Anatomy of the Knee

The knee is a complex hinge-type joint that can be thought of as having two separate joints – the main hinge and a second joint between the patella (knee cap) and the front of the femur (thigh bone).
These joints are, however, contained within one capsule and so any increase in fluid within the knee (effusion/swelling) affects both joints.
The knee is made up of the following components:

Bones

  • The femur [thigh bone] is the largest bone in the body.

  • The tibia [shin bone] is the larger bone of the lower leg.

  • The fibula is the smaller bone in the lower leg, sitting on the outer aspect of the tibia.

  • The patella [knee cap] is located in front of the femur and tibia and slides within its own groove on the femur as the knee moves

Ligaments

There are two main groups of knee ligaments and these connect the bones of the upper and lower leg. They are made up of strong groups of fibres that help to provide stability to the knee, allow joint movement and prevent excessive or abnormal motion.

Collateral ligaments:


The medial and lateral collateral ligaments [MCL and LCL] lie on the inner and outer sides of the knee respectively. They help to prevent sideways motion.


Cruciate ligaments:
The anterior cruciate ligament [ACL] and posterior cruciate ligament [PCL] lie in the centre of the knee crossing each other (hence cruciate). The ACL helps to limit rotation and forward motion of the tibia whilst the PCL located just behind the ACL limits the backward motion of the tibia.

In addition, there is a complex area called the posterolateral corner (PLC), which is made up of a group of linked ligaments, tendons and capsular fibres that assist in rotational (twisting) stability.


Muscles and Tendons

The quadriceps [‘quads’] are four muscles in front of the thigh that act to straighten the knee.
The hamstrings are the muscles in the back of the thigh that work together to bend the knee.
Tendons are structures that attach muscles to bones. The four quadriceps form into one tendon called ‘the quadriceps tendon’ which surrounds the patella and then becomes the patellar tendon as it attaches to the tibia below the knee.


Cartilage

Articular cartilage
The end surfaces of each bone are covered with smooth articular (joint surface) cartilage that reduces friction between the moving surfaces of the joint and also helps to spread the loads that are applied to the joint.
Meniscal cartilage, meniscus




Between the femur and the tibia are two ‘C’ shaped wedges called menisci. These are situated one on either side of the joint, and act as shock absorbing cushions between the two main bones, in addition to other functions.


Torn Cartilage
When a meniscus is damaged, it is often referred to as a ‘torn cartilage’. This can be a confusing term, but when people talk about a torn cartilage, they usually mean the meniscus and not the articular cartilage.

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