Reconstruction of the Skull Base

In recent years, a variety of innovative skull base craniofacial approaches including anterior, anterolateral, and posterolateral routes have been developed to access skull base meningiomas. Recently, technical advances and scientific progress have led to a progressive reduction of the invasiveness of these approaches and the possibility to access different areas of the skull base.

In these terms, it should be noticed that the reconstruction segment of any surgical procedure at the level of the skull base represents one of the most important steps of this kind of surgery; indeed, the development of reliable reconstructive materials/techniques has definitely proceeded throughout the evolution of skull base surgery. Moreover, the constant refinements of reconstruction techniques have also improved aesthetic outcomes, thus improving patients’ satisfaction.

An effective watertight closure is mandatory to restore the natural intra- and extradural compartments and to prevent postoperative cerebrospinal fluid (CSF) leakage. Failure to create adequate reconstruction may lead to significant complications, such as meningitis, brain herniation, and tension pneumocephalus.

In the present manuscript, the materials used and nuances of techniques of reconstruction of skull base are presented and discussed, also in regard to the surgical approaches for skull base meningiomas.

17.1 Introduction

Skull base surgery represents a highly specialized discipline that gathers the expertise of different specialties, that is, ENT surgeons, neurosurgeons, radiotherapists, oncologists. In the last decades, it has been rapidly evolving, thanks to developments in surgical technique, technological advancements, and, accordingly, expanded surgical indications. A variety of innovative skull base craniofacial approaches including anterior, anterolateral, and posterolateral routes have been developed over the past years to reach deep-seated lesion, while reducing the need for brain retraction. ¹^, ²^, ³^, ⁴^, ⁵^, ⁶^, ⁷^, ⁸^, ⁹^, ¹⁰^, ¹¹^, ¹²^, ¹³^, ¹⁴^, ¹⁵^, ¹⁶^, ¹⁷ Recently, technological advances (endoscopy, neuronavigation, monitoring, intraoperative imaging) have led to a progressive reduction in the invasiveness of these approaches and the possibility to access deep lesions at the skull base through small opening, together with improved visualization and control of critical neurovascular structures.

As with any surgical procedure, closure and reconstruction in the skull base is one of the most important steps of the surgery. With the increasing use of expanded approaches and more radical tumor resections, the need for more complex reconstructions has arose. Conversely, the development of more reliable reconstructive materials/techniques has definitely participated in the renewal and expanding of certain surgical corridors (i.e., the endonasal route following the popularization of the Hadad flap). ¹⁸^, ¹⁹^, ²⁰ Moreover, the constant refinements of reconstruction techniques have also improved aesthetic outcomes, thus improving patients’ satisfaction.

To achieve a successful reconstruction after skull base surgery, the following points should be addressed: (1) isolation of the intradural compartment; (2) water and airtight closure to prevent CSF leak, pneumocephalus, ascending meningitis, and other intracranial infections; (3) obliteration of dead space; (4) coverage of critical extradural structure to avoid infection; (5) promotion of the healing process; (6) preservation and rehabilitation of function and cosmesis; (7) management of risk factors of increased intracranial pressure (ICP). ²¹

Accordingly, the reconstruction of the surgical pathways can be performed choosing different suitable, autologous, or heterologous materials that can be used in various methods, individually or combined according to the pathology targeted and the route chosen to approach it. ²² In this chapter, we will review the materials used and discuss nuances of techniques of reconstruction, in regard to the surgical approaches for skull base meningiomas.

17.2 Reconstruction Techniques and Materials

Reconstruction of the skull base after tumor removal should be devised in a way that recreates the anatomical and functional integrity of each of the compartments transgressed. The choice and planning of the approach for tumor resection should also take into account the expected skull base defect and materials and tissues that will be available to achieve an adequate reconstruction. In fact, reconstruction of defect begins during the opening stage of the surgery. An adequate skin incision, careful dissection of the soft tissues, along with temporalis muscle, and preservation of neurovascular pedicles can make closure at the end of the surgery much simpler. Avoiding overly aggressive and unnecessarily wide exposures is an important general principle. The bigger the opening, the bigger the closure!

