Normative measurements of orbital structures using CT

OBJECTIVE. The purposes of this study were to establish criteria for the diameters of normal extraocular muscles, to determine the normal position of the globe as revealed by CT, and to investigate the effects of age and sex on these structures. SUBJECTS AND METHODS. Diameters of extraocular muscles, distance from the interzygomatic line to the posterior margin of the globe, width of the optic nerve-sheath complex, and length of the interzygomatic line were calculated for 200 normal orbits of 100 patients on axial and direct coronal CT images. Effects of age and sex on muscle diameters and globe position were analyzed. RESULTS. Normal ranges for the diameters (mean +/- 2SDs) of extraocular muscles were medial rectus, 3.3-5.0 mm; lateral rectus, 1.7-4.8 mm; inferior rectus, 3.2-6.5 mm; and superior group, 3.2-6.1 mm. The normal position of the globe was 9.4 mm behind the interzygomatic line (range, 5.9-12.8 mm). The mean diameters of the extraocular muscles and the length of the interzygomatic line in male patients were significantly larger than in female patients (p < .001). Statistically significant correlation was found between age and the diameters of the inferior and lateral rectus muscles (p = .32, p = .013; and r = .23, p = .048, respectively). CONCLUSION. Our results may be important in interpreting CT scans of the orbit because, to our knowledge, no reliable normative data exist regarding these orbital structures.