While most mastoid surgery is conducted for infection, mastoidectomy is also performed to create a pathway to the deeper recesses of the temporal bone. These include surgery for neoplasia, vestibular procedures, and access to the dural interfaces for repair of CSF leakage and encephalocele. Mastoidectomy can be “complete” with exenteration of all mastoid pneumatization or limited as in the case of antrotomy sometimes also called “simple” mastoidectomy.
Acute and chronic ear infections are not typically limited to the middle ear; they involve the entire pneumatized spaces of the temporal bone. The mastoid consists of a large cell medially, called the antrum, which connects to the epitympanum via the aditus-ad-antrum. A variable extent of peripheral air cells may be present. Most mastoid surgery done for infection occurs in older children and adults and is in the setting of chronic otitis media in which entrapped infection has led to formation of granulation tissue and polyps. Acute mastoiditis is uncommon in the antibiotic era. When antibiotics have proven ineffective and/or when the air cells have become coalescent due to breakdown of their septa, mastoidectomy for abscess drainage is indicated. Special care is needed with postauricular incision in the first few years of life due to incomplete development of the mastoid tip which places the facial nerve in jeopardy in the lower limb of the incision. In infants and toddlers, it is best to make a more transverse incision above the ear rather than the traditional C-shaped incision curving behind the postauricular sulcus.
The mastoid anatomy has both consistent aspects and considerable variability. While the antrum is a consistent feature and the facial nerve and semicircular canals are consistently located, the extent of peripheral pneumatization and the size of the mastoid are highly variable. In poorly pneumatized temporal bones, only an antrum may remain. In such cases, the mastoid is said to be “contracted” and the temporal dura is low and the sigmoid sinus is forwardly placed.
When the mastoid is exteriorized and made confluent with the ear canal, a cavity of variable size is created (see section 8.3 Canal Wall Down Mastoidectomy in Chapter 8). When the mastoid is extensively pneumatized, a large cavity results that can accumulate debris and is more likely to discharge. In such cases, an effort is made to reduce the size of the cavity by mastoid obliteration. This can be done with autologous bone paté alone or in combination with various designs of soft-tissue rotational flaps.
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7.2 Intact Canal Wall Mastoidectomy
Fig. 7.1 Anatomical relationships of the mastoid antrum and air cells: lateral view. LSCC, lateral semicircular canal; PSCC, posterior semicircular canal; SSCC, superior semicircular canal.
Fig. 7.2 Intact canal wall mastoidectomy bounded superiorly by the tegmen, posteriorly by the sigmoid sinus, and anteriorly by the ear canal.
Fig. 7.3 Anatomical relationships of the mastoid antrum and air cells: axial view. CN V, trigeminal nerve; CN VII, facial nerve.
Fig. 7.4 Intact canal wall mastoidectomy connecting to the middle ear via the aditus-ad-antrum.
Fig. 7.5 Mastoidectomy and its relationships. Note the temporal lobe above the tegmen and cerebellum behind the sigmoid sinus. The second genu, vertical segment, and stylomastoid foramen of the facial nerve are situated in the floor of the mastoid as are the three semicircular canals.
Fig. 7.6 Exposure of the mastoid in preparation for mastoidectomy. Note the surface landmarks.
Fig. 7.7 Initial removal of the mastoid cortex behind and above the ear canal is conducted with a cutting burr.
Fig. 7.8 Removal of the outer cortex reveals the peripheral air cell system in the well pneumatized temporal bone.
Fig. 7.9 Thinning of the bony ear canal by removing air cells.
Fig. 7.10 As a general principle, the deepest bone excavation is always kept high and forward approaching the tegmen mastoideum and toward the epitympanum. This is the surest way of encountering the antrum and avoiding possible injury to the facial nerve at its second genu.
Fig. 7.11 Identifying the tegmen mastoideum: note the smooth sheet of bone. The drill noise may become higher pitched as the tegmen is approached.
Fig. 7.12 The safest way to approach the antrum is high up along the tegmen and forward toward the epitympanum. The floor of the opening at this stage sometimes is a flat sheet of bone, called Koerner’s septum, which is a false bottom and not the actual floor of the antrum.
Fig. 7.13 Koerner’s septum needs to be penetrated to enter the antrum.
Fig. 7.14 Initial entry into the mastoid antrum.
Fig. 7.15 Further opening of the antrum brings the lateral semicircular canal into view.
Fig. 7.16 Thinning of the bony shelf over the ossicles in the epitympanum.
Fig. 7.17 Flaking off the last thin sheet of bone over the incus and malleus with a stapes knife or curette protects the ossicles from contact with the drill which could cause cochlear injury.
Fig. 7.18 Completion of the antrotomy and exposure of the epitympanum including the body of the incus and the head of the malleus. Note that the opening anteriorly fully exposes the head of the malleus and the anterior epitympanum.
Fig. 7.19 Thinning of air cells over the sigmoid sinus and the sinodural angle (also known as Citelli’s angle).
Fig. 7.20 Tip cells are opened as necessitated by the extent of pneumatization.
Fig. 7.21 Beveling of the edges of the mastoid is important to avoid sharp edges which may cause discomfort, especially in eyeglass wearers.
Fig. 7.22 Completed mastoidectomy. Completion of the tip cell removal has exposed the digastric ridge.
7.3 Anatomical Variations in Mastoidectomy
Fig. 7.23 While the mastoid antrum is consistent, much variation exists in the pattern and extent of the peripheral air cell system. This is the average degree of pneumatization.
Fig. 7.24 An extensively pneumatized temporal bone with cells extending into the root of the zygoma and retrosigmoid regions. Note the pneumatization of the petrous apex deep to the inner ear.
Fig. 7.25 A severely hypopneumatic temporal bone of the type often seen in chronic otitis media. Note the preservation of the antrum.
Fig. 7.26 Coronal illustration of variability of mastoid air cells.