Human Anatomy and Physiology: Articulations

Functions of Joints

• Connect two bones.
• Enable movement via muscles and tendons.
• Provide stability, especially in the skull.
• Allow long bones to lengthen at the epiphyseal plate.

Classes of Joints

Functional Classification:

• Synarthrosis: No movement; high stability.
• Amphiarthrosis: Small movement; significant stability.
• Diarthrosis: Freely moveable; least stable.

Structural Classification:

• Fibrous Joints: Dense connective tissue; no joint space; synarthroses or amphiarthroses.
• Cartilaginous Joints: Cartilage between bones; no joint space; synarthroses or amphiarthroses.
• Synovial Joints: Joint cavity with fluid; diarthroses.

Fibrous Joints

• Collagen fibers provide stability.
• Types include sutures, gomphoses, and syndesmoses.
• Sutures: Skull bones connected by short collagen fibers; synarthroses; may fuse into synostosis.
• Gomphoses: Teeth attached to alveoli via periodontal ligament; synarthroses.
• Syndesmoses: Bones joined by interosseous membrane or ligament; amphiarthroses (e.g., between radius/ulna).

Cartilaginous Joints

• Lack a joint cavity; allow little motion.
• Types include synchondroses and symphyses.
• Synchondroses: Hyaline cartilage unites bones; synarthroses (e.g., epiphyseal plate, first sternocostal joint).
• Symphyses: Fibrocartilage pad unites bones; amphiarthroses (e.g., intervertebral discs, pubic symphysis).

Synovial Joints: Joint Cavity

• Space between articulating bones.
• Articular Capsule: Double-layered; fibrous layer (dense connective tissue) and synovial membrane (loose connective tissue).
• Synovial Fluid: Lubrication, metabolic support, shock absorption.
• Articular Cartilage: Hyaline cartilage reduces friction/absorbs shock; avascular.
• Other Components: Adipose tissue, blood vessels, nerves.

Stabilizing and Supportive Structures

• Ligaments: Bone-to-bone connection; intrinsic (within capsule) or extrinsic (outside capsule).
• Tendons: Muscle-to-bone connection; stabilize joints via muscle tone.
• Bursae: Synovial fluid-filled sacs minimize friction.
• Tendon Sheaths: Long bursae protect tendons.

Joint Inflammation

• Bursitis: Inflammation of a bursa.
• Arthritis: Inflammation of joints, breakdown of cartilage.
• Osteoarthritis: Wear and tear.
• Rheumatoid Arthritis: Autoimmune.
• Gouty Arthritis: Uric acid crystal deposits.

Functional Classes of Synovial Joints

• Based on axes of motion.
• Nonaxial Joints: Motion in one or more planes, no axis.
• Uniaxial Joints: Motion around one axis.
• Biaxial Joints: Motion around two axes.
• Multiaxial Joints: Motion around three axes.

Movements at Synovial Joints

• Gliding Movements: Nonaxial sliding motion.
• Angular Movements: Change angle between bones.
• Flexion: Decreases angle.
• Extension: Increases angle.
• Abduction: Away from midline.
• Adduction: Toward midline.
• Circumduction: Cone-shaped motion.
• Rotation: Pivoting motion.
• Internal (Medial) Rotation: Toward midline.
• External (Lateral) Rotation: Away from midline.
• Special Movements: Unique to certain joints.
• Opposition/Reposition: Thumb movement.
• Depression/Elevation: Inferior/superior movement.
• Protraction/Retraction: Anterior/posterior movement.
• Inversion/Eversion: Foot rotation.
• Dorsiflexion/Plantarflexion: Foot/ankle movement.
• Supination/Pronation: Forearm rotation.

Structural Classes of Synovial Joints

• Plane Joint: Nonaxial, flat surfaces.
• Hinge Joint: Uniaxial, convex/concave surfaces.
• Pivot Joint: Uniaxial, rounded surface in a groove.
• Condylar Joint: Biaxial, oval convex/concave surfaces.
• Saddle Joint: Biaxial, both convex and concave regions.
• Ball-and-Socket Joint: Multiaxial, ball-shaped surface in a socket.

Specific Hinge Joints: Elbow and Knee

Elbow

• Humeroulnar and humeroradial articulations.
• Supported by radial, ulnar collateral, and anular ligaments.

Knee

• Tibiofemoral and patellofemoral joints.
• Menisci and ligaments enhance stability.
• Anterior Cruciate Ligament (ACL): Prevents anterior tibial movement.
• Posterior Cruciate Ligament (PCL): Prevents posterior tibial movement.

Specific Ball-and-Socket Joints: Shoulder and Hip

Shoulder

• Glenohumeral joint (humeral head and glenoid cavity).
• Stabilized by ligaments, tendons (rotator cuff), and glenoid labrum.

Hip

• Acetabulum and femoral head.
• Stabilized by acetabular labrum, strong capsule, and ligaments (iliofemoral, ischiofemoral, pubofemoral).

Clinical notes

• Knee Injuries: Lateral blows can cause the "Unhappy Triad" (ACL, MCL, and medial meniscus tears).
• Shoulder Dislocations: Glenohumeral joint displacement.
• Hip Replacement: Surgical replacement of damaged hip joint with prosthetic device.