A joint is formed when two bones are brought together and held in place by supporting tissues. Joints may have large ranges of movement, as in the shoulder and hip joints, or have very little movement, as in the joints between the bones in the skull.
Like people, dogs and cats have three different types of joints that are classified by the type of tissues that connect the bones. These include synovial, fibrous and cartilaginous joints.
Synovial Joints
Of all joint types, synovial joints generally have the greatest range of movement. In a synovial joint, also known as a diarthrodial joint, the bone ends are covered with cartilage. Tough, fibrous tissue encloses the area between 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, either 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. Examples of synovial joints include the stifle (knee), elbow, shoulder and temporomandibular joint which unites the skull and the lower jaw.
Fibrous Joints
Fibrous joints allow for very little movement. The bones are held together tightly by tough, fibrous tissue. Fibrous joints include those that join the bones of the skull together. The skull is made up of over 40 different bones, all tightly held together by this fibrous tissue. Another example of a fibrous joint includes the mandibular symphysis, which joins the two mandibles (lower jaw bones).
Cartilaginous Joints
Cartilaginous joints allow for some movement and are formed when two or more bones are joined by cartilage. The joints formed between each vertebra in the vertebral column are cartilaginous joints. Intervertebral discs have cartilage on their outer surfaces and function to joint two vertebrae together and cushion any impact between these bones.