What Are Diarthrotic Joints?

Diarthrotic joints are both the commonest and the most mobile joints in a skeleton. They consist of at least two bones that have cartilage covering the ends at the joint to reduce friction, ligaments that connect the bones together and tendons that connect the bones to the muscles that move them. They also have a joint capsule around them, formed by a synovial membrane that secretes a fluid that acts as a lubricant to promote smooth movement of the joint. This membrane gives them their alternate name: synovial joints. Diarthrotic or synovial joints can be divided into three categories, according to the planes in which they move, and these categories divide further into types of movement.

Uniaxial

• Uniaxial joints move in only one plane -- they open (extend) and close (flex) in an arc of motion rather than a range of motion. Hinge joints appear in the human elbow and knee. Pivot joints occur where the atlas and axial vertebrae and the radius and ulna come together.

Biaxial

• Condyloid joints join human fingers to the bones of the hand at the knuckles, and a saddle joint -- in which one bone has a part sticking out that fits into the other bone at a matching depression -- connects the thumb to the hand. These joints can move in two directions, which allows a greater range of motion.

Triaxial

• The joints of the human body with the greatest range of motion are in the hip and shoulder, which move in three planes -- right/left, forward/back and up/down, the way an airplane can move in the sky. Both are ball-and-socket joints, in which the rounded end of one bone fits into a cup in the other. A simpler type of triaxial joint is found between the bones of the fingers at the plane joints, where the ends of two bones just slide across one another.

Joint Problems

• Joints can have problems if they are overstressed or used too much, but can also have problems from being used too little. They can be damaged by injury or attacked by disease. Joints allow flexibility and motion, without which the human body cannot function properly, and should be protected and used correctly. For joint pain or malfunction, see a physician.