Luxating patella (or Patellar luxation)

Luxating patella, or trick knee, is a condition in which the patella, or kneecap, dislocates or moves out of its normal location. The luxation is usually medial, but can be lateral. It can be caused by some form of blunt trauma, or may be a congenital defect. In congenital cases, it is usually bilateral. Patellar luxation is a common condition in dogs, particularly small and miniature breeds. The condition usually becomes evident between the ages of 4 to 6 months. It also occurs in cats, especially the Domestic Shorthair. Diagnosis is made through palpation of the knee. X-rays are necessary in some cases. The luxating patella often causes no or very slight symptoms. There may be intermittent limping in the rear leg. Osteoarthritis can develop secondarily.

There are four diagnostic grades of patellar luxation, each more severe than the previous:

Grade I - the patella can be manually luxated but is reduced (returns to the normal position) when released;
Grade II - the patella can be manually luxated or it can spontaneously luxate with flexion of the stifle joint. The patella remains luxated until it is manually reduced or when the animal extends the joint and de rotates the tibia in the opposite direction of luxation;
Grade III - the patella remains luxated most of the time but can be manually reduced with the stifle joint in extension. Flexion and extension of the stifle results in reluxation of the patella;
Grade IV - the patella is permanently luxated and cannot be manually repositioned. There may be up to 90¼ of rotation of the proximal tibial plateau. The femoral trochlear groove is shallow or absent, and there is displacement of the quadriceps muscle group in the direction of luxation.
Grades III and IV, as well as most grade II cases, require surgery to correct, if the animal has difficulty walking. The surgery involves a sulcoplasty, a deepening of the trochlear sulcus that the patella sits in.

Additional help can be given with the use of pet ramps, stairs, or steps. These can help the animal travel from one place to another, especially up and down, without adding any pain or damage to the patella.