What is the most common site of an orbital blowout fracture?

Explanation

## Correct Answer: A. Floor An orbital blowout fracture is a break in the orbital wall caused by blunt trauma to the globe or orbit, typically from a fist, ball, or motor vehicle accident. The **orbital floor** is the most common site of blowout fracture because it is the thinnest and weakest part of the orbital skeleton. The floor is composed primarily of the maxilla and is inherently more fragile than the thicker frontal bone of the roof or the sturdy zygomatic bone of the lateral wall. When blunt force is applied to the orbit, the pressure increases within the confined space, and the floor—being the path of least resistance—fractures first. This mechanism is well-documented in Indian ophthalmology practice and trauma centers. Floor fractures account for approximately 50–60% of all orbital blowout fractures. The clinical consequence is entrapment of the inferior rectus muscle and orbital fat in the fracture site, leading to vertical diplopia (especially on upgaze), enophthalmos, and infraorbital nerve hypoesthesia. Imaging (CT orbit) confirms the diagnosis and guides surgical intervention, which is indicated when there is significant enophthalmos, restrictive strabismus, or acute entrapment with oculomotor dysfunction. ## Why the other options are wrong **B. Medial wall** — The medial wall (lamina papyracea of ethmoid) is thin but is the second most common site of blowout fracture (20–30% of cases), not the most common. It is protected somewhat by the nasal bridge and medial canthal anatomy. Medial wall fractures cause entrapment of the medial rectus, leading to horizontal diplopia. This option traps students who confuse 'thin' with 'most common'—thinness alone does not determine fracture frequency; biomechanical force distribution matters more. **C. Roof** — The orbital roof is composed of thick frontal bone and is the strongest part of the orbital skeleton, making it the least common site of blowout fracture (<5% of cases). Roof fractures require very high-impact trauma and are often associated with serious intracranial injury. This is a distractor for students who incorrectly assume that 'blowout' means upward pressure; in reality, most blunt orbital trauma is anterior-to-posterior, favoring floor fracture. **D. Lateral wall** — The lateral wall, formed by the zygomatic bone and greater wing of sphenoid, is thick and robust—the strongest orbital wall. Lateral wall fractures are rare (<10% of blowout fractures) and typically occur only with severe high-velocity trauma. Students may select this if they confuse orbital wall fractures with zygomatic fractures (which are common in facial trauma). The lateral wall's strength makes it resistant to the pressure gradients created by typical blunt orbital injury. ## High-Yield Facts - **Orbital floor** is the most common site of blowout fracture (~50–60% of cases) due to its thin maxillary composition and biomechanical vulnerability to anterior blunt force. - **Inferior rectus entrapment** in floor fractures causes vertical diplopia on upgaze and is the primary indication for urgent surgical repair. - **Medial wall (lamina papyracea)** is the second most common site (~20–30%), causing medial rectus entrapment and horizontal diplopia. - **CT orbit** (coronal and axial views) is the gold standard imaging to confirm blowout fracture, assess entrapment, and guide surgical planning. - **Enophthalmos** (posterior displacement of globe) and **infraorbital nerve hypoesthesia** are cardinal signs of orbital floor fracture. - **Surgical repair** is indicated for acute entrapment with oculomotor dysfunction, significant enophthalmos (>3 mm), or restrictive strabismus affecting vision. ## Mnemonics **FLOOR is WEAK (Biomechanics of Orbital Blowout)** **F**ragile maxilla | **L**east resistant | **O**rbital pressure → **O**utward force | **R**oof is thick. The floor is the path of least resistance when blunt force increases intra-orbital pressure. **Orbital Wall Strength (Thick to Thin)** **Lateral** (zygomatic—thick) > **Roof** (frontal—thick) > **Medial** (ethmoid—thin) > **Floor** (maxilla—thinnest). Weakest = most common fracture site. ## NBE Trap NBE pairs 'thin' with 'most common' to lure students into selecting the medial wall (lamina papyracea), which is indeed thin but is only the second most common site. The discriminator is biomechanical force distribution, not absolute bone thickness. ## Clinical Pearl In Indian trauma centers, orbital floor fractures from road traffic accidents and interpersonal violence are the most frequent orbital injury. A patient presenting with vertical diplopia and enophthalmos after blunt eye trauma should be imaged urgently; delayed repair (>2 weeks) risks permanent strabismus and vision loss due to muscle fibrosis. _Reference: Robbins & Cotran Pathologic Basis of Disease (Orbit section); Harrison's Principles of Internal Medicine Ch. 397 (Ophthalmology); Parson's Diseases of the Eye (Orbital Trauma)_