Orbital anatomy and thyroid eye disease

Surgical Anatomy of the Orbit

The bony orbit is a quadrilateral pyramid whose apex points posteromedially toward the optic canal and whose base opens anteriorly onto the facial skeleton. The adult orbital volume is approximately $30\text{ mL}$, of which the globe occupies roughly $7\text{ mL}$ ($~23\%$), with the remainder occupied by extraocular muscles, orbital fat compartments, cranial nerves, vascular networks, and the lacrimal apparatus.

The Four Bony Walls

1. Orbital Roof (Superior Wall):

   • Constituent Bones: Formed predominantly by the orbital plate of the frontal bone anteriorly and the lesser wing of the sphenoid bone posteriorly.
   • Landmarks:
     • Lacrimal gland fossa: Located anterolaterally, housing the orbital lobe of the lacrimal gland.
     • Trochlear fovea/spine: Located anteromedially (4 mm behind the orbital margin), serving as the cartilaginous pulley for the superior oblique tendon.
   • Clinical Relevance: Separates the orbit from the anterior cranial fossa and frontal sinus. Thinning in elderly patients or erosion by frontal sinus mucoceles can expose the dura mater.

2. Medial Orbital Wall:

   • Constituent Bones: Formed by four bones from anterior to posterior: the frontal process of the maxilla, the lacrimal bone, the ethmoid bone (orbital lamina or lamina papyracea), and the lesser contribution from the body of the sphenoid.
   • Lamina Papyracea: Measuring only $0.2-0.4\text{ mm}$ in thickness, this translucent wafer of bone separates the orbit from the ethmoidal air cells. It is the thinnest wall of the orbit.
   • Lacrimal Fossa: Formed by the lacrimal bone and the frontal process of the maxilla, bounded by the anterior and posterior lacrimal crests, accommodating the lacrimal sac.
   • Ethmoidal Foramina: Located at the frontoethmoidal suture line (at the level of the cribriform plate). The anterior ethmoidal foramen lies approximately $24\text{ mm}$ posterior to the anterior lacrimal crest; the posterior ethmoidal foramen lies roughly $12\text{ mm}$ further back; and the optic canal lies roughly $6\text{ mm}$ behind that ("24-12-6 Rule"). These landmarks mark the surgical roof level during medial wall decompression or ethmoidectomy; dissection above this line risks entering the anterior cranial fossa and causing cerebrospinal fluid (CSF) rhinorrhoea.

3. Orbital Floor (Inferior Wall):

   • Constituent Bones: Formed by three bones: the orbital surface of the maxilla (largest component), the orbital surface of the zygomatic bone anterolaterally, and the orbital process of the palatine bone at the posterior apex.
   • Landmarks: The infraorbital groove traverses the floor, transitioning anteriorly into the infraorbital canal and opening onto the maxilla at the infraorbital foramen, transmitting the infraorbital nerve (cranial nerve $V_2$) and infraorbital artery.
   • Blowout Fractures: The floor is the most common site of blunt orbital trauma fractures, particularly within the thin wafer of bone posteromedial to the infraorbital canal. Entrapment of the inferior rectus muscle, inferior oblique muscle, or adjacent capsulopalpebral fascia within the fracture gap produces tethered upgaze/downgaze diplopia, enophthalmos, and ipsilateral infraorbital nerve hypoesthesia (affecting the ipsilateral lower eyelid, cheek, lateral nose, and upper incisors/gingiva).

