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. Author manuscript; available in PMC: 2022 Apr 3.
Published in final edited form as: Otolaryngol Head Neck Surg. 2021 Jul 20;166(4):738–745. doi: 10.1177/01945998211029541

The Mechanics of Total Drum Replacement Tympanoplasty Studied with Wideband Acoustic Immittance

Kristine Elisabeth Eberhard 1,2, Salwa Fatima Masud 1, Inge M Knudson 3, Keshinisuthan Kirubalingam 3,4, Hamza Khalid 3, Aaron K Remenschneider 3,5, Hideko Heidi Nakajima 1
PMCID: PMC8912255  NIHMSID: NIHMS1783524  PMID: 34281437

Abstract

Objective:

Poor hearing outcomes often persist following total drum replacement tympanoplasty. To understand the mechanics of the reconstructed eardrum, we measured wideband acoustic immittance, and compared the mechanical characteristics of fascia grafted ears to the normal tympanic membrane.

Study Design:

Prospective comparison study.

Setting:

Tertiary care center.

Methods:

Patients who underwent uncomplicated total drum replacement with temporalis fascia grafts were identified. Ears with healed grafts, an aerated middle ear and no other conductive abnormalities were included. All patients underwent pre- and postoperative audiometry. Wideband acoustic immittance was measured with absorbance and impedance computed. Fascia grafted ears were compared to normal unoperated ears.

Results:

Eleven fascia grafted ears without complications were included. Postoperatively, median air-bone gap was 15 dB (250-4000 Hz), with variation across frequency and between ears. Fifty-six normal control ears were included. Absorbance of fascia grafted ears was significantly lower than normal ears at 1-4 kHz (p<0.05), but similar below 1 kHz. Impedance magnitude demonstrated deeper and sharper resonant notches in fascia grafted ears than normal ears (p<0.05), suggesting lower mechanical resistance of the fascia graft.

Conclusion:

The mechanics of fascia grafted ears differ from the normal tympanic membrane by having lower absorbance at mid-to-high frequencies, thus poor sound transmission at these frequencies. The lower resistance in fascia grafted ears may be due to poor coupling of the graft to the malleus. To improve sound transmission, grafts for tympanic membrane reconstructions would benefit from refined mechanical properties.

Keywords: tympanoplasty, total drum replacement, wideband acoustic immittance, conductive hearing loss, reflectance, absorbance, fascia graft

INTRODUCTION

Tympanoplasty type I is a commonly performed procedure that involves graft material to reconstruct the tympanic membrane (TM). In the case of large perforations with an intact ossicular chain, a technique of lateral graft tympanoplasty, or total drum replacement, may be performed with temporalis fascia.1,2 While perforation closure rates using a fascia graft and/or lateral graft technique have been reported to be >85%,3–7 postoperative hearing outcomes are variable and often suboptimal.3,4,8–11 Literature reports that for the majority of patients, postoperative air-bone gap (ABG) is >10 dB and on average ~15 dB.3,4,6,7,10,11 Interestingly, studies demonstrate frequency-dependent hearing outcomes with the lack of improvement in the high frequency ranges,10–12 possibly due to specific graft materials.12–14

The native three-layered TM’s middle connective tissue layer has radial (type II) and circumferential (type III) collagen fibers15,16 that allow for complex vibrational patterns.17,18 Contrary, fascia is composed primarily of linear type I collagen,19 and the fascia grafts remain significantly thick compared to native TMs.14 Thus, fascia grafts with different structure and mechanical properties to the native TM would also differ in acoustic transmission.

