For many hearing-impaired individuals, the benefits of conventional amplification may be limited by acoustic feedback, occlusion effect, and/or ear discomfort. The MAXUM system and other implantable hearing devices have been developed as an option for patients who derive inadequate assistance from traditional HAs, but who are not yet candidates for cochlear implants. The MAXUM system is based on the SOUNDTEC Direct System technology, which has been shown to provide improved functional gain as well as reduced feedback and occlusion effect compared to hearing aids. This and other implantable hearing devices may have increasing importance as future aural rehabilitation options.
Key points
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The MAXUM system (Ototronix LLC, Houston, TX) is a semi-implantable hearing device that provides hearing-impaired patients with an alternative to conventional hearing aids.
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Patients implanted with the MAXUM system technology have demonstrated a functional gain in hearing significantly greater than that of conventional hearing aids.
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Patient questionnaire has shown a subjective reduction in acoustic feedback and the occlusion effect for MAXUM technology users relative to conventional hearing aids.
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The MAXUM system may be surgically implanted in a short procedure performed under local anesthesia, with a low risk of procedure-related adverse events.
Introduction
Hearing loss affects approximately 30 million people in the United States, including up to one-third of individuals older than 65 years. An estimated 75% to 80% of people who could potentially benefit from hearing aids (HAs) do not seek amplification; of the patients who are fitted with HAs, many are poorly compliant in using them. Reasons for this lack of compliance include acoustic feedback/distortion, occlusion effect, ear canal discomfort/irritation, and social stigma. Although new technologies have reduced some of these drawbacks, they have been largely unable to overcome the difficulties that many patients with conventional aids still face when listening in crowded, noisy environments.
Implantable and semi-implantable hearing devices (IHDs) have been developed as an option for patients who derive limited benefit from traditional HAs but who are not yet candidates for cochlear implants. All IHDs have in common the ability to provide amplification via a direct driver to the ossicles, which is stimulated to vibrate by either electromagnetic or piezoelectric energy. Because IHDs do not use a speaker to amplify ambient noise, they eliminate the acoustic feedback seen with conventional aids. The review of recent literature has suggested that IHDs can amplify sound as good or perhaps greater than conventional HAs.
The Ototronix MAXUM system (Ototronix LLC, Houston, TX) is a semi-implantable device that amplifies sound using electromagnetic energy transferred from an external ear canal mold to an internal surgically implanted magnet. The system is based on technology that was previously marketed as the SOUNDTEC Direct Drive Hearing System (SOUNDTEC Inc, Oklahoma City, OK), which received approval from the Food and Drug Administration in 2001. Although the SOUNDTEC device used a behind-the-ear processor, the MAXUM system differs in that it has a combined digital sound processor and electromagnetic coil worn in the ear canal (known as the integrated processor and coil [IPC]). The internal component is the same for both devices and consists of a permanent magnetic implant attached to the ossicular chain.
Sound presented to the external component of the MAXUM system is received by the microphone, amplified, and processed into an electrical signal, which is then delivered to the electromagnetic coil in the ear canal mold. The charged coil then produces an electromagnetic field in the middle ear space, which stimulates the magnet attached to the incudostapedial (IS) joint. Vibrations of the magnet are synchronous to the original sound input, which are then transmitted to the stapes and on to the cochlea.
Introduction
Hearing loss affects approximately 30 million people in the United States, including up to one-third of individuals older than 65 years. An estimated 75% to 80% of people who could potentially benefit from hearing aids (HAs) do not seek amplification; of the patients who are fitted with HAs, many are poorly compliant in using them. Reasons for this lack of compliance include acoustic feedback/distortion, occlusion effect, ear canal discomfort/irritation, and social stigma. Although new technologies have reduced some of these drawbacks, they have been largely unable to overcome the difficulties that many patients with conventional aids still face when listening in crowded, noisy environments.
Implantable and semi-implantable hearing devices (IHDs) have been developed as an option for patients who derive limited benefit from traditional HAs but who are not yet candidates for cochlear implants. All IHDs have in common the ability to provide amplification via a direct driver to the ossicles, which is stimulated to vibrate by either electromagnetic or piezoelectric energy. Because IHDs do not use a speaker to amplify ambient noise, they eliminate the acoustic feedback seen with conventional aids. The review of recent literature has suggested that IHDs can amplify sound as good or perhaps greater than conventional HAs.
The Ototronix MAXUM system (Ototronix LLC, Houston, TX) is a semi-implantable device that amplifies sound using electromagnetic energy transferred from an external ear canal mold to an internal surgically implanted magnet. The system is based on technology that was previously marketed as the SOUNDTEC Direct Drive Hearing System (SOUNDTEC Inc, Oklahoma City, OK), which received approval from the Food and Drug Administration in 2001. Although the SOUNDTEC device used a behind-the-ear processor, the MAXUM system differs in that it has a combined digital sound processor and electromagnetic coil worn in the ear canal (known as the integrated processor and coil [IPC]). The internal component is the same for both devices and consists of a permanent magnetic implant attached to the ossicular chain.
