A joint is formed when two bones meet, to allow articulation and movement.
The majority of joints in the human body are synovial joints, where synovial fluid acts as a shock absorber, cushioning and lubricating the joint and helping to prevent the bones from rubbing together.
A synovial joint is made of four main components which protect and maintain movement.
The Joint Capsule is an envelope surrounding the joint and consists of fibrous tissue helping to hold the joint together.
The Synovial Membrane is the inner layer of the Joint Capsule. It consists of loose soft, porous connective tissue which covers the Synovial Cavity. The Synovial Membrane produces Hyaluronic Acid which is secreted into the joint cavity - this increases the thickness of the Synovial Fluid, helping to lubricate the joints and allowing molecules to pass through from the adjacent skin and muscles into the Joint Membrane.
The Synovial Fluid (also known as Joint Fluid) fills the Joint Cavity. It is a thick lubricant material that contains proteins and the “goo” molecule Hyaluronic Acid and creates a cushioning environment to allow free joint movement. It acts as a shock absorber during movement.
The Cartilage covers the ends of the movable bones in the joint. It is a dense connective tissue (containing the protein collagen) which is tough and slippery. It protects the bones by preventing them from rubbing against each other and enabling the joint to move smoothly. The Cartilage also contains Hyaluronic Acid which lubricates and cushions the joints.
Other useful related words:
Collagen is a major component of cartilage. Collagen is a dense fibrous protein with great tensile strength and provides strength and elasticity to connective tissues such as cartilage.
Hyaluronic Acid (HA) is a key component within the Synovial Fluid and has been widely referred to as the "goo" molecule. HA helps to lubricate and cushion joints as part of normal daily functioning.
Tendons, or sinews, connect muscle to bone. The bones in a person's skeleton enable them to walk, run, jump, lift, carry, and do other important physical activities. Without the connection between the muscles and bones that are responsible for controlling these actions, it would be impossible for the body to move in the way it does.
Ligaments are stretchy bands of tissue that, in most cases, hold one bone to another. They are similar to tendons. They can withstand tension and work in conjunction with muscles and transmit force. They are made of collagen and are very stretchy. Some ligaments limit the mobilty of articulations or prevent certain movements altogether. Capsular ligaments are part of the articular capsule that surrounds synovial joints. There are many types of tendon injuries, often caused by over-use, such as tendinosis (non inflammatory) and tendinitis (with inflammation).