The Internal Acoustic Meatus And Its Meningeal Architecture: An Anatomic Study Using Plastinated Histological Sections

Objective The internal acoustic meatus is located in the petrous part of the temporal bone,with a narrow depth and complex anatomy.The arachnoid structure within it has been controversial.The purposes of this article are to reveal the distribution and extension of the arachnoid membrane of the internal acoustic meatus.The purpose of this study is to find out the delicate structure and spatial position in the internal acoustic meatus.It is also to provide anatomical basis for clinical and imaging diagnosis and clinical microsurgery.Methods Observed regional anatomic specimens from 3 human cadaveric heads(bone structure and cranial nerve of the posterior cranial fossa).Three sheet plastination slices and thirtysix temporal bone tissue blocks containing the internal acoustic meatus region were obtained from 21 human cadaveric heads(Forty-two slices).These specimens were plastinated and cut into serial sections.The slicers were scanned clearly and examined.Stained the ultra-thin sections and used to reconstruct a three-dimensional visualization model by 3D-slicer software.This study was collected MRI horizontal sectional images of 15 cases(30 sides)to compare with ultra-thin plastic sections.Results(1)The internal acoustic meatus is a cylindrical bony canal.It opens on the inside of petrous part of the temporal bone and faces to cerebellopontine cistem.The side is the fundus of the internal acoustic meatus,which forms the inner wall of vestibule and cochlea.The transverse crest divides the fundus of the internal acoustic meatus into two regions.The upper part contains the facial nerve region(located in the front)and the superior vestibular nerve region(located in the posterior)by the vertical crest(Bills crest);the lower half includes the cochlear region(located in the front),the inferior vestibular region posterior,and the single canal(located in the infer posterior portion).There are small holes in the transverse crest.(2)The facial nerve and vestibular cochlear nerve originate from the cerebellopontine angle and outside of the bulb pontine sulcus.They finally enter the internal acoustic meatus.The facial nerve is located in the anterior superior part of the porus.The vestibular cochlear nerve is located in the posterior inferior part of the facial nerve.The intermediate nerve exists between vestibular nerve and facial nerve.The vestibulocochlear facial nerve complex is parallel to the long axis of the internal acoustic meatus and converges into the areas at the fundus of the internal acoustic meatus.(3)The labyrinthine artery is the main blood vessel in the internal acoustic meatus,most of which originated from the anterior inferior cerebellar artery.The vascular loops exist outside the porus and the internal acoustic meatus.Of these,30.56 percent were located outside the internal acoustic meatus,36.11 percent were located in the internal acoustic meatus and 30.56 percent had no vascular loops.The extension of vascular loop into the internal acoustic meatus is different.61.54 percent of the elongation does not exceed half the length of the internal acoustic meatus.The half the length of the internal acoustic meatus is 6.26±1.53 mm,and 38.46 percent of the elongation exceeds half the length of the internal acoustic meatus.In one case,the vascular loop extends to the fundus of the internal acoustic meatus.(4)The dura of the petrous bone was into the internal acoustic meatus at the porus of the internal acoustic meatus.The dura covers the bone wall as closely as the periosteum.Near the fundus of the internal acoustic meatus,the dura becomes more and more thinner and covers the bone wall and the various regions.The arachnoid membrane envelops the facial nerve,vestibulocochlear nerve,and small labyrinthine artery into the internal acoustic meatus.There are arachnoid trabeculae at the fundus of the internal acoustic meatus.The arachnoid membrane enters into the fundus of the internal acoustic meatus.There are arachnoid trabeculae at the fundus of the internal acoustic meatus.Six cases of arachnoid folding occurred at the porus of the internal acoustic meatus.At the posterior porus of the internal acoustic meatus,the arachnoid did not follow the dura mater into the internal acoustic meatus,but appeared folded and extended forward and medial.The part of the extension comes into contact with the facial nerve and vestibular cochlear nerve.(5)The length of the internal acoustic meatus was measured about 12.52 ± 1.53 mm.The diameter was measured about 4.8±0.11 mm.The shortest distance from the internal acoustic meatus to the Internal carotid artery is about 3.6mm(2.7-6.1 mm).The shortest distance from internal acoustic meatus to the jugular bulb is about 4.7mm(2.9-6.8 mm).(6)The shape of the internal acoustic meatus,nerves and blood vessels was shown in the MRI image are same with the results of the anatomical study in this article,but the extension and termination of the arachnoid membrane in the internal acoustic meatus cannot be seen.Conclusions 1.The facial nerve,vestibular cochlear nerve and labyrinth artery exist in the internal acoustic meatus;The facial nerve is located in the anterior superior part of the porus.The vestibular cochlear nerve is located in the posterior inferior part of the facial nerve;The vascular loops exist at the internal acoustic meatus and the extension of vascular loop is different.2.The arachnoid in the internal acoustic meatus surrounds the vestibular cochlear facial nerve complex and enters the fundus of the internal acoustic meatus.The arachnoid membrane ends in the various regions of the internal acoustic meatus and fuses with nerves.3.Arachnoid membrane does not have double-layer replication in the internal acoustic meatus,but arachnoid folds at the porus of the internal acoustic meatus.