The modern era of neurotologic transtemporal skull base surgery began in 1961 when William House introduced the operating microscope and multidisciplinary surgery for the removal of acoustic neuromas with low mortality rate and enhanced facial nerve preservation rate.  An array of neurotologic procedures provide safe exposure of the mid brain,clivus, CPA, petrous apex and infratemporal fossa.

The objective of transtemporal surgery is obtaining wide skull base exposure by precise dissection of the temporal bone.   These collaborative techniques by neurotologist and neurosurgeon provide wide surgical exposure and minimize brain retraction. Knowing theanatomy of the temporal bone is essential to understand about these approaches.

Anatomy of Temporal Bone: 

The temporal bone contains and is surrounded by many important structures.  It articulates with five other  cranial bones: the frontal, parietal, sphenoid, occipital and zygomatic.
It can be divided into four parts: the squamous,mastoid, petrous and tympanic.

a) Squamous portion:

The lateral surface defines the boundary of the middle cranial fossa.  It extends medially to join the superior surface of the petrous bone in the region of the tegmen.

b)   Mastoid portion:

Pneumatization within the mastoid process is variable.  The squama of the temporal bone forms the lateral wall of the central air containing space, the antrum, which communicates with middle ear by the aditus.  The suprameatal spine and cribrifom area provide important landmarks for surgical access to the anturm.  From here pneumatization may extend inferiorly into the tip of the mastoid process.  Pneumatization also extends into the perilabyrinthine region and petrous portion of the temporal bone.

c) Petrous portion: 

The petrous portion of thetemporal bone roughly assumes the configuration of a four-sided pyramid.  Within the body of the petrous bone is found the labyrinth and internal carotid artery, CN VII and CN VIII, all penetrate the bone substance.  The medial wall of the middle ear cavity contains the first turn of the cochlea.

d) Tympanic portion: 

The tympanic part of the temporal bone forms the anterior and inferior walls and part of the posterior wall of the external auditory meatus.It is separated anteriorly from the squamous bone by the tympanosquamous suture more medially from the petrous bone by the petrotympanic fissure and posteriorly from the mastoid portion of the petrous bone by the tympanomastoid fissure.   The inner part of the  tympanic  ring  is grooved and is called the tympanic sulcus, which accomodates the tympanic membrane annulus.   The inferior aspect of the tympanic bone is elongated into a vaginal process immediately anterior to the styloid process.

Superior and anterior surface: 

This forms part of the middle cranial fossa.  The foramen lacerum is found between the apex of the petrous bone and sphenoid bone and contains but does not transmit the ICA.  Near the apex is a small depression which lodges the trigeminal ganglion.  The arcuate eminence of the petrous bone overlies the superior semi-circular canal.  The tegmen tympani is lateral to the eminence.  The opening of the hiatus of the facial canal is anterior and medial to the arcuate eminence; this transmits the superficial petrosal branch of the middle meningeal artery and the greater petrosal nerve.


Posterior cerebellar surface: 

Posterior surface of the petrous bone forms the anterolateral surface of the posterior fossa.  A sulcus for the superior petrosal sinus defines its superior border.Posteriorly it articulates with the occipital bone.
Approximately midway between the apex and the anterior border of sigmoid sulcus is the IAM.  It is a short canal begins medially at the internal acoustic pore.A bony plate which is also part of the medial wall of the cochlea and vestibule closes the lateral end.  A horizontal ridge of bone, the transverse crest,divides thepore into upper and lower areas.  Theanterior portion of the superior division contains the facial nerve which is separated from the superior vestibular  nerve in the posterior portion of the upper division by a small, vertical crest of bone,known as ' Bill's bar'.  It serves as an important landmark during the translabyrinthine approach.  The cochlear nerve  lies in the anterior portion and the inferior vestibular nerve in the posterior 

portion of the lower division.  Midway between the meatus and sigmoid sulcus, is the vestibular aqueduct which transmits the endolymphatic sac and duct.

Inferior surface: 

Most irregular of the petrous bone'ssurfaces. The opening of the carotid canal is aboutmidway between the apex and base; this is the entrance for the ICA  and its plexus of veins and sympathetic nerves. The canal courses in a cephalad direction along the anterior wall of the tympanic cavity to the bony eustachian  tube and then bends horizontally,ending at the apex of the petrous bone and the occipital bone.  Carotid ridge is a sharp bone separating  the carotid and jugular foramen.  The lateral part of the foramen contains the sigmoid portion of the transverse sinus; the medial part contains the inferior petrosal sinus and the glossophayrngeal ,vagus and accessory nerve. Anterior to the lateral compartment is the broad fossa for  the jugular bulb. Posterior and lateral to it is the  styloid process.  Lateral to its base is the stylomastoid foramen transmitting 

facial nerve.

