M01.02.007 The Mastoid Fossa (MacEwen’s Triangle)

The mastoid fossa, also known as MacEwen’s triangle or supra meatal triangle, is a small but crucial landmark on the temporal bone. Understanding its location and significance is essential in ear surgery, particularly cortical mastoidectomy.

Borders

This triangular depression has well-defined borders:

• Superior: Supramastoid crest, an extension of the zygomatic process.
• Anterior: Suprameatal spine (spine of Henle), a bony projection near the external ear canal.
• Posterior: An imaginary vertical line along the midpoint of the posterior external auditory canal wall.

Contents

While seemingly unremarkable, the mastoid fossa serves a critical purpose:

• Mastoid Antrum: The fossa overlies the mastoid antrum, the largest air cell within the mastoid bone.
• Surgical Safe Zone: The fossa itself is devoid of major structures, making it a valuable “safe zone” for surgical access.

Clinical Relevance: Cortical Mastoidectomy

Cortical mastoidectomy is a surgical procedure to remove infected mastoid air cells, often in cases of severe acute mastoiditis (middle ear infection spreading to the mastoid bone). The mastoid fossa plays a starring role:

• Surgical Entry Point: As it overlies the mastoid antrum, the fossa provides the entry point for the surgeon’s drill during the traditional cortical mastoidectomy approach.
• Minimized Risk: Because the fossa lacks significant structures, it minimizes the risk of injuring nerves or blood vessels during surgery.

Figures

Fig 1.0 – Borders of the Mastoid Fossa

Fig 1.1 – Contents of the Mastoid Fossa

Fig 1.2 – Clinical Relevance of the Mastoid Fossa

Remember

• Vital Landmark: The mastoid fossa, though small, is a vital landmark for ear surgeons.
• Safe Access: Its location guides safe access to the mastoid antrum during crucial procedures like cortical mastoidectomy.