Clinical Applied Anatomic Study Of Endoscopic Expanded Endonasal Skull Base Surgery

BackgroundThe characteristics of lesions locating at the midline skull base include their deep positions, adjoining crucial neurovascular structures. To treat these lesions with conventional craniotomy needs the sequelae of a skin incision, facial bone flap or craniotomy, and brain retraction, and even destroying of normal structures which are sometimes vital to people, to access the lesions. The procedure usually results in extensive injury and massive bleeding. All the same, when entering into the skull base where the lesion located, it is often be obstructed by some crucial neurovascular structures. So, it’s difficult to expose the lesion sufficiently. Adding surgeons have to remove the lesion between the crucial neurovascular structures, it is difficult for surgeons to completely resect it. As a result, the possibility of damaging the neurovascular structures and tumor recurrence after operation increases.Expanded Endoscopic Endonasal Approach (EEEA) for skull base surgery was developed in the past decade, which illuminated the operating field by a rigid endoscope, resected lesions in the midline skull base by a transnasal approach. EEEA uses a natural surgical corridor such as the nasal cavity, sphenoid sinus, ethmoid sinus to access lesions located at the central skull base and remove them. The approach’s characteristic is that the corridor is direct, by a transnasal approach, through simply burring the thin bone of skull base, can access the lesions. Avoiding an extensive skin incision, facial bone flap or craniotomy compared with conventional microscopic craniotomy. Furthermore, avoidance of cerebral lobes retraction is another advantage of this approach, and it also avoid encounter of crucial neurovascular structures when accessing the lesions. As a result, the possibility of damaging the neurovascular structures decreases, and the morbidity decreases either. A rigid endoscopes,4 mm in diameter, is used when performing EEEA. The endoscope can be placed in proper position to have a better view, adding angled endoscopes can be used to detect corners of the operating field, in operating, the surgeons can have a panoramic view and resect lesions accurately and thorouthly, leading a lower recurrence rate. As the above advantages, the EEEA is preferred by more and more neurosurgeons and otolaryngologists.However, the surgical approach also has shortcomings. Firstly, anatomical structures observed under endoscope are different from that observed under microscope and naked eyes. An endoscope produces 2-D image which interfere with proper depth perception, so the image is distorted. Adding that the neurosurgeons are not familiar with the ventral skull base anatomical characteristics, all these form an obstruction to extensively use EEEA in clinic. Secondly, lesions on the skull base often involve the bone and dura of skull base and even brain tissue, resection of these will lead to bone and dura defects, which communicate the sterile cranial cavity with the unsterile nasal cavity, will lead to cerebrospinal fluid leak and intracranial infection. Thirdly, the EEEA is performed through the two nostrils. The outer opening of the approach, the two nostrils, is small and narrow, which limits maneuverability during operation. So, the surgeons must have skilled surgical techniques and be familiar with the ventral anatomical structures of skull base.This study from the perspective of expanded endoscopic endonasal skull base surgery, aims at the shortcomings of EEEA, performs a cadaveric anatomical research. We hope it will make the surgeons to know of the characteristics of the approach, anatomical landmarks in the surgical areas, and reconstructed method of skull base defects, so as to make it easy to master the approach and develop its advantage. Objectives:1, To study the limitation of exposure, anatomical landmarks and relationships with adjacent structures of the ventral skull base exhibited by EEEA, provide these as elementary knowledge to surgeons to widely spread the using of EEEA.2, To study the vascularization in nasal cavity so as to explore the feasibility of posterior pedicled inferior turbinate-nasoseptal flap (PPITNF) which based on the posterior lateral nasal artery, composed by the mucoperiosteum and mucoperichondrium of the inferior turbinate, lateral nasal wall, nasal floor, and nasal septum.3, To study the feasibility to reconstruct the extensive skull base defect resulted from EEEA using PPITNF.4, To help surgeons to locate the intracranial anatomic landmarks when performing endoscopic endonasal tanssphenoidal surgery using measurements based on 3-D CT reconstruction images. Materials and Methods:1, Five human cadaver heads were dissected imitating the extanded endoscopic endonasal approach for skull base surgery. Ⅰ, preparation of human cadaver heads: Five formalin fixed adult cadaveric heads were prepared, the arteries were injected with red latex, the nasal cavities were cleaned with swabs. Ⅱ, imitating EEEA for skull base surgery:Rod-lens endoscope with 0 degree lens was