Methods and reference data for middle ear transfer functions.

Human temporal bone specimens are used in experiments measuring the sound transfer of the middle ear, which is the standard method used in the development of active and passive middle ear implants. Statistical analyses of these experiments usually require that the TB samples are representative of the population of non-pathological middle ears. Specifically, this means that the specimens must be mechanically well-characterized. We present an in-depth statistical analysis of 478 data sets of middle ear transfer functions (METFs) from different laboratories. The data sets are preprocessed and various contributions to the variance of the data are evaluated. We then derive a statistical range as a reference against which individual METF measurements may be validated. The range is calculated as the two-sided 95% tolerance interval at audiological frequencies. In addition, the mean and 95% confidence interval of the mean are given as references for assessing the validity of a sample group. Finally, we provide a suggested procedure for measuring METFs using the methods described herein.

[Coupling of active middle ear implants-biomechanical aspects]. / Ankopplung aktiver Mittelohrimplantate ­ biomechanische Aspekte.

Active middle ear implants or implantable hearing aids are used to treat sensorineural or combined hearing loss. Their coupling to the middle ear structures has a large impact on the success of rehabilitation. Practical issues such as the coupling site, influence of middle ear status, and forward and backward excitation of the inner ear are discussed in the context of biomechanics. For this purpose, experimental studies, model simulations, and current literature data are evaluated. The explanations are intended to contribute to a better understanding of certain procedures in hearing rehabilitation with active implants.

Can piezoelectric ultrasound osteotomies result in serious noise trauma?

The use of ultrasound to cut bone in oral and craniofacial surgery has increased. There is concern that the application of ultrasound to the craniofacial skeleton might represent a potential hazard to the inner ear because of sound transmission by bone conduction resulting in hearing trauma. Conventional and ultrasound osteotomies were performed on human specimens of temporal bone containing an intact middle and inner ear. The equivalent sound pressure was measured with a microphone at the round window, which had been calibrated with a bone conduction audiometer. Conventional osteotomy with a rose burr resulted in maximum sound pressures of 125dB(A) consisting of major frequency components at 2100, 7600, and 9300Hz. Ultrasound osteotomy resulted in maximum sound pressures of 122dB(A) and exhibited major frequency components at around 10kHz, 20kHz, and 26.5kHz. Ultrasound osteotomies have no acoustic advantage over conventional osteotomies. Both osteotomy techniques can produce noise-induced hearing trauma, especially when applied over longer durations of time. This appears to be more relevant for ultrasound osteotomies, because the bone cutting efficiency is usually poorer than in conventional osteotomies. Surgeons should consider the risk of noise-induced potential damage to the inner ear when selecting the method of osteotomy.

[The Vibrant Soundbridge as an active implant in middle ear surgery]. / Die Vibrant Soundbridge als aktives Implantat in der Mittelohrchirurgie.

Implantable hearing aids are not only gaining importance for the treatment of sensorineural hearing loss, but also for treatment of mixed hearing loss. The most frequently used active middle ear implant is the Vibrant Soundbridge (VSB) system (Fa. MED-EL, Innsbruck, Österrreich). Following widening of the spectrum of indications for the VBS, various new coupling systems have been established. Based on the literature, available petrosal bone investigations and finite element model (FEM) calculations, this article summarizes the current knowledge concerning mechanical excitation by the VSB. Important concomitant aspects related to coupling, transmission and measurement are also discussed.

Experiments on the coupling of an active middle ear implant to the stapes footplate.

BACKGROUND: Although the function of active middle ear implants in cases of intact ossicular chains and ventilated middle ears is well known, information about sound transfer function to the inner ear in cases of chronic middle ear effusion and defective middle ear structures is needed. A temporal bone model was developed to measure (1) the coupling of the active middle ear implant Vibrant Soundbridge in cases of nonventilated radical cavities, and (2) the effect of effusion and cartilage shielding. METHODS: Three fresh human temporal bone specimens were studied. After preparation of a radical cavity, the floating mass transducer was coupled to the stapes footplate. The transducer was stimulated with 50 mV multisinus signals and inner ear fluid vibration was measured using a microphone in the round window niche. Several coupling conditions were simulated with mass and stiffness variations and cartilage shielding. RESULTS: Coupling modality and prestress have the most influence on the sound transfer function to the inner ear. Cartilage shielding may ensure better coupling of the FMT to the footplate. The effect of middle ear effusion is negligible. CONCLUSION: The Vibrant Soundbridge provides good sound transfer to the inner ear not only in cases of coupling onto an intact ossicular chain in a ventilated middle ear but also in cases of coupling to the stapes footplate in non-ventilated radical cavities.

[Experimental investigations on middle ear prostheses with an integrated micro joint]. / Experimentelle Untersuchungen zu Mittelohrimplantaten mit integriertem mikromechanischem Gelenk.

