Chapter 9: Articulations (Joints) – Structure, Function, and Classification

Chapter 9: Articulations (Joints)

Introduction to Articulations

Articulations, or joints, are anatomical structures where two or more bones meet. They play a crucial role in holding the skeleton together and, in many cases, permitting movement. The structure and function of joints reflect a balance between the need for strength and the need for mobility.

• Definition: An articulation (joint) is the site where two bones interconnect.

• Functions:

  • Hold bones together

  • Permit varying degrees of movement

• Range of Motion (ROM): The degree of movement a joint allows; influenced by joint structure, ligaments, and muscle tone.

Classification of Joints

Structural Classification

Structural classification is based on the material binding the bones together and the presence or absence of a joint cavity.

• Bony fusion: Bones fuse and boundary disappears (e.g., synostosis).

• Fibrous joints: Bones joined by dense fibrous connective tissue (e.g., sutures, gomphoses).

• Cartilaginous joints: Bones joined by cartilage (e.g., synchondroses, symphyses).

• Synovial joints: Bones separated by a fluid-filled joint cavity (e.g., most limb joints).

Functional Classification

Functional classification is based on the amount of movement allowed by the joint.

• Synarthroses: Immovable joints (fibrous or cartilaginous, may fuse over time).

• Amphiarthroses: Slightly movable joints (fibrous or cartilaginous).

• Diarthroses: Freely movable joints (synovial).

Summary Table: Types of Joints

Type	Subtype	Structural Material	Example	Mobility
Synarthroses	Sutures	Fibrous (dense CT)	Skull bones	Immovable
Synarthroses	Gomphoses	Fibrous (periodontal ligament)	Teeth in jaws	Immovable
Synarthroses	Synchondroses	Cartilaginous (hyaline cartilage)	Epiphyseal plates	Immovable
Synarthroses	Synostoses	Bony fusion	Epiphyseal lines	Immovable
Amphiarthroses	Syndesmoses	Fibrous (ligament)	Tibia & fibula	Slightly movable
Amphiarthroses	Symphysis	Cartilaginous (fibrocartilage)	Pubic symphysis, intervertebral discs	Slightly movable
Amphiarthroses	Interosseous membrane	Fibrous	Between radius & ulna	Slightly movable
Diarthroses	Synovial	Fibrous capsule, synovial fluid	Shoulder, knee, elbow	Freely movable

Detailed Structure of Synovial Joints (Diarthroses)

Key Features

• Articular cartilage: Hyaline cartilage covering bone ends; reduces friction and absorbs shock. Lacks perichondrium and has higher water content than typical hyaline cartilage.

• Joint (synovial) cavity: Space filled with synovial fluid.

• Articular capsule: Encloses the joint cavity; consists of:

  • Fibrous layer: Continuous with periosteum of articulating bones.

  • Synovial membrane: Inner layer; produces synovial fluid, contains areolar tissue, synoviocytes, and macrophages.

• Synovial fluid: Clear, viscous solution secreted by synoviocytes; functions include:

  • Lubrication

  • Nutrient distribution (from blood vessels in areolar tissue)

  • Shock absorption

Accessory Structures

• Articular discs/menisci: Wedges of fibrocartilage that separate articular surfaces and channel synovial fluid.

• Fat pads: Packing material between capsule and bone; covered by synovial membrane.

• Ligaments: Bone-to-bone connections that reinforce and support the joint.

• Bursae: Small, fluid-filled sacs that reduce friction and cushion movement.

• Tendon sheaths: Tubular bursae that wrap around tendons to limit and support joint's range of motion.

Factors Stabilizing Synovial Joints

• Shape of articular surfaces

• Collagen fibers of the joint capsule and ligaments

• Presence of other bones, muscles, fat pads, or cartilage

• Tension in tendons and muscle contraction (muscle tone)

Joint Injuries and Homeostatic Imbalances

• Sprain: Ligament is stretched or torn; may self-repair if partial, but complete tears can destabilize the joint.

• Cartilage injuries: Slow to heal due to poor blood supply; fragments can interfere with joint function.

• Dislocation (Luxation): Bones forced out of alignment; can damage cartilage, ligaments, and capsule.

• Bursitis/Tendonitis: Inflammation of bursae or tendon sheaths, often due to friction or overuse.

• Arthritis: Inflammatory or degenerative joint disease.

  • Osteoarthritis: Degenerative, age-related; articular cartilage breaks down, bones may rub together.

  • Rheumatoid arthritis: Autoimmune; inflammation, swelling, and possible ossification of synovial membrane.

Types of Synovial Joints (by Shape and Movement)

Type	Movement Allowed	Example
Ball-and-socket	Multiaxial (all angular and rotational movements)	Shoulder, hip
Ellipsoidal (Condyloid)	Biaxial (flexion/extension, abduction/adduction)	Wrist, metacarpophalangeal joints
Saddle	Biaxial (greater range than condyloid)	Thumb (carpometacarpal joint)
Hinge	Uniaxial (flexion/extension)	Elbow, knee, interphalangeal joints
Pivot	Uniaxial (rotation)	Proximal radioulnar joint, atlas-axis joint
Gliding (Plane)	Nonaxial or multiaxial (sliding movements)	Intercarpal, intertarsal joints

Types of Joint Movements

• Gliding: Flat surfaces slide past each other.

• Angular movements: Change the angle between bones (flexion, extension, abduction, adduction, hyperextension).

• Circumduction: Circular movement combining flexion, extension, abduction, and adduction.

• Rotation: Bone turns around its own long axis (medial/lateral rotation, pronation/supination).

Clinical Connections

• Hypermobility: Excessive range of motion, often due to abnormal bone shape, weak or stretched ligaments, or altered muscle tone.

• Double-jointed: Informal term for hypermobility; not an actual doubling of joints.

Example: The shoulder (glenohumeral) joint is a ball-and-socket synovial joint with high mobility but lower stability, making it prone to dislocation (luxation).

Additional info: Sensory neurons in joint capsules, ligaments, and tendons detect pain but are not present in the joint cavity itself.