Wideband acoustic immittance (WAI) measures the biomechanical properties of the middle-ear system and can describe acoustic sound transmission.20,21 It can also be used for differential diagnosis of conductive pathologies of the ear and to determine stability after treatment.22–25 It is a rapid non-invasive assay, similar to standard 226 Hz tympanometry where static pressure is varied. However, WAI assesses across a wide range of sound frequencies (not just 226 Hz). When sound is presented to the external ear canal, a frequency-dependent amount of sound will be absorbed by the ear while a portion of the sound is reflected back to the ear canal (Fig. 1).20,21,26 Absorbance refers to the proportion of the forward stimulus sound energy that is absorbed by the middle and inner ear.20,21,26 By analyzing WAI measures such as absorbance and impedance, we can gain insight regarding mechanical factors that influence sound transmission in reconstructed ears.

Figure 1.

Figure 1.

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Wideband acoustic immittance: When a sound stimulus presents to the ear canal, a frequency-dependent amount of sound is absorbed by the ear, while the rest is reflected back.

To better understand the mechanical properties of fascia grafts, we measured WAI in ears following total drum replacement with fascia. The mechanics of fascia TMs were then compared to normal unoperated TMs.

METHODS

Subjects & Ears

This prospective study recruited patients from the Otology Clinic at Massachusetts Eye and Ear between Jan.2011-Dec.2019. Patients were included if the ear with fascia reconstructed drum had perforation closure and stable healed and healthy-appearing graft without evidence of lateralization. Strict exclusion criteria were: (1) use of other graft materials such as cartilage; (2) evidence of other conductive abnormality such as ossicular pathology, effusion, Eustachian tube dysfunction or cholesteatoma; (3) report of a surgical procedure that could alter sound transmission by affecting the middle-ear ossicles (i.e. ossiculoplasty); (4) evidence of other conductive abnormality such as superior canal dehiscence if computed tomography was available. Thus, our strict criteria for WAI measurement minimized confounding effects and only included the effects of uncomplicated fascia repair in well-aerated middle ear.

Institutional Review Board approval was obtained from Human Subjects Committee of the Institutional Review Board at Massachusetts General Brigham (protocol number: 2019P003517).

Surgery

Surgical details were obtained from the subjects’ medical records. The type I tympanoplasties were performed using a standardized lateral graft total drum replacement technique.27,28 Six surgeons performed the eleven procedures, with a single surgeon carrying out five. A postauricular or endaural incision was made to harvest the temporalis fascia graft with the superficial fascial layer removed. All TM tissue including the fibrous tympanic annulus was removed, but in Fascia 3 and 10 the anterior annulus was preserved. Canaloplasty reducing the anterior and inferior bony ear canal was performed to improve visualization. The middle ear ossicles were examined to confirm they were intact, articulating and mobile. The fascia graft was placed lateral to the bony annulus but deep to the manubrium of the malleus. Split-thickness skin grafts were positioned to cover the exposed bony ear canal wall and the lateral parts of the fascia graft to support the tympanomeatal angle and minimize the risk of anterior blunting and graft lateralization. Finally, a rosebud dressing of silk strips soaked in mineral oil was applied, and the ear canal was packed with antibiotic-impregnated cotton.

Wideband Acoustic Immittance

WAI was measured using the Mimosa Acoustics Hear ID system (Mimosa Acoustics Inc., Champaign, IL, USA).29 The system was calibrated daily and each foam tip calibrated as described in Rosowski et al. 2012.26 Measurements were made as described in Rosowski et al. 2012,26 where the foam tip with an inner tubing coupled to a sound source and a microphone was inserted into the ear canal such that the outer surface of the foam tip was in plane with the opening of the ear canal and a complete seal was achieved. A chirp stimulus (60 dB SPL, 210-6000 Hz) was presented to the ear canal. The subject was instructed to swallow before the first measurement to equalize the middle-ear cavity to atmospheric pressure. At least two measurements without evidence of noise artifact or acoustic leak were performed. Acceptable measurements passed the following criteria between 210-500 Hz: (1) average absorbance <0.3; (2) a negative slope of impedance magnitude; and (3) a flat and negative impedance phase gradually increasing with frequency.30

Audiometric Evaluation

The preoperative audiogram from the medical records closest to the date of surgery was chosen. Postoperatively, the audiogram closest to the date of WAI was utilized. All audiograms had both air and bone conduction thresholds measured during the same session. Pre- and postoperative audiometric results were reported in scattergrams relating pure tone average (PTA) ABG to word recognition score (WRS), according to AAO-HNSF standard reporting format.31 PTA thresholds for both air and bone conduction were calculated by averaging the pure tone thresholds at 500, 1000, 2000, and 3000 Hz. For 3000 Hz, an estimate was calculated using interpolation.

