The knee joint is a complex structure designed to allow normal function whilst withstanding the huge stresses we put through it during everyday activities and sport. The main weight bearing part of the joint is formed between the two rounded ‘condyles’ on the end of the femur (the thigh bone) and the flatter top surface of the tibia (the shin bone).
The space between the condyles of the femur is called the ‘intercondylar notch’; this provides the surface attachments for two of the main ligaments in the knee. The notch extends forwards forming a groove at the front of the bone (the trochlea), the kneecap (patella) is situated at the front of the knee and is shaped to sit and glide in the trochlea during movement of the joint.
The knee is a ‘synovial hinge joint’ – this means that movement is restricted to one plane – ie. bending and straightening in a forward / backward direction. This however, is rather too simplistic – the knee is a complex structure where some rotation also occurs, thus allowing a locking and unlocking mechanism to take place.
All joints in the body that allow voluntary movement, are enclosed by a fibrous capsule; this is lined with a special ‘synovial’ membrane which produces fluid to keep the joint surfaces lubricated and healthy.
Other Important Structures
There are two different types of cartilage in the knee and this can cause some confusion if you don’t understand which is which.
Articular (hyaline) Cartilage:
This cartilage covers the ends of all bones in the body where they form mobile joints. It provides protection for the bone and has a very smooth surface, keeping friction to a minimum during movement. Articular cartilage varies in thickness depending where it is in the body, but tends to be thickest in the areas of weight bearing joints that are exposed to greatest stress, ie. the femoral condyles in the knee. This cartilage receives nutrition from the synovial fluid but, if damaged, is unable to readily heal itself.
If you look at an X’ray of the knee, it appears that there is a space between the bones – this is because an X’ray will only show up bone – the apparent space is in fact occupied by articular cartilage covering the surface of both bones. The joint space should look even however, in advanced stages of arthritis, the space becomes narrowed and irregular as the quality and depth of cartilage is reduced.
If you hear about a sportsman ‘tearing a cartilage’, this is probably what they are referring to.
We have two menisci in each knee – they are semi-circular, wedge shaped structures situated on the inside (medial) and outside (lateral) of the top of the tibia. They are made of fibrocartilage and function like shock-absorbers and spacers in the knee. The cross-sectional shape also adds to the stability of the femoral condyles on the relatively flat surfaces of the tibia.
Ligaments attach bone to bone and basically hold the joint together. There is a small amount of give in a ligament but it is not essentially an elastic structure. Some ligaments appear to be thickened reinforced areas of the joint capsule, other ligaments are separate entities in themselves. Ligaments prevent unwanted movement and are very important in providing stability during movement. There are four major ligaments in the knee – the co-lateral ligaments which are outside the joint on either side, and the cruciate ligaments which are in the centre of the joint and form a cross, hence the name.
The medial co-lateral ligament – this is a large broad ligament, reinforcing the capsule on the inside of the knee. Its’ main function is to prevent sideways movement of the joint, ie. the tibia moving out to the side in relation to the femur. Stress in this direction is called ‘valgus’, and damage to the medial ligament can lead to excess ‘valgus’ movement and even a valgus deformity (knock knee) on weightbearing.
The lateral co-lateral ligament – this is a separate cord-like structure on the outside of the joint. Its’ function is to restrict ‘varus’ movement (the tibia moving in towards the other leg, in relation to the femur). Damage to this ligament can lead to excess varus movement and the appearance of being bow legged on weightbearing.
The anterior cruciate ligament – (ACL) is in the middle of the joint and passes from the centre front of the tibia, upwards and backwards to attach to the lateral side of the intercondylar notch. It is responsible for controlling forward glide of the tibia in relation to the femur, this movement is known as ‘anterior tibial translation’.
The posterior cruciate ligament – (PCL) is also in the middle of the joint and passes from the centre back of the tibia, upwards and inwards to attach to the medial side of the intercondylar notch. It is responsible for controlling backward glide of the tibia in relation to the femur, this is known as ‘posterior tibial translation’.
All the ligaments work together to control rotation or twisting of the joint.
Providing all four major ligaments are competent, the amount of play in the joint is controlled, thus giving added protection to the joint surfaces during activity.
Muscles and Tendons
Tendons attach muscle to bone. There are many muscles /tendons which pass across, and therefore have an effect on the knee joint, however two large muscle groups are responsible for the primary movements of flexion (bending the knee) and extension (straightening the knee).
The Quadriceps – This group, as its name suggests, consists of four separate muscles that join together to form one tendon. These muscles are responsible for straightening the knee or to control bend of the knee in a weight-bearing position, such as squatting, where gravity would otherwise have us in a heap on the floor.
Within this group there are three large muscles on the front of the thigh – two in the centre and one situated more to the outside. The fourth muscle is smaller and positioned on the inside of the front of the thigh; part of this muscle is at an oblique angle as it inserts on the kneecap.
All four muscles converge into the ‘quadriceps tendon’ which passes over the front of the knee and attaches to a prominent bump on the front of the tibia, just below the joint.
The patella is a small bone embedded in the underside of the tendon and is shaped to move within the trochlea groove on the front of the femur. The patella acts a little like a pulley and also serves to alter the angle at which the tendon attaches to the bone, thus allowing more efficient muscle action.
The area of tendon between the patella and the tibia is called the ‘patellar tendon’ even though it is continuous with the quadriceps tendon.
Usually all four quadriceps muscles work together, however the three large muscles have a tendency to drag the patella laterally (to the outside) as the knee straightens. The smaller inner muscle has the function of counteracting this pull and thus stabilising the patella in the trochlea during movement. The way the patella moves in the trochlea when the knee bends and straightens is called ‘patella tracking’.
Hamstring Muscles – We have three hamstring muscles which originate on the pelvis and therefore pass across the back of the hip joint as well as the knee, to attach to the tibia or fibula. The muscles pass down the back of the thigh and are responsible for bending the knee ( they also contribute to extending the hip).
As discussed earlier, joints are enclosed within a fibrous capsule, however in the knee, the patella and tendon are at the front of the joint. The capsule is extended up to attach to the side edges of the patella – these fibrous extensions are known as retinaculae (one medial and one lateral) and have the added function of contributing to the stability of the patella.