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CROS, BiCROS and bone-conduction devices (BAHA and implants)
When one ear cannot be aided, sound can be routed from its side to the good ear (CROS), or the outer and middle ear can be bypassed by vibrating the skull straight to the cochlea (bone conduction). Both give back sound from the poor side — but not binaural hearing.
Why it matters in the exam
CROS puts a microphone on the unaidable ear and sends the signal to the good ear, which is normal; this removes head shadow from the poor side and lets a talker on that side be heard, but there is no localisation and no binaural benefit. BiCROS suits a better ear that also needs amplification: for example a mild-to-moderate loss on the right and a profound one on the left. Hearing in the left ear a sound that arrived from the right means CROS or streaming between the aids.
A bone-anchored device (BAHA, or a processor on a softband for young children) vibrates the skull and reaches the cochlea directly. The classic indications: conductive or mixed loss when a conventional aid cannot be worn — canal atresia with microtia, an ear that keeps discharging from chronic otitis — and also single-sided deafness (as a CROS through the skull). It suits ears whose bone-conduction thresholds fall within the processor's range. It does not suit a severe bilateral sensorineural loss, because there is no cochlear reserve. Drawbacks: limited maximum output, weaker transmission at high frequencies and skin problems around the implant. Because the skull carries vibration to both sides with little loss, two bone-conduction devices each drive both cochleae — localising sound becomes very hard.
Key points
- CROS: microphone on the poor side, the good ear normal.
- BiCROS: the better ear has a loss.
- BAHA: atresia, a discharging ear, single-sided deafness.
- Limits: output, highs, skin; no localisation.
- Not for a severe bilateral sensorineural loss.
Summary table
| Solution | For whom | Limitation |
|---|---|---|
| CROS | One unaidable ear, the other normal | No localisation and no binaural benefit |
| BiCROS | One unaidable ear, the other with a loss | Same limit; the better ear also gets gain |
| BAHA (bone-anchored) | Conductive or mixed loss with no conventional option | Limited output, weak highs, skin |
| Softband | Young children before implantation | The same bone conduction without surgery |
| Bone conduction in single-sided deafness | Sends sound through the skull to the good cochlea | Like CROS — no localisation |
| Not suitable | Severe bilateral sensorineural loss | No cochlear reserve; consider a cochlear implant |
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Sources
- Dillon, Hearing Aids, 2nd edition (2012) — CROS, BiCROS and bone-conduction hearing aids
- Snik et al., Clinical Otolaryngology, 2005 — Consensus statements on the BAHA system
- Reinfeldt, Håkansson, Taghavi & Eeg-Olofsson, Medical Devices: Evidence and Research, 2015 — New developments in bone-conduction hearing implants
Sources are given for further reading; the text is written afresh and not copied from them.
Reviewed by the Examqo AI Professor · 2026-11-02. The text may contain errors; when in doubt, check the source.