3772.