Durity Skull Base Workshop

With the advent of skull base surgery and advancement in surgical technique, it has become possible to access deep areas in the base of the skull with minimal retraction and disruption of neurological structures. The concept is to remove bone, displace extra cranial soft tissue to gain access, and control the vascular supply to lesions early; to make deep seated skull base lesions behave like more readily accessible convexity lesions.

The Durity Skull Base Neurosurgery Resident Workshop is a 2-day program comprising of a formal high speed drill course and hands-on dissections of select skull base approaches and their variations. It is designed to introduce neurosurgery residents to the anatomic intricacies of skull base anatomy and principles of skull base surgery and to provide an understanding of skull base surgical approaches and reconstructive options.

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Given the intensity of the course, it is advised that participants familiarize themselves with some background knowledge that would facilitate their learning during the course. Explore the links below to obtain a fundamental framework to the approaches employed in skull base neurosurgery.

The anterior and anterolateral approaches involve using a corridor through the orbital bones, reducing brain retraction and often improving the maneuverability of the optic nerve (which is often compromised) with unroofing the optic nerve. Tumors that may require these approaches include meningiomas with and without orbital extension, pituitary tumors, aneurysms, and craniopharyngiomas. These approaches can be modified to access predominantly middle, infratemporal, and posterior fossa lesions by removing varying amounts of the zygoma and anterior temporal bone: chondrosarcomas, trigeminal schwannomas, cholesterol cysts, paragangliomas and jugular foramen schwannomas. The superior orbital osteotomy and orbito-zygomatic osteotomy are probably the most common skull base type of approaches and will be discussed here.

All of these operations start with the skin incision. These procedures can be preformed through a standard front-temporal curvilinear skin incision behind the hair line (or a eye brow incision when just a limited superior exposure is required). The former is preferred by the author for its familiarity, good cosmetic result and ability to modify the size of the exposure if it is deemed to be required intraoperatively. The curvilinear skin incision starts in front of the tragus of the ear and extends just past midline a centimeter behind the hairline. The majority of the time the lesions of interest require only limited exposure of the frontal and temporal bone. The incision needs to go past midline so that adequate exposure of the orbital rim and zygoma can be achieved once the skin is flipped down. It is the preference of the author to perform a ‘cheveron’ staggered incision into the tragus to break up the incision line where it is visible below the hair line for a better cosmetic result. The lower limit of the incision is 5mm or so below the lower border of the root of the zygoma and the incision is kept within a finger breath of the tragus, to prevent injury to the frontalis branch of the facial nerve that runs in front of the ear and about a centimeter or so below the zygoma within the upper parotid. If the skin incision needs to be extended inferiorly, the facial nerve in the upper aspect of the parotid gland need to be dissected out. The incision is started at the tragus, going through the galea and identifying the frontal branch of the superficial temporal artery which can be coagulated and divided with the bipolar. The dissecting in the subgaleal plane, temporalis muscle and fascia is left intact and the pericranium is left on the bone to take down as a separate layer. The pericranium is turned down subperiosteally, dissecting in the subgaleal plane posterior to the skin incision to obtain a large piece of pericranium. The pericranium is very good material to use in reconstruction of the dura, and more importantly, it can also be used as a vascularized flap to seal off the frontal sinus which is often entered with the osteotomy. Once the pericranium is turned down, it should be wrapped in a soaked sponge to keep it from drying out during surgery. The supra orbital nerve can be identified with its vascular bundle exiting the supra-orbital notch or often foramen. The nerve can be mobilized out of the foramen, using an osteotome since the foramen is very short. The supraorbital nerve can be preserved in this fashion to maintain sensation to the forehead. Once the root of the zygoma and the orbital rim above the temporalis muscle can be palpated, the superficial temporalis fascial dissection needs to be preformed to preserve the frontalis branch of the facial nerve which passes within the fat pad between the superficial and deep layers of the temporalis fascia, within a centimeter behind the lateral orbital rim. A gentle curved incision is made extending from the root of the zygoma to the anterior superior attachment of the temporalis muscle. If this incision is made too posterior, one will go through the temporalis fascia before it splits between the deep and superficial layer and you will see muscle. If one has to error, it is better to error on the deep side and takes the dissection down below the deep fascia layer exposing muscle to preserve the facial nerve. Once the fat pad is taken down with the superficial temporalis fascia, the entire lateral orbital rim/zygoma can be exposed in a subperiosteal fashion, while maintaining the integrity of the facial nerve. Care should be taken if coagulation is required in this area and skin hooks should avoid going through the fat.

