Definition of a Safe Drilling Zone in the Mastoid: Anatomic Insights From CT-Based Facial Nerve Mapping

Anatomic variability and poor mastoid pneumatization increase the risk of facial nerve injury during mastoidectomy by reducing the reliability of conventional anatomic landmarks. Currently, no standardized numerical guidelines define safe drilling depth relative to the facial canal. The objective of this study was to analyze the mastoid segment of the facial nerve on noncontrast head computed tomography (NCCT) to quantify facial nerve depth at 3 reproducible mastoid reference points and to define a practical, imaging-based safe-drilling zone for mastoidectomy.

We retrospectively reviewed 100 adult NCCT examinations (200 sides; 65 women, 35 men) acquired with bone windows, submillimetric slice thickness (≤0.6 mm), and multiplanar reformations aligned to the orbitomeatal line (Reid's baseline). Bilateral measurements captured the perpendicular cortex-facial canal distance at 3 anatomic landmarks: (1) incus level, (2) midpoint between the incus and the stylomastoid foramen, and (3) stylomastoid level. Distances (mm) were summarized using 2.5th to 97.5th percentiles.

Across 200 sides, cortex-facial canal distances were 15.81 to 24.22 mm at the incus level, 11.53 to 20.22 mm at the midpoint, and 10.34 to 19.77 mm at the stylomastoid level (2.5th-97.5th percentiles). For intraoperative usability and safety, we rounded the 2.5th percentiles down to define safe-drilling limits of 15 mm (incus), 11 mm (midpoint), and 10 mm (stylomastoid).

NCCT-based percentile mapping yielded reproducible thresholds for drill depth relative to the facial canal. Limiting depth to ≤15 mm at the incus, ≤11 mm at the midpoint, and ≤10 mm at the stylomastoid level provides an anatomically grounded reference that may support operative depth awareness and safer mastoid drilling, particularly when conventional landmarks are unreliable.