Postoperative Orbital Volume Measurement by MRI

Purpose: The exact source of enophthalmos as a late complication after orbital fracture is poorly investigated. 3-D orbital volume measurement has been the domain of multi-slice computed tomography to this date, but 3D-magnetic resonance imaging is supposed to be a superior alternative, because of the well known drawbacks of MSCT concerning limited soft tissue depiction, radiation and artefacts. Thus, the key goal of this prospective study was to establish a soft tissue and bony orbital volume calculation method based on different MRI weightings to assess the source of late postoperative enophthalmos. Nonrandomized prospective comparative study 37 patients with reconstructed complex orbital fractures.

Methods: Morphologic and dimensional changes of the orbit, eye globes, extraocular muscles and fat contents were measured 3-4 months postoperatively. Together 2D-CT and MRI images as well as the respective 3D-reconstructions were used to assess the site and size of bony and soft tissue changes in the traumatized orbits. Dimensional changes in the eye globe-orbital apex distance were used to assess the criteria for enophthalmos. Results: All enophthalmic orbits revealed a significant volume increase compared to the contralateral side. Orbital volume enlargement of 1 cm3 lead to 0.93 mm enophthalmos (p< 0.05). The retrobulbar fat presented itself fragmented and in most cases dislocated into the medial posterior region. However, almost no fatty atrophy was determined by MRI.

Conclusions: In several aspects of orbital volume measurements MRI investigations of the posttraumatic orbit revealed to be superior to MSCT. By this MRI technique the source and the degree of enophthalmos could be determined much more precisely due to the better soft tissue depiction for further planning of secondary correction. Proton density weighting proved to be the superior sequence for soft tissue volume segmentation allowing distinguishing the cause of posttraumatic enophthalmos.