The dissection is then continued down over the orbital rim to dissect the periorbita. Often it is easier to start the dissection laterally, taking down the lacrimal gland and then working medially. Depending on how much of the orbital bone is to be removed, the periorbita can be taken down laterally until the inferior orbital fissure is identified.

As the dissection is carried deeper, the inferior orbital fissure is identified inferiorly and the superior orbital fissure should be avoided from the orbital side. On the nasal/medial side, if required, the anterior ethmoid artery is identified first, which can be coagulated and divided. The posterior ethmoid is found approximately 6mm deep to the anterior ethmoid and can also be bipolared and divided. These vessels supply the dura and lesions such as meningiomas in this area, which aids in resection. When dividing these vessels, it is important to divide the vessels close to the bone and to cut the vessel carefully to prevent the vessel retracting into the orbital fat and causing a hematoma as it is a branch of the ophthalmic artery. The optic nerve can be recognized another 6 mm or so deep to the posterior ethmoid artery, it can be differentiated from the posterior ethmoid from its size and direction (care needs to be taken to be certain you are coagulating the posterior ethmoid and not the optic nerve). Once the periorbital dissection is accomplished, the planes can be maintained using large cotton patties, which protect the periorbita when making the boney cuts. The periobita can often be abnormal when involved in tumor, it is favorable to spend some time with the dissection to preserve the periorbital layer.

The amount of orbital bone removed as an osteotomy and the craniotomy performed is tailored to the lesion. For access to lesions of the ipsi-lateral frontal floor/peri-sellar areas, a small fontal craniotomy over the orbit and a superior orbital osteotomy may give enough access. For lesions extending into the middle fossa, an orbito-zygomatic osteotomy and a fronto-temporal craniotomy maybe required. If contra-lateral extension is required, the skin incision and the medial cut can go across midline to include varying amounts of contra-lateral orbital bar. These cuts are made after pacing and predrilling holes for the titanium plates used in reconstruction to get an anatomical re-approximation of the bone the end of the procedure. In most instances, the medial boney cut can be made at the level of the superior orbital notch, taking the osteotomy cut back to a level just shy of the superior orbital fissure. The cut is kept lateral to the down sloping area of the medial frontal floor to avoid the ethmoid sinuses. The author prefers to use a reciprocating saw while protecting the periobita with cotton patties and a malleable retractor. This cut often involves the frontal air sinus which will need to be dealt with in the reconstruction. The sinus is packed with a cotton patty soaked in antibiotic containing irrigation solution until the reconstruction. The lateral cut is usually made mainly in one of two places depending on what is required. To just remove the ipsi-lateral orbital rim, the lateral cut can be made few millimeters inferior to the orbito- zygomatic suture, again just shy of the superior orbital fissure. If the zygoma also needs to be removed, the lateral cut can be made through the body of the zygoma down into the inferior orbital fissure; the cut at the root of the zygoma is made on an angle in parallel to the boney attachment. The cut in the orbital roof and lateral sphenoid is again made just shy of the superior orbital fissure, connecting these cuts into the inferior orbital fissure for the orbito-zygomatic osteotomy. These approaches also give good access to the middle and infratemporal fossa, lateral to and including the V3/foramen ovale (trigeminal schwannoma).

For certain peri-sellar tumors that may benefit from mobilization of the optic nerve ie. Diaphragm sella meningiomas (that push the optic nerve up from underneath), the author prefers to remove the greater and lesser wing of the sphenoid, as well as completely decompressing the optic canal/removal of the clinoid. The superior orbital fissure is unroofed from below using a rongeur, removing the greater wing of the sphenoid to decompress the superior orbital fissure; it is important to not put any instruments into the superior orbital fissure prior to doing this. The removal of greater wing can be carried medially to the level of foramen rotundum which sits a few millimeters inferior to the most medial aspect of the superior orbital fissure. Removal of bone deep to the foramen rotundum will lead to the sphenoid sinus.

