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TitIe: Sound transmission by cartilage conduction in the ear with fibrotic aural

atresia

Short Title: Cartilage conduction 加:fi brotic atresia

Chihiro Morimoto" ， Tad 郎副 Nishimura" 人 Hiroshi Hosoi" ， Osamu Saito" ， Fumi Fukuda a， Ryota

Shimokura a， Toshiaki Yamanaka a

a Depar 回ent of Otolaryngology-Head and Neck surgeη ， Nara Medical University ， 840 s同0・cho

Kashihara ， N ara 634 ・8522 ，Japan

* Coηesponding au也or.

Address: Depar加entofOtolaryngology~Head and Neck surger ぁNa:r aMedical University ， 840 s同0・cho

Kashihara ， Nara 634-8522 ， Japan

Tel.: +8ト744~22-3051;

F阻: +81-744-24 ・6844;

E-mail: t圃凶由主n@n 紅amed-u.ac.jp

Abstract

A hearing aid using cartilage conduction (CC) has been proposed by Hosoi (2004) as 姐 altemative to

bone conduction (BC) hearing aids. The 位姐sducer d巴veloped for this application is lightweight ， requires

a much sma l1er :fi xation force 也ana bone conductor ， and is more convenient to use. CC can be of great

bene :fi t to patients with :fi brotic aural a回 sia. Fibrotic tissue connected to the ossicles provides an

additional pathway ， (termed :fi brotic tissue p柚 way) for sound to reach the coc h1ea by means of CC. To

address the function of :fi brotic tissue pathway ， BC and CC thresholds were measured in six ears wi由

自brotic aural 柑 esia.τ 'herelationship between the CC白resholds and the res u1ts of computed tomography

was investigated. In the e訂swith the presence of a :fi brotic tissue pathway ， the CC thresholds were lower

than the BC由resholds at 0.5 and 1 ld王Z. At 2 kHz， no signi :fi cant di能rence was observed between the

BC and CC 也resholds. The current :fin dings suggest that sound in the low to middle 企'equency range is

transmi 抗edmore e盟ciently by CC via a :fi brotic tissue pathway 也anBC. The development of heぽ包g

devices using CC c姐 contributeωrehab i1i tation ，partic 凶arly in patients w地価rotic aural 甜 'esia.

Key words: Acqu 出 daural atresia ， Air-bome sound ， Binaural hearing ， Bone anchored hearing aid ， Bone

conduction ， Extemal auditory canal ， Hearing aid ， Occ1usion effect ， Fibrotic tissue pathway ， Soft tissue

pathway

1

Abbreviations

AC=air conduction ， BC=bone conduction ， CC=cartilage conduction ， CT=computed tomography

Introduction

Hearing loss results in reduced communication in daily life and is a major factor contributing to

reduced quality of life. Aural a仕'esia is a heぽ担gdisorder that is difficult to trea t. Conventional hearing

aids using air conduction (AC) provide little benefit owing toせlefixation problem ， feedback oscillation ，

and insufficient gain [1]. Me也ods of intervention include surgical 廿ea加lent ，or fi従加gofbone conduction

(BC) hearing aids [2]. However ， the sぽgical 位回国ent involves 也erisk associated with 也eoperation

[3，4]， and sometimes results in the stenosis and lateralization of the repaired ear canal ， and the hearing

improvement is worsened after long term observation [5，6]. For BC hearing aids ， the 仕組sducer h拙 tobe

tigh t1y pressed against the mastoid [2] or direc t1y fixed with an attaclnnent screw embedded in the bone ，

国fe汀ed to as a bone anchored he釘ing aid [7，8]. Unfortunatel ぁboth methods involve respective

disadvantages. For conventional BC hearing aids ， the 仕組sducer has a relatively large mass and a large

fixation force is needed for the device to function prope r1y. Long term use can also cause skin irritation ，

long -continued depressions 也 the skin ヲ組ddiscomfort [2]. For bone anchored hearing aids ， surgery is

required ， the portion of the 加pl組 texposed to open air can induce infection ，' and some cases are required

for revision surgery owing to skin overgrowing 也eabu 加 ent [9，10].

