The synovial joints, also known as diarthroses, comprise most of the joints in the human body. These movable joints, or articulations, are named for their structure, which consists of connective tissue and secreted fluid that hold together the bones while allowing smooth movement.
The Structure of Synovial Joints
The ends of the bones joined by a diarthrosis are covered in articular cartilage. The articulation is enclosed by fibrous connective tissues, called a joint capsule. The capsule is lined with synovial membrane, which secretes synovial fluid into the joint cavity. Joints that have tendons crossing outside the capsule have an additional sac of synovial fluid between the joint and tendon, called a bursa.
How Movable Joints Function
The synovial joints are constructed in such a way that the bones can move freely against each other. The articular cartilage and synovial fluid reduce friction and protect the ends of the bones during movement. The connective tissues, ligaments, hold the bones together and determine the limitation of the movement the joint can make. Tendons that cross the joint pull on the bone with one end based on the contraction of the muscle at the other end of the connective tissue, actually causing the movement of the bone and determining which direction the joint bends.
Types of Synovial Joints
The diarthroses are categorized by the movement they are capable of achieving under normal conditions. There are six main types. Ball and socket joints consist of a cup on one bone that fits over a ball like structure at the end of the other bone. This allows for rotational and multi-plane movement. Hinge joints only allow bending for movement in one plane. Condyloid joints also move in only one plane, but have some lateral flexibility because the bones fit together in an ellipsoid manner. Pivot joints rotate. Gliding joints move side to side, with the bones moving along each other. Finally, saddle joints move in many planes, but do not have the full range of motion as ball and socket joints because the bones do not fit together in a cup-like manner.
What follows is a list of the categories specific diarthroses are considered to fall under.
- Ball and socket: Shoulder (humerus and scapula), hip (hip and femur)
- Hinge: Knee (femur and tibia), elbow (humerus and ulna), fingers and toes (phalanges)
- Condyloid: Jaw (mandible), hand (metacarpals and phalanges)
- Pivot: Neck (atlas and axis), elbow (radius and ulna)
- Gliding: Foot (tarsals, metatarsals), ankle (tibia, fibia, and talus), hand (carpals), wrist (radius, ulna, and carpals)
- Saddle: Thumb (carpometacarpal)
For more information on the joints of the human body, see “Definition of joints of the Body, principle” at Medterms (accessed April 10, 2010).
Reference:
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.
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