QUANTIFICATION OF IMPORTANT MEASUREMENTS OF THE PETROUS BONE IN COMPUTED-TOMOGRAPHY PRECEDING COCHLEAR IMPLANT

In this investigation the following anatomical measurements of the pet rous bone preceding cochlear implant were evaluated using computed tom ography (CT): the width of the basal coil of the cochlea, the thicknes s of the promontory, the distance between the sigmoid sinus and the po sterior wall of the external auditory canal and the thickness of the s quamous part of the temporal bone in the area of operation. Initially it was necessary to ascertain how accurately CT represents the anatomi cal structures of the petrous bone. The procedure of CT investigation in adult patients was simulated using the base of a skull. The CT plan e in which the above mentioned measurements preceding cochlear implant can be made was represented and measurements were taken. Subsequently the same plane in CT was represented macroscopically with a specially developed method. The results show that anatomical structures measure d in CT are magnified by 5% in contrast to the original. This magnific ation is presumably attributable to the computing of connective tissue which is adjacent to bone and the incapacity of CT to image structure s of different density in the same voxel. In the second part of this i nvestigation we evaluated 66 preoperative CT's of patients who were gi ven a cochlear implant. The results must be adjusted by 5% to allow fo r this error in magnification. In conclusion a preoperative radiologic evaluation of the anatomical structures relevant to cochlear implant is without doubt tenable. Also the negligible difference between CT an d the anatomical original requires no operative consequences. Because cochleostomy is carried out with a diamond burr of 1 mm diameter, a te nth of a millimeter is negligible according to our experience.