10When the Sound Cannot Get In: Conductive and Mixed Loss

FTwo ways to be deaf, and why it matters

Hearing loss is not one problem but two very different ones, and the distinction decides which device can help. In sensorineural loss the cochlea itself is damaged, so even sound that reaches it is poorly transduced. In conductive loss the cochlea is healthy but sound cannot reach it because the outer or middle ear is blocking the way: a perforated drum, a discharging mastoid cavity, an absent ear canal, or a fixed stapes. A mixed loss combines both, with a damaged cochlea sitting behind an obstructed conductive pathway.

The single most useful number for this chapter is the bone-conduction threshold. Air-conduction testing measures the whole pathway, but bone conduction bypasses the outer and middle ear and stimulates the cochlea through the skull, so it reports how good the inner ear actually is. The gap between the air and bone curves on the audiogram, the air-bone gap, is the size of the purely conductive problem. A large air-bone gap with good bone thresholds is the signature of an ear that an implantable device can rescue.

The crucial insight is that a conductive or mixed ear has wasted potential. The cochlea works, but the sound is being lost on the way in. Rather than amplifying through a broken or unusable conductive route, a bone-conduction device sends vibration straight to the skull, and a middle-ear implant drives the ossicles or round window directly. Both strategies bypass the diseased conductive apparatus and deliver clean energy to a cochlea that is ready to use it.[2015][2005]

TReading the air-bone gap and choosing by bone threshold

Device selection in conductive and mixed loss follows the bone-conduction pure-tone average, usually taken across 0.5, 1, 2 and 3 or 4 kHz. The logic is simple: a bone-conduction or middle-ear device first overcomes the air-bone gap, then must still have enough output left over to push past whatever sensorineural component remains. The worse the bone thresholds, the more output the device must muster, and the sooner it runs out of headroom.

Passive percutaneous and active transcutaneous bone-conduction systems are typically indicated up to bone averages of roughly 45 to 65 dB HL depending on the specific implant and transducer power, with newer active devices reaching the upper end of that range. Active middle-ear implants such as the floating-mass transducer driving the round window or ossicular chain are fitted against published gain targets, and an objective measure of output and dynamic range, rather than a single threshold, defines the safe boundary of who can be helped.

Because the conductive component is corrected by the device itself, two ears with identical air-conduction audiograms can need very different solutions: the one with excellent bone thresholds is an easy candidate, while the one with poor bone thresholds may have outgrown bone-conduction options and edge toward a cochlear implant. Always plot both curves; the bone line, not the air line, is the gatekeeper.[2014][2015]

CThe ears that send patients to these devices

Chronic otitis media is the classic referral. A draining or repeatedly infected ear, an open mastoid cavity, or a canal that will not tolerate a mould makes a conventional aid impractical, yet the cochlea behind it is often good. A bone-conduction implant lifts the transducer off the diseased ear entirely, sidestepping moisture, occlusion and feedback while the ear continues to be managed surgically or medically.

Congenital aural atresia and microtia are the other archetype, especially in children. With no canal to deliver air-conducted sound and frequently a normal inner ear, these ears are perfectly suited to a bone-conduction route, which is why softband bone conduction is offered in infancy and an implant later. Otosclerosis sits at the harder end: stapes surgery is the usual answer, but when surgery has failed, is refused, or the ear is a far-advanced mixed loss, an implantable device or even a cochlear implant becomes the rehabilitation path. Ossicular discontinuity or fixation that cannot be reconstructed rounds out the list.

Across all of these, the common thread is an ear whose conductive machinery cannot be made to work normally and cannot wear a standard hearing aid, paired with a cochlea worth driving. That combination, not the diagnosis label, is what defines the candidate.[2025][2026]

CPutting it together at the chairside

In practice the work-up is a short, disciplined sequence. Confirm the loss is conductive or mixed with a clear air-bone gap, document the bone thresholds frequency by frequency, and establish why a conventional aid is not an option, whether anatomical, medical or by repeated failure. Only then does the device shortlist open up.

The bone average then narrows the choice: good bone thresholds allow the full range of bone-conduction and middle-ear options, while deteriorating bone thresholds shift the conversation toward higher-output devices and, eventually, cochlear implantation. A preoperative trial, on a softband or test rod for bone conduction, lets the patient hear the benefit before committing, and a CT scan checks that the bone, mastoid and middle ear will accept the chosen hardware. Counselling should set the expectation honestly: these devices restore access to sound for the affected ear, but they do not turn a damaged cochlea into a normal one.[2005][2014]