Skull base defects can be broadly divided into extradural and transdural, the latter implying a transgression of the intradural compartment and, frequently, opening of the subarachnoid spaces. The level of intraoperative CSF leak can then be subdivided into low- and high-flow CSF leaks, depending if the communication also involves multiple subarachnoid cisterns or a ventricle.

At least three levels of interest are encountered and needs to be addressed during closure: the intradural space, the osteodural defect, and the extracranial tissues.

Prior to the repair, the surgical site to be reconstructed needs to be prepared to receive the grafts, flaps, and/or any other reconstruction material. In order to favor integration, especially of free or pedicled fascial and/or myofascial flaps, the target site should be ridden of any residual nonviable or interposed tissue (mucosa, necrotic, infected material, etc.).

Several factors should be considered when reconstructing a skull base:

Size and location of the bony and dural defects.
High- versus low-flow CSF leaks.
Prior radiation therapy or scheduled postoperative radiotherapy.
Previous surgery.
Presence of comorbidities or imaging findings that may be associated with high ventricular pressure: morbid obesity, dilated ventricles, empty sella, or dilated optic nerves.

Current options available to reconstruct the skull base bone and dura can be divided in heterologous and autologous, the latter can then be subdivided in free or vascularized grafts. Autologous grafts represent the most valuable materials as they are easily harvested from the main surgical site or eventually from distant donor sites. Above all, the ability of such tissues to rapidly interact with the surrounding physiological structures quickens the recovery of the anatomical barrier and reduces the potential for infectious complications. On the other side, the development of new materials either heterologous and/or synthetic has provided a wide array of alternative solutions that help avoid morbidity related to the second surgical site. These materials can be used in various ways, individually or in combination in a multilayer fashion. ²¹^, ²²

17.2.1 Free Autografts

Use of autologous materials is the ideal choice for reconstruction; they are perfectly biocompatible, do not provoke any immune or inflammatory response, and integrate quickly. However, they often require additional incision(s) and approach(es), are associated with potential morbidities, and, in some cases, increase postoperative pain or discomfort.

Free autografting indicates the harvesting of tissue from an autologous donor site, thereafter transferred and implanted in a recipient site. Free tissue grafts do not have their own blood supply, they thus require a well-vascularized recipient bed to allow integration.

Free autografts most often used in neurosurgical reconstruction procedures include muscle fascia (i.e., fascia lata and/or temporalis fascia), the galea capitis and/or periosteal layers, cartilage/bone and fat.

The abdominal fat is usually harvested from the periumbilical or pelvic areas and can be useful alone or as adjunct to other materials to fill the surgical cavity, dead spaces, and/or any gap created by extensive bone removal (e.g., mastoidectomy, orbital roof removal, frontal sinus opening) (▶ Fig. 17.1, ▶ Fig. 17.2).

Fig. 17.1 Fat graft used to fill the osteodural gap and the mastoid cells after a combined petrosal approach. A temporal craniotomy and a mastoidectomy are previously performed. The temporalis muscle has been overturned and the galea capitis has been preserved during the approach. (a) Artistic drawing. (b) Intraoperative picture. F, fat; GC, galea capitis; TM, temporalis muscle.

Fig. 17.2 Osteodural defect filled by the fat graft after the repositioning of the bone flap. The bone is fixed to the cranium by screw and miniplates.

Free abdominal fat may also serve as a radiological spacer to enable contrast between the soft tissues used in reconstruction and the tumor bed postoperatively. This improves the ability of the radiation oncologist to target any residual tumor and to minimize collateral radiation injury to adjacent organs at risk. ²¹ In a recent study, fat graft showed signs of progressive reabsorption. The evolution of its magnetic resonance (MR) characteristics was found to be easily misdiagnosed as residual or recurrent tumor. Besides, fat evolves into a strong scar and generates fibrosis, eventually hindering the identification and dissection of neurovascular structures. ²¹ This fact should be taken into consideration when dealing with pathology that has the propensity to recur and may need (multiple) reoperation(s), in high-grade meningiomas for instance. Fascia lata is a valid and versatile graft for dural substitution, which was adopted for various reconstructive procedures, thanks to its resilience and resistance. Anatomically, the fascia lata is constituted by the deep fascia covering the thigh muscle, forming the outer limit of its fascial compartments. Usually, it is thickened at its most lateral aspect, where it defines the iliotibial tract, a structure that runs to the tibia serving as site of muscle attachment. Care should be taken when harvesting the fascia lata as procedural errors can result in delayed donor site morbidity. Most commonly, injury to the lateral femoral cutaneous nerve can cause neuropathic pain or resection of an overly large portion of the fascia can cause muscle prolapse. ²³

