Mastering “The Mystic Mastoid” Part 2

Previous blog was about the brief introduction to the methodology of the artificial temporal bones and their specific role in gaining the dissection skills. Now the critical details of the anatomy of the temporal bone will be discussed. Only those details that are pertinent to performing the surgeries on the bone with minimal rate of complications. The features of anatomy that are not present in the artificial bone will also be mentioned in this discussion.

Eyes see only what the brain knows!

As discussed in the previous blog, the anatomy of the temporal bone is very complex and compact. Barely in the space of less than an inch square, all the inner ear inclusive of the cochlea for hearing and the vestibule for the balance mechanism of our body is fitted. To add to the complexities, the main nerve of our emotional facial features, that means the seventh cranial nerve, swerves through the bone alongside and hugging the inner ear. Also the direct relation of the cochlea with the internal carotid artery, the main blood supply to brain, is very remarkable. That is why having a hands-on dissection practice will only help the learning surgeon to attain certain skills of surgery.

Let us discuss these complex relations in details. Unless there is a clear picture of them in mind, further surgical dissection will not be successful.

Why named ‘temporal’?

Temporal bone, seated at the temples or temporal areas of the skull, supports the brain in a lateral and basal fashion. ‘Temporal’ means related with time. The area of the scalp related to time, in a way, meaning the area where the aging starts, the greying of hair starts first.

This temporal bone forms in five different parts, viz. Squamous, petrous, mastoid, tympanic, and styloid. All these fuse ultimately to form one bone. And inside the petromastoid part grows the otic capsule, the inner ear. Hence there are many fusion points or suture lines or fissures in the bone. These are the joints between the different parts.

The squamous part

The squamous part is a membranous bone that grows fast in a superoinferior direction. Lying most laterally, it limits the expansion of the petrous pyramid. The most constant feature of the squamous part is its anterior extension in the form of the zygomatic process. This is so constant that the zygomatic process may be the only known landmark in the cases of total atresia of the ears. Taking help of this process and its glenoid fossa, where the mandible condyle fits, one has to reconstruct the middle ear in the cases of atresia.

As is well discussed, to approach the inner intricacies of the temporal bone, we have to go from one known landmark to the next. The next obvious landmark is the opening of the external auditory canal, completed in its lower portion by the attachment of the tympanic plate. Somewhat oval in shape, the opening has a distinct spine of Henle at its posterosuperior margin. The spine may be well formed or rudimentary, but has the significance of its relation to the mastoid antrum. The antrum always lies posterosuperior to the spine. At the meeting point of the squamous part with the mastoid part, in the suture line, is the mastoid emissary foramen. Not only the emissary vein, it also carries a branch inside from the occipital artery to the dura. Being valve less, the emissaries are always a conduit for easy transfer of infections. In this case, up to the sigmoid sinus from the scalp outside.

Tympanic part

The tympanic part is a plate that fits from below to the squamous part and makes almost full anterior wall and half of the posterior wall of the external auditory canal. Thus having a horizontal tympanosquamous suture line on the anterior wall and tympanomastoid suture line on the posterior wall, this tympanic plate has a sulcus, a shallow rim-like groove, at its medial end. In this sulcus fits the annulus of the tympanic membrane. The dehiscence of the tympanic plate is very common in the early childhood, though it fuses fast with the development. Having bony overhangs on its walls, this portion can restrict a direct view of the tympanic membrane and needs to be widened by drilling it out, the procedure named as canaloplasty.

Photograph: ear canal, membrane & ossicles — appears on the live site

Down the canal of the model: the annulus seated in its sulcus, with chorda tympani and ossicular detail behind the membrane.

Koerner’s septum — the most important guide

Surgically more relevant is the fusion between the squamous part on its medial aspect with the petromastoid part. As said earlier, the squamous part is the limit for the lateral expansion of the petromastoid part. And during growth, this petromastoid portion gets splayed over the squamous, leading to its retention in the form of a plate. This plate is the Koerner’s septum.

