Synovial Joints

The synovial joint is the most common type of joint in the body. The structure of a synovial joint allows for the absorption of shock during movements and prevents bones from coming into direct contact. This joint is distinguishable from other types of joints because the synovial joint contains a fluid-filled enclosed cavity called a joint cavity. The joint cavity is enclosed and protected by the articular capsule. The articular capsule consists of two layers that surround and enclose a synovial joint. The outer layer is a spongy, fibrous connective tissue, and the inner layer is the synovial membrane. The synovial membrane is a specialized, secretory connective tissue membrane. This inner synovial membrane produces and secretes synovial fluid, a lubricating fluid that fills the joint cavity. Synovial fluid minimizes friction to protect joints, provides nutrients to the articular cartilage, and absorbs shock. Articular cartilage is hyaline cartilage that covers the articulating surface of the bones found in the synovial joint cavity.

Several synovial joints of the body are compartmentalized by specialized structures. An articular disc is a small, oval fibrocartilage structure that divides the synovial joint cavity. Because of the strong and flexible nature of fibrocartilage, an articular disc can unite the bones of the joint to each other or can provide cushioning between the bones. An articular disc can also smooth movements between connected bones, as seen at the temporomandibular joint. A meniscus is a large, C-shaped fibrocartilage structure that differs from an articular disc in that it only partly divides a synovial joint cavity. Another structure, a bursa, is a synovial fluid–filled thin connective tissue sac that serves to reduce friction between adjacent structures, such as ligaments, tendons, muscles, and bones, to prevent them from rubbing against each other. There are six types of synovial joints: pivot, hinge, saddle, plane, condylar, and ball-and-socket.

• Pivot joints allow rotation about a single axis. These joints are formed between a rounded portion of a bone and a ring of another bone. The rounded bone can rotate, or pivot, within this ring. An example of a pivot joint is the joint in the neck between the first cervical vertebra (atlas) and the second cervical vertebra (axis) that allows rotation of the head from side to side.
• A hinge joint permits angular motion in a single plane. A depression on the end of one bone will form a hinge joint with the concave end of another bone. Hinge joints allow for bending and straightening motions along a single axis. Examples include the elbow, knee, and ankle joints.
• Saddle joints allow for angular motion in two planes and circumduction but not rotation. At a saddle joint, the convex shape of one bone fits in the concave shape of another bone. The joint at the base of the thumb is a saddle joint. This joint gives us "opposable thumbs," allowing the thumb to move away from the palm of the hand along two planes.
• Plane joints have flat or slightly curved faces that allow bones to slide past each other. Examples of plane joints are found between the carpal bones of the hand or cuneiform tarsal bones of the foot.
• Condylar joints allow angular motion in two planes and circumduction. At a condylar joint (also called an ellipsoid joint), there is an attachment between one shallow concave bone and the rounded end of another bone. Condylar joints are found at the finger knuckles between the phalanges (fingers) and the metacarpals (bones in the palm of the hand). This joint allows fingers to flex and extend and spread apart.
• Ball-and-socket joints allow angular and rotational movement and provide the greatest range of motion of any joint. At these joints, one bone has a ball shape at the end that fits into a socket of an adjacent bone. The hip joint and the shoulder joint are the only ball-and-socket joints of the body.

Each of these joints provides support for different motions of the appendages. The various motions permitted by the different joints can be described in relation to other parts of the body, the direction a bone moves, or the angle of a joint after movement. For example, moving the arms up to the side and away from the body is called abduction. Bringing the arms back towards the body is called adduction. Moving a bone upwards is called elevation. An example of elevation is when the hands are raised above the head. In contrast, bringing the bone downwards is called depression. Each of these movements is due to the fact that bones connect with each other in different places along their bodies and are held together with flexible tissues.