Orbital Volume Change Measured by Computed Tomography in Blow Out Fracture

ABSTRACT

Enophthalmos is a common result of blow out fracture of the orbit. Prior to CT volume analysis no reliable measurement of the degree of bony and soft tissue deformity was available to identify patients who would develop enophthalmos. Evaluation of orbital volume expansion and volume expansion percentage were performed in 23 patients with blow out fracture, after 1-4 days of injury. All studies were performed on a CT (Somatom Plus, Germany, Siemens), using transaxial scan technique(140 kVp, 206 mA, 3 mm contiguous sections). The radiological boundaries of the orbit were defined anteriorly by a line mrrecting the anterior surface of the zygomaticofrontal process to the nasomaxillary suture and posteriorly to the optic foramen. The Hounsfield Unit(HU) ranges were -100 to 0 and 0 to +100. It is difficult to distinguish blood from fat in the orbital volume measurement. We used three methods to obtain more accurate orbital volume. First, The concave margin or the air meniscus surface area is blood and the convex margin area is fat. Second, It can be distingushed blood from fat by Hounsfield Unit of specific area pixel. Third, The homogenous area is blood and inhomogenous area is fat or mixture of fat and blood. The patients(> or =13% orbital volume expansion) are managed surgically. 15 patients were classified in operative group and 8 patients were in conservative treatment group. The volume expansion percentage is 12.7% to 28.8% in operative group and 4.2% to 11.2% in conservative group. There was no enophthamos in each groups after 3 months of operation and injury. CT measurements of orbital volume expansion and volume expansion percentage can predict the final degree of the late enophthalmos and may facilitate the planning of surgical intervention. To obtain more accurate prediction of enophthalmos, we consider not olny volume expansion but also volume expansion percentage, because everyone doesn't have the same normal orbital volume.