Mastoid Process

MASTOID PROCESS, processus mastoideus, is a part of the temporal bone located behind the external auditory canal, posterior to the sutura squamomastoidea, and connecting with the squamous and tympanic parts of the temporal bone. The upper edge of the mastoid process connects with the parietal bone, and the posterior edge with the occipital bone (Fig. 1). The external surface of the mastoid process is convex and rough, as five muscles attach to the mastoid process at this location: mm. splenius capitis, sterno-cleido-mastoideus, occipitalis, auricularis post., and complexus min. The internal surface is concave and smooth. Here, there is a wide, curved groove for the sinus sigmoideus. On the external surface, there are several

Figure 1. External view of the temporal bone and mastoid process: 1-occipito-mastoid suture; 2-groove for m. biventer; 3-cells of the mastoid process; 4-mastoid process; 5-tympanic antrum; 6-spina suprameatum; 7-cavum tympani; 8 and 11-Eustachian tube; 9-styloid process; 10-fossa mandibularis; 12-nasopharyngeal opening of the Eustachian tube; 13-lateral plate of the pterygoid process; 14-articular tubercle; 15-proc. zygomaticus; 16-greater wing of the sphenoid bone.

Figure 2. Pneumatic type of mastoid process: 1-groove for m. biventer; 2-wall of the mastoid process; 3-cellulae mastoideae; 4-canalis n. facialis; 5-proc. cochleariformis; 6-oval window; 7 and 8-canalis n. facialis; 9-pyramid; 10-aditus ad antrum; 11-antrum.

small openings (foramina mastoidea), the number, shape, and direction of which are inconstant. They open on the internal surface into the sinus groove. These are emissary veins through which the veins of the outer part of the skull communicate with the intracranial veins and the sinus. From the medial part, the mastoid process is encompassed by a deep notch (incisura mastoidea), where the posterior belly of the digastric muscle originates and passes. Somewhat posterior to the incisura mastoidea and parallel to it is a groove for the a. occipitalis. Between the mastoid process and the squamous part of the temporal bone is the incisura parietalis, into which the mastoid part of the occipital bone extends for its connection with the temporal bone. The apex of the mastoid process is directed downward. The mastoid process appears only in the first years of life. From the end of the first year of life, air-filled bone cells (cellulae mastoideae) appear and develop within it, parallel to the formation of the process.

Figure 3. Transitional pneumatic type of mastoid process: 1-groove for m. digastricus; 2-wall of the mastoid process; 3 and 6-diploe (spongy bone tissue); 4-canal for n. facialis; 5-proc. styloideus; 7-Eustachian tube; 8-promontorium; 9-cochleariform process; 10 and 11-facial nerve canal; 12-pyramid; 13-entrance to the antrum and prominence of the lateral semicircular canal; 14-antrum.

Figure 4. Transitional sclerotic type of mastoid process: 1-groove for m. digastricus; 2-wall of the mastoid process; 3-diploe; 4-cellulae; 5-styloid process; 6-Eustachian tube; 7-pr. cochleariformis; 8-promontorium; 9-oval window; 10 and 11-canalis n. facialis; 12-pyramid; 13-entrance to the antrum and prominence of the lateral semicircular canal; 14-antrum.

In adults, two types of mastoid process are distinguished: pneumatic and sclerotic (Figs. 2–4). The former is rich in cells lined with a mucous membrane of the same structure as the mucosa of the tympanic cavity. The cells communicate with each other, and one of them, of somewhat larger size, located anteriorly and superiorly (antrum mastoideum, antrum tympanicum), communicates with the recessus epitympanicus of the tympanic cavity through a wide opening (aditus ad antrum tympanicum). The sclerotic (senile) type usually does not contain cells and consists of a compact bone mass. The cells of the mastoid process are arranged in a fan shape around the antrum tympanicum; most of them are located inferiorly and laterally. While the antrum mastoideum is already present at the moment of birth, the cellulae mastoideae begin to develop only toward the end of the first year of life, so it is quite correct to hold the opinion that the antrum mastoideum actually represents a part of the tympanic cavity, and the cellulae mastoideae are only an appendage of the tympanic cavity. The cells lie very close to the transverse sinus and to the facial nerve canal.

Figure 5. Sclerotic type of mastoid process: 1-groove for m. digastricus; 2-sclerotized tissue; 3-wall of the mastoid process; 4-styloid process; 5-Eustachian tube; 6 and 7-pr. cochleariformis; 8 and 9-canalis n. facialis; 10 and 11-entrance to the antrum; 12-pyramid; 13-antrum.

It is important for the otiatrist and the practical surgeon to remember that during trepanation of the mastoid process, one must chisel the bone only within the limits of the Shipo triangle, the sides of which are formed superiorly by the temporal line (the continuation of the root of the zygomatic process), posteriorly by a perpendicular line dropped from this line to the apex of the mastoid process (this line often coincides with the crista mastoidea), and anteriorly by a line connecting the apex of the process and the spina suprameatum (see Mastoiditis).

