The canine stifle joint is a complex joint made up of 3 main bones and 2 main ligaments surrounded by a sealed joint capsule, which retains joint fluid to lubricate the joint. The femur (thigh bone) has a rounded base and sits on the normally flat surface of the tibia (shin bone). There are 2 small crescent shaped pieces of cartilage that sit either side of the tibia acting as cushions or shock absorbers in the knee joint. There are many ligaments within the knee joint but the 2 most critical are the anterior or cranial cruciate ligament (ACL) and the posterior or caudal cruciate ligament (PCL). As the name suggests they cross over inside the joint forming the major supporting structures internally. Running over these ligaments is the patellar or kneecap, which joins the quadriceps muscle via the patellar ligament to the front of the tibia. The cranial cruciate ligament has 3 main functions. It limits rotation of the tibia (shin bone equivalent) with respect to the femur (thigh bone) Prevents hyperextension of the knee Prevents forward movement of the tibia relative to the femur An odd feature of the canine stifle is that the weight bearing surface (the tibial plateau) slopes backwards at an angle. Whenever a dog puts weight on their knee the cranial cruciate ligament must therefore take the strain. It’s like parking a car on a slope and relying on the handbrake to keep it there. If the cranial cruciate ligament fails, the tibia slides forwards and the joint capsule is stretched. We call this motion tibial thrust or cranial drawer, and it hurts! It’s why when a cruciate ligament is ruptured a dog won’t put much weight on the leg.