Boundaries and fascial layers
The mastoid region is covered by the auricle. Its superior boundary is the line continuing the zygomatic process of the temporal bone posteriorly (linea temporalis). The remaining contours match the shape of the mastoid process itself, which is easily palpated beneath the skin.
Blood vessels run in the subcutaneous tissue: the posterior auricular artery and vein (a. et v. auriculares posteriores). Nerves are also located here: the sensory posterior branch of the great auricular nerve from the cervical plexus (n. auricularis magnus) and the motor posterior auricular nerve (n. auricularis posterior) from the facial nerve (n. facialis). The periosteum in this area is firmly fused with the outer surface of the bone. The only exception is the area of Schuške's triangle, where it is easily detached. Neck muscles attach to the process: m. sternocleidomastoideus, the posterior belly of m. digastricus, and m. splenius. Beneath the muscular layer, in the mastoid notch, lies the occipital artery (a. occipitalis).
Projection of intraosseous structures
For an accurate understanding of where important anatomical structures are located inside the mastoid process, its outer surface is conventionally divided into four quadrants. This is done using two lines: vertical (from the apex of the process to the middle of its base) and horizontal (dividing the vertical line in half).
- Anterosuperior quadrant: the mastoid antrum projects here (antrum mastoideum).
- Anteroinferior quadrant: the site of projection of the bony canal of the facial nerve (canalis facialis).
- Posterosuperior quadrant: corresponds to the posterior cranial fossa.
- Posteroinferior quadrant: conceals the sigmoid venous sinus (sinus sigmoideus).
Schuške Trepanation Triangle
Schuške's triangle is a crucial landmark for performing antrotomy (mastoidectomy). The mastoid antrum is located precisely within its limits.
The boundaries of the triangle are clearly defined by anatomical structures:
- Anteriorly: posterior margin of the external acoustic meatus and suprameatal spine (spina suprameatica).
- Posteriorly: mastoid crest (crista mastoidea).
- Superiorly: horizontal line continuing the zygomatic process posteriorly (linea temporalis).
Topography of the Mastoid Antrum
The mastoid antrum (antrum mastoideum) is a constant cavity measuring approximately 12 by 7 mm. It communicates with the tympanic cavity through a special opening (aditus ad antrum).
The walls of the antrum have important syntopy:
- Superior wall: forms the boundary with the middle cranial fossa.
- Medial wall: contains the aditus ad antrum and two prominences: the lateral semicircular canal (prominentia canalis semicircularis lateralis) and the facial nerve canal (prominentia canalis facialis).
- Posterior wall: lies adjacent to the sigmoid venous sinus. If the process is poorly developed or in brachycephalics, the sinus may lie very close to the antrum.
Briefly: answers to topical questions
Which muscles attach to the mastoid process?
Four muscles attach to or originate from the mastoid process.
- Sternocleidomastoid muscle (m. sternocleidomastoideus) — originates from the process.
- Digastric muscle (m. digastricus) — its posterior belly originates from the process.
- Splenius muscle (m. splenius) — originates from the process.
- Longissimus muscle (m. longissimus) — attaches to the process.
Which nerves run in the subcutaneous tissue of the mastoid region?
The following pass through the subcutaneous tissue of the mastoid region:
- Posterior branch of the great auricular nerve (n. auricularis magnus) — sensory, belonging to the cervical plexus.
- Posterior auricular nerve (n. auricularis posterior) — branch of the facial nerve (n. facialis).
What structures border all the walls of the mastoid antrum?
The walls of the mastoid antrum (antrum mastoideum) have the following syntopy with adjacent anatomical formations:
- Superior wall — separates the antrum from the middle cranial fossa.
- Medial wall — contains the aditus ad antrum (aditus ad antrum) and two prominences: the lateral semicircular canal (prominentia canalis semicircularis lateralis) and the facial nerve canal (prominentia canalis facialis).
- Posterior wall — borders the sigmoid venous sinus.
The topography of the anterior, inferior, and lateral walls is not described in the sources.
What types of structure (pneumatization) of the mastoid process are distinguished?
Three types of mastoid process structure (pneumatization) are distinguished depending on the degree of development of the air cells.
- Pneumatic type — characterized by a well-developed cell system.
- Diploic type — represents a spongy bone structure.
- Sclerotic type — distinguished by dense bone with a minimal number of air cavities.
The degree of pneumatization is individually variable and depends on genetics and past illnesses.
Class and exam questions
In which quadrant of the mastoid process is the sigmoid sinus located?
The sigmoid venous sinus projects into the posteroinferior quadrant of the mastoid process.
Where should mastoid trepanation be performed?
Trepanation (antrotomy) is performed within Schuške's triangle because the mastoid antrum projects precisely there and the periosteum easily detaches from the bone.
What structures border the mastoid antrum?
Superiorly it borders the middle cranial fossa, posteriorly — the sigmoid sinus, and on its medial wall are the prominences of the semicircular canal and the facial nerve canal.
What else to study on this topic
- Anatomy of the facial nerve within the mastoid region
- Variants of mastoid process pneumatization
- Technique of antrotomy
- Syntopy of the sigmoid venous sinus
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