Joints - Anatomy & Physiology

Introduction

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

Sutures of the skull- Copyright Thegreenj

• 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

Pelvic Symphysis, Gray's Anatomy - Wikimedia Commons 2008

• 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 courtesy Bristol Biomed Image Archive)
  • 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

Types of Joints

• Plane Joint: describes translational movement; in reality, nonexistent, as all articular surfaces are curved
• Hinge Joint: movement allowed in one plane only, inhibited by collateral ligaments and/or bony protuberances (eg. elbow joint)
• Pivot Joint: comprises a peg fitted with a ring, movement occurs about the long axis of the peg (eg. radioulnar joint)
• Condylar Joint: knuckle-shaped condyles vary in distance from one another allowing uniaxial movement with limited rotation (eg.femorotibial joint)
• Ellipsoidal Joint: ovoid convex articulation allows movement in two planes at right angles with limited rotation (eg. radiocarpal joint)
• Saddle Joint: also biaxial with a greater scope for rotation
• Spheroidal Joint: (aka ball-and-socket) multiaxial movement allows for rotational movement in several planes (eg hip joint)

Joint Formation

Joints begin to form as the mesenchyme is condensing. Joint-forming cells develop different characteristics to cartilage precursors; they are flat and densely packed. The region where these joint-forming cells exist is called the "interzone". There are three layers in the interzone, a middle layer of low density with regions of high density either side. The high density regions form cartilage. The middle layer is eliminated by cell death, forming the joint space.

Links

• Joint Pathology