Guide to Cortical Mastoidectomy

Author: Mr Vik Veer, ENT SpR – March 2009

This is a simple guide to performing cortical mastoidectomy. It should be used as a starting point for understanding how to practise on a temporal bone course, or for understanding an operation you are observing. It is not a substitute for hands-on supervised training.

Indications for Cortical Mastoidectomy

The cortical mastoidectomy (also called a simple mastoidectomy) is the foundational mastoid operation. It is performed to:

• Drain and eradicate acute coalescent mastoiditis when antibiotics have failed
• Provide access for modified radical or radical mastoidectomy in cases of cholesteatoma
• Create access for cochlear implant surgery (the mastoid is drilled to create a posterior tympanotomy approach)
• Treat chronic suppurative otitis media with unsafe disease

Types of Mastoidectomy — A Brief Overview

• Cortical (simple) mastoidectomy: The posterior canal wall is preserved. The mastoid air cells are exenterated and the antrum is opened, but the middle ear is not formally entered. This is the procedure described in this guide.
• Modified radical mastoidectomy (canal wall down): The posterior canal wall is taken down, creating a common cavity between the mastoid, antrum, and external ear canal. Used for cholesteatoma when the canal wall cannot be safely preserved.
• Radical mastoidectomy: As above but the middle ear cleft is also obliterated. Rarely used in modern practice.

Starting Point — The Cadaveric Ear

Below is a picture of a cadaveric ear specimen showing the external landmarks before surgery begins.

Step 1 — Post-Auricular Incision

We begin with a post-auricular incision. The ear is retracted forward to expose the mastoid area.

Feel this bone on yourself to get a sense of where it is. The incision should follow the bone down near the mastoid tip and curve gently around the posterior ear crease. Do not allow your incision to enter the post-auricular crease itself because this will:

• Make it difficult to suture at the end
• Result in poor wound healing
• Make it harder to expose the mastoid bone properly

Step 2 — Raising the Periosteal Flap

Dissect down through the skin and the post-auricular muscles until you reach the fibrous periosteal layer. Follow this layer forward and expose the operative area. If you require a temporalis fascia graft, this can be harvested at this stage from the superior end of the incision.

Maintaining the periosteal plane is useful during closure, as it ensures the ear returns to its correct anatomical position. Extend the flap anteriorly until you are near the ear canal. Place your finger in the canal at this point to confirm you are not inadvertently entering it.

Step 3 — Exposing the Mastoid Cortex

Incise the periosteal layer and expose the mastoid surface as widely as possible.

You need to expose the following structures clearly:

• The mastoid tip (the inferior bony prominence)
• The temporal line (the superior bony ridge — see below)
• The root of the zygoma
• The posterior lip of the external ear canal

Use a periosteal elevator to scrape the periosteum off, leaving the bone as clean as possible. At this point most of the landmarks needed to proceed safely should be visible.

Key Anatomy — Landmarks Before Drilling

The Spine of Henle

The spine of Henle is a small bony outcrop found within the ear canal at the inferior posterior corner. It roughly marks the spot, deep to which the mastoid antrum will be found. It is not always clearly visible on images but is an important tactile landmark intraoperatively.

The Temporal Line

The temporal line is a bony ridge that represents the continuation of the zygomatic arch as it attaches just above the ear canal. It has two important surgical implications:

1. It is the attachment of the temporalis muscle above
2. It marks the approximate level of the dura of the middle cranial fossa below

Always remain inferior to this line during drilling. Breaching the dura here risks a CSF leak.

MacEwen's Triangle

MacEwen's triangle (also called the suprameatal triangle) is a bony triangle formed by the posterior wall of the external auditory canal, the zygomatic arch, and a line tangential to the posterior wall of the ear canal. The mastoid antrum lies approximately 1.5 cm deep to this triangle. It is a useful surface guide when beginning drilling.

Step 4 — Initial Drilling

Pick up the largest cutting burr available (typically size 5) on your drill. With copious irrigation and wide-bore suction, begin drilling just inferior to the temporal line. Use the equator of the burr rather than its tip — this is the point of maximal cutting speed and efficiency. Hold the drill firmly but do not press hard into the bone. Allow the burr to cut using minimal downward force. Using the tip or applying excessive pressure will cause the drill to jump off the bone unpredictably.

Step 5 — Establishing MacEwen's Triangle

Make a second line of drilling at right angles to the first, descending towards the mastoid tip and staying just posterior to the ear canal. This creates a 'V' shape. When the free ends of these two lines are joined, they mark out MacEwen's triangle and keep your dissection within the mastoid cavity.

