Algonquin College
5.4 - Joint Structure and Function
Joints connect the components of the skeletal system together. They give the skeletal system flexibility, and allow muscles to direct movements by moving bones in different directions.

Joints can be classified structurally or functionally.

There are three basic functional classifications for joints:
  • Synarthroses are immovable joints; these joints are common where protection of delicate internal structures (such as the brain and spinal cord) is important.
  • Amphiarthroses are slightly movable joints; these joints are common where protection of delicate internal structures (such as the brain and spinal cord) is important.
  • Diarthroses are freely movable joints; these joints dominate in the limbs and areas of the body where movement is important.
There are three basic structural classifications for joints: fibrous, cartilaginous and synovial joints.

Fibrous joints allow very little movement, and are composed of fibrous (dense) connective tissue. The skull sutures and syndesmoses such as the connection between the tibia and fibula are fibrous joints.

Cartilaginous joints allow very little or no movement, and are characterized by a connection between adjoining bones made of cartilage. The pubic symphysis, intervertebral joints and connection between the first rib and sternum are slightly movable cartilaginous joints. The epiphyseal plate of growing bones is an immovable cartilaginous joint.

Synovial joints are the most complex of the joint types. They are characterized by articular (hyaline) cartilage covering the ends of bones, a fibrous articular capsule (composed of fibrous connective tissue) lined with synovial membrane, a joint cavity containing synovial fluid and reinforcing ligaments to hold the bones together. Synovial joints are found in between the bones of the limbs, and are freely movable.

Synovial joints are also associated with bursae, which are flattened fibrous sacs lined with synovial membrane that develop in areas of friction. Tendon sheaths are special bursae that wrap around tendons in areas of friction.

Figure 5.4.1 Joint types.



Figure 5.4.2 Structure of a synovial joint.

There are several types of synovial joints, each specialized in specific types of movement.

Plane joints connect two flat surfaces of bone to one another, and only allow side-to-side movement with no rotation. Since plane joints allow no rotation, they are called nonaxial joints.The short bones of the wrist move against one another with plane joints.

Hinge joints connect a cylindrical bone end to a concave (indented) portion of another bone. Rotation can occur in only one plane (or axis), much like a door hinge, and are thus called uniaxial joints. The elbow and ankle joints are hinge joints.

Pivot joints connect the rounded end of one bone to a ring or sheath formed by another bone. Pivot joints are uniaxial joints. The joint between the radius and ulna at the elbow is a pivot joint.

Condyloid joints fit the rounded convex articular surface of one bone into the rounded concave surface of another bone. Because both bone ends are rounded and fit closely together, condyloid joints allow side to side and forwards-backwards movements, but no rotation, similarly to saddle joints. Movement occurs in two planes, making condyloid joints biaxial joints. The joints of the knuckles are condyloid joints.

Saddle joints are characterized by concave and convex surfaces on both articular surfaces. Saddle joints allow side to side and forwards-backwards movements, but no rotation, similarly to condyloid joints. Movement occurs in two planes, making saddle joints biaxial joints. The joint between the carpal and metacarpal of the thumb is a saddle joint.

Ball-and-socket joints join the spherical end of one bone to the concave, rounded socket of another bone. These joints allow movement in all axes and rotation, and are therefore multiaxial joints. They are the most movable of all synovial joints, and are found in the shoulder and hip.

Figure 5.4.3 Movement in each type of synovial joint.