Articular System

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Joints 101 - The Body's Connectors

  • Definition: Site where ≥2 bones (or bone & cartilage) meet; an articulation.
  • Functions: Confer mobility & stability to the skeleton.

I. Structural Classification (Material & Cavity):

  • Fibrous: Dense fibrous CT; no cavity; mostly synarthrotic.
    • Sutures (skull), Syndesmoses (tibia-fibula), Gomphoses (teeth).
  • Cartilaginous: Cartilage; no cavity; synarthrotic/amphiarthrotic.
    • Synchondroses (hyaline; 1st rib-sternum), Symphyses (fibrocartilage; pubic symphysis).
  • Synovial: Joint cavity with synovial fluid; diarthrotic; most common.
    • Features: Articular cartilage (hyaline), articular capsule, synovial membrane.

Fibrous, Cartilaginous, and Synovial Joint Types

II. Functional Classification (Movement):

  • Synarthrosis: Immovable (e.g., sutures).
  • Amphiarthrosis: Slightly movable (e.g., pubic symphysis).
  • Diarthrosis: Freely movable (all synovial joints).

Hilton's Law: The nerve supplying a joint also supplies the muscles moving the joint and the skin over their insertion.

Immovable & Slightly Movable - Fibrous & Cartilaginous Joints

  • Fibrous Joints: United by fibrous CT; no joint cavity. 📌 S.S.G. (Suture, Syndesmosis, Gomphosis).
    • Sutures: Skull; interlocking. Immovable (synarthrosis).
      • Synostosis: Ossified suture (e.g., metopic suture).
    • Syndesmoses: Bones linked by ligaments/interosseous membrane (e.g., distal tibiofibular). Slightly movable (amphiarthrosis).
    • Gomphoses: Teeth in sockets; periodontal ligament. Immovable (synarthrosis). Fibrous Joint Types: Suture, Syndesmosis, Gomphosis and cartilaginous joint types (synchondrosis, symphysis))
  • Cartilaginous Joints: United by cartilage; no joint cavity. 📌 S.S. (Synchondrosis, Symphysis).
    • Synchondroses (Primary): Hyaline cartilage (e.g., epiphyseal plate, 1st sternocostal). Immovable (synarthrosis); often temporary.
    • Symphyses (Secondary): Fibrocartilage (e.g., pubic symphysis, IVDs). Midline; slightly movable (amphiarthrosis).

⭐ The first sternocostal joint is a synchondrosis (primary cartilaginous joint), while other sternocostal joints (2nd-7th) are synovial.

Freely Movable Marvels - Synovial Joints Unpacked

Synovial joints (diarthroses): Characterized by free mobility. Key features: articular cartilage (hyaline), joint capsule (outer fibrous layer, inner synovial membrane), synovial cavity with synovial fluid (lubrication, nutrition, shock absorption). Anatomy of a Synovial Joint

  • Essential Components:

    • Articular Cartilage: Covers articular surfaces; smooth, reduces friction. Avascular.
    • Synovial Membrane: Lines capsule; secretes synovial fluid.
    • Joint Capsule: Fibrous tissue enclosing joint cavity.
    • Ligaments: Reinforce joint (e.g., capsular, extracapsular, intracapsular).
    • Accessory Structures: Articular discs/menisci (e.g., knee), bursae (reduce friction), fat pads.
  • Classification by Articular Shape & Movement (Examples):

    • Plane: Gliding (e.g., intercarpal, acromioclavicular).
    • Hinge (Uniaxial): Flexion/Extension (e.g., elbow, knee, interphalangeal).
    • Pivot (Uniaxial): Rotation (e.g., atlanto-axial, proximal radioulnar).
    • Condyloid (Biaxial): Flex/Ext, Abd/Add (e.g., radiocarpal, metacarpophalangeal).
    • Saddle (Biaxial): Like condyloid + opposition (e.g., 1st carpometacarpal joint - thumb).
    • Ball & Socket (Multiaxial): Universal movement (e.g., shoulder, hip).

Hilton's Law: The nerve supplying a joint also supplies the muscles moving the joint and the skin over the attachments of these muscles. This is clinically important for referred pain from joints.

Joint Lifelines & Troubles - Neurovascular & Clinical Snippets

  • Innervation: Hilton"s Law (nerve to muscle also supplies joint & overlying skin).
  • Blood Supply: Periarticular arterial plexuses from nearby arteries; epiphyseal & metaphyseal branches.
  • Lymphatics: Drain to regional nodes.
  • Clinical Issues:
    • Sprains (ligaments), Strains (muscles/tendons).
    • Dislocations (complete loss of contact), Subluxations (partial).
    • Osteoarthritis, Rheumatoid Arthritis.
    • Referred pain (e.g., hip to knee via obturator nerve).

⭐ Avascular necrosis (AVN) commonly affects scaphoid & femoral head due to tenuous blood supply.

High‑Yield Points - ⚡ Biggest Takeaways

  • Synovial joints: Have articular cartilage, synovial membrane, capsule; most mobile.
  • Hilton's Law: Nerve to joint muscles also supplies joint & overlying skin.
  • Primary cartilaginous (synchondrosis): Growth (e.g., epiphyseal plate); Secondary (symphysis): Midline, strong (e.g., IV disc).
  • Saddle joint (thumb CMC): Allows opposition. Ball-and-socket (hip): Multiaxial movement.
  • Osteoarthritis: Commonest; involves articular cartilage degeneration.
  • Fibrous joints: Sutures (skull), syndesmoses (inf. tibiofibular), gomphoses (teeth).

Practice Questions: Articular System

Test your understanding with these related questions

Which of the following is not a ball and socket type of joint?

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Flashcards: Articular System

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_____ long bones have an elongated shaft (diaphysis) and two expanded ends (epiphysis).

TAP TO REVEAL ANSWER

_____ long bones have an elongated shaft (diaphysis) and two expanded ends (epiphysis).

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