Joints - Anatomy & Physiology
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BACK TO MUSCULOSKELETAL ANATOMY AND PHYSIOLOGY
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
- Synovial membrane: pink connective tissue sheet, vascular and sensitive
- Often the synovial membrane is reinforced by a fibrous capsule and ligaments restricting joint movement and providing stability
- Outer fibrous layer
- Encloses bone and muscle insertions within joint capsule
- Supplied by blood vessels and nerve endings
- Inner synovial layer
- Covers surfeces except for articular cartilage
- Very thin
- Has surface projections - villi
- Outer fibrous layer
- Articular cartilage covers the articular surfaces
- 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)
- Articular cartilage is avascular and insensitive
- 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?
- In young animals overlies epiphyseal cartilage