Summary
This chapter describes the multitude of reconstructive options, both endoscopic and open, that have been created in the last twenty years to adapt to the expanding role of endoscopic endonasal surgery for a wide variety of skull base defects and lesions. These reconstructive techniques include grafts, local and regional vascularized flaps, and microvascular free flaps. Indications, advantages, and disadvantages of each reconstructive option are discussed. Details of techniques for harvest and placement of each type of reconstruction are outlined. Patient selection, postoperative care, potential complications, and current data detailing outcomes of these reconstructive options are also described.
7 Reconstruction of Skull Base Defects
7.1 Introduction
In the past 20 years, endoscopic endonasal surgery has expanded to become an approach for resection of both extradural and intradural skull base lesions. The goals of this approach are to completely resect skull base tumors with negative margins and minimal morbidity, as well as to reconstruct residual skull base defects in a reliable manner. These methods of reconstruction must be watertight if they are to prevent postoperative complications, including cerebrospinal fluid (CSF) leak, meningitis, pneumocephalus, and death.
Reconstruction of the skull base directly correlates with the type and extent of surgical defect that has been created. In the past, these reconstructions were performed primarily using cellular or acellular grafts. However, as endoscopic techniques have expanded and the sizes of these residual defects have increased, such grafts have been unable to provide adequate closure. Accordingly, the harvest of vascularized tissue was explored as an innovative option for reconstruction. Examples of vascularized tissue used in both endoscopic and open techniques include the nasoseptal flap, pericranial flap, and turbinate flaps.
Thus, as the indications for endoscopic skull base resections have expanded, the need for advanced endoscopic reconstructive techniques has increased as well. In this chapter, we discuss various options for endoscopic reconstruction, including cellular and acellular grafts, as well as vascularized regional flaps. In addition, we address open techniques for skull base defects that cannot be repaired endoscopically due to either failure of prior reconstruction or anatomic limitations.
7.2 Goals of Reconstruction
Skull base surgeons must keep in mind several goals of reconstruction when formulating their surgical plan.1 The primary objective is to separate the cranial cavity and brain from the sinonasal tract while also providing an adequate seal. In addition, the reconstructive choice must protect the brain and intracranial neurovasculature from infection and desiccation. By providing this watertight seal, the reconstructive technique can also help accelerate the healing process. Dead spaces that have been created should be obliterated. Finally, both function and cosmesis should be preserved and/or restored. With these key aspects of reconstruction in mind, the skull base surgeon must choose a proper technique that achieves these goals.
7.3 Types of Defects
The nature of the skull base defect itself is a key factor in the decision of the proper reconstructive technique. First, defects can be categorized by size. Small defects are usually less than 1 cm in size, whereas larger defects are greater than 3 cm in size. For small defects, acellular grafts and nonvascularized tissue grafts are usually adequate for reconstruction. For medium-sized defects, local flaps are a more suitable option. For larger defects, regional or free flaps are generally used.
Next, defects are characterized by their extension.1 , 2 Skull base defects can be intradural or extradural. Whereas extradural defects involve resection of skull base with intact dura, and thus without leakage of CSF, intradural defects involve violation of dura and can be divided into two groups: extra-arachnoidal and intra-arachnoidal. By definition, cases of intra-arachnoidal defect involve CSF leak. The distinguishing factor between high- and low-flow CSF leak is whether a cistern was directly opened.
7.4 Patient Selection
The decision to perform endoscopic skull base surgery requires careful patient selection and preoperative planning that includes examination of multiple tumor characteristics, including size, extent, location, and relationship to surrounding structures and neurovasculature. During this planning stage, reconstructive options are addressed. Patient factors that can lead to poor healing outcomes include prior radiation and smoking. Additionally, intracranial hypertension and obesity can increase the difficulty of the reconstruction. Certain defects, depending on their location and size, can be associated with a greater risk of CSF leakage, and the chosen reconstructive option must address this.
