Congenital patella luxation (MPL; LPL)
Medial patella luxation (MPL), Lateral patella luxation (LPL)
Coxa vara (bowleg), and excessive retroversion of the femoral head causes of MPL. Coxa valga (knock-kneed) cause LPL. Traumatic
patella luxation is not discussed here.
The patella is a type A (primary function is articulation) sesamoid bone located in the tendon of insertion of the quadraceps
muscles. The origins of the quadriceps muscles are the proximal femur and immediately cranial to the acetabulurn (rectus
femoris m.). The quadriceps m. follows a straight line, by necessity, to its insertion at the tibial crest. If the quadriceps
muscles are displaced medial, then the patella must also be displaced medially. Coxa vara, and to some extent excessive retroversion,
displace the origin of the quadriceps m. medially. In the 6 week old dog, altered quadriceps muscle pull causes permanent
bone changes in 2-4 weeks. The resulting deformities are bowing and torsion of the distal femur, bowing and torsion of the
proximal tibia due to compression from the medially located quadriceps. The trochlear groove is shallow because pressure
from the patella on the articular-epiphyseal growth plate is necessary to retard bone growth locally and form a trochlear
groove. Absence of the patella due to luxation results in failure of the trochlear groove to achieve a normal depth. Rotational
instability of the stifle occurs because of stretching of the lateral joint capsule and other lateral supporting structures.
The severity of MPL is progressive until about 6 months of age. Grade II - IV MPL in a juvenile dog can be expected to progress
and surgical correction should not be delayed. The grade of MPL is stable after 6 months of age. However, DJD will be progressive
and there is a report of 15% increased risk of cranial cruciate ligament rupture in dogs with MPL.
MPL is a very common juvenile joint disease which necessarily develops in dogs less than 6 months of age. MPL and LPL are
developmental diseases, not "congenital" and the conventional name indicates. However, dogs may not be presented until months
or years later when DJD or cruciate rupture become clinical. Predominantly small breeds of dogs are affected, although it
does occur in large dogs. Lateral patella luxation is a less common occurrence and typically occurs with coxa valga associated
with hip dysplasia in large breeds of dogs. Feline patella luxation is rare and occurs in association with bilateral congenital
hip luxation. There is no sex predisposition. A review of 124 cases of patella luxation reported that in small breeds 98%
are medial and 2% are lateral; in medium breeds 90% are medial and 10% lateral; large breeds have 80% medial and 20% lateral;
giant breeds 66% medial and 33% lateral.
Grade 1 and 2 MPL are often an incidental finding on orthopedic examination. History often reveals the dog carries his leg
for a few steps then returns to weight-bearing, without evidence of pain. Onset is insidious. Dogs with grades 3 and 4 patella
luxation have more severe and consistent lameness and deformity of the leg(s).