In This Orbital Fracture, the Soft Tissues Show the Most Important Finding

You’re reading a facial CT after an assault, and the orbital floor fracture is easy. The fragment hangs down into the maxillary sinus, there’s blood layering beneath it, and the report is already writing itself. It’s tempting to describe the fracture, template the rest, and move on.

The finding that changes what happens tonight is somewhere else: the coronal soft tissue series, in the shape of a muscle.

How an Orbital Floor Fracture Happens#

The orbit is a bony cone holding pressurized soft tissue: the globe, the four rectus muscles, and the fat that suspends them. When a fist or a ball strikes the rim and the globe, pressure inside the cone spikes, and something has to give. The globe itself rarely ruptures. The force vents instead through the weakest bone available, and the two weakest walls are the floor, a millimeter of bone roofing the maxillary sinus and thinned further by the infraorbital canal, and the medial wall, the lamina papyracea, which is about as thick as its name suggests.

So the floor blows out downward, into the sinus. The fracture works as a pressure vent, and that venting is probably why the globe survives most of these injuries. Orbital fat follows the fragment down, and on coronal images it hangs through the defect into the top of the sinus.

The bone is the easy part. The question the surgeon will ask you is about what went down with it, and what got caught on the way.

What Entrapment Looks Like on CT#

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Work the coronal soft tissue series and look at the inferior rectus on each side. The orbit gives you a built-in control, because the normal side sits on the same image. A normal inferior rectus runs flat along the floor, a ribbon of muscle with a straight belly. On this exam the right side is that ribbon. The left inferior rectus is rounded and edematous compared to its partner, sitting at the defect.

That shape change is the finding, and it has a mechanical explanation. A muscle rounds up when it’s tethered. If the belly or the fascial sling around it is caught at the fracture line, the muscle can’t lie flat anymore. It pulls taut, and its cross-section goes from ribbon to cord. The edema is the injury itself, because a caught muscle is a congested, swelling muscle. Rounding plus edema on the fractured side means entrapment until proven otherwise.

The spectrum runs from obvious to nearly invisible. At the obvious end, the muscle frankly herniates through the floor and you can follow it down into the sinus. At the other end, the muscle stays above the floor, just rounder and denser than its partner, over a defect that can look trivial. That quiet end is where the diagnosis gets missed.

The amount of displaced bone does not scale with the danger to the muscle. A wide blowout vents the orbit, and the herniated tissue usually moves freely. The dangerous pattern is the small linear fracture that hinges open under load and snaps shut, a trapdoor. Young bone does this best, because it bends before it breaks. The trapdoor catches whatever came through it, and on CT the bone can look nearly normal over a muscle that is being strangled.

You need both windows to do this well. The bone window finds the fracture and maps the defect (the same width-and-level trade covered in Window and Level), but at that width the muscles wash out. The soft tissue window is where you can see a rounded inferior rectus at all. Compare the recti side to side on the coronals of every facial CT you read.

Why Entrapment Cannot Wait#

An entrapped extraocular muscle is a surgical urgency.¹ The mechanics are the same as any incarcerated tissue. The trapdoor is a hernia ring, and the muscle inside it goes from congested to ischemic to infarcted. Muscle that infarcts scars, and a scarred inferior rectus means fixed diplopia that no late repair fully undoes. You have hours, not days.

There’s also a reflex to know about. Traction on an entrapped muscle fires the oculocardiac reflex: bradycardia, nausea, vomiting, sometimes syncope when the patient tries to look up. In a kid who took an elbow to the eye and is now pale and vomiting, the story reads like a concussion until someone checks upgaze.

What changes management is the word itself. A floor fracture without entrapment gets measured and watched, and repair becomes a decision made over days to weeks, driven by enophthalmos, defect size, and whether diplopia persists. Entrapment moves the same fracture to urgent repair, within a day or two and faster in children.¹ Ophthalmology confirms it at the bedside with the motility exam and forced duction, and they examine the patient tonight because you raised it. Put concern for inferior rectus entrapment in the impression, and then actually call. The decision to operate belongs to the surgeon. Making sure the question gets asked tonight belongs to you.

The Bottom Line#

Read the bone windows for the fracture and map the floor. Then change windows, go to the coronals, and compare the inferior recti side to side. A flat ribbon on both sides with freely herniated fat is a fracture you can describe calmly. A rounded, edematous muscle on the fractured side means entrapment until proven otherwise, and that means an urgent consult, not a recommendation for follow-up.

Notes

1. Hood CM, Gardiner MF, Bleicher ID, Lev MH, Jutras M. Imaging, Management, and Treatment of Orbital Trauma. Radiologic Clinics of North America. 2026;64(4)

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Hood CM, Gardiner MF, Bleicher ID, Lev MH, Jutras M. Imaging, Management, and Treatment of Orbital Trauma. Radiologic Clinics of North America. 2026;64(4)

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