Step 6 — Deepening the Cavity

Drill within this triangle and progressively deepen the mastoid cavity. In a well-pneumatised mastoid you will begin to encounter air cells, which provides some reassurance that you are in the correct plane. However, do not simply follow air cells — always use your surface landmarks to guide progress.

Critical Structures to Avoid

Several vital structures are at risk during mastoid drilling. Understanding their positions before you begin is essential for safe surgery.

• The sigmoid sinus is located in the posterior border of the mastoid cavity. It contains venous blood at high volume and pressure — injury is life-threatening. As you approach it, the bone thins and takes on a slightly bluish hue. Ensure your irrigation is adequate so you can detect this colour change early.
• The sinodural angle is the angle between the sigmoid sinus posteriorly and the dural plate superiorly. This should be fully drilled to provide adequate exposure of the antrum. The mastoid tip acts as a guide for dissection in this region.
• The dural plate (tegmen) is the thin bony plate separating the mastoid from the middle cranial fossa. It is slightly harder than surrounding mastoid bone and bleeds a little more when approached. Identifying it confirms your superior limit of dissection.
• The facial nerve and chorda tympani are found in the anterior wall of the mastoid cavity. The facial nerve is the most feared structure in ear surgery. Its position relative to the lateral semicircular canal is described below.
• The labyrinthine structures — the semicircular canals and cochlea — occupy the floor of the mastoid cavity. They are encased in dense otic capsule bone (the hardest bone in the body). Injury here causes permanent sensorineural hearing loss.
• The jugular bulb lies inferior to the labyrinth and can be unusually high-riding in some individuals, where it may be encountered during inferior dissection.

Practical Drilling Tips

• Diamond burr vs. cutting burr: Switch from the cutting burr to a diamond paste burr as you approach delicate structures such as the facial nerve, semicircular canals, or sigmoid sinus. The diamond burr cuts far more slowly but is much safer in these critical zones. It also has the advantage of sealing small bleeders from the dural wall.
• Drill along, not across: When working close to a structure such as the facial nerve, sweep the drill along the course of the structure rather than drilling perpendicular to it. This dramatically reduces the risk of inadvertent transection.
• Visualise before you advance: Always drill from inside to outside rather than advancing blindly into a space. Never drill around a corner where you cannot see what you are doing.
• Saucerisation: This is the technique of rounding off and smoothing the edges of the cavity as you advance. It serves two purposes — it improves visualisation of deep structures by allowing better line of sight, and it encourages the cavity to self-clean and contract during the healing phase.

Step 7 — Identifying the Antrum and Ossicular Chain

With the dura, sinodural angle, sigmoid sinus, and mastoid tip all identified, extend your dissection slightly anteriorly towards the zygomatic root and superior to the posterior canal wall. The mastoid antrum — a space of variable size communicating with the middle ear — will be found below this point.

Take great care when opening the antrum because:

• The short process of the incus points directly towards you from the superior border of the antrum, with its tip directed towards the sinodural angle
• The second genu (bend) of the facial nerve lies just deep to the tip of the short process of the incus
• You must not touch the ossicular chain at all — vibrations transmitted by the burr travel down to the stapes footplate and can cause permanent sensorineural hearing loss

Step 8 — The Horizontal Semicircular Canal and Facial Nerve

The floor of the antrum is approximately at the same depth as the labyrinth. The horizontal (lateral) semicircular canal is normally the first canal to become visible — it appears as a slightly paler, denser ridge of bone. This is an extremely important surgical landmark because the start of the descending (vertical) portion of the facial nerve lies directly anterior to this canal.

The facial nerve continues to descend towards the mastoid tip, running along the anterior wall of the mastoid cavity. There are air cells deep to the nerve (retrofacial cells), but unless disease is present in these cells they should be left undisturbed — drilling into the retrofacial space would leave a mastoid cavity with a high facial ridge that would not self-clean, resulting in a chronically discharging ear.

Complications of Mastoidectomy

• Facial nerve injury: This is the most feared complication and can range from temporary neuropraxia to permanent palsy. Risk is minimised by thorough knowledge of anatomy, the use of diamond burrs near the nerve, and the use of facial nerve monitoring intraoperatively.
• Sigmoid sinus injury: Causes brisk venous haemorrhage. Controlled with direct pressure (bone wax or a muscle plug), rarely requires vascular surgical input.
• CSF leak (dural breach): Most leaks from the tegmen are small and seal spontaneously. Larger leaks require repair with temporalis fascia or fat obliteration.
• Labyrinthine injury: Causes permanent sensorineural hearing loss and may cause vertigo. This is an uncommon but catastrophic complication.
• Chorda tympani injury: Causes a metallic or altered taste on the ipsilateral side of the tongue. This often resolves with time but can be permanent.
• Post-operative wound infection: Managed with antibiotics; rarely requires further surgery.