Normal Control Ears

For comparison, 56 ears with normal hearing and normal native TM was used from our previous study.19 Control ears had: (1) no significant history of middle-ear disease or residual consequence hereof; (2) no history of otologic surgery or residual consequence of previous tympanostomy tube or myringotomy; (3) no abnormalities on otoscopic exam; (4) normal audiometry (air conduction thresholds ≤20 dB HL from 0.25-8 kHz; ABG <15 dB at 250 Hz and <10 dB from 0.5-8 kHz); (5) normal tympanograms).

Analyses & Statistics

WAI measurement with the estimate of the Thévenin equivalent source impedance from calibration allowed calculations of impedance and absorbance as described previously.21

Because of the small sample of fascia grafted ears, individual data, median, and interquartile range (IQR) were reported. Medians of normalized impedance were made only for frequencies with data of at least five ears. To compare dichotomous and continuous demographic data between groups, non-parametric tests, Fisher’s Exact test and Wilcoxon Rank Sum test were used.

To confirm significant difference in absorbance and impedance between groups, a permutation test with 10,000 permutations was performed. The permutation test estimated the probability of getting the observed separation of the quartiles (upper quartile of one group and lower quartile of the other) between the fascia grafted and the normal TM group by chance. To reduce the number of frequency points to test, only the frequencies demonstrating separation of the quartiles were tested and multiple frequency points were reduced to frequency bands in which absorbance and impedance were averaged.

Significant differences between pre- and postoperative ABG for each of the five frequencies were tested using Wilcoxon Signed Rank test, and to adjust for multiple testing the p-values were multiplied by five.

Statistical significance was set at p<0.05. All analyses and statistics were performed using MATLAB R2018a and RStudio-v1.1.463. Statistical methods and interpretations were recommended by a statistician in our institution, especially in regards to limited data.

RESULTS

Subjects

Eleven subjects who underwent total drum replacement with fascia graft met inclusion criteria. The median time from surgery to WAI measurement was 3.75 years with an IQR of 0.2-8.5 years. All fascia grafts were well-healed and healthy-appearing on otoscopic evaluation on the day of WAI. No ears showed thickening of the graft or moderate or severe blunting, however Fascia 4 and 8 demonstrated a mild anterior blunting. In Fascia 2 and 5, a canal wall up mastoidectomy was performed in conjunction with the tympanoplasty procedure. Characteristics of subjects and ears are described in Table 1.

Table 1.

Characteristics of Subjects & Ears and Relative Timing of Measurements

Fascia Grafted Ears Normal Ears
Ears / subjects 11 ears / 11 subjects 56 ears / 29 subjects
Age at surgerya 35 yrs (29-59; 23-62) Not Applicable
Age at WAIa,*1 41 yrs (35-61; 23-71) 33 yrs (29-42; 22-63)
SexΔ2
  Female 5 (45%) 31 (55%)
  Male 6 (55%) 25 (45%)
Perforation etiology
  Trauma 1 (9%) Not Applicable
  Chronic otitis media 10 (91%) Not Applicable
Ear
  Right 8 (73%) 29 (52%)
  Left 3 (27%) 27 (48%)
Surgical approach
  Postauricular 7 (64%) Not Applicable
  Endaural 4 (36%)
(Fascia 1, 6, 7, 11)		 Not Applicable
Time from surgery to WAI measurementa 3.75 yrs
(0.8-8.5; 0.2-10.2)		 Not Applicable
Time from surgery to postoperative audiometrya 1.33 yrs
(0.5-3.29; 0.25-11.08)		 Not Applicable
Timing of WAI relative to audiometry (minus is audiometry before WAI)a −1 month
((−16)-0; (−76)-12)		 Always within 1 week
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WAI, wideband acoustic immittance; yrs, years.

a

Values presented as: median (IQR; total range);

*1

Statistically significant difference in age at WAI measurement between groups, p=0.02;

Δ2

No statistically significant difference in sex between groups, p=0.75.

