## 4 Discussion
SOE was found to be an accurate diagnostic tool for tip fold-over, detecting all four cases (5.5% incidence). Nevertheless, imaging may be superior to SOE testing for two reasons: firstly, SOE testing requires an in-depth knowledge of the different manufacturers' software and is performed by an expert audiologist; secondly, imaging is diagnostic for a variety of misplacements other than fold-over. In our report, the two patients with tip fold-over had normal NR and impedance. The patient in case 1 was one of the first patients nationally implanted with the CI512 device and SOE testing was conducted for academic purposes. Nevertheless, the abnormality was seen in the radiograph, and reinsertion would have been performed regardless of the SOE results. For extracochlear misplacements, NR may be more sensitive, although for intralabyrinth-extracochlear locations (vestibular labyrinth), normal responses may be found [5; 19]. Case 3 further illustrates the limitation of electrophysiologic testing. Even after the successful full intracochlear reinsertion, no responses were measured. Upon switch-on and in further mapping sessions, neural responses were obtained from all electrodes. In our experience, it is not uncommon to receive partial responses or even no responses intraoperatively but then have complete neural responses later at the time of switching on the device. In contrast to the two-dimensional image in plain radiographs, computed tomography provides three-dimensional information of the temporal bone and cochlear implant electrode position, and is mainly used worldwide for pre- and postoperative imaging. Mostly the high-resolution CT (HRCT) or the less radiating cone beam CT (CBCT) is used. The utility of intraoperative CT was illustrated in some reports in cases of inner ear malformations [3; 20]; however, we believe it has no clear advantage in ears with normal anatomy. As mentioned, the inner ear anatomy including the vestibular part can be seen in the plain radiographs. Two disadvantages of CT are higher radiation exposure and cost. The radiation doses of CT are much higher than those of plain radiographs, regardless of the type of system being used. The exposure dose for high resolution CT is approximately 1000 \(\upmu\)Sv, while doses of cone beam CT are about 50 \(\upmu\)Sv [21]. When compared to doses of around 2 \(\upmu\)Sv of a plain radiograph, the difference is significant, especially in the pediatric population which are anticipated to undergo re-implantations in the future and possible additional imaging for other reasons. In addition, prepping and scanning the patient takes about 8 min as estimated in one study, and the overall cost of use of CT per surgery was USD 721 [6]. When specifically addressing the cone beam CT, it should be noted that it is superior to the plain X-ray in the accuracy it can provide regarding the location of the electrode array. In contrast to the plain X-ray, the cone beam CT can determine if the electrode array is in the scala tympani or scala vestibuli. Nevertheless, the plain X-ray is still much easier to perform intraoperatively, and it is cheaper, quicker and emits less radiation. Importantly, the intraoperative cone beam CT is not available in many centers that perform CI, and therefore we believe the plain traditional X-ray is still a good and viable option for detecting the misplacement of the electrode array. This review presents four examples in which abnormal electrode position was detected intraoperatively by the use of plain radiographs and was successfully repositioned. The routine usage of intraoperative radiographs in 100% of cochlear implant patients with normal anatomy further validates this report by avoiding sampling bias. The three patients (two with tip fold-overs and one with incomplete insertion) would have probably gained benefit from their misplaced implant to some degree in terms of hearing and communication. Most probably after months or years, less then optimal function would warrant investigation.