Anatomy and radiology observation of spatial relationship between tympanic annulus plane and vertical segment facial nerve

Objective To examine the spatial relationship between the tympanic annulus plane and the vertical segment facial nerve, and then explore the feasibility of the tympanic annulus plane as the anatomical landmark for the vertical segment facial nerve. Methods Eleven formalin-fixed adult cadavers(22 temporal bones)were used in this study from January to October 2014. All the tympanic membranes were coated with iohexol injection in advance, then all the cadavers were scanned by multi-slice spiral computed tomography. The tympanic annulus plane and the vertical segment facial nerve were reconstructed using the technique of 3D-volume rendering(3D-VR) and skeletonized by microanatomy. Finally, data measurements and statistical analysis were accomplished. Results All the tympanic membranes could be clearly displayed on MSCT film. The spatial relationship between the tympanic annulus plane and the vertical segment facial nerve were reconstructed successfully by the technique of 3D-VR. For most of the cases (12/22), 8 o'clock(right ear) or 4 o'clock(left ear) of the tympanic annulus plane was the nearest to the vertical facial nerve, and the shortest distance was 3.58 mm(micro-anatomy/3D-VR) on average. The angle between the tympanic annulus plane and superior vertical segment of the facial nerve were 39.14°(micro-anatomy) and 39.41°(3D-VR). There were no significant difference between the measurements obtained by CT and micro-anatomy (all P values>0.05). In the postero-inferior quadrant of tympanic membrane, the facial nerve coursed lateral to the plane of the tympanic annulus in 63.64%(14/22) specimen for 3D-VR and 68.18%(15/22) specimen for microanatomy, in the meantime, the facial nerve also coursed anterior to the posterior edge of the plane in 10/14 specimen for 3D-VR and 10/15 specimen for microanatomy respectively. The two methods had no significant difference in determining the cross phenomenon between the tympanic annulus plane and the facial nerve(κ=0.899, 0.874). Conclusions The tympanic annulus plane is a relatively constant anatomical landmark for the vertical segment of the facial nerve. The facial nerve is most vulnerable to injury in the postero-inferior quadrant of the tympanic membrane during trans-canal surgery. 3D-VR can truly reflect the spatial relationships between the tympanic annulus plane and the vertical segment facial nerve. Key words: Ear, middle; Tympanic annulus; Facial nerve; Volume rendering; Tympanic bone; Anatomy