Wideband Acoustic Immittance

Absorbance, an indicator of how much sound is absorbed by the ear, was compared between fascia grafted and normal ears. Figure 2 compares individual absorbance of fascia grafted ears (thick colored curves) with normal ears (thin gray curves). For frequencies below 1 kHz, absorbances of fascia grafts were generally similar to normal TMs. However, above 1 kHz, absorbances of all fascia grafts were significantly lower than normal TMs. Figure 3 plots the median and IQR of the two groups. Permutation test for statistical significance was performed between 1-4 kHz (averaged into third-octave frequency bands). Above 4 kHz, there is uncertainty of absorbance data due to possible speaker-microphone crosstalk.32 The separation of the quartiles was statistically significant between 1-4 kHz (p<0.017 in the 1 kHz band; p<0.001 in the six bands between 1.3-4 kHz).

Figure 2.

Figure 2.

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Absorbances of fascia TM (colored) are lower than normal TM (gray, n=56) for frequencies >1 kHz. Both fascia and normal TM absorbances are similar below 1 kHz.

Figure 3.

Figure 3.

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Absorbance median and interquartile range (IQR) of the fascia and normal TMs. Significant separation of the quartiles from 1-4 kHz between groups. Shaded green area could have absorbance artifact.

Acoustic impedance (the complex ratio between pressure and volume velocity) provides biomechanical information of the ear. Impedance magnitude and phase also demonstrated similarity of fascia grafted ears and normal ears at frequencies below 1 kHz (Fig. 4A). At these low frequencies, the impedance magnitude decreased about a tenth for each decade of frequency and the phase was around −0.25 periods, consistent with dominance of mechanical compliance. At higher frequencies, resonances can be observed in the plot as notches in magnitude and phase shifts from about −0.25 to +0.25 period. The resonant frequencies were generally between 3-5 kHz but varied across ears in both groups.

Figure 4.

Figure 4

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A. Individual ear impedances. Pink line shows resonance for Fascia 10. (B) Impedances normalized to resonant frequency. Here, fascia TMs have sharper magnitude notches and steeper phase transition than normal TMs.

To better compare impedance patterns, magnitude notches and phase transitions were aligned by normalizing the impedance curves to their resonant frequencies as value of one (Fig. 4B). Figure 5 plots the median and IQR of impedances in Fig. 4B. Both Figs. 4B and 5 show that the impedances of fascia grafts exhibited deeper and sharper magnitude notches and more rapid phase transitions with steeper slopes at resonant frequencies than the normal TMs. This is consistent with the fascia graft having less resistance (less friction), and the separation of quartiles at the notch was significant with p<0.001 (over the five normalized frequency bands from 0.8-1.2). At frequencies below 0.5 resonant frequency, the impedance of the fascia graft was similar to the normal TM and showed a compliance dominated pattern in both groups. Comparison between the groups above resonance was not possible because few fascia grafted ears provided impedance data above 1.5 normalized resonant frequency because the majority of ears resonated at frequencies too close to the upper frequency limit of the WAI.

Figure 5.

Figure 5.

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Impedance median and interquartile range (IQR) of the fascia grafted and normal ears. Fascia grafts demonstrate deeper and sharper magnitude notches and steeper phase transition consistent with lower resistance.

