Otology Seminar 90-8-8
Middle ear mechanics for diseased and reconstructed ears
R3 葉春風
A. Introduction
 John J. Rosowski Ph.D. (Eaton-Peabody Lab of MEEI), Saumil N. Merchant
M.D.(ENT of MEEI)
 Mathematically describe the contribution of each part of the ear’s structure to
sound flow through the ear
 Establish a model to expain and quantify the pathophysiology of conductive
hearing loss, quantify structure-function relationship
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Predict the result of middle ear reconstructive surgeryif match with clinical
observationconfirm the model
B. Sound transmission of normal human middle ear
 Can be quantified by 3 quantities
 The conductive hearing loss can be predicted by how these quantities are altered.
Ossicular coupling—Ps
 Refered to the sound pressure gain(stapes vs ear canal) through TM and ossicle
chain
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Traditional acoustic transformer model : area ratio, ossicular lever (27-30 dB),
catenary lever (34 dB), gain is frequency independent
It’s not accurate, TM movement not like a rigid piston, not all TM motion
transferred to stapes motion.
Middle ear pressure gain smaller than believed: 20 dB for 250 to 0.5k Hz, 25 dB
at 1 kHz, decrease at 6dB per octave at > 1kHzfig2
Pressure gain is frequency dependent
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Middle ear aeration is important for ossicular coupling. Nonaeration lead to
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reduction of or loss of ossicular coupling.
TM motion is driven by pressure difference Pec-Pme
Normally aerated ears
Redduction of ME aerationincrease ME impedanceincrease Pmereduction
of Pec-Pmereduction of TM and ossicle motion
To keep ossicular coupling 10 dB within normal, at least 0.5 cc is needed.
Acoustic coupling-- P
 The difference of sound pressure directly acting on oval and round window
 EEC sound pressureTM motionsound pressure in MEPow and Prw are
nearly the same but not identical
P=Pow-Prw
 Relatively very small and can be omitted in normal ears. But important in some
diseased and reconstructed ears
 Ossicular coupling dominant in normal ears. If losing it , there will be large
hearing loss.
 Bekesy measured it in temporal bone without TM or ossicles P=Ps – 30 to 50
dB
 Normal ear, with intact TM and ossicle chain : 10-20 dB less than Bekesy, 60dB
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smaller than Ps
Maximal P= ~40dB above normal ear, ~20dB smaller than Ps. Occurred when
one window is equal to EEC pressure with acoustic shield to greatly reduce other
window pressure.
Acoustic coupling includes relative magnitude and phase between oval and round
window. But in ideal type IV tympanoplasty, magnitude difference existed. The
cochlea input depends on magnitude-difference. ( phase-difference is not so
important)
Stapes-cochlear impedance--Zsc
 Zsc = cochlear load = Opposition of footplate movement by annular ligament,
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cochlear fluid, cochlear partition, and R-W membrane
In normal ear, the R-W membrane impedance is small
Otosclerosisstapes fixationZsc increase
Non aerated middle earfluid or fibrous tissue near R-W nicheincrease R-W
impedanceZsc increase
量化中耳傳音機轉
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Stapes volume velocity (Us)as a measure of middle ear performance
(EQN 1) Us = ( Ps +
P) / Zsc
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Degree of conductive hearing loss can be predicted by normal Us/altered Us and
convert to dB
In diseased middle ear
Ossicular interuption with intact TM
 No ossicular coupling, only acoustic coupling provides cochlea input
 Fig2Ps - P in normal ear = 50-60dB
 EQN1predict ABG 50-60 dB
Loss of TM, malleus, incus
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No ossicular coupling, all depends on acoustic coupling
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EQN1ABG 40-50 dB
Similar to large perforation, ossicle interuption
Fixation of ossicles
 Stapes fixation in otosclerosisZsc increaseUs decrease
 At < 1kHz, Zsc dominated by annular ligamentany change in ligament is first
seen on low tone. eg. Otosclerosis ABG5~50 dB
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Malleus fixation, usually from bony spur from epitympanic wall to malleus
headthe point of fixation was near the malleus rotation axismalleus
movement only partially reducedABG 15-25 dB
Atelectasis of TM
 Clinically TM atelectasis with intact mobile ossicles can produce 0-50 dB ABG
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ME & RW aerated
50% decrease in ossicular coupling will produce 6 dB ABG
