Joints in the Human Skeleton

Joints

• Joints are the meeting, joining, union, or connection between different parts of the skeleton, specifically bones or cartilage
• Types of Joints:
  • Fibrous Joints:
    • Two bones connected by fibrous tissue
    • Mostly inelastic white fibers connecting the bony ends
    • Non-movable ("fixed" joints)
    • Examples: sutures of the skull (coronal, sagittal, etc.), Gomphosis (dentoalveolar syndesmosis), Inferior tibio-fibular joint (Syndesmosis)
  • Cartilaginous Joints:
    • Two bones connected by hyaline cartilage (primary) or fibro-cartilage (secondary), connecting the bony ends
    • Little to limited movement due to cartilage
    • More common than fibrous joints
    • Types of cartilaginous joints:
      • I. Primary Cartilaginous Joints (Synchondroses)
        • Hyaline cartilage, ossifies later in life (turns to bone)
        • No movement
        • Examples: epiphyseal plate of long bones, spheno-occipital (basi-spheoid) joint, between costal cartilage of first rib and manubrium sterni
      • II. Secondary Cartilaginous Joints (Symphyses)
        • Nature of articulation preserved throughout life and is an example of intervertebral discs as well as Sympysis pubis and Manubrio-sternal joint
  • Synovial Joints:
    • Most common and most movable type of joint in the body
    • Structure:
      • Bony articular ends: Two or more bony ends, usually large in size
      • Articular cartilage: Thin smooth layer of hyaline cartilage covering the opposing articular surfaces, providing a smooth surface for articulation and flexibility between articulating bones, facilitating movement.
      • Fibrous capsule: Extends and connects opposing bony ends, may be thickened to form ligaments or weakened by openings in certain areas. Contains intra-capsular ligaments.
      • Synovial membrane: Lines the inner surface of the fibrous capsule, and covers all intracapsular structures except articular surfaces. Contains synovial cells that secrete synovial fluid
      • Synovial fluid: Acts as a lubricant preventing friction between opposing bones, facilitates movement and plays a significant role in nourishing the articular cartilage. -Subsidiary structures inside synovial joints: Articular discs of fibro-cartilage (e.g., temporomandibular, sterno-clavicular), intra-capsular ligaments (e.g., cruciate ligaments of the knee joint), menisci of cartilage in knee joints, tendons of muscles (e.g., long head of biceps brachii), and pads of fat inside the joint cavity. -Blood supply: Articular twigs come from surrounding vessels and pierce the fibrous capsule to reach the joint
    • Nerve Supply: Capsule & ligaments receive an abundant sensory nerve supply. Sensory nerves also supply muscles moving the joint and the skin overlying the insertions of these muscles (Hilton's Law).

Stability of Joints

• Factors: shape, size, arrangement of articular surfaces, ligaments, and tone of muscles around the joint
  • Articular Surfaces: Shape, size, and arrangement are important for stability (e.g., ball-and-socket in hip and ankle joints).
  • Ligaments: Fibrous ligaments prevent excessive movement, but prolonged stress can cause stretching. Elastic ligaments, like those in the auditory ossicles, return to their original length after stretching and assist the return of bones to original position after movement.
  • Muscle tone: Important for stability, particularly in joints with less bony support/stability (e.g., shoulder, knee).

Classification of Synovial Joints

• Based on axis of movement:
  • Uni-axial: Move around a single axis (e.g., elbow, ankle)
  • Bi-axial: Move around two axes (e.g., wrist, metacarpo-phalangeal)
  • Poly-axial: Move in three axes (e.g., shoulder, hip)
• Based on shape of articulating surfaces:
  • Ball and socket: Poly-axial
  • Hinge: Uni-axial
  • Saddle: Bi-axial
  • Gliding (plane): Non-axial
  • Condyloid: Bi-axial/modified hinge
  • Ellipsoid: Bi-axial