Cochlear implants may provoke new bone formation linked to further hearing loss

Cochlear implants are linked to the formation of extra bone in the ear, according to a new study. This new bone could in turn cause increased hearing loss, and further medical complications.

Cochlear implants have been used for over 40 years to mitigate some of the effects of hearing loss. They work by implanting a device under the skin to stimulate nerves in a section of the inner ear (the cochlea), which sends information to the brain. An external sound processor sends information to the implant.

While cochlear implants can help people recognise speech, they don’t completely replace normal hearing.

Two diagrams of inner ear showing cochlear implant
(A) a right ear with a cochlear implant. (B) new bone formation observed in the study, shown in grey. Credit: Radiological Society of North America

The implants rarely cause complications when inserted, but some post-mortem studies have found that they can cause inflammation, fibrosis, and the formation of new bone material. But so far, because of the implants’ size and location, it’s been very difficult to spot these effects in vivo (in living people).

“Such subtle changes are challenging to visualise in vivo, in particular in the vicinity of a metallic implant causing artifacts on computed tomography images,” says Dr Floris Heutink from the Department of Otorhinolaryngology at Radboud University Medical Centre, in the Netherlands.

Heutink, along with some fellow researchers, addressed this by using a new X-ray technique to see the implants better.

Four ct scans showing cochlear implants and associated bone growth
A series of CT scans, showing new bone formation in red – from four different people. This bone appeared in slightly different places from person to person. Credit: Radiological Society of North America

The researchers took ultra-high spatial resolution CT scans (UHRCT) of 123 people, each of whom had a cochlear implant.

Out of the 123 patients, 83 (68%) had new bone formation – mostly at the base of the cochlea in the inner ear. This group was significantly more likely to have long-term residual hearing loss.

“As indicated by our study, there is a correlation between new bone formation and long-term residual hearing loss,” says collaborator Dr Berit Verbist, from both the Departments of Radiology at Radboud and Leiden University Medical Centre in the Netherlands.

Read more: Say what? How hearing loops amplify one voice above the rest

The researchers believe this extra bone can interfere with the electrical current from the cochlear implant, making the device fit less well and reduce its performance. It could also make it harder to use other therapies in future.

“Last but not least, new bone formation may complicate reimplantation surgery,” says Verbist.

The researchers say that more detection and monitoring of this effect is urgently needed in people with cochlear implants. At the moment, there’s not enough data to decide whether this bone formation needs treatments.

Their study is described in a paper in Radiology.

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