Microanatomical Study Of The Facial Nerve In The Temporal Bone Via Mastoid-middle Cranial Fossa Approach

Objective To study the microanatomy of the facial nerve(FN) in the temporal bone and its adjacent structures, in order to provide microanatomical datas for the clinical operation, disease-diagnosing, operation-simulating, imaging anatomy and three dimensional(3D) reconstruction of FN in the temporal bone and its adjacent structures. Methods In 20 adult wet skull specimens(40 temporal bones), all the segments of FN and their adjacent structures in temporal bones were observed according to operational layers by simulating transmastoid-transmiddle cranial fossa combined approach. Results (1)FN in the temporal bone exhibited 3 obvious changes in direction: Between internal acoustic meatus segment and labyrinthine segment, there was an angle of 115.5+6.88(110.6-118.5) opening towards anterior and medial direction; Between labyrinthine segment and horizontal segment there was an angle of 73.5 5.66(70.5-79.6) opening towards posterior, inferior and lateral direction; The angleopening towards anterior direction between vertical segment and horizontal segment was 115.5 6.89(109.5-128.6). (2)The vertical distance between suprameatal spine and frontal blade of sigmoid sinus was 13.28 1.87(9.20-17.60)mm and the number of this distance was under 10mm in 3 temporal bones(7.5%). (3)The angle towards superior direction between chorda tympani nerve and verical segment of FN was 38.6 1.99 (28.5-52.5) .The distance between stylomastoid foramen and the point from which chorda tympani nerve departed FN was 5.99 0.74 (5.01-7.90) mm. (4)The depth of facial nerve recess(FNR) was 1.96 1.03(0.74-4.46)mm. (5)From the summit of pyramidal segment of FN to the eminence of the lateral semicircular canal, the distance was 2.86 0.31(2.23-3.56)mm. The distsnce between the summit of pyramidal segment and the tip of short limb of incus was 2.55 0.21(2.10-2.90)mm. (6)There were 3 specimens(3/40, 7.5%) whose base ring of cochlea extensed into internal acoustic meatus(IAM). The angle towards medial direction between base ring of cochlea andlongitudinal axis of LAM was 44.5 5.8(41.5-46.8). From fundus of IAM to base ring of cochlea, there was, a distance of 1.43 ?0.50 (-0.11-1.92)mm. Conclusions (1)Three turns through the total FN in temporal bone are very important in clinical operations. (2)The distance between posterior wall of external auditory meatus and sigmoid sinus must be made clear before operation. (3)Short limb of incus(or incudal fossa) and lateral semicircular canal are important landmarks by which pyramidal segment of FN might be located in manipulation of mastoid. (4)The structures such as spinous foramen, hiatus of facial canal, geniculate ganglion, arcuate eminence, superior semicircularcanal, external auditory canal opening, head of malleus serve as very important anatomical guides to locate IAM. ㊣t is more accurate to locate cochlea according to the triangle before IAM . Manipulating outside of this region is safe and able to avoid injuring cochlea. ㏕he relations of all the segments of FN and their adjacent structures are complicated ,compact. It will be good that the operation in this region be fulfilled under microscope. The manipulater should be practiced in operational technique and absorb a great deal of anatomical knowledge about temporal bone. ㏕he parameters of all the segments of FN and their adjacent structures can only be thought as reference and must be remembered well. But it is also necessary to vary in accordance of individual in real process.