The Envoy Esteem (Envoy Medical, St. Paul, MN) received FDA approval for use in the US in 2010 for adults ages 18 or over who have a stable bilateral moderate to severe sensorineural hearing loss with unaided word recognition scores of >40 % and a normal tympanic membrane [16, 17]. Patients must also have enough space for the implanted device and undergo at least a 30-day trial with hearing aids prior to implantation. Co-existing otologic pathologies are a contraindication to the Esteem.
The Envoy Esteem is a fully implantable device that consists of two piezoelectric bimorph crystals, the sensor and the driver, attached to a processor. The speech processor and non-rechargeable battery are hermetically sealed within titanium casing . The sensor is attached to the incus and acts as the system’s microphone, using input from the tympanic membrane . The driver is connected to the stapes. Sound is funneled through the ear canal and vibrates the tympanic membrane, which in turn moves the malleus and incus. The sensor, using the motion of the incus, sends electric impulses to the speech processor, which modifies the signal. The modified signal is sent from the speech processor to the driver, which vibrates the stapes.
Surgery and Outcomes
Surgery has been reported as having a steep learning curve indicated by the time spent in the operating room, with reports ranging from ~4–8 h [16, 18, 19]. Surgery includes a mastoidectomy and epitymanpanectomy to ensure enough room for the device components. The ossicles are disarticulated at the incudostapedial joint, and resection of the long process of the incus is performed with a laser. The transducers are cemented to the appropriate ossicles, and the system is tested intraoperatively using a laser Doppler vibrometer. The device is then turned off until activation.
Adverse events and adverse device effects noted in patients implanted with the Envoy Esteem ranged from mild to serious and included wound infections, delayed-onset facial paresis or numbness, delayed fibrosis that impeded device function, taste disturbance, tinnitus, vertigo, and headache [16, 18]. Reports on audibility with the Esteem indicate improvement in air conduction thresholds over both preoperative unaided and aided (with a conventional hearing aid) scores, although not always a statistically significant difference [17, 18].
Literature regarding the Envoy Esteem has been limited and includes small sample sizes and restricted follow-up durations. Patients should appreciate the ease of use, full-time wearability, and invisible equipment. Unlike surgical procedures of other implantable hearing systems, the removal of the long process of the incus during Esteem implantation creates additional hearing loss and could limit options should the patient decide not to use the Esteem in the future. Surgical changing of the battery is also a factor to consider as exposure to noisy environments and use of high gain can significantly shorten the expected battery life. Battery life expectancy is reported by the company at 4.5–9 years with reports in the literature of several patients with battery changes between 28 and 39 months .
Otologics MET and Carina
The Middle Ear Transducer (MET) (Otologics, Boulder, CO) is a semi-implantable hearing device that has been used in adults with moderate to severe sensorineural hearing loss . Otologics later updated the MET to a fully implanted hearing device named Carina for use in adults with moderate to severe sensorineural hearing loss, which later received a CE Mark in Europe for use in conductive and mixed hearing losses [22, 23]. Because of the flexibility of the internal components, Carina candidacy can encompass patients that have certain middle-ear anomalies, which would have excluded them from the Envoy Esteem implantable hearing aid .
The Otologics MET is comprised of both internal and external components. External components consist of a Button Audio Processor that houses the signal-processing system, microphone, battery, and transmitter coil . Internal components include the electronics capsule and mechanical transducer . The lead of the MET could be disconnected from other internal components to allow conversion to the fully implanted device without disturbing transducer placement, once such a device was made available. The Button Audio Processor processes and transmits the signal to the internal capsule transcutaneously by means of FM transmission [24, 25].
The fully implantable Otologics Carina includes an internal magnet, battery, signal processor, receiver coil, and connector all housed in an electronics capsule, as well as a microphone and the transducer system [23, 26]. There are multiple transducer types with the selection of a transducer type dependent on where the transducer is to be placed, determined by the patient’s middle ear anatomy [26, 27]. The receiver coil allows for transcutaneous programming of the device and recharging of the battery. To charge the battery, a special external charger is placed over the implant site for ~1–1.5 h. A remote control is also available to adjust volume and power off and on the device.