Hearing Outcomes

The median time from surgery to postoperative audiometry was 1.33 years with IQR of 0.5-3.29 years. The median time interval between WAI and postoperative audiometry was one month (Table 1). Figure 6 shows postoperative ABG for ears with fascia graft. Note that only one ear (Fascia 11) had closure of the ABG to less than 10 dB at all measured frequencies. The remaining ears showed persistent ABG with inter-ear variability across all frequencies. Median postoperative ABGs were 20, 15, 20, 5 and 15 dB at 250, 500, 1000, 2000 and 4000 Hz.

Figure 6.

Figure 6.

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Postoperative air-bone gap (ABG) of the 11 fascia grafted ears including median and interquartile range. Most ears show persistent and frequency-dependent ABG postoperatively.

Comparison between postoperative and preoperative ABGs (Fig. 7) showed that fascia grafted TMs improved conductive hearing significantly at low frequencies, with median of 25 dB at 250 Hz and 10 dB at 500 Hz (p=0.005; p=0.029). There was no statistically significant improvement at higher frequencies (1-4 kHz). These findings are similar to Polanik et al. 2020.12 In general, bone conduction thresholds were stable between pre- and postoperative for measured frequencies (250-4000 Hz).

Figure 7.

Figure 7.

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Change in air-bone gap between after and before tympanoplasty. Low-frequency hearing improved following tympanoplasty, whereas lack of improvement is seen ≥1 kHz. There is large inter-ear variability. *Statistically significant improvement.

Scattergrams relating preoperative ABG and WRS as well as postoperative ABG and WRS changes are shown in Figs. 8A+B. They demonstrate that ABG in general improved with tympanoplasty and that WRS was stable.

Figure 8.

Figure 8.

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Scattergrams relating air-bone gap (ABG) and word recognition score. (A) shows the preoperative condition, and (B) the change between postoperative and preoperative conditions.

DISCUSSION

Key Findings

This study compares the mechanical characteristics of uncomplicated fascia reconstructed TMs and normal TMs with WAI measurements. Absorbances computed from WAI are similar between fascia grafted and normal TM below 1 kHz. However, above 1 kHz, less sound is absorbed and transmitted beyond the eardrum in all fascia grafted TMs as compared to normal TMs (Figs. 2 and 3).

Hearing outcomes of fascia tympanoplasty are similar to previous reports showing variability across ears.10–12 While a significant postoperative improvement in ABG is observed at 250-500 Hz as compared to preoperative condition, no statistically significant difference is observed above 1 kHz (Fig. 7).

From WAI measurements, computing and interpreting impedance magnitude and phase allow us to understand biomechanical properties. The impedance magnitude and phase are similar and consistent with a compliance-dominated mechanical system at low frequencies (<1 kHz) for both fascia graft and normal TM (Figs. 4 and 5). The resonant frequency varies slightly across measurements. This variation can be due to differences in distance between the foam tip and the eardrum, as well as differences in frequency where transition occurs between mechanical compliance and mass. When the impedances are normalized to the resonant frequencies, fascia grafts show deeper and sharper magnitude notches and steeper phase transitions compared to normal TMs. This is consistent with fascia graft having lower mechanical resistance (lower friction). Poor coupling of the malleus to the fascia graft or low resistance areas of the fascia may explain this lower resistance of fascia grafted TMs.

Other Mechanical Properties

The fibrous annulus was removed at surgery (except in Fascia 3 and 10 where the anterior fibrous annulus was preserved). Postoperatively, the fascia will heal along the circumference of the bony annulus. Normally, the fibrous annulus provides an anchor for the native TM and influences TM motion.33,34 Different surgical techniques with and without annulus removal may influence mechanical properties.

Fascia 3 shows an atypical low-frequency peak in absorbance (Fig. 2) and rapid shifts in impedance magnitude and phase (Fig. 4A). This pattern could be due to an abnormally compliant TM.24

Previous studies report that fascia grafts are thicker than the native TM, thereby altering the middle-ear system by increasing its mass.14 Mass effects are seen in the high frequencies of WAI, above resonance, and can theoretically contribute to the poor audiometric outcomes at high frequencies seen in this and other studies.10–12 Only few fascia grafted ears provided impedance data above resonant frequency, thus we were unable to compare high-frequency mass-related effects of the TM.

