A joint is formed when two bones are brought together and held in place by supporting tissues. Joints may have large ranges of movement such as the shoulder and hip joints, or have very little movement such as the joints between the bones in the skull.
There are three types of joints based upon the type of tissues that connect the bones:
Synovial Joints
Synovial joints generally have the greatest range of movement. In a synovial joint, the bone ends are covered with cartilage which prevents the bones from rubbing directly against each other. Tough, fibrous tissue encloses the area around the bone ends and is called the joint capsule. Ligaments, which are also made of tough fibrous tissue, help hold the bones in alignment. The ligaments may be part of the joint capsule, inside of it, or outside of it. The area inside the joint capsule is called the joint cavity and is filled with a fluid called synovial (joint) fluid which lubricates the joint and reduces friction within the joint. Examples of synovial joints include the joints in the legs and the temporomandibular joint which joins the skull to the lower jaw (mandible).
Fibrous Joints
Fibrous joints allow very little to no movement. The bones are held together tightly by tough, fibrous connective tissue. Fibrous joints do not have a joint capsule or synovial fluid. Fibrous joints include those that join the bones of the skull together. The canine skull is actually made up of over 40 different bones, all tightly held together by this fibrous tissue.
Cartilaginous Joints
Cartilaginous joints allow some movement as compared to a fibrous joint, but less movement than a synovial joint. Cartilaginous joints are formed when two or more bones are joined together entirely by cartilage. The joints formed between each vertebra in the spine are cartilaginous joints. The intervertebral disc is actually cartilage, which joins two vertebrae together. Another example of cartilaginous joints are the joints where the ribs meet the sternum.