After removing the greater wing, the lesser wing can be rongeured off, taking care to bite the bone off rather than twisting, as the lesser wing can potentially fracture through the optic strut and into the optic canal. Once the lateral portions of the lesser wing is removed, the optic canal and anterior clinoid can be drilled out using an self irrigating high speed drill with a diamond bit. The optic nerve exiting the canal intracranially can be identified extra-dually and the optic nerve entering the canal from the orbit side can be identified by the trajectory of the apical contents and the gentle convexity in the bone as the canal forms at the apex. Occasionally the canal can be dehiscent and almost continuous with the dura, care should be taken when dissecting extra durally in this area. The canal is thinned out with the diamond drill and egg-shelled off the optic nerve, thus completely decompressing the nerve. It is important not to put any instruments in to the canal prior to being decompressed. The anterior clinoid is then drilled from inside out, working from the cancellous bone out into the cortical bone of the optic strut, again egg shelling the bone of the strut prior to fracturing it, releasing the anterior clinoid which can then be dissected off the dura and removed. Care should be taken when doing this drilling; on one side of the strut is the optic nerve and on the other side is the carotid artery. As intracranial tumors in this area often cause tethering of the nerve at the level of the optic canal, completely removing the clinoid and decompressing the nerve can allow precious few millimeters of mobilization and removal of tumors (ie. Meningiomas) extending into or involving the boney canal.

For certain perisellar and pericavernous sinus tumors, it may be beneficial to monitor the cranial nerves. The removal of the orbital bone via osteotomy allows a unique opportunity to monitor the cranial nerves since they are easily accessible. The main methods for monitoring nerves include free running electromyography, evoked compound muscle action potentials and motor evoked potentials of cranial nerves. The nerves of particular interest for tumors requiring these approaches for the neurosurgeon are the oculomotor, tro and abducens nerves. To monitor these nerves, recording electrodes need to be placed in or near the enervated muscles. Free running EMG alerts the surgeon when nerves are irritated identifying potential injurious maneuvers. Evoked compound muscle potentials allows surgeons to stimulate and map out locations of nerves intra-cranially, the anatomy of which is often distorted by the underlying pathology. These monitoring techniques are believed to improve the outcome in patient with certain tumors in these areas.

If more medial and inferior exposure is required to the middle fossa (more than what can be accessed through drilling of Kawase’s triangle), medial to the petrous carotid artery, the condylar fossa can be removed with the zygoma to give access medial to the petrous carotid (Chondrosarcomas/cholesterol cysts/jugular foramen schwannomas/paragangliomas). The lateral middle fossa dura is elevated. Foramen spinosum and ovale is identified. Sometimes the bone over the horizontal portion of the petrous carotid artery is deficient in this area and care should be taken while dissecting in this area. The mandibular condyle is dissected off the undersurface of the condylar fossa, identifying anterior and posterior limits of the condylar fossa from the curvature of the bone. It is important to not enter the external ear canal posteriorly since any holes made into the canal though the skin is very difficult to repair and will be a source of difficult csf leak and possible infection. Instead of making the cut at the root of the zygoma, to remove the condylar fossa with the zygoma, the osteotomy cut including the condyle is made in a triangular fashion, apexing the cut at the level of the foramen spinosum. This avoids injury to the carotid which is medial to this structure.

