MacEwen's (Suprameatal) Triangle: Anatomical Landmark and Surgical Significance
MacEwen's triangle (also called the suprameatal triangle) is a critical surface anatomical landmark on the lateral mastoid cortex that serves as the primary safe entry point for cortical mastoidectomy, lying posterosuperior to the external auditory canal and bounded by the posterior canal wall anteriorly, the temporal line (linea temporalis) superiorly, and the posterosuperior spine of Henle (suprameatal crest) inferiorly.
Anatomical Definition and Boundaries
The triangle is formed by three key structures 1:
- Anterior border: Posterior wall of the external auditory canal
- Superior border: Temporal line (linea temporalis)
- Inferior border: Posterosuperior spine of Henle (suprameatal crest/suprameatal spine)
The triangle overlies the mastoid antrum, which typically lies 12-15mm deep to the cortical surface in adults.
Clinical and Surgical Relevance
Primary Surgical Access Point
MacEwen's triangle represents the safest initial drilling site for mastoidectomy because 2:
- It provides direct access to the mastoid antrum
- The underlying anatomy is relatively predictable
- It avoids critical structures when proper technique is used
Critical Adjacent Structures to Protect
When drilling through this triangle during mastoidectomy, surgeons must be aware of 3, 2:
- Tegmen mastoideum (dural plate) - superiorly
- Sigmoid sinus - posteriorly
- Lateral semicircular canal - medially/deep
- Facial nerve - anteromedially (in the deeper dissection)
Important Clinical Pitfalls
Anatomical Variations
A critical caveat: Congenital defects in the lateral mastoid wall within MacEwen's triangle can occur, though rare 3. A 2025 case report documented bilateral focally deficient lateral mastoid walls in this region with large adjacent medial air cells—an anatomical variant that significantly increases surgical risk if unrecognized. This emphasizes the importance of:
- Preoperative high-resolution CT (HRCT) imaging before mastoidectomy
- Careful review of the lateral mastoid cortex integrity
- Recognition that apparent "defects" without adjacent pathology or surgical history suggest congenital variants rather than disease
Surgical Implications
Failure to identify such variants preoperatively can lead to 3:
- Inadvertent dural injury during drilling
- Lateral semicircular canal violation
- Unexpected entry into abnormally positioned air cells
Practical Application
When performing mastoidectomy, the surgical approach through MacEwen's triangle should 2:
- Begin with identification of the triangle's surface landmarks
- Initiate drilling within the triangle boundaries
- Proceed with controlled depth awareness (antrum typically 12-15mm deep)
- Continuously orient to the tegmen superiorly and sigmoid sinus posteriorly
- Identify the lateral semicircular canal as a key deep landmark
The triangle serves as both an entry point and a constant reference for spatial orientation throughout the mastoid dissection 1, 2.