Head and Neck ImagingBlow-out fracture
fracture produced by a blow by an object too large to enter the orbit. The force is absorbed by the orbital rim and transmitted to the weaker medial orbital wall and especially to the orbital floor, causing
fracture. As the intraorbital pressure is raised by the backward displacement of the eye, the orbital content is squeezed out of the orbit, blowing the fractured floor out into the
maxillary sinus. Usually only fat herniates into the
sinus, but the inferior and sometimes the medial rectus muscle can be displaced inferiorly. These muscles are only occasionally entrapped by bony fragments. Diplopia is the most frequent complaint; this may occur solely because of periorbital oedema and
haemorrhage, possibly exerting pressure on the globe and resolving in several days (while entrapment diplopia remains) (
Fig.1). There are some other possible causes for eye motility disturbances after an orbital blow-out
fracture, for example post-traumatic oculomotor nerve palsy, gross enophthalmos with posterior and inferior displacement of the eyeball, incarceration of orbital fat itself and scar formation. Sometimes only indirect signs of a blow-out
fracture are present on conventional X-rays, consisting of soft tissue swelling over the inferior orbital rim and antral opacification. The presence of an opacified maxillary
sinus can be misleading, as it can be due to inflammatory disease or haemosinus caused by a mucosal tear. Coronal
CT clearly shows the actual
fracture and the
herniation of the orbital contents.
Pure ethmoidal blow-out fractures are more rarely seen (usually occurring together with an orbital floor fracture); entrapment of the medial rectus muscle is rare. Medial wall fracture can be inferred by the presence of orbital emphysema, developing after trauma when the patient blows his nose (Fig.2).
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Coronal CT images through the orbit. The bone algorithm image shows the displaced orbital floor fracture (asterisk); associated blow-out fracture of the medial orbital wall (arrow). The soft tissue window shows downwards displacement of the orbital fat, thickening and displacement of the inferior rectus muscle, with perimuscular oedema/haematoma (arrowhead). Residual haematoma in the maxillary sinus.
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Blow-out fracture, Fig.1 (a) | | Blow-out fracture, Fig.1 (b) | | Blow-out fracture, Fig.2 (a) |
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Blow-out fracture, Fig.2 (b) | |