Rheumatoid arthritis causes the body’s immune system to attack the joints. White blood cells, which act as the body’s natural defenses, target the synovium, resulting in synovitis, an inflammation of the synovium. This disease is characterized by pain, swelling, and reddening and warmth of the joint.
The disease is further indicated by a thickened synovium which causes the knee to swell and enlarge. Further along in its progression, rheumatoid arthritis allows abnormal synovial cells to attack the joint and its cartilage, causing it to erode and deteriorate. This is the reason why rheumatoid arthritis is commonly known as the “wear-and-tear” arthritis. In severe cases, the cartilage shell wears away, eventually causing the bones of the knee - both the tibia and femur, to rub against each other.
This leads to pain, swelling and difficulty of movement. The joint gradually becomes deformed, causing the bordering muscles, ligaments and tendons to weaken. In addition, bone spurs known as osteophytes start to form along the joint perimeter, causing fragments of bone and cartilage to break away and drift within the joint area, bringing about an increase in pain and adding to joint destruction.
The above information thankfully comes from the arthritiskneepain.co.uk at the following link.