Mastoid part of temporal bone
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The mastoid part of the temporal bone is its posterior component. The inferior conical projection of the mastoid part is called the mastoid process.
An irregular cavity within the anterosuperior aspect of the bone is called the mastoid (or tympanic) antrum, which communicates with the attic of the tympanic cavity 1. Pneumatization extends from the antrum as the mastoid air cells.
The roof of the mastoid antrum is called the tegmen mastoideum, which is a posterior extension of the tegmen tympani. The tegmen mastoideum forms part of the floor of the middle cranial fossa, separating the mastoid from the cranial cavity.
The lateral extracranial surface of the mastoid is sometimes called the mastoid cortex. The anterior landmark is the MacEwen triangle including the spine of Henle. The posterior landmark is the asterion.
The medial extracranial surface of the mastoid process contains a deep groove called the digastric fossa (mastoid notch), which originates the digastric muscle. Medial to that is the occipital groove, which is traversed by the occipital artery.
At the posterior intracranial surface of the mastoid, the sigmoid sulcus lodges the sigmoid sinus and partially the transverse sinus 2. The sigmoid plate is the thin lamina of bone that separates mastoid air cells from the dural venous sinuses. Around this area is the inconstant mastoid foramen, which transmits an emissary vein and occipital artery branch 1,2.
The mastoid part of the temporal bone articulates with the following bones 1,2:
Lateral attachments (from superior to inferior) 1:
occipitalis muscle (occipital belly of occipito-frontalis 2)
Medial attachments 1:
digastric muscle (posterior belly)
Relations and/or Boundaries
Anteroinferiorly, the mastoid contributes to the posterior bony wall of the external auditory canal and tympanic cavity (retrotympanum) 1. The anterior margin is partly defined by the tympanic part of the temporal bone at the plane of the tympanomastoid suture.
The mastoid part is fused anterosuperiorly with the descending process of the squamous temporal bone and medially with the petrous temporal bone 2. The boundaries of the mastoid part at these sites are imprecisely defined based on the semicircular canals medially and the external auditory canal anteriorly 6,7.
The degree of mastoid pneumatization is variable, which may be categorized as pneumatic (full air cell development), sclerotic (solid bone), diploic (marrow), or mixed (air cells and marrow) 2,3.
The mastoid process is absent at birth 2. Starting in infancy and continuing into puberty, the mastoid develops from the squamous and petrous parts of the temporal portion. The anterolateral third belongs to the squamous part (squamomastoid) and the posteromedial two-thirds belongs to the petrous part (petromastoid) 3,4. The dividing walls are largely resorbed in development by 5 years of age 5. However, the margin of fusion often remains discernable in adults at some sites as the Koerner septum internally and petrosquamous fissure externally 4.
Pneumatization (creation of mastoid air cells) occurs in 80% of people by age 4 years 3. Mastoid pneumatization can in some cases proceed into other areas of the squamosa and petrosa.
Mastoidectomy is the surgical procedure performed to evacuate pathology involving the mastoid, such as cholesteatoma, granulation tissue, or infection.
History and etymology
Mastoid derives from the Greek word μᾰστοειδής, meaning "breast-like".
mastoid air cell opacification (differential)
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