J Neurol Surg B Skull Base 2015; 76 - P018
DOI: 10.1055/s-0035-1546646

Expanded Endonasal Approach to the Intraconal Orbit: A Comparative Anatomic Study Before and After Release of the Annulus of Zinn

Lewis J. Overton 1, Rounak B. Rawal 1, Ana M. Lemos 1, Zainab Farzal 1, Deanna Sasaki-Adams 1, Matthew Ewend 1, Charles S. Ebert Jr.1, Brent A. Senior 1, Adam M. Zanation 1
  • 1University of North Carolina–Chapel Hill, North Carolina, United States

Background: The expanded endonasal approach (EEA) has previously been described to access the medial orbit for a variety of pathologies; however, intraconal approaches are still novel and evolving. In this study, we displace the inferior rectus and medial rectus via external control, and measure the degree of endoscopic access allowed on fresh cadaveric specimens before and after the release of the Annulus of Zinn (AZ). Measurements are also made indicating the increased anterior–posterior visualization of the medial orbit and optic nerve before and after release.

Study Design: This study is a comparative anatomic study on human cadaveric heads.

Methods: Endoscopic approaches to the medial and intraconal orbit were performed. Measurements of the inferior and medial rectus corridor at the level of the anterior ethmoid artery (AEA) and posterior ethmoid artery (PEA) and the extent of optic nerve visualization were performed before and after release of the AZ.

Results: A total of four human cadavers were dissected bilaterally (n = 8). The mean medial rectus length and standard deviation before and after AZ release were 17 ± 2.7 mm, and 22 ± 1.8 mm, respectively (p = 0.004). The mean optic nerve length and standard deviation before and after AZ release were 8.6 ± 2.7 and 14 ± 2.7 mm, respectively (p < 0.001). Mean caudal-rostral distance between the rectus muscles at the level of the AEA and PEA before AZ release was 3.4 ± 0.75 and 1.31 ± 0.34 mm, respectively (p < 0.001). Mean caudal-rostral distance between the rectus muscles at the level of the AEA and PEA after AZ release was 4.0 ± 0.82 and 1.7 ± 0.36 mm, respectively (p < 0.001). The mean differences in the caudal-rostral distance measured before and after AZ release at the level of the AEA and PEA were 1.46 ± 0.61 and 0.39 ± 0.17 mm (p = 0.005).

Conclusion: Greater access to orbital contents via an expanded endonasal approach is afforded by AZ release. The magnitude of increased access is greater at the level of the anterior ethmoid artery compared with the posterior ethmoid artery. In addition, with the release of the AZ, significantly longer anterior–posterior segments of the optic nerve and medial rectus muscles can be identified. This may aid in endoscopic surgical planning for intraconal lesions.