The Orbital Angle: Understanding the Role of Skull Base Topography in Determining Access to the Midline via a Supraorbital Craniotomy

Introduction: The role the topography of the anterior skull base (i.e., the angulation of the orbital roof with the cribriform plate) plays in determining access to the midline via the supraorbital craniotomy (via an eyebrow incision) has not been studied. To quantify the feasibility of this approach to various midline points in the anterior cranial fossa, we analyzed the CT images of 25 patients (50 orbits) to understand the angulation of the orbital roof with the cribriform plate and the role of individual patient variation.

Methods: Using a rotated coronal plane, which included the proposed supraorbital craniotomy and the target point on the midline anterior skull base, we approximated the line of sight required to access the skull base at 0.5-cm increments anterior to the tuberculum sella, measured on an axial cut parallel to the planum sphenoidale. We then measured the vertical requirement (VR), which is the distance from point A (intersection of the axial plane at the superior margin of the orbit, sagittal plane at the lateral margin of the orbit, and the skull) to point B (intersection of the line of sight and the coronal plane defined by point A). A larger VR indicates that the craniotomy would need to be larger to access the specified point; hence, not accessible via a standard supraorbital craniotomy.

Results: The typical orbit topography is relatively flat between the supraorbital craniotomy and the tuberculum sella, becoming more obstructed by the orbit when moving anteriorly until a peak of 3 cm, and becoming less obstructed when progressing past 3 cm. Only three orbits had a maximum VR at 0–2 cm from the anterior margin of the tuberculum. The most common distance to maximum VR was at 3 cm (21 orbits). Another 21 orbits had a maximum VR at 4–6 cm. This reflects the typical trend of VR, which starts at an average of 0.6 ± 0.2 cm at the anterior margin of the sella, and reaches 2.3 ± 0.3 cm at 3 cm anteriorly. Using linear interpolation between each pair of adjacent measurements, a craniotomy measuring 2 cm vertically would be able to access as far anteriorly as 2.36 cm on the average orbit. In addition, the average skull base flattens out anterior to the 3-cm point, and the VR reaches 1.5 ± 0.2 cm at 5 cm anterior to the sella.

Conclusions: These results indicate that the angle at which the orbit intersects with the cribriform plate varies in an anterior-posterior direction and in between patients; these factors must be contemplated when considering the supraorbital craniotomy for midline anterior cranial fossa lesions.