Surgery and Outcomes
Microphone and transducer positioning are very important during surgical placement of the Otologics Carina device. The microphone placement can be in the temporalis region, the retroauricular region, or on the mastoid tip, avoiding areas that are prone to tissue thickness changes and contraction effects of the temporalis and sternocleidomastoid muscles [26, 27]. Just as there are many available transducer tips, there are also many configurations for transducer placement. Reports of transducer placement have included mounting directly onto the incus, into a hole drilled into the body of the incus, at other ossicular sites including the footplate of the stapes, and at the round window membrane [21, 23, 26–28]. Efficient coupling is important at the transducer site to prevent unnecessary battery drain, achieve appropriate gain, and avoid additional hearing loss [27, 28]. Adverse events and adverse device effects reported throughout the literature include partial or total device or microphone extrusion, a fullness or pressure sensation, conductive hearing loss, lightheadedness, tinnitus, middle ear effusion, feedback, device failure, and extended length of time for the battery to recharge [26–28].
Improvement in hearing and speech perception when compared to preoperative hearing testing has been noted on multiple occasions, although thresholds below 30 dB HL were not regularly observed [21, 23, 26]. Subjective benefit using the APHAB questionnaire indicated that patients prefer the Carina over traditional amplification or their preoperative unaided condition, but there was also an increase in aversiveness to sound with the Carina [23, 26].
The fully implantable Otologics Carina introduces options for patients with abnormal middle ear anatomy due to the variety of available transducer tips. However, the Carina is not MRI compatible, and surgery is needed to change the battery. Literature is limited, and additional studies comparing outcomes and long-term effects of the Carina and MET are warranted. In June 2013, Otologics was purchased by Cochlear Ltd., which plans to incorporate the technology into acoustical implant development (Cochlear Annual Report, 2013).
Vibrant SoundBridge (VSB)
The Vibrant Soundbridge (MED-EL, Innsbruck, Austria) is a semi-implantable hearing system that received FDA approval for use in the US in 2000 . The device is approved for adults with moderate to severe sensorineural hearing loss who cannot benefit from a conventional hearing aid [29–32]. Audiometric candidacy criteria include bone conduction thresholds up to 65 dB HL at 500 Hz and 85 dB HL at 6,000 Hz . Uses for both children and mixed hearing loss have also been investigated [29, 30, 33]. Like the Otologics MET and Carina, flexibility of the internal component placement has the potential to make this device appropriate for patients with a wide range of middle ear anomalies.
The Vibrant Soundbridge (VSB) consists of an external audio processor and an internal Vibrating Ossicular Prosthesis (VORP) [32, 34]. The audio processor contains a microphone, speech-processing capabilities, transmitting coil, magnet, and battery [29, 35]. The internal VORP is comprised of a receiver module, magnet, conductor link, and floating mass transducer (FMT). Magnetic force between the audio processor and internal device keeps the external unit in place. Speech is processed in the audio processor, which transmits the signal transcutaneously to the internal receiver unit. The signal then travels to the electromagnetic FMT where vibrational energy is realized and either the ossicular chain or inner ear is stimulated directly, depending on FMT placement.
Surgery and Outcomes
Traditional placement of the FMT for patients with sensorineural hearing loss is to clip the device to the long process of the incus. There are concerns regarding possible necrosis of the ossicle using a clip; however, significant widespread cases of necrosis were not identified in a literature search [30, 32]. Investigations of alternative coupling sites in the middle ear have been explored to address instances where conductive or mixed hearing loss is present because of ossicular structural abnormalities. FMT placement sites include placement at the stapes head, crura, mobile footplate, or oval window and can be used in conjunction with a PORP, TORP, or other coupler [29, 31, 36•].
In cases where the stapes footplate is not mobile or not intact, the FMT can be coupled to the round window [37–39]. Placement of the FMT at the round window has been shown to be a safe and effective method for overcoming conductive or mixed hearing loss [36•, 37]. This unique placement can provide amplification in cases where ossicles or a tympanic membrane are not present such as in canal wall down cavities. Potential complications for all approaches include FMT displacement, additional hearing loss, dizziness, device failure, facial nerve paralysis, postoperative wound-healing issues, and gustatory disorders, although some studies fail to report any complications [29, 31, 33, 36•,37, 40].
Few or no changes in unaided air conduction thresholds or bone conduction thresholds have been reported postoperatively [30, 31, 33, 36•, 37]. Improvement in hearing thresholds and word recognition scores were noted when comparing use of the VSB over an unaided condition. Boheim and colleagues reported that long-term postoperative audiologic results using the VSB were stable at an average of 40 months when compared to postoperative scores at 3 months [36•]. Performance with the VSB has shown comparable hearing benefit to use with conventional hearing aids [41••].
