Joints - Anatomy & Physiology

Joints comprise broadly two categories:

Joint Function:

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

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

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

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

Microscopically, surface has depressions - for synovial fluid?