Objectives:To calculate the volume of bone in 3 areas of the deep lateral orbit that are available for removal in decompression surgery and to demonstrate these 3 areas within a 3-dimensional computed tomographic reconstruction of the orbit. Design:The 3 areas of bone in the deep lateral orbit were designated the lacrimal keyhole, the sphenoid door jamb, and the basin of the inferior orbital fissure. By means of digitized computed tomographic scans, these 3 areas of bone were analyzed by measuring preoperative and postoperative orbital volumes and predicted bony expansion volumes in 9 patients (17 orbits) who underwent deep lateral orbital decompression surgery. We also calculated the volume of bone that could be removed from 11 normal orbits. A 3-dimensional computer reconstruction of an orbital computed tomographic scan was created, and the 3 areas of potential bone were delineated within it. Results:The average volumes of the basin of the inferior orbital fissure, the sphenoid door jamb, the lacrimal keyhole, and the total of the 3 regions were 1.2, 2.9, 1.5, and 5.6 cm 3 , respectively. The 3 areas of bone contributed variably to the total, with the door jamb contributing the most volume of the 3, nearly twice the value of the other 2. There was, however, a significant amount of interpatient variability, especially for the door jamb region. Conclusion:Orbital decompression surgery of the deep lateral wall can provide adequate volume expansion because of the amount and location of potential space that exists in the 3 areas of deep bone.

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