With the advent of new technologies and better understanding of the anatomy, skull base defects have become increasingly larger and more complex to reconstruct. Several options now exist from avascular free grafts to large axially perfused local-regional flaps. Some of the vascularized options are harvested traditionally through open approaches, but several endonasal options are available. The concept for both remains the same: a watertight closure to reconstruct the barrier separating the intracranial cavity from external spaces and the aerodigestive tract and prevent postoperative complications. Most vascularized flaps arise from external carotid artery branches, except for the pericranial flap which receives vascular supply from branches of the ophthalmic artery, itself a branch of the internal carotid artery. Reconstructive choice depends on the defect location, size, surgical approach, disease process, patient factors, and surgeon’s preference. Ideally, surgeons will be comfortable with several options to offer tailored reconstruction for optimal results.
Key wordsEndoscopic – reconstruction – skull base – vascularized flap
4 Vascular Supply of Local-Regional Flaps in Skull Base Surgery
4.1 Key Learning Points
Reconstruction of skull base defects is needed to create a protective barrier around the cranial cavity.
Extranasal flap options include: anterior pericranial flap, temporoparietal fascial flap, temporalis muscle flap, facial artery musculomucosal (FAMM) flap, and occipital pericranial flap.
Endonasal flap options include: nasoseptal flap, inferior turbinate/lateral nasal wall flap, and middle turbinate flap.
Flap selection depends on the underlying pathology, patient co-morbidities, prior therapy, approach extension, and defect location and size.
Reconstruction of the skull base is critical to re-establish a barrier between the cranial cavity and the sinonasal tract and to prevent postoperative cerebrospinal fluid (CSF) leaks. Failed reconstruction can lead to pneumocephalus, meningitis, abscess formation, and ventriculitis. Significant advances in technology and the advancement of endoscopic endonasal techniques have required endonasal reconstruction of larger and more complex skull base defects. Reconstructive techniques have also evolved, and multiple options are now available to skull base surgeons. The reconstructive algorithm depends on defect size and location, surgical approach, pathologic diagnosis, patient factors, and prior therapy. Small defects (<1 cm) of the ventral skull base can be repaired with >90% success rates with multilayered nonvascularized free grafts. 1 Larger dural defects or those that are exposed to high-flow CSF leaks were found to have CSF leak rates of 16.7 and 16.2% with endoscopic endonasal approaches and open craniofacial resections, respectively. 2 , 3 Vascularized tissue was found to significantly reduce CSF leak rates in such high risk defects. 4 This chapter presents the most commonly used vascularized reconstructive flaps in transcranial and endoscopic endonasal skull base surgery. ▶ Table 4.1 presents the major characteristics for each flap.
4.3 Extranasal Reconstructive Flaps
For transcranial approaches, vascularized scalp flaps are effective at re-establishing separation of the intracranial space. Preoperative surgical planning of scalp incisions aids in the harvest and preservation of the vascular supply for these reconstructive options. With open approaches, primary repair of the dural defect with suturing of a fascial graft and inset of the flap facilitates a watertight seal. For reconstruction of endonasal skull base defects, extranasal flaps have the advantage of being harvested at distance from the primary pathology. This is most significant for malignant sinus and skull base tumors where involvement of local tissues can jeopardize the vascular supply of a flap or compromise oncologic resection. Furthermore, if the patient has received prior radiation therapy to the nasal cavity or skull base, the flap and its vascular supply are typically beyond the radiation field. 5
4.3.1 Anterior Pericranial Flap
The anterior pericranial flap (PCF) receives blood flow from the supraorbital and supratrochlear arteries, both branches of the ophthalmic artery. It is the only locoregional skull base reconstructive flap that receives its blood supply from the internal carotid artery (ICA). It can be harvested unilaterally or bilaterally depending on defect size and arc of rotation and can be extended posteriorly beyond the bicoronal skin incision if needed. A standard PCF combines the periosteum and the superficial loose areolar connective tissue. A galeo-pericranial flap also includes the galeal layer but is rarely employed due to increased risk of overlying necrosis. When using anteriorly based scalp flaps, it is advisable to preserve blood supply to the anterior scalp to reduce the risk of skin necrosis, especially in previously irradiated patients. In these patients, the bicoronal scalp incision should be planned to preserve the parietal branch of the superficial temporal artery (STA) for increased vascular supply to the frontal scalp. 6 PCFs are traditionally utilized for skull base defects from the frontal sinus to the planum sphenoidale. This flap can be inserted intracranially via a frontal craniotomy, and its utility in endonasal surgery is increasingly noted. It can be transferred endonasally below the plane of the skull base (extracranially) through an osteotomy at the nasion (Fig. 4.1). Care must be taken during this step not to twist the pedicle and compromise the vascular supply to the flap. Endoscopic-assisted harvesting of the PCF has been described and has the potential for reduced morbidity. 7 Disadvantages of the PCF include the external incision and delayed mucosalization of the flap which leads to prolonged nasal crusting, especially with postoperative radiation therapy (see ▶ Table 4.1).
