Direct comparison of mastoidal and retrosigmoidal placement of a transcutaneous bone conduction device after canal wall down tympanoplasty

Cholesteatoma and chronic otitis media are common and regularly treated in surgical otology centers. Repeated surgeries are often required in order to eliminate residual disease and reconstruct functional hearing. Frequently, the posterior wall of the auditory canal has to be resected resulting in a canal wall down (CWD) technique with variable hearing outcome. This article is protected by copyright. All rights reserved.


Direct comparison of mastoidal and retrosigmoidal placement of a transcutaneous bone conduction device after canal wall down tympanoplasty 1 | INTRODUCTION
Cholesteatoma and chronic otitis media are common and regularly treated in surgical otology centres. Repeated surgeries are often required in order to eliminate residual disease and reconstruct functional hearing. Frequently, the posterior wall of the auditory canal has to be resected resulting in a canal wall down (CWD) technique with variable hearing outcome. 1 Subsequently, strategies such as incus interposition and alloplastic middle ear prostheses are regularly used for air conduction restoration. However, results are often unsatisfactory due to recurrence of disease, prosthesis extrusion or insufficient air conduction capability of the utilised materials. 2 Even with the support of conventional hearing aids, hearing rehabilitation may be limited resulting in a reduction of quality of life. For these patients, bone anchored active implants might be a viable option to restore hearing if bone conduction hearing values are no higher than 45 dB in the affected ear. 3 Percutaneous bone anchored hearing aids (BAHA) and transcutaneous bone conduction implants (BCI) offer surgeons the possibility to improve functional hearing. While the BAHA has been in clinical use for decades, the BCI has only recently been added to the surgeon's repertoire. 4 The BCI consists of an implantable electromagnetic floating mass transducer (BC-FMT), which is fixated to the bony structures of the skull with two self-tapping screws. To ensure stability and safety, the BC-FMT can be supported by screw fixation lifts, which enable the BC-FMT to be fitted snugly and securely into a previously drilled boney excavation. The BC-FMT has a height of 8.7 mm and a diameter of 15.8 mm. These measures imply that the surgeon would need enough space within the boney surgical field to secure the BC-FMT. In selected cases, the BCI may also be placed retrosigmoidally ( Figure 1). 5 To date, the majority of implants have been implanted into the mastoid ( Figure 2). 6 To the best of our knowledge, no studies have directly compared the hearing outcomes of both of these placement options in cases of canal wall down tympanoplasties at the time of performing this study. It is not yet clear whether the decreased amount of boney mastoid, which implies less bone to conduct the vibratory stimulation of the BC-FMT towards the inner ear, impacts the postoperative outcome. Therefore, the aim of this study was to (a) compare the audiological benefit, as well as to (b) evaluate surgical The copyright line for this article was changed on 7 January 2019 after original online publication.

| Ethical considerations
Due to the retrospective nature, an anonymous analyses, the authors had no ethical concerns in performing this study. All patients were intensively counselled and gave informed consent before treatment.
The study had no influence on patient treatment or follow-up. The study was submitted to the local review board for approval.

| Patients and surgical indication
We performed a retrospective chart review of a total of 20 cases,

| Treatment groups
Cases were divided into two groups: In Group A (n = 9) the bone conduction implant had been placed retrosigmoidally (

| Surgery
In general anaesthesia, a retroauricular approach was used in all cases. In cases with extensive mastoid defects due to previous surgeries, the BC-FMT was placed retrosigmoidally with dural expo-

| Statistical analyses
The difference between the preoperative bone conduction value and the postoperative aided free field measurement is considered as the primary endpoint, the difference between the unaided and aided free field readings as secondary outcome of interest. To account for the fact that full independence of the observations cannot be assumed, as one patient is included with two observations (ie. ears), the permutation test was used for comparisons of the primary and secondary endpoints between treatment groups (A vs B). Furthermore, bootstrap percentile confidence intervals were calculated for the mean differences (group A minus group B). Both resampling methods are based on 10 000 replications. Two-sided P-values <0.05 were considered as indicating statistical significance.

| Hearing outcome
Free field unaided and preoperative bone conduction values are shown in Figure 3 for both groups.

Keypoints
• After extensive cholesteatoma surgery, a transcutaneous bone conduction implant is a valuable hearing rehabilitation tool in patients where conventional middle ear reconstruction is ineffective or impossible.
• Both retrosigmoidal and mastoidal implantation achieve good postoperative hearing results, with the possibility of retrosigmoidal superiority.
• Postoperative free field values resembled preoperative bone conduction results, implying optimal air-bone gap closure.
• No adverse effects were reported.
• The greater distance between the inner ear and the ret- There was a strong increase in free field hearing outcome across the whole patient collective. The difference between aided and unaided free field readings for the PTA is estimated by a mean improvement of −28 dB for Group A and a mean improvement of −16 dB for Group B ( Figure 5). The bootstrap median difference (A minus B) was −12.5 (95% CI -28.6; 6.8), the difference is not statistically significant (P > 0.05).

| Adverse effects
There were no adverse effects reported in either group.

| DISCUSSION
To the best of our knowledge, this is the first study directly com- we could identify a possibility that soft tissue compression, which regularly becomes necessary in the retrosigmoidal approach, may potentially benefit hearing outcome (Figure 4). The confidence intervals and bootstrap charts of our data can exclude the mastoid placement of achieving superior hearing results even in a patient collective. Seeing that the data hints at the possibility of improved F I G U R E 4 Bootstrap median differences and confidence intervals for the differences (A minus B). The assumption of the similarity of mastoid placement and retrosigmoidal BC_FMT placement could not be rejected, while hinting at the possibility that retrosigmoidal implantation might yield a slightly better hearing outcome F I G U R E 5 Pure Tone Average (PTA) improvement for pre-and postoperative free field measurements F I G U R E 3 Preoperative bone conduction and free field measures, showing the similarity between the groups