66% decrease 10 dB ABG
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ME & RW loss of aeration loss of ossicular coupling, TM invaginate to oval
and round window niche, the pressure be near ear canal pressure40-50 dB
ABG
Perforation of TM
 Clinical observation—ABG range from 0-40 dB, ABG are greater for low
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frequencies(< 1-2 kHz)
Mechanism 1—loss of pressure-force couplingreduction of ossicular
couplingreduction is proportional to size
Mechanism 2—loss of pressure difference across TM, this effect is greater at low
tone
C. In the post-tympanomastoid surgery ears
Tympanoplasty classification:modified by Nadol & Schuknecht from Wullstein
Decreased middle ear air space volume
 Middle ear air space = air in tympanic cavity + mastoid air cell , in the model 6 cc
 Sound pressurecompress the air + overcome the resistance of others
 Air volume largeZcav small Zt determined by Ztoclarge middle ear air
volume had little effect on middle ear mechanics
 Air volume smallZcav can be larger than Ztoc
 1.5 cc2 dB ABG below 1 kHz
 0.4 ccnearly 10 dB low tone ABG
 0.1 cc19 dB below 0.8k
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0.001cca small air bubbleABG at least 60 dB
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Canal down mastoidectemyresidual air only in mesotympanumonly 0.5
cc12 fold volume reductionpredicted ABG < 10 dB
Further reduction of air space in poor E tube functioncause larger ABGFig 7
Tympanomastoid surgery should provide air space at least 0.5 cc, better at 1 cc
for best acoustic result
Air space > 1cc provide no significant additional acoustic benefit
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Without ossicular linkage—type IV, V
 Depends solely on acoustic coupling, no ossicular coupling
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Type IVSound directly onto stapes footplate, a fascia graft put between
round window and ear canalthe effect is to reduce the round window sound
pressure magnitude
Four elements model include impedance of footplate, round window air
space(cavum minor), shield
Model analyses sugst the best hearing result normal footplate mobility, stiff
enough acoustic shield, aeration of cavum minor at least 0.1 cc to achieve
maximal acoustic coupling, EQN 1 got ABG = 20 dB, consistent of reported best
result of Wullstein
Shield impedancesnme as TM got >40 dB hearing loss30 to 300 fold TM
impedance got optimal result
Impedance is related to the third power of shield thicknessoptimal is 1 mm
thickness
Leave the footplate as mobile as possible by cover it with a thin STSG not a
fascia graft
Keep cavum minor aeration at least 0.03 cc
Type V—if footplate ankylosed, remove it and replace by a fat graft
With preservation of ossicular linkage—type I, II, III
 Type I—repair of TM perforation with mobile and intact ossicle
 Type II—use of bone strut to continue between incus long process and stapes
head
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Type III—1. Stapes columella-TM graft onto stapes head. 2.minor columella-a
strut from stapes head to manubrium/TM. 3.Major columella-a strut from
footplate to manubrium/TM
Good hearing result depends on ME and R-W aeration to keep Ps, P, Zsc
normal or near normal
Adequately aeratedpostop ABG <= 20-25 dB wth suboptimal TM-ossicle
configuration
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Non-aerated filled with fluid or fibrous tissuePs decrease,
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increasepost op ABG 50-60 dB
To further improveenhance ossicular couplingthe following 4 aspects
P unchange, Zsc
Stiffness
 not important if strut stiffness >>>Zsc with good fixation
 columella with ossicle, cortical bone, or hydroxapatite is enough
Ossicular mass
 Some limited experiments suggest that ossicular mass do not limit ME function.
 Stapes mass 3 mg—16 folds increase produces less than 10 dB ABG, mainly at
high tone.
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Malleus and incus—model analog 7.9 mg, but real mass 60 mg--it may be due to
the M & I rotate with a center of gravity and got a smaller rotational inertial
mass.
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Ossicular prosthesis—do not rotate, like stapes—TORP 48 mg produces hearing
result like 16 folds-normal stapes mass
Mass is not a major concern for stapedotomy or ossicular prosthesis
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Position of columella
 Vlaming & Feenstra:stapes and prosthesis angle < 45 satisfactory sound
transmission
Optimal tension between TM/malleus and stapes head/footplate
 Too high or low tension will decrease sound transmission