Joints: Structural and Functional Classifications

Classification of Joints

• Joints are sites where bones meet, providing mobility and holding the skeleton together.

• Classifications:

  • Structural: Fibrous, Cartilaginous, Synovial (based on binding material).

  • Functional: Synarthroses (immovable), Amphiarthroses (slightly movable), Diarthroses (freely movable).

Fibrous Joints

• Bones joined by dense fibrous connective tissue; no joint cavity.

• Types:

  • Sutures: Skull joints that ossify in middle age.

  • Syndesmoses: Bones connected by ligaments (e.g., tibiofibular joint, interosseous membrane).

  • Gomphoses: Peg-in-socket joints (teeth in sockets).

Cartilaginous Joints

• Bones united by cartilage; no joint cavity, not highly movable.

• Types:

  • Synchondroses: Hyaline cartilage unites bones (e.g., epiphyseal plate).

  • Symphyses: Fibrocartilage unites bones (e.g., intervertebral joints).

Synovial Joints

• Bones separated by a fluid-filled joint cavity; all are diarthrotic.

• Features:

  • Articular cartilage: Hyaline cartilage prevents crushing.

  • Joint cavity: Fluid-filled space.

  • Articular capsule: Fibrous layer and synovial membrane.

  • Synovial fluid: Lubricates and nourishes.

  • Ligaments: Reinforce the joint.

  • Nerves and vessels: Detect pain and supply filtrate.

• Other features: Fatty pads and articular discs (menisci).

Bursae and Tendon Sheaths

• Synovial fluid bags that reduce friction.

• Bursae: Between ligaments, muscles, skin, tendons, or bones.

• Tendon sheaths: Wrap around tendons.

Stability of Synovial Joints

• Influenced by articular surface shape, ligament number, and muscle tone.

Movements Allowed

• Nonaxial, Uniaxial, Biaxial, Multiaxial.

• Types:

  • Gliding: Flat surfaces slip.

  • Angular: Flexion, Extension, Abduction, Adduction, Circumduction.

  • Rotation: Medial or lateral.

Special Movements

• Supination/Pronation, Dorsiflexion/Plantar flexion, Inversion/Eversion, Protraction/Retraction, Elevation/Depression, Opposition.

Types of Synovial Joints

• Plane, Hinge, Pivot, Condylar, Saddle, Ball-and-socket.

Selected Synovial Joints

• Jaw (TMJ), Shoulder, Elbow, Hip, Knee.

Temporomandibular Joint (TMJ)

• Hinge joint for depression/elevation and gliding for lateral excursion.

• Prone to dislocation; symptoms include pain and stiffness.

Shoulder (Glenohumeral) Joint

• Freely moving; stability sacrificed.

• Reinforced by rotator cuff tendons.

• Common anterior/inferior dislocations.

Elbow Joint

• Hinge joint for flexion/extension.

• Ulnar and radial collateral ligaments.

Hip (Coxal) Joint

• Ball-and-socket; deep socket limits motion but enhances stability.

Knee Joint

• Complex joint with femoropatellar and tibiofemoral components.

• Stabilized by cruciate and collateral ligaments.

• Vulnerable to injuries (3 C’s).

Disorders of Joints

Common Joint Injuries

• Cartilage tears: Repaired with arthroscopic surgery.

• Sprains: Ligament tears; treated with repair or grafts.

• Dislocations: Bones out of alignment; must be reduced.

Inflammatory/Degenerative Conditions

• Bursitis: Bursa inflammation.

• Tendonitis: Tendon sheath inflammation.

• Arthritis: >100 types; Osteoarthritis, Rheumatoid arthritis, Gouty arthritis, Lyme disease.

Developmental Aspects

• Synovial joints resemble adult joints by week 8 of development.

• Use modifies joint size and flexibility.

• Aging affects joints; exercise postpones problems.