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Indian Journal of Plastic Surgery, Vol. 40, No. Supp. 1, 2007, pp. 52-59 Review Article Reconstruction in skull base surgery: Review of current concepts Trivedi NiravP, Kuriakose MoniA, Iyer Subramania Division of Reconstructive Surgery, Head and Neck Institute, Amrita Institute of Medical Sciences, Kochi - 682026, Kerala Correspondence Address: Dr. Subramania Iyer, Head and Neck Institute, Amrita Institute of Medical Sciences and Research Centre, Elamakkara, Kochi - 682 026, Kerala, India. E-mail: subu@aims.amrita.edu Code Number: pl07031 Abstract Surgical resection plays a main role in the management of complex tumors involving the skull base. Advances in reconstructive surgery has made these resections to be carried out with less morbidity and better outcome in terms of disease control and functional rehabilitation. The reconstruction methods range from utilizing local galeal tissue to distant tissues transferred as free flaps. The reconstructive methods can be grouped according to the area of the skull base reconstructed, namely, the anterior, middle or posterior skull base. The article reviews the anatomical tissue requirement at each of these subsites and the suitable method available. At the end of the article, the readers should be able to understand the anatomical and functional requirements after skull base resection and should be able to plan the reconstruction using an algorithmic approach.Keywords: Craniofacial resection, free flaps, local flaps, skull base reconstruction Complex anatomy of the region, varied and aggressive pathology, difficult surgery, complex reconstruction and need for intense peri-operative care makes skull base surgery interesting and challenging. Limited options of reconstruction at the skull base region have particularly been the rate-limiting steps in the past. Reconstruction in skull base surgery has evolved greatly in the last four to five decades with the advent of vascularised free tissue transfer. Ever since the first series of craniofacial resection was published in the 1960s, reconstructive options have evolved continuously. Initial attempts at skull base reconstruction, which consisted of closure of dural defects with skin grafting, yielded hazardous results with complications like cerebrospinal fluid leaks reaching up to 71%. [1] Subsequent techniques used local flaps like glabellar, forehead, temporalis and pericranial for reconstruction, which gave better outcome but still did not fulfil all the requirements. Inadequate volume to support the skull base after complex resection was one of main limiting factors. In the 1970s, the use of myocutaneous flaps like pectoralis major, latissimus dorsi and trapezius became popular. Though bulk was not a problem with these flaps, adequate reach at distant skull base sites was difficult at times. Necrosis of the distalmost portion of flap and detachment from primary site due to weight dragging the flap down were major problems. During the 1980s and 1990s, microvascular techniques improved and their use in skull base reconstruction became popular. With better understanding of flap design and improvement in microvascular technique, results have steadily improved. [1],[2],[3],[4],[5],[6],[7],[8],[9],[10],[11],[12],[13],[14] Neligen et al. in 1996, [8] described a single institute′s experience of skull base reconstruction in 90 patients using local, regional or free flap. The overall complication rate in the local flap and free flap group was approximately similar (38.8% vs. 33.5%). But the complication rate in the regional flap group was significantly higher (75%). They concluded that the use of a combination of local and free flap according to the requirement of an individual case probably gave best results. Anatomical and Functional Requirements of Skull Base Reconstruction The reconstructive procedures carried out for skull base resections should aim at providing the following; Seal of dura Cranio-nasal separation and support to dura Cover to carotid artery Support to orbital contents Oro-nasal separation Good cosmetic and functional outcome. Cerebrospinal fluid (CSF) leak is probably the most important postoperative complication, which leads to various central nervous system (CNS) complications and to increased morbidity and mortality. Watertight seal of dura is of utmost importance. Small rents in the dura can be closed primarily but multiple rents or larger defects need to be re-enforced. To achieve this, either local flaps like the pericranial-galea flap and temporalis fascia flap or distant flaps like fascia lata can be used. Both are equally effective in efficiently sealing the dura. Fibrin glue has also been used with these flaps to effectively seal the dura. Use of fasciocutaneous or fascia only free flaps for dural closure has become popular recently particularly in post-surgery or post-RT redo cases. After resection of skull base tumour, the cranial cavity is often exposed to the adjacent nasal or oral cavity, which serves as potent source of infection. Resulting dead space can cause haematoma and serous collection. It is advisable to separate the cranial cavity from adjacent open cavities to prevent hazardous infection-related complications. Local flaps