Articular System

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.

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). 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).

• 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).