Mastoid
bowl. I’s a funny term. The residents who rotate with me scratch
their heads when I mention this and a few patients who themselves possess this
anomaly also appear perplexed when I utter those two words, having never heard
them before. But I’m not making this up—it’s a for-real
medical entity. Recently, an elderly
gentleman with a mastoid bowl asked me to explain this and the type of surgery
that was done to create it. His surgery
was over 30 years ago! The aftermath of
this explanation with my crude drawings subsequently lead me to create this
article.
A mastoid bowl (MB) results from drilling out the mastoid from behind the ear (postauricular approach) but also removing the posterior (back portion of) the ear canal, often with removal of all or part of the ear drum (tympanic membrane or TM) and some or all of the ossicles (those tiny bones that transmit sound from the TM to the inner ear). This surgery, called a canal-wall down tympanomastoidectomy, is typically done for more extensive cholesteatomas where monitoring for and preventing recurrence is much simplified.
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The
Surgery
Following is the surgical approach, in a nutshell: Incisions are made inside the ear canal to raise a flap of tissue that includes the back (posterior) part of the canal and the TM. If one or more ossicles have been destroyed or distorted by the disease-process, they too are removed.
Next, attention is directed behind the outer ear (postauricular area), a long curvilinear incision is made (which hides well upon healing) and the mastoid bone is exposed. High speed drills, using a variety of spherical-headed burrs, cut away bone in incremental fashion. Care must be taken to avoid cutting into: 1. The facial nerve as it travels vertically along the front part of the mastoid, 2. The semicircular canals, 3. The sigmoid sinus, which drains blood from the brain down into the large internal jugular vein below. Once the mastoid is drilled out, the middle ear space is also readily visible through the mastoid.
If this were an intact-canal-wall tympanomastoidectomy, then the bone removal part of the surgery stops here. However, if there is extensive disease that cannot be removed adequately, then the posterior wall is removed with the mastoid drill. This creates a large cavity that includes the rest of the canal, the drilled-out mastoid, and the exposed middle ear.
If the TM was removed, a graft can be placed usually with fascia (a tough sheet of tissue that covers muscle) harvested within the postauricular incision created at the onset. This graft is used to reconstruct the TM. If the ossicles were removed, some of them could be reshaped to serve as a natural prosthesis or other autologous materials such as small pieces of ear cartilage can be used for this purpose. Otherwise, a synthetic ossicular reconstruction prosthesis (often made from hydroxyapatite, a biocompatible calcium ceramic) is used. A skin flap is made from the incised posterior canal and allowed to line part of the mastoid cavity, and voila, the result is a…
The
Mastoid Bowl
The resulting bowl allows for visualization into the mastoid cavity from peering into the canal from outside the ear. Once healed it is easy to inspect and clean the MB and remainder of the ear, and the chances for a recurring cholesteatoma are much reduced, or can be removed in its early stages. However, the MB is not self-cleaning, i.e., the ear does not move out debris and wax as it naturally does with a normal, unaltered ear. Thus, a patient with a MB must have regular exams for debridement under the microscope. Typically once well healed, these patients are seem every 6-12 months, sooner if needed (infections sometimes occurs, but are usually more easily treated with debridement or cleaning, sometimes needing topical treatment of a variety of medicated ear drops or powders).
©Randall
S. Fong, M.D.
For more topics on
medicine, health and the weirdness of life in general, check out the rest of
the blog site at randallfong.blogspot.com
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