like pericranial and temporal fascia flaps effectively do this in most cases. If the defect is larger or posterior, then distant vascularised tissue in the form of free radial forearm flap or free rectus abdominis flap is needed. These flaps provide vascularised tissue that prevents spread of infection, fills the defect and also supports the cranial base. Bony framework is rarely required to support the cranial base unless extensive area of bone is removed. The aim of bony reconstruction is mainly to give support to the cranial base and to achieve good cosmetic outcome. The removal of a major part of the orbital roof with the cribriform plate, frontal bar or malar eminence needs bony reconstruction. Outer calvarial grafts with pericranial flap or free bone grafts can be used. Titanium plate also serves the same function in certain cases. Free vascularised bone flaps are used in cases where adjuvant radiation is likely. Carotid blow-out due to spread of infection is a dreaded complication and to be avoided at any cost. It is essential to provide cover to carotid with vascularised tissue, particularly when it is exposed over a considerable length after extensive resection. Vascularised muscle flap is an ideal flap for this purpose. The orbital plate is sacrificed in maxillary tumour involving the orbital floor, orbit contents sink in if not supported and this results in diplopia. Orbital floor reconstruction is one of important aspects in post-maxillectomy reconstruction. One of the combined soft tissue-bone flaps can be used. The temporalis muscle with outer calvarial bone as a regional flap has been described. Free fibula, radial forearm with bone or rectus abdominis with rib has also been described and each has its own merits and demerits. Oro-nasal separation is important for functional rehabilitation of post-maxillectomy patients. Various methods ranging from use of obturator to soft tissue or bone flaps are described. Dental rehabilitation should be kept in mind in all cases while selecting the reconstructive option. Most patients undergoing skull base resection would receive radiation. This is an important factor in selecting the reconstructive option as non-vascularised tissue or implant-related complication rate is higher with radiation. Defects resulting from resection of skull base tumour are complex and their reconstruction is a challenging task. The aim of reconstruction in each case should be to achieve maximum cosmetic and functional result. The availability of various flaps, advancement in microsurgical techniques and improvement in functional rehabilitation has provided a sound platform. Role of Free Flap Transfer in Skull Base Reconstruction Use of free tissue transfer has a specific role in skull base reconstruction. Transfer of vascularised tissue in larger defects has significantly reduced complications. It can be used either in combination with local flaps or alone. Local flaps are ill advised when previous surgery has been performed in the same region or in previously irradiated patients and free tissue transfer is probably the only option left. Most patients receive postoperative radiation and long-term complication rates are lower with free vascularised tissue. Large complex defects needing multiple tissue components are best reconstructed with combination of flaps and a variety of free flaps are used for that. Larger defects are best repaired using free flaps. Califano et al. [15] shared their experience from Memorial Sloan Kettering Cancer centre showing good results with the use of vascularised free tissue for large defects. In patients receiving adjuvant radiation, the rate of long-term major complications is much higher in the local flap group compared to the free flap group. Heth et al. [16] compared the results of local vs. free flap use in anterior skull base reconstruction in 67 patients. The rate of late major complications was 23% in the local flap group and 0% in the free flap group. It was thought to be due to effects of radiation. Some other studies also support this finding. [17] Alloplastic Implants in Skull Base Reconstruction In the past 10 years, advances in bone implant technology have yielded several new and exciting alloplastic materials with applications to skull base surgery. [18],[19],[20],[21],[22],[23],[24],[25],[26] They include titanium plate, hydroxyapatite cement, porous polyethylene and resorbable plate fixation. Implants should be used with caution as most of these patients would receive radiation at some point and implant extrusion rates are high in these circumstances. As a rule, implants should be covered by vascularised tissue. In our experience, it is advisable to use free vascularised tissue and avoid implants as far as possible in patients receiving radiation. Titanium plates are biocompatible and can easily be adapted to fit the bony curvature. Titanium mesh is used as rigid construction for bony defect at the skull base orbital rim and frontal bone. Hydroxyapatite cement is a bone-substitute material composed of interlinked calcium phosphate chains. It is supplied as powder and liquid, which is to be mixed intraoperatively to yield mouldable gel. Once this is moulded into the required shape and left there for some time, it fixes nicely. It