Medial Patellar Luxation (also known as Kneecap Luxation and Slipped Stifles) may be congenital or acquired. The congenital form is most common in the toy and miniature breeds, and may occur simultaneously with other pelvic limb deformities. Most researchers believe luxated patellas to be heritable (inherited) as well, though the exact mode of inheritance is not known.
When patellar luxations are present early in life, the major muscle groups of the thigh pull toward the inside of the leg, putting abnormal pressure on the knee joint cartilage. The result is a bowlegged stance and an abnormal pull on the patella.
When the patella is in its normal position, its cartilage surface glides smoothly and painlessly along the cartilage surface of the trochlear groove with little or no discomfort. As the patella "pops out" of its groove these cartilage surfaces improperly rub each other. The dog may cry out and try to straighten the leg to "pop it back in" or may hold the limb up until muscle relaxation allows the kneecap to reposition itself. There is little or no discomfort until the cartilage is effectively "rubbed off" or eroded to a point where bone touches bone. From this point on, each time the patella "pops out" into its abnormal, luxated position it will cause pain.
Additional Information:
Patellar Luxation in Small Breed Dogs by Teri Dickinson, DVM
Patellar Luxation — Canine Inherited Disorders Database
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