7.5 Endoscopic Techniques
After smooth induction of anesthesia and endotracheal intubation, the patient is positioned appropriately for both resection and reconstruction. One of the main concerns to be relayed to the anesthesia team is that of avoiding intraoperative hypertension and hypotension. The patient is kept supine, the head of the bed is slightly elevated, and padding is placed around the heels and elbows to prevent postoperative peripheral neuropathies. All potential reconstructive donor sites are exposed. If a lumbar drain will be used, the neurosurgery team should place it during this preparatory time. Next the image guidance system must be set up and the patient registered into the system. By this point, CT and MRI images should already be loaded. Axial, coronal, and sagittal cuts must be visible and linked. Finally, all surgical sites are sterilely prepped and draped.
7.5.1 Acellular and Cellular Grafts
Acellular and cellular grafts generally have a role in most skull base procedures, but they can be used alone for small defects. Acellular grafting materials can be made from a collagen matrix (Duragen, Integra Life Sciences) or dermal matrix (AlloDerm, LifeCell). Duragen is usually placed in an inlay fashion, either in the epidural or in the subdural plane. To prevent CSF leakage, the graft should be placed 0.5 to 1 cm past the dural margin. If the resection includes areas where there are limited bony edges, or if an inlay graft cannot be placed, AlloDerm can be placed in onlay fashion in the subdural or epidural plane after removal of all underlying mucosa. Multilayer reconstruction and bolstering techniques are described hereafter. The intranasal edges of the AlloDerm are usually bolstered by oxidized cellulose (or absorbable packing), with sequential application of biologic or synthetic glue and absorbable gelatin sponge.
Another graft option is the use of cellular tissue for repair. Examples of cellular grafts include free mucosal grafts, abdominal fat grafts, and dermal fat grafts. Free mucosal grafts are very commonly used and can be taken from the nasal floor, septum, or middle turbinate. The advantage of this graft is that there is no need for a second donor site, so donor site morbidity is rare. The middle turbinate is used quite often for moderate-sized skull base defects. The mucosa of this structure is stripped and then used as a graft. It is placed onto the skull base after the mucosa has been cleared from bony ledges. Although it provides a scaffold for healing, its small size limits its use in larger skull base resections. Another cellular graft that is commonly used is the abdominal free fat graft. Often employed as a bolster of biologic dressing in a multilayered reconstruction technique, it can also be used to obliterate spaces such as the clival recess or a nasopharyngeal defect after tumor resection. Options for donor sites include the periumbilical region, the right or left lower abdominal quadrants, and the lateral hip. Harvest is performed by making a small incision in any one of these regions and circumferentially dissecting an appropriate volume of fat. The specimen is then placed in saline prior to use. Often the dermis is taken along with fat in what is termed a dermal fat graft. To harvest this, an elliptical incision is performed and carried through the dermis without violating the fat. The epidermis is removed, and the specimen is again circumferentially dissected to remove the desired volume of fat while keeping the dermis in continuity. Although these cellular grafts have shown great value in small and sometimes moderate-sized defects, their use in larger resections (i.e., greater than 3 cm in size) has been associated with a higher rate of postoperative CSF leakage, ultimately prompting development of the innovation of vascularized reconstruction.3
7.5.2 Nasoseptal Flap
The nasoseptal flap is the primary workhorse for most skull base reconstruction surgery. It was first described by Hadad et al in 2006, in whose retrospective review of 43 patients who underwent endoscopic reconstruction of large anterior dural defects only 5% had postoperative CSF leakage.3 The vascular supply of this flap is the posterior septal artery, which is a branch of the sphenopalatine artery. The extra length provided by this pedicle allows use of this flap for multiple types of defects. The flap itself is composed of mucoperiosteum and mucoperichondrium.
