Orbital Anatomy

Bony Orbit & Openings - Bone Box Boundaries

• Pyramidal, ~30 mL volume. Base anterior, apex posterior.
• 7 Bones: 📌 Many Friendly Zebras Eat Lovely Salty Peanuts
  • Maxilla
  • Frontal
  • Zygomatic
  • Ethmoid
  • Lacrimal
  • Sphenoid (GWS, LWS)
  • Palatine (orbital proc.)
• Boundaries:
  • Roof: Frontal, Lesser wing of Sphenoid (LWS).
    • Lacrimal fossa (anterolat), Trochlear fossa (anteromed).
  • Floor: Maxilla, Zygomatic, Palatine.
    • Infraorbital groove & canal (V2).
    • Links to maxillary sinus.
  • Medial: 📌 SMEL- Sphenoid (body), Maxilla, Ethmoid (lamina papyracea), Lacrimal.
    • Thinnest: Lamina papyracea.
    • Lacrimal sac fossa. Ant/Post ethmoidal foramina.
  • Lateral: Zygomatic, Greater wing of Sphenoid (GWS).
    • Strongest. Whitnall"s tubercle.
  • Apex: Optic canal (LWS).

⭐ Orbital floor (maxilla) & medial wall (ethmoid lamina papyracea) are commonest blowout fracture sites (thin).

Muscles & Fascia - Movers & Sheaths

• Extraocular Muscles (EOMs): 7 (4 Recti, 2 Obliques, 1 Levator)
  • Recti (SR, IR, MR, LR): Origin - Annulus of Zinn. Insertion - Sclera (anterior).
  • Obliques (SO, IO): Origin - SO: Sphenoid; IO: Maxilla. Insertion - Sclera (posterior).
  • Levator Palpebrae Superioris (LPS): Elevates upper lid.
• Innervation: 📌 LR6SO4R3 (LR: CN VI; SO: CN IV; Rest: CN III).
• Primary Actions:
  • Adduction: MR
  • Abduction: LR
  • Elevation: SR, IO
  • Depression: IR, SO
  • Intorsion: SR, SO
  • Extorsion: IR, IO
• Orbital Fascia:
  • Tenon's Capsule (Fascia Bulbi): Ensheaths globe.
  • Muscular Fasciae: Cover EOMs.
  • Check Ligaments: Limit movement (Medial & Lateral).
  • Suspensory Ligament of Lockwood: Supports globe (fascial sling from IR & IO).

⭐ The Inferior Rectus (IR) is the most commonly involved muscle in Thyroid Eye Disease (Graves' Orbitopathy).

Nerves of the Orbit - Wired for Sight

• Optic Nerve (CN II): Special sensory for vision. Exits via Optic Canal.
• Motor Nerves (enter via SOF):
  • Oculomotor (CN III): Innervates SR, IR, MR, IO, Levator Palpebrae Superioris. Carries parasympathetic fibers to ciliary ganglion (for sphincter pupillae, ciliary muscle).
  • Trochlear (CN IV): Innervates Superior Oblique (SO). Longest intracranial course.
  • Abducens (CN VI): Innervates Lateral Rectus (LR).
• Sensory Nerve (V1 - Ophthalmic division of Trigeminal CN V): Enters via SOF. 📌 SOF Contents Mnemonic: "Live Frankly To See Absolutely No Insult" (Lacrimal, Frontal, Trochlear, Superior division CN III, Abducens, Nasociliary, Inferior division CN III).

⭐ Parasympathetic Pathway to Pupil: Preganglionic fibers (Edinger-Westphal nucleus, CN III) → Ciliary Ganglion (synapse) → Postganglionic Short Ciliary Nerves → Sphincter Pupillae (miosis) & Ciliary Muscle (accommodation).

Orbital Vasculature & Fat - Pipes & Packing

• Arterial Supply:
  • Ophthalmic Artery (OA): Main supply.
    • Via Optic Canal.
    • Branches: CRA, Lacrimal, Muscular, SPCAs/LPCAs, Ethmoidal, Supraorbital.

    ⭐ OA from Internal Carotid Artery (ICA).

  • CRA: enters optic nerve ~10-12 mm from globe; end artery.
• Venous Drainage: Valveless.
  • Superior Ophthalmic Vein (SOV): Main drainage; to Cavernous Sinus (SOF).
  • Inferior Ophthalmic Vein (IOV): To Cavernous Sinus (SOF) or Pterygoid Plexus (IOF).
  • Central Retinal Vein (CRV): To Cavernous Sinus (direct/SOV).
• Orbital Fat:
  • Cushions & supports contents.
  • Intraconal & extraconal.

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High‑Yield Points - ⚡ Biggest Takeaways

• Orbit: 7 bones (Frontal, Sphenoid, Zygomatic, Maxillary, Ethmoid, Lacrimal, Palatine).
• Optic canal: Transmits CN II & ophthalmic artery.
• Superior Orbital Fissure: Transmits CN III, IV, V1, VI & superior ophthalmic vein.
• Orbital floor: Weakest wall; site of blowout fractures. Involves maxillary, zygomatic, palatine.
• Medial wall: Thinnest wall; primarily lamina papyracea of ethmoid.
• Lateral wall: Strongest wall; zygomatic & greater wing of sphenoid.
• Annulus of Zinn: Common origin for recti muscles.