Reconstruction of the skull base : a review of personal techniques.
With increasing technological and surgical sophistication in cranial base and craniofacial surgery, reconstructive efforts are challenged to provide a reliable means of compartmentalisation. Some improvised methods of basal reconstruction with vascularised pedicle flap are described in this presentation. The dependable blood supply, long length, ease of harvesting and the ability to alter the arc of rotation of the described flaps make them versatile for lining skull base. The flaps may be used to cover defects, may be folded for bulk and may be used to carry blood supply to poorly vascularised recipient sites. The techniques are presented.
Reconstruction is an integral part of any neurosurgical operation and must be adequately planned and performed. Preoperative planning of the reconstruction methodology and suitably altering the operating steps is crucially important for success of the surgical procedure. Adequate reconstructive procedures are necessary to provide for rigid compartmentalisation after an elaborate skull base operation.,,, There are various inherent problems involved in skull base reconstruction. Some of these are listed as follows:
1. The proximity of the skull base to potentially infective spaces of paranasal sinuses, nasal, oral and pharyngeal pathways and external ear canal. Postoperatively it is important to seal off the cranial cavity from these spaces to avoid ascending infections.
2. The basal dura is relatively thin, friable and densely stuck to the bone. Approximation of edges and water tight suturing often may not be possible, especially in areas surrounding or near vessels and nerve transits.
3. Occasionally, following a surgical procedure, the site of CSF fistula cannot be deciphered. It is pertinent, therefore, that all the potential sites of CSF leakage be recognised during the surgery and adequately taken care off.
4. Some tumours involve the basal bone and dura, and for achieving a radical resection, these structures have to be removed elaborately. Opening up of paranasal air sinuses, middle ear cavity and eustachian tube, can subject the patient to risks of cerebrospinal fluid fistula.
5. Frequently, there are large dead spaces that need to be filled in after basal bone, soft tissue and tumour resection.
6. Many skull basal procedures are of long duration. Many surgeons are involved in the operation and extensive instrumentation is used. All these factors add to the risk of infections.
7. The reconstruction begins at the end of a relatively long operation when the operative team may be exhausted and errors of omission may be made. Some prefer plastic and reconstructive surgical team to assist in the reconstruction.
Sometimes persistent post-surgical or traumatic cerebrospinal fluid fistula can pose a formidable surgical challenge and super-added infection can be a life threatening condition. Familiarity with various methods of reconstruction and their satisfactory performance can be a gratifying surgical procedure. In the following discussion, some methods of reconstruction are described. These methods are relatively simple to perform and plastic and reconstructive surgical team's help is unnecessary.
Reconstruction of bone defects
Reconstruction of bone defects following skull base surgery is a controversial subject. All bone defects do not require reconstruction. The principle purpose of reconstructing bone defects is to avoid possible herniation of the brain matter into the aerodigestive tracts, ear, orbit, or into the space resulting following a large tumour resection or after treating basal encephalocele. The possibility of such an eventuality is rare and the decision regarding reconstruction of basal bone should be a calculated one. In the presence of intact basal dura, even large bone defects can be tolerated without any consequence. One third to half of the orbital roof can be removed without any problem of pulsating exophthalmos. Large clival, petrous and sphenoid bone defects can safely be left unrepaired. Often it is safer not to perform bone reconstruction at the end of a long surgical procedure for the fear of infection and consequently complicating the entire surgical procedure.
Bone defects should preferably be reconstructed in the following situations:-
1. Larger defects over the tegmen tympani and orbital roof should preferably be replaced with bone as the dura in this region is relatively thin and the region is deprived of large basal cerebrospinal fluid cisterns. Consequently, the brain pulsations can be transmitted initially and later the brain can herniate into these spaces.
2. Support for brain tissue may be necessary where large bone defects are created following the operation.
3. Reconstruction of the bone is also carried out to support the dura and help in the provision of water-tight sealing of the base and avoiding formation of cerebrospinal fluid fistulae.
4. Protection of the brain from future injury and restoration of a reasonable appearance and quality of life are other important objectives.
