Arthritis is the swelling and tenderness of one or more joints. The main symptoms of arthritis are joint pain and stiffness, which typically worsen with age. The most common types of arthritis are osteoarthritis and rheumatoid arthritis
Normal wear and tear causes OA, one of the most common forms of arthritis. An infection or injury to the joints can exacerbate this natural breakdown of cartilage tissue. Cartilage is a firm but flexible connective tissue in your joints.
An autoimmune disorder, rheumatoid arthritis occurs when your immune system mistakenly attacks your own body's tissues.

Two wedge-shaped pieces of cartilage called meniscus act as "shock absorbers" between your thighbone and shinbone. They are tough and rubbery to help cushion the joint and keep it stable.

The knee is the largest and strongest joint in your body. It is made up of the lower end of the femur (thighbone), the upper end of the tibia (shinbone), and the patella (kneecap). The ends of the three bones that form the knee joint are covered with articular cartilage, a smooth, slippery substance that protects and cushions the bones as you bend and straighten your knee.

What does arthritis pain feel like?

In general, the first sign of arthritis is pain, also called arthralgia. This can feel like a dull ache or a burning sensation. Often, pain starts after you've used the joint a lot, for example, if you've been gardening or if you just walked up a flight of stairs. Some people feel soreness first thing in the morning.

What are the parts of a joint?

Joints get cushioned and supported by soft tissues that prevent your bones from rubbing against each other. A connective tissue called articular cartilage plays a key role. It helps your joints move smoothly without friction or pain. Some joints have a synovial membrane, a padded pocket of fluid that lubricates the joints. Many joints, such as your knees, get supported by tendons and ligaments. Tendons connect muscles to your bones, while ligaments connect bones to other bones.

What are the different types of arthritis?

Arthritis is a broad term that describes more than 100 different joint conditions. The most common types of arthritis include:

• Osteoarthritis, or “wear and tear” arthritis, which develops when joint cartilage breaks down from repeated stress. It’s the most common form of arthritis.
• Ankylosing spondylitis, or arthritis of the spine (usually your lower back).
• Juvenile arthritis (JA), a disorder where the immune system attacks the tissue around joints. JA typically affects children 16 or younger.
• Gout, a disease that causes hard crystals of uric acid to form in your joints.
• Psoriatic arthritis, joint inflammation that develops in people with psoriasis (autoimmune disorder that causes skin irritation).
• Rheumatoid arthritis, a disease that causes the immune system to attack synovial membranes in your joints.

Rheumatoid Arthritis

Rheumatoid arthritis is a chronic disease that attacks multiple joints throughout the body, including the knee joint. It is symmetrical, meaning that it usually affects the same joint on both sides of the body. In rheumatoid arthritis, the synovial membrane that covers the knee joint begins to swell. This results in knee pain and stiffness. Rheumatoid arthritis is an autoimmune disease. This means that the immune system attacks its own tissues. The immune system damages normal tissue (such as cartilage and ligaments) and softens the bone.

Posttraumatic Arthritis

Posttraumatic arthritis is form of arthritis that develops after an injury to the knee. For example, a broken bone may damage the joint surface and lead to arthritis years after the injury. Meniscal tears and ligament injuries can cause instability and additional wear on the knee joint which, over time, can result in arthritis.