In human anatomy, a joint is the physical point of connection between two bones. For example, the knee joint is the point of connection between the femur (thigh bone) and the tibia (shin bone).
Joints contain a variety of fibrous connective tissue. Ligaments connect the bones to each other. Tendons connect muscle to bone. Cartilage covers the ends of bones and provides cushioning.
Immovable and Slightly Movable Joints
Fixed joints have no joint cavity, but the bones are connected by fibrous tissue (mostly collagen). These include the bones of the skull, which are connected flexibly in the infant but later fuse together in suture joints and eventually ossify (turn to bone). Fibrous tissue also connects the bone of your teeth to their sockets in your jaw.
In cartilaginous joints, the bones are held together by cartilage, with no joint cavity. The ends of the long bones have a cartilaginous joint in childhood, which later closes. The pubic symphysis, where the pubic bones meet, is a slightly-mobile cartilaginous joint.
The most common joints are freely movable joints in the body called synovial joints. Synovial joints are surrounded by fibrous tissue or sac called the joint capsule. The lining of this capsule secretes synovial fluid, which lubricates the tissues and spaces within this capsule. There are several types of synovial joints that allow different forms of motion.
Ball and Socket Joints
This type of joint allows for a wide range of rotation and movement, including rotation. Your shoulder and hip are examples of ball and socket joints.
The jaw and fingers both have condyloid joints. These joints don't allow rotation, but are versatile; think of the way a joystick moves on a video game console.
You have this kind of joint, which allows bones to glide around and past each other in your spine, ankles, and wrists.
Just like the name suggests, these joints work like hinges. Think of your knee and the part of your elbow that bends (the ulna). These are hinge joints.
Your neck and elbow both have pivot joints, which allow bones to pivot or twist around other bones.
The best example of a saddle joint and what it does is found in the base of the thumb. Saddle joints allow side to side and back and forth motion, but don't fully rotate.
Range of Motion
A majority of the human body’s joints allow for movement. A few, like joints in the skull, do not. Joints that do allow for motion, such as the knee or ankle, have a predetermined range of motion, which is basically how far is each direction that joint can move or bend comfortably.
The range of motion of a joint is usually measured in degrees. Typically, the extension of a joint is limited to 180 degrees or less. In other words, that joint can be opened until it is straight. Think of your arm or leg as an example: they can be bent until they're just about straight, but can't be pushed beyond 180 degrees without pain or damage.
Extension is an action in which the bones forming the joint are moved farther apart, or straightened from a bent position. This increases the angle between the bones of the limb at a joint.
Flexion occurs when the bones that form a joint are pulled closer together. During flexion, the angle between the bones of a limb at a joint is decreased. Muscles contract and bones are moved into a bent position at the joint.
Conditions Affecting the Joints
Arthritis is an inflammatory condition of a synovial joint. One type is osteoarthritis, in which the cartilage is damaged over time and thins until pressure between the bones causes pain. Rheumatoid arthritis is an autoimmune condition in which the immune system attacks the tissues of the joints, causing damage.
Gout occurs when uric acid crystals build up in a synovial joint (usually the big toe), causing pain. The synovial membrane may also become inflamed with overuse, resulting in synovitis.