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Orbital anatomy
Anatomy & Optics
Key Points
- •The orbit is formed by 7 bones (frontal, zygomatic, maxilla, lacrimal, ethmoid, sphenoid, palatine); medial wall (lamina papyracea) is thinnest (0.2-0.4 mm); lateral wall (zygomatic + greater sphenoid) is thickest/strongest; volume ~30 mL
- •Three critical openings: optic canal (CN II + ophthalmic artery), SOF (CN III, IV, V1, VI, SOV), IOF (V2, zygomatic nerve, IOV); the annulus of Zinn surrounds the optic canal + central SOF and gives origin to 4 recti + SO; IO is the only EOM NOT from the apex (arises from orbital floor)
- •SOF syndrome: total ophthalmoplegia (CN III, IV, VI) + V1 loss + NO optic nerve involvement; Orbital apex syndrome: SOF syndrome PLUS optic neuropathy (CN II also involved); both caused by tumour, infection, inflammation at the apex
- •Intraconal lesions → axial proptosis (optic nerve glioma, cavernous haemangioma, lymphoma); extraconal lesions → eccentric proptosis (lacrimal gland tumour, dermoid); Spiral of Tillaux: MR 5.5, IR 6.5, LR 6.9, SR 7.7 mm from limbus
- •Orbital floor blowout fracture: most common orbital fracture; IR entrapment → restricted upgaze + diplopia; infraorbital nerve → cheek numbness; enophthalmos (late); pediatric trapdoor fracture = surgical emergency (ischaemic IR); orbital cellulitis: most commonly from ethmoid sinusitis through lamina papyracea
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13 more sections are included with the Clinical plan.
- Definition
- Epidemiology
- Aetiology and causes
- Pathogenesis
- Classification
- Clinical features