Patella is kneecap while luxation is dislocation. Patellar luxation refers to a condition in which kneecap or patella of a dog dislocates or moves out of its normal anatomic position. Also known as trick knee, subluxation of patella, floating patella, or floating kneecap, the condition occurs when the kneecap slides out of its normal position. It is mostly a congienital problem caused by a defect in hind limb conformation although genetics and trauma are also associated with this common joint disorder in dogs.
The knee is a complex structure consisting of muscles, ligaments, tendons, cartilage, and bones. These components must align properly and interact harmoniously in order to function properly. Three bones are included in the knee: the femur, the tibia, and the patella (kneecap). In patellar luxation, the kneecap does not align properly with the femur and tibia. The condition can be temporary or permanent and range from complete dislocation to mild patellar instability. The dislocation can occur laterally (toward the outside of the knee joint), medially (toward the inside), or in both directions.
A dog's kneecaps are an important component of a normally-functioning knee joint. These kneecaps (= patella) are meant to ride in a groove on the face of the femur. The patella acts as a pulley, giving leverage to extend the knee as the dog walks. Once the kneecap is dislocated in the groove of thigh bone , it can return to its normal position only when the quadriceps muscles in the hind legs of the dog relax and lengthen. That is why most dogs with this condition hold up their hind legs for a short time. The condition can progress to become more frequent or even permanent due to the erosion of the articular cartilage and the development of secondary arthritis, or because of rupture of the cranial cruciate ligament.
There are 4 types of patella luxations.
Grade 1 – The patella is positioned normally but can be luxated with slight manual pressure.
Grade 2 – Spontaneous luxation occurs; however, it can reduces spontaneously or can be replaced manually.
Grade 3 – The patella is luxated most of the time; however it can be replaced manually.
Grade 4 – The patella cannot be reduced manually.
The most common being the medial patella luxation which frequently occurs in miniature and toy breeds including miniature and toy Poodles, Maltese, Jack Russell Terriers, Yorkshire Terriers, Pomeranians, Pekingese, patterdale terrier, Chihuahuas, Cavalier King Charles Spaniels, Papillons, Boston Terriers, Plummer Terriers and Teddy Roosevelt Terriers. The condition usually becomes evident between the ages of 4 to 6 months. Some large breeds such as labrador retrievers are also predisposed to this condition. The condition usually becomes evident between the ages of 4 to 6 months. In older dogs, patella luxation appears suddenly and is often the result of a traumatic injury.
Although, clinical signs associated with patellar luxation vary greatly with the severity of the disease, the initial symptoms include occasional limping, an intermittent skip in the gait, sudden loss of support on the limb, abnormal sitting posture with the knee placed outward, intermittent or consistent lameness, an abnormally bowlegged stance, a disinclination to run and jump or a tendency to hold a rear leg out to the side when walking .