inserted into the right nostril to identify the anatomic landmarks included the nasoseptum, inferior turbinate, middle turbinate, superior turbinate, choana, nasopharynx, opening of eustachian tube, torus tubarius, sphenoethmoidal recess and sphenoid ostium. Then, inserted the endoscope into the left nostril to identify the above landmarks in the left nasal cavity, and lateralized the middle turbinate in the left nostril. After these, inserted the endoscope into the right nostril to have removals of the right middle turbinate and approximately 2 cm of the posterior portion of nasal septum, then, removed the whole anterior wall of sphenoid sinus. After the completion of that, dissected the anterior skull base, sphenoid sinus, clivus and ventral cranio-cervical junction imitating EEEA for skull base surgery.2, To study the feasibility of posterior pedicled inferior turbinate-nasoseptal flap. I, preparation of human cadaver heads:Eight frozen fresh adult cadaver heads were perfused with red latex via the common carotid arteries after defrosting at room temperature. Then they were placed into a refrigerator with temperature of negative 20 degrees, and defrosted at room temperature after 24 hours. II, Gross anatomy:firstly, removed the mandible; secondly, removed the palatine process of the maxilla and horizontal plate of bony palate from each specimen using the help of a microscope with caution of protecting the mucoperiosteum on nasal floor; thirdly, the nasoseptal mucoperichondrium and mucoperiosteum were elevated from the septal cartilage and bone on both sides with the help of a microscope up to the skull base, detached their attachment on the ventral anterior skull base carefully, then, the septal cartilage and bone were removed, and the specimen was sectioned sagittally along the midline between the bilateral nasoseptal mucoperichondrium and mucoperiosteum to get half-cadaver heads with intact mucoperichondrium and mucoperiosteum of the nasal floor and nasoseptum. Ⅲ, Harvesting a posterior pedicled inferior turbinate-nasoseptal flap (PPITNF):The definition and harvest of the flap were performed by the following nine incisions. The first incision was made in the sagittal plane from the junction of the anterior wall of the sphenoid sinus and anterior skull base to the base of the nasal bone. The second incision was made from the base to the tip of the nasal bone, and the third incision was made from the tip of the nasal bone to the nasal spine of the maxilla. The fourth incision was made in the coronal plane from the junction of the anterior wall of the sphenoid sinus and the anterior skull base to the nasal floor on the nasoseptal side of the choana. Then, the fifth incision was made laterally along the nasal floor from the nasal spine of the maxilla, by passing the posterior edge of the nasolacrimal duct opening, and then passing anterior to the inferior edge of the head of the inferior turbinate. The sixth incision was made in the coronal plane from the inferior to superior edge of the head of the inferior turbinate, the seventh incision was in the sagittal plane from the superior edge of the head of the inferior turbinate to a point approximately 1.5 cm from the inferior turbinate’s posterior tip, and the eighth incision was in the coronal plane from this point to the anterior edge of the sphenopalatine foramen. Then, the ninth incision was made laterally along the nasal floor from the nasoseptal side of the choana to the lateral wall of the nasal cavity, then following the coronal plane, by passing the posterior tip of the inferior turbinate, to the posterior edge of the sphenopalatine foramen. After the incisions were performed, fractured the inferior concha laterally, took out the concha bone piece by piece, caution must be paid not to destroy the inferior turbinate artery. IV, Harvesting a invitro flap:transected the pedicle of the PPITNF at sphenopalatine forame to have a invitro flap. V, Microdissection of the flap. Dissected the invitro PPITNF using a microscope to observe the artery distribution of the flap.3, To study the reconstruction of the extensive skull base bone and dura defects resulted from expanded endoscopic endonasal skull base surgery using a PPITNF. I, Three frozen fresh adult cadaver heads were perfused with red latex via the common carotid arteries after defrosting at room temperature. Harvested a posterior pedicles inferior turbinate-nasoseptal flap using a 0 degree endoscope in the right nasal cavity with the method mentioned above, placed the flap into the right maxillary sinus. Ⅱ, Removed the middle turbinate on one side of nasal cavity and the 2cm of the posterior portion of the nasal septum, lateralized the middle turbinate in the other nostril to create the surgical corridor of expanded endoscopic endonasal approach. Then, removed the anterior and inferior wall of the sphenoid sinus, the clivus and the ethmoidal sinus to expose the ventral aspect of anterior, middle and posterior skull base. Ⅲ, Rotated the PPITNF about 180 degree around the sphenopalatine forame to cove the anterior, middle and posterior ventral skull base. The nasoseptal part