The knee is considered to be one large hinge joint between the thigh and the lower leg, but actually consists of two separate joints (tibiofemoral and patellofemoral), both sitting in the same fluid filled capsule. These joints produce a modified hinge that works not only to allow the knee to bend and straighten, but also to rotate slightly from side to side.
The knee is part of a chain of lower limb components, working together with the pelvis, hip, upper and lower leg, ankle and foot to bring about complex movements.
It is the largest joint in the human body, and is needed to provide both flexibility of movement and stability in standing, whilst supporting the body’s weight.
The main components of the knee are:
- Muscles and Tendons
- Femur (thigh bone): the largest and longest bone in the body. The two rounded knobs at the end are the condyles, which together form the patellofemoral groove.
- Tibia (shin bone): runs from the knee to the ankle. At the top there are two distinct surfaces forming the tibial plateau, and a protruding tibial tubercle.
- Fibula (finer lower leg bone): runs alongside the tibia
- Patella (kneecap): flat triangular shaped bone that glides along the patellofemoral groove and protects the knee joint.
The ligaments are strong bundles of fibres that connect the bones of the leg together in order to provide stability, allowing controlled movement at the knee joint.
There are two groups of ligaments:
- Collateral ligaments
- Cruciate ligaments
Collateral Ligaments consist of Medial Collateral Ligament (MCL) on the inner side of the knee and Lateral Collateral Ligament (LCL) on the outer side of the knee. They prevent excessive sideways movement at the knee joint.
Cruciate ligaments consist of Anterior Cruciate Ligament (ACL) and Posterior Cruciate Ligament (PCL) that lie at the centre of the knee, crossing over one another (hence the name cruciate).
The ACL limits forward movement of the tibia relative to the femur, and the PCL limits backward movement. In addition the ACL also limits rotation (twisting at the knee) and provides the brain with information about the position of the knee (proprioception).
Also contributing to knee stability is the Posterolateral Corner (PLC). This is made up of a collection of ligaments, tendons and fibres and help with rotational mechanisms.
Muscles and Tendons
Quadriceps: 4 muscles at the front of the thigh used to straighten the knee. They come together as the quadriceps tendon that surrounds the patella, and continue as the patella tendon, attaching to the tibia below the knee.
Hamstrings: muscles at the back of the thigh that bend the knee.
There are two important types of cartilage in the knee:
Articular Cartilage: a smooth, white, shiny surface between the bones allowing the joint surfaces to glide over one another.
Menisci: The C-shaped meniscal cartilages are known as “shock absorbers’ or ‘footballers’ cartilage’. There are two meniscal cartilages; medial (inner side) and lateral (outer side). They are tough and rubbery and help cushion and stabilize the knee joint.
This is a thick fibrous structure that is wrapped around the knee joint. There are many folds of this capsule, some are embryological remnants called plicae. A knee plica can sometimes be the cause of problems within the knee joint. The synovial membrane is also considered to be part of the capsule. This membrane produces synovial fluid – the lubricating fluid present in most joints.