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. It serves as an important anatomical landmark in otologic surgery. In this article, we shall look at the anatomy of MacEwen’s triangle – its borders, contents and clinical relevance. By TeachMeSeries Ltd (2025) Fig 1The mastoid fossa (shaded yellow) is a surface landmark of the external surface of the temporal bone Pro Feature - 3D Model You've Discovered a Pro Feature Access our 3D Model Library Explore, cut, dissect, annotate and manipulate our 3D models to visualise anatomy in a dynamic, interactive way. Learn More Borders The mastoid fossa is a triangular shaped area with superior, anterior, and posterior borders: Superior – Supramastoid crest Extension of the upper border of the posterior root of the zygomatic process. Anterior – Suprameatal spine (spine of Henle) Projection of bone at the posterosuperior aspect of the opening of the external acoustic meatus, immediately inferior to the root of the zygomatic process. Posterior – Hypothetical vertical line Tangential to the mid-point of the posterior wall of the external auditory canal By TeachMeSeries Ltd (2025) Fig 2The mastoid fossa is triangle shaped. It has superior, anterior, and posterior borders. Contents The mastoid fossa is a bony landmark on the external surface of the temporal bone. It overlies the mastoid antrum – the largest of the mastoid air cells. The mastoid fossa does not contain any other important structures. This is why it is so significant in otologic surgery – it demarks a ‘safe area‘ to begin the traditional drilling approach during a cortical mastoidectomy. By TeachMeSeries Ltd (2025) Fig 3The mastoid fossa highlighted in yellow with the underlying mastoid antrum. Clinical Relevance Cortical Mastoidectomy A cortical mastoidectomy is a surgical procedure used to remove the mastoid air cells of the temporal bone. It is typically used to treat severe cases of acute mastoiditis. This is a condition where infection from the middle ear spreads to the mastoid air cells. The mastoid fossa is an important landmark in a cortical mastoidectomy. It overlies the mastoid antrum, and demarks a ‘safe area‘ to begin the traditional drilling approach. Do you think you’re ready? Take the quiz below Pro Feature - Quiz The Mastoid Fossa (MacEwen’s Triangle) Question 1 of 3 Submitting... Skip Next Rate question: You scored 0% Skipped: 0/3 1800 More Questions Available Upgrade to TeachMeAnatomy Pro Challenge yourself with over 1800 multiple-choice questions to reinforce learning Learn More Print Article Rate This Article