38 THE HEARING JOURNAL The future of implantable hearing devices AUGUST 2008 • VOL. 61 • NO. 8

The future of implantable hearing devices:Introduction to a two-part special sectionBy Marshall ChasinGuest Editor

A brief history of middle ear implantsBy Marshall Chasin

The last decade has seen astonishing advances inthe technology of hearing aids. A comparabledegree of technological innovation is now tak-ing place in a related field: implantable devices.

In this context, an implantable hearing deviceis any instrument that is entirely or partiallyimplanted in the temporal bone or the middleear. Partially implanted devices include the bone-anchored hearing aid or Baha®, in which thevibratory “receiver” is implanted in the tempo-ral bone, just behind the pinna. The Baha is

especially useful for conductive and most mixed hearinglosses. There is also a range of middle ear implants in whichthe “receiver” transducer is implanted on the ossicular chain.

The greatest change is taking place among fully implanteddevices. Historically, these were restricted to piezo-electricdevices that used a very small transducer. However, in thenear future we will see electro-magnetic devices that offerthe benefits of being completely in the ear.

The benefits of the explosion in hearing aid technologyhas not passed implantable devices by. We will see many

of the same innovations found in modern hearing aidsbeing used in implantable devices.

The following special section, which begins in this issueand will continue in the September issue, is based on apanel presentation, “New Developments in ImplantableDevices,” presented at the American Academy of Audiol-ogy’s AudiologyNOW! 2008. I would like to thank theacademy for organizing the panel session that broughttogether many of the leading researchers and audiologistsworking in this exciting area.

This section primarily focuses on the near future inimplantable devices. In doing so, it provides hearing health-care providers with a preview of what’s in the pipelinetoday that will be able to help their patients tomorrow.However, before looking at the future, we’ll be taking abrief look at the developments that led us to where we aretoday.

Marshall Chasin,AuD, Guest Editor of this special section, moderated the panel

session, “New developments in implantable devices,” held at AudiologyNOW! 2008.

Readers may contact Dr. Chasin at Marshall.Chasin@rogers.com.

In 1935 Alvar Wilska sprinkled iron filings onto the tym-panic membrane of a patient lying on his side on a med-ical table.1 Dr. Wilska placed an earphone over the man’sear that produced no sound—only an electro-magneticsignal—and the patient reported “hearing.”

In this experiment, the iron filings moved around insynchrony with the changing magnetic field (the alternat-ing current or ac flux). These filings caused the tympanicmembrane to move, which in turn transduced the vibra-tion to the cochlea. This was the first reported example ofan implantable hearing device. In this case, the iron filingstook the place of the receiver. In the 1950s, it took 28,000mA to produce an 85-dB SPL signal. Today less than 3 mAcan produce this same level of signal.

Goode defined a middle ear implant (MEI) as a hear-ing device part or all of which is implanted in the middleear.2 MEIs offer cosmetic advantages (since they are invis-ible when worn), and they can also provide greater high-

frequency amplification because of the separation of themicrophone from the receiver. In addition, an MEI cre-ates little or no occlusion effect because very little, if any,of the device is situated in the ear canal.

The first MEI to come to market was designed forpatients with chronic dysfunction of the middle ear, andit replaced many of the middle ear’s functions.3 Possiblybecause of the complicated and irreversible nature of thesurgery, this MEI approach did not gain widespread useoutside of Japan.

MORE RECENT APPROACHESThe 1990s saw a return to MEIs in which only the receivertransducer was placed in the middle ear. The major thrustof modern implementations of the device is for treatingpeople with purely sensorineural hearing losses (and nor-mally functioning ossicular chains), although some man-ufacturers are modifying their products for patients with

Marshall Chasin

40 THE HEARING JOURNAL The future of implantable hearing devices AUGUST 2008 • VOL. 61 • NO. 8

mixed or conductive hearing losses as well.Although various approaches have

been tried, most current MEIs use areceiver element situated on or near theincudo-stapedial joint, which Jack Hough,MD, and colleagues have shown to be theoptimal location. It’s medial enough totake advantage of the rotary characteris-tics of the ossicles and thus ensure suffi-cient high-frequency transduction, yetlateral enough to be close to the drivingcoil, which is situated either in the earcanal or behind the ear.4 Coating theimplanted receiver magnet in a titaniumcase wards off moisture, which was a prob-lem with earlier implants.

