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Mac ewens triangle

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McEwen's triangle mastoid antrum anatomy surgical landmark

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https://teachmeanatomy.info/head/areas/mastoid-fossa

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Here is a complete summary of McEwen's Triangle (also written MacEwen's or Macewen's triangle):

McEwen's (Suprameatal) Triangle

Synonym: Suprameatal triangle, Mastoid fossa
Location: A triangular-shaped depression on the external surface of the temporal bone, positioned posterior and superior to the external auditory canal (EAC).

Borders

BorderLandmark
SuperiorSupramastoid crest (temporal line)
AnteriorSuprameatal spine (spine of Henle)
PosteriorA vertical tangential line drawn along the midpoint of the posterior wall of the EAC

Contents

The triangle itself does not contain any major structures. It is a bony surface landmark. Deep to it lies the mastoid antrum - the largest of the mastoid air cells - at approximately 1.5 cm depth in adults (slightly less in children).

Special feature in children

In children, the floor of the triangle shows small perforations called the cribrosa area, through which small blood vessels pass.

Clinical Significance

  1. Cortical mastoidectomy - McEwen's triangle is the classic "safe zone" for beginning the drill during cortical mastoidectomy. Drilling here allows access to the mastoid antrum while avoiding critical structures.
  2. Why it is safe:
    • The area overlies the mastoid antrum with no critical neurovascular structures at the surface
    • The surgeon must remain below the supramastoid crest (temporal line) to avoid the middle cranial fossa dura
    • The surgeon must remain posterior to the suprameatal spine to protect the ossicles and EAC
  3. Depth to antrum: ~1.5 cm in adults, less in children (due to smaller mastoid size).

Related Triangles in Temporal Bone Surgery

  • Trautmann's triangle - demarcated by the bony labyrinth, sigmoid sinus, and superior petrosal sinus/dura
  • Citelli's angle - the sinodural angle between sigmoid sinus and middle fossa dura plate
Source: K.J. Lee's Essential Otolaryngology, p. 266 (suprameatal triangle of Macewen); TeachMeAnatomy - Mastoid Fossa
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