What Is the Definition of a Joint?

Joints Are What Allow Bones in the Body to Move

X-ray radiograph of 51 year-old hip
An example of ball and socket joints. SMC Images/Getty Images

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, or thigh bone, and the tibia, or shin bone.

Joints contain a variety of fibrous connective tissue. Ligaments connect the bones to each other; tendons connect muscle to bone and cartilage covers the ends of bones and provides cushioning.

Synovial Joints

The most common joints are freely movable joints in the body called synovial joints.

Synovial joints are surrounded by a fibrous tissue or sac called the joint capsule. This capsule surrounds the joint and is filled with a fluid called synovial fluid that lubricates the tissues and spaces within this capsule.

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.

Condyloid 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. 

Gliding Joints

You have this kind of joint, which allows bones to glide around and past each other in your spine, ankles, and wrists. 

Hinge Joints

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. 

Pivot Joints

Your neck and elbow both have pivot joints, which allow bones to pivot or twist around other bones.

 

Saddle Joint

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 of a Joint

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 of Joints

Extension is the bending of a joint so that the bones forming the joint are moved farther apart, or straightened. This is a physical position that decreases the angle between the bones of the limb at a joint. It occurs when muscles contract and bones move the joint into a bent position.

Flexion of Joints

Flexion is the bending of a particular joint so that the bones that form that joint are pulled closer together. During flexion, the angle between the bones of a limb at a joint is decreased. Flexion typically occurs when muscles contract and the bones thus move the nearby joint into a curved position. Flexion is a physical position that decreases the angle between the bones of the limb at a joint.

It occurs when muscles contract and bones move the joint into a bent position.

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