Hosoi found that a clear sound can be'heard when a vibration signal is delivered to the aural cartilage

from a transducer [11]. 百lI sform of si伊al 仕組smission is referred to as "ca:抗 ilage conduction (CC)".

Uti1i zing CC， a novel hearing device was developed for patients with conductive hearing loss and for

whom a conventional AC hearing aid was not e民ctive [12， 13]. A later study demons 回 ted superior

benefit of a CC hearing device especially in patients wifu postoperative aural a仕esia [14]. 百lese results

suggest that novel hearing devices can be developed using CC as an altemative to BC in hearing aids and

other audiological instrunlents.

When the CC 仕組sducer is placed on the aural cartilage ， sound is trans nlI tted to the coc h1ea via 出ree

possible routes in an anatomically normal ear ，田ShO Wfl in Figure 1 (め.In the first pathway ， vibrations of

the transducer produce air-bome sounds ， some of which reach the ear canal 姐 dare transmitted to the

coc h1ea via the conventional pathway for AC. Such s回ysound is also radiated by BC transducers [15 ，

16]. This pathway is termed “Direct AC". In the second pathway ， vibrations of the' aural c副i1 age 間

仕組snlI甘 edto也ecar 副aginous portion of the ear c姐 al. These vibrations induce an acoustic signal in the

canal which is transmitted by AC to 也eeardrum. This p抑a血wa可yiおs t旬ermed

di江:ffi島 er問'err 凶ltpa 瓜，thway 也a伺.t isnoωtpar 抗tofei 也eぽ:rthe AC 0ωrBCpat 白hwa:可ys. In仕 1巴也ird pathway ， vibrations of the

aural cartilage are transmitted to the coc h1ea via 也巴skull bone. This pathway is termed “Cartilage BC".

There ぽeimport 粗_t differences between the proposed method of delivering sound by means of CC 姐 d

也econventional rnethod of delivering sound by means ofBC. A major 岨 erence between CC and BC is

2

性le :fix ation position of the tr姐 sducer. CC allows for a small lightweight 仕組sducer to be placed

conveniently on the aural cartilage for sound 仕組smission. In BC， the fixation posi 世on is either the

mastoid or forehead bone which is some distance fro lIl the ear. There is also the problem of凶 nscranial

transmission with BC. The precision of transducer placement is also not well con 廿olled for mastoid

placement leading to relatively large test-retest variability 並BCmeasurements. A second ， more important

di:ff erence is the :fix ation force. A small fixation force of 0.06 N is su:ffi cient for sound transmission with a

CC transducer. In con なast ，a BC回 nsducer requ 国s a :fix ation force of 5.4 N which is almost a hundred

times larger and is a source of discomfort 恒 BChe 紅泊gaids.

In an ear with aural a仕esia ，most of也eili:r -borne sound in the ear canal cannot reach the cochlea 出

with direct AC.τnere is， however ， an additional pathway for CC in an ea'r with 宣brotic aural a甘esia. In a

previous study by Nishimura et al. [14] ， the ear canal was occluded with fibrotic tissue not bony tissue. In

addition ，也efibrotic tissue was connected to the stapes ， thereby _troviding a fourth pathway for CC sound

to reach 批 cochlea. Thi s four 血pa血wayi:h fibrotic aural a仕esia is termed the “fibrotic tissue pa也way 問

。fCC (Figure 1(b). Nishimura et al. [14] obtained a large gain below 2 kHZ in patients with a fibrotic

tissue pathway using the prototype CC heat 姐g'aid.

The above observation le拙 tothe imderiying rationale for 批 curtelit 蜘 dy. It was hypothesized that

for those ears showing a fibrotic tissue link to血eossicles ， the CC也reshold wi1l be lower than that for

BC低企equencies below 2 kHz. lli ord ぽ totest 血ishypo 曲目is，BC and CC thresholds were measured for

outpatients in 0ぽ hospital with fibrotic autal a世田iawhich had already been diagnosed with computed

tomography (CT scans)

Methods

Six patients with acquired autal atresia pぽticipated 担the study. The characteristics ofthe subjects are

shown in Table l.Theit ear cana1 s were occluded with :l:i bro 伝ctissue ， which was induced after surgical

operation in five subjects. The 1針ぽality of the autal atresia was right i:h a11也esu時ects by chance. The

exper 加lental procedure was approved by the ethics commi 仕ee of Nara Medical University. Participants

provided writteri inform 巴dconsen t.

