The skull base is an extremely complex anatomical region in which a variety of heterogeneous tumor types can occur, with a range of clinical and biological features. Tumor types include congenital, inflammatory, and vascular lesions, as well as benign and malignant tissue growths (neoplasms). They may be intradural, extradural or both, but all occur in an area of the body that requires a multidisciplinary approach to treatment.

The cranial base is classically divided into the anterior, middle and posterior segments. The tumor's location within these segments will dictate the surgical approaches one can utilize to remove the tumor and minimize morbidity, regardless of the tumor type. 

The anterior base extends from the frontal sinus to the lesser wing of the sphenoid and the planum sphenoidale. The greater wing of the sphenoid forms the anterior wall of the middle skull base while the posterior limit is the clivus. Laterally, it extends to the posterior surface of the petrous bone.

NOTE: Access osteotomies are not utilized for approaches to the posterior skull base and are not discussed in this article.

Axial and coronal CT scans are necessary and useful for delineating the integrity of the bony architecture of the paranasal sinuses and skull base. Magnetic resonance imaging (MRI) delineates the soft tissue planes between tumor and vital adjacent structures, distinguishes tumor from mucous and inflamed mucosa, and is critical in assessing tumor respectability. These imaging studies will help the surgeon design a surgical approach that encompasses all of the tumor's margins.

In addition to precise imaging, operating microscopes and instrumentation as well as better anesthetic and post-operative care techniques have allowed these tumors to be resected safely. The surgical approach employed to obtain adequate exposure of the skull base may be extracranial, intracranial, or in many instances, a combination of both.

Tumors that arise from, or extend to the base of the skull pose a significant challenge. The surgery of tumors in this area can be difficult because of restrictive access, distortion of normal anatomy and proximity of vital structures. In general, when tumors penetrate the cranial fossa anteriorly (nasal roof, cribiform plate, frontal lobes), fewer vital neurovascular structures are involved, and cure rates are higher than tumors which invade near the cavernous sinus and/or carotid artery.

The goal of the skull base surgery should be to provide optimal exposure to the tumor so that all of it can be safely excised producing a minimal amount of deformity. When the lesion is malignant, surgery must be designed to excise the entire tumor with a margin of healthy tissue. Anything less guarantees a high incidence of local recurrence even with adjunctive postoperative radiation and/or chemotherapy.

Optimal surgical access depends on matching the anatomic location and pathologic character of the growth to the versatility of the surgical approach. In considering exposure, the following general factors are relevant.

1. Take the shortest route from the skin to the target, bypassing and/or displacing structures essential for functional survival of the patient.
2. Use pre-existing or potential ‘surgical’ avenues if possible.
3. Use osteotomies through the facial and skull bones to reduce brain retraction and improve exposure to the lesion.
4. Identify and preserve adjacent vital neurovascular structures.
5. Consider previous incisions in relation to vascularity of flaps.
6. Control the blood supply to the growth (neoplasm) to reduce bleeding.
7. Plan access in relation to the reconstruction so that the dura can be repaired and separation of the intracranial and extracranial cavities can be achieved.
8. Respect cosmesis and preserve function.
9. Consider the possibility of future surgery.
   
   

Access to the skull base is achieved via a coronal incision from above and a transfacial and/or transoral incision from below. With the advent of endoscopic techniques to the skull base, inferior access can be achieved in many cases without the need for facial incisions.

Prior to the surgery, in most cases, a lumbar drain is placed in the patient to allow for release of cerebro-spinal fluid during the procedure to minimize brain retraction. For tumors of the anterior cranial fossa, a bi-coronal incision is preferred from pre-tragal area to pre-tragal area about 15 cm above the nasion instead of following the hairline. This permits a greater length of pericranium to be available for reconstruction and camouflage of the incision in hair bearing tissue. The scalp is elevated forward to obtain exposure of the orbits, nasal roof, the zygoma and zygomatic arches.

Care must be utilized to preserve the frontal branches of the facial nerve as well as the sensory branches to the forehead. 