The size of the defect and its site, extent of the resection of the associated structures, history of previous operative procedures and endovascular and radiation treatment are important variables that determine the appropriate reconstructive procedure. Various methods of reconstruction of basal bone have been described. Reconstruction of bone defects in the cranial base should preferably be done with the help of bone. In sterile fields, free small or large bone pieces can be used for this purpose. Ribs, iliac crest and scapular grafts have been successfully used. Acrylic and metal plates can also be used in such a situation. However in potentially infective fields which are more frequently encountered after a skull base operation, use of a bone flap based on a vascularised pedicle is recommended.
Vascularised osteomyoplastic flap : Various experimental studies have been conducted on the evolution of temporalis osteomuscular flaps compared with that of calvarial free bone grafts.[6-8] The studies confirmed that vascularised bone flaps remain viable and are characterised by normal evolution, while the free bone grafts show typical signs of necrosis and resorption. Such vascularised bone flaps based on a muscle pedicle have been used in cosmetic facial surgery and mandibular, maxillary and palatal reconstruction and mastoid cavity obliteration.,
The temporalis muscle has an extensive blood supply to the calvarium through multiple perforators. Elevation of the bone flap with wide attachment to the muscle pedicle results in a well-vascularised flap [Figure. 1a] and [Figure. 1B]. Vascularised bone flap can also be based on muscles of the nape of the neck. Judicious use of split cranial grafts can fill up the resulting defects in the skull. Osteomyoplastic flaps can be used for reconstruction of middle fossa and petrous bone defects with relative ease. The fascial layers covering the temporalis muscle can be used to seal the defects in the basal dura while the muscle and bone occupy the area of bone defect. Unlike the pericranial and galeal flaps which can be raised either before or after the surgical resection, osteomyoplastic flaps, on most occasions, have to be planned and fashioned in the exposure stage of the operation. The incisions and dissection in the muscle and fascial planes are modified to avoid transection of blood supply which could be crucial to sustain the bone flap.
Pericranium based bone flaps., Pericranial and galeal flaps have been described for reconstruction of anterior cranial base and craniofacial deformities. The pericranium comprises of an outer layer of loose areolar tissue and an inner layer of osteoblasts and contains extensive vascular network. The pericranium derives blood supply anteriorly from the supratrochlear and supraorbital arteries and laterally from the superficial temporal arteries. The pericranial layer pedicled flaps can be based on either of these vessels, and accordingly rotated anteriorly or laterally. The pericranium can sustain the calvarial flap by means of multiple small, vertical perforators. Studies have shown that the calvarial flaps can safely be pedicled on the pericranial layer or galeo-pericranial layer.[13-16] Bone flaps of the calvarium can be either of full thickness or only of the outer table. A split thickness osteo-galeopericranial flap, as shown in the [Figure. 2a] and [Figure. 2B], can be used to reconstruct the anterior cranial fossa base. The bone piece can be pedicled on a large base of pericranium and can be additionally nourished by the galeal flaps. Whenever necessary, the bone flap can be turned upside down so that the bone surface is not directly exposed to the paranasal sinuses. The flap can even be sandwiched between the pericranial layers. The bone piece can be fractured into two or more pieces retaining its attachment to the pericranium so that its contour can be adjusted to suit the local environment.
Long vascular pedicle composite cranial flap. The flap described is a split or full thickness cranial bone flap based on the pericranial layer which receives its vascular nourishment from the temporalis muscle and its overlying fascial layers. The outer table of the skull bone is split, preserving the overlying pericranium. A part of the temporalis muscle (and fascial layers) along with the pericranium is elevated and rotated as required [Figure. 3]. The superficial fascial layer over the temporalis muscle is a part of the pericranial aponeurosis whilst the deep temporal fascial layer completely invests the superficial aspect of the temporalis muscle. The temporalis muscle and its fascial envelope and the pericranium receive their blood supply from the anterior, middle and posterior deep temporal and superficial temporal arteries. Subperiosteal elevation of the temporalis muscle along with the fascial layers preserves the integrity of all the major vascular supply. The split or full thickness cranial bone based on pericranial layer and the temporalis musculofascial layer as described, results in a viable flap with abundance of vascular supply. Vertical splitting of the temporalis muscle into half or one-third would still preserve at least one or two of the main feeding vessels. This splitting would not adversely affect the circulation to the flap and could help in preservation of the function of the temporalis muscle. The skull bone is thicker superior to the temporal line which marks the attachment of the temporalis muscle. Splitting of the skull bone is easier in the paramedian parietal bone due to the well formed diploic channels. The split thickness graft can be broken into pieces and can be suitably contoured, preserving the attachment of the pericranium.
