Chapter 4: Cortical Mastoidectomy

Cortical Mastoidectomy is a surgical procedure aimed at removing diseased air cells from the mastoid process while preserving critical anatomical structures. This procedure is vital for managing chronic otitis media, mastoiditis, and other temporal bone pathologies. The following detailed guide outlines the steps, techniques, and precautions associated with mastoidectomy, adhering to established surgical principles and anatomical landmarks.

Key Anatomical Landmarks

Understanding the anatomy of the temporal bone is crucial for a successful mastoidectomy. The essential landmarks include:

1. Surface Landmarks:

• External auditory canal (EAC): Defines the anterior boundary.
• Spine of Henle: The suprameatal spine, posterior to the EAC, marks the lateral boundary.
• Macewen’s triangle: Posterior and superior to the spine of Henle, this guides access to the mastoid antrum (1, 7).
• Temporal line: Continuous with the superior border of the zygomatic arch, this indicates the superior boundary.
• Mastoid tip: The inferior boundary where the digastric ridge can be identified (1, 3, 12).

2. Deep Landmarks:

• Middle fossa dura (tegmen tympani): The roof of the mastoid cavity and the superior limit of dissection.
• Sigmoid sinus: The posterior boundary, visible as a bluish discoloration under the bone.
• Sinodural angle: The junction of the tegmen tympani and sigmoid sinus.
• Lateral semicircular canal: A key structure posterior to the antrum, critical for preserving inner ear function.
• Facial nerve: Located medially and anteriorly, requiring careful identification and preservation (2, 3, 12).

Macewen’s Triangle 

Macewen’s triangle, also known as the suprameatal triangle, is a critical surface landmark for accessing the mastoid antrum during mastoidectomy. Its boundaries are defined as follows:

1. Anterior Boundary:
   • A vertical tangent line drawn posterior to the external auditory canal (EAC).
2. Superior Boundary:
   • A horizontal tangent drawn posteriorly from the level of the temporal line or the superior edge of the EAC.
3. Posterior Boundary:
   • A line connecting the posterosuperior margin of the EAC to the temporal line in an oblique direction.

Key Features:

• Location: Macewen’s triangle is situated immediately behind the spine of Henle.
• Depth: The mastoid antrum typically lies 1.5–2 cm deep to this triangle.
• Utility: It serves as a guide for safe drilling, providing access to the antrum without risking injury to deeper structures like the sigmoid sinus or dura (1, 3, 7).

Understanding these boundaries ensures precise orientation and facilitates safer and more effective mastoidectomy procedures.

Surgical Steps

1. Initial Exposure

• Positioning and Preparation:
  • Stabilize the temporal bone and remove soft tissue.
  • Orient the specimen in the surgical position and clear the periosteum from the lateral surface.

• Surface Anatomy:
  • Identify landmarks such as the EAC, spine of Henle, and temporal line and Macewen’s triangle to establish surgical orientation (1, 7, 11).

• Initial bone cuts: 

• Initial bone cuts using drill are made along the boundaries of the Macewen’s triangle. 

2. Saucerization and Cortical Removal

• Begin with a large cutting burr to create a wide saucerized cavity. This ensures improved visibility and access to deeper structures.
• Define the boundaries:
  • Anterior: Posterior wall of the EAC.
  • Superior: Temporal line and tegmen tympani.
  • Posterior: Sigmoid sinus.
  • Inferior: Mastoid tip (1, 3, 7).

• Drilling should progress from lateral to medial, avoiding deep holes and maintaining even cavity depth. Keep the posterior canal wall intact while thinning it sufficiently to reveal underlying structures.

3. Identifying the Tegmen Tympani

• The tegmen tympani marks the superior boundary of the dissection. Drill parallel to its plane, observing changes in bone color to identify the dura (pink hue) without breaching it.
• Thin the bone over the tegmen to enhance visibility into the antrum (1, 2, 3).

4. Identifying the Sigmoid Sinus and Sinodural Angle

• Progress posteriorly to locate the sigmoid sinus, using changes in color (bluish hue) and drill sound (higher pitch as bone thins) as cues.
• The sinodural angle, formed by the tegmen and anterior margin of the sigmoid sinus, serves as a key guide for deeper dissection (2, 3, 12).

5. Opening the Mastoid Antrum

• Drill anteriorly and medially from the sinodural angle to access the mastoid antrum. This cavity lies approximately 2–3 cm deep to Macewen’s triangle and contains larger air cells.
• Remove Koerner’s septum if present, taking care not to breach the horizontal semicircular canal located anteriorly and medially (7, 12).

6. Preserving the Facial Nerve

• The facial nerve lies more superficially in the mastoid portion, especially in pediatric patients. Identify it under a thin bony layer by drilling parallel to its course.
• Use diamond burrs near the facial recess and labyrinthine structures to minimize risk (2, 3, 7, 12).

7. Skeletonizing the Sigmoid Sinus

• Thin the bone over the sigmoid sinus, ensuring the posterior boundary is well-defined without compromising its integrity.
• Progress inferiorly to delineate the digastric ridge, which helps identify the stylomastoid foramen and the facial nerve’s vertical segment (3, 7, 12).

8. Widening the Cavity

• Remove air cells widely, ensuring the cavity is funnel-shaped, with broader lateral access. This step improves visibility of deeper structures and prevents overhangs that hinder access (1, 7).

9. Completing the Mastoidectomy

• Exenterate all air cells, including perilabyrinthine and retrofacial cells, to ensure pathology clearance.
• Confirm the exposure of key structures, including:
  • Tegmen tympani
  • Sigmoid sinus
  • Sinodural angle
  • Digastric ridge
  • Lateral semicircular canal
  • Fossa incudis and incus (1, 3, 7, 12).

Special Considerations

Pediatric Anatomy

• The facial nerve lies more laterally and superficially in children due to incomplete mastoid development. Extra caution is required during drilling and incision-making (2).

Coalescent Mastoiditis

• Diseased bone may be softened by inflammation. In suspected dura or sinus involvement, expose and evaluate these structures for granulation tissue, pus, or thrombosis (2).

Safe Drilling Practices

• Always drill parallel to critical structures, transitioning to diamond burrs near sensitive areas like the facial nerve and semicircular canals.
• Observe sound and visual cues to avoid breaching dura or sinus walls (1, 3, 7, 12).

Conclusion

A successful mastoidectomy requires precise anatomical understanding and careful surgical execution. By adhering to established landmarks, employing systematic saucerization, and maintaining vigilance near critical structures, surgeons can effectively manage pathology while minimizing complications. This step-by-step approach ensures safe navigation through the temporal bone and optimal surgical outcomes (1, 2, 3, 7, 12).