martedì 20 gennaio 2009

Orbital blow out fracture


Figure 1: The x-ray demonstrates a fracture of the right orbital floor and an air-fluid level in the right maxillary sinus suspicious for hematoma.
Figure 2: Coronal CT image demonstrates a fracture of the left orbital floor along with herniation of the orbital fat. Air-fluid level in the left maxillary sinus containing high attenuation is consistent with hematoma.

Diagnosis: Orbital blow out fracture

An orbital blow-out fracture is a fracture of the orbital floor caused by blunt trauma to the orbit. Blow-out fractures are usually caused by round or oval object with a diameter slightly larger than the orbital diameter such as a baseball, snowball, tennis ball or fist. The mechanism of fracture is controversial. The two most accepted theories are: 1) The "retropulsion" theory which proposes the fracture is a result of a sudden increase in intraorbital pressure when the globe is pushed posteriorly. 2) The "buckling" theory states that the fracture is secondary to the force causing the orbital rim to buckle and transmitting the force into the orbital bones.

One may expect medial wall fractures to be more common than orbital floor fractures as the medial wall is thinner; however, the reverse is true. Proponents of the "retropulsion" theory attribute this to the honeycomb structure of the ethmoid air cell septae, which support the lamina papyracea, thus allowing it to withstand the sudden rise in intraorbital hydraulic pressure better than the orbital floor. Proponents of the “buckling” theory propose that the orbital floor is particularly vulnerable as the infraorbital canal further weakens the floor’s already delicate bony structure.

Routine facial views obtained should include the Caldwell and Waters view. The Caldwell projection is used to evaluate the lateral orbital wall and ethmoid bone, while the Waters view is useful for visualizing the inferior orbital wall and maxillary sinuses. The teardrop sign is secondary to opacification of the upper maxillary sinus from herniated orbital fat.

CT has become the modality of choice to evaluate orbital fractures. Herniation of orbital fat, the inferior rectus muscle, and the inferior oblique muscle may occur and herniated extraocular muscles entrapped between the fragmented bone segments may lead to diplopia. Edema and hemorrhage from the trauma can also cause diplopia; however this should resolve in a few days in the absence of entrapment. Immediate surgical intervention may be required in cases of entrapped extraocular muscles or acute enopthalmos. Chronic enopthalmos may also develop in fractures with extensive orbital fat herniation.

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