The underlying pericranium is mobilized to the supraorbital foramen (preserving supraorbital blood supply) and is used later to resurface any anterior fossa defect. To reach the planum sphenoidale with this flap, adequate length is achieved by elevating the flap in a subgaleal plane posterior to the bicoronal incision for 3-4 cm and raising the pericranium anteriorly from there. The periorbita at the roof and upper parts of the medial and lateral walls are elevated in continuity with the pericranium. If present, the supraorbital foramina are converted into notches by fracturing the floor with a small osteotome and the neurovascular bundles are carefully dissected inferiorally with the periorbita. The temporalis muscles are elevated from the anterior portion to the temporal fossa leaving a small cuff for their reattachment. Transfrontal access to tumors penetrating to and through the cribiform plate is established traditionally via a frontal craniotomy alone, or with additional osteotomies.

A bifrontal craniotomy can be made in one or two pieces and there are no natural lines of osteosynthesis as the craniotomy is dictated by the amount of exposure necessary to remove the tumor. Typically the inferior extent is through the frontal sinus. After tumor extirpation the frontal sinus is cranialized and the frontal bone is fixated with miniplates, either 1.3mm or 1.5 mm.

Frequently, in order to get optimal exposure to the planum sphenoidale and the optic chiasm, an extended frontal approach is used to minimize brain retraction and increase the angle of viewing. In these situations, after the frontal craniotomy has been performed, a second series of osteotomies is made to remove the fronto-orbital bar. This is designed along the lines of osteosynthesis at the fronto- zygomatic suture laterally and the nasion medially. Osteotomies are made through these areas and then completed utilizing a high-speed drill from lateral to medial across the orbital roofs to meet anterior to the cribiform plate. The osteotomy is completed by making a horizontal cut with a saw from outside, across the nasion, at the level of the anterior ethmoidal arteries. The entire piece of bone is dislodged and removed with the assistance of a small osteotome. Once the tumor is removed and the skull base is sealed, the frontal orbital bar is replaced and fixated at the nasion and across the fronto-zygomatic suture with mini plates. The frontal bone is replaced as described previously.

Although the anterior subcranial approach was initially developed in 1978 for anterior skull base trauma, indications now include access for resection of a variety of skull base tumors. It differs significantly from the above-mentioned approaches in that it allows for broad exposure of the anterior skull base from below as opposed to the transfrontal superior route. Osteotomy of the frontal-nasal-orbital external skeletal frame provides optimum anterior access to the orbital and sphenoid planes and into the nasal and sinonasal cavities, while avoiding frontal lobe retraction and external facial incisions. Similar to the transfrontal approaches, a bi-coronal flap is raised in the standard fashion. Two types of osteotomy techniques have been described. In the Type I technique, an osteotomy of the fronto-nasal segment leaving the posterior frontal wall intact to be removed in a second step is performed. This procedure is indicated if the tumor involves a posterior frontal sinus wall. The external skeletal frame is removed after dissection of the posterior wall under direct vision, safely and far enough from the tumor borders. In the Type II technique, a one-stage removal of the fronto- nasal segment, including the posterior frontal sinuses is done. This is useful when tumors do not involve the posterior frontal sinus wall or have major intracranial extension. The osteotomy borders can be extended cranially and laterally to include segments of the orbital roof as necessary. Prior to making the osteotomies and depending on the size and the extent of both the lesion and the frontal sinus, the outline of the naso-frontal segment is planned.

Titanium miniplates for subsequent bone fixation are adapted and drilled so as to allow for proper adaptation of the bone plate once the tumor is removed. Bilateral ethmoidectomies and sphenoidotomies are performed to allow exposure of the ethmoidal roof and central skull base from below. Accurate delineation of tumor margins and dural excision are facilitated by this exposure and oncologic resection takes place. Appropriate dural reconstruction is then performed and closure proceeds as in the previously described approaches.

Another access osteotomy which can be used alone or in combination with approaches from the frontal route is the Le Fort I maxillotomy. Although this has been well described for correction of dental-facial deformities, it is particularly useful for clival tumors and juvenile nasal angio-fibromas. An upper buccal incision is made from pre-molar to pre-molar and dissection continues directly to the maxilla. The anterior face of the maxilla is exposed with particular attention to the lateral margin of the pyriform aperture and the zygomatic buttress region which will provide for the lines of osteosynthesis and fixation. With careful protection of the nasal floor, an osteotomy is made through the anterior face of the maxilla, at least 5 mm above the root apices to the junction of the maxilla and pterygoid plates. Miniplates are applied at the lateral aspect of the pyriform margin and zygomatic buttress on each side. The holes are drilled and screws placed. The plates are then removed and the osteotomies through the medial maxillary wall and pterygoid plates are done to allow for down fracture of the maxilla, exposure of the tumor, and it’s ultimate removal. Once the tumor is resected the maxilla is replaced and the pre-bent plates are reapplied to allow for proper fixation of the maxilla and maintenance of proper occlusion. Although transfacial maxillotomy procedures have been described, the advent of minimally invasive techniques has limited their current utilization.