The' 也resholds of AC 姐 dBC were measured by a conventional pure tblie audiometer (AA ・78，Rion，

To匂70，Japan). The AC and BC s出iu1i were presented' to 也ee紅 and mastoid using earphones (A:下 02，

Rion， Tokyo ， Japan) and a bone vibrator (BR ・41，Rion) ， respectively. The earphones and bone vibrator

were calibrated with a sound pressure meter (AG-64; Rion) 姐 dartificial mastoid (τ'yp e 4930; Bruel &

勾ar，Narum ， Denmark) according to IS0 389 幽1and 389-3 ， respectively [17 ， 18]. For CC， the transducer

was placed on the cavity ofthe concha except for subject 3. In subject 3， it was :fix ed on the tragus with a

commercial tape because it could not be hung on 也ecavity of也econcha due to the postoperative

deformation. The prope 均7of the transducer is described later.

百resholds were obtained at企equencies of 0.5 ， 1，2， and 4証Iz， respectively. Tone bursts of 300 ms

3

including rise/fall r姐 lpS of 50 ms were employed for the st凶叫us. The signals were generated by a

白nction generator (WF1946 ， NF Elec 位onic Ihs truments ， Yokohama ， Japan) 阻 dthe 也tensity was

con 仕olled by a programmable a枕enuator (pA5.0， Tucker-Davis Technologies ， Gainesville ， FL). The

threshold was detem 血edby the same ascend 泊gmethod as in conventional audiome 句.Th巴 opposite ear

was masked by a narrow band noise using a plateau method. The experiment was performed in a sound

proofroom.

Figure 2 shows the CC 甘佃sducer. The outtut level of也eCC transducer was calibrated with 也e

artificial mastoid (Type 4930) in the same rila凶i~r as BC. In the calibration of the BC 仕組sducer ，it is

fixed to也巴 artificial mastoid with the fixation Iorce of 5.4 N， which is the same as the :fix ation to the

mastoid for the threshold measuremen t. In con 廿aSt，the CC仕ansducer was held in place by a combination

of its own weight and the sti 血 ess of批 cohchalωtilage. the じCtransducer weighs 6 g姐 dthe fixation

force excluding the sti 盛岡S8of the conchal c副ilage w創出tiniated to be approximately 0.06 N. The force

exe 巾 dby the sti 血 ess of the conchal caitilage is similady relative low. However ， the 回 nsducer has to

be tightly :fix ed to血ea士tificial ma8toid i並order td measure tlie force leve l. Thus ，也eCC transducer was

also :fix ed to the arti fIci81 rtlastoid with the :tixa tion fdrce of 5.4 N in the same manner as the BC

位制sducer. The outt 1it ievei 01 CC Was re帥 s出 ed in hearing level based on 1SO 389-3 [18]. The

:fix ation force is 凶port 阻t品ctor for 血esou 刷出nsmission via BC [19 ， 20]. B巴cause 也eCC仕組sducer

was placed on 出ecavicy df也eco:htha wi也afo:rce rtluch less 也姐 5.4 N; 血e e血ciency of sound

conduction from cc刷出ducer to cartilage is ex:tected to be less 白anthat fur Bc甘ansducer to bone.

Restilts

Figure 3 shows 也eaudibgratils for 批町switJi fibrotic aural a佐esia. All 印刷iograms show a 1紅ge

air-bone gap d田 tofue atii' rtl a出血 Figure 4 shdws 也eresuHs of CT. Soft 白s;ue d~出ity was observed in

the ear canals imply h1g fibrotic aural atresia. For subjed 1， the bo1iy po抗ion Was rtlaint cUn ed， and fibrotic

tissue did not exist 也也ebony portion. In con 佐田む Ior subject 2，也ebo姐ypdttion was ulled with 宣brotic

tissue induced by irritation and inflammation. For sti 吋ects 3， 4， 5， arid 61也ebonypo 此ionof 也ee紅 canal

was resected 血 the operation of carcinoma of tí:i~ ei:tr canal. Wi白 reg 批dto 出e cOffil ection between

occluding fibrotic tissue and ossicles ， the CT scans for subjects 2， 4， 5， and 6 show a substantial

connection of occluding fibrotic tissue with 也eossicles ， implying the presence of a fibrotic tissue

pathway. There is no such connection evident in the CT scans for subjects 1 and 3.