In 10/11 fascia grafted ears, the etiology of the perforation was chronic otitis media, which could affect the ossicular chain by erosion or scarring (note, ears with evidence of middle-ear pathology were excluded). However, fascia grafts and normal TMs show similarities in absorbance and impedance below 1 kHz. This similarity in low frequencies indicates no major difference in stiffness/compliance properties, thus major middle-ear pathology affecting stiffness in our fascia grafted ears is unlikely.22,23,35

Limitations

This study is limited by its small number of fascia grafted ears; however, our findings were unambiguous and statistically significant, and it is likely that they do reflect the true mechanical properties of uncomplicated fascia grafted TMs. To enable a study minimizing confounding factors, strict inclusion and exclusion criteria reduced bias from factors such as different surgical techniques, other graft materials, and concurrent non-TM mechanical pathologies (e.g. middle-ear pathology).

Fascia 2 and 5 had undergone canal wall up mastoidectomy, which increases middle-ear cavity volume and alters the mastoid space. This can affect WAI but these two ears were not systematically different from the other fascia grafted ears and did not show WAI patterns consistent with increased volume.36,37

A standard ear canal cross-sectional area is used in computation of impedance and absorbance. If the true ear canal cross-sectional area is larger than the area used in the computation, it may falsely produce a slightly lower absorbance at low frequency.38 However, Figs. 2 and 3 do not show systematically lower absorbance below 1 kHz in fascia grafted ears, suggesting that slight difference in ear canal area at a limited location of canaloplasty did not play a noticeable role in absorbance.

Effect of negative middle-ear pressure possibly affecting our measurements is of concern. We were careful to not include subjects with active Eustachian tube dysfunction, and all subjects were asked to swallow prior to WAI measurements to equalize the middle-ear cavity to atmospheric pressure. Postoperative 226 Hz tympanometry (not on the same day as WAI measurement) was performed in five of the fascia grafted ears, and all demonstrated normal middle-ear pressure. Negative middle-ear pressure is known to stiffen the TM, affecting absorbance by decreasing it at low-frequencies (<1.5 kHz) and increasing it at high frequencies (>2.6 kHz).39,40 The absorbances measured in fascia grafted ears were not consistent with negative middle-ear pressure.

Conclusion

WAI measurements demonstrate differences in mechanical properties between the fascia grafted TM and the normal unoperated TM. The normal TM allows for better sound transmission at frequencies above 1 kHz as compared to fascia graft, as seen in the absorbance data. Impedance characteristics at resonant frequency are consistent with lower resistance in the fascia grafted ears. This may be explained by poor coupling of the malleus and the fascia graft or low resistance areas of the fascia. Fascia grafted and normal TMs provide similar mechanical properties below 1 kHz, evidenced by both absorbance and impedance data. Development of TM grafts with optimized mechanical properties, especially to improve mid-to-high frequency mechanics, will be valuable for improving hearing outcomes for patients.

Acknowledgement:

This work was conducted with statistical support from Harvard Catalyst and Dr. Hang Lee.

Funding:

Funding was provided by NIH/NIDCD: R01 DC004798, K08 DC018575, F31 DC016761; the William Demant Foundation; Copenhagen Hearing & Balance Centre at the Department of Otorhinolaryngology, Head and Neck Surgery & Audiology, Copenhagen University Hospital - Rigshospitalet; Thomas M. and Louise A. Brown Research Studentship Fund, Queen’s University School of Medicine.

Footnotes

Conflicts of Interest Statement: The authors have no conflicts of interest to disclose.

Institutional Review Board (IRB) Approval: The study was approved by the Human Subjects Committee of our Institutional Review Board at Mass General Brigham, protocol number: 2019P003517.

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