Ruptured Anterior (Cranial)
The knee is a fairly complicated joint. It consists of the femur above, the tibia below, the kneecap (patella) in front, and the bean-like fabellae behind. Chunks of cartilage called the medial and lateral menisci fit between the femur and tibia like cushions. An assortment of ligaments holds everything together, allowing the knee to bend the way it should and keep it from bending the way it shouldn’t.
There are two cruciate ligaments that cross inside the knee joint: the anterior (or, more correctly in animals, cranial) cruciate and the posterior (in animals called the caudal) cruciate. They are named for the side of the knee (front or back) where their lower attachment is found. The anterior cruciate ligament prevents the tibia from slipping forward out from under the femur.
Medial Luxating Patella
The medial luxating patella, commonly called a trick knee, is an extremely common problem in toy breed dogs. An owner typically notices a little skip in the dog’s step. The dog may even run on three legs, holding one hind leg up, and then miraculously be back on four legs as if nothing has happened.
In fact, something has happened: the kneecap (patella) has slipped out of the smooth groove in which it normally rides up and down. It has slipped medially, which is to say towards the opposite leg, as opposed to laterally, which would be away from the dog entirely. With the patella dislocated (or luxated) medially, the knee cannot extend properly and stays bent. Hopefully, the patient will be able to slip the kneecap back where it belongs and be back to normal in only a few steps. For some dogs, getting a kneecap back where it belongs and normal extension of the rear leg is a mere dream only attainable with surgical correction.