Once the orbital zygomatic osteotomy with the condylar fossa is removed, the middle fossa dura is elevated from posterior to anterior identifying the arcuate eminence and the greater superficial petrosal nerve (GSPN). If you are monitoring the facial nerve, the facial nerve can be stimulated through the GSPN and geniculate ganglion to help with identification. The dura over the GSPN is split to preserve it’s integrity. The middle fossa dura will be tethered at the foramen spinosum and foramen ovale. The foramen spinosum can be drilled out and coagulated/divided to allow elevation of the dura in this area. The dural over ovale can also be split to allow more mobilization of the middle fossa dura. The carotid artery in the petrous bone can be identified in a couple of ways. The carotid is located below the level of the greater superficial petrosal nerve and is medial to foramen ovale. As you are drilling the bone medial to your osteotomy cut, you will encounter the Eustachian tube which is lateral to the carotid. One can drill through the eustachain tube but the opening anteriorly into the nasopharynx is noted since it will need to be packed off and the cartilaginous portion closed with a stich to avoid csf leak. The horizontal petrous carotid is medial to the Eustachian tube. The vertical portion of the petrous carotid artery is located deep to the vertical portion of the petrous bone deep to the condylar fossa, and can be found by drilling through it. It is important to remove the bone and unroof the bone of the foramen ovale since the carotid artery when mobilized laterally is hinged on the bone in this area and the carotid can be kinked by sharp bone edges. Inferiorly as the carotid enters the petrous bone, there is a cartilaginous ‘ring’ that has to be cut open to allow mobilization of the carotid more freely. Unless the surgeon is very experienced, it may be prudent when doing this procedure to expose the carotid in the neck for proximal control. The bone medial and anterior to the carotid can be drilled out, exposing the posterior fossa dura.

Chondrosarcomas can often be followed in to the clivus to the contralateral petrous apex. Traveling in the dura in this area is the abducens nerve. The posterior limit of the bone drilling is limited by the cochlea, which sits just behind and superior to the curve of the petrous carotid from the vertical to horizontal petrous segment. In patients who don’t have hearing, the posterior drilling and exposure is limited by the geniculate ganglion/facial nerve and anterior aspect of the internal acoustic meatus.

There are options for reconstruction of these defects based on a number of principles. If there is a dural defect, it should be closed in a water tight fashion. This can be done primarily when the dura does not require resection. If some of the dura needs to be resected, a piece of the pericranium can be used as a free graft. An option is to use an artificial dural substitute (there are a number of commercially available acceptable materials; the author prefers to use material that can be sutured versus on lay type material). Pericranium can also be used to repair any periorbital defects. Often the frontal air sinus will be entered with the medial orbital cut and need to be addressed. There is some variation in how surgeons manage exposure of the frontal sinus. The author prefers to remove the sinus mucosa in both the orbital osteotomy bone piece and the exposed frontal sinus. The frontal sinus ostium is pack with fat or a piece of temporalis muscle and sealed with dural glue (number of acceptable commercially available materials). The vascularized pericranial flap is place over the defect in the frontal sinus and periorbita, sealing the frontal sinus from the osteotomy bone, which is place over the pericranial flap. The vascularized pericranium is thus ‘enveloped’ between the boney cut. If more than a third of the surface area of the orbital bone required resection and is not available for the reconstruction, it is the author’s preference to replace this bone. This can be done using split calvarial graft or more conveniently Medpore bone substitute which can be plated to the bone. When the orbital / orbito-zygomatic osteotomy bone is replaced, care should be taken to avoid trapping of periorbita and orbital fat between the bone edges. The osteotomy piece is reattached using titanium plates and the predrilled holes (that were made prior to the bone cuts) for an anatomical reconstruction. The titanium plate is placed on top of and screws placed though the pericranial flap (when required to seal off the frontal sinus). The craniotomy bone pieces are also replaced using titanium plates, preferring to avoid boney deficit along the cuts anteriorly; larger deficits posteriorly are acceptable behind the hairline. The temporalis muscle is reapproximated to the superficial temporal line and the superficial temporalis fascia reconstructed with sutures, over compensating anteriorly in anticipation of some temporalis atrophy.

The retrosigmoid approach is a workhorse in neurosurgery. It allow access to the entire CP angle from the lateral foramen magnum to the tentorium/meckles cave. Although there are a number of acceptable incisions, the author prefers the gentle C shaped retromastoid incision, elevating the skin in the subgaleal (above nuchal line) and subinvesting fascial (below nuchal line) planes. The muscles are elevated subperiostealy, off the mastoid and occipital bones. Emissary veins leading to the sigmoid are often encountered and the bone needs to be waxed. The craniotomy is performed after identifying landmarks and suture lines that predict the transverse and sigmoid sinuses. The junction of the occipito-mastoid, lambdoid, and parieto-mastoid marks the transverse sigmoid junction. The bone is taken down to the flat part of the occipital bone and laterally the bone drilled out to expose a few mm of the sigmoid sinus. Mastoid air cells are waxed. CSF can be drained though a small low dural incision with gentle retraction of the cerebellar hemisphere prior to opening the reset of the dura to allow brain relaxation.

