The natural knee joint is formed by the coming together of the lower end of thigh bone (the femur), the upper end of the shin bone (the tibia) and the knee cap (patella). The ends of the bone have a special lining of white spongy gristly material (the articular cartilage) that enable the bones to move on one another smoothly and without friction. The bones are held together by strong ligaments inside the knee (the cruciate ligaments) and outside the knee (the collateral ligaments and the joint capsule). The muscles that span the knee joint help bend and straighten the knee. A thin membrane (synovial membrane) covers the bones of the joint and provides small amounts of lubricating fluid which also nourishes the articular cartilage. The normal knee joint can move from a fully straight position to about 135 degrees of bend (flexion).
The bones: The lower end of the thigh bone is shaped almost like a half-circle when seen from the side. The upper end of the shin bone on the other hand is quite flat. The knee joint is thus quite different from the hip joint in that it has much less inherent stability built in from the shape of its bones. It therefore relies heavily on the integrity of the ligaments and muscles around the joint for stability. Also, the design of the joint is such that it functions almost like a hinge with most of the movement occurring only in one plane - bending and straightening. The hip joint however, is a ball and socket joint and thus is much more mobile.
The cruciate ligaments: The posterior cruciate ligament is one of two strong ligaments present inside in the middle of the knee joint. These ligaments 'connect' the thigh bone to the leg bone and prevent abnormal backward slippage of the shin bone relative to the lower end of the femur bone. The anterior cruciate ligament is the second ligament and this ligament prevent abnormal forward slippage of the shin bone relative to the femur. The anterior cruciate is often found damaged at the time of surgery in most patients. If it is not, then it has to be removed to provide access to the inside of the knee and allow the artificial replacement to go ahead.
The collateral ligaments: The natural knee has two collateral ligaments; one on the inner side of the knee (the medial collateral) and the other on the outer side (the lateral collateral). These ligaments help to prevent one bone moving side ways with respect to the other i.e., to prevent a knock-knee or a bow-leg type of movement.
The muscles: The main muscles around the knee are the quadriceps or the bulky muscles on the front of the thigh which help mainly to straighten the joint and the hamstrings (the muscles on the back of the thigh) whose job is mainly to bend the knee joint. Good and strong muscles help to preserve function of the knee joint.
The menisci: The meniscus is a C-shaped structure made of dense scar like tissue which seem to function like a washer in the knee joint. There is one of these on the inner side of the knee and another on the outer side. Both are fixed to the top surface of the shin bone. In common usage they are known as 'cartilage' and it is these which are often torn during sport injuries and may require removal through keyhole surgery. At the time of the knee replacement operation, these menisci or their remnants are removed as part of the standard procedure.