Overview

Dog, cats and humans are affected. Patellar luxation (PL) is usually a congenital condition in which the kneecap, or patella, dislocates outside of its normal groove. 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 may results in pain, cartilage damage, and arthritis, but also may not cause any pain or distress depending on the grade of luxation. There are varying degrees of severity of this disease, and surgery may or may not be needed.

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. Dogs are frequently presented to a veterinarian for intermittent lameness, often because it is becoming more frequent or severe. In many cases however patellar luxation is diagnosed as an incidental finding during routine examination.

When the patella, or kneecap, pops out of its normal groove, the dog will struggle to bear weight , and owners may report a hitch in the gait. The dog will frequently extend the knee out from the body in order to get the patella to pop back in to the groove. This is usually a mechanical lameness and often no pain is present. As PL progresses, the structures that hold the patella in place become looser, and thus the problem becomes more frequent. with time the shearing forces acting on the cartilage result in damage, pain and later in arthritis.

Unfortunately, many of the severe Grade III or IV cases go unnoticed for months or years because the affected animals are usually small breeds that are often carried much of the time by their owners. Their inability to jump or straighten out their hind legs may go unnoticed because of their small size and sedentary lifestyle. The larger breeds tend not to cope so well with this condition and surgical intervention is more often required.Many of the milder grade I & II may cause no clinical problems.

It is much more common for the kneecap to ride on the inside than on the outside surface of the knee. This condition typically affects small and miniature breeds such as the poodle, Pomeranian, Chihuahua, Schipperke, Bichon frise, Yorkshire terrier and pug. It is also seen in the cocker spaniel, Staffordshire bull terriers, golden retriever, Labrador retriever,  and mixed breeds.

Outside dislocation of the kneecap, or lateral patellar luxation, occurs more often in large breeds such as the Great Dane, Saint Bernard, and Bernese mountain dog. Lateral luxation is often accompanied by malformation of the femur, the large bone in the thigh. In these cases the prognosis is more uncertain, as major limb reconstructive surgery may be needed. Generally, the prognosis is better when corrective surgery is performed early in the course of the disease; many of the malformations in the bones and joint occur over time and can be prevented with early correction.

Clinical signs

Lameness that is often intermittent, and may be unilateral or bilateral. The degree of lameness is determined by the severity and duration of the disease, as well as the extent of existing arthritis.  thick, swollen stifles, palpable luxation; inability to jump or walk normally; bowing of the distal third of the femur. Some will not show any symptoms until later in life.

Diagnosis

The examining veterinarian will often make a diagnosis from a physical examination and history. However, x-rays are needed to determine the degree of arthritis, and evaluate for any malformation of the femur and tibia, the two major bones in the leg, which are joined together at the knee.

Treatment

Conservative treatment is based on nutritional supplements, medications and exercise programs such as hydrotherapy and exercise control. Special diets like hills j/d diet and other medicated diets form an integral part of the management of this condition. This treatment however will not correct the anatomical problem that result in the luxation of the knee cap. It will only ease the discomfort that may be associated with this condition.

Surgical treatment involves replacing the kneecap into the groove, and preventing it from popping in and out. The following procedures can be used alone or in combination as necessary to maintain the proper function of the knee.

Imbrication

Tightening the joint capsule, known as imbrication, is done on the opposite side of the luxation to prevent the kneecap from having enough slack to pop out of the trochlear groove. Thus a medial patellar luxation is treated with a lateral imbrication, and vice-versa. Additionally, the joint capsule can be loosened on the side of the luxation; this is called a release incision. This procedure relieves the tension that the joint capsule is placing on the patella, thus allowing it to ride in the groove (trochlea).

Trochleoplasty
Deepening of the trochlear groove, or trochleoplasty, can be accomplished with a variety of techniques. A chondroplasty technique involves cutting out a wedge or a block of cartilage, removing a small portion of bone beneath it, and then replacing the cartilage The result is a deeper groove

Tibial Tuberosity Transposition

The kneecap attaches to the lower leg via its patellar tendon at a bony site called the tibial tuberosity. Many times this site forms abnormally on the inside, as with MPL, or on the outside, as with LPL. In this procedure, the surgeon moves the tibial tuberosity back into proper alignment and secures it in place with an orthopaedic pin &/or wire. Realigning the joint, kneecap, and tendon prevents dislocation from reoccurring.

Corrective Osteotomy

In severe cases, with malformation of the tibia or femur, corrective bone cuts known as corrective osteotomies may be required.

Prognosis

The prognosis for a Grade I patellar luxation is very good  These dogs may not need surgery. However close observation for signs of worsening is important. If surgery is indicated and performed early on, most animals regain normal functionality. The prognosis for Grade IV patellar luxation is guarded. Most of these animals have moderate to severe bony malformations and significant arthritis. If correction is performed, it is important to initiate early physical therapy to help restore function.

The prognosis for Grades II and III depends on how much arthritis and malformation have occurred. If caught and treated early, both have a good to excellent prognosis. If there is significant bony malformation or arthritis, the prognosis is guarded to fair. the decision for surgery is not based only on the grade of luxation but the breed, the age, the character and life style of the dog and family.

Prevention

Early detection and correction is the best way to prevent severe lameness and dysfunction. Breeding affected animals should be discouraged; however, the disease is so prevalent in some breeds that this may not be practical.