Author - Mr Vik Veer ENT SpR - March 2009

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Guide to Cortical Mastoidectomy

This is a simple guide to performing cortical mastoidectomy. This should be used as start to understanding how to practice whilst on a temporal bone course or understanding an operation you are observing.

Below is a picture of a ear (cadaver specimen)

We will start with a post auricular incision, and before you will be the ear which you need to pull forward to expose the mastoid area (see picture below).

Feel this bone on yourself to give you an idea where it is. The idea is to make an incision that follows this bone down near the tip and gently curves around the posterior ear crease. Don’t allow your incision to go into the post auricular crease as that will be:

As you dissect down through the skin, and the post auricular muscles you will come to a fibrous layer (periostium) which you should follow forward and expose your area. If you need a temporalis fascia graft you can collect it at this point from the superior end of the incision (I will cover this in the endaural incision guide). Keeping the periostial plane can be useful at the end when suturing this back as it ensures the ear returns to its correct position. Extend this flap forwards until you near the ear canal. Put your finger in the canal to make sure you don’t injury it and you know how far you have to go.

At this point you can incise the periostial layer and expose as widely as you can.

You need to expose the mastoid tip, the temporal line, the root of the zygoma, the posterior lip of the external ear canal and the rest of the mastoid bone. Use a periostial elevator to scratch the periostium off to leave the bone as clean as possible. At this point you will normally see most of the landmarks needed to continue.

The spine of Henle is a small out crop of bone just within the ear canal at the inferior posterior corner. This roughly marks the spot deep to which the mastoid antrum will be found. Sadly this is not visible on the image below. The temporal line is a small ridge of bone that is the continuation of the zygomatic arch as it attaches just above the ear canal. This line is the attachment of the temporalis muscle and also the level of the dura in the middle fossa of the brain. We clearly need to stay inferior to this and not breach this layer as it will cause a CSF leak.

The first step therefore is to pick up the largest cutting burr available (size 5?) for your drill and with plenty of irrigation and wide bore suction, drill just inferior to this line. Use the ‘equator’ of the burr rather than the tip. This is point of maximal cutting, and the fastest point on the burr. Hold the drill firmly but don’t press hard on the bone. Allow the burr to cut the bone using minimal effort. Using the tip or pressing too hard will frequently cause the drill to jump off the bone dangerously. A slow methodical approach is best.

The next step is to make another line at right angles to your first cut and descend down towards the mastoid tip, staying just posterior to the ear canal. Always use the same drilling technique as mentioned before. You may need to change position to do this comfortably with control.

You should have a ‘V’ shape now which when the free ends are joined marks out MacEwans triangle which helps you stay within the mastoid cavity.

Drill within this triangle and work your way deeper into the mastoid. You will start to see mastoid air cells in normal bones, which can give an element of reassurance. Don’t fall into the mistake of following air cells, you should be always mindful of your landmarks and use these to guide your progress.

There are several structures you need to avoid when drilling into the mastoid cavity:

Avoiding damaging these structures are important to completing the operation safely. There are some useful tips that may help your dissection:

With the dura, sinodural angle, sigmoid sinus and mastoid tip all exposed you should now be able to extend your dissection slightly towards the zygomatic root (superior posterior to the ear canal) below which the antrum will be found. Staying close to the dura, work your way down until you reach the antrum which is an open space of variable size leading into the middle ear. Be careful here as the short process of the incus will pointing towards you and the second genu of the facial nerve is located just deep to its tip. The incus is normally seen in the superior border of the antrum with the tip of the short process of the incus pointing towards the sinodural angle. You must not touch the ossicular chain as the vibrations from the burr would travel down to the stapes and may cause sensorineural hearing loss.

The floor of the antrum is roughly at the same level of the labyrinth which is encased in a much harder whiter bone. The horizontal semi-circular canal is normally the first canal to be found. The start of the descending portion of the facial nerve lies directly anterior to this canal.

The facial nerve continues to descend towards the mastoid tip skirting along the anterior wall of the mastoid cavity. There are cells deep to the facial nerve (retrofacial cells) but if they are not diseased it is probably better to leave them alone to avoid having a mastoid cavity with a high facial ridge. This would leave a non-cleaning cavity that would constantly discharge and give your patient problems.

Further Reading

Otolaryngology Houston - good pictures of sigmoid sinus and a completed mastoidectomy

Temporal Bone Surgical Dissection Manual by Ralph Nelson - is an excellent book and a worthwhile purchase


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