Medial patella luxation ( MPL ) is a common developmental orthopaedic condition of the canine patient . Small breed dogs are more commonly affected by the condition than large breed dogs . Patella luxation also occurs in large breeds dogs with medial luxation occurring more commonly than lateral luxation . Medial patella luxation can also be seen in cats the surgical management is similar to that of dogs .
Medial patella luxation is easily detected clinically by palpating the patella with the leg in extension and feeling the patella luxating medially to the trochlear groove . These patients are usually young , under a year of age .
Radiographs are generally used to rule out any other conditions which may be present . It is essential to radiograph these patients for 2 reasons . Firstly , in middle age patients it is less likely for the patella to be clinically significant and the clinician should make sure there is no other condition such as cranial cruciate ligament disease . The other reason for radiographs is that the patella luxation may be the result of more complicated conformational conditions . These patients will usually have associated musculoskeletal abnormalities , including medial displacement of the quadriceps muscle group , distal femoral varus ( Fig 1 ), hypoplasia of the medial condyle and rotation tibial deformity .
Clinically these patients have been graded as to the degree of patella luxation through the full range of motion ( Table 1 ). This grading system takes into account the effects of the other components of the condition by their effect on the patella . The grading system does not how-
ever quantify the underlying components of the condition which may have lead to the patella luxation . The big concern is that a grade 2 or 3 luxation can be present in a patient with a marked femoral varus and in another patient with only a mild femoral varus . This grading does not thus allow us to accurately determine the best way forward in regards to surgical treatment of each individual patient . Grade 4 patients mostly have moderate to severe bony deformities that need correcting .
There are many suggestions put forward to explain how MPL occurs . Some authors suggest that the pathogenesis differs in small breed and large breed dogs . A reasonable suggestion is that MPL develops from a decreased angle of inclination of the femoral neck , coxa vara . This leads to marked angular deformities of the distal femur from bowing of the distal femur , genu varum . This causes a relative tibial varus and internal rotation of the tibia on the femur . The patella is then forced medially due to the pull of the medial thigh muscles and hypoplasia of the medial vet360