The Knee

Understanding the Knee

The knee is one of the largest and most complex joints in the body. The knee joins the thigh bone (femur) to the shin bone (tibia). The smaller bone that runs alongside the tibia (fibula) and the kneecap (patella) are the other bones that make the knee joint.


The main knee muscles can be divided into two groups, the quadriceps and hamstrings. They are responsible for initiating and controlling movement of the knee and the kneecap, and they also work with the thigh and calf muscles to control the hip and foot.
The Quadriceps are four muscles located on the top of the knee running up to your thighs. Their main function is to straighten your leg. Some common activities that engage this muscle is getting up from a chair or walking up stairs.


The Hamstrings are located at the back of your knee, they run all the way up to your hip. The hamstring’s main role is to bend your leg. These muscles engage when you are bending or twisting your knee, for example whilst running.
There are many more muscles, however we will stick to just one more of key importance. There is a muscle behind the knee joint itself called popliteus which helps the knee to twist, aids stability of the knee and helps protect the lateral meniscus.


Tendons are elastic tissues that connect muscles to bones. The tendons main role is to stabilize the knee. There are two major tendons in the knee—the quadriceps and patellar.

The quadriceps tendon connects the quadriceps muscles of the thigh to the kneecap and provides the power for straightening the knee. It also helps hold the knee cap in the dedicated groove of the femur. The patellar tendon connects the kneecap to the shinbone.

At the back of the knee, there is the hamstring tendon.

Notice the colour of the tendons, they are white. Whereas muscles are red, this is due to blood flow.


Ligaments are designed to connect one bone to another. They are soft tissue structures that are strong, but not particularly flexible. Once stretched, they tend to stay stretched, if they stretch too far, they can snap.
Ligaments join the knee bones and provide stability to the knee.

The anterior cruciate ligament (ACL) prevents the femur from sliding backward on the tibia (or the tibia sliding forward on the femur).
The posterior cruciate ligament (PCL) prevents the femur from sliding forward on the tibia (or the tibia from sliding backward on the femur).


This is the cushioning that is provided to your knee joint, ultimately, they act as a shock absorber, cartilage wears out and cannot repair itself.

  • The medial meniscus is made of fibrous, crescent shaped cartilage and attached to the tibia, on the inside of the knee.
  • The lateral meniscus, same as medial however located on the outside of the knee.
  • Articular cartilage is found on the ends of the bones, it covers the ends of the femur and tibia and the back of the patella. The articular cartilage is kept slippery by synovial fluid (which looks like egg white) made by the synovial membrane (joint lining). Since the cartilage is smooth and slippery, the bones move against each other easily and without pain.

Joint Capsule (lubrication membrane)

The capsule is a thick, fibrous structure that wraps around the knee joint. Inside the capsule is the synovial membrane which is lined by the synovium, a soft tissue that secretes synovial fluid when it gets inflamed and provides lubrication for the knee.

Joint Sacs

There are many bursa sacks of various sizes in and around the knee. These sacs are filled with fluid to cushion the joint and reduce friction between muscles, bones, tendons and ligaments. The prepatellar bursa is one of the most significant bursa and is located on the front of the knee just under the skin. It protects the kneecap. In addition to bursae, there is a infra patellar fat pad that helps cushion the kneecap. If these become inflamed, they will swell and cause a lot of pain.

Pathological Conditions of the Knee

Knee osteoarthritis:

Osteoarthritis is the most common form of arthritis, and often affects the knees. Caused by aging and wear and tear of cartilage, osteoarthritis symptoms may include knee pain, stiffness, and swelling. Osteoarthritis is not simple to understand, there are many links with the wear and tear of cartilage that also affects every other part of your joint. We discuss this in our treatment information pack.

Chondromalacia patella:

Irritation of the cartilage on the underside of the kneecap (patella), causing knee pain. This is a common cause of knee pain in young people. Also known as ‘runners knee’.

Septic arthritis:

An infection caused by bacteria, a virus, or fungus inside the knee can cause inflammation, pain, swelling, and difficulty moving the knee. Although uncommon, septic arthritis is a serious condition that usually gets worse quickly without treatment.


A form of arthritis caused by buildup of uric acid crystals in a joint. The knees may be affected, causing episodes of severe pain and swelling.


A form of arthritis similar to gout, caused by calcium pyrophosphate crystals depositing in the knee or other joints.

Issues that often occur with Osteoarthritis

Knee effusion:

Fluid buildup inside the knee, usually from inflammation. Any form of arthritis or injury may cause a knee effusion.


Inflammation of the tendons that are connecting muscle to bone. As previously discussed tendonitis is very common. It can occur due to simple overuse, overstrain or even underuse. Tendons can become weak very quickly and they take a very long time to repair due to lack of blood flow.

Cartilage tear:

The most common type of cartilage tear in the occurs on the meniscus. Damage to a meniscus, the cartilage that cushions the knee, often occurs with twisting the knee. Large tears may cause the knee to lock. Tears can occur due to injury or long-term degeneration of the cartilage due to osteoarthritis.

Ligament strain or tear:

The ACL is responsible for a large part of the knee’s stability. An ACL tear often leads to the knee “giving out”. PCL tears can cause pain, swelling, and knee instability. These injuries are less common than ACL tears.

Patellar subluxation:

The kneecap slides abnormally or dislocates along the thigh bone during activity. Knee pain around the kneecap results.


Pain, swelling, and warmth in any of the bursae of the knee. Bursitis often occurs from overuse or injury. Also referred to as ‘housewife’s knee’

Baker’s cyst:

Collection of fluid in the back of the knee. Baker’s cysts usually develop from a persistent effusion as in conditions such as arthritis.


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