Fig ぽ'e 5 shows 社le comparison of BC 姐 dCC thresholds. In也 eears with a fibrotic tissue pa血way

，(subjects 2，4.5 ，姐d6)，社leCC thresholds were lower than 白eBC 也氏sholds at frequencies of 0.5 組 d 1

証主z.At 2 kHz， no significant difference w;出 observed between the BC and CC thresholds. At 4 kHz ， the

BC 白reshold was lower in subjects 2， 4， and 5. In the ears without a fibrotic tissue pathw 可 (su 町田ts 1

and 3)， the CC thresholds were lower 也組theBC 仕rresholds at 0.5 kHz， but not at higher frequencies.

4

Discussion

The main hypothesis is supported in that ears with a. fibrotic tissue pathway ， as determined by the CT

scans ， showed lower CC thresholds than BC thresholds at企equ 巴ncies below 2 KHz. It is argued that the

connection of the fibrotic tissue to the ossicles created a foぽth pathway for CC sound to reach the

cochlea (the fibrotic tissue pathway) thereby lowering the CC thresholds. It should also be noted that the

CC thresholds at 4ほ-Iz were substantially poorer than those for BC. Our previous study showed low ga加

at 4 kHz for the prototype CC hearing aid [14]. The curr 巴nt results are consistent with the gain of the

prototype CC hearing aid as a function of企equency.

明局len the fibrotic tissue is not connected to the ossicles ラthe transmission pathway to the cochlea has to

involve the sk:u ll bone or the air cavity between the fibrotic tissue and ossicles. In the case ofthe cartilage

BC pathway ， the fixation force is an important factor for efficient sound transmission particularly at high

frequencies [19 ，20]. The lQW fixation force ofthe CC仕組sducer may account for the poor CC thresholds

athigh 企equencies. In the case ofthe cartilage AC pathway; air-bome sounds 企omthe fibrotic tissue have

to vibrate the ossicles without the tympanic membrane. Considering th巴巴levation in thresholds for the 巴ar

with the lateralized tympanic membrane [2リラ sound transmission without a fibrotic tissue pathway is not

e妊icien t.

Despite the ine 伍cient 仕組smission without a fibrotic tissue pathway ， the CC threshold at 0.5 kHz was

lower 也組出atof BC. A possible explanation for the low CC也reshold at 0.5 妊Izis the occlusion e茸ect.

When the ear canal is occluded ， a low 企equency resonance is introduced such that the threshold for

air.bome sounds in the canal is lowered in the region 0.4-1. 3 kHz [22]. There are several factors that

con 仕ibute to the measured CC白reshold and it is not clear which is the dominant factor as a function of

frequency. The fmdings of this s印dy identi 命 afactor that has not been considered 泊 previous

investigations ， that of a fibrotic tissue pathway in ears with fibrotic aural atresia.

The current findings demonstrated the function of a白brotic tissue pathway. Ati ear 泊which a fibrotic

tissue pathway is present has characteristics that ate advantageous with respect to the development of an

improved CC hearing aid. Compar 吋 to AC， sound is deliver 吋 byvibrating the aural cartilage ， which is

not mediated by the air. The acoustic feedback result Il1 g企omthe impedance mismatch be何 een the air

and fibrotic tissue is substantially less than that for an anatomically nor n1al ear. As a consequence ， the

gain of the hearing aid can be greater for CC than for conventional air conduction before the onset of

uncontrolled acoustic feedback (whistling). Compared to BC， the lower CC thresholds indicate more

efficient sound transmission at low to rniddle 企equencies. Vibration of血e sk:u U bone Is not needed for

sound transmission in CC. The output level from the CC仕組sducer is su妊icient if it can vibrate the aural

cartilage and fibrotic tissue. For 仕lI sapplication ， the smaller size and lower weight of the CC transducer

relative to a BC transducer are significant advantages. A more substantial advantage is that the fixation

force for the CC transducer is about one thousandth of that required for a BC仕組sducer (0.06 N vs 5.4 N).

