Anatomy of the Knee
The knee is the largest and most complex joint in the body. Nearly normal knee function is needed to perform routine everyday activities.
The knee is made up of the lower end of the thigh bone (femur), which rotates on the upper end of the shin bone (tibia), and the knee cap (patella), which slides in a groove on the end of the femur. The joint surfaces where these three bones touch are covered with articular cartilage, a smooth substance that cushions the bones and enables them to move easily without friction.
The joint is surrounded and enclosed by a thick ligamentous joint capsule. Inside this is a smooth tissue liner called the synovial membrane. This membrane releases a special fluid that lubricates the knee, reducing friction to nearly zero in a healthy knee.
The infrapatellar fat pad and bursa are structures that act as cushions, protecting the knee against outside forces.
Large ligaments, each with a particular function, are attached to the knee to provide stability.
These are the Medial Collateral Ligment (MCL) which attaches to and runs between the inner (medial) surfaces of the femur and tibia. The innermost band is attached to the medial meniscus. The MCL prevents the knee from collapsing inwards and protects the knee from valgus or outwards forces.
The Lateral Collateral Ligament (LCL joins the outer (lateral) surface of the femur to the head of the fibula. It resists impact from the inner surface of the knee known as a varus force.
The Anterior Cruciate Ligament (ACL) and the Posterior Cruciate Ligament (PCL) are a pair of stabilising ligaments located in the centre of the knee. They are strong rope like structures connecting the femur to the tibia through the intercondylar notch. The ACL and PCL prevent the femur from abnormal sliding and rotating movements on the tibia. One of the main functions of the ACL is to provide stability during rotational movements such as turning, twisting, and sidestepping. The PCL attaches posterior to the ACL and prevents the knee being forced backwards (hyperextension)
Acting as shock absorbers and correcting weight distribution between the joint surfaces of each knee joint are the medial and lateral menisci. These are two crescent shaped pieces of specialised fibrocartilage.
The two main muscle groups of the knee joint are the quadriceps and the hamstrings. Both play a vital role in moving and stabilizing the knee joint and providing strength.
The quadriceps muscle group is made up of four different individual muscles which join together forming the quadriceps tendon. This thick tendon connects the muscle to the patella which in turn connects to the tibia via the patellar tendon. Contraction of the quadriceps pulls the patella upwards and extends the knee. The quadriceps muscles consist of the rectus femoris, vastus medialis, vastus intermedius and vastus lateralis muscles.
The hamstring muscles at the back of the thigh function to flex or bend the knee as well as providing stability on either side of the joint line. The hamstring muscles consist of the biceps femoris, semitendinosus and semimembranosus, and gracilis.