The surgical rehabilitation of mixed hearing losses can be performed by coupling the floating mass transducer of the Vibrant Soundbridge to the round window. The quality of coupling the floating mass transducer to the round window is crucial for the audiological outcome. It was the aim of this study to further observe the different patterns of floating mass transducer position at the round window. We compared twenty patients with mixed hearing loss implanted with a floating mass transducer attached to the round window and 24 surgeries between 5/2007 and 6/2010. An evaluation of the chronological observation of the flat panel angiography-controlled position of the floating mass transducer at the round window with relation to the surgical report and the audiological outcome was done. We observed no changes in the mean pre- and postbone conduction thresholds. The floating mass transducer position was variable and could be radiologically classified and correlated with the audiologically outcome. A learning curve was observed from the earlier to later implantations. Postoperative, radiological evaluation of the location and angle of the floating mass transducer by means of flat panel tomography allowed us to classify the floating mass transducer position at the round window into 4 groups.
The Vibrant Soundbridge middle ear implant (VSB) was introduced in the late 90s for the treatment of high-frequency, purely sensorineural hearing loss (SNHL) as well as for patients with SNHL who were unable to use conventional hearing aids due to problems such as chronic otitis externa. Colletti et al. [
Various centers have described their experiences with attaching the FMT at the RW [
Postoperative, radiological evaluation of the position of otological implants is a useful tool for quality control, for example, in cochlear implantation [
It was therefore the aim of the present study to define the different patterns of FMT position at the RW, to correlate these findings with the functional outcome, to compare radiological and functional outcome data with the chronology of implantation and the different surgical techniques applied.
Between May 2007 and June 2010, 24 round window attachments of the FMT were performed in 20 patients and included in this study. All patients had a previous history of multiple middle ear operations. The surgeries were performed by a transcanal approach (surgeries no. 3, 6, 7, 8, 9, 10, 12, 16, 17, 19, 22, and 24) or by a two step surgery in canal wall down patients (surgeries no. 1, 2, 4, 5, 11, 13, 14, 15, 18, 20, 21, and 23). The first step was a decrease of the cavity size by a flap followed by a second step after 3–6 month with the implantation of the FMT.
In the retrospective part of the study, 7 implantations using different attachment techniques (between May 2007 and May 2009) were performed (see Section
In the prospective portion of the study, 17 operations (between May 2009 and February 2010) were included using a standardized surgical approach. This approach consisted of the following steps. The RW was visualized by removing the promontory lip. Low-speed drilling away from the promontory lip was performed at 1000 r/sec. Ivalon (a PVA sponge) was placed between the FMT and the round window. The FMT was stabilized at the distal end with cartilage and covered with fascia. This construction was covered with fibrin glue.
Because they were fitted with the new Amade audio processor (AP), patients number 20 to 24 were only evaluated radiologically.
Determination of the FMT-RW position was performed with an Allura Xper FD20 system (Philips Medical Systems, Best, Netherlands), using a flat panel detector. The system’s parameters were as follows: entrance field of 22 cm, 274 mAs, 95 kV, 180° rotation, 241 projections, and filter 0.90 mm Cu + 1.00 mm Al and postero-anterior (p.a.). The focus panel distance was constant during the whole rotation at a frequency of 30 pic/s. The 3D angiography was performed in the unsubtracted mode. From this volume data set, the temporal bones were secondarily enlarged (FoV of 100 mm), digitally stored, and sent to an external workstation (Extended Brilliance Workspace, Philips, Cleveland, USA) for the 2D- and 3D-reconstruction. The actual classification of the single scans was performed independently by two ENT surgeons and one radiologist.
Since a correlation between the radiologic classification and the functional gain could not be observed (Figure
The AF should serve as an indicator for the quality of RW coupling. Two major determinants were considered, that is, the functional gain (calculated as warble tone threshold at the patients preferred volume settings minus postoperative bone conduction threshold) and the so-called audio processor reserve. For a better visualization of the functional gain, the value is multiplied by −1. The audio processor reserve was measured after the fitting by using an audio processor Type 404 (AP 404) in Kuppler mode (with a 2 ccm chamber at 65 dB). The Kuppler value in dB recorded to reach the aided threshold at 2 kHz was subtracted from the Kuppler value to reach the maximum possible AP gain. This value was called AP reserve (APR). The AF was calculated by adding this APR value to the functional gain at 2 kHz. The measurements and calculations were done at and referred to 2 kHz to eliminate a bias induced by a deprivation-related hypersensitivity to higher frequencies [
No significant difference (Freq: 0.5; 1; 2; 4; 49.0 dB +/− 14.4 versus 51.5 dB +/− 16.9) was found between the pre- and postop BC thresholds (Figure
Comparison of the pre- and postoperative BC.
A mean functional gain of 10.4 dB with a maximum value at 1,5 kHz, 2 kHz, and 3 kHz of 35 dB and a minimum value at 500 Hz of −25 dB was observed. The individual mean functional gain (.5 Hz, 1 kHz, 2 kHz, 4 kHz/4) is presented (Figure
The FMT position in the round window niche as seen on the flat panel angiography could be categorized into 4 different patterns. Figure
Type IV coupling—FMT directly at the RW in a rectangular position.
Type III coupling—FMT in contact to RW, not rectangular.
