Patellar luxation is usually a congenital condition in which the kneecap, or patella, dislocates outside of its normal trochlear groove. This dislocation, clinically referred to as luxation, can occur on either the medial, or inside surface, or the lateral, or outside surface, of the knee. There are varying degrees of patellar luxation that are graded depending on whether the patella is intermittently or constantly luxated. This abnormal displacement of the kneecap results in pain, cartilage damage, and arthritis. There are varying degrees of severity of this disease, and surgery may be needed. This condition is common occurs on cats, dogs and humans. On dogs, this condition typically affects small and miniature breeds such as the poodle, Pomeranian, Chihuahua, Schipperke, Bichon Frise, and pug. It is also seen in the cocker spaniel, golden retriever, Labrador retriever, and mixed breeds. On the rare occasions, this condition on larger dog breeds, the kneecap is as likely to move to the outside (lateral) side of the legs as to the inside.

Clinical signs of medial patellar luxation are lameness that is often intermittent, and may be unilateral or bilateral; thick, swollen stifles; pain on range-of-motion; crepitus; palpable luxation; inability to jump or walk normally; medial displacement of quadriceps muscle group; lateral bowing of the distal third of the femur. Common symptoms are intermittent or consistent lameness; bowlegged stance; reluctance to walk or jump; occasionally holding a rear leg out to the side when walking.

Medial patellar luxation, or MPL, is a very common disease of small or toy and miniature breeds in which the kneecap occasionally rides on the inside of its normal groove. Primarily congenital, although occasionally acquired through trauma, MPL causes lameness in one or both rear limbs. The degree of lameness is determined by the severity and duration of the disease, as well as the extent of existing arthritis. Patellar luxation is graded on a scale from I to IV, with IV being the most severe. The disease can progress from the less severe to more severe grades over time. The more severe forms are often accompanied by malformation of the femur and tibia, as well as varying amounts of arthritis.

Some veterinarians and medical experts can identify dogs with these condition as early as eight weeks of age. They explain that the problem is a genetic defects so they are not advisable to be bred. On treating this condition, when the problem occurs only due to my manipulation or only occasionally on its own, no surgery is necessary and no medicines need to be administered. When the knee locks up frequently or the dog exhibits pain it its knee surgery is required. There are a number of surgical techniques that attempt to fix this problem. Some veterinarians relocate the patellar ligament and a small portion of bone (the tibial tuberosity) where it attaches to the tibia or shinbone. Others remove a portion of the medial patellar ligament and reinforce the lateral patellar ligament with suture. Often, the groove in which the patella rides is deepened. Because it is impossible to cleans and maintain an animal as aseptically as a human being, the vet begin all orthopedic cases on a broad-spectrum antibiotic several days prior to surgery. They continue this medicine for a week following the surgery. The affected leg or legs are bandaged for three days following the surgery and the pet is limited to short leashed walks for an additional two weeks. Because the surgery is relatively straight forward, few post surgical complications occur. Vet will try to do both legs at the same time even if the current problem is confined to a single leg. This is because left unattended, problems with the lesser-affected knee or sub clinical problems will lead to arthritis of that knee.