Technology has been relatively slow to conquer a problem that affects millions of people worldwide: hearing loss and deafness. Although 36 million American adults report some degree of hearing loss, only one out of five people who could benefit from a hearing aid actually wears one, according to the National Institute on Deafness and Other Communication Disorders (NIDCD; Baltimore, Maryland).

Ideally, most people would prefer a device that's not visible, but the most advanced type of hearing aid, cochlear implants, are reserved for people with profound hearing loss or total deafness. Yet these devices aren't entirely implanted and only a handful have come on the market around the world since the first one was approved two decades ago, each of which has been somewhat fraught with difficulties along the way.

"Cochlear implants have two major disadvantages," cochlear implant expert Hugh McDermott, PhD, professor of auditory communication and signal processing in the Department of Otolaryngology, University of Melbourne (Australia), told Medical Device Daily. "They work quite well for people listening to speech when it's quiet and if there's not much background noise. But the ability of the person to understand speech in a noisy place, like a restaurant, is difficult.

"The other problem is pitch perception, which is important for following music and for people who speak tonal languages such as Cantonese and Mandarin because they contain tones that equate meaning. Cochlear implants are poor about giving information about the pitch," he said.

The clarity and usefulness of these little devices boils down to the number of electrodes that can be used in an implant. Most implants have up to 24 electrodes. But they are intended to make up for the estimated 16,000 delicate hair cells that are part of a normal hearing ear. Cost, too, is another obstacle. Most implants average $60,000 including surgery and hardware, according to the NIDCD.

Also known as bionic ears, these devices aren't just souped-up hearing aids, which amplify sound. Instead, they stimulate functioning auditory nerves inside the cochlea with an electric field. The connected external components include a microphone and speech processor.

McDermott, whose research centers on the development of cochlear implants and advanced hearing aids, invented the widely used speech-processing strategies known as SPEAK and ACE. (Cochlear implants are capable of delivering stimulation in several different ways, each of which is called a speech strategy.)

Most implants on the market today are made by just a few companies, with one that dominates, according to McDermott: Cochlear (Melbourne, Australia) makes the Nucleus Cochlear Implant System, which was FDA approved in 1998, now a fourth-generation version.

The number of devices implanted in humans varies, according to the source, from 112,000 up to 170,000. But that's still a drop in the bucket compared to the millions of people who could potentially benefit.

Other implant makers include:

• Advanced Bionics (Sylmar, California) received FDA approval to market the Clarion Multi-Strategy Cochlear Implant in 1997.

• Med-El (Research Triangle Park, North Carolina) makes the FDA-approved Combi 40+ Cochlear Implant System.

• Symphonix Devices (San Jose, California) makes the Vibrant Soundbridge, FDA approved in 2000. At the time, the agency called it the first implantable hearing device approved in the U.S. to treat moderate to severe sensorineural hearing loss the result of hair cells, or nerves in the inner ear, being damaged. This type of hearing loss affects the vast majority of people with hearing loss.

"These products are all quite similar on a basic level," McDermott said. "They all have an external device that looks like a hearing aid and it extracts information about sounds and transmits through the skin to the implant, which picks up the signal and generates pulses that are delivered to the inner ear. The cochlear sound is converted into neural activity."

Two other companies have been working on technology that compares to the Symphonix implant, including Implex (Ismaning, Germany) and Otologics (Boulder, Colorado).

In addition to technological challenges faced by these implant makers, they have been faced with other difficulties. For instance, the FDA reached a settlement with Advanced Bionics over alleged violations involving the failure to notify the agency of a change of outside supplier or vendor, which may have exposed patients to unnecessary health risks, such as device failure and surgery, according to the FDA (MDD, July 18, 2008). The company had to pay a penalty of $1.1 million. In March 2006, Advanced Bionics conducted a recall of the unimplanted devices containing components from the unapproved supplier, because of excessive moisture that could leak into the devices and cause device failure and possible surgery.

Last year, the FDA sent a warning letter to Cochlear's Swedish subsidiary (Molnlyeke, Sweden) with citations related to deviations from good manufacturing practices (MDD, Aug. 27, 2008).

Then, a study published just two years ago confirmed what many physicians have frequently speculated, that the presence of cochlear implants increases the risk of bacterial infections that can cause meningitis (MDD, April 12, 2007). The report, which appeared in Otolaryngology-Head and Neck Surgery said that the finding increases the need to educate the public on the necessity for meningitis vaccinations in potential cochlear implant recipients. At the time, 90 of the 60,000 people receiving cochlear implants were stricken with meningitis, drawing concern within the international medical community.

So what's on the horizon in terms of improved devices, possibly one that's completely implanted yet with better clarity?

"There is a bit of controversy about totally implanted devices," McDermott said. "A lot of people would like to have a totally implanted device, giving them ability to hear 24 hours a day and when swimming or showering because the external component can't get wet. One of the controversies is that some people with hearing impairment prefer that it's partially visible because they want people to know they have a hearing impairment because cochlear implants and hearing aids don't restore normal hearing. So if you're in a noisy situation it might be helpful for the person to know you have a hearing impairment to speak clearly and directly."

He explained that with current technology, there would be a trade-off with a totally implanted version.

"You get the benefit [of a device that's totally implanted], but you lose performance. As soon as you implant a microphone under the skin, the ability to pick up sound is lost and the internal noises increase, such as breathing, chewing and other body noises," McDermott said. "It's why totally implanted devices haven't moved ahead of the experimental stage."

Better devices, he said, come down to the number of electrodes and will require radical rethinking.

"The problem is that we don't understand enough about how normal hearing works," he said. "We're trying to emulate a natural process that people don't really understand. It's also possible that it will never work. When you use electrical stimulation in the cochlea with an impairment, it's not normal. The auditory nerve may be too small or damaged to give good information. If that's the case, there's a completely different line of research on the way to make the neurons grow or to help existing neurons to survive."

Further down the line, McDermott says the real answer may be found in gene or stem cell therapy.

"Forget about implants, this approach would have cells inside the cochlear regrow for those that are damaged or not there," he said.