The Koerner’s septum is the distinction between the groups of air cells. Those air cells lateral to the septum are of superficial group and those medial to it are the deeper or petrous air cell groups. Remembering that the air cells are never single but always in chains, to follow the specific air cell tracts is very essential for eliminating the mastoid disease. Though the Koerner’s septum is not a constant feature, some or the other portion of it is always present. The differential feature is the thickness of the septum as compared to that of the air cell walls. The septum is three to five times thicker as compared to the air cell walls.

Also, remember that this septum forms the lateral wall of the mastoid antrum. So following and removing this septum anteromedially from the sinodural angle to the fossa incudis is the correct way of finding the antrum. Another importance is that the facial nerve will always be medial to this septum. Often there is a similar septum between the squamous and mastoid parts, in the lower portions. Even the cog in the roof of the middle ear is nothing but the portion of this squamo-petrosal septum.

Petromastoid part

The petrous and the mastoid parts fuse in early foetal life and can rarely be labelled apart. Holding the otic capsule, this petrous part, the second toughest bone in the body after the dentin of teeth, gives a rock-support cover to the precious organ of inner ear. As written by Portmann in his book ‘The Internal Auditory Meatus’, this portion of the temporal bone is directly or indirectly related to four out of five special senses of our body. The nerves passing through the internal auditory meatus are concerned with hearing, taste, vision (by way of proper lacrimation), and smell (by way of nasal mucosal secretions). The inner ear, especially mammalian inner ear, must be the most beautiful organ ever evolved. Its two portions, the cochlea and the vestibule, are like conjoint twins. Most amazingly, both these portions carry similar chemicals and proteins, both are having basic functional units as hair cells, both have same composition fluids that can interplay, and still both subserve very different functions. One deals with that special sense of hearing and the other makes us balance ourselves for all the activities.

The inner ear, especially mammalian inner ear, must be the most beautiful organ ever evolved. Its two portions, the cochlea and the vestibule, are like conjoint twins.

Artificial Temporal Bone — anatomy details

This model of the temporal bone has all the named parts of the bone. The external as well as internal anatomical features are correctly positioned and having a faithful replica of the normal bone as per the CT scans.

Main feature of this bone is that all the nerve and vessel canals, the cochlea, and the three semicircular canals are patent. The ossicular chain is printed in one go and correctly placed. The facial nerve has chorda tympani as well as the greater superficial petrosal nerve. The internal auditory meatus has all the four nerves. The sigmoid sinus and the internal carotid artery are perfectly colour coded. The layer of pink dura gives a realistic hue when blue-lined during dissection.

Photograph: sigmoid sinus, nerves & dura — appears on the live site

Colour-true vessels and nerve pathways run through the bone, exactly where surgery will find them.

The basic complete model of the temporal bone is extensively pneumatized and has many retrosigmoid air cells as well. This bone is easier to learn the hands-on dissection. Any fresher is recommended to start with this model.

The different models, a variety of them in the catalogue, have different features. There are paediatric bones, normal as well as with congenital cochlear anomalies; adult bones having variations of pneumatization, and of various difficulty levels of drilling.

The artificial bone can also be customised for a specific case based on the CT scan findings. If the scans are of suitable resolution, minimum thickness of slices being 0.6mm, and in the triplanar mode, meaning axial, coronal, and sagittal sections, then the quality of the artificial bone will be more accurate as far as the details of anatomy is concerned. Sometimes, rarely as in congenital anomalies, the MRI scans are also needed, especially to know the exact layout of the facial nerve and the chorda tympani.

The hands-on workshops held on these artificial bones are under various headings. Basic temporal bone dissection, Advanced temporal bone dissection and Cochlear implant surgery run as scheduled programmes. Advanced cochlear implant surgery, Facial nerve surgeries, Tympanoplasty methods, Stapedotomy surgery, and others can be arranged on request.

Assured are the gains in confidence of doing live surgeries after attending these workshops, as they are held under the constant and stepwise guidance of the renowned faculties.

Train on this bone

The models and programmes described above, at ProGuide.

• The temporal bone models— basic, complete and task-based
• Your own model from a CT scan— 0.6mm triplanar slices, as described above
• KBI SkillBridge workshops— dates, fees and registration
• Ask a question— our programme experts answer seven days a week

Writer

Dr Prashant M Naik

MS (ENT), DLO

Nagpur, India