Use of bone dust and debris : With the use of modern craniotomes for making burr holes, a large amount of bone dust can be obtained. The bone dust can be flattened and placed in the form of a sheet over the site of defect. Small bone chips and bone debris which are generally available during a craniotomy can be placed interspersed in the bone dust. This forms a template over which new bone and fibrous tissue is laid and a firm, strong and resistant barrier is formed.
Reconstruction of skull base with free bone flap : Whenever possible, bone defects should be filled up with a vascularized pedicle bone flap. However, such a vascularized bone flap is sometimes not available and in this situation a free bone flap may be required. A free bone flap should not be directly exposed to the paranasal sinuses or mucosal surfaces. Whenever such a free bone flap is placed in the skull base, it should be as small in size as possible and care should be taken that it is covered on both sides with vascularised and viable soft tissue. Such a 'sandwich' free bone flap can be placed between muscle and fascial-pericranial layers. Post-operative collection of haematoma around the bone flap should also be avoided and,
whenever necessary, external drains can be placed. Tenting scalp stitches can be used whenever possible and indicated.
Placement of soft tissue in place of bone, although not consistent with the generally taught principles of reconstruction, can form a firm barrier of fibrous tissue and serve the purpose in cases of basal bone defects. Rotation of a vascularised pedicle muscle, pericranial or fascia flap or free fat graft or muscle can be performed to occupy the area of bone defect and empty spaces.
Subgaleal preservation of bone flaps : Convexity and basal bone flaps can be preserved in some rare situations like brain swelling, potentially infective operative field and other such conditions and can be replaced to the original site after a period of time. Various techniques of preservation of bone flaps, both inside and outside the body, have been described. The flaps have been more frequently and successfully stored in the subcutaneous plane of the abdominal wall or in the thigh. External storage methods include deep freezing, autoclaving it while replacing, storing after radiation treatment and using antibiotic solutions. An alternative method of preservation of the skull bone flap in the subgaleal space was found to be effective. The subgaleal plane adjacent to the site of operation is exposed widely using blunt dissection. The bone flap is placed in this plane, taking care that the curvature of the bone flap does not excessively elevate the scalp or cause tension over the scalp edges. In cases where larger bone flaps are placed subgalealy, the curvature may necessitate fracturing of the bone flap in two or more pieces. The relative avascularity of the subgaleal plane helps in limiting the rate of reabsorption of the bone. The subgaleal space can be easily exposed widely, providing a large area for bone placement. Frequently the curvature of the bone flap matches with that of the adjacent bone which assists in the ease of placement. The flap is placed and replaced to the original site in one operative field. As no additional incision is necessary, the procedure is quick to perform.
Reconstruction of the dural and soft tissue defects
Fat and free muscle as a packing material : Small and large pieces of fat have been effectively used for this purpose. The fat globules get vascularised early from the surrounding tissues. Whenever free muscle is being used as a packing material, it should be ground into small pieces so that these pieces can get vascularised from the surrounding tissues. If a large piece of muscle is placed, it gets vascularised from the periphery, and the central portions of the tissue can get necrosed. Such a necrosed tissue can get absorbed or even pose a problem of infection. A free tissue should never be placed over a free tissue as this procedure can lead to ischaemic necrosis of the tissue most distant from the vascularised structures.
Lyophilised dura and fascia lata : These dural substitutes have been used extensively and successfully for basal reconstruction. However, whenever possible, a live and vascularised pedicle fascial material may be used to obtain a compact closure.