The large fixation force required for BC tr佃 sducers is a major source of discomfort with

5

bone-conduction hearing aids. Sound transmission in bone is more efficient than in cartilage which has

both advantages and disadvantages. For example ， a仕enuation of sound ac1'oss the skull is small r巴sulting

in significant 甘anscranial stimulation 泊 aBC hearing aid [23 ， 24]. Cross-over stimulation results in

additional stimulation of the cochka contralate 1'al to the ea1' with the BC transducer ， thereby reducing the

efficacy ofbinaural hea 1'ing [25]. In contrast ， with 仕le fib 1'otic tissue pathway ， because the force levels at

the thresholds for CC were lowe 1' than those fo1' BC， the transmission of CC sound is dominantly

mediated by not the skull bone but the 五b1'otic tissue which connects to the ipsilate 1'al cochlea.

Consequ 巴ntly ，CC sound is perceived by the ipsilateral ear with negligible crossover to the contralat 巴ral

ea1'. A binaural CC hearing aid can thus mainta :lti the benefits uf binaural hearing ， unlike the loss of these

benefits with a binau 1'al BC hearing aid [l3].

Conclusions

In the ear with fibrotic aural 跡目ia，社1巴connection of the f1brotic tissue to the ossicles con 仕ibutes to

more e伍cient sound transmission by means of CC. Iri the tresence of this fibrotic tissue pathway ， CC is

more e伍cient than BC， while a1so provid il1 g advantages over BC in terms of 仕組sducer weight ，

substantially smaller fixatioh forc 巴 and greaie 1' cdnvenience and ωmf01i. The development of hearing

devices using CC can contribute to rehabilitation ， particularly in patients with fibrotic aural atresia.

Whe1'eas it is recognized that the incidence of aural a住居sia is relatively lbw with an estimated annual

incidence of“0.6 cases p巴r100 ，000 凶labitallts" [26] ， the estimated numbet 6f new cases per year in the

USA is 2，000 which is 1:l. ot an insignificant number. It is also 1日ωly that veterans with hearing damage

resulting from blast injuries will have a significantly higher incidence of acquired aural atresia as a result

of damage to the ear and l'e1ated surgical interve Iition.

Acknowledgements

Author Contribtitio J1s:

Study concept and desigh: T. Nishimura ， II. FIosbi

Acquisition of data: C. Morimotb ， O. Saito ， E Fukuda

Analysis and interpretatio :h of data: R. Shimokura ， T. Yamana1 正a

Drafting of manuscript: C. Mor 訂110tO ，T， Nishimura

Study supervision: H. Hosoi

Financial Disclosures: The authors have d巴claτed that no competing inter 巴sts exis t.

Funding/Support: This 1'esearch was supported by a Health and Labour Science Research Grants fo1' the

Sensory and Communicative Diso 1'd巴rs 企om the Ministry of Health ， Labour and Welfare ， Japan. This

study was also supported by JSPS KAKENHI Grant Number 23791924.

Additional Contributions: We thank D1'. Harry Levitt (professor Emeritus ofthe City University ofNew

Yo1'k) fol' his inva1uab1e help ， and M1'. Takashi Iwakura and Mr. Kyoji Yoshikawa (Ri on Co. ， Ltd.) for

6

deve10pment of the c紅ti1age conduction experiment device.

Institutional Review: The e出 cs committee of Nara Medical University reviewed and approved 白is

research. All participants provided informed consen t.

Participant Follow 同Up: The authors do not p1an to inform the participants of the publication of也is

study due to a 1ack of contact information.

