The skeletal system is made up of more than 200 separate bones, which are held together at joints. The joints between bones, or articulations, are held together by connective tissue. The human skeletal joints are classified based on the amount of bone movement they allow.
Immovable Joints – Synarthrosis
A synarthrosis is an immovable joint in which the bones are sutured together by solid, fibrous connective tissue. Synarthroses are found between the cranial bones and the immovable facial bones. The cranial bones actually begin as separate bones with temporary cartilaginous joints called sychondroses, which allow limited flexibility for cranial growth. The bones then fuse together in infancy.
Flexible Joints – Amphiarthrosis
An amphiarthrosis is a fibrocartilaginous joint with limited movement. A disc of fibrous cartilage, called a symphysis, is found between the bones. Amphiarthroses are found in the pubic bones and the spinal column. The symphyses between the vertebrae allow some flexibility to the back. However, too much movement can herniate, or slip, a disc, a jelly-like cushion between the vertebrae. This condition results in back pain, but can also put pressure on the spinal cord, which runs inside the vertebral column. Also, increased pressure on the pubic bones, such as with pregnancy, can cause stress on the amphiarthroses, resulting in pelvic pain. The condition is known as symphysis pubic dysfunction.
Synovial Joints - Diarthrosis
A diarthrosis is a freely movable joint, also known as a synovial joint because of their structure. Articular cartilage is found on the end of both bones meeting at the joint. The articulation is enclosed in a joint capsule made of fibrous connective tissue and lined with a synovial membrane that secretes synovial fluid. This construction allows for lubricated movement of a well held together joint.
Diarthroses are further categorized based on the type of movement they allow:
- Ball and socket – full range of motion (all planes), examples are the shoulder and hip
- Hinge – movement in one plane (up and down or side to side, but not both), examples are the knee and elbow
- Pivot – rotation, an example is the atlas and axis bones of the neck
- Gliding – side to side, an example is the small fingers of the hand (carpals)
- Saddle – movement in many planes, but not as full range as ball and socket, an example is the thumb where it joins the hand
- Condyloid – movement in one plane, but flexibility for a second, an example is the jaw
For a list of the categories specific joints belong to, see Human Skeletal Movement – Synovial Joints
Additional References:
American Association of Neurological Surgeons
MedlinePlus
Scanlon and Sanders. Essentials of Anatomy and Physiology, 4th edition. 2003.
Alicia Mae Prater - Alicia received her doctorate in Experimental Pathology in 2007. She has been a freelance writer and scientific editor since 2008.
Join the Conversation