As with all implantable hearing devices, the risk of surgery must be weighed with expected benefit. This is particularly true when outcomes do not show any statistical performance benefit over conventional amplification. Use of MRI is currently contraindicated with the VSB. Although the FDA has approved this device for use in the US, insurance coverage continues to be a barrier to treatment. The VSB has shown utility for patients with all types of hearing loss and, unlike the Envoy Esteem, can be used when the ossicular chain is not intact.
Direct Acoustic Cochlear Stimulator Partial Implant (DACS)/Codacs
The DACS (Phonak Acoustic Implants SA, Switzerland) is a partially implantable hearing device for moderate to severe mixed hearing loss [42, 43]. Both Phonak Acoustic Implants SA and Cochlear originally collaborated on the development of the system . The DACS (a.k.a. DACS-PI) was primarily investigated for use with advanced otosclerosis using the idea of applying a power-driven stapes piston to stimulate the cochlea directly rather than performing a traditional stapedectomy and then utilizing a conventional hearing aid to overcome any remaining conductive or sensorineural hearing loss [42, 43, 45]. Recommended audiometric criteria for bone conduction thresholds are between 40 and 80 dB . The Codacs (Cochlear Ltd., Sydney, Australia) is a modified version of the DACS [46•].
The DACS system consists of both an external audio processor and internal components . The original DACS audio processor included two microphones, a battery, and a sound processor . The audio processor would connect to the internal system by means of an implanted percutaneous plug. The external processor was later modified to include a magnet and RF coil to allow for transcutaneous signal delivery to the internal device, negating the need for the percutaneous plug. The internal system consisted of a percutaneous plug that was later replaced by an RF link, a fixation system, a transducer with an artificial incus, and a piston prosthesis.
The Codacs device includes a modified Cochlear Nucleus Freedom sound processor (Cochlear Ltd., Sydney, Australia) and RF coil as its external components. Internally, the device consists of a receiver coil, transducer, electronics, and fixation system.
Surgery and Outcomes
Limited reports on the DACS make comprehensive surgical information difficult to report. Hausler and colleagues reported surgical times ranging from 5 to 2.5 h, with time reduction as surgical experience was gained . The DACS transducer is surgically placed behind the ear and the actuator is placed near the incus. The footplate of the stapes is removed, and a conventional prosthetic stapes is crimped onto the actuator. In order to have the patient achieve improved hearing even with the DACS system turned off, the surgeon has the option to include a second prostheses attached to the patient’s incus parallel to the first prosthetic . Postoperative complications reported in the literature range from no complications to infections around the percutaneous plug in the older device, pain, dizziness, and need for ear canal reconstruction [43–45].
No significant changes in bone conduction thresholds were reported postoperatively, and thresholds remained stable for up to 2 years after surgery [44, 45]. Busch et al.  compared postoperative hearing between the activated DACS system and a conventional hearing aid and found improved APHAB scores with the DACS over the hearing aid as well as improved speech scores and soundfield thresholds, although results were not always statistically significant.
Lenarz et al. [46•] indicate that, while the actuator of the Codacs is the same as the DACS device, the fixation system has been modified to include a ball joint, which is intended to improve device placement options. Adverse events that occurred in the investigational device studies included strange sound sensations, inferior sound quality with the device compared to a hearing aid, nausea, tinnitus, sensation at receiver site, deterioration of bone conduction thresholds, facial palsy, and feedback from the device. Audiometric results showed improvement in word recognition abilities and the ability to amplify high frequencies compared to both the unaided condition and a conventional hearing aid. APHAB scores improved in all subscales except Averseness to Sound (AV) subscale, which showed no changes.
The DACS-PI showed promising results comparing subjects using the DACS system and a conventional hearing aid on the same ear postoperatively with a sample size of nine patients . Lack of larger sample sizes and long-term follow-up, inconsistent testing methods, and limited published literature prevent further evaluation of both the DACS system and Codacs and can invite bias when compiling results. To the authors’ knowledge, Phonak Acoustic Implants is no longer producing the DACS. Codacs has recently received the CE Mark for use in Europe, and only one study could be found regarding the device.