Navigating the Intersection Between the Orbit and the Skull Base: The "Mirror" McCarty Keyhole During Transorbital Approach: An Anatomic Study With Surgical Implications.

Abstract

Background and objectives: McCarty keyhole (MCK) is the most important entry point during orbitocranial and cranio-orbital approaches; nevertheless, its anatomic coordinates have never been detailedly described from transorbital perspective. To provide the spatial coordinates for intraorbital projection of the "mirror" MCK by using the well-established main anatomic-surgical bony landmarks met along transorbital corridor.

Methods: MCK was identified in 15 adult dry skulls (30 sides) on exocranial surface of pterional region based on the well-defined external bony landmarks: on the frontosphenoid suture, 5 to 6 mm behind the joining point (JP) of frontozygomatic suture (FZS), frontosphenoid suture (FSS), and sphenozygomatic suture (SZS). A 1-mm burr hole was performed and progressively enlarged to identify the intracranial and intraorbital compartments. Exit site of the intraorbital part of burr hole was referenced to the FZS on the orbital rim, the superior orbital fissure, and the inferior orbital fissure and to the JP of FZS, FSS, and SZS. To electronically validate the results, 3-dimensional photorealistic and interactive models were reconstructed with photogrammetry. Finally, for a further validation, McCarty mirror keyhole was also exposed, based on results achieved, through endoscopic transorbital approach in 10 head specimens (20 sides).

Results: Intraorbital projection of MCK was identified on the FSS on intraorbital surface, 1.5 ± 0.5 mm posterior to JP, 11.5 ± 1.1 mm posterior to the FZS on orbital rim following the suture, 13.0 ± 1.2 mm from most anterior end of superior orbital fissure, 15.5 ± 1.4 mm from the most anterior end of the inferior orbital fissure in vertical line, on measurements under direct macroscopic visualization (mean ± SD). These values were electronically confirmed on the photogrammetric models with mean difference within 1 mm.

Conclusion: To be aware of exact position of intraorbital projection of MCK during an early stage of transorbital approaches provides several surgical, clinical, and aesthetic advantages.