Lateral Rotation of the Basal Turn of the Cochlea with its Impact on Cochlear Implant Feasibility

Abstract
Purpose: To investigate variation in cochlear orientation, with respect to rotation of basal turn, and its implication to cochlear implantation.
Methods: Prospective observational study including 25 patients that underwent cochlear implantation at Tanta Cochlear Implant Centre (TCIC), Otolaryngology, Head & Neck Surgery (ORL-HNS) Department, Faculty of Medicine, Tanta Univer-sity, that had been done from April 2015 to December 2015, with available charts including Computerized Tomography (C.T.) scan of temporal bone. Axial cut with maximum visu-alization of the round window was identified, then angle of axial rotation of the cochlea was measured using two methods: Angle 1 between the midline and line through the long axis of the basal turn and angle 2 between the long axis of the basal turn and the long axis of horizontal segment of Internal Carotid Artery (I.C.A.) at the same axial level. All angles were measured using computer software. All cases had cochlear implantation using standard trans-mastoid technique. Round window visibility through facial recess was assessed after maximal widening of the facial recess to achieve best exposure. Then, cases were assigned into one of two groups according to the degree of round window visibility, Group I: Case with full exposed Round Window (R.W.) membrane. Group II: Cases with partially exposed R.W. membrane or invisible R.W. membrane.
Results: Following review of the Computed Tomography (CT) images of the two patients (Group II) in whom cochlear implant was difficult. It appeared that the orientation of the basal turn of the cochlea relative to the midline (angle 1) was more obtuse or more acute than in other patients with p- value =0.007 & 0.002 respectively. It also showed that the orientation of the basal turn of the cochlear in relation to long axis of horizontal part of internal carotid artery (angle 2) was more obtuse relative to other patient with p-value=0.002 & 0.001 respectively.
Conclusion: Our work highlights the importance of pre-operative measuring basal turn angel relative to fixed plane, as the mid-sagittal plane in pre-operative CT, instead of other anatomical structure that could be subjected to developmental remodeling and its impact on surgery of cochlear implantation.