What is a joint?
Joints are the areas where two or more bones meet. Most joints are mobile, allowing the bones to move. Joints consist of the following:

• cartilage - at the joint, the bones are covered with cartilage (a connective tissue), which is made up of cells and fibers and is wear-resistant. Cartilage helps reduce the friction of movement.
  
  

• synovial membrane - a tissue called the synovial membrane lines the joint and seals it into a joint capsule. The synovial membrane secretes synovial fluid (a clear, sticky fluid) around the joint to lubricate it.
  
  

• ligaments - strong ligaments (tough, elastic bands of connective tissue) surround the joint to give support and limit the joint’s movement.
  
  

• tendons - tendons (another type of tough connective tissue) on each side of a joint attach to muscles that control movement of the joint.
  
  

• bursas - fluid-filled sacs, called bursas, between bones, ligaments, or other adjacent structures help cushion the friction in a joint.
  
  

• synovial fluid - a clear, sticky fluid secreted by the synovial membrane.
  
  

• femur - the thighbone.
  
  

• tibia - the shin bone.
  
  

• patella - the kneecap.
  
  

• meniscus - a curved part of cartilage in the knees and other joints.

Examples of the hip and knee joints follow:

What are the different types of joints?  
There are many types of joints, including joints that do not move in adults, such as the suture joints in the skull. Joints that do not move are called “fixed.” Other joints may move a little, such as the vertebrae. Examples of mobile joints include the following:

• ball-and-socket joints
  Ball-and-socket joints, such as the shoulder and hip joints, allow backward, forward, sideways, and rotating movements.
  
  

• hinge joints
  Hinge joints, such as in the fingers, knees, elbows, and toes, allow only bending and straightening movements.
  
  

• pivot joints
  Pivot joints, such as the neck joints, allow limited rotating movements.
  
  

• ellipsoidal joints
  Ellipsoidal joints, such as the wrist joint, allows all types of movement except pivotal movements

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