If you are the owner of a small breed dog that limps or even occasionally holds up a back leg, you should call your veterinarian and have them perform a physical exam. If an MPL is diagnosed, your veterinarian can prescribe the best treatment options for your pet. Medial patella luxation (MPL) is a condition in which the patella (kneecap) no longer glides within its natural groove (sulcus) in the femur, the upper bone of the knee joint. It becomes displaced to the inside of the joint and can be partial or complete, intermittent or permanent.