Indications for surgery and preparation
The primary indication for mastoidectomy is purulent otitis media complicated by the spread of infection to the air cells of the mastoid process. This pathological condition is termed mastoiditis. Under such conditions, conservative therapy proves ineffective, and mechanical removal of the accumulated purulent exudate and pathological granulations is required.
Prior to surgical manipulations, adequate anesthesia must be provided. The choice of method depends on the clinical situation: either general anesthesia (narcosis) or local infiltration anesthesia (using a 0.5% novocaine solution) is used.
The patient is placed on the operating table in the supine position. The head must be turned strictly to the healthy side and securely fixed to prevent accidental movements while working with the chisel. The auricle on the affected side is retracted anteriorly by the assistant, exposing the postauricular region to the surgeon.
Topographic landmarks and surgical approach
The operation begins with the creation of the proper surgical approach. The surgeon makes an incision through the skin and subcutaneous adipose tissue. The incision line must run strictly parallel to the attachment line of the auricle, 1 centimeter posterior to it.
The key anatomical landmark for safe bone opening is the suprameatal triangle (MacEwen's triangle). The projection of the skin incision should fall precisely on the middle of this surgical approach.
For precise localization, the surgeon uses palpation:
- A necessary landmark is the suprameatal spine.
- This spine is located at the junction of the superior and posterior walls of the bony external acoustic meatus.
- Within the boundaries of MacEwen's triangle, a smooth bony area is visualized, which represents the anterior surface of the upper-inner quadrant of the mastoid process. It is here that work with bone instruments will begin.
Technique of trepanation and debridement
After making the incision, the wound is widely retracted. The periosteum is carefully detached from the bone using a raspatory, exposing the trepanation area.
Stages of chisel work:
- At the initial stage, a wide trough-shaped chisel is used. It is used to carefully remove the outer cortical layer of the bone. Chisel movements are performed from above, then from below from the apex of the mastoid process, and anteriorly — strictly parallel to the direction of the posterior wall of the external acoustic meatus.
- As the surgeon goes deeper into the cancellous bone, the instrument is changed to a narrow chisel. The direction of work changes: it is necessary to go inward and anteriorly, continuing to keep the instrument parallel to the posterior wall of the acoustic meatus.
Revision of the antrum: Opening the mastoid antrum (antrum mastoideum) requires mandatory control. For this, a probe is inserted into the created opening. When the approach is performed correctly, the probe should pass freely through the aditus ad antrum directly into the tympanic cavity.
Having verified the correctness of the approach, the surgeon takes a sharp surgical curette and thoroughly cleans all accumulated purulent exudate and granulations from the cavity of the antrum and opened cells. The intervention is completed by suturing the wound, with a drain — a strip of rubber dam — necessarily left above and below the sutures for the outflow of wound discharge.
Potential errors and topography-related anatomical complications
Mastoid trepanation requires perfect knowledge of topographic anatomy, as the slightest deviation of the chisel beyond the safe zone of MacEwen's triangle leads to severe iatrogenic injuries of adjacent structures.
Consequences of instrument deviation:
- Superiorly: destruction of the superior wall of the antrum will lead to breakthrough into the middle cranial fossa, which threatens injury to the meninges and brain tissue.
- Posteriorly: exit of the instrument beyond the posterior boundary is fraught with injury to a major venous collector — the sigmoid sinus (at the site of transition of the transverse sinus), which causes profuse bleeding.
- Inferiorly: deviation in the inferior direction will inevitably lead to the transection or compression of the facial nerve running in its own bony canal.
Briefly: answers to topical questions
What anatomical structures bound the sides of MacEwen's triangle?
The sides of MacEwen's triangle are bounded by the following bone anatomical structures:
- Anteriorly — the posterior margin of the external acoustic meatus and the suprameatal spine (spina suprameatica).
- Posteriorly — the mastoid crest (crista mastoidea).
- Superiorly — the horizontal line (linea temporalis) drawn posteriorly from the zygomatic process of the temporal bone.
What anatomical structures form the walls of the mastoid antrum (antrum mastoideum)?
The walls of the mastoid antrum (antrum mastoideum) are formed by the following anatomical structures:
- 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.
What tissue layers are sequentially incised during the surgical approach to the mastoid bone?
During the surgical approach to the mastoid bone, the following tissue layers are sequentially incised and reflected:
- Skin — thin and mobile.
- Subcutaneous tissue — contains vessels and nerves (posterior auricular vessels, branches of the great auricular and posterior auricular nerves).
- Periosteum (periosteum) — within MacEwen's triangle, it is loosely connected to the bone and easily detached with a raspatory.
After detachment of the periosteum, the compact bone plate is exposed.
What types of structure of the mastoid process are distinguished depending on the degree of pneumatization?
Depending on the degree of pneumatization, three structural variants of the mastoid process are distinguished. These include:
- Pneumatic structure;
- Sclerotic structure;
- Diploetic structure.
Regardless of the structural type, a constant air-containing cavity — the mastoid antrum (antrum mastoideum) — is located in the process at a depth of 1.5–2.0 cm from the bone surface.
Which nerves provide sensory innervation to the skin in the postauricular region?
Sensory innervation of the skin in the postauricular region (behind the auricle) is provided by branches of the cervical plexus:
- Great auricular nerve (n. auricularis magnus, C₂–C₃) — its posterior branch innervates the auricle and the skin behind and below it.
- Lesser occipital nerve (n. occipitalis minor, C₂) — innervates the area immediately behind the auricle.
Hazards during chisel displacement: UP — brain (middle cranial fossa), BACK — blood (sigmoid sinus), DOWN — nerve (facial nerve).
Class and exam questions
What is the purpose of using a button-ended probe during surgery?
A probe with a button end is necessary for safe orientation in the wound and monitoring the opening of the mastoid antrum. It must pass through the aditus ad antrum, confirming communication with the tympanic cavity.
How to properly position the patient on the operating table?
The patient lies supine. Their head must be turned to the healthy side and fixed, and the auricle on the surgical side is retracted anteriorly.
What instruments are needed for bone opening?
Chisels and osteotomes from a special Voyacek set are used. First, a wide trough-shaped chisel is used for the outer layer, then a narrow one for going deeper inward.
What else to study on this topic
- Exact anatomical boundaries of MacEwen's triangle
- Structure and topography of the mastoid antrum (antrum mastoideum)
- The anatomical pathway of the aditus ad antrum and its connection with the tympanic cavity
- Topography of the facial nerve canal in the mastoid process
- Structure of the dural venous sinuses of the posterior cranial fossa
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