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)
  • Usually, this is Hyaline, although Fibrocartilage or fibrous tissue can substitute
  • 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

Joint Movements

• Translation: Flat surfaces slide against each other, producing no change in orientation of their bodies
• Rotation: Moving bone turns on an axis perpendicular to articulation
• Swing: Moving bone turns on an axis parallel to articulation
• Flexion:(aka palmar flexion) Angle between two segments of a limb is reduced
• Extension: Angle between two segments of a limb is increased
• Overextension: (aka dorsal flexion) Eg, Posture of equine fetlock standing at rest
• Adduction: Pendular movement toward the median plane
• Abduction: Pendular movement away from the median plane
• Circumduction: Combination of flexion and extension that allows a limb to create a circular movement

Links

• Joint Pathology