The two most common knee diseases we see in practice are patellar luxation and cruciate ligament tears. Patellar luxation is usually seen in small dogs, especially Yorkshire Terriers, Chihuahuas, Miniature Pinschers, and Toy Poodles. The patella, or kneecap, is a small oval bone in the tendon of the quadriceps muscle. Normally it rides in a groove in the end of the femur, just above the knee joint. A ridge of bone on either side of the groove keeps the patella centered in the groove.
Patellar luxation occurs when the patella slips out of the groove and over the ridge of bone.
When this occurs toward the outer half of the leg it is termed lateral luxation, displacement of the patella to the inside of the leg is medial luxation. Medial patellar luxation is the type usually seen in small breed dogs. Patellar luxation is graded from 1 to 4, with Grade 1 being a very mild, intermittent problem. Grade 4 patellar luxation occurs when the patella is fixated outside its normal location and can not be manually returned to the femoral groove. The primary sign of patellar luxation is hindlimb lameness.