This type of MEI is referred to as anelectro-magnetic or electro-mechanicalimplant. As the name suggests, it requirestwo parts: an implanted magnet and anexternal coil coupled to the microphone.Like any electro-magnetic combination,the external coil can’t be too far from theinternally implanted magnet. An advan-tage of this system is that it can achievevery high outputs (up to 130 dB SPL),although in most cases outputs have tobe limited to 110 dB SPL (usually becauseof Food and Drug Administration regu-latory concerns). A disadvantage of theelectro-magnetic approach is its relativelylarge size. That meant that the MEI imple-mentations of the 1990s and early 2000scould be only partially implanted.

A second approach developed in the1990s (similar to Suzuki and Yanagihara’sidea) used a piezo-electric form of

transduction. As with electro-magneticMEIs, candidates for this more modernpiezo-electric approach had purely sen-sorineural hearing losses.

When a piezo-electric crystal is bentor flexed, a current is generated, and theconverse is also true. Thus, it could beused as a microphone and a receiver, albeitwith different degrees of efficiency. Anadvantage of these piezo-electric MEIs istheir small size, which allows them to befully implanted in the middle ear.

However, a serious drawback is thattheir maximum output is typically lim-ited to about 110 dB SPL (with maxi-mum use gain on the order of 30-35 dB).Therefore, the devices cannot be fitted onpatients with more than moderate sen-sorineural hearing loss, a range well withinthe capability of the smaller CIC hearingaids. In the piezo-electric approach, a smallceramic crystal is implanted on the ossic-ular chain (usually surgically placed in ornear the incudo-stapedial joint). This isphysically joined either to an externalmicrophone surgically located in the (pos-terior) wall of the ear canal or to thepatient’s tympanic membrane, whichserves as the microphone.

THE BAHAThe bone-anchored hearing aid (Baha®)has been widely used since the late 1980s.Designed primarily for patients withconductive or mixed hearing losses (withbone-conduction scores no worse than45 dB HL), the Baha is fitted such that

only the receiver is implanted in thetemporal bone and sound is sent by boneconduction to the relatively intact cochlea.

Historically, there have been both tran-scutaneous (over the skin) and percuta-neous (through the skin) devices, the latterbeing the now-discontinued Xomed Audi-ent and the former being the only Baharemaining on the market, which is madeby Cochlear Corp. There were inherentproblems with a transcutaneous approachbecause the transduced signal may be oflow intensity, and in some cases could notstay in contact with the skull.5

In contrast, the percutaneousapproach, which uses an abutment thatprotrudes through the skin behind theear, has better mechanical transductioncharacteristics, which make it suitable fora wider range of patients.

ON TO THE FUTURE…The rest of the articles in this special sec-tion are based on presentations given atAudiologyNOW! 2008. They discusswhat is just down the road for middle earimplants. No longer are MEIs too largefor a fully implantable system and, thanksto sophisticated new digital algorithmsand digital processing, today’s MEIs candraw on the same electroacoustic bene-fits that are available with conventionalhearing aids.

Marshall Chasin, AuD, is is Director of Auditory

Research at Musicians’ Clinics of Canada. Readers may con-

tact Dr. Chasin at Marshall.Chasin@rogers.com.

Soundbridge users report better sound qualityBy Tim Campos

The Vibrant Soundbridge®, which hasbeen approved for use by adults in theU.S. since 2001, is intended for personswho have moderate-to-severe sen-sorineural hearing loss and cannotachieve success or adequate benefit fromacoustic hearing aids or are unableto tolerate hearing aids due to conditionssuch as chronic perichondritis of the ear,chronic otitis externa, atresia of the earcanal, congenital malformations, or skinreaction from the earmold or hearing aidcase.

Additional candidate selection criteriainclude: normal middle ear function, aspeech-recognition score of at least 50%at the implant ear under headphones,no retrocochlear or central involvement,no skin conditions preventing attachmentof the audio processor, and realistic expec-tations.

The Soundbridge consists of two pri-mary components, as seen in Figure 1:� a surgically implanted vibrating ossic-

ular prosthesis (VORP) containing amagnet, receiving coil, demodulator

package, conductor link, and float-ing mass transducer (FMT).

� the external audio processor (AP).The AP is worn outside the head,

behind and above the pinna. It is held tothe scalp by attraction between magnetsin the VORP and in the AP. The APalso contains a microphone, a signalprocessor, telemetry electronics, and astandard 675-zinc air battery that powersthe system.

The FMT is a totally enclosed trans-ducer that uses inertial drive to impart