For transcranial open approaches, temporalis fascia is a strong dural substitute comparable to the fascia lata. It is thicker and stronger than the pericranium. It can be harvested together with the fat located between the superficial and deep temporalis fascia. Care should be taken to preserve the frontalis branch of the facial nerve. The muscle fibers are preserved and the temporalis muscle innervation and vascularization is not impaired.

When an endoscopic endonasal approach has been performed, free autografts can be harvested from the structures encountered during nasal exposure. A bony-cartilaginous buttress can be created from the vomer and/or perpendicular plate of the ethmoid for instance. This solid barrier, used in conjunction with other materials, can be very useful in cases in which a wide craniectomy has been created or in patients with increased ICP (e.g., obese patients, suffering of sleep apnea) to prevent the occurrence of an encephalocele or meningocele. On the other side, it is worth remembering that bone autografts can undergo radiation necrosis and that they are not recommended when adjuvant radiotherapy is expected. Similarly, in cases where the middle turbinate has been removed to get more space inside one nostril, its mucoperichondrium can be used to cover the skull base defect (▶ Fig. 17.3).

Fig. 17.3 Middle turbinate mucoperichondrium covering the osteodural defect after endoscopic endonasal transsphenoidal approach.

17.2.2 Nonautologous Materials

The use of nonautologous materials, either heterologous or synthetic, for reconstruction of skull base defects is a viable alternative in cases when autologous grafts are either unavailable or not suitable for the defect. Moreover, they have the advantage of being readily available and avoid the donor site morbidity altogether. The ideal biomaterial should be safe in terms of infectious disease transmission, present a high biocompatibility, provide adequate bolstering, be malleable and easy to handle, in order to be molded into the specifically tailored shape. Moreover, nonautologous materials intended to be used as dural substitutes should be impermeable to ensure a watertight closure. Most contemporary grafts do not create severe adhesion with the surrounding tissues, which is helpful to maintain distinct anatomical planes.

Different materials, for example, collagen matrix, bovine and/or porcine pericardium, equine tendon, are currently available and widely adopted in reconstruction after skull base approaches. They can be used either as replacement for the dura or as supplemental layer over a primary closure of the dura mater.

In the vast majority of the procedures, the craniotomy flap can be positioned back in place to restore skull continuity and preserve from aesthetic deformities. ²⁴^, ²⁵^, ²⁶^, ²⁷^, ²⁸ A variety of options are available for bone flap fixation, including nonabsorbable sutures, plates and screws, clamps, etc. (▶ Fig. 17.2). Low-profile plating systems are probably preferable in the frontal, pterional, or orbitozygomatic region. In some cases, when the tumor has invaded or eroded the bone or after some skull base approaches, an artificial material for cranial bone reconstruction is required. ²⁹^, ³⁰^, ³¹ There are many options for calvarial bone substitution, including titanium, polyetheretherketone (PEEK), hydroxyapatite, polymethyl methacrylate (PMMA). ³²^, ³³^, ³⁴^, ³⁵ With some materials, the implant can be molded during the surgery, while others are custom made before the surgery based on a volumetric computed tomography (CT) scan with 1- to 2-mm slices. To determine which implant is best suited, a variety of factors should be taken into account, including the size, location, and shape of the defect, as well as the indication and cost/availability of the cranioplasty implant. ²⁹^, ³⁶^, ³⁷^, ³⁸^, ³⁹

Titanium mesh is a great example of a readily available material that can be molded during the case to fit a variety of small-sized defects. In our practice (S.F.), it is often used to supplement the bone flap in the pterional region in order to avoid hollowing in the keyhole region and compensate for temporalis muscle atrophy. Other options include calcium pyrophosphate putty or PMMA cement to fill the craniotomy lines.

