Surgical Anatomy Of The Anterior Transpetrosal Approach To Petroclival Region

Objective Surgical access to lesions of petroclival region represents a challenge, both because of the depth of the lesion site and the vicinity of complex anatomic structures. The anterior transpetrosal approach(Kawase's approach) is one of the most important approach to petroclival region. However, the methods to precisely locate the coc-hlea, internal carotid artery(ICA) and geniculate ganglion(GG) are not generally agreed on, which have affected the clinical application of anterior transpetrosal approach greatly. The purpose of this study is to understand the detailed microsugical anatomy of this approach and explore the better method and landmarks locating cochlea, internal acoustic canal(IAC) and ICA by studying the topographic relationships of petrous and peripetrous structures. We are also trying to provide reliable basis for drilling petrous pyramid and maximizing petroclival exposure, and provide anatomic information for the operative technique and clinical application of anterior transpetrosal approach. Methods Ten cases of dry human adult skulls(20 temporal bones) were used. the surface structures such as the eminentia arcuata(AE), the greater superficial petrosal nerve(GSPN) groove, the hiatus facialis, the trigeminal impression, the foramen spinosum(FS) and the internal acoustic meatus(IAM) were observed, and then the petrous bone were drilled and the cochlea, the bony semicircular canal and the fundus of IAC were identified, the relationships of the main structures were observed and measured. Ten formalin-fixed human adult cadaveric head (20 sides) were examined, blood vessels were injected with color silicone. the cerebral hemisphere, diencephalon and the middle fossa dura were removed, keeping the brain stem, cerebellum and the inte-grality of the tentorium of cerebellum. The FS, the trigeminal nerve, GSPN, AE, the petrous ridge and the superior petrosal sinus were identified, the petrous segment of ICA,GG and cochlea were exposed after drilling the petrous pyramid, and then we observed the relat-ionships and measured the distance between these structures. Lastly, Six formalin-fixed human adult cadaveric head(12 sides) were examined. The anterior transpetrosal approach was mimicked below at 6×to 25×magnification operating microscope and the petroclival exposure in the course of operation was observed. All measurements were made in millimeters, using vernier caliper and compass, and getting the average for three times. Then the data was statistically analyzed and my personal experience in dissection was summarized. Results (1)Kawase's triangle is bordered anterior by the trigeminal nerve, laterally by GSPN groove and medially by petrous ridge, and it is divided into two triangles by IAC. The anterior trigone is surrounded by the trigeminal nerve, GSPN and IAC, the cochlea and the horizontal ICA lie within the lateral one half of the trigone,the posterior trigone is surrounded by AE, petrous ridge and IAC,and there are vestibule and semicircular canals within it. (2)The superior aspect of the superior semicircular canal(SSC) lay beneath the floor of the middle cranial fossa, the depth was about 1.92±0.51mm. 12 out of 20 specimens SSC were found beneath the central parts of the AE, and of the rest of eight specimens, SSC were found beneath the medial parts of the AE. The cochlea locates lateral to the line from the intersecting point of the mandibular nerve and GSPN to the lateral lip of the IAM in all 30 specimens, and we can precisely localize the perimeter of the cochlea with a guadrangle in this approach. (3)The distance between ICA andpetrous ridge is 9.88±1.69mm,and ICA will not be damaged when pyramid resection is limited to within this range. (4)29 AE out of 52 specimens had prominent eminence,14 specimens had slight eminence and 9 specimens had flat surface with no prominent eminence. In 20 out of 52 specimens the apex of AE could be defined easily, whilst it is difficult in the rest of 32 specimens. (5)The length of the anterior wall and posterior wall of IAM were 12.58±1.54mm and 8.39±1.43mm respectively. The VII-VIII cranial nerves will not be damaged when drilling the petrous pyramid anterior to the line that is perpendicular to the petrous ridge and locates 5.31±1.19mm posterior to the porus trigeminus. (6)The area of drilling is surround by Dorello's canal anteriorly, the SSC posteriorly, IAC laterally and the inferior petrosal sinus inferiorly (the depth was 14.43±1.05mm). Conclusion (1)FS and GSPN were important landmarks and we can define the other instructures in petrous bone according to their location. (2)AE should not be used as a substitute for the SSC because AE had great variation, and it is not proper that the AE is regard as a constant landmark in this approach.(3)The middle fossa dura should be elevated in anterior-to-posterior direction to expose GSPN from the mandibular nerve untill the AE is identified.(4)New methods to localize the peri-meter of the cochlea, IAC and the petrous segment ICA have been put forward.(5)If we are familiar with the anatomy of the petrous bone and related landmarks,the cochlea and ICA can be protected very well in operations by anterior transpetrosal approach with fewer complica-tions.