Figure 6. Relation of the mastoid process to the middle ear: 1-apex of the pyramid; 2-canal of the a. carotis int.; 3-semicanal for the tensor tympani muscle; 4-septum of the musculotubal canal; 5-Eustachian canal; 6-pneumatic cells near the Eustachian tube; 7-promontorium; 8-oval window; 9-promontorium; 10-sinus tympani; 11-styloid process; 12-eminentia pyramidalis; 13-mastoid process; 14-cell of the mastoid process; 15-antrum mastoideum; 16-prominence of the lateral semicircular canal; 17-tegmen tympani; 18-elevation of the facial nerve canal; 19-proc. cochleariformis (cochleariform process); 20-hiatus canalis facialis; 21-sulcus n. petrosi superficialis major.

Going superiorly from the temporal line, one can damage the dura mater and enter the middle cranial fossa; posteriorly, one can damage the sinus and thereby cause significant hemorrhage, and subsequently phlebitis of the sinus, thrombophlebitis, general

Figure 7. Projection of the temporal gyri, tympanic antrum, transverse sinus, cerebellum, and the 3rd vertical part of the n. facialis: 1-sinus transversus; 2-cerebellum; 3-antrum tympanicum; 4-gyrus temporalis inf.; 5-spina suprameatum; 6-canalis facialis; 7-external auditory meatus; 8-linea temporalis.

sepsis, and very rapid death of the patient; anteriorly, damage to the facial nerve results in subsequent paralysis of the muscles of the corresponding half of the face. It is exceptionally important for the surgeon to know in advance with which type of mastoid process he is dealing.

Figure 8. Relations between the antrum tympanicum and the sinus transversus, which protrudes far laterally and occupies almost the entire anterior part of the pars mastoidea ossis temporalis, and with a complete absence of mastoid cells: 1-occipito-mastoid suture; 2-sinus transversus; 3-groove for m. biventer; 4-mastoid process; 5-bone layer 2 mm thick; 6-external auditory meatus; 7-spina suprameatum; 8-antrum tympani; 9-parieto-mastoid suture; 10-bone layer 1 mm thick.

The danger of injuring the sinus is especially great in the case of its anterior displacement. The degree of displacement can be judged by the thickness of the mastoid process, determined by the position of the incisura mastoidea: the thicker the mastoid process, the further the sinus is located, and consequently, the less the danger of its damage. The facial nerve canal is bent or

Figure 9. Relations between the cellulae mastoid. and the antrum tympanicum on one side and the sinus transversus on the other, with strong development of cellulae mastoideae and moderate depth of the sulcus transv. The cells of the mastoid process are exposed by removing the lateral wall: 1-occipito-mastoid suture; 2-projection of the sinus transversus; 3-groove for m. biventer; 4-cells of the mastoid process; 5-external auditory meatus; 6-spina suprameatum; 7-antrum tympanicum; 8-mastoid-parietal suture. Figure 10. Mastoid part of the temporal bone on the right side with a strongly developed antrum tympanicum and cellulae mastoideae. The entire mastoid part is occupied by cells, leaving only a paper-thin plate; the cells are separated from the sinus transversus only by a very thin bone layer: 1-entrance to the antrum of the tympanic cavity.

at a large angle or at an angle approaching a right angle. In the first case, when the canal has a more oblique course, its wall is very thin and easily traumatized. The shape of the triangle of Schiff somewhat guides: a triangle with a small height corresponds to the cellular type of pneumatization, and the angle of the facial nerve canal approaches a right angle all the more, the smaller the height of the triangle and the larger its base. The posterior side of the triangle corresponds to the position of the sinus, the upper side to the temporal lobe of the brain, the anterior side to the facial nerve canal (fig. 4-10). So-called 'dangerous' temporal bones are known. They are observed on the right side significantly more often than on the left, and the mastoid process in these cases is poorly developed ('infantile'). Since in children the sigmoid sinus is usually separated from the outer surface only by a thin bone layer, such cases during operations require the greatest caution, as the chisel can easily pierce the lateral wall of the sinus and open it. The blood supply to the mastoid process occurs from the a. meningea med. and a. stylo-mastoidea. The veins flow into the sinus petrosus superior and the sinus sigmoideus. The innervation of the mastoid process consists exclusively of sensory nerves and sympathetic nerves of the same origin as the nerves of the tympanic cavity (see Middle ear). The lymphatic vessels are little studied. Operations in the area of the mastoid process: 1) mastoidectomy - antrumoperation, 2) radical operation of Schwarz-Stucke, 3) opening of the sinus throughout the entire length of bulbi jugularis.

p. shuf'yan.