When the knee flexes or bends, the LCL and MCL are relaxed and the ACL and MCL are taut. Both the ACL and MCL are slightly unwound. As the knee extends, the femoral condyles glide into position and the knee rotates inward very slightly due to the shape of the inner femoral condyle. When the knee is extended or straight, the LCL and MCL, as well as the anterior part of the ACL, are taut. When the knee moves, it does not just bend and straighten (flex and extend). It protects the outer side of the knee from an inside bending force. The LCL stretches from the outer portion of the femur to the head of fibula. IIt protects the medial (inner) side of the knee from being bent open by a stress applied to the lateral (outer) side of the knee. The MCL stretches straight from the inner portion of the femur to the inner portion of the tibia and is partially fused to the meniscus. The ACL locks the knee when the leg is straight and the PCL prevents the femur from shunting forwards on the tibia when the knee is flexed or bent. The ACL is lateral (or outer) and the PCL is medial (or inner). The anterior cruciate ligament (ACL) and the posterior cruciate ligament (PCL) are two strong, rounded bands that extend from the head of the tibia to the intercondyloid notch (the space between the two round condyles) of the femur. The ligaments surrounding the knee joint offer stability by limiting movements and, together with several menisci and bursae, protect the articular capsule. They are the anterior cruciate ligament (ACL), the posterior cruciate ligament (PCL), the medial collateral ligament (MCL) and the lateral collateral ligament (LCL). LigamentsĪs mentioned above, there are four main ligaments which strap the bones of the knee joint together. There are two c-shaped menisci in the knee – the medial meniscus and the lateral meniscus. The meniscus can often be torn in forceful, twisting injuries. Meniscal cartilage is fibrous with a good tensile strength, separating the bones of the knee and acting as a shock absorber. Meniscus & Ligaments Illustration ( image courtesy of: Smith & Nephew) Cartilage also ensures that joint surfaces slide easily over each other for smooth knee movement.Īll bones that make up the knee joint have articular or hyaline cartilage covering the end of each bony surface.ĭamage to the articular cartilage is usually gradual ‘wear and tear’ damage caused by overuse, aging, and can be accelerated by poor knee tracking. Articular CartilageĬartilage is a thin, elastic connective tissue protects the bone by absorbing shock. Numerous bursae or small fluid sacs are like folds in the capsule around the knee joint which provide extra flexibilty and stretch to the capsule. This fluid lubricates the joint and nourishes the articular cartilage that coats the ends of the bones in the joint. It has a tough, fibrous outer membrane and an inner synovial membrane, which produces joint fluid (synovial fluid).

Many other muscles are also concerned with the function of the knee joint. Hamstrings are the muscles at the back of the thigh or femur. Quads extend (or straighten) the knee joint. The quadriceps muscles lie on the top of the femur at the front of the knee. The principle muscles involved in the knee joint are the quadriceps and the hamstrings. Whilst mostly buried with it the tendon, the back side or posterior of the patella articulates with the patella groove in the femur. The patella is part of the tendon that rises out of the quadriceps or thigh muscle. The tibia also has a pair of flattened condyles at the top where the bone articulates (or joins) the femur. The shape of these condyles permits the sliding and rolling and turning motions in the flexing knee. The portions of the femur which help create the knee joint are its lateral and medial condyles – the round portion at the end of the bone. Anatomy of the knee ( image courtesy of: Smith & Nephew)

Knee compartments