There are many good descriptions of knee anatomy available on the internet. This information covers areas that are important in understanding the problems of knee injury and disease and areas that patients often find difficult to understand.
The knee is the largest joint in the human body. Its function relies on a complicated combination of structures to produce a joint which is mobile yet stable and works under remarkable loads.
The knee is not a hinge - it relies on the shape of the joint surfaces, the ligaments and muscles to maintain stability.
The knee is the joint between the femur and tibia with the knee-cap (patella) sited in the tendon of the thigh muscle (quadriceps) at the front of the knee. The patella glides over the front of the femur during flexion and extension.
The bone structure near the joint is spongy with a thin outer layer of cortex, unlike its structure in the shaft of a long bone which is a thick cylinder of cortical bone. This means that the bone is relatively soft at this site and can be damaged without actually being fractured (bone bruise).
This is a resilient layer of tissue that covers the ends of the bone where they slide over each other during movement. This includes the back of the patella. It normally has a shiny pearl-like appearance and because of its unique properties, it is remarkably well lubricated and provides little resistance to movement (friction) even under heavy loads. As a consequence of its specialised structure, it is very poor at repairing itself after injury and it is open to degeneration and age related changes.
There are two in each knee joint - the medial and lateral menisci. They are often referred to as the cartilages, but they are quite separate to the articular cartilage and this can cause a lot of confusion for patients. When someone is told they have a "torn cartilage", this usually means they have a torn meniscus. Damage to the articular cartilage is a completely different issue.
The menisci are horse-shoe shaped when viewed from above and triangular in cross section. They sit between the curved surface of the femur and the flatter surface of the tibia to fill in the resulting gap and spread the load between the two bones so it is diffused over a large area. They also act as shock absorbers during impact such as when running. They have a rather rubbery texture and also a specialised structure that does not heal well or regenerate. If a meniscus is torn, it is only likely to heal again under specific circumstances (see torn meniscus).
This is a thick fluid that is secreted by the joint lining (synovium). It helps to lubricate the joint and provides nutrition to the cells in the articular cartilage (chondrocytes), so keeping the joint surface healthy. It is constantly produced and absorbed. Many diseases and injuries cause the accumulation of excess fluid in the knee (an effusion). Simply removing the fluid is rarely an answer to the problem, as the effusion will recur unless the cause is put right.
Ligaments are strips of slightly elastic tissue that join one bone to another across a joint. There are several important ones in the knee that are described in more detail in the sections on ligament injuries. Ligaments are important in providing stability to the knee. They can be stretched or torn in injuries and some heal better than others. They are often damaged in combination rather than individually. As well as giving mechanical stability, ligaments also contain the ends of nerve fibres that allow a degree of automatic control of the knee - the subconscious ability to know where the joint is and what it is doing (proprioception). Regaining proprioception is a vital part of rehabilitation after knee ligament injuries.
These are tough fibrous structures that connect muscle to bone. They vary in shape, size and strength depending on the muscle and its function. The quadriceps tendon is a large wide and flat tendon extending from the muscle at the front of the thigh (quadriceps) to the tibial tubercle with the patella set in its centre. Above the patella it is the quadriceps tendon and below it is known as the patellar tendon. It can be ruptured from acute injury or become inflamed and degenerate (tendonitis). Behind the knee are the tendons running from the hamstring muscles to the tibia (hamstring tendons). There are 4 in total, and one or two are often removed to be used as grafts to repair ligaments in the knee.
The muscles are the motors that move the knee. They are vital to good knee function. As well as producing the fast movements needed for running, they also give static stability of the knee as in standing. The fine control of the knee needed for rapid twisting and turning relies on proprioception (see above). Muscle weakness or incoordination will give a subjective sense of instability even if the ligaments and other structures around the knee are functioning normally. This is another reason why rehabilitation is so vital in recovery after injury or surgery.