Use of pericranial and galeal flaps : Pericranial and galeal flaps have been extensively used for reconstruction of basal defects. Their use is more effectively possible for reconstruction of bone and dural defects in the anterior cranial base.
Use of rotation of muscle flap : Temporalis muscle can be effectively rotated to cover middle fossa defects. Sternocleidomastoid and other muscles of the nape of the neck can be rotated superiorly to cover defects in the posterior cranial basal defects.
Extended vascularised temporalis muscle-fascia flap : The deep layer of the temporalis fascia is everted partially over the temporalis muscle preserving its vascularity. A composite flap comprising of temporalis muscle and its fascia attached along its superior border, is rotated for reconstruction [Figure. 4b]. Partial eversion of the temporalis fascia over the muscle increases the length of the flap while retaining its vascular pedicle. The long length vascularised pedicle flap can be used to cover defects, may be folded for bulk and may be used to carry blood supply to poorly vascularised recipient sites. The deep layer of the temporalis fascia is nourished by multiple small branches which traverse through the muscle. Partial eversion of the deep layer of fascia with an attempt to preserve small vascular connections can result in a long vascularised flap.
Muscle-fascia flap : The temporalis muscle along with its fascial coverings can be rotated along with the pericranium to form a long flap. The fascial cover of the temporalis muscle can be everted to form a thick peripheral flap.
Extended subgaleal fascia - pericranial temporalis flap : A long pericranial flap can be used to line an anterior skull base defect. The temporalis muscle is rotated along the pericranial flap and forms the vascular pedicle of the flap. Subgaleal fascia is preserved to enhance the vascularity of this long flap. Subgaleal fascia, known previously as 'loose areolar tissue layer' of scalp and believed to be a relatively avascular zone, has now been shown to have its independent and extensive blood supply entering into it at the base of the cranial vault circumferentially. Casanova et al called the same layer as innominate fascia. Superficial temporal artery and its branches which supply the galea also give minute but many branches to subgaleal fascia which is closely approximated to the pericranial layer. The blood supply to the subgaleal fascia is also enhanced by the vascular connections with the branches of the deep temporal arteries which supply the temporalis muscle and its fascial coverings. Pericranial layer and subgaleal fascia are continuous laterally with the superficial layer of the temporalis muscle. Pericranial layer is supplied by the perforating branches of the superficial temporal artery which traverse the subgaleal fascia and those of deep temporal arteries which traverse through the temporalis muscle and its fascial envelope. Preservation of the subgaleal fascia and subperiosteal elevation of the temporalis muscle along with the fascial layers preserves the integrity of the major vascular supply to the pericranial layer. It is presumed that such a twin source of vascular supply could sustain a long length of pericranial flap. Although laterally based pericranial flaps are described for covering anterior central skull base defects, successful usage of such a flap taken beyond midline has not been reported. While elevating the pericranial flap some of subgaleal fascia is almost always included. Specific attempt to preserve this layer by conscious and careful dissection from galea in an attempt to preserve additional vascular supply to the pericranial layer could be critically important in long pericranial flaps, as described.
Multilayer reconstruction of middle fossa base : The procedure involves use of an osteomyoplastic flap which is placed over one of the two described types of temporalis muscle-fascia flap. Elevation of the pericranial layer along with the temporalis muscle and fascial layers as shown in the [Figure. 4a] results in a long viable flap with abundance of vascular supply. In situations where this flap is sufficiently thick it can be used as the primary flap. Whenever the pericranial part of the flap is not very thick, or for some reason cannot be harvested, a muscle-fascia flap (described earlier) can be rotated. The peripheral part of the flap now has three fascial layers comprising of pericranium and superficial and deep layers of temporalis fascia. This makes the distal end of the flap relatively thick. The proximal end of the flap is formed by the full thickness of the muscle. The result is a long and thick flap. The full thickness or anterior or the posterior part of the split temporalis muscle can be used to harvest a vascularised pedicle osteomyoplastic flap [Figure. 4a]. This forms the secondary flap. Split or full-thickness bone can be used. The bone piece can be fractured into two or more pieces retaining its attachment to the muscle so that its contour can be adjusted to suit the local environment. The primary flap consisting of either of the two types of muscle-fascial flap is first placed over the site of middle fossa bone defect [Figure. 4b], over which the secondary osteomyoplastic flap is placed [Figure. 4c]. This forms a compact and multilayered construct of vascularised pedicle flaps. Whenever it is felt that bone replacement over the site of the defect is not essential, only soft tissue flaps could be used.
