The interzygomatic line is a commonly used reference standard for the evaluation of proptosis due to various etiologies on CT/MRI scans.
Technique
A horizontal line should be drawn between the most anterior parts of the zygomatic bones in the axial plane.
The normal distance of the interzygomatic line from the posterior sclera:
- 9.9 +/- 1.7 mm (angled parallel to the orbitomeatal plane) 1
- male: 11.2 +/- 4.8 mm (angled -15° to 15° to the orbitomeatal plane)
- female: 10.8 +/- 1.9 mm (angled -15° to 15° to the orbitomeatal plane) 2
Mean length of the line: 103 mm in men and 96 mm in women (angled -15 to 15° to the orbitomeatal plane) 3.
Practical points
The interzygomatic line can only be used as a reference if the angle of the acquisition relative to the orbitomeatal plane is standardized. Unfortunately various studies haves used different scanning parameters ranging from acquisitions parallel to the orbitomeatal plane 1, to angled axial scans (-15° to 15°) 2-4. Knowledge of this discrepancy between the potential reference standards is crucial, and thus it is also important to perform cross-sectional imaging of the orbits in an internally consistent manner.
Furthermore, it is a good practice to ask patients to keep gazing forward, and to maintain only a gentle eye closure during the scan, in order to avoid globe displacement by the forced contraction of the extraocular muscles 3.