Facial nerve:

The facial nerve lies in the anterosuperior part of IAM, anterior to  Bill's bar.  It passes laterally over the labyrinth (Labyrinthine segment) to reach  the  geniculate ganglion.There it makes as acute bend, running posteriorly upto the lateral semicircular canal (tympanic segment).  There it takes 90 degree  bend to run in the inferior direction (mastoid segment) before it exits through the stylomastoid foramen.

Surgical approaches:

Approaches that traverse the otic capsule (Transcapsular) permit wide exposure but sacrifice hearing: translabyrinthine (TL), Trans otic (TO) and Transcochlear (TC).  

Posterior approaches that spare the otic capsule (Retro capsular) provide varying degrees of CPA exposure with an opportunity for hearing preservation: retro labyrinthine (RL), retro sigmoid (RS). 

Superior approaches (Supra Capsular) permit unroofing of the internal auditory canal (IAC) with varying degrees of petrous apex exposure and an opportunity for hearing preservation: middle fossa (MF) and extended middle fossa (EMF).

The inferior approaches: infracochlear (IC) and infralabyrinthine (IL).Removal of the otic capsule provides the most direct
route to the IAC and CPA without the need for brain retraction.

1) TRANSLABYRINTHINE APPROACH:  click for intra-operative video clippings

The TL approach is applicable for CPA and IAC lesions of all sizes especially in patients with poor hearing. Even though this approach is popularized for acoustic neuromas, it is suited for any  neoplasm requiring exposure of the CPA.  In patients without useful hearing, the TL approach is also useful for facial nerve tumours and vestibular neurectomy.
 

Technique: Surgical Highlights · Retro auricular incision · Cortical Mastoidectomy · Posterior labyrinthectomy · Exposure of the internal auditory canal · Identification of the Facial nerve at the meatal foramen
. Removal of lesion or nerve section · Aditus, mastoid cavity and vestibule obliterated with musculo facial graft and fat.		extent of bone removal in translab.approach
The patient is supine with the head turned to the opposite side. A curved post auricular incision is made with the apex 3 to 4 cm posterior to the post auricular crease. Complete cortical mastoidectomy is performed. The tegmen and  posterior fossa dural plate are identified  and the sigmoid sinus is skeletonized. Exposure of the retrosigmoid posterior fossa dura for at least 1 cm behind the sigmoid sinus is important. Anteriorly the facial nerve is identified in its vertical segment but left covered with bone for protection against inadvertent  burr trauma.
Next step is labyrinthectomy by drilling out the semicircular canals. Care is taken to leave the anterior  wall of the  lateral canal, and the most anterior  part of the ampulla of the superior canal in order to protect the tympanic and labyrinthine portions  of the facial		lateral, posterior & superior semicircular canals seen after cortical mastoidectomy
nerve and to serve as a landmark for the superior vestibular  nerve.    All IAC bone removal is completed before the dura is opened and the  IAC neural structures are exposed.  IAC skeletonization begins by drilling a trough along the inferior edge of the vestibule until the jugular  bulb is identified - the inferior limit of dissection.     The anterior limit of dissection is the cochlear aqueduct.  Once the inferior border of the IAC is identified, a superior trough is drilled along  the superior edge of IAC. A full of 270 degree skeletonization  of the IAC dura is critical to  prevent bony edges from interfering  with adequate tumor removal. Particular attention is required along the lateral  extent of
the IAC to preserve crucial landmarks for facial nerve dissection		labyrinthectomy in progress
Tumor Removal: In small lesions the tumor is exposed by opening the dura of  IAC.  The superior vestibular nerve is transected by placing an angled instrument adjacent to Bill's bar and reflecting the superior vestibular nerve and it identifies  the lateral plane between the facial nerve and the tumor.  Sharp and blunt dissection can proceed without actually placing traction on the facial nerve.
In large  tumors, CPA exposure is necessary.  Intracapsular tumour debulking is completed before the tumor is dissected  directly from the facial nerve. After tumor removal eustachian  tube is packed with surgicel and temporalis muscle.		bone surrounding the IAM removed for 270 degrees. Tumor in the IAM seen
The dural defect is loosely approximated with sutures and the mastoidectomy defect is filled with strips of abdominal fat. The TL approach provides wide and direct access to CPA tumors with minimal cerebellar retraction.  It permits identification of the facial nerve - laterally at the fundus and medially at the brain stem, which helps in anatomic preservation of the facial nerve.
The disadvantage of this approach is that  hearing cannot be preserved.		extent of bone removal in transcochlear approach
2) TRANSCOCHLEAR APPROACHES:   click for intra-operative video clippings  While TL approach offers wide exposure of the CPA,the cochlea and petrous apex block access to the anterior aspects of the CPA and the ventral brain stem.   A spectrum of transcochlear approaches provide access to the ventral brain stem, beginning with the transotic (TO) and extending to a true transcochlear (TC), with the widest exposure being the transpetrous (TP).
All these approaches, by definition, remove the cochlea following a TL approach to extend the exposure anteriorly. The facial nerve remains in situ (although skeletonized) in the TO approach,		subtotal petrosectomy done. semiocircular canals being drilled out.;tympanic & mastoid segment of facial nerve decompressed
The facial nerve is transposed posteriorly in the TC approach. The TP approach includes the full TC with the addition of an infra temporal fossa approach and even transposition of the petrous carotid artery in certain cases. The disadvantage of this approach is that  hearing cannot be preserved. 3) TRANSOTIC APPROACH:
Surgical Highlights:		facial nerve exposed;exposure of geniculate ganglion & GSPN
Retro auricula-temporal incision .Blind closure of external auditory canal .Subtotal petrosectomy .Tympanic and mastoid fallopian canal left as a  bridge over the cavity   Otic capsule removed to expose the complete medial surface of the temporal bone. Maximal trans temporal exposure of the internal auditory canal and CP angle. Direct anterior approach  to the intrameatal  and intracranial facial nerve. Dura reconstructed with musculo facial graf.
Cavity obliterated with fat and temporalis muscle flap. Through a postauricular incision, ear is reflected anteriorly, and the external auditory canal		posteriorly rerouted facial nerve lying on the posterior fossa dura
is transected and closed in two layers.  The soft tissue exposure, complete mastoidectomy, labyrinthectomy,posterior and middle fossa dura decompression, and skeletonization  from the geniculate ganglion to the stylomastoid foramen, while maintaining  a thin egg shell of bone on the  nerve are carried out.   The retrofacial air-cell tract is also  dissected, permitting 360-degree skeletonization of the facial  nerve in the vertical and tympanic segments.  Then the cochlea is drilled out and the petrous carotid artery is the anterior limit of the exposure.  By working around the facial nerve, the surgeon has access to lesions of the IAC, CPA, clivus and jugular foramen.   Closure is performed with  obliteration of the defect with abdominal fat.
		cochlea is drilled out.