also acts as bone-conductive material and is said to undergo incorporation and eventually replacement by bony ingrowth. To what extent this happens is doubtful. Porous polyethylene implants are characterized by ingrowth of fibro vascular tissue and have been used for long in cranio-facial trauma cases. Resorbable plates and screws are useful for the paediatric age group where growth of skeleton is not restricted by rigid plates. External prosthesis can also be used alone or in combination with flaps to give better cosmetic outcome. Nasal prosthesis and orbital prosthesis with spectacles are used frequently. Anterior Skull Base Reconstruction Tumours of the nose and para-nasal sinus or orbit can involve ethmoids, dura, cribriform plate, fovea ethmoidalis, sphenoid sinus or frontal sinus. This requires craniofacial resection combining one of the trans-facial approaches with the trans-cranial approach. Depending on the extent of resection and structured removed, reconstruction is planned. Main focus in the anterior skull base reconstruction is to achieve watertight closure of dura and provide adequate bulk and support to the cranial base, which also separates the cranial cavity from the nasal cavity. Re-alignment of cosmetic integrity is also equally important. Orbital floor reconstruction is essential to prevent diplopia. Oro-nasal separation enables patients to take food orally. Local-regional or free tissue can be used alone or in combination to achieve all goals. Local and regional flaps Small rents in the dura can be repaired primarily but any defect needs reinforcement. For smaller defects, the most commonly used flap is the pericranial flap [27],[28] or galeo-pericranial flap [29],[30],[31] [Figure - 1],[Figure - 2]. The pericranial flap consists of periosteum and subaponeurotic connective tissue and is based on supratrochleal or supraorbital arteries, though the vascularity of pericranial only flaps is not always good. This flap can also be based laterally on the branches of the deep temporal artery. [32] Galea and part of the frontalis muscle is included along with the pericranial flap when more bulk is required and this has a better vascularity. Contour deformity of forehead or necrosis of skin can rarely occur if a galeo-pericranial flap is used. Previous surgery or radiotherapy can be a relative contraindication for use of local flaps. The flap can be passed either: (1) beneath the supraorbital rim if one-piece fronto-orbital osteotomy is used: or (2) above the supraorbital rim in the gap between the frontal bone and orbital rim, if classical anterior craniofacial resection is done. The need for multi-layer dural repair is still controversial. Larger defects at the anterior skull base can pose a theoretical risk of postoperative meningoencephalocele. More rigid fixation combining pericranial flap with other tissues and implants is described by some authors. [33],[34],[35] Gok et al. , [33] published a series of 17 patients with anterior skull base defects. He used pericranial flap "sandwiched" between two layers of non-vascularised fascia lata in a three-layered repair of dura. He noticed very good results with no CSF leak, meningitis or brain herniation. Badie et al., [34] used titanium plate with pericranial flap for reconstruction. Incidence of CSF leak was approximately 15% in this series. Sinha et al. [35] presented a series of 20 patients with anterior skull base defects. Patients had more extensive defects combining dural defect with bony defect. Five of these patients had bilateral roof defects. He used calvarial bone grafts supported with titanium plates. These structures were wrapped in a pericranial flap that was sealed at the edge of the dural defect with fibrin glue. After one year, no patients had CSF leak, meningitis or implant extrusion. Only six patients received postoperative RT in this group. Free flaps The choice of free flap is dictated by the extent of resection done, its volume and the different tissue components required. Different free flaps can offer different tissue components like muscle, bone and skin. Choice of flap can be made based on which tissue is required at which place. The use of the radial forearm flap is suitable for defects where local tissue is not available and volume or bony defects are not extensive [Figure - 3]. It is also ideal where skin of the nose and forehead is also excised. Its distinct advantages include pedicle length and pliability of tissue, can be raised as fascio-cutaneous or fascia only flap. Schwartz et al reported use of radial forearm flap in 10 cases of anterior skull base. Author reports only one early CSF leak and no other complications. [26] Another paper from the University of Michigan reported 20 cases of free radial forearm flaps in the anterior and lateral skull base that had prior surgery or radiation. Cerebrospinal fluid leak and meningitis was reported in 5% and overall complication rate was 15%. Free anterolateral thigh flap can also be used in larger defects where area required to cover the defect is more but excess volume is not required. [36],[37] It is also very useful in a complex excision where large skin paddle is required, can be used with other flaps in combination. It has better donor site morbidity than the