Medial patellar luxation, or MPL, is a very common disease of small and miniature breeds in which the kneecap occasionally rides on the inside of its normal groove. Primarily congenital, although occasionally acquired through trauma, MPL causes lameness in one or both rear limbs. The degree of lameness is determined by the severity and duration of the disease, as well as the extent of existing arthritis.
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. When the patella, or kneecap, pops out of its normal trochlear groove, the dog feels pain, 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 trochlear groove. As MPL progresses, the structures that hold the patella in place become looser, and thus the problem becomes more frequent. This dislocation causes pain, and as the frequency increases, so does the lameness.
Unfortunately, many of the severe Grade III or IV cases go unnoticed for months or years because the affected animals are usually miniature 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.
Lateral patellar luxation, or LPL, is less common than MPL and occurs when the kneecap occasionally rides on the outside of its normal groove. It, too, can be congenital or acquired, with the congenital form again being more common. While it can occur in any dog, it is more common in large and giant breeds. LPL is frequently accompanied by malformation of the femur and/or tibia. The disease can produce marked lameness and progress to crippling arthritis. Because of the accompanying bony malformations, extensive surgery may be required to correct this problem.
