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The Implantable Middle Ear Amplifier
Implantable middle ear amplifiers represent the most recent breakthrough in the combined medical and rehabilitative treatment of sensorineural hearing loss. Although traditional hearing aid technology remains a viable option, this technology evolved as a result of the inherent shortcomings of conventional hearing aids and the associated consumer dissatisfaction with them.1 Traditional hearing aids can cause hearing aid-induced signal distortion, feedback, and the negative consequences of signal fidelity attributable to the occlusion effect, particularly those with greater degrees of sensorineural hearing loss.2 In other words, individuals approaching moderately severe or severe degrees of sensorineural hearing loss, who are most reliant on high-fidelity signals to achieve necessary functional benefit, are most susceptible to receiving the poorest fidelity amplification.
Middle ear implants were designed to overcome these disadvantages. By driving the ossicular chain directly, the output speaker component of hearing aids is eliminated, providing a more natural, less distorted signal to be introduced to the cochlea. With more energy directed to the inner ear, acoustic feedback is eliminated or significantly reduced. In addition, because the majority of middle ear implants do not require any components in the external auditory canal, the ear remains open, eliminating the occlusion effect while enhancing sound quality via the natural resonance of an open ear canal.
■ Transducer Technology
Three types of technologies, each with inherent advantages or disadvantages, may be applied to middle ear implants: (1) piezoelectric, (2) electromagnetic, and (3) electromechanical. Piezoelectric devices use small crystals that are either connected to the ossicular chain or, in some instances, replace lateral portions of the ossicular chain.3 When a piezoelectric crystal is bent, it generates an electrical voltage; likewise, when an electrical voltage is applied, the crystal bends.4 Ultimately, the deformation of the piezoelectric crystals provides the mechanical energy needed to stimulate the ossicular chain. Piezoelectric transducers are very efficient in transmitting high-frequency energy; however, output for low frequencies is dependent upon the size of the crystal. Due to the relative small space of the middle ear, piezoelectric transducers are likely limited in low-frequency output.
In contrast, electromagnetic transducers consist of both an energizing coil and a magnet. The coil, energized by an electrical input, creates a magnetic field, causing the magnet to vibrate. The vibration of the magnet, which makes contact with one of the middle ear bones, results in the direct stimulation of the ossicular chain. With electromagnetic technologies, the two main components must be maintained in close proximity to achieve an efficient system because the associated force generated by this arrangement is inversely proportional to the cube of the distance between the coil and the magnet. In other words, as the distance between the magnet and the coil increases, the associated output significantly decreases. Lastly, electromechanical transducers function similarly to electromagnetic transducers with one significant difference. Unlike electromagnetic systems, the energizing coil and magnet of the electromechanical transducer are housed within the same assembly. In this way, the efficient output of the device does not vary because the coil and the magnet always remain in a fixed position relative to one another.
■ Middle Ear Implants: Available Systems and How They Work
Two middle ear implants, the Direct Drive Hearing System (DDHS) (SoundTec, Inc., Oklahoma City, Oklahoma), and the Vibrant Soundbridge (MED-EL Corporation, Durham, North Carolina), are commercially available. The Middle Ear Transducer (MET) Ossicular Stimulator (Otologics, LLC, Boulder, Colorado) and the Envoy system (St. Croix Medical, Minneapolis, Minnesota) are in different stages of Food and Drug Administration (FDA) clinical trials and anticipated for commercial availability in the near future (Table 29 The implantation of each of these devices can be surgically challenging. Table 29–3 considers possible points of difficulty during the surgical procedures.
Direct Drive Hearing System
The DDHS is a semi-implantable device consisting of both internal (implantable) and external components. The internal component consists of a magnet hermetically sealed in a titanium canister. A wire-form attachment ring encompasses the canister so that it can be surgically placed over the head of the stapes. Because the implant resides at the level of the incudostapedial joint, disarticulation of the ossicular chain is required. Two external components are also required with this system: (1) an earmold/coil assembly (ECA) housed within a deep-fitting completely-in-the-canal casing, and (2) a speech processor available in either a behind-the-ear (BTE) or in-the-ear (ITE) model. The speech processor, an analog, class D, two-channel wide-dynamic-range circuit, detects and amplifies sound, sending the electronic signal to the transmitter coil residing within the ECA.5 The ECA induces an electromagnetic field that activates the magnet, vibrating the ossicular chain.
Vibrant Soundbridge
The Vibrant Soundbridge consists of an external audio processor and an electromagnetic system referred to as
the vibrating ossicular prosthesis (VORP). The VORP contains a receiver, conductor link, and the floating mass transducer (FMT). The FMT consists of two energizing coils wrapped around a hermetically sealed titanium housing. A small magnet resides within the housing, supported by a pair of springs. A titanium clip extending from the housing is used to attach the FMT to the long process of the incus. Sound picked up by the audio processor is digitally processed, amplified, and sent via radio frequency signals transcutaneously to the VORP. The VORP receives the signal and transmits the information to the FMT that, via an electromagnetic field, directly vibrates the ossicular chain.
The MET Ossicular Stimulator
The MET Ossicular Stimulator from Otologics is a semi-implantable device consisting of the implantable electromechanical system and the externally worn button audio processor. The MET Ossicular Stimulator consists of an electronics capsule, transducer lead, and the proprietary middle ear transducer (Fig. 29–1A). The transducer lead is uniquely attached to the electronics capsule using IS-1 connector technology, the standard connector incorporated in pacemaker technology ( Fig. 29–1B). The advantage of this feature is that the transducer lead can be disconnected from the electronics capsule, allowing later upgrades to fully implantable models without requiring removal of the original transducer. Sound is picked up by the microphone of the Button Audio Processor, digitally modified according to the amplification needs of the wearer, and transcutaneously transmitted to the electronics capsule. The information is fed down the transducer lead, activating the MET Ossicular Stimulator. The transducer, housing both the energizing coil and the magnet, activates a probe tip that is coupled to the body of the incus. The mechanical motion of the probe tip directly drives the ossicular chain.
The Envoy System
The Envoy system (St. Croix Medical) is a fully implantable, piezoelectric system consisting of a sensor, internal sound processor, and driver. The tip of the sensor makes contact with the malleus. As the tympanic membrane moves, the vibrations of the ossicular chain are detected by the sensor, which in turn generates a voltage proportional to malleus vibration.4 The voltage is routed to the sound processor via the sensor lead, where the signal is processed and amplified. The sound processor sends the information to the driver, whose tip makes contact with the head of the stapes. Voltage applied from the sound processor to the driver results in direct vibration of the stapes.
FIGURE 29-1 (A) The Otologics MET Implantable Hearing Device. (B) The application of IS-1 connector technology allows for disconnection of the transducer and transducer lead from the electronics capsule. (Courtesy of Otologics LLC, Boulder, Colorado.)
■ Surgical Procedures
Implantation of the Direct Drive Hearing System
The DDHS is implanted through a transcanal approach performed as a same-day surgery procedure under local anesthesia.5 A tympanomeatal flap is raised to visualize the incudostapedial joint.6 The incudostapedial joint is disarticulated to place the wire-form magnet on the stapes.
As described by Hough et al, the incudostapedial joint is disarticulated utilizing the suture retraction technique, where a 4-0 suture with a 1 mm steel ball on one end is placed on one side of the incus and maneuvered into position into the oval window niche.6
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