7How Ossicular Defects Degrade Sound Transmission

FTwo routes to the cochlea

Sound reaches the inner ear by two parallel pathways, and the whole logic of ossicular defects follows from the contest between them. The first and dominant pathway is ossicular coupling: the tympanic membrane gathers sound, the malleus and incus move as a lever, and the stapes footplate pumps the cochlear fluid. The second is acoustic coupling— the small, direct difference in sound pressure between the oval and round windows acting on the cochlea without help from the chain. In a healthy ear, ossicular coupling so far outweighs acoustic coupling that the acoustic route is effectively silent [1992]. The transformer mechanism of the middle ear exists precisely to overcome the impedance mismatch between air and the fluid-filled cochlea; the bare acoustic route does almost nothing to bridge that mismatch.

This matters because every ossicular defect changes the balance between the two routes, and it does so in a way that the audiogram can read. When the chain is disrupted, the cochlea is thrown back on the weak acoustic route, and a large conductive loss appears. When the chain is merely stiffened or partially eroded, ossicular coupling survives but is degraded in a frequency-selective way. The difference in strength between the two routes also sets a ceiling: the maximal conductive loss from a purely mechanical lesion, with a normal cochlea, is only about 50 to 60 dB, because residual acoustic coupling always provides a faint floor of transmission [1992, 1998]. A conductive loss cannot become total. Hold that number; it is the yardstick against which every defect pattern is judged.

FDiscontinuity: the maximal, flat gap

Complete ossicular discontinuity is the cleanest experiment in middle-ear mechanics, and the commonest culprit is separation at the incudostapedial joint— whether from erosion of the long process in chronic ear disease or from trauma. Once the chain is broken, the drum is free to move, but its movement leads nowhere; the footplate sees almost nothing. With ossicular coupling abolished, only acoustic coupling remains, and the air-bone gap climbs to the 50 to 60 dB ceiling described above [1998].

Two features make the discontinuous ear recognisable. First, the gap is large and broadband— roughly flat across the spectrum rather than tilted to one end — because the failure is a wholesale loss of coupling rather than a frequency-selective change in stiffness or mass. Second, the drum, no longer loaded by the chain behind it, becomes hypermobile. On a 226 Hz tympanogram this shows as a tall, deep peak (type Ad), and the acoustic reflex is absent because the chain cannot transmit the stapedius contraction. The classic presentation is therefore an intact, normal drum with a near-maximal conductive loss, a hypermobile tympanogram and absent reflexes — a combination that should always raise discontinuity rather than reassure. A large air-bone gap behind an intact membrane is, until proven otherwise, a broken chain.

TFixation: a low-frequency stiffness lesion

Fixation degrades transmission in an entirely different way and therefore prints a different audiometric signature. When tympanosclerosis cements the malleus head in the attic, or when otosclerotic bone locks the stapes footplate in the oval window, the chain is not broken — it is stiffened. The mechanical consequence follows from how impedance scales with frequency: a stiffness load opposes motion most strongly at low frequencies, where the system is naturally stiffness-controlled. So the air-bone gap of a fixation lesion is largest in the bass and tapers as frequency rises[1998]. This is the mirror image of the partial-erosion pattern below, and the contrast is one of the most useful in otology.

The tympanogram reflects the same stiffening. A fixed chain is less compliant than normal, so the 226 Hz tracing tends to be shallow(type As) rather than hypermobile. Where discontinuity unloads the drum, fixation over-loads it. The acoustic reflex is again typically absent in stapes fixation, but for the opposite mechanical reason — the footplate cannot move when the stapedius pulls. Putting the pieces together, a slowly progressive low-frequency-weighted conductive loss behind a normal drum, with a shallow tympanogram and absent reflexes, is the textbook picture of stapes fixation from otosclerosis, while a similar but less progressive pattern with a history of chronic ear disease points to tympanosclerotic fixation.

