Mastoid Antrum

INTRODUCTION:
  • This air-containing space (9 mm height, 14 mm width and 7 mm depth) is situated in the upper part of mastoid. Vol – 1ml
  • Mastoid is not a pneumatic bone
  • Mastoid antrum is present in petrous part of temporal bone
  • Development of the mastoid air cell system does not occur until afterbirth, with about 90% of air cell formation being completed by the age of six with the remaining 10% taking place up to age of 18
  • Shortest skull diameter is Bimastoid
BOUNDARIES -
  • Roof: It is formed by the tegmen antri, which separates mastoid antrum from the middle cranial fossa.
  • Lateral wall: It is formed by a 15mm thick plate of squamous part of temporal bone which is marked on the lateral surface of mastoid by suprameatal (Macewen’s) triangle. It is covered by postaural skin.
  • Boundaries of Macewen’s triangle
  1. Linea temporalis (temporal line): A ridge of bone extending posteriorly from the zygomatic process (marking the lower margin of temporalis muscle and approximating the floor of middle cranial fossa)
  2. EAC: Posterosuperior margin of EAC.
  3. Tangent: A tangent to the posterior margin of EAC
  4. At birth its 2mm thick and increases at rate of 1 mm/yr to attain full thickness of 12-15mm
  • Medial wall: It is formed by the petrous bone and related to the
  1. Posterior semicircular canal
  2. Endolymphatic sac
  3. Dura of posterior cranial fossa
  • Anterior: Anteriorly mastoid antrum communicates with the attic through the aditus ad antrum. Medial to lateral relations are following:
  1. Facial nerve canal
  2. Aditus ad antrum and facial recess lie between tympanum and mastoid antrum
  3. Deep bony external auditory canal (EAC)
  • Posterior wall: It is formed by mastoid bone and communicates with mastoid air cells.Sigmoid sinus curves downwards.
  • Floor: It is formed by mastoid bone and communicates with mastoid air cells. Other deeper relations from medial to lateral sides are
  1. Jugular bulb medial to facial canal.
  2. Digastric ridge which gives origin of posterior belly of digastric muscle.
  3. Origin of sternocleidomastoid muscle.
TYPES of MASTOID:
  • The mastoid consists of “honeycomb” air cells, which lie underneath the bony cortex. Depending on its development, three types of mastoid are described: cellular, diploeic and acellular.
  1. Cellular (Well-pneumatized): Mastoid cells are well developed with thin intervening septa.
  2. Diploeic: Mainly there are marrow spaces with few air cells
  3. Acellular (Sclerotic): There are neither cells nor marrow spaces.
MASTOID AIR CELLS
  • The mastoid air cells are traditionally divided into several groups, which include:
  1. Zygomatic cells: In the root of zygoma.
  2. Tegmen cells: In the tegmen tympani.
  3. Perisinus cells: Present over the sinus plate.
  4. Retrofacial cells: Present round the fallopian canal of facial nerve.

  5. Perilabyrinthine cells: They are located above, below and behind the labyrinth.The cells, which are present in the arch of superior semicircular canal, may communicate with the petrous apex.
  6. Peritubal cells: They are present around the eustachian tube. These and the hypotympanic cells communicate with the petrous apex.
  7. Tip cells: These large cells lie in the tip of mastoid medial and lateral to the digastric ridge
  8. Marginal cells: These cells, which lie behind the sinus plate, may extend into the occipital bone.
  9. Squamous cells: They lie in the squamous part of temporal bone.

Exam Question
  • Mac Ewan's triangle is the landmark for mastoid antrum
  • The suprameatal triangle overlies mastoid antrum
  • Development of the mastoid air cell system does not occur until afterbirth
  • Mastoid is not a pneumatic bone
  • Suprameatal triangle forms lateral wall of mastoid antrum
  • Shortest skull diameter is Bimastoid 
  • Lateral wall of mastoid antrum is related to External auditory canal
  • Mastoid antrum is present in petrous part of temporal bone

Don't Forget to Solve all the previous Year Question asked on Mastoid Antrum