Joints occur where at least two bones meet, and they allow the bones to move with respect to one another. For example, the elbow is the joint between the one upper and two lower bones of the arm. The job of a joint is to make repeated movement between the bones smooth, safe, and efficient.
Joints are made up of several parts, each one needed to allow the joint to function properly.
- Cartilage: The ends of bones that meet at the joint are covered by a smooth substance (cartilage) that serves both as a shock absorber and as a tough coat to prevent damage to the underlying bone.
- Muscles: Bones are linked together by muscles, strong tissues that provide the force to move the bones.
- Ligaments: Bones are also linked together by ligaments, strong tissues that form the outer covering, or capsule, of the joint.
- Joint fluid: Specialized cells called synoviocytes that line the inner surface of the joint capsule fill the joint with a thin cushion of fluid (synovial fluid), which absorbs shocks and prevents the bones from hitting each other.
- Bursa: The joint is surrounded by bursae, fluid-filled pockets that provide buffering where there might otherwise be friction between the skin and joint, between two bones, and between a tendon or a ligament and a bone.
The importance of shock absorption
If you have healthy joints, when you stomp your foot down on the floor, you do not get a sensation of searing pain shooting through your knee. This is quite remarkable, since the force of your thigh muscles ought to slam the bones of your upper leg crashing down onto the bones of your lower leg. This doesn't happen because tissues within the joint serve as shock absorbers, like springs that absorb energy, and thus slow down the transfer of force from one bone to the next.
The shock-absorbing properties of the joint are due primarily to the cartilage and the thin cushion of fluid that fills the space between the bones.
The cartilage covering the ends of all the bones that meet at a joint is a Teflon-like substance that is both very hard and very smooth. Cartilage is made up of cells that receive their nourishment from a solution called synovial fluid that fills the inside of the joint space. Synovial fluid contains proteins and sugars and is produced by a layer of cells lining the joint. Synovial fluid is thick like molasses, enabling it to protect the joint from transmitting the normal forces associated with movement to the underlying bones.
The mix of cells and surrounding solution acts much like a sponge. For example, with each step downward, fluid is squeezed out of the cartilage of the knee. When pressure is released as the leg comes up off the ground, fluid rushes back into the cartilage, and it springs back into shape.
Holding the joint together
Joints function as a way to move two bones with respect to one another. In order for this to work, the bones that meet at the joint must be attached to each other. The attachment needs to be firm enough to hold the joint together, yet flexible enough to allow the bones to move. Damage to these supporting structures of the joint can allow bones to ram into one another, damaging the smooth cartilage that lines the joint.
Bones are attached by strong bands called ligaments. Muscles are attached to the bones by bands called tendons. Ligaments and tendons are made of tissue that is strong enough to hold the joint in place but flexible enough not to tear under normal movement. The placement of tendons and ligaments determines how different joints are able to move. For example, the knee can bend forward but not backward. Abnormal movements can lead to damage to these supporting structures, with long-term consequences for the joint. For example, a common football injury occurs when a player who has his leg extended forward while running is contacted on the outer side of the knee by another player running full tilt at him from the side. The knee, which is supposed to move forward to back, is suddenly wrenched side to side, tearing the supports.
To keep the skin that covers the joint from restricting movement, the movements of the bones within the joint must be isolated from the skin. The structures that separate the joint from the overlying tissues are small, mobile sacs of fluid called bursae. Like other components of the joint, damage to these structures can cause joint pain (bursitis).