4) TRANS COCHLEAR APPROACH :

Like TO, the TC approach combines the TL with removal of the cochlea; however wide access to the anterior CPA is provided by posterior transposition of the facial nerve.  Thus the exposure extends from the sigmoid sinus posteriorly to the petrous carotidartery anteriorly. 

The posterior transposition of the facial nerve results in an obligatory temporary facial paralysis that produces some degree of aberrant regeneration. The fifth,seventh, ninth, tenth, and eleventh cranial nerves, the  clivus, both the vertebral arteries and the basilar artery are routinely seen. 

 One added advantage of the approach is that during bony dissection to obtain exposure, the blood supply and the tumor

 base are removed, which is particularly important in petrous ridge meningiomas.

The principle indications for this approach are large petro-clival meningiomas, epidermoids, extensive gliomas, jugular 

tumors and even temporal bone malignancies. 

Technique:

Surgical Highlights:

 Retro auriculo temporal incision
 Blind closure of external auditory canal
 Subtotal petrosectomy
 Posterior labyrinthectomy
 Exposure of internal auditory canal
 Decompression of mastoid, tympanic and labyrinthine segments of facial nerve and geniculate ganglion
 Division of greater superficial petrosal nerve and posterior rerouting
 Drilling out of anterior wall of IAC, cochlea,petrous tip and clivus
 Removal of tumor
 Cavity obliterated with fat and temporalis muscle flap

Posterior transposition of the tympanic and vertical segments of the facial nerve requires transection of the greater superficial petrosal nerve.  Following facial nerve transposition, cochlea and tympanic ring removal exposes the carotid artery anteriorly, the jugular bulb and inferior petrosal sinus inferiorly and the superior petrosal sinus superiorly.

This approach provides direct access to the base of implantation and blood supply of tumors arising from petrous tip and petroclival junction.  Temporary facial nerve paralysis occurs uniformly with the posterior transposition of the facial nerve which is most likely the consequence of devascularization of the perigeniculate segments of the nerve caused by transection of the greater superficial petrosal nerve and its accompanying vessels.

5) TRANSPETROUS APPROACH

In this procedure, full TC exposure is combined with infratemporal fossa approach, orbitozygotomy and even transposition of petrous carotid artery.  This approach is indicated in only the most extreme extension of tumor into the lateral cranial base and intracranially.