“Endoscopic endonasal skull base surgery has dramatically changed and expanded over recent years due to significant advancements in instrumentation, techniques, and anatomic understanding. With these advances, the need for more robust skull base reconstructive techniques was vital. In this article, reconstructive options ranging from acellular grafts to vascular flaps are described, including the strengths, weaknesses, and common uses.”
Key points
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This article describes an array of options for skull base reconstruction.
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Techniques used for acellular grafting, cellular grafting, and vascularized flap reconstruction are described, as well as benefits and limitations of each.
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A standard approach to patient management, from preoperative evaluation to postoperative care is also described.
Introduction
Endoscopic skull base surgery has become increasingly complex over recent years. As approaches to the skull base have expanded, reconstructive options have broadened and diversified. A multitude of reconstruction techniques are discussed in the literature. Most recently, vascularized grafts have been used for reconstruction. As endoscopic techniques have expanded to include large intradural and even intra-arachnoidal surgery, combinations of these reconstructive options have been used in tandem.
Introduction
Endoscopic skull base surgery has become increasingly complex over recent years. As approaches to the skull base have expanded, reconstructive options have broadened and diversified. A multitude of reconstruction techniques are discussed in the literature. Most recently, vascularized grafts have been used for reconstruction. As endoscopic techniques have expanded to include large intradural and even intra-arachnoidal surgery, combinations of these reconstructive options have been used in tandem.
Treatment goals and planned outcomes
The primary goal of the reconstructive surgeon is to provide a watertight separation between the sinonasal tract and intradural space to prevent postoperative cerebrospinal fluid (CSF) leak, thereby decreasing the risk of devastating sequelae like pneumocephalus and/or meningitis while promoting timely and uncomplicated wound healing.
Preoperative planning and preparation
Before surgery, careful consideration of the tumor characteristics, including its type, proximity to other structures, and expected surgical defect, must be undertaken. In addition, patient factors that could affect postoperative healing must be considered, including other underlying health problems, smoking history, prior radiotherapy, and obesity.
Patient positioning
Patients undergoing endoscopic endonasal skull base surgery, either extradural or intradural, are largely managed in a standardized fashion with few very specific modifications that are outside the scope of this discussion. Once general anesthesia has been established, meticulous positioning and preparation are undertaken before the commencement of the procedure. In a select group of patients, including those with known elevated intracranial pressures, morbid obesity, and/or those in whom large dural defects with resultant high-flow CSF leaks are expected, consideration for placement of a lumbar drain before starting the procedure should be undertaken. With the evolution of skull base surgery over the past decade, it became common to have lumbar drains placed before surgery for CSF diversion. It was thought, from experience with open cranial cases, that diversion would relieve pressure in the setting of postoperative edema. However, like all interventions, lumbar drains came with a unique and separate set of risks and complications, including headache, meningitis, tension pneumocephalus, and herniation. The literature reports a 3% risk of major complications, and 5% risk of minor complications associated with lumbar drains. Because of this, several recent studies have been performed to assess the need for lumbar drains in the preoperative period for endoscopic skull base resections. Garcia-Navarro and colleagues reviewed 46 cases in which 67% of patients had lumbar drains placed. Only 2 patients had postoperative CSF leaks, and they found no significant relationship between lumbar drain usage and postoperative CSF leak rate. Ransom and colleagues retrospectively reviewed 65 patients who had lumbar drains placed at the time of surgery. They found a postoperative CSF leak rate of 6.2%, whereas their lumbar drain complication rate was 12.3%, and recommended very judicious us of lumbar drains to avoid further complications. Because of this, the use of lumbar drains should be restricted to only very specific patients at the discretion of the surgeon.
Once the decision for lumbar placement has been made and performed, the bed is then turned 90° away from the anesthesia team. Next, at the discretion of the surgeon, the patient may remain flat or be placed in a modified beach chair position, with the head of bed elevated and feet lowered. A degree of reverse Trendelenberg also may be used to optimize positioning. It is our standard practice to not place the patient in pin immobilization, but scenarios exist when this immobilization is used, particularly when a combined endonasal and transcranial approach is required. Finally, depending on the proposed reconstruction, additional required surgical sites (eg, abdomen, lateral thigh, and scalp) are prepped and draped in the standard fashion to facilitate surgical access. Once this is completed, the image guidance system is brought into the field and positioned in the standard fashion following patient registration.
Reconstruction
Grafts
Skull base defects may be repaired using acellular or cellular free grafts. Acellular grafts, such as those composed of a noncellular dermal matrix, and cellular grafts, such as mucoperichondrium/mucoperiosteum, fat, dermal fat, or fascia, have been described. These techniques were initially adopted from work that repaired CSF leaks resulting from endoscopic sinus surgery or trauma. As endoscopic endonasal surgery developed, free grafts were expanded to be used for larger dural defects.