of the PPITNF on the anterior skull base, the nasal floor part of the PPITNF on the upper livus and the turbinate part of the PPITNF on the middle and lower livus. To observe the extent of coverage of the PPITNF.4, Located the cranial anatomical landmarks using 3-D reconstructed CT images for expanded endoscopic endonasal skull base surgery. Ⅰ, The multiplanar CT images of Ⅲ adult Chinese patients were obtained for the clinical trial, and reconstructed 0.41-mm-thick gapless sagittal and coronal CT images with mimics 15.0. Then, located the following basic points:the nasal spine, the posterior midpoint of hard palate, the line between the nasal spine and the posterior midpoint of hard palate was the basic line. Located the observed points:the left and right optic canals, the left and right medial edge of foramen lacerums, the midpoint of tuberculum sellae and the posterior-inferior midpoint of sellar floor. Ⅱ, Measured the distances between the observed points and the nasal spine, measured the angles between the line from the observed points to the nasal spine and the basic line, measured the distances between the left and right optic canals, foramen lacerums and the distance from the midpoint of tuberculum sellae to the posterior-inferior midpoint of sellar floor. Ⅲ, Statistical analysis of all the above measurements were performed using the Statistics Package for Social Sciences software (SPSS 20.0 for Windows).Results:1, Anatomic landmarks in the nasal cavity for expanded endoscopic endonasal skull base surgery included:the inferior turbinate, the posterior part of nasal septum, the choana, the sphenoethmoid recess, the sphenoid ostium. Anatomic landmarks of the anterior skull base for expanded endoscopic endonasal skull base surgery included: the frontal recess, the crista galli and the medial wall of orbit. Anatomic landmarks in the sphenoid sinus for expanded endoscopic endonasal skull base surgery included: the optic protuberances, the carotid protuberances, the optocarotid recesses, the sellar floor, the clivus recess, the posterior wall of sphenoid sinus. The anatomic landmarks for the exposure of clivus included the inferior wall of sphenoid sinus, the foramen lacerums and the vidian canals. The anatomic landmarks suggested the exposure of cranio-cervical junction included:the tubal torus, the occipital condyle, the hypoglossal canal, the anterior arch of atlas, the odontoid process. The extent of the exposure of the ventral midsagittal skull base:in the anterior skull base, the frontier limit was the frontal recess and the lateral limit was medial wall of orbit; at the level of planum sphenoidale, the lateral limit could extende to the lateral edge of the superior orbital fissure; at the level of the sellar region, the lateral limit could extende to the foramen rotundum; at the level of the clivus, the lateral limit was the internal opening of the carotid artery canal; at the level of the the occipital condyle, the lateral limit was anterior edge of the hypoglossal canal; the inferior limit of the exposure of the ventral midsagittal skull base was the superior edge of the dentata. The exposed neurovascular structures included the gyrus rectus, the longitudinal cerebral fissure, the anterior cerebral artery, the anterior communicating artery, the olfactory bulb, the olfactory tract, the optic nerve, the optic chiasma, the ophthalmic artery, the internal carotid artery, the pituitary gland, the pituitary stalk, the mastoid body, the diaterma, the posterior cerebral artery, the posterior communicating artery, the basilar artery, the oculomotor nerve, the superior cerebellar artery, the trochlear nerve, the ophthalmic nerve, the maxillary nerve, the abducent nerve, the pontine arteries, the vertebral artery, the posterior inferior cerebellar artery, the anterior spinal artery, the facial nerve, the vestibulocochlear nerve, the glossopharyngeal nerve, the vagus nerve, the accessory nerve, the hypoglossal nerve, the first and second cervical nerves, the pons, the medulla, the superior cervical spinal ventral.2, The posterior lateral nasal artery gave off 2.50±0.52 inferior turbinate arteries and the posterior nasoseptal artery branched into 2.50±0.52 nasoseptal arteries; the inferior turbinate arteries and the nasoseptal arteries gave anastomosing arteries to form the inferior turbinate artery net and nasoseptal artery net; 3.19±1.47 anastomosing arteries existed consistently between the inferior turbinate artery net and nasoseptal artery net, the mean outer diameter of largest branch was 0.40±0.10 mm (range,0.24-0.60mm). The mean flap area of the posterior pedicled inferior turbinate-nasoseptal flap was 3090.69±288.08 (range,2612.97-3880.09) mm2. The pedicle of the flap was 11.21±2.40 (range, 5.00mm-14.74) mm; the mean flap length was 100.65±5.61 (range,91.43-109.44) mm and minimum and maximum widths were 25.21±2.29 (range,22.36-30.23) mm and 44.53±5.02 (range,36.45-54.10) mm, respectively.3, The posterior pedicled inferior turbinate-nasoseptal flap provided coverage of the ventral midsagittal skull base from the posterior wall of the frontal sinus to the foramen magnum, and