Ossification of the cochlea can provide a surgical challenge. When ossification is well established, the cochlear lumen can be difficult to identify. Even in partial obstruction, removal of soft bone to provide an adequate lumen for the electrode array can be challenging. The relative rarity of this presentation, especially with declining rates of meningitis, limits the opportunity to gain surgical experience or training in the techniques required. However, second-side electrode placement for post-meningitic patients is becoming more desirable, transiently increasing the number of ossified cochleas requiring implants.14
We have developed a realistic cadaveric model of cochlear ossification using bone cement, which we have found to be very beneficial in surgical training.
This model produces a heterogeneous ‘ossification’ pattern ranging from just the basal 3-4 mm of the scala tympani to the entire scala tympani being obliterated. This pattern of ossification is similar to that seen in labyrinthitis ossificans.15
Otosclerosis also causes ossification of the cochlea, which predominates in the first part of the basal turn. Significant apical and middle-turn ossification is uncommon in the absence of basal turn ossification.15
This rarer pattern of obstruction was not produced by our model, but could be achieved by cementing the cochlear from an apical or middle fossa cochleostomy. The model is suitable for training scala vestibuli insertion and use of the double array, which are important techniques in implantation of the ossified cochlea.16,17
Imaging with CBCT and diagnostic CT provided good assessment of the completeness of obliteration in the model in 4 out of 5 heads (9 out of 10 cochleas). The single exception was understaged in both CT and CBCT. In a study by Young et al in 2000,18
assessing the pre-operative evaluation of ossified cochleas, 20 high resolution CT scans obtained post cochlear implantation were reviewed and compared to surgical findings and the pre-operative CT scan. Ninety percent of patients required drilling of the ossified bone within the basal turn at surgery. High-resolution pre-operative CT scans predicted ossification within the basal turn in 45% of cases (50% sensitivity). Five of 6 cases without radiographic evidence of ossification had positive findings at surgery. Interestingly, this study found that lateral semicircular canal ossification was a more sensitive measure for predicting cochlear ossification. Immediate pre- or intra-operative CBCT would appear to be an appropriate means to detect progression of ossification, which may continue to develop after diagnostic imaging has been obtained. As mentioned above, CBCT demonstrates sub-mm spatial resolution and soft-tissue contrast visibility, but its performance does not match that of diagnostic CT due to a variety physical limitations (e.g., x-ray scatter and detector efficiency). Of course, the CBCT system described here is clearly not intended as a replacement of preoperative/diagnostic imaging, and its imaging performance appears suitable to tasks of image guidance, as demonstrated here and in previous work.
We have found intra-operative CBCT to be valuable in guiding drilling and electrode insertion in the more obstructed cochlea models. We acknowledge that the questionnaire used in this study was subjective, but still felt it an appropriate, valuable means to evaluate the surgeons' perspective of utility of the imaging tool. While helpful in the simulation setting, we envisage that this could be of great benefit surgically when electrode insertion is more difficult – in congenitally abnormal cochlea as well as the ossified cochlea. With the C-arm CBCT system, intraoperative scanning provided detailed information regarding electrode position and allowed an opportunity to re-position the electrode if required. Surgical navigation systems have been described for electrode placement in the ossified cochlea19
but do not offer the advantage of intra-operative CBCT in showing surgical changes in anatomy.
Post-operative imaging is widely used to confirm electrode placement, typically with plain radiographs. Though not routinely used in all centers,20
it is highly advisable in the ossified or congenitally abnormal cochlea due to the increased chance of poor placement. Interpretation of electrode position can be difficult with 2D radiographs, especially in bilateral implantation where the implants overlie each other on lateral views. The advantage of CBCT over a conventional x-ray radiograph is the increased 3D volumetric information provided. The radiation dose is significantly less than diagnostic CT (in the order of 3-10 mGy, compared to 50-100 mGy).9
In addition, CBCT is acquired over only ~180° (with the x-ray tube orbiting posterior to the head) so that radiation-sensitive organs, including the eyes, receive even lower doses. Finally, intraoperative CBCT can be performed with a relatively short acquisition time, with volumetric images available within ~1 minute. The ability to visualize implant placement intraoperatively, in a manner consistent with time and radiation dose constraints, is particularly valuable in the ossified or congenitally abnormal cochlea.
The pattern of metal streak artifact in CT and CBCT images depended strongly on the design of cochlear implant used. This is due to the different metallic components in each case. Specifically, Implant #3 (HiFocus 1j, Advanced Bionics) incorporated titanium components that were significantly more CT-friendly than heavier metals incorporated in other implants. The streak artifact is primarily a result of “beam-hardening” (i.e., attenuation such that the mean energy of the x-ray beam is increased in a manner not accounted by the reconstruction algorithm) and very low x-ray transmission through such dense materials (for which the detector records a very low signal and the 3D reconstruction algorithm backprojects a very high attenuation value) – each resulting in white streaks through the electrodes.