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Anatomy of the Orbit

Anatomy

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Clinical Analogy

Think of the orbit as a seven-walled fortress. Each wall is contributed by a different bone - some thick (providing structural rigidity), others paper-thin (the medial wall, a natural crumple zone). The fortress has three gates: the superior, inferior, and lateral orbital fissures - each allowing passage of critical neurovascular cables. The optic canal is the private VIP tunnel connecting the brain to the eye. Herniation of fat through the medial or floor 'crumple zones' in trauma is the fortress wall giving way under siege pressure. Every orbital disease can be mapped to this architecture.

Mnemonic: "My Favourite Zygoma Loves Eating Peanut Sandwiches" → Maxilla, Frontal, Zygomatic, Lacrimal, Ethmoid, Palatine, Sphenoid

Bone	Wall Contribution	Key Feature / Clinical Relevance
Frontal	Roof (orbital plate)	• Lacrimal fossa (superolateral) • trochlear fossa (superomedial) • Sinus above → mucocele.
Zygomatic	Lateral wall + floor (part)	• Strongest lateral wall. • Zygomaticofacial & zygomaticotemporal foramina may be present.
Maxilla	Floor + medial wall (part)	• Infraorbital groove/canal. • The floor is thin and is a common blow-out fracture site.
Lacrimal	Medial wall (anterior)	• Lacrimal groove → nasolacrimal duct. • Thin; the ethmoid lamina papyracea is the key route for sinusitis spread.
Ethmoid	Medial wall (posterior - lamina papyracea)	• Lamina papyracea is thin. • Adjacent air cells → spread of orbital cellulitis.
Palatine	Posterior floor	• Small; contributes to the pterygopalatine fossa.
Sphenoid (lesser + greater wing)	Apex, roof (lesser), lateral (greater)	• Optic canal (lesser wing and body) • Superior orbital fissure (SOF; between wings) • Inferior orbital fissure (SIF) partly.

Bone	Wall Contribution	Key Feature / Clinical Relevance
Frontal	Roof (orbital plate)	• Lacrimal fossa (superolateral) • trochlear fossa (superomedial) • Sinus above → mucocele.
Zygomatic	Lateral wall + floor (part)	• Strongest lateral wall. • Zygomaticofacial & zygomaticotemporal foramina may be present.
Maxilla	Floor + medial wall (part)	• Infraorbital groove/canal. • The floor is thin and is a common blow-out fracture site.
Lacrimal	Medial wall (anterior)	• Lacrimal groove → nasolacrimal duct. • Thin; the ethmoid lamina papyracea is the key route for sinusitis spread.
Ethmoid	Medial wall (posterior - lamina papyracea)	• Lamina papyracea is thin. • Adjacent air cells → spread of orbital cellulitis.
Palatine	Posterior floor	• Small; contributes to the pterygopalatine fossa.
Sphenoid (lesser + greater wing)	Apex, roof (lesser), lateral (greater)	• Optic canal (lesser wing and body) • Superior orbital fissure (SOF; between wings) • Inferior orbital fissure (SIF) partly.

Wall	Bones Forming	Thickness	Adjacent Structure	Clinical Implication
• Roof	• Frontal (orbital plate) • lesser sphenoid wing	• Thick anteriorly, thinner posteriorly	• Anterior cranial fossa • frontal sinus	• Frontal sinus mucocele • sphenoid-wing meningioma with orbital and bone involvement
• Floor	• Maxilla • zygomatic • palatine	• Thinnest at posteromedial (maxillary sinus roof)	• Maxillary sinus (Antrum)	• Blow-out fracture → enophthalmos • diplopia • infraorbital nerve anaesthesia
• Medial	• Maxilla • lacrimal • ethmoid (lamina papyracea) • sphenoid body	• Very thin (0.2–0.4 millimetres)	• Ethmoidal air cells • medial rectus alongside	• Orbital cellulitis spread • medial wall blow-out
• Lateral	• Zygomatic • greater sphenoid wing	• Thickest (3–4 millimetres)	• Middle cranial fossa (posterior) • temporal fossa (anterior)	• Sphenoid wing meningioma • surgical access - lateral orbitotomy

Wall	Bones Forming	Thickness	Adjacent Structure	Clinical Implication
• Roof	• Frontal (orbital plate) • lesser sphenoid wing	• Thick anteriorly, thinner posteriorly	• Anterior cranial fossa • frontal sinus	• Frontal sinus mucocele • sphenoid-wing meningioma with orbital and bone involvement
• Floor	• Maxilla • zygomatic • palatine	• Thinnest at posteromedial (maxillary sinus roof)	• Maxillary sinus (Antrum)	• Blow-out fracture → enophthalmos • diplopia • infraorbital nerve anaesthesia
• Medial	• Maxilla • lacrimal • ethmoid (lamina papyracea) • sphenoid body	• Very thin (0.2–0.4 millimetres)	• Ethmoidal air cells • medial rectus alongside	• Orbital cellulitis spread • medial wall blow-out
• Lateral	• Zygomatic • greater sphenoid wing	• Thickest (3–4 millimetres)	• Middle cranial fossa (posterior) • temporal fossa (anterior)	• Sphenoid wing meningioma • surgical access - lateral orbitotomy

Parameter	Value
Volume	~30 mL
Depth (anterior rim to apex)	~40–45 mm
Width (at widest point)	~45 mm (wider than height)
Height	~35 mm
Opening (orbital rim width)	~40 mm
Optic canal diameter	~6.5 mm (< 7 mm normal)
Annulus of Zinn inner diameter	~6.5 mm

Parameter	Value
Volume	~30 mL
Depth (anterior rim to apex)	~40–45 mm
Width (at widest point)	~45 mm (wider than height)
Height	~35 mm
Opening (orbital rim width)	~40 mm
Optic canal diameter	~6.5 mm (< 7 mm normal)
Annulus of Zinn inner diameter	~6.5 mm

Shape: Quadrilateral pyramid with apex posteriorly at optic canal.
The medial walls run parallel to each other and to the sagittal plane.
The lateral walls form a 90° angle to each other.
A lateral blow may cause medial impaction of the ipsilateral lateral orbital wall.

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