Patella is the medical term for the kneecap. The medical term "patellar luxation" refers to "dislocation of the kneecap". The kneecap is simply a small bone located on the underside of the tendon of the quadriceps muscle.
The condition of patellar luxation is quite common, particularly the congenital form (i.e. that form present from birth) in which both hind legs are often affected (20-25% of cases).
The direction in which the kneecap slips may vary, but mostly (in 75-80% of cases) it is towards the inner (medial) side of the leg.
There is also the much less common acquired form of kneecap dislocation resulting from a direct injury to the stifle ("knee") joint.
Because the forces causing or perpetuating the kneecap dislocation vary, there is a range of surgical procedures used to correct the problem. The original deformities causing the abnormal forces may be found in the hip bone (pelvis), the thigh bone (femur) or the shin bone (tibia).
The effects of the kneecap dislocation on the knee joint of the dog may be short term and/or long term.
In the short term there may be lameness manifest as not putting the foot to the ground and with the leg locked in a flexed position. Sometimes an acute lameness is accompanied by obvious sharp pain seen as yelping or screaming.
A few dogs will carry the affected leg most of the time. Some dogs are reluctant to jump. Some dogs stretch their legs out backwards in an effort to relocate the kneecap.
In the longer term there may be arthritic soreness (manifest as weakness and a dull ache) and secondary rupture of other ligaments.
Dogs that are either born with the condition or develop it at an early age often show minimal lameness until arthritis is the major sign. These dogs may have grinding or clicking sensations in the joint but are not lame, probably because they have become desensitised. An owner may actually hear the click whilst the dog is walking, or may see the kneecap popping from side to side. But when these dogs are older the legs are typically bowed and arthritic, and the gait appears stiff and awkward. The knees are also more liable to develop a secondary rupture of the cruciate ligaments. Sometimes there can be stress on the hip joint leading to arthritis or dislocation there also.
Four grades of severity are described (number 4 being the worst), but the distinctions between grades are not always clear.
When determining the best surgical treatment, each affected knee is assessed individually according to the various deformities that are causing the kneecap to dislocate.
The most common treatment is a deepening of the groove in which the kneecap glides combined with a tightening the supportive soft tissues on one side of the joint (that which is opposite the dislocation).
Sometimes a small piece of bone, the tibial crest, needs to be rotated or moved to straighten the patellar tendon. Very rarely the femur needs to be cut and straightened. As a last resort the kneecap may need to be removed completely.
The success of surgery in preventing dislocation of the kneecap has been estimated at 60-70%. The success in reducing lameness, however, is higher. The overall success rate is improving as techniques are being better refined.
Surgery is indicated in dogs showing signs of lameness. It is also indicated in some non-lame dogs: those puppies in which severe dislocation is detected, and those dogs which belong to the large breeds. In young puppies surgery is performed early (at 3-4 months of age) so as to realign the kneecap during growth and so reduce the overall deformity of the limb and prevent irreparable contracture of the quadriceps muscle.
In large breed dogs, the early prevention of arthritis is more important than in smaller breeds. Whilst these recommendations for surgery in non-lame dogs may be quite valid, they are not often taken up by owners because there is no problem apparent to the owner at that time.
If dogs are not operated on they should at least be kept from excessive stresses on their joints by preventing them from becoming overweight.
Needless to say, dogs with the hereditary form of the condition should not be used for breeding.