4. Lateral Orbital Wall:

   • Constituent Bones: Formed by the orbital surface of the zygomatic bone anteriorly and the greater wing of the sphenoid bone posteriorly.
   • Structural Properties: The lateral wall is the thickest and strongest orbital wall, particularly at its anterior rim (the zygomaticofrontal junction), shielding the globe from lateral trauma.
   • Whitnall's Tubercle: A small bony projection on the inner aspect of the zygomatic bone, situated approximately $11\text{ mm}$ inferior to the frontozygomatic suture and $4-5\text{ mm}$ internal to the orbital margin. It provides anatomical anchoring for five critical structures:
     1. Lateral canthal tendon (crus superius and inferius)
     2. Lateral check ligament of the lateral rectus muscle
     3. Lateral horn of the levator palpebrae superioris aponeurosis
     4. Whitnall's superior transverse suspensory ligament
     5. Suspensory ligament of Lockwood

Anatomical Architecture of the Orbital Walls and Apex Foramina

The Orbital Apex & Superior Orbital Fissure (SOF)

The apex represents the convergence of the four orbital walls and constitutes the main neural and vascular gateway between the intracranial vault and the orbit:

• Optic Canal: Situated entirely within the lesser wing of the sphenoid bone. It transmits the optic nerve (CN II) sheathed by all three meningeal layers and CSF, the ophthalmic artery (inferolateral to the nerve), and postganglionic sympathetic vasomotor fibers.
• Superior Orbital Fissure (SOF): A cleft located between the greater and lesser wings of the sphenoid bone. It is partitioned into intraconal and extraconal sectors by the Annulus of Zinn (common tendinous ring), which serves as the fibrous origin for the four rectus muscles.

Thyroid Eye Disease (TED)

TED reflects autoimmune orbital inflammation with extraocular-muscle and fat involvement, often associated with Graves disease but possible in other thyroid states. Assess thyroid history, smoking, exposure, lid retraction, proptosis, diplopia, motility and optic function. Obtain acuity, colour vision, pupils, fields and corneal examination. Severity triage comes before activity scoring: corneal breakdown or dysthyroid optic neuropathy (DON) can threaten vision even with a low Clinical Activity Score (CAS).

CAS describes inflammatory activity, while mild, moderate-to-severe and sight-threatening categories describe consequences. A low score does not guarantee safe inactivity. Imaging commonly shows muscle-belly enlargement with relative tendon sparing, but tendon patterns overlap with other disorders and are not characteristic. An atypical unilateral pattern, marked pain or unusual imaging may require consideration of lymphoma, vascular disease or other orbital inflammation.

Restore and maintain appropriate thyroid control with endocrine care, encourage smoking cessation and manage exposure with lubrication and selected lid protection. Selenium has evidence for recent mild active disease, particularly in selenium-deficient regions; routine supplementation is not automatically beneficial in every population and excess can be harmful. Discuss radioiodine-associated risk and prevention with the endocrine team where relevant.

For active moderate-to-severe TED, EUGOGO 2021 recommends IV methylprednisolone combined with mycophenolate as a first-line strategy in suitable patients. A common IV course totals 4.5 g over twelve weekly infusions. Regimen choice depends on severity, contraindications and local care; assess infection, liver and cardiovascular risk and cumulative steroid exposure. Rehabilitative decompression, strabismus and lid surgery generally follow stable inactive disease, in sequence dictated by the anatomy.

DON requires urgent specialist treatment, often high-dose IV steroid pulses and urgent decompression if response is inadequate or vision is deteriorating. Do not wait through a full elective twelve-week course. Check corneal integrity and other explanations for poor vision. Severe orbital congestion, apical crowding and restricted motility may be informative, but proptosis alone neither diagnoses nor excludes DON.

Teprotumumab targets IGF-1 receptor pathways. Tepezza received EU authorisation for moderate-to-severe TED on 19 June 2025; availability and reimbursement still vary. Assess hearing, glycaemia, inflammatory bowel disease, pregnancy and other product-specific risks with the specialist team. An authorisation is not a rule to replace every established regimen. See EUGOGO guidance and EMA Tepezza.

At follow-up, document function as well as appearance: diplopia fields, corneal protection, optic tests and daily-life limitations. Activity, severity, thyroid status and response can change at different rates. Explain urgent return symptoms such as reduced acuity, altered colour, escalating pain or exposure symptoms.