TPartial erosion: the high-frequency clue

Between an intact chain and a frankly broken one lies the most under-recognised lesion: partial (incomplete) ossicular discontinuity. Here the long process of the incus has been thinned by chronic disease so that bony continuity is lost, but the two fragments remain joined by a strand of fibrous soft tissue. That bridge is not rigid; it is a compliant, slightly slippery coupling. Its behaviour is exquisitely frequency-dependent. At low frequencies the chain moves slowly and the soft-tissue bridge has time to follow, so transmission is nearly normal. At high frequencies the chain must reverse direction many times each millisecond, and the bridge stretches and slips rather than driving the stapes, so velocity collapses. The result is a conductive loss that is worst at high frequencies— the inverse of fixation[2016].

This high-frequency-dominant gap is a genuinely useful clinical test. Defining the high-frequency conductive hearing-losssign as the air-bone gap at 4 kHz minus the mean gap at 0.25 and 0.5 kHz, one large series found that a positive sign predicted incomplete discontinuity at surgery with about 83% sensitivity and 92% specificity — far better than chance, and detectable on an ordinary audiogram [2017]. The catch is that this pattern is also the one most easily mistaken for sensorineural loss if bone conduction is not measured carefully with proper masking, because a down-sloping high-frequency curve looks cochlear until the preserved bone-conduction thresholds reveal the air-bone gap. Two further clues complete the picture of incomplete discontinuity: the loss often fluctuates, and hearing may briefly improve after a Valsalva manoeuvre that momentarily tightens the bridge [2013].

TCarhart’s notch and the limits of the audiogram

One audiometric finding deserves separate attention because it is so often over-interpreted. In fixation, bone-conduction thresholds themselves can show a dip of about 10 to 15 dB centred on 2 kHz — Carhart’s notch. This is not true cochlear damage. Bone-conducted sound normally reaches the cochlea partly through inertial movement of the ossicular chain near its resonant frequency, around 2 kHz; when the chain is fixed, that inertial contribution is lost and the bone-conduction threshold worsens at that frequency. The proof that it is mechanical, not sensorineural, is that the notch typically resolves after successful surgery that frees the chain.

The important trainee-level caveat is specificity. The Carhart notch is real and supports a fixation diagnosis, but it does not identify which element is fixed. In a study of congenital ossicular anomalies and otosclerosis, the 2 kHz dip occurred at similar rates whether the lesion was stapes fixation, incudostapedial detachment or fixation of the malleus or incus, so it is a non-definitive predictor of stapes fixation[2011]. The broader lesson is that the audiogram describes a mechanical behaviour— loss of coupling, added stiffness, added compliance — not a specific anatomical lesion. It narrows the differential powerfully but rarely closes it; the final answer is found at exploration.

CReading the pattern in the clinic

For the operating surgeon these mechanical signatures are not academic; they are the pre-operative map. Before lifting the drum you can usually predict what lies beneath it by combining the shape of the air-bone gap, the tympanogram, and the reflexes. A broad, near-maximal gap behind an intact, hypermobile drum says discontinuity and a likely need to bridge or replace a segment. A low-frequency-weighted gap with a shallow tympanogram says fixation, and warns you that mobilisation, stapedotomy or careful release of sclerotic plaque — not simply slotting a prosthesis — will be required. A high-frequency-weighted gap, perhaps fluctuating, says partial erosion with a soft-tissue bridge, often the long process of the incus with the stapes still intact and mobile — the very situation in which bone cement or incus interposition excels.

The final caution is the one that governs counselling and prosthesis choice alike. The audiogram tells you the mechanical lesion, but the achievable result after reconstruction is dictated by the surviving scaffold— above all an intact, mobile stapes and a preserved malleus handle — together with middle-ear aeration and the absence of active disease. Staging systems built from large ossiculoplasty series show these anatomical factors predict the post-operative air-bone gap better than the material chosen, which is exactly why classifying the defect by what remains, rather than only by what is lost, is the foundation of rational reconstruction [2001, 1971]. Read the pattern, name the mechanism, then look at the anatomy you have left to rebuild with.