The mastoid part of the temporal bone is its posterior component. The inferior conical projection of the mastoid part is called the mastoid process.

Gross anatomy

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. Starting around birth and continuing through childhood, pneumatization develops outwardsPneumatization extends from the mastoid antrum, resulting inantrum 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 surface of the mastoid process is sometimes called the mastoid cortex.

The medial 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 inner posterior 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.

Articulations

The mastoid part of the temporal bone articulates with the following bones ^1,2: 

• superiorly: mastoid angle of the parietal bone via the parietomastoid suture
• posteriorly: squamous part of the occipital bone via the occipitomastoid suture
• anteriorly: tympanic part of the temporal bone via the tympanomastoid suture

Attachments

Lateral attachments (from superior to inferior) ¹:

• occipitalis muscle (occipital belly of occipito-frontalis ²)
• posterior auricular muscle
• sternocleidomastoid muscle
• splenius capitis muscle
• longissimus capitis muscle

Medial attachments ¹:

• digastric muscle (posterior belly)

Relations and/or Boundaries

The mastoid is derived from the squamous and petrous parts of the temporal portion. The anterior portion belongs to the squamous part and the posterior portion belongs to the petrous part ³. Therefore, the mastoid part is fused anterosuperiorly with the descending process of the squamous temporal bone and anteromedially with the petrous temporal bone ².

Anteroinferiorly, the mastoid forms the posterior bony wall of the external auditory canal and tympanic cavity (retrotympanum) ¹.

Variant anatomy

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. 

Development

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 anterior portion belongs to the squamous part and the posterior portion belongs to the petrous part ³. Pneumatization occurs in 80% of people by age 4 years ³.