The far and extreme lateral approaches are for lesions in relationship to the vertebral artery. Lesions anterior to the vertebral artery such as anterior foramen magnum meningiomas can be resected more readily by using an extreme lateral approach and mobilizing/opening the dura anterior to the vertebral artery. This is more of an upper cervical spine exposure, working from below to above, and in front of the lower cranial nerves from below. There is no or minimal need for cerebellar retraction or manipulation.

The patient is positioned full lateral with neck straight in slight flexion.
The author prefers a large C shaped incision, extending almost to midline. Similar to the restrosigmoid opening, the skin flap is taken down in the subgaleal and subinvesting fascial planes. The para nucal muscles are taken down in layers to expose the bone and identify the vertebral artery. Often fibers of the sternocleomastod are taken down with the skin and investing fascia. The splenius capitus is detached from the mastoid and taken down based medially dissecting a plane between it and semispinalis capitus which lays underneath. The occipital artery runs between these muscles and will need to be taken. Semispinalis capitus is detached from the occiput and tuned down, taking with it few fibers to trapezius near the midline, down to the spinus process of C 2. Semispinalis is dissected off the muscles of the suboccipital triangle as it is turned down. The suboccipital triangle marks the posterior curve of the vertebral artery as it exits the transverse foramen of C1 and turns around the C1/occipital condyle. The superior oblique muscle is detached from the occiput and turned down attached at the transverse process of C1. Inferior obique is detached from c1 transverse process and turned down attached at C2 spinous process. Rectus capitus minor and major are detached from the occiput turned down on to spinus process of C1 and C2 respectively. During these exposures, there maybe significant venous bleeding which can be controlled with sugicell or more satisfactory, tissel injected into the veins through the

point of bleeding. Care should be taken to not inject too much, potentially causing venous obstruction if the injection spills into major veins. The vertebral artery may not be easily identified yet. It may be palpated laterally just above the C1 arch. The upper border of C1 is followed laterally and a depression in the upper margin marks the vicinity of the vertebral. The C1 hemi-arch is removed, identifying the dura. The lateral edge of the dura marks the pedicle of C1 and the transverse foramen can be identified by following the bone laterally. The C1 root is identified, which can be taken, then the vertebral artery within the transverse foramen is identified. The transverse foramen is unroofed and the vertebral artery mobilized under the microscope. The upper margin of the hemi-lamina of C2 is also removed. A low lateral craniotomy is preformed opening into the foramen magnum. The author prefers to perform the craniotomy into the foramen magnum using the foot plate. With the vertebral artery mobilized out of the tranverse foramen, the occipital and C1 condyle can be drilled out. The bone should be drilled until one has access to the dura anterior to the vertebral artery penetration of the dural. Care should be taken to look for branches of the vertebral artery since rarely the posterior inferior cerebellar artery can take off prior to the vertebral entering the dura (this may be identified on preop imaging). Removal of a full 1/3 of the condyle is probably safe. This is done by identifying the hypoglossal canal and the boney removal is kept posterior to it. The hypoglossal canal is more cephalad in this area then one may think. Once happy with the boney removal and exposure of the pre vertebral dura, the dura is opened linearly first behind the vertebral. Few of the dentate ligaments are taken, as is rootlets of Cl as needed. The dural around the vertebral artery is cut with a cuff and the vertebral artery is mobilized medially and held with vessel loops. This allows access to the dura of the anterior foramen magnum, working from a caudal to cephalad direction, the cranial nerves are out of the way and fibers of accessory nerve is mobilized medially with the vertebral. The only nerve that one needs to be mindful of is the hypoglossal nerve above.

Removal of these tumors off delicate structures requires more than just surgical approach and access. Skull base surgery is a philosophy. Specific maneuvers are done to reduce injury to structures including use of sharp dissection, boney decompression and minimal manipulation of structures. Surgeons need to be experts in manipulating the operating table and microscope to take advantage of the line of sight available when these exposures are made.

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