Implants that are prepared before the surgery can be made using different materials, most commonly titanium, PMMA, hydroxyapatite, or PEEK. ³²^, ³³ Titanium alloy cranioplasty is easy to prepare and put in place and its use can thus shorten the operative time. ²⁷^, ³³^, ³⁸ On the other hand, it may be associated with a higher risk of postoperative infection and implant exposure. Moreover, it causes significant artefacts on MRI and CT scanner and should be avoided in tumor cases that need neuroimaging follow-up. Hydroxyapatite has high biocompatibility and a lower risk of infection. It also has osteoconductive properties that may accelerate integration. ⁴⁰^, ⁴¹ Conversely, it is more expensive and intraoperative positioning is time consuming, as fine adjustments are usually required. ³²

17.2.3 Vascularized Flaps

Over the years, along with the development of different corridors to access multiple areas of the skull base, many local and regional vascularized flaps have also been described to improve the reconstruction phase of the surgical procedure. A thorough knowledge of the surgical anatomy of myofascial and/or mucosa layers with underlying feeding vessels is crucial in order to use these vascularized grafts optimally. More recently, the use of pedicled or free flap for covering osteodural defects has been found effective in reducing the rate of postoperative CSF leakage in endoscopic endonasal surgery. ²¹ Indeed, intranasal vascularized flaps include posterior pedicled nasoseptal flap, reverse flap, inferior turbinate flap, middle turbinate flap, anteriorly based lateral nasal wall flap, and posteriorly based lateral nasal wall flap. The most popular and widely used is the nasoseptal flap (NSF) (Hadad–Bassagasteguy flap); its main features are a consistent vascular supply, based on a fairly long and robust pedicle made of the branches of the sphenopalatine artery, the ease of harvest, and the possibility of tailoring the mucosa surface according to the size of the defect. ⁴²^, ⁴³

Several other flaps have been described usually to increase the surface of the graft or to replace the NSF if it has been compromised. Among these, there is the rescue flap, a modification of the original posterior pedicle NSF, ⁴⁴ the inferior turbinate flap, ⁴⁵^, ⁴⁶ the vascularized middle turbinate flap, ⁴⁷^, ⁴⁸^, ⁴⁹ etc.

On the other side, vascularized flaps which can be used to cover defect in transcranial surgery include transfrontal pericranial flap, transpterygoid temporoparietal fascia flap, occipital galeopericranial flap, facial artery myomucosal (or mucosal) flap, along with tailored galea sheet harvested from the proximity of the approach. In multioperated cases in which an endonasal mucosal flap is no longer available, regional vascularized flaps have been developed (pericranial flap, palatal flap, and temporoparietal fascia flap) and can have good results even in complex reconstruction scenarios (active infection, CSF fistula, postradiation, etc.).

The transfrontal pericranial flap is a regional pedicled flap that can cover defects from the frontal sinus to the sella and from one orbit to the other. It is extremely useful for large anterior skull base defects. ⁵⁰ This area of pericranium has strong neurovascular supply by both the supraorbital and supratrochlear bundles.

The temporoparietal fascia flap also relies on very consistent vascular support, the anterior branch of the superficial temporal artery. It can be extended to provide a very large coverage surface. It is constituted of a strong fascial layer connecting the overlying fibrous septae of the subcutaneous tissue at the temporoparietal region of the skull. ⁵¹

17.3 Current Reconstruction Techniques

17.3.1 Transcranial Approaches

After tumor removal and meticulous hemostasis have been completed, the repair can be initiated. Reconstruction aims at covering exposed dura, preventing CSF leakage and brain herniation, and replacing surgical dead space with healthy, vascularized tissue. Careful and watertight closure of the dura should be the goal. It is worth reminding that throughout the initial exposure, pericranial, galeal, and/or muscular layers have to be identified, protected, and mobilized as they often are the most adequate tissues for reconstruction. According to the surgical route used, proper maneuvers can facilitate harvesting of these tissues at the end of the case. It is important to ensure their viability all along the procedure by avoiding desiccation and excessive traction. Similarly, carefully positioned osteotomies and bone resection can make closure easier (i.e., placement of flap). Hereafter, we will present specific considerations in reconstruction after most frequently used approaches to different skull base areas.