4.3.2 Temporoparietal Fascial Flap
The STA, a branch of the external carotid artery (ECA), provides blood supply to the temporoparietal fascial flap (TPFF). The STA is accompanied by one or two veins, both superficial or within the temporoparietal fascia. The TPFF extends in a fan-like fashion superficial to the temporalis muscle fascia, and is in continuity with the galea aponeurotica. 8 Preoperative planning of the scalp incision is important to preserve this flap and its STA supply as the flap must be dissected during incision. Significant disadvantages of this flap are risk to hair follicles with subsequent alopecia, the need for transposition through the pterygopalatine fossa to reach the ventral skull base, and risk to the frontal branch of the facial nerve. 9 , 10 It can also lead to devascularization of the anterior scalp in patients who have had previous surgeries or irradiation. Fig. 4.2 presents an example of a TPFF and a PCF, both harvested in the same patient for reconstruction of a complex skull base defect.
4.3.3 Temporalis Muscle Flap
The temporalis muscle receives vascular supply from the deep temporal branch of the internal maxillary artery which has a main anterior and posterior division deep to the muscle. This flap provides robust blood supply and bulk but has a limited arc of rotation and is mostly used for reconstruction of lateral skull base and orbital exenteration defects. The entire muscle can be rotated for reconstruction, or it can be divided into an anterior or posterior component to limit cosmetic temporal depression without compromise of its blood supply.
4.3.4 Occipital Pericranial Flap
The occipital artery supplying the pericranium has a convoluted course that usually originates just deep to the insertion of the posterior belly of the digastric muscle. Preserving a wide vascular pedicle base instead of dissecting the artery itself may prevent injury. Reconstructive surface of up to 4 cm × 11 cm can be harvested and transposed to reconstruct lateral and posterior skull base defects. Alternatively, the flap can be rotated below the mastoid tip and tunneled deep to the mandible and through the PPF into the nasal cavity to reconstruct mid-clival defects. 11
4.3.5 Facial Artery Musculomucosal (FAMM) Flap
Initially described in 1992, the FAMM flap receives vascular supply from retrograde flow of the facial artery. 12 It is harvested intraorally and tunneled through the gingiva-buccal sulcus into a maxillary antrotomy (Caldwell-Luc). 13 Once in the nasal cavity, it can be rotated to reconstruct anterior, middle, or posterior cranial fossa defects. 14 , 15 , 16 The strong vascular supply of the FAMM flap makes it ideal to cover areas of skull base osteoradionecrosis. The disadvantages of this flap are its thickness, pliability, and potential for intraoral morbidity (trismus, oronasal fistula, harvest bed dehiscence). Although its use in oral cavity defect reconstruction is well accepted, only limited data supports its use in skull base reconstruction. 5 , 14 , 15 , 16 , 17
4.4 Endonasal Reconstructive Flaps
Endonasal skull base defects are conveniently reconstructed using endonasal techniques. Most small, low-flow defects (<1 cm) can be repaired with free nonvascularized grafts with >90% success rate. 1 Larger or high-flow defects are more challenging to reconstruct, and the use of vascularized tissue as a reconstructive barrier has been shown to significantly reduce the risk of postoperative CSF leak when compared to nonvascularized grafts. 4 Although options are limited to the nasoseptal flap (NSF), inferior turbinate/lateral nasal wall flap, and middle turbinate flap, the harvested flap size can be tailored to the surgical approach to optimize dural reconstruction.
4.4.1 Nasoseptal Flap
Also called the Hadad-Bassagasteguy flap, the NSF is vascularized by the posterior septal artery, a branch of the sphenopalatine artery. 18 , 19 The vascular supply to the flap can be assessed intraoperatively, in both primary and revision surgery, with Doppler ultrasonography or indocyanine green fluorescence angiography (Fig. 4.3 and Fig. 4.4). 20 Its long, robust, and centrally located pedicle allows it to reach ventral defects of the anterior, middle, and posterior cranial fossa. The flap can be extended to include the nasal floor to increase the reconstructive surface area. This is generally supplied by two arteries that extend from the nasal septum to the nasal floor (Fig. 4.5). Care must be taken during the surgery to protect the flap from traumatic injury, especially when a high-speed drill is used in proximity to the pedicle. The NSF is preferentially elevated on the side opposite to powered instrumentation (drill, ultrasonic curette) to avoid trauma. In some circumstances, the side of flap elevation is dictated by the approach. For example, a transpterygoid approach classically entails lateralization of the pterygopalatine fossa contents and therefore requires elevating the flap on the contralateral side.
The NSF remains the workhorse of endonasal skull base reconstruction as it is versatile, pliable, and has a long pedicle that provides strong vascular supply. The potential disadvantages of this flap are loss of vascular supply from prior endonasal surgery and potential microscopic tumor invasion in cases of endonasal malignancy. In addition, the flap or its pedicle must be harvested or preserved throughout the surgery. This places the pedicle at constant risk. The potential for early increased nasal morbidity remains controversial; however, its use in skull base reconstruction was not found to significantly increase long-term postoperative morbidity. 21 Initial concerns of reduced olfaction have been disproved with olfactory mucosa sparing techniques. 22 Several strategies to expedite septal healing have been described: mucosal or fascial grafts, silicone sheet splinting, and reverse septal mucosal flap (Caicedo flap). 17 , 23 , 24