The knee is pivoting joint composed of the combined bones of the femur and tibia. There are four ligaments that hold it together:

• medial collateral ligament
• lateral collateral ligament
• anterior cruciate ligament
• posterior cruciate ligament

The kneecap, or patella, forms the anterior part of the knee. Underneath the patella, and along the edges of the femur and tibia, is the smooth and slick surface of the articular cartilage. This tough elastic tissue enables non-abrasive movement without pain.
The synovium, a membrane that surrounds the knee, supplies tiny quantities of a viscous fluid that moisturizes the articular cartilage to keep it smooth and slippery. The synovium is surrounded by the joint capsule, a sturdy external cover that shields and reinforces the knee.