Orbital Trauma — Blowout Fracture

Overview

The orbital floor, thin bone overlying the maxillary sinus, is the most common site of blowout fracture in blunt orbital trauma; the medial wall (the paper-thin lamina papyracea of the ethmoid) is a close second. A sudden rise in intraorbital pressure from a blunt object roughly the size of the orbital opening (a fist, a ball, an elbow) transmits force to the globe and orbital contents, and because the floor is the weakest wall, it gives way before the sturdier, thicker walls (the zygomatic-formed lateral wall, the frontal-bone roof) do. The fracture is protective in its own right: it absorbs energy that would otherwise reach the globe directly, lowering the risk of rupture. But it creates its own urgent problems.

A blowout fracture's clinical significance ranges enormously, from an asymptomatic radiographic finding needing no intervention to a true surgical emergency. Which end of that range a given patient lands on depends almost entirely on one variable: whether extraocular muscle tissue has become entrapped in the fracture site. Recognizing entrapment, and recognizing it fastest in children, where it can be present despite a strikingly unremarkable external exam, is the central skill this topic tests.

Mechanism

On coronal CT the orbital floor slopes downward roughly 20 degrees from posterior to anterior. That contour, combined with the bone's thinness over the maxillary sinus, makes it the point of least resistance when the orbit is struck. Two mechanisms are classically described for how a floor fracture forms: a direct hydraulic mechanism, in which a sudden rise in intraorbital pressure from the blunt impact is transmitted straight down through the orbital contents onto the floor, and a buckling mechanism, in which the force travels through the bony orbital rim and buckles the thin floor without the globe itself being struck directly. Most real-world fractures likely involve elements of both.

Once the floor gives way, orbital contents, fat, connective tissue, and potentially the inferior rectus muscle itself, can herniate downward into the maxillary sinus through the fracture defect. How much tissue herniates, and whether any of it becomes mechanically trapped in the fracture edges as the bone springs back, determines what happens clinically.

Muscle Entrapment

The inferior rectus is the extraocular muscle most commonly entrapped in an orbital floor fracture, simply because of its direct anatomic relationship to the floor immediately beneath it. Entrapment, the muscle or its surrounding fibrous connective tissue becoming mechanically caught in the fracture line rather than just displaced, restricts elevation of the globe and produces vertical diplopia, classically worse on attempted upgaze. On exam this has to be distinguished from a cranial nerve palsy causing a similar-looking restriction. Forced duction testing, physically grasping and moving the globe to see whether the restriction is mechanical or neurogenic, makes that distinction: a mechanically entrapped muscle resists passive movement in a way a paretic one does not.

True entrapment is a surgical emergency, and the reason is ischemic rather than purely mechanical. A muscle physically trapped in a bony fracture line can lose its own blood supply, and prolonged entrapment risks permanent fibrosis and contracture: a restriction that starts out reversible can become a fixed, permanent strabismus if release is delayed. This is also the setting for the oculocardiac (trigeminovagal) reflex. Forced traction on an entrapped muscle, especially on attempted upgaze, can trigger vagally-mediated bradycardia, nausea, and vomiting, itself another sign that surgery shouldn't wait.

When entrapment and the oculocardiac reflex are present, that urgency translates into a real number: the muscle should be released within 24 to 48 hours, since the odds of recovering full motility drop sharply once ischemia runs past that window. Fractures without entrapment are handled very differently. A small defect with a negative forced duction test, no enophthalmos, and resolving diplopia can simply be observed. A larger defect, persistent diplopia, or enophthalmos beyond 2mm calls for repair, usually scheduled one to two weeks out so acute edema can subside first and the surgeon gets an accurate read on eye position and motility before committing to titanium mesh, porous polyethylene, or a resorbable implant.

The Pediatric "White-Eyed" Trapdoor Fracture

This is the single highest-yield concept on this topic. In children and young adults the orbital floor bone is more elastic and less brittle than in older adults. Instead of fracturing into distinct, displaced fragments, it can crack, bow open just long enough for orbital tissue to herniate through, and then spring back almost to its original position, like a trapdoor. The result is a fracture that traps the inferior rectus muscle, or the connective tissue overlying it, tightly in a narrow, often linear fracture line, while leaving remarkably little external evidence of trauma: minimal periorbital ecchymosis, minimal eyelid swelling, and often a relatively unremarkable-looking eye. Hence "white-eyed."

That external mildness is the trap. A child with a white-eyed trapdoor fracture can look, at a glance, like they've had a minor bump. Underneath, the inferior rectus can be acutely and tightly entrapped, with real muscle ischemia already in progress. The exam findings that reveal what's actually happening are functional rather than visual: restricted upgaze, vertical diplopia, and, often the most striking clue, a vagally-mediated oculocardiac reflex with nausea, vomiting, and bradycardia specifically provoked by attempted upward eye movement. The external exam underplays the severity, which makes this presentation a classic source of missed or delayed diagnosis. Any child with post-traumatic restricted upgaze needs to be taken seriously no matter how mild the eyelid bruising looks.

Infraorbital Nerve Injury & Fat Herniation

The infraorbital nerve, a branch of the trigeminal nerve's V2 (maxillary) division, runs within a canal in the orbital floor itself, directly in the path of a floor fracture. Damage to it produces hypoesthesia, numbness, in its sensory distribution: the cheek, the upper lip, and the ipsilateral upper gum and teeth. Checking for infraorbital numbness should be routine in any patient with suspected orbital floor trauma. It confirms a floor fracture is present, and it's something patients often don't volunteer on their own.

Orbital fat can also herniate through the floor defect into the maxillary sinus without any muscle being trapped at all. When enough orbital volume is lost this way, the eye can develop enophthalmos, a sunken appearance, as a delayed finding once acute swelling resolves. This is a distinct problem from entrapment: isolated fat herniation doesn't restrict eye movement or trigger an oculocardiac reflex, but it can still warrant surgery if the resulting enophthalmos or globe malposition becomes cosmetically or functionally significant.

Imaging: The Teardrop Sign

CT, both axial and coronal cuts, is the first-line imaging study for suspected orbital floor fracture, chosen for its superior bony detail and its ability to show acute hemorrhage; MRI's strength lies more in soft-tissue characterization elsewhere in the orbit. On coronal CT, herniated orbital fat and inferior rectus tissue protruding downward through the floor defect into the roof of the maxillary sinus produces a rounded, tear-shaped soft tissue density: the classic teardrop sign, a direct radiographic signature of orbital floor blowout fracture.

High-Yield Pearls

• Entrapment is a surgical emergency because of muscle ischemia risk, not just mechanical restriction: delay risks permanent fibrosis/contracture.
• Enophthalmos after blunt trauma without entrapment signals a pure fat herniation / volume loss mechanism.
• CT (axial + coronal) is first-line for any suspected orbital floor fracture.
• Always document infraorbital sensation on exam: patients often don't volunteer it.

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