radial forearm flap and pedicle length can be achieved up to 20 cm. Channa et al., [38] reported seven cases where free Anterolateral thigh (ALT) flaps were used for reconstruction with no CNS complications. They also harvested fascia lata to achieve vascularised layer for dural closure. The rectus abdominis myocutaneous flap is the most widely used free flap in anterior skull base defects. It provides large vascularised muscle that can be used to provide volume, support skull base and cover carotids. Large skin paddle can also be raised on various axes that are useful in complex defects [Figure - 4]. The rectus has a long pedicle and harvest is simple and simultaneous. It can be harvested with rib that can be used for orbital floor reconstruction [Figure - 5]. All these factors make the rectus an ideal flap for large volume and complex defects. A large series of 35 patients published by Teknos et al., [39] included reconstruction with free rectus abdominis flap in 20 patients. Cerebrospinal fluid leak was reported in 8.6% and meningitis in 2.9%. Endoscopic reconstruction for anterior skull base Endoscopic reconstruction of small anterior skull base and sphenoid defects (< 3 cm) has gained popularity in recent years. Defects resulting from sinus surgery, tumour extirpation and trauma are now routinely repaired with endoscopic approach using a variety of materials like septal cartilage, nasal mucosa, temporalis fascia or cranial bone. Lateral Skull Base Reconstruction Resection of extensive tumours of the parotid, external auditory canal, temporal bone, pterygopalatine space or maxillary and oral cavity tumour going into the infratemporal fossa often results in extensive and complex defects. Defects are generally complex with large volume loss and needing multiple tissue components [Figure - 6]. The main aim of reconstruction is to achieve dural closure and provide adequate vascular tissue to fill the defect and cover carotid vessels. Skin paddle is often required for external skin and pharyngeal mucosa lining. Vascularised free muscle flaps with skin paddle are ideally suited for this [Figure - 7]. For limited defects in this region, the temporalis muscle is frequently used. The temporalis system can be raised as a muscle only flap, myofacial flap including the temporo-parietal fascia and as facial flap. The outer table of the skull can be included to harvest as muscle-bone composite flap. Use of the temporalis system leaves a hollow in the temporal region. In many instances, blood supply to the temporalis muscle is damaged during resection and hence this can′t be used in reconstruction. Gal et al., [40] reported skull base reconstruction with various loco-regional and free flaps. Author did not note any complication with the temporalis flap but two of the trapezius flaps had wound-related complication. Schramm and Imola [41] reported their experience in 20 patients. Two of seven patients who underwent temporalis muscle flap developed flap necrosis. Rectus abdominis was used in six patients, gastro-omental flap in six and latissimus dorsi in one patient. Memorial Sloan-Kettering institute′s experience of 18 patients undergoing lateral skull base resection showed free rectus abdominis flap as the most common flap used (14 cases). Latissimus dorsi was used in two and ALT flap in one case. There were no major complications and the minor complication rate was 33%, which settled with conservative management. [42] Posterior Skull Base Reconstruction Defects of the posterior skull base are typically limited to the bone of the squamous portion of the temporal bone, occipital bone and the cervical vertebrae. They are usually not associated with the soft tissue. The main aim of reconstruction is to seal the dural defect wherever possible or boost it with free fat grafts when repair is not possible due to limited access. The temporalis flap is used as vascularised soft tissue if needed. Bony defects can be covered with an alloplastic material like hydroxyapatite cement. If overlying skin is also sacrificed, free rectus abdominis or latissimus dorsi flaps can be used. For extensive tumour of the cervical vertebra, median glossotomy approach is suitable for resection and vertebral defect can be bridged with free fibula bone flap [Figure - 8]. Conclusion Skull base reconstruction has evolved greatly in the last few decades with the advent of microvascular free tissue transfer. Reliable techniques in reconstruction have enabled surgeons to remove large and complex tumours at skull base regions. Free tissue transfer can provide dependable and vascularised tissue for effective reinforcement to dural closure, separating the intracranial compartment from adjacent cavities like the nasal and oral cavity, as well as having enough volume to replace the excised tissue and to provide support to skull base. It can be used alone or in combination with local tissue. In certain cases local tissue alone can be used to achieve satisfactory reconstruction. Regional flaps are rarely used now and should be reserved for cases where other options are not available. Implants and prosthesis can be used to improve the overall functional and cosmetic outcome of the procedure. References
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