laterally, at the level of the anterior skull base to the medial wall of the orbit, at the level of the sellar region to cover the bilateral cavernous sinuses, and at the level of the clivus the flap covered the clivus from one internal carotid artery to the other. As pulling of the pedicle of the flap, the part of the contralateral upper clivus can’t be completely covered. For the flaps on the left nasal cavity, the arc of rotation was clockwise. Likewise, the rotation was counterclockwise for the flaps on the right nasal cavity.4, The mean distance from nasal spine to optic canal is 73.12±4.10 mm, and the mean angle between the line connecting the optic canals to the nasal spine and the line connecting the nasal spine to the midpoint of the posterior hard palate is 39.79±3.13 degree. The distance from the medial edge of foramen lacerum to nasal spine is 79.91±4.01 mm, and the mean angle between the line connecting the foramen lacerums to the nasal spine and the line connecting the nasal spine to the midpoint of the posterior hard palate is 23.27±2.89 degree. The distances from midpoints of the tuberculum sellae and posterior-inferior sellar floor to nasal spine are 76.16±4.56mm and 82.05±4.81mm, and the mean angles between the line connecting the tuberculum sellae/posterior-inferior sellar floor to the nasal spine and the line connecting the nasal spine to the midpoint of the posterior hard palate angles are 34.97±3.24 degree and 26.39±3.51 degree respectively. The distances between both foramen lacerums and optic canals are 22.54±3.25mm and 23.44±3.49mm respectively. The distance from the midpoint of tuberculum sellae to posterior-inferior sellar floor is 13.33±1.87mm.Conclusion:1, The major advantages of expanded endoscopic endonasal skull base surgery are that it provides extensive exposure of the whole ventral midline skull base with direct anatomical access and panoramic view, avoiding the brain retraction and obstruction of some important neurovascular structures. The expanded endoscopic endonasal skull base surgery can serve as a effective surgical method to treat lesions of the ventral midline skull base, including lesions of the ventral midline of anterior skull base, middle skull base, clivus and the ventral cranio-cervical junction; especially to those lesions that can’t be resected by other surgical approaches, the expanded endoscopic endonasal approaches can serve as the first selective method. The most common surgery-related problems of this approach are related to the prevention of postoperative CSF leakage and treatment of venous bleeding from the nasal cavity or the cavernous sinus impairing visualization, or potentially catastrophic arterial bleeding from the larger arteries including the internal carotids. The development of the expanded endoscopic endonasal skull base surgery need more applicable surgical instruments, more effective hemostatic materials and reconstruction methods. These approaches should be performed only by experienced neurosurgeons with adequate endoscopic skill training, which can be obtained after dissection studies in cadavers and completion of many endoscopic endonasal surgeries, with the close cooperation and collaboration of ear, nose, and throat surgeons.2, Our study provided the solid anatomic evidence of anastomosing arteries between the inferior turbinate arteries and the nasoseptal arteries, harvesting of the posterior pedicled inferior turbinate-nasoseptal flap based on the posterior lateral nasal artery is feasible. Based on previous reports and our anatomic dissections we believe that the posterior lateral nasal artery can supply sufficient blood to the posterior pedicled inferior turbinate-nasoseptal flap and avoid ischemia and necrosis. The posterior pedicled inferior turbinate-nasoseptal flap is the largest described potential intranasal pedicled flap reported to date.3, The posterior pedicled inferior turbinate-nasoseptal flap located in the nasal cavity, it can cover extensive skull base defects resulted from the endoscopic expanded endonasal skull base surgery and avoid external scalp incision and transposition which are needed in performing ectorhinal pedicled flaps, and decreasing the relevant complications. The posterior pedicled inferior turbinate-nasoseptal flap can serve as a potential suitable option of endonasal pedicled flap for vascularized reconstruction of extensive ventral skull base defects.4, The optic canals, foramen lacerums, tuberculum sellae and the posterior-inferior midpoint of sellar floor are the important anatomic landmarks for the expanded endoscopic endonasal skull base surgery, we can locate and assess these landmarks preoperatively using reconstructive three-dimensional slice CT images to help surgeons identify them during operation, so as to avoid damage to critical neurovascular structures and reduce relevant complications, especially to a presellar or conchal sphenoid sinus. Assessing these anatomical landmarks preoperatively on three-dimensional CT images for each patient separately will increase the chance of success in performing endoscopic expanded endonasal skull base surgery.