Orbital fat volume in the inferolateral quadrant in Japanese: a guide for orbital fat decompression without injury to the oculomotor nerve.

PURPOSE: The inferior oblique muscle branch of the oculomotor nerve is susceptible to injury during orbital fat removal from the inferolateral quadrant of the orbit. Understanding the amount of removable orbital fat volume in this quadrant may be helpful in achieving maximum fat decompression without causing nerve injury. The aim of this study was to calculate the orbital fat volume in the inferolateral quadrant using computed tomographic (CT) images.

METHODS: In this retrospective, observational study, contiguous 1-mm coronal CT images were obtained from 53 sides (30 patients). The cross-sectional areas of the orbital fat in the inferolateral quadrant were measured from the level just behind the inferior oblique muscle to the orbital apex. The cross-sectional areas reached the inferior oblique muscle branch medially, the lateral orbital wall laterally, the inferior edge of the lateral rectus muscle superiorly, and the orbital floor inferiorly. An integrated value between the cross-sectional areas and the CT slice thickness was calculated.

RESULTS: The mean orbital fat volume in the inferolateral quadrant was 3.9 ± 1.4 mL (range: 1.3-7.0 mL). Multiple regression analysis demonstrated a significant relationship between orbital fat volume and Hertel exophthalmometry measurement (adjusted r 2  = 0.101; P = 0.012), although the thickness of the lateral and inferior recti muscles was deleted from the regression equation by stepwise process.

CONCLUSIONS: The orbital fat volume in the inferolateral quadrant can serve as a guide for orbital fat removal without causing injury to the inferior oblique muscle nerve branch.