Anterior Skull Base Reconstruction

Reconstruction starts with careful duraplasty. Small dural defects are sutured, whereas larger defects resulting from intradural tumor involvement are patched either with pericranium and/or temporal fascia. Watertight closure is mandatory and the best primary protection against CSF leak. Lyophilized homograft dura or bovine pericardium layer can also be used to repair larger defects, but pericranium harvested locally over and around the bone flap is a very efficient closure material. The use of thin 5–0 sutures reduce the risk of leakage around the hole needle. Whether suturing is complete or not, the dural closure can by supplemented by fibrin glue on certain occasions (open ventricle, opened frontal sinuses, etc.). After resection of anterior cranial base meningiomas invading the cribriform plate, the floor of anterior fossa also needs to be reconstructed. In these cases, a flap of pericranium, pedicled on the supraorbital and supratrochlear arteries, can be wedged between the cranial floor bone and the overlying dura and secured with sutures through the bone or anchored with fibrin glue.

If the frontal, ethmoid, or sphenoid sinuses are opened during the approach or removed because of involvement with disease, communication between the paranasal sinuses and intradural space must absolutely be prevented. To do so, two main strategies have been employed: sinus “cranialization” and sinus exclusion with maintenance of adequate drainage. The main concern and the objective of both techniques are the occurrence of delayed mucocele, which happens when mucosa is isolated from its drainage path. When doing sinus cranialization, the sinus mucosa must be carefully and totally resected. Drilling of the inner surface of the bony structure of the sinus should be done with a diamond burr to make sure that all the mucosa has been resected. When the sinus has been breached during craniotomy, the mucosa in the bone flap should not be forgotten. The nasofrontal canal mucosa is the infolded and pushed toward the nasal cavity without disrupting it. A piece of temporalis muscle or fascia can be added to secure the occlusion of the nasofrontal pathway. The sinus is then packed with fat and fibrin glue and the pericranial flap is turned over the frontal sinus and extended over any defect in the floor of the anterior fossa. When the sinus is excluded, the drainage path of the sinus must first be explored to ensure that it is free from obstruction. Its mucosa is then carefully dissected and its edges are then either sutured together or coagulated to create infolding of the mucosa. Additionally, a thin layer of bone wax can then be used to isolate the sinus, but packing of the sinus should be avoided. Whenever applicable, the area of mucosa that is on the side of the bone flap should be removed when the sinus is excluded. Ideally, this should be done during the opening and before durotomy to reduce the risk of intracranial infection.

After dural closure, the bone flap is placed back into its position, usually with low-profile plates. The burr holes should be covered, either with titanium burr hole covers or with commercially available plugs. In the frontal area, craniotomy lines can sometimes be seen under the skin, which is obviously displeasing aesthetically. This phenomenon occurs when adhesions develop between the galea capitis and the dura and retraction ensures. Careful positioning of the bone flap and the use of cement, calcium pyrophosphate, or any other nonabsorbable filling agent is thus encouraged. When removal of the orbital wall and roof was necessary to manage tumor, it can be reconstructed with lyophilized cartilage grafts, collagen sponges, or equine tendon graft (Tachosil) (▶ Fig. 17.4). However, in spheno-orbital meningiomas, even when there is extensive exposure of the periorbital after resection of the tumor, when the periorbital is intact, we have not found that patient complain of pulsatile exophthalmos. When the periorbital is opened or disrupted, its reconstruction is mandatory, with the use of thin sutures (6–0) that progressively reapproximate the margins, combined with a layer of equine tendon graft (Tachosil).

Fig. 17.4 Anterior skull base reconstruction after a cranio-orbital approach. (a) Collagen sponge as a sustainer of the skull base defect. The periorbital fat is visible after the drilling of orbital rim and roof and the opening of the periorbita. (b) Collagen sponge is covering the periorbital fat, while fibrin glue is injected on the dura matter.

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