Skeletal System: Joints (Articulations)
Joints (Articulations)
Introduction to Joints
Definition: Joints, also called articulations, are the places where two bones meet.
Function: They give the skeleton its mobility and hold it together.
Vulnerability: Due to the mobility they provide, joints are the weakest parts of the skeleton.
Classification: Every joint has both a functional and a structural classification.
Functional Classification
Based on the amount of movement allowed at the joint.
Synarthrotic Joints: Completely immovable (e.g., most joints in the axial skeleton for support).
Amphiarthrotic Joints: Slightly movable (in-between immovable and freely movable).
Diarthrotic Joints: Freely movable (e.g., most joints in the limbs).
Structural Classification
Focuses on the material that binds the two bones together and whether a joint cavity is present.
Three main structural classifications:
Fibrous Joints
Cartilaginous Joints
Synovial Joints
Fibrous Joints
Description: Bones are joined together by dense fibrous connective tissue.
Joint Cavity: There is no joint cavity (no opening) between the bones.
Movement: The amount of movement depends on the length of the fibers.
Types of Fibrous Joints:
Sutures:
Description: Seam-like joints found only between the bones of the skull, characterized by wavy, interlocking bone edges.
Functional Classification: Synarthrotic (completely immovable).
Development: Initially open to allow brain growth, later become joints, and fuse completely in adulthood to form synostoses.
Syndesmoses:
Description: Bones are connected by ligaments (bands of fibrous tissue that connect bone to bone). The fibers are longer than those in sutures.
Functional Classification: Can be synarthrotic (no movement) or amphiarthrotic (slightly movable), depending on fiber length (e.g., distal tibia and fibula ligament allows little to no movement, but can become wiggly if rolled).
Gomphoses:
Description: A peg-in-socket joint, resembling a nail or bolt hammered into a socket (Greek for nail/bolt).
Example: The only example is a tooth sitting in an alveolar socket.
Connection: The periodontal ligament provides the fibrous connection, making it a fibrous joint.
Cartilaginous Joints
Description: Bones are united by cartilage.
Joint Cavity: There is no joint cavity.
Functional Classification: Either synarthrotic or amphiarthrotic (not highly movable).
Types of Cartilaginous Joints:
Synchondroses:
Description: Bones are joined by a bar or plate of hyaline cartilage.
Functional Classification: Virtually all are synarthrotic (no movement).
Examples:
Temporary Epiphyseal Plate: Binds the epiphysis to the diaphysis (a temporary synchondrosis that ossifies).
First Rib to Manubrium: The joint between the first rib and the sternum's manubrium.
Symphyses:
Description: Articular surfaces are covered with hyaline cartilage, which in turn is fused to a fibrocartilage pad. Fibrocartilage unites the bones, acting as a shock absorber and allowing limited movement.
Functional Classification: Amphiarthrotic (slightly movable).
Examples:
Intervertebral Discs: Discs between vertebrae.
Pubic Symphysis: The joint connecting the two pubic bones. Hormones during pregnancy soften this pad to allow wider hip opening.
Synovial Joints
Description: Bones are separated by a fluid-filled joint cavity.
Joint Cavity: This is the first structural classification to feature a joint cavity (an opening between bones).
Functional Classification: All synovial joints are diarthrotic (freely movable).
Location: Includes almost all joints in the limbs and most joints in the body.
Six Distinguishing Features of Synovial Joints
Articular Cartilage:
Composition: Made of hyaline cartilage, typically about millimeter () thick.
Function: Covers the ends of both bones; prevents crushing of bone surfaces and absorbs compression placed on the joint.
Joint (Synovial) Cavity:
Description: A small space between the bones that is filled with fluid.
Articular Capsule (Joint Capsule):
Description: A double-layered capsule that encloses the joint cavity.
Layers:
External Fibrous Layer: Tough, dense irregular connective tissue, continuous with the periosteum of the bone, providing structural strength.
Inner Synovial Membrane: Loose connective tissue that lines the inside surface of the joint capsule (everywhere except where there is cartilage) and produces synovial fluid.
Synovial Fluid:
Composition: Viscous (thick) filtrate of blood plasma, appearing like egg white.
Production: Formed by filtering blood through capillaries in the synovial membrane.
Function: Reduces friction between bone ends, lubricates the joint, and nourishes the cells of the articular cartilage (squeezes out with compression to nourish cells).
Viscosity: Thins out and becomes less viscous (clearer) when warmed, which is why warming up before exercise is beneficial.
Reinforcing Ligaments:
Function: Strengthen the articular capsule and help hold bones together.
Location: Can be found outside the capsule (extracapsular), intrinsic to the capsule (thickened parts of the fibrous layer), or inside the capsule (intracapsular, e.g., in the knee).
Nerves and Blood Vessels:
Innervation: Synovial joints are richly supplied with nerve fibers that detect pain and monitor joint position and stretch, preventing hyperextension.
Vascularization: Well-vascularized to filter blood for synovial fluid production and to remove old synovial fluid.
Other Special Features of Some Synovial Joints
Fatty Pads: Found in some joints (e.g., hip, knee) between the fibrous layer of the capsule and the synovial membrane; provide cushioning.
Articular Discs (Menisci): Wedges of fibrocartilage that partially or completely separate the articular surfaces.
Function: Improve the fit between bone ends, make the joint more stable, and reduce wear and tear (e.g., in the knee).
Bursae and Tendon Sheaths: Both are sacs of lubricant, reducing friction.
Bursae: Flattened fibrous sacs lined with synovial membrane, containing synovial fluid. They act like