Joints - Anatomy & Physiology

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Joints comprise broadly two categories:

  • Synarthroses form joints that are relatively rigid
  • Diarthroses form joints that are freely movable

Joint Function:

  • Absorb force of impact, transfer force via cartilage to bone
  • Allow a variable degree of movement

Fibrous Joints

  • Most occur in the skull: known as sutures
    • Key in development: allow extension of individual bones during growth
    • Gradually eliminated as ossification progresses
  • Syndesmoses: facing areas of two bones joined by connective tissue ligaments, very limited movement allowed
    • Eg. Joints of the metacarpus in the horse
  • Gomphosis: attachment of tooth to bone within its socket

Cartilaginous Joints

  • Synchondroses: eg. joints between epiphyses and diaphyses of juvenile long bones, disappear on maturity
    • Permanent synchondroses: the joint between the skull and hyoid
  • Symphysis: articulating bones are divided by a succession of tissues, with cartilage covering the bones or the tissue between
    • Eg. mandibular, pelvic, vertebral
    • Fibrocartilagenous joints
      • Form major union between vertebrae, except first two cervical vertebrae
      • Nucleus pulposus is position eccentrically within annulus fibrosis
      • Vertebrae in thoracic region have conjugal ligaments
        • Extend from rib to rib on opposite sides
        • Strenghten the area over the discs

Synovial Joints

  • Articulating joints are separated by a fluid-filled joint cavity, which is bounded by a synovial membrane
    • Synovial membrane: pink connective tissue sheet, vascular and sensitive
      • Can be unsupported (membrane may pouch, allowing remote access), resting on an outer fibrous capsule, or separated from capsule by pads of fat
      • No continuous covering of cells
      • Where cells exist, they produce lubricant (aminoglycans) of synovial fluid
    • Synovial fluid: Nourishes and lubricates articular cartilage
      • Derived from synovial membrane cells and blood plasma
      • Normal amount in canine joint - 0.01 - 1.0 ml; possible in equine/bovine: 20-40mL
      • Transparent to light yellow (horses)
      • Usually very thick due to high hyaluronic acid, forms strands
      • Windrowing of cells on smear
      • Normal protein < 25g/l (all species)
      • Normal cell count: Large mononuclear cells, <12% neutrophils, <11% lymphocytes
        • Small animals - < 3 x 10e9/L
        • Horses - < 0.5 x 10e9/L
        • Cows - < 1 x 10e9/L
  • Often the synovial membrane is reinforced by a fibrous capsule and ligaments restricting joint movement and providing stability
    • Encloses bone and muscle insertions within joint capsule
    • Supplied by blood vessels and nerve endings
  • Articular cartilage covers the articular surfaces
Normal joint cartilage (Image sourced from Bristol Biomed Image Archive with permission)
    • Articular cartilage is avascular and insensitive
      • Nutrients via diffusion from synovial fluid and nearby vessels (adjoining tissue and marrow cavities)
  • Some joints possess intracapsular discs or menisci to provide congruence and enable complicated movements
    • Eg. Temperomandibular joint, paired menisci of the stifle joint
    • Limited response to injury, Little repair capacity


  • In large animals - normal bilaterally symmetrical depressions on the surface
  • Obtains nutrition from synovial fluid and less so from subchondral vessels

Microscopically, surface has depressions - for synovial fluid?

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