Harvest of the nasoseptal flap is initiated at the beginning of the case prior to tumor resection because of the location of the pedicle (Fig. 7.1). The flap must be harvested and the vascular supply protected prior to sphenoidotomy and septectomy. First the inferior turbinates are outfractured bilaterally and the ipsilateral middle turbinate is excised. To protect the olfactory epithelium, the superior harvest incision is made 1 to 2 cm from the most superior portion of the septum. The inferior incision is made below the floor of the sphenoid sinus and across the posterior choana. This incision should extend along the nasal floor. Both the superior and inferior incisions are connected with a vertical incision at the level of the head of the inferior turbinate. All incisions should be made prior to elevation of the flap, both to prevent tearing and because orienting the tissue and maintaining tension can be difficult once it has been elevated. Elevation of the flap should begin anteriorly with either a Cottle elevator or suction dissector. The flap is elevated posteriorly to the sphenoid face. After ensuring that the vascular supply is intact following elevation, the nasoseptal flap can be placed in the nasopharynx or ipsilateral maxillary sinus to prevent inadvertent damage during the ablative portion of the procedure.
One of the major advantages of this flap is that there is no need for a second surgical site. However, a significant disadvantage is that it needs to be harvested prior to surgical resection, particularly the sphenoidotomy and septectomy portions of the procedure. To circumvent this problem, the “rescue” technique, or partial harvest, was created.4 In this technique, the superior incision is made extending from the sphenoid os to the superior aspect of the septum, about 1 cm below its cranialmost aspect. The incision is then extended 2 cm anteriorly, after which the flap is reflected inferiorly using a Cottle elevator to expose the sphenoid rostrum. The vascular pedicle is protected during this process. Once the partial harvest has been completed, the posterior septectomy and remainder of the resection can be performed without compromising flap viability. Once the flap is placed in proper orientation and position, it is supplemented with a multilayer bolster.
7.5.3 Posterior Pedicled Inferior Turbinate Flap
The nasoseptal flap is not a viable option for skull base reconstruction in certain patients, such as those who have had prior septectomy or wide sphenoidotomies. In these instances, the vascular supply to the nasoseptal flap has been compromised and an alternative solution is required for endoscopic reconstruction. One option is the posterior pedicled inferior turbinate flap (PPITF), which is based on the inferior turbinate artery.5 This artery arises from a branch of the posterior lateral nasal artery (PLNA), which itself arises from the sphenopalatine artery. The PLNA travels in a descending course over the perpendicular plate of the ascending process of the palatine bone. It gives off a branch that supplies the middle turbinate. The remainder courses inferiorly and enters the inferior turbinate at its superolateral attachment. It then pierces the bone and soft tissue before giving off several branches.
The PPITF should be harvested ipsilateral to the defect. The flap has a much smaller volume than the nasoseptal flap, so the entire inferior turbinate should be harvested to ensure adequate coverage. Harvest begins by identifying the sphenopalatine artery as it exits the sphenopalatine foramen. This is followed distally until the PLNA branches. Two parallel incisions are then made—one just superior to the inferior turbinate and the other following the caudal margin of the turbinate. A vertical incision at the anterior head of the inferior turbinate is made to connect the two parallel incisions. Next the mucoperiosteum of the inferior turbinate is elevated from anterior to posterior. Preserving the vascular pedicle as it enters the superior aspect of the lateral attachment of the inferior turbinate and the PLNA is essential.
When placing the flap, any nonvascularized tissue between the margins of the defect and the flap must be removed. Biologic glue can then be applied, followed by a multilayer bolster. A 16 Fr coudet catheter is placed to provide pressure against the PPITF. A limitation of this flap is its smaller coverage area, which is approximately 60% of the anterior cranial fossa per prior endoscopic analysis.6 To compensate for this, the flap can be supplemented with free grafts. Bilateral flaps can be used for larger defects. Another disadvantage is the shorter length of the flap and its pedicle, as well as its restricted angle of rotation. Thus this flap is more useful for posterior defects of the sellar, pansellar, and midclival regions.
Related posts:
5 Anesthesia and Intraoperative Monitoring of Patients with Skull Base Tumors
4 Head, Neck, and Neuro-otologic Assessment of Patients with Skull Base Tumors
10 Management of Benign Skull Base Tumors in Neuro-oncology: Systemic Cytotoxic and Targeted Therapies
9 Clinical Applications of Radiotherapy for Skull Base Tumors
8 Radiotherapy for Skull Base Tumors: Principles and Techniques
15 Pediatric Skull Base Surgery
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