The knee is the largest joint in the human body. A healthy knee joint allows you to move freely, such as walking, turning or many other normal activities, without any pain. Bones, cartilage, ligaments, muscles and tendons form a complex network working together to make the knee joint flexible. There are 3 bones in the knee joint. The thigh bone (femur) sits on top of the lower leg bone (tibia). When you bend or straighten your knee, the rounded end of the femur goes around and slides over the relatively flat upper surface of the tibia. The third bone is commonly referred to as the kneecap (patella), and the patella attaches to the muscle that allows the knee to straighten. The kneecap acts as a lever that reduces muscle tension. In a healthy knee joint, the surface of these bones is very smooth and has a hard protective tissue of cartilage covering its surface. Ligaments (another soft tissue) are located along the sides and back of the knee joint and hold the bones of the knee in place. These ligaments work in coordination with muscles, bones, and tendons to allow you to bend and straighten your knee. A fluid-filled sac (bursa) cushions the area where skin or tendons slide over the bone. The knee has a lining (synovium) that secretes a clear fluid – joint fluid. This fluid lubricates the joint, which further reduces friction and makes movement easier. As you might expect, there are many causes of knee pain, including injury, infection, and arthritis. Osteoarthritis (OA) The most common cause of knee pain is osteoarthritis (OA), a degenerative bone disease that can lead to damage to the cartilage in the joint. When the cartilage – the tissue that “cushions” the joint and protects the surface of the bones – is damaged or worn away, the bones rub against each other, resulting in pain. Pain can be felt when climbing stairs, working in the garden, or just sitting with your knees bent. The pain can even make it difficult for you to sleep. Osteoarthritis can damage your whole knee or just one side of it. If you have pain on only one side or area of your knee, your doctor may diagnose you with unicondylitis. If you have pain in your lower kneecap, your doctor may diagnose patellofemoral arthritis. This is not uncommon; studies have shown that one in ten patients over the age of 40 have patellofemoral arthritis. Factors that contribute to the development and progression of osteoarthritis include age, obesity, joint damage and a family history of arthritis (genetics). Although there is currently no cure, early diagnosis and treatment are key to slowing or preventing more damage to the joints. Rheumatoid Arthritis In rheumatoid arthritis, the synovium (lining of the joints) becomes red and swollen. This inflammation releases a chemical that thickens the synovium of the affected joint and causes cartilage damage. Inflammation of the synovium leads to pain and swelling. Rheumatoid arthritis can be cured. Rheumatoid arthritis is a disease that worsens over time, so multiple treatments are needed, and the program is always adjusted. For some people, lifestyle changes, medications, and non-surgical treatments such as walkers help reduce pain. For others, tissue grafts to replace lost cartilage tissue can help restore normal function. But for most people, knee replacement surgery may be the only long-term solution. In short, you and your doctor can decide on the best treatment option for you.