23 Infratemporal Fossa “Type A” Approach
Definition
Following a subtotal petrosectomy, permanent anterior transposition of the facial nerve, and performing procedures on the structures located medial to the facial canal. At the end of this approach, abdominal fat is put into the cavity and a “blind sac” closure of the external auditory canal is carried out.
Indications
An infratemporal fossa “type A” approach is mainly carried out for removal of glomus jugulare tumors and neuromas of cranial nerves IX–XII.
Anatomical Orientation
Surgical Steps
The procedure will be carried out on the temporal bone where a modified radical mastoidectomy and exposure of the facial nerve have already been performed. In the first stage, the incudostapedial joint is separated with a micro hook.
II: Micro hook
MIL: Stapes head
EAC: External auditory canal
I: Incus
M: Malleus
MCF: Middle cranial fossa (dural plate)
S: Stapes
TM: Tympanic membrane
The tympanic membrane is separated from the tympanic annulus by means of a micro elevator and positioned superiorly. The anatomical structures encountered during this stage are indicated in this and the previous figure.
II: Micro elevator
MIL: Tympanicannulus
FN: Facial nerve
HSC: Horizontal semicircular canal
PSC: Posterior semicircular canal
SSC: Superior semicircular canal
TA: Tympanic annulus
TM: Tympanic membrane
After separating the tympanic membrane from the tympanic annulus circumferentially, and making a cut to the tendon of the tensor tympani muscle, the tympanic membrane, including the malleus and incus, is removed as a whole unit.
I: Incus
M: Malleus
TM: Tympanic membrane
Definitions and Tips
- The infratemporal fossa “type A” approach affords the most extensive, most direct, and an unobscured exposure of the jugular foramen area, which is hidden medial to the facial nerve, without sacrificing the facial nerve itself.
- There are two principal approaches, where the facial nerve is transposed from the facial canal to gain unobstructed access to the medially located petrous parts and the cerebellopontine angle. The infratemporal fossa “type A” approach involves anterior transposition of the facial nerve and the transcochlear approach involves posterior transposition of the facial nerve.
The bony overhang anterior to the mastoid facial nerve is removed. The anterior and inferior walls of the tympanic bone are drilled by means of a medium-sized cutting burr. The direction of burr rotation should be away from the facial nerve.
II: 3–4mm cutting burr
MIL: Facial nerve
Drilling of the anterior part of the temporal bone is continued and the anterior wall of the tympanic bone is thinned until a bluish-pink discoloration indicates the temporomandibular joint reflection. Alternatively, the periosteum of the temporomandibular joint can be directly exposed, depending on the needs of the individual procedure.
II: 3–4mm cutting and diamond burrs
MIL: Temporomandibular joint periosteum
DR: Digastric ridge
MCF: Middle cranial fossa (dural plate)
SS: Sigmoid sinus (dural plate)
TB: Tympanic bone
TMP: Temporomandibular joint periosteum (plate)
Following this, the inferior part of the tympanic bone is drilled extensively. When working in this area, the blue-colored reflection of the jugular bulb should be sought, in both the laboratory and the operation.
II: 3–4mm cutting and diamond burrs
MIL: Jugular bulb reflection
Definitions and Tips
- During the operation, the crura of the stapes may be cut and the stapes suprastructure removed, leaving only the stapes footplate in place. This maneuver reduces the risk of dislocation of the footplate during tumor removal or packing the middle ear with fat, and preserves the functions of the inner ear.
- The facial nerve should not be exposed until the final stages of the subtotal petrosectomy. The reason for this is to prevent inadvertent damage to the facial nerve during the extensive drilling process. Once the facial nerve is uncovered, it is immediately transposed anteriorly into its prepared groove.
In the next stage, the tensor tympani muscle and its tendon are separated from the cochleariform process and moved anteriorly using a curved micro dissector. Afterward, the proximal part of the tympanic facial nerve is exposed up to the geniculate ganglion.
II: Curved micro dissector
MIL: Cochleariform process
ET: Eustachian tube
TTM: Tensor tympani muscle
TTTM: Tendon of tensor tympani muscle
At this point, using medium-sized cutting and diamond burrs, the bone at the zygomatic root is drilled and a groove is made, beginning at the geniculate ganglion and ending at the squamosa. The facial nerve will later be transposed and placed into this groove.
II: 2–3mm cutting and diamond burrs
MIL: Geniculate ganglion
This completes the subtotal petrosectomy, the full exposure of the facial nerve, and the creation of the “groove.” The figure shows a wide-angle view of the surgical site. In the operation, the extratemporal facial nerve trunk should be found, and its two main branches exposed in the parotid gland, prior to the execution of the aforementioned procedures.