Multilayer reconstruction of the anterior cranial fossa floor : Multiple combinations of flaps can be used to cover anterior cranial basal defects. The flaps could comprise only of soft tissues [Figure. 5] or may include a bone piece. Bicoronal incision is taken and scalp flaps are reflected both anteriorly and posteriorly in the subgaleal plane. The pericranium over the frontal bosses is relatively thin and harvesting an intact pericranial flap could be difficult. Whenever such an elevation is possible, the frontal pericranium can be rotated over the defect in the anterior cranial base. Everting the posterior part of the scalp results in a wide exposure of the pericranium over the parietal or even occipital region. Harvesting a long pericranial flap based on the temporalis muscle and its fascial cover from both sides and rotating them over the anterior cranial fossa defect can result in a multilayer closure [Figure. 5a], [Figure. 5B]. The long length of the flaps can be useful in placing the flaps loosely or even doublebreasting it over the defect.
Whenever it is felt that addition of bone is necessary for the adequate reconstruction, a vascularised pericranial based bone flap as described earlier can be used. The split or full thickness bone flap can also be pedicled on a laterally based pericranial flap. A long pericranial flap is harvested from the contralateral side as shown in the [Figure. 5b]. Both these pericranial flaps (one containing bone piece) are then rotated along with the temporalis muscle and its fascial coverings. On both sides the temporalis muscle can be split vertically. On one side the anterior portion of the split muscle is used as the base of pericranial flap over anterior parietal region, while on the other side the posterior portion of the split muscle forms the base of the pericranial flap over the posterior parietal region. A specific attempt is made to preserve the subgaleal fascia to enhance the vascularity of the designed flaps. Both these flaps are rotated anteriorly and laid down one over the other in the region of the defect in the base. The vascularised pedicle bone flap is placed superior to these flaps so as to avoid direct exposure of the bone to the paranasal sinuses.
Use of outer layer of dura as a pedicled flap : Cranial dura is formed by two layers - the outer endosteal layer and the inner meningeal layer. These layers are well defined and 'separable' by manual dissection, particularly in younger individuals. The dura is supplied by multiple small and medium sized arteries circumferentially. The meningeal blood vessels are largely located in the endosteal layer. When preserved intact, the outer endosteal layer can be rotated and used to cover defects in the proximity. The principal advantage of using such a material is that it may be used as a vascularised pedicle flap or a free graft. The consistency and quality of the material matches with that of adjacent dura. Local availability and ease of rotation of this flap are the other advantages. Despite the limitations in using this flap due to technical difficulties in separating the layers, it can be useful in an occasional patient.
Tenting scalp stitches : Post-operative collections underneath the scalp sometimes can be a source of secondary complications such a wound breakdown and infection. Such complications are particularly relevant after a long skull base operation. The problem can occur after an osteoplastic craniotomy when the collection develops between the galea and the pericranium, or following free bone flap exposure in which case the collection is between the pericranial layer and the bone flap. Suction drains and tight head dressings are frequently used at the time of closure to avoid this problem. Occasionally, there is a need to drain the collection and approximate the scalp layers by a large bore needle and sometimes lumbar drainage of CSF may be necessary. Suction or gravity drains often become clogged, or only approximate the layers of the scalp in a limited area. They cannot be used for long periods because of the risk of infection.
'Tenting' sutures of the dura are routine after every craniotomy. These stitches help to prevent the occurrence and expansion of extradural collections. Tenting scalp stitches are similar in principle and action for the scalp. This is a simple and non-time consuming technique. The sutures help to seal surgically created space between the layers of the scalp and act as internal compression. The closer approximation of the scalp to the bone improves the vascularity of the bone by providing an additional pathway for blood supply in the initial post-operative phase.