What is a joint?

A joint is the location where two bones meet in the body. They prevent bones from rubbing against bones in the moving parts of our skeletons. They protect and cushion the bones from damage.

In the body there are many different types of joints, from the ball-and-socket construction of your shoulders and hip joints to the hinge mechanism of your knees. Your neck works on a pivot system allowing for limited rotation. Even though the mechanisms may differ, these joints have similar roles.

The knee may look like a simple joint, but is one of the most complex joints in the body. It is essential to the movement of the leg and for walking. The knee is also a weight-bearing joint and locks into position so we can stand upright. It also has the ability to rotate and bend slightly under the guidance of certain thigh muscles. This flexibility makes us able to withstand extreme stresses when we run or play sport. The downside is that it is also more likely to be injured than any other joint in the body.

Healthy joints are important for us to maintain our mobility and flexibility. The shoulder must be flexible to allow for lifting, pushing and pulling. The hip joint has the second largest range of movement and supports the weight of the body, arms and head.

How does a healthy joint work?

All joints, including the knee and hip are similar and have cartilage, synovial fluid and various connective tissue (such as ligaments and tendons).

Cartilage

At the end of each of our bones, there is a connective tissue covering called cartilage. This slippery tissue allows the bones to move against each other without causing any friction or damage. In the knee, cartilage covers both the shins and the end of the thigh, and in the hip, cartilage covers both the femur and pelvic bone. Cartilage acts as a shock absorber and spreads the forces evenly when you put pressure on a joint. There are no nerves or blood vessels in cartilage. Therefore when this tissue is damaged, it heals more slowly. Like with many body processes, cartilage repair becomes less efficient with age.

Synovium and synovial fluid

Healthy joints are also surrounded by a thin membrane called the synovium, which produces a clear thick fluid. This is known as the synovial fluid and it acts as a lubricant.

Ligaments and the capsule

Surrounding the synovium is another layer – a tough outer layer called the capsule, which has a dual function. It provides a watertight case for the fluid and ensures the bones do not move around too much. The bones within a joint are kept firmly in place by strong bands of connective tissue, which run within or just outside the capsule. Together with the capsule, these bands, known as ligaments, prevent the bones moving too much or dislocating. Ligaments also act like shock absorbers connecting bone to bone.

Tendons

Tendons are similar to ligaments. They help to connect the muscles to the bones. A tendon is flexible, but fibrous and tough, and helps to keep the joint in place. When a muscle contracts, it shortens and this pulls the bone and makes the joint move.

What happens to a joint in rheumatoid arthritis?

Arthritis is the name for a group of conditions where there is damage caused to the joints of the body. Joints are crucial to your mobility, allowing your limbs and body to move in certain directions.

Rheumatoid arthritis affects the cells that line and normally lubricate the joints. This type of arthritis is caused by autoimmune reaction, which means that the body’s immune system attacks itself, instead of protecting the joint and its membranes.

In rheumatoid arthritis, the area around the joint becomes inflamed and swollen. The inflammation leads to redness and the inflamed joint may feel warm to touch.

In severe rheumatoid arthritis, the inflammation may cause the joint to lose its shape and alignment, leading to more pain, and loss of movement. In rheumatoid arthritis, several joints may be inflamed on both sides of the body.

What’s the difference between rheumatoid arthritis and osteoarthritis?

Osteoarthritis is different to rheumatoid arthritis. It is caused by wear and tear of cartilage over the years. The cartilage gradually thins, and the bone underneath thickens.

In severe osteoarthritis, the cartilage can become so thin that it exposes the bone underneath, which starts to wear down. The joint may then change shape.