Acellular grafts may be used in an inlay and/or onlay fashion. In cases with dural resection, a collagen matrix (Duragen; Integra Life Sciences, Plainsboro, NJ) is often used as an inlay graft. This graft, laid either between dura and the osseous skull base (epidural plane) or brain and dura (subdural plane), should extend approximately 5 to 10 mm beyond the dural margins in all directions and is used to obliterate dead space and in many cases abate any CSF flow. This can be followed by an onlay graft or flap. For ease of discussion in this section, we focus on onlay grafts, recognizing that the surgeon’s preference dictates onlay technique. Acellular dermal matrix (AlloDerm Life Cell, Branchburg, NJ) also may be placed in the epidural plane or subdural plane. The graft is prepared by hydration in normal saline and is sized to extend beyond the edges of the defect. If the size of the defect prevents an inlay graft or prior inlay technique has already been used, the acellular dermal matrix may be used as an onlay graft after complete removal of underlying mucosa. It is vital that all mucosa be removed to prevent any mucocele formation.
Cellular grafts may be derived from a multitude of locations. Free mucosal grafts may be taken from any site in the nose, but in our practice is typically taken from the middle turbinate that is often removed during the initial approach to the skull base to facilitate a bimanual technique. Used as an onlay graft, this is placed onto the skull base defect after mucosa has been cleared from the bony ledges. The graft provides an excellent scaffold for wound healing; however, the small size limits its use in larger skull base resections. Other cellular grafting techniques include the abdominal free fat graft. To harvest, a small incision is made in the periumbilical region, right or left lower abdominal quadrant, or the lateral hip. The required volume of fat is then circumferentially dissected, removed, and placed in saline solution until the completion of the extirpative portion of the procedure. Fat is typically used to help obliterate space, thereby creating a laminar skull base defect. It may be used in conjunction with other reconstructive techniques further described in other portions of this article. Recently, dermal fat grafts have come into more frequent use. To harvest, an elliptical incision is performed and carried through the dermis. Before proceeding with fat removal, the epidermis is removed, thereby leaving the dermis attached to the underlying fat. The required volume of fat is then circumferentially removed while keeping the dermis in continuity. The use of dermis with fat allows for improved manipulation of the graft in situ and the ability to create a more laminar surface for the use of subsequent multilayered reconstruction. The harvest of fat or dermal fat grafts adds a second surgical site and adds potential donor site complications, including hematoma formation, seroma formation, and/or wound infection; therefore, meticulous sterile technique and multilayer closure should be used.
Flaps
More robust repair of large intradural and intra-arachnoidal defects often requires vascularized flap reconstruction. Vascularized reconstructive techniques began with the development of the Hadad-Bassagasteguy flap in 2006. This flap, more commonly known as the nasoseptal flap (NSF), has become the workhorse of large skull base defect repair. Pedicled on the posterior septal artery, the NSF is composed of mucoperiosteum and mucoperichondrium and is characterized by a long robust pedicle, making it extensively mobile along the skull base. The size of the NSF also can be enlarged with extension onto the nasal floor, allowing it to span from orbit to orbit and from sella to frontal sinus. In some cases, the need for a nasoseptal flap for reconstruction is not known at the beginning of the case. In these cases, to preserve the vascular supply to the NSF before sphenoidotomy, a nasoseptal “rescue” flap may be elevated. This technique has also been shown to reduce healing time and overall postoperative recovery.
Nasoseptal flap
After standard positioning, the nasal cavity is prepared with the placement of 0.05% oxymetazoline-soaked pledgets and thorough decongestion is allowed to take place. To assist in the approach, the inferior turbinates are outfractured bilaterally and the middle turbinate ipsilateral to the side of NSF harvest is removed. Attention is then turned to the flap elevation. The flap is harvested based on the anticipated size of the defect and is typically overestimated to ensure proper coverage. Using needle tip electrocautery, 2 parallel incisions are made. The superior incision is made 1 to 2 cm from the most superior portion of the septum to preserve the olfactory epithelium. The inferior incision is made across the posterior choana below the floor of the sphenoid sinus, and extends along the nasal floor ( Fig. 1 ) This incision can be modified to make a larger flap. A vertical incision is then placed to connect the 2 previous incisions at the level of the head of the inferior turbinate. This can be extended as far as the mucocutaneous junction. All incisions should be completed before undertaking elevation of the NSF to prevent tearing. Elevation of the NSF is performed using a Cottle elevator or suction dissector, and is started anteriorly. This is done with care to maintain the integrity of the flap without creating perforations. The flap is elevated posteriorly back to the sphenoid face with preservation of the vascular pedicle ( Fig. 2 ) The flap is then placed in the nasopharynx or ipsilateral maxillary sinus for preservation until the reconstruction portion of the procedure. Should the need for the NSF be unknown at the beginning of the case, a nasoseptal “rescue” flap may be performed. For the rescue flap, partial harvest is done at the beginning of the case. In these cases, the superior incision is performed extending from the sphenoid os to the superior aspect of the septum, approximately 1 cm below the cranial-most aspect to prevent damage to olfactory filaments. This incision is extended 2 cm anteriorly, in contrast to the NSF previously described. A Cottle elevator is then used to reflect the flap inferiorly to expose the sphenoid rostrum, while protecting the vascular pedicle should a flap be required for reconstruction later in the case. Once protected, a posterior septectomy may be performed and the skull base surgery may be completed. On completion of the extirpative portion of the procedure, attention is then turned to multilayer reconstruction of the skull base defect ( Fig. 3 ) If a rescue flap was used and the flap is required for reconstruction, completion of the flap is undertaken as detailed previously.