Type II coupling—FMT in RW niche, no direct contact.
Type I coupling—FMT not in RW niche.
The first retrospective series (surgeries 1–7) showed the occurrence of a learning curve in terms of optimizing the FMT position within the RW niche (Figure
Chronology of implantations. Relationship between radiologic classification (red line) and AF values (blue line). Circles indicate cases of bad coupling. The
The temporal changes of the AF at the first fitting showed an increase with the number of surgeries over time (Figure
Except for patients 2 and 3, the AF and the radiological FMT position (according to the classification) were directly related. In cases 2 and 3, the AF decreased after the first fitting (Figure
We considered different aspects which corresponded to the various ways in which the FMT was positioned.
In some of our patients, the promontory lip was not or not completely removed and the FMT was merely pushed into the RW niche. This led to a worse radiological position than in the patients where a complete visualization of the RW was possible. This mainly involved the patients in the retrospective group (subjects 2, 3, and 6), but it also occurred in the prospective part of the study (patients 11, 12).
The stabilization of the FMT in the RW niche was performed in different ways. In patients 4, 5, and 8–24, the FMT was supported at the distal end with cartilage, covered with fascia, and stabilized with fibrin glue. Case 7 differs since the cartilage was placed on the fascia. In patients 1, 2, 3, and 6, the FMT was supported with fascia only, which is responsible for the large variability in the retrospective arm of the study. These patients showed worse results in the radiological classification.
Fascia was used to connect the FMT to the RW in patients 1–7 (the retrospective arm of the study), whereas in patients 8–24 Ivalon was used (prospective arm). Patient 4 experienced a migration of the FMT away from the RW and underwent revision surgery to further stabilize the coupling (the radiograph after revision is depicted in Figure
Revision surgeries after radiological evaluation without decoupling. Stars with the same colour indicate revised cases before and after the revision. The red diamond indicates a case which was revised but had no scan available from after the first surgery (retrospective case). AF values in dB.
Exemplar estimation of the AF for surgeries 5 and 14 with the specific information attached.
Mean functional gain (overclosure) in dB.
The patients were clinically reevaluated after the radiological examination of the FMT position. In some cases, this led to a transtympanic repositioning of the FMT at the RW. An optimized radiological position was achieved in all this patients (Figure
Colletti’s suggestion to position the FMT at the RW extended the indication range for middle ear implants from pure SNHL to mixed hearing losses.
The validity of positioning the FMT at the RW niche with respect to the clinical outcome is obvious since different groups [
However, the surgical challenges linked to the FMT-RW niche coupling account for a high variability in the outcome and should not be underestimated. Preservation of the cochlear integrity is of central importance in this specific approach. Drilling at the ossicular chain or the promontory can lead to a noise exposure of more than 130 db SPL [
Postoperative radiological control is helpful to improve the quality of the surgical approach, to monitor the audiological outcome, and to be able to decide which revisions may be required after cochlear implantation and stapes surgery [
One author suggested monitoring the FMT transfer function at the RW by eBERA or EcoG [
Although not double-checked by eBERA or EcoG, the close correlation between AF and radiological classification led us to assume that it might provide valuable information about the coupling quality of the FMT at the RW. Because of the inhomogeneous audiological indication criteria, in comparison to the classical incus coupling, a correlation between the radiological classification and the pure FG was not observed (Figure
Additionally, the measured APR cannot be added to the FG to calculate a hypothetical threshold. The individual presence of APR gives information about a relative amount of reserve. The reachable aided threshold is individually variable and influenced by further factors.
The temporal changes of the AF and the radiological classification over time clearly point to the occurrence of a learning curve in terms of improving the FMT-RW coupling. Patients 2 and 3 (retrospective group) emphasize this learning curve. Interestingly, the individual AF of these two patients deteriorated over time after the first AP fitting (Figure
When considering the radiological classification, the necessity of drilling away the promontory lip becomes obvious. This is demonstrated in cases 2, 3, 6, 11, and 12. With or without a partial removal of the promontory lip, only a type 1 or type 2 position was achieved.
In the rare case of a wide RW niche [
Another important aspect in achieving optimum coupling is the fixation at the distal end of the FMT in the hypotympanum. In the first 7 cases of our series, only fascia was used for this purpose. In cases 8–24, the fixation was performed with cartilage, which proved to be a stable coupling mode that kept the AF stable as well.
At the moment, there is no long-term data available in our series to define the influence of fascia, cartilage, and/or their resorption on the AF. The same holds true for the Ivalon placed between the FMT and the RW. While fascia was applied in the first 7 patients, the latter material was used in patients 8–24. It has already been described that these PVA sponges can be invaded by granulomatous tissue [
In our revision cases (Figure
The postoperative and retrospective scanning of the FMT led us to revise a number of cases (Figure
The clinical findings of our series are in line with the temporal bone studies reported elsewhere [
Comparing the effective radiation dose of a flat panel observation is 1/3 of a temporal bone CT scan [
Based on our data, a postoperative scan of the FMT position can be strongly recommended for various reasons. This scanning is used as a tool for quality control of the FMT position, and reliably improves the surgeon’s learning curve. It should definitely be performed in patients with poor functional gain to help the surgeon make a decision about whether revision surgery may be necessary for positional reasons.
The authors declare that there is no conflict of interests.