Joints: Type and Classification

Synovial joints permit movement and are categorized by the type of movement they allow:

1. Ball-and-socket joint: Also known as an enarthrodial joint, this joint allows a wide range of movement in many directions. Examples are the shoulder and hip joints.

2. Saddle joint: The sellar or saddle joints are like ball-and-socket joints but cannot rotate. The thumbs are the only sellar joints in the human body.

3. Hinge joint: The hinge or ginglymus joints include the elbows, ankles, and knee joints. They allow a wide range of movement in one plane.

4. Gliding joint: The arthrodial or gliding joints of the body include the tarsometatarsal joints of the foot. In these joints, two flat bones press against each other.

5. Pivot joint: Trochoidal or pivot joints rotate around a long axis. The radioulnar joint of the forearm is a pivot joint.

6. Condyloid joint: Also known as an ellipsoid joint, these joints move through two planes of motion. Rotation is not allowed in these joints. The radiocarpal joint of the wrist is one.

Joint Classification

There are three classifications of joints in the human body.

Fibrous joints - (synarthrosis) Connected by dense connective tissue consisting mainly of collagen. Fibrous joints are called “fixed” or “immovable” joints because they do not move.

• Synarthrodial joints are found in the skull. During birth, sutures are flexible to allow the fetus to pass through the birth canal. However, they become more rigid as the child grows.

• Syndesmoses are found between the long bones. These are only slightly movable.

• Gomphosis joints attach teeth to the sockets of the maxilla and mandible.

Cartilaginous joints - (amphiarthrosis) are joined by either fibrocartilage or hyaline cartilage. These are slightly movable. Cartilaginous joints are divided into primary and secondary joints and allow more movement between bones than a fibrous joint but less than synovial joints. Epiphyseal (growth) plates are primary. Secondary cartilaginous joints include the manubriosternal joint, intervertebral discs, and the symphysis pubis.

Synovial joints (diarthrosis) are the most movable joints. Bones are separated by a synovial cavity made of dense, irregular connective tissue. They join bones with a fibrous joint capsule that is continuous with the periosteum of the joined bones, constitute the outer boundary of a synovial cavity, and surround the bones’ articulating surfaces.

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