Compartmental Endoscopic Surgical Anatomy of the Inferior Intraconal Orbital Space

Background: The role of the endoscopic endonasal approach for management of orbital lesions has significantly expanded in recent years. The endoscopic approach to lesions of the inferomedial intraconal space is technically demanding due to the small area, limited retraction for access, and variable anatomy of critical structures. The purpose of this study is to define the anatomy of the inferior intraconal space in terms of its neurovascular structures and compartmentalize the space to provide guidance for surgical planning.

Methods: This was an endoscopic anatomical study of 8 cadaveric orbits. After dissection of the inferior intraconal space, the branches and insertion of the oculomotor nerve (OMN) and ophthalmic artery (OA) were described relative to the fixed landmark of the posterior maxillary wall. Arterial structures were assessed for their point of insertion on the inferior rectus muscle (IRM) (medial, mid, or lateral belly). The insertion of the OMN branches to the IRM and point of lateral excursion of the inferior oblique muscle (IOM) branch were analyzed.

Results: A mean of 2.6 OA branches to the IRM per orbit were identified (standard deviation [SD]±0.53). The mean distance of the insertion of the OA branches from the posterior maxillary wall was 7.11mm (range: 1mm-20mm, SD±5.65mm). 39% of branches were in the medial position, 50% were in the mid position, and 11% were in the lateral position relative to the IRM belly. The mean distance of the insertion of the OA branches into the IRM from the posterior maxillary wall was greater in the vessels in the mid-belly location (9.89mm) compared with those in the medial location (4.29mm). Overall, there were 15 branches of the OA that were medial and 3 branches lateral to the OMN among the 8 orbits.

The mean number of OMN branches to the IRM was 1.63 (SD±0.74) with a mean insertion distance from the posterior maxillary wall of 1.88mm (range: 0mm-6mm, SD±1.89mm). The mean distance from the origin to the point at which the OMN branch supplying the IOM crossed to the lateral surface of the IRM was 5.38mm (range: 0mm-14mm, SD±5.42mm).

Conclusions: This cadaveric study provides a guideline on the variability of the neurovascular structures that supply the IRM within the inferior intraconal space. These critical structures and their inter-relationship enable the conception of different compartments with varying risks of dissection relative to the position in the globe.