The Medial Patellofemoral Ligament of the Knee
The medial patellofemoral ligament (MPFL) is one of several ligaments on the medial aspect of the knee. The MPFL originates between the medial epicondyle of the femur and the adductor tubercle (just anterior and distal to the adductor tubercle and superior to the superficial medial collateral ligament). The MPFL inserts into the superomedial aspect of the patella.
On an x-ray, there is a radiographic point that is reproducible, which is about 1mm anterior to the posterior cortex extension line. This point is also about 2.5 mm distal to the posterior margin of the medial femoral condyle. The point of the ligament will be distal to the posterior margin of the medial femoral condyle, and is proximal to the level of the posterior point of the Blumensaat line on a lateral x-ray. The easiest way to find that point is just anterior to the posterior cortex of the femur and proximal to the Blumensaat line on a lateral x-ray.
The medial patellofemoral ligament helps to keep the patella centered along the front of the knee within the patella femoral groove. The MPFL provides restraint to any movement towards the outside of the knee (lateral patellar subluxation). The MPFL resists lateral migration of the patella. It provides about 50–80% of the restraining force to lateral patellar dislocation. The MPFL is most effective between 0–30° of flexion. The trochlea becomes the primary restraint with any additional flexion.
Rupture of the medial patellofemoral ligament may allow lateral dislocation of the patella. The MPFL usually ruptures from its femoral origin. There will be pain and tenderness along the medial retinaculum and this needs to be differentiated from an MCL tear. Traumatic injury or laxity of the MPFL can cause future patellar instability. Rupture of the MPFL and dislocation of the patella may cause direct impact injury to the medial patella and to the lateral femoral condyle. Lateral dislocation or subluxation of the patella can also be seen in the “sunrise” view. Tearing of the Vastus Medialis Obliquus (VMO) muscle and anterior tearing of the medial retinaculum may occur in association with lateral dislocation of the patella.
An osteochondral fracture of the lateral femoral condyle and fracture of the medial patellar facet may be seen in association with patellar dislocation. An MRI may be useful to confirm the tear of the ligament and the presence of associated classic bony contusions.