Understanding how your knee works

The knee is the largest joint in the body. A healthy knee moves easily, allowing you to walk, turn, and do many other activities without pain. A complex network of bones, cartilage, ligaments, muscles, and tendons work together to make a knee flexible.

There are three bones in your knee joint. Your thighbone (femur) sits on top of your shinbone (tibia). When you bend or straighten your knee, the rounded end of your thighbone rolls and glides across the relatively flat upper surface of your shinbone. The third bone is often called the kneecap (patella), which is attached to the muscles, allowing you to straighten your knee. Your kneecap provides leverage and reduces strain on these muscles.

In a healthy knee joint, the surfaces of these bones are very smooth and covered with a tough protective tissue called cartilage.

Ligaments (another type of soft tissue) lie along the sides and back of the knee, holding the bones of the knee joint in place. These ligaments work with the muscles, bones and tendons so that you can bend and straighten your knee. Fluid-filled sacs (bursae) cushion the area where skin or tendons glide across bone. The knee also has a lining (synovium) that secretes a clear liquid called synovial fluid. This fluid lubricates the joint, further reducing friction and making movement easier.

As you might expect, there are many different reasons why you could be feeling knee pain, including injury, infection, and arthritis.

Osteoarthritis (OA)

The most common cause of knee pain is osteoarthritis (OA), a degenerative joint disease that causes the cartilage in your joints to break down. When that layer of cartilage — which is meant to “cushion” the joints and protect the surface of the bones — is damaged or worn away, your bones grind against one another, and that grinding hurts. You can feel it climbing stairs, working in the garden, or just bending your knees to sit. It may even keep you up at night.

OA can damage the entire knee or be limited to just one side of the knee. If you experience pain only on one side, or compartment, of your knee, your doctor may diagnose you with unicompartmental OA.

If you experience pain under the knee cap, your doctor may diagnose you with patello-femoral OA. This is not uncommon, as studies have shown that about one out of every 10 patients over the age of 40 have patello-femoral disease.

The factors leading to the development and progression of OA include aging, obesity, joint injuries, and a family history of arthritis (genetics). Although there is no cure, early diagnosis and treatment are crucial in slowing or preventing more damage to your joints.

Rheumatoid arthritis

In rheumatoid arthritis, the synovium (lining of the joint) becomes inflamed. This inflammation causes chemicals to be released that thicken the synovium and damage the cartilage and bone of the affected joint. This inflammation of the synovium causes pain and swelling.
The good news about rheumatoid arthritis in the knee is that it can be treated. Rheumatoid arthritis is a disease that typically worsens over the years, so it is common for treatment to involve more than one approach and to change over time. For some people, nonsurgical treatments such as lifestyle changes, medications, and walking aids help alleviate the pain. For others, replacing lost cartilage with tissue grafts may help restore normal function. And for many, knee replacement surgery may be the only long-term solution. Together, you and your doctor can determine the best treatment options for you.