The knee is the largest joint in the body and carries almost half of your weight.
The knee joint consists of the lower end of the thigh bone (femur) and the top of the shin bone (tibia), with the knee cap (patella) at the front. There is a complex type of hinge between the femur and tibia, with the kneecap moving upward and downward in a groove in front of the femur as the knee bends and straightens.
The bones are held together by ligaments and the joint capsule. The Medial Collateral Ligament (MCL) stabilises the inner knee joint, and the Lateral Collateral Ligament (LCL) the outer knee. There are two cruciate ligaments, the Anterior Cruciate Ligament (ACL) and the Posterior Cruciate Ligament (PCL). The cruciate ligaments lie within the central knee and control rotation and the front and back movements between the femur and tibia.
In a healthy knee, the bony surfaces of the joint are lined with articular (chondral) cartilage. This is a very smooth, firm surface which allows the bones to move against each other with minimal friction. Between the two bones (the femur and tibia) are two meniscal cartilages (medial and lateral menisci). The menisci are two semicircular or moon shaped cartilages which act as shock absorbers and stabilisers.
Damage to any of the ligaments of the knee can lead to instability and joint degeneration. As well as instability, the cartilage surfaces and menisci can degenerate over time, leading to arthritis. Arthritis of the knee is characterised by pain, stiffness and reduced range of motion.