Sound presented to the external component of the MAXUM system is received by the microphone, amplified, and processed into an electrical signal, which is then delivered to the electromagnetic coil in the ear canal mold. The charged coil then produces an electromagnetic field in the middle ear space, which stimulates the magnet attached to the incudostapedial (IS) joint. Vibrations of the magnet are synchronous to the original sound input, which are then transmitted to the stapes and on to the cochlea.
Candidacy for surgery
Candidates for the MAXUM system include adults 18 years of age and older with moderate to moderately severe sensorineural hearing loss (SNHL), ideally with a high-frequency pure tone average (1, 2, and 4 kHz) between 35 and 70 dB. A necessary prerequisite for candidacy is a normal conductive hearing system. Bone conduction thresholds should be within 10 dB of air conduction values, and word recognition scores should be 60% or better. Patients must have normal middle ear anatomy, no history of middle ear surgery, and no evidence of acute/chronic otitis media, retrocochlear lesions, or central auditory disorders.
Preoperative planning, preparation, and counseling
All patients should undergo a thorough otologic examination and audiometric evaluation before being considered for surgery. MRI can be helpful to exclude retrocochlear pathology in cases of asymmetric hearing loss, whereas computerized tomography can evaluate the extent of middle ear/mastoid pathology when the otologic examination suggests abnormalities. All candidates should be provided with other options for hearing rehabilitation, including the use of conventional HAs. Validated subjective questionnaires, such as the Hough Ear Institute Profile, may reveal problems with conventional aids, including acoustic feedback, which can be reduced with an IHD. Additional counseling before surgery should ensure that patients have realistic expectations of the device advantages and limitations as well as a thorough understanding of surgical risks inherent to middle ear surgery.
The choice of ear to implant is based on several considerations. In patients with objective asymmetry in air conduction thresholds, word discrimination scores, or speech reception thresholds, the poorer-hearing ear is implanted first. When no objective difference is observed, the ear not used to talk on the telephone is chosen. If no preference exists, then patient choice alone is used as a deciding factor.
The size and shape of the external ear canal must be assessed before surgery to ensure functionality of the device. Before surgical implantation, a deep ear impression is taken. In order for the ear canal is able to accommodate the IPC, the following ear canal dimensions have been specified as a prerequisite for implantation: 20-mm canal length from aperture to medial canal, 4-mm width at the canal aperture, and 3-mm canal diameter from the second bend of the canal to the medial portion.
Operative procedure
For most individuals, the MAXUM system is implanted under local anesthesia using a transcanal stapedectomy-type approach. Intravenous access is obtained, and oral and/or intravenous sedation is administered. Patients are positioned supine with the head resting on a foam cushion and tilted away from the surgeon. The surgical site is then prepped and draped so that monitoring of the facial nerve can take place. A mixture of 1% lidocaine with 1:30,000 epinephrine is used for pain control and hemostasis. Injections are made superiorly and inferiorly in the soft tissues of the cartilaginous ear canal and then more medially in the posterior vascular strip.
An incision is made along the posterior canal wall, and a tympanomeatal flap is elevated. The annulus of the tympanic membrane is identified and elevated out of its sulcus to enter the middle ear space. Posterosuperior bone of the medial bony canal is curetted, taking care not to injure the chorda tympani nerve. Bone should be removed until the IS joint, posterior stapes crura, and pyramidal process can be clearly visualized. The oval window niche and mobility of the ossicular chain are now evaluated, along with the position of the facial nerve. Attention is then directed toward the IS joint, the site of attachment of the MAXUM implant.
The implant magnet is composed of neodymium iron boron, housed in a titanium cylinder, and attached to an open wire-form ring ( Fig. 1 ). The cylinder measures 2 mm in length and 1.35 mm in diameter; the implant weighs 27 mg. To minimize contamination of the implant, the surgeon should not directly handle the implant. Nonmagnetic MAXUM surgical instruments are available for implant handling during package removal and insertion, including a cylinder-holding forceps ( Fig. 2 ) and/or a suction insertion tool, controlled by a foot pedal allowing the surgeon to vary suction strength ( Fig. 3 ). Once the implant has been introduced to the middle ear, the open portion of the attachment coil is placed around the IS joint ( Fig. 4 ). This coil is composed of nitinol, a memory alloy that, when exposed to heat, will form a closed coil around the IS joint. The attachment coil should not be manually crimped. One commercially available low-temperature heating device that may be used for closure of the coil is the SMart Piston Heating Device (Gyrus ENT, Stamford, CT, Fig. 5 ). A standard laser is also an option for coil closure. Although no specific manufacturer recommendations for laser settings exist, the similarity of the implant’s nitinol material to the stapes SMart prosthesis suggests that the same laser settings may be used. One previously reported laser setting with the argon laser in SMart piston stapedotomy is 0.7 W with a pulse duration of 0.1 second. Older versions of the implant used a full coil ( Fig. 6 ), which required separation of the IS joint to secure the implant. Surgical technique with the full coil has previously been summarized.
