Articulations (Joints)

General Terms

Articulations, or joints, are points where two or more bones meet. They play a crucial role in providing mobility and stability to the skeleton.

• Joint (Articulation): A connection between bones, allowing for movement and providing mechanical support.

Synovial Joints

Structure of Synovial Joints

Synovial joints are the most movable type of joint in the body, characterized by a fluid-filled cavity.

• Synovial Joint: A freely movable joint with a synovial cavity between articulating bones.

• Articular Capsule: A double-layered structure surrounding the joint, providing strength and enclosing the joint cavity.

• Fibrous Outer Layer: The tough, external layer of the articular capsule made of dense connective tissue, providing stability.

• Synovial Membrane: The inner layer of the capsule that secretes synovial fluid.

• Synoviocyte: Specialized cells in the synovial membrane that produce synovial fluid.

• Synovial Fluid: A viscous, lubricating fluid that reduces friction, nourishes cartilage, and absorbs shock.

• Joint Cavity: The space within the joint capsule that contains synovial fluid.

• Articular Cartilage: Smooth hyaline cartilage covering the ends of bones, reducing friction and absorbing shock.

Supportive Structures of the Synovial Cavity

• Ligament: Dense regular connective tissue connecting bone to bone, stabilizing joints.

• Tendon: Dense regular connective tissue connecting muscle to bone, transmitting muscle force to bones.

• Bursa: A small, fluid-filled sac reducing friction between moving structures (e.g., tendons and bones).

• Tendon Sheath: An elongated bursa wrapping around a tendon, especially in areas of high friction.

Functional Classes of Synovial Joints

Synovial joints are classified by the number of axes (planes) along which movement occurs.

• Nonaxial Joint: Allows slipping or gliding movements only; no axis of rotation (e.g., intercarpal joints).

• Uniaxial Joint: Movement in one plane or axis (e.g., elbow joint).

• Biaxial Joint: Movement in two planes or axes (e.g., metacarpophalangeal joints).

• Multiaxial (Triaxial) Joint: Movement in three planes or axes (e.g., shoulder joint).

Movements at Synovial Joints

Synovial joints allow a variety of movements, classified as gliding, angular, rotational, or special movements.

• Gliding: Flat bone surfaces slide past each other (e.g., carpal bones).

• Angular Movement: Change in the angle between bones.

• Flexion: Decreases the angle between articulating bones (e.g., bending the elbow).

• Extension: Increases the angle between articulating bones (e.g., straightening the knee).

• Hyperextension: Extension beyond the anatomical position (e.g., bending the head backward).

• Abduction: Movement away from the midline (e.g., raising the arm sideways).

• Adduction: Movement toward the midline (e.g., lowering the arm to the side).

• Circumduction: Circular movement combining flexion, extension, abduction, and adduction (e.g., arm circles).

• Rotation: Bone turns around its own longitudinal axis (e.g., turning the head side to side).

• Special Movements: Unique to certain joints.

• Opposition: Movement of the thumb to touch the fingertips.

• Reposition: Returning the thumb from opposition to anatomical position.

• Depression: Lowering a body part (e.g., opening the mouth).

• Elevation: Raising a body part (e.g., closing the mouth).

• Protraction: Moving a body part forward (e.g., jutting the jaw forward).

• Retraction: Moving a body part backward (e.g., pulling the jaw back).

• Inversion: Turning the sole of the foot medially.

• Eversion: Turning the sole of the foot laterally.

• Dorsiflexion: Lifting the foot upward toward the shin.

• Plantarflexion: Pointing the foot downward (standing on tiptoe).

• Supination: Rotating the forearm so the palm faces upward or anteriorly.

• Pronation: Rotating the forearm so the palm faces downward or posteriorly.

Structural Classes of Synovial Joints

Synovial joints are also classified by the shape of their articulating surfaces and the type of movement they allow.

Example: Shoulder Joint

• The shoulder joint is a classic example of a ball-and-socket joint, allowing a wide range of motion in multiple planes.

Additional info: Synovial joints are the most common type of joint in the body and are essential for complex movements required in daily activities and athletic performance. Disorders of synovial joints, such as arthritis, can significantly impair mobility and quality of life.