EXPERIENCE WITH THE VIBRANT SOUND BRIDGE IMPLANT, INDICATIONS AND RESULTS
Introduction
Conventional hearing aids are not sufficiently satisfactory or are not well tolerated by a significant number of patients with severe sensorineural or mixed hearing loss1 because of a chronic, otological pathology in the middle and external ear. Active middle-ear implants (AMEI) could be an excellent option in these cases. The Vibrant Sound Bridge (VSB) is the most extended AMEI system of the last decade.2–4 This device processes and amplifies sound stimuli through an electrical-magnetic transducer called the Floating Mass Transducer (FMT), which transfers sound vibrations to the cochlea when applied to an element of the tympanic ossicular chain or in the proximity of the windows.
In the last few years, several authors5,6 have published studies of patients in whom the FMT of the VSB has been placed on the round window (RW) membrane. They have assumed that the impact of this vibration on the basilar membrane is similar to air conduction through the ossicular chain to the oval window.7–9
Initially, there are several advantages in placing the FMT on the RW. On this location, the external and middle ear are bypassed. Because of this, for example, this device may be used in ears with sequelae of chronic otitis media (COM), where the function has not been successfully restored. The indication for ears treated with open techniques is especially remarkable since the device can work without air spaces in the middle ear. The objective of the study is to analyze results obtained with the VSB in a group of patients in whom the original purpose was to place the FMT in the RW.
Materials and methods
Population
There has been a retrospective study (from 2007 to present) on the indications and results obtained by individuals implanted at our center with the VSB. Table 1 shows demographic data of the patients studied and the primary pathology on their implanted ear. The follow-up period ranged from five to 64 months (an average of 41.2 months). Eight patients had a history of simple chronic otitis media (COM), four patients had cho-lesteatomatous COM and one patient suffered from acquired stenosis in the external auditory canal (EAC), associated with tympanosclerosis.
Description of the surgery
In six patients, the implant was placed by means of an already existing radical mastoidectomy. A simple mas-toidectomy and a posterior tympanotomy to expose the RW were carried out on the remaining seven patients. To correctly place the FMT on the membrane of the window, the titanium clip designed to place the FMT on the ossicular chain was cut off.
Table 1. Demographic data of the patients included in the study
The edges of the RW were carved until the RW membrane was exposed completely. Afterwards, the fossula ante fenestram was shaped to accommodate the FMT in front of the RW membrane. The FMT connection cable was arranged on the walls of the mastoid cavity or the inferior wall of the EAC. The cable and the FMT were handled with non-ferromagnetic microsurgical material. A sheet of fascia temporalis was placed between the FMT and the RW membrane. The correct contact was checked and secured with a wedge of cartilage placed between the FMT lower end and the bone recess drilled in the fossula ante fenestram. Moreover, a graft of fascia was used to insulate the fossula ante fenestram from the remaining middle ear spaces.
In patient #3, the FMT was placed onto the oval window, since the surgery revealed that the round window was ossified.
A canal was drilled in the bone with a 1.5 mm diamond burr to lodge the device’s cable in patients with a history of radical mastoidectomy. The cable’s passage through the middle ear spaces was protected with cartilage sheets. The canal was drilled through the facial wall in the first patients. Afterwards, the canal was drilled through the inferior wall of the EAC, at the level of the tympanic bone, following the trajectory towards the tip of the mastoid. For patients undergoing a closed mastoidectomy, the cable was secured with temporal muscle grafts in the mastoid cavity.
Assessments pre- and post-operation
All patients underwent a pre-operatory otological study, including a micro-otoscopy, CT of petrous bones, air and bone-conduction (AC and BC respectively) pure-tone audiometry, and speech-recognition test with bisyllabic words.
After surgery, pure-tone air-conduction thresholds with headsets were tested, without the VSB external component. Pure-tone and speech audiometries using bisyllabic words were performed in free field with the VSB disconnected and connected. The unaided ear was masked using a TDH 39 headset with a NB sound at 70dB SPL. All tests were performed in a soundproof booth.
The VSB audioprocessor (Siemens Signia) has eight frequency bands. It can provide a maximum gain of 70 dB and a maximum output of 113 dB. The initial adjustment was made with the CONNEXX software, referenced to pre-surgery BC thresholds. In subsequent checkups, the gain increased progressively in all eight frequencies up to a comfortable level for the patient. The optimal performance of the system was usually reached three to four months after the first activation.
