Chapter 8: Joints – Classification, Structure, and Function

Joints (Articulations)

Introduction to Joints

Joints, or articulations, are sites where two or more bones meet. They play a crucial role in providing the skeleton with mobility and holding the skeleton together.

• Function of joints: Enable movement and provide structural support.

• Classification: Joints are classified by their structure and the type of movement they allow.

Primary Joint Classifications

Structural Classifications

Joints are structurally classified based on the material binding the bones and the presence or absence of a joint cavity.

• Fibrous Joints: Bones joined by dense fibrous connective tissue; no joint cavity; most are immovable.

• Cartilaginous Joints: Bones united by cartilage; no joint cavity; not highly movable.

• Synovial Joints: Bones separated by a fluid-filled joint cavity; freely movable.

Functional Classifications

Joints are also classified by the amount of movement they allow:

• Synarthroses: Immovable joints.

• Amphiarthroses: Slightly movable joints.

• Diarthroses: Freely movable joints.

Types of Fibrous Joints

• Sutures: Rigid, interlocking joints of the skull; allow for growth during youth and fuse in adulthood.

• Syndesmoses: Bones connected by ligaments (e.g., inferior tibiofibular joint, interosseous membrane between radius and ulna); movement varies by length of fibers.

• Gomphoses: Peg-in-socket joints (e.g., tooth in its socket); held by the periodontal ligament.

Types of Cartilaginous Joints

• Synchondroses: Bar or plate of hyaline cartilage unites bones (e.g., epiphyseal plates in children, cartilage of 1st rib with manubrium of sternum); mostly immovable.

• Symphyses: Fibrocartilage unites bones (e.g., intervertebral joints, pubic symphysis); strong, slightly movable.

Structural Characteristics of Synovial Joints

General Features

Synovial joints are characterized by the presence of a fluid-filled joint cavity and are the most movable type of joint, found in nearly all limb joints.

• Articular cartilage: Hyaline cartilage covers the ends of bones, reducing friction and absorbing shock.

• Joint (articular) cavity: Small, fluid-filled space unique to synovial joints.

• Articular capsule: Two-layered capsule enclosing the joint cavity:

  • Fibrous layer: Strengthens the joint.

  • Synovial membrane: Produces synovial fluid and contains phagocytic cells.

• Synovial fluid: Viscous, slippery filtrate of plasma and hyaluronic acid; lubricates and nourishes articular cartilage.

• Reinforcing ligaments: Strengthen and stabilize the joint.

• Nerves and blood vessels: Detect pain, monitor joint position, and supply nutrients for synovial fluid.

Bursae and Tendon Sheaths

These structures reduce friction in synovial joints:

• Bursae: Flattened, fibrous sacs lined with synovial membrane and containing synovial fluid; found where ligaments, muscles, skin, tendons, or bones rub together.

• Tendon sheaths: Elongated bursae that wrap around tendons subjected to friction.

Factors Influencing Joint Stability

Joint stability is essential to prevent dislocations and is determined by:

• Articular surface: Shape and fit of bone ends (minor role; shallow surfaces are less stable).

• Ligaments: Number and location; more ligaments generally mean stronger joints.

• Muscle tone: Most important factor; keeps tendons taut as they cross joints.

Movements Allowed by Synovial Joints

Types of Movements

• Gliding: One flat bone surface glides or slips over another (e.g., intercarpal joints).

• Angular movements: Change the angle between bones.

  • Flexion: Decreases the angle of the joint.

  • Extension: Increases the angle of the joint.

  • Hyperextension: Extension beyond anatomical position.

  • Abduction: Movement away from the midline.

  • Adduction: Movement toward the midline.

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

• Rotation: Turning of a bone around its own long axis (e.g., rotation of the head, humerus, or femur).

• Special movements:

  • Supination/Pronation: Rotation of the forearm (palm up/palm down).

  • Dorsiflexion/Plantar flexion: Upward/downward movement of the foot.

  • Inversion/Eversion: Sole of foot turns medially/laterally.

  • Protraction/Retraction: Anterior/posterior movement (e.g., jaw).

  • Elevation/Depression: Lifting/lowering a body part (e.g., shrugging shoulders).

  • Opposition: Movement of the thumb to touch the tips of other fingers.

Types of Synovial Joints

Synovial joints are classified by the shape of their articular surfaces and the type of movement they allow:

Type	Movement	Example
Plane	Nonaxial (gliding)	Intercarpal joints, joints between vertebral articular surfaces
Hinge	Uniaxial (flexion/extension)	Elbow, knee, interphalangeal joints
Pivot	Uniaxial (rotation)	Proximal radioulnar joint, atlantoaxial joint
Condylar (Ellipsoid)	Biaxial (flexion/extension, adduction/abduction)	Metacarpophalangeal (knuckle) joints, wrist joints
Saddle	Biaxial (greater freedom than condylar)	Carpometacarpal joint of the thumb
Ball-and-socket	Multiaxial (flexion/extension, adduction/abduction, rotation)	Shoulder, hip joints

Clinical Implications: Joint Injuries and Disorders

Common Joint Injuries

• Sprains: Ligaments are stretched or torn; common in ankle, knee, and lumbar spine; heal slowly due to poor blood supply.

• Dislocations (luxations): Bones forced out of alignment; accompanied by sprains, inflammation, and difficulty moving the joint; often caused by falls or contact sports.

• Subluxation: Partial dislocation of a joint.

Shoulder Dislocation

• Shoulder is prone to dislocation due to its mobility and weak anterior/inferior reinforcement.

• Dislocations often occur when the humerus is rotated laterally and force is applied (e.g., during a tackle in football).

Arthritis and Other Disorders

• Arthritis: Inflammatory or degenerative diseases that damage joints (e.g., osteoarthritis, rheumatoid arthritis, gouty arthritis).

• Joint injuries may cause locking or binding, and articular cartilage rarely repairs itself.

Additional info: Synovial joints are the most common and functionally important joints in the body, allowing for a wide range of movements necessary for daily activities and athletic performance.