Orbital Blow-out Fractures

The CLASSIC BLOWOUT FRACTURE involves the floor of the orbit, usually sparing the orbital rim. Frequently, orbital tissues are trapped in the fracture site and ocular motility disturbances have been ascribed to entrapment of one or both inferior EOMs, but may be also due to orbital hematoma / inflammation.

The term orbital blow-out fracture describes the injury that results from a blow to the orbit by an object that is too large to enter the orbit (fist, baseball, etc.). The force of the blow is absorbed by the orbital rim and is transmitted to the thinner orbital floor, which shatters, usually in the middle third near the infraorbital canal. As the eye is pushed back into the conical orbital apex, it increases intraorbital pressure and this ‘‘blows out’’ the fractured floor into the maxillary sinus.

Pure blow-out Vs. Impure blow-out fractures

Usually the orbital rim is not fractured (pure blow-out fracture) and the globe remains undamaged. Less commonly the inferior orbital rim also is fractured; this is referred to as an impure blow-out fracture. 

Herniation of orbital fat, inferior rectus muscle, and inferior oblique muscle can occur with occasional muscle entrapment in the fracture line, resulting in diplopia on upward gaze.

Diplopia is the most frequent complaint in all patients with blow-out fractures and may occur solely because of periorbital edema and hemorrhage, which exert pressure on the globe. This type of diplopia resolves in several days, whereas entrapment diplopia remains.

Ref : Head and Neck Imaging, 4^th Edition, Peter.N.Som.

Terms 'Blow-out / Blow-in' fractures can also be used in cases of Medial wall or rarely roof fractures also. Medial wall fractures are more likely to cause diplopia than a floor blow-out. Roof fractures might require an intracranial approach of treatment because of the breach of dura.

Sometimes the fractured floor, can sprang back into place, after the inferior rectus has herniated, trapping the muscle, this is called as a 'Trapdoor Fracture' (See the image below)

CT coronal section (soft tissue window) showing the entrapped inferior rectus, with the fractured floor back into anatomical position -- TRAPDOOR fracture.

Ref : RG 2006; 26:783-793 - 'Diagnosis of Midface fractures with CT'.

 
The extra-ocular muscle that herniates in the floor blow-out fracure is almost always the Inferior Rectus - Oblique. But below is a rare case, where the medial rectus has herniated through the floor defect into the maxillary sinus.

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