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8

F今、

¥¥

Table 1. Characteristics ofpatients

Subject Age Sex Laterality Cause of fibrotic aural atresia Condition of the

opposite ear

1 10 Fem a1e 脳出t Re-atresia after the operation of Normal

congenital fibrotic aural atresia

組 dcholesteatoma in the occ1uded

ear can a1

2 70 Female Right Chronic 町 itation 組 d Chronic otitis media

inflammation

3 47 Male Right Operation of the carcinoma of the Normal

ear canal

4 76 Female Right Operation of也ecarcinoma of the Profoundly deaf

ear ca担a1

5 45 Fem a1e Right Operation df the carcinoimi of the No:t'm al

ear cai1a1

6 Right Open l.tion 6f也ecarcinoma of the Sensorineur a1 he紅'ing

6紅 crlnal loss

Figure legends

Figure 1. Sound trans :riii ssicln pathway of cartilage conduction

In a norm a1 anato :th ical ear ， soufid is tr姐抽出ed to 也ecochlea via 由ree tossible rbutes (件In the 悶

with fibrotic aur a1 a出 sia， fibrotic tissue ili 也日己批 can a1blocks air cbnduction. However ， if the fibrotic

tissue is conn 吋 tofue ossi 伽，則的蜘smi 恥 d削 he connecticln to the ∞chlea ゆ). This おur血

pa也way is termed 助rbtic tissue pathway.

Figure 2. Cartilage trailsducet

The transducer ccimprises a piezoelec 仕ic包加b中h組dcoveti 且gmateri a1. A l'泊 gmatle of acrylic acid i"esin

is glued to由6伽.i1 sduCer tip. The duter and inner d1attieters ofthe ririg are 16 and g mm， respec 託vel 子It s

也ickness is 5 :tt:tI:t1.百 let()tal weight of也etransduce 士is6 g.

Figure 3. Audiograms in six subjects

Arr ows means that 社le 也reshold was higher 也姐也emasked leve l. It was not determined within 也e

currentm 組出umoutput leve l.

Figure 4. Results of computed tomography

The 位iangles indicate the connection of the fibrotic tissue to也eossi c1es.

Figure 5. Comparison of仕rreshold in force level between cartilage and bone conduction

Arr ows me阻 S也at the 也reshold was higher 也阻也emasked leve l. It was not determined within the

curr 阻 tm砿 imum output leve 1.

9

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(b) Fibrotic aural atresia (a) Normal ear

Direct air conduction

Cartilage air conduction

Cartirage bone conduction

Fibrotic tissue pathway

①②③④

Figure2

(b) Subject 2

三p・40

~ -30 ネー20!: -10 :R 0 -= 10 壱 20

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550 f=. 60 70

(a) Subject 1

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550 f=. 60 70

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(a} Subject 1

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ぞー205・10..'l! 0 Q> 10 ・ξ20!ll 30 E 40 ∞50 ~6日百 70空 80ui 90 !l:' 100 戸刊。120 0.1250. 250.5 1 2 4

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(c) Su同ect 3 さ~-20芸-10A 0 Q> 10 .2: 20 l.'i 30 E 40 ∞50 ~ 60 :c 70 Q 80 ui 90 ~100 戸刊。120 0.1250 .250.5 1 2 4

Frequency (kHz)

(e) Subject 5

4

0.5 1 2 4 Frequency (kHz)

(d) Subject 4

0.5 1 2 4 Frequency (kHz)

(c) Subject 3

8 8

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0.5 1 2 4 Frequency (kHz)

(f) Subjecl 6

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8

さ>20;;: -10 A 0 Q> 10 ・E 20 l.'i 30 ヰ 40∞50 主主 60宝 70Q 80 o 90 e100 戸110120 0，1250 ，250 ι5 1 2 4

Frequency (kHz)

(り Subject 6

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会-4 0~ -30 二-20e -10 l.'i 0 士 10豆 20Eヨ30540 史 50戸 6070

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1

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0.5 1 2 4 Frequency (kHz)

0.5 1 2 4 Frequency (kHz)

8 s

ぞ ・20::; -10 ..'l! 0 Q> 10 壬 20~ 30 1三 40∞50 :s. 60 -0 70 Q 80 ui 90 ~100 ξ110 120 0.1250.250 ，5 1 2 4

Frequency (kHz)

o air conduction

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Figure5 Figure3

(a) Subject 1 (b) Subject 2

(c) Subject 3 (d) Subject 4

(e) Subject 5 (f) Subject 6

Figure4