Statistical procedure
To compare results before and after the surgery, a Student T for related samples has been carried out, using the SPSS 20.0 computer program.
Auditory results
Figure 1 shows the average values of AC and BC thresholds recorded for all patients before and after the surgery. In this case, it illustrates AC thresholds obtained with the VSB post-activation.
Fig. 1. Mean BC and AC thresholds before and after VSB.
Fig. 2. Mean maximum speech recognition score, speech recognition score and speech detection threshold before and after VSB.
A statistically significant gain (p < 0.001) was observed in average tone thresholds of all frequencies studied (0.5–6 kHz) when comparing AC before and after activating the VSB. This gain was more evident in 1, 2 and 4 kHz frequencies. The average auditory gain (Preop AC vs VSB AC) in the frequency range 0.5–6 kHz was 44.07 dB.
All patients had sensorineural hearing loss to some extent. The average AC audiometric performance after activating the device is significantly better than the pre-operatory BC performance in ten out of 13 patients studied. The average gain was 11.3 dB (p = 0.005).
When comparing the pre- and post-surgery BC, no statistically significant differences (p = 0.201) have been found.
The speech understanding performance with and without the VSB using bisyllabic words is represented in Figure 2. The detection threshold was significaly reduced (p = 0.012). The maximum speech recognition score, and the speech recognition score with 65 dB SPL also showed a significant improvement (p = 0,024 and p = 0,004 respectively) when using the VSB.
In patient #7, the implant cable was partially exposed in the mastoidectomy cavity two years after implantation. It was covered with free skin graft under local anesthesia. After one year, the cable became exposed again, and, once again, it was covered with cartilage graft. The performance of the device was stable at all times. Patient #8 required a surgical check up under local anesthesia twice (five and 13 months after the operation, respectively). An exploratory tympanotomy revealed the FMT was moved so contact with the RW was not optimal. It was correctly adjusted. The device’s performance immediately improved, and this gain was sustained over time.
Patient #6’s device failed twice. The metallic cable connecting the FMT to the processor was broken in the second time, although the silicon sheath was intact. The device was explanted, and with the new implantation, the auditory gain of the patient is now similar to the gain acquired with the device before it failed.
Patient #5 did not obtain a satisfactory performance from his device, despite the fact that it was working correctly. The audiological profile included profound hearing loss in the 2, 3 and 4 KHz frequencies and the inability to recognize words. The VSB performance was somewhat beneficial, and the air conduction thresholds reached 70 dB in the free-field-tone audiometry. However, the device did not improve his performance in the speech recognition test. Figures 3 and 4 shows the audiometric results of this patient in the affected ear. Finally, the team decided to place a cochlear implant in the left ear. The results obtained were excellent, as illustrated by the same figures.
Fig. 3. AC and BC threshold of patient # 5 before BSB, AC threshold with VSB and AC threshold with CI.
Fig. 4. Speech recognition score of patient # 5 with VSB, with the CI, and with the hearing aid in the contralateral ear.
Discussion
Patients included in this study showed medical or audiological contra-indications to the use of conventional hearing aids or bone-anchored hearing aids. Most patients suffered from severe to profound hearing losses, some even had hearing loss patterns in some frequencies beyond the recommended criteria set by the VSB implant manufacturing company.
It is important to point out that this study has evidenced an improvement of the audiometric performance beyond expectations, since the air-bone gap was closed in ten out of 13 cases. Other bone-integrated or bone-conduction implants do not replicate this improvement. This may be due to the location of the FMT on the round window, which can convey vibration directly to the fluids of the inner ear. Likewise, the VSB processor may provide additional sound amplification.
This gain above the air-bone gap varied among patients. Several factors may have an impact, such as the anatomy of the round window, the degree of cochlear damage, the stapes mobility or the coupling of the FMT onto the RW. It is obvious that if the total surface of the FMT is not in contact with the RW membrane, the intensity of the sound transmission will be worse. The fixation of the stapes has been linked to poorer audiological results.6,9
The audiological results are highly positive. Patients reported a high level of satisfaction in spoken word comprehension. In the group of patients with severe to profound mixed hearing losses, results have been very rewarding, since the evolution of these subjects had not been satisfactory with other treatments (tympanoplasty, hearing aids) and they were rescued by the VSB implantation.
