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Hearing Preservation

Bone-conduction preservation

Bone conduction bypasses the middle ear and tests the cochlea directly. It is the one line on the audiogram that surgery should never make worse — and the one that proves whether preservation succeeded.

Audiogram Pure-tone audiogram showing air conduction thresholds around 40 to 50 decibels and bone conduction thresholds around 10 to 25 decibels. The shaded area between them is the air-bone gap. 0 20 40 60 80 100 120 250 500 1k 2k 4k 8k Frequency (Hz) Hearing level (dB HL) <<<<< air-bone gap
Air conduction <Bone conduction Air-bone gap

What bone conduction measures

A vibrator on the bone behind the ear sets the skull moving, and with it the cochlea. The sound reaches the hair cells without passing the ear canal, drum or ossicles. Bone-conduction thresholds therefore reflect the inner ear and nerve — the part no operation can repair — and they are the ceiling for what closing an air-bone gap can achieve.

Why it is the preservation marker

If bone conduction is the same after surgery as before, the cochlea has been preserved, whatever happened to the air-conduction line. If it is worse, the inner ear has been damaged, even when air conduction has improved overall. An honest statement of surgical results reports both.

A change of up to 5 dB is within test variability. A shift of 10 dB or more in the average, or of 15 dB at any frequency, is generally regarded as a real change. High frequencies (4 and 8 kHz) are the most vulnerable and the most often omitted from reports.

The Carhart notch — the exception that must be understood

In otosclerosis, bone conduction often shows a dip at 2 kHz of 10–15 dB that is not inner-ear damage but a mechanical effect of the fixed stapes on the way skull vibration reaches the cochlea. After successful surgery this dip often disappears — bone conduction improves. This is welcome, but it complicates measurement: comparing post-operative air conduction with pre-operative bone conduction can make a result look better than it is. The correct comparison uses post-operative bone conduction.

Measurement pitfalls

Masking is the commonest source of error in bone conduction; interaural attenuation for bone is effectively zero, so the non-test ear must be masked whenever an air-bone gap is present on either side. Vibrator placement (mastoid versus forehead) and occlusion of the test ear (Occlusion effect at low frequencies) change thresholds systematically; keep them consistent before and after. Bone-conduction ceilings (typically 60–70 dB HL) limit measurement in severe mixed loss. Post-operative bone conduction at 2 kHz may improve by 10–15 dB after stapedotomy (Carhart recovery) and at 4 kHz may fall by a similar amount; averaging across 0.5–4 kHz can mask both. Report frequency-specific changes.

What a shift means for the person

A small high-frequency loss may be inaudible in daily life but reduces the reserve the ear has for the future. A shift of 15–20 dB across the speech frequencies is a lost margin that a hearing aid will now have to make up. A profound loss is the outcome everyone is trying to avoid and, when it happens, is usually permanent. Knowing the bone-conduction result is how the person and the surgeon know which of these has occurred — and it should be explained plainly at the post-operative review.

This page provides general educational information. It cannot replace an individual assessment, which depends on a full history, examination, audiometry and, where relevant, imaging. If you have sudden hearing loss, severe pain, facial weakness, severe dizziness or discharge with fever, seek medical care promptly.