Interzygomatic line

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)
  • 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)

Mean length of the line: 103 mm in men and 96 mm in women (angled -15 to 15° to the orbitomeatal plane) .

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 , to angled axial scans (-15° to 15°) . 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 .