The final audiological results depend on several factors, among others, previously existing cochlear function, the frequency-response curves and the programming of the processor.
Regarding the possible appearance of an acoustic trauma or direct damage to the cochlea, no significant differences have been detected in BC thresholds pre- and post-VSB to date in any of the subjects studied.
The surgery to place a VSB implant on the RW has a medium-high difficulty, although results obtained are on par with the challenge, as already mentioned. There are several characteristics, based on the experience acquired with this group of patients worth mentioning:
•By placing the FMT on the round window membrane, you can use it in ears after a radical mastoidectomy. Therefore, ossicular residues or ventilation of the middle ear spaces are not necessary for its proper functioning. Because of these characteristics, it is very useful for patients with severe sequelae in the context of COM due to tubal dysfunctions.
•Seeking to prevent the exposure of the connecting cable in the mastoidectomy cavities, it is recommended to drill a channel in the inferior area of the pathway, at the level of the tympanic bone, not on the facial wall. Once the cable is placed, it will be covered with bone paté and cartilage. This solution has reported excellent results, and it contributes to reduce the risk of materials fatigue that end up breaking the connecting cable.
•In case the FMT is displaced, it is possible and recommendable to do the check-up surgery under local anesthesia. Aside from being a well-tolerated procedure, it provides immediate intra-operative feedback on the correct positioning of the FMT, by activating the implant during the surgery and gathering the impressions of the patient.
Conclusions
The surgery to implant the VSB requires experience and skill. The FMT must be perfectly coupled onto the RW membrane to attain an optimal performance of the device.
This study suggests the VSB middle-ear implant provides a satisfactory audiological gain in high frequencies to patients with moderate to severe mixed hearing losses. Patients attained greater comfort than with conventional hearing aids.
The VSB is considered to be an appropriate device to be used by patients with BC losses up to 70 dB (a threshold beyond the manufacturing company’s recommendation). There must be present hearing levels in all frequencies between 0.5 and 4 KHz, and the maximum speech recognition score must be beyond 60% in the ear chosen for implantation.
1.Boeheim K, Pok SM, Schloegel M, Filzmoser P. Active middle ear implant compared with open-fit hearing aid in sloping high-frequency sensorineural hearing loss. Otol Neurotol 31(3):424–429, 2010
2.Uziel A, Mondain M, Hagen P, Dejean F, Doucet G. Rehabilitation for high-frequency sensorineural hearing impairment in adults with the Symphonix Vibrant Soundbridge: a comparative study. Otol Neurotol 24:775Y83, 2003
3.Sterkers O, Boucarra D, Labassi S, et al. A middle ear implant, the Symphonix Vibrant Soundbridge: retrospective study of the first 125 patients implanted in France. Otol Neurotol 24:427Y36, 2003
4.Schmuziger N, Schimmann F, aWengen D, Patscheke J, Probst R. Long-term assessment after implantation of the Vibrant Soundbridge device. Otol Neurotol 27:183Y8, 2006
5.Colletti V, Soli SD, Carner M, Colletti L. Treatment of mixed hearing losses via implantation of a vibratory transducer on the round window. Int J Audiol 45:600Y5, 2006
6.Beltrame AM, Martini A, Prosser S, Giarbini N, Streitberger C. Coupling the Vibrant Soundbridge to cochlea round window: auditory results in patients with mixed hearing loss. Otol Neurotol 30(2):194–201, 2009
7.Garcia-Ibanez. Sonoinversion: A New Audiosurgical System. Arch Otolaryngol 73:268–272, 1961
8.Wever G, Lawrence M, Smith K. The middle ear in sound conduction. Arch Otolaryngol 48:19Y35, 1948
9.Stenfelt S, Hato N, Goode RL. Fluid volume displacement at the oval and round windows with air and bone conduction stimulation. J Acoust Soc Am 115:797Y812, 2004
Address for correspondence: Dr. Manuel Manrique Rodriguez, Otorhinolaryngology Department, University Clinic of Navarra, Pio XII 36. Pamplona 31008, Navarra, Spain. mmanrique@unav.es
Cholesteatoma and Ear Surgery – An Update, pp. 285–290
Edited by Haruo Takahashi
2013 © Kugler Publications, Amsterdam, The Netherlands