Basic Structure and Function of Joints

Overview of Joint Structure and Function

• Joints classified by anatomic structure and movement potential.
• Synovial joints analyzed through mechanical analogy.

Classification of Joints

• Synarthrosis:

  • Connective tissue junction with minimal movement.
  • Functions to bind bones and transmit forces.
  • Examples: sutures in the skull, distal tibio-fibular joint.

• Amphiarthrosis:

  • Formed by fibrocartilage or hyaline cartilage allowing restrained movement.
  • Functions to transmit and disperse forces.
  • Examples: intervertebral discs, pubic symphysis.

• Diarthrosis:

  • Known as synovial joints, featuring fluid-filled joint cavities.
  • Specialized for movement; prominent in upper and lower extremities.

Features of Synovial Joints

• Contains synovial fluid for nutrition and lubrication.
• Articular cartilage provides shock absorption and covers bone ends.
• Articular capsule protects the joint with two connective tissue layers.
• Synovial membrane produces lubricating hyaluronate and glycoproteins.
• Capsular ligaments resist motion and support joint structure.
• Blood vessels supply nutrition and support through capillary beds.
• Sensory nerves in the fibrous capsule provide pain and proprioception feedback.

Additional Structural Features in Some Synovial Joints

• Intraarticular Discs/Menisci:

  • Pads of fibrocartilage that enhance joint congruency and force dissipation.

• Labrum:

  • Fibrocartilage structure that deepens the articulation in joints like the shoulder and hip.

• Fat Pads:

  • Located within joint capsules; thicken capsule and manage synovial fluid volume.

• Synovial Plicae:

  • Pleats of tissue that increase surface area for synovial fluid without excess tension.

Mechanical Analogies for Synovial Joints

• Recognize seven types of synovial joints based on function:

  • Hinge Joint: Angular motion primarily in one plane. Examples: humeroulnar, interphalangeal joints.

  • Pivot Joint: Spin motion around a single axis. Examples: humeroradial, atlanto-axial joints.

  • Ellipsoid Joint: Convex and concave surfaces allowing bi-planar motion. Example: radiocarpal joint.

  • Ball-and-Socket Joint: Allows motion in all three planes, including spin. Examples: glenohumeral, hip joints.

  • Plane Joint: Movement through sliding and slight rotation; lacks a defined axis. Examples: carpometacarpal joints.

  • Saddle Joint: Allows bi-planar motion with limited spin; both surfaces concave and convex. Example: carpometacarpal joint of the thumb.

  • Condyloid Joint: Offers two degrees of freedom with a comparatively shallow concave surface. Examples: tibiofemoral, atlanto-occipital joints.

Instantaneous Axis of Rotation

• The axis changes position with angles during motion; path of movement is referred to as the evolute.
• Less congruence in joint surfaces results in a longer evolute path.