For more laterally based tumors and those extending to the middle crania fossa, (cavernous sinus, parasellar area optic chiasm) access via orbitozygomatic osteotomies with a pterygonal craniotomy can be done. These fronto-lateral approaches incorporate the orbital roof and provide greater access to tumors underneath the frontal and temporal lobes. With modifications of the cranial bone flap (i.e. changing the position and size of the flap), incorporation of a posterior orbitotomy and/or zygomatic osteotomy, the cranio-orbital approaches can be specifically tailored to optimize the exposure of a given tumor. As in the anterior frontal, a bi-coronal or hemi-coronal incision can be utilized. To protect the frontal branch of the facial nerve and expose the zygomatic arch, an incision is made in the superficial temporal fascia 1-2 cm above the zygomatic arch and dissection continues through the deep temporal fascia to the superior border of the zygomatic arch. The periosteum is then elevated to expose both the arch and the body of the zygomatic bone. The zygomatico- mandibular portion of the temporal muscle is incised along the lower border of the zygomatic arch and the arch itself is undermined. The temporalis muscle is then detached from the lateral rim of the orbit and lateral skull so that it can be retracted posteriorally. Utilizing an oscillating saw in the temporal fossa an osteotomy is made along the zygomatico-sphenoid suture. This osteotomy begins at the base of the zygomatic process of the frontal bone and extends downward to the inferior orbital suture. A second osteotomy is made at the posterior extension of the zygomatic arch, and finally a third osteotomy is made at the body of the zygomatic bone 1 cm posterior and parallel to the zygomatical maxillary suture. Titanium mini plates are placed across the osteotomy sites and holes re-drilled to allow for proper anatomic replacement of the segment after the tumor is removed. Now the infratemporal fossa is completely exposed. The craniotomy can be performed before or after this osteotomy to allow adequate tumor removal. Once the tumor is extirpated and the dura is sealed, the orbitozygomatic and cranial bones can be replaced and the plates applied. The temporalis muscle is then fixated in its proper anatomic position and the wound is closed.

Craniofacial resection of skull-base tumors has a high degree of morbidity which can be related to the approach, resection, and repair, either alone or in combination. The general morbidity rate is 1.5% - 3.9% but can be as high as 12% for advanced malignancies which require extended skull base dissection. Central to many of the approaches is dissection of the peri-orbital tissues, and protection of the orbital contents is paramount. In addition, significant neurovascular structures are encountered, particularly as one approaches the cavernous sinus and absolute knowledge of this anatomy is necessary to protect these structures. Retraction of the frontal and / or temporal lobes is necessary to a degree in every resection. Choice of the approach for each particular tumor can allow for minimization of retraction injuries.

Equally as critical as the approach and resection of skull base tumors is the repair and reconstruction. Specifically the dura must be repaired or restored to reduce the risk of CSF leak and pneumocephalus (2-14% of patients). In addition, separation of the sinonasal cavity from the intracranial cavity will minimize the risk of meningitis (1-5% of patients). Vascularized tissue in the form of a pericraniocranial flap, pedicle muscle flap, or free-tissue transfer will minimize these risks in every case. Accurate fixation of the bone segments will allow for accurate anatomic reconstruction of the craniofacial skeleton and maintain proper occlusion when tooth-bearing segments are osteotomized. Proper re-draping and suspension of soft tissues is necessary to achieve an optimal aesthetic result. Persistent nasal crusting is common and is easily treated with self-administered nasal saline irrigations.

With these approaches “unresectable” is now a relative term in the management of skull base neoplasms. The surgery must be offered, however, with the clear understanding by the patient for the potential perioperative complications that can occur as well as a realistic assessment of the quality of life that may be achieved.