Mastoid Anatomy

Mastoid Fossa – MacEwen's Triangle

The mastoid fossa (also known as MacEwen's triangle or suprameatal triangle) is a triangular shaped depression in the external surface of the temporal bone, serving as an essential anatomical landmark in otologic surgery. In this article, we will discuss the anatomy of MacEwen's triangle, including its borders, contents, and clinical relevance.

Borders

The mastoid fossa is a triangular area with three distinct borders:

• Superior – Supramastoid crest. This is created by the extension of the upper border of the posterior root of the zygomatic process.
• Anterior – Suprameatal spine (spine of Henle). This is a projection of bone at the posterosuperior aspect of the opening of the external acoustic meatus, located just inferior to the root of the zygomatic process.
• Posterior – Hypothetical vertical line. This line is tangential to the mid-point of the posterior wall of the external auditory canal.

Contents

The mastoid fossa is a bony landmark on the external surface of the temporal bone. It overlies the mastoid antrum – the largest of all the mastoid air cells. Other than this structure, the mastoid fossa does not contain any significant structures. This is the reason it is such a crucial landmark for otologic surgery, as it marks a “safe area” to begin the traditional drilling approach during a cortical mastoidectomy.

Clinical Relevance – Cortical Mastoidectomy

Cortical mastoidectomy is a surgical procedure used to remove the mastoid air cells of the temporal bone. The mastoid fossa is an essential landmark for this procedure, as it overlies the mastoid antrum, demarking a “safe area” to begin the traditional drilling approach. It is commonly used for severe cases of acute mastoiditis – a condition when the infection from the middle ear spreads to the mastoid air cells.

In sum, the mastoid fossa (MacEwen’s triangle) is a triangular depression in the external surface of the temporal bone, serving as an important anatomical landmark for otologic surgery. Its borders, contents, and clinical relevance are discussed above. It is a safe area to begin the traditional drilling approach during a cortical mastoidectomy, and is used to treat severe cases of acute mastoiditis.