Articulations and Joint Classification

Articulations

• Articulations are unions or junctions between bones or rigid parts of the skeleton.
• Articulations can be classified by function or structure.
• Functional classification is based on the amount of movement permitted.
  • Synarthroses are immovable joints, common in the axial skeleton.
  • Amphiarthroses are slightly movable joints, common in the axial skeleton.
  • Diarthroses are freely movable joints, common in the appendicular skeleton; all synovial joints are diarthroses.
• Structural classification is based on the material that binds the bones together.
  • Fibrous joints are connected by fibrous connective tissue.
  • Cartilaginous joints are connected by cartilage.
  • Synovial joints have bones connected by a synovial membrane.

Types of Fibrous Joints

• Sutures are short fibers between bones, and are immovable (synarthroses).
  • Examples are sutures in the skull.
• Syndesmoses have long fibers between bones, and are slightly movable (amphiarthroses).
  • Examples are joints between the radius and ulna, or the tibia and fibula.
• Gomphosis are periodontal ligaments between bones and are immovable (synarthroses).
  • Examples are the roots of teeth in the jaw.

Types of Cartilaginous Joints

• Synchondroses are hyaline cartilage between bones and are immovable (synarthroses).
  • Examples are costochondral joints and epiphyseal plates in long bones.
• Symphyses are fibrocartilage between bones and are slightly movable (amphiarthroses).
  • Examples are intervertebral discs and the pubic symphysis.

Summary of Joint Classes

• Fibrous:
  • Adjoining bones are united by collagenic fibers.
  • Types: sutures, syndesmoses, gomphoses.
  • Mobility: Immobile (sutures), slightly mobile (syndesmoses), immobile (gomphoses).
• Cartilaginous:
  • Adjoining bones are united by cartilage.
  • Types: synchondroses, symphyses.
  • Mobility: Immobile (synchondroses), slightly movable (symphyses).
• Synovial:
  • Adjoining bones are separated by a joint cavity, covered with articular cartilage, and lined with a synovial membrane.
  • Types: plane, hinge, pivot, condylar, saddle, ball-and-socket.
  • Mobility: Freely movable (diarthroses).

Synovial Joints

• All synovial joints are diarthroses.
• Key components of every synovial joint:
  • Synovial fluid-filled joint cavity
  • Articular cartilage- covers articulating surfaces
  • Articular capsule with fibrous and synovial layers
  • Reinforcing ligaments
• Some synovial joints contain an articular disc.
• Classification is based on the shape of joint surfaces, which determines the types of movements.

Movements Allowed by Synovial Joints

• Gliding - one bone glides over another; no change in the angle.
• Angular movements (change the angle between the bones)
  • Flexion and extension- bending and straightening
  • Abduction and adduction- moving away from or towards the mid-line
  • Circumduction- combination of several angular movements that create a circular motion
• Rotation- bone pivots around its own long axis

Synovial Joints Classified by Shape

• Plane joints: Articular surfaces are flat planes; examples include intercarpal, intertarsal, and acromioclavicular joints; gliding is the primary movement type;
• Hinge joints: Uniaxial, permitting only flexion and extension; Examples include elbow, knee, and interphalangeal joints;
• Pivot joints: Uniaxial, allowing only rotation; Examples include the radioulnar and atlantoaxial joints
• Condylar joints (ellipsoid): Biaxial, allowing flexion, extension, abduction, adduction, and circumduction; examples include radiocarpal and metacarpophalangeal joints.
• Saddle joints: Biaxial, allowing flexion, extension, abduction, adduction and circumduction, and opposition; example including thumb (1st carpometacarpal) joint
• Ball-and-socket joints: Multiaxial, allowing movement in all axes; examples include shoulder and hip joints

Selected Synovial Joints

• Shoulder: most freely movable joint; lacks stability; articular capsule is thin and loose; glenohumeral ligaments, coracoacromial ligament, coracohumeral ligament stabilize this joint; muscles also aid in stability.
• Elbow: hinge joint with the humeroulnar joint; allows for flexion/extension; radio-ulnar joints also allow supination and pronation; important ligaments are ulnar collateral, radial collateral and annular.
• Hip: ball and socket structure; limited movement by ligaments and acetabulum; head of femur fits into acetabulum; stability comes from acetabulum and ligaments – iliofemoral, pubofemoral, ischiofemoral.
• Knee: acts primarily as hinge joint; two fibrocartilage menisci within cavity; femoral-patellar joint lets the patella glide; important ligaments are the anterior and posterior cruciate ligaments, and collateral ligaments.

Bursa and Tendon Sheaths

• Bursae are membrane-bound pockets filled with synovial fluid found where tendons or ligaments rub against bones.
• Reduce friction, shock absorption.
• Tendon sheaths are bursae that extend alongside tendons.