BACKGROUND: Quasi static pressure changes as they occur in altitude changes or Valsalva's manoeuvre are causing great tympanic membrane displacements. These can produce a prosthesis dislocation from the stapes footplate, in case of using a TORP for reconstruction. Additionally, prostheses extrusions can occur in the chronically poor ventilated middle ear. Anatomical studies on the ostrich demonstrate that the flexible columella in the avian middle ear provides a good sound transmission and features an effective inner ear protection in pressure changes. MATERIAL AND METHODS: Focusing on this bionic attempt, we designed a new micro joint TORP by interposing a silicone ball joint in the prosthesis shaft. This prototype was used for frequency response measurements with the laser Doppler vibrometer in the ventilated middle ear and under conditions of increased pressure in the external auditory canal. RESULTS: Frequency response measurements showed comparable results to those using a conventional TORP. Investigations on pressure changes in the external auditory canal demonstrated a significant decrease in footplate excursion after reconstruction with the new micro-joint prosthesis then compared to a common TORP. CONCLUSIONS: The insertion of the new silicone bordered micro joint into a TORP shaft provides an effective method to decrease the potentially damaging stapes footplate excursions after complete ossicular reconstruction. Limited deflections of the prosthesis shaft reduce the occurring forces on the stapes footplate as they partly bore the quasi static pressure changes. Thus, the micro joint can reduce the risk of prostheses dislocation and annular ligament damage and is additionally providing a proper sound transmission through the reconstructed middle ear. Moreover, the bionic modification of a TORP reflects the remaining possibilities for further improvements in prosthetic reconstruction.

Reflection of bounce phenomenon in TEOAE in humans: dependence upon exposure parameters.

The bounce phenomenon is an alteration of hearing acuity after presentation of loud tones. Generally, it implies the improvement in hearing acuity that followed by the worsening. The dependence of the bounce upon exposure stimulus parameters has been evaluated in the present study. Normally hearing subjects were investigated, while transiently evoked otoacoustic emission, TEOAE, has been recorded to estimate the event objectively. In the 1st experiment, the bounce indices have been compared under different intensities, 55-100 dB SPL, of exposure-tone of 250-Hz frequency. At lower intensities, 65-75 dB SPL, the augmentation phase in TEOAE bounce reflections exceeded the depression one. At higher intensities, 80-95 dB SPL, both phases of the bounce were of similar magnitudes. The event appeared thus bipolar and symmetric. At the highest intensity, 100 dB SPL, TEOAE drop has hardly been preceded by any augmentation. In the 2nd experiment, the bounce manifestations have been compared under different exposure-tone frequencies, 250, 500, and 2000 Hz. Under 250- and 500-Hz frequencies, the bounce was of similar magnitudes. Under 2000-Hz frequency, a conventional bounce has not been seen, a trend to the overall TEOAE decrement being observed only. The bounce mechanism is discussed and its complex rather than simple nature is considered.

[A new model for training in tympanoplasty]. / Entwicklung eines Tympanoplastik-Trainingsmodells.

BACKGROUND: Functional simulation of middle ear reconstruction is a valuable tools for training in otosurgery. We introduce a new experimental model which provides a direct acoustic feedback of the functional quality of ossicular chain reconstruction. METHOD: In this model the tympanic membrane and the ossicular chain have to be reconstructed for proper sound transmission to an artificial inner ear receptor. The received signal is converted into acoustic information and consecutively provided via headphone. RESULTS: Any changes in the reconstruction (e. g. tilting the prosthesis) can be experienced online and immediately optimized by the surgeon or a trainee. CONCLUSION: The experimental model can be used for demonstration and training in otosurgery. This model is also suitable for comparing measurements of transfer functions in a calibrated version and can be applied to development and critical evaluation of middle ear prostheses.

[Thermal stress on the healthy laryngeal mucosa caused by CO(2) laser treatments]. / Thermische Belastung der gesunden Larynxschleimhaut bei Einsatz des CO(2)-Lasers*

BACKGROUND: The carbon dioxide (CO(2)) laser in endolaryngeal surgery is a successful tool because of its implementation of the thermally induced hematostatic incision technique. We investigated the problem of thermal damage of surrounding tissue during laser surgery. Until now a potential damage of surrounding healthy mucosa caused by laser surgery through general heating up, hot gases and steam has scarcely been noticed. During prolonged tumor surgery the tender mucosa of the surrounding healthy tissue can be irreversible affected and damaged due to thermal exposure above 45 degrees C. METHOD: The study was performed on the larynx of a pig cadaver. Temperature probes were placed under the epithelium of the vocal cords on the opposite side of the surgical field to detect any increase in temperature during laser application. RESULTS: A rise in temperature of up to 32 degrees C was observed in the vocal cord opposite the surgical field after 70 s of laser surgery (output power: 4 W, continuous wave mode) even with continuous aspiration. Protection of the healthy mucosa against thermal radiation caused a rise in temperature of only 3.5 degrees C. An increase in temperature of up to 16.1 degrees C was observed, if intermittent exposures (output power: 4 W, pulse length: 0.5 second, repeat time: 0.1 second) were used. CONCLUSION: Transferring the presented cadaver studies to in vivo surgery, the laryngologist should take into account heating of collateral healthy tissue up to 69 degrees C (37 degrees C body temperature plus 32 degrees C heating by thermal radiation).

Biomechanical aspects in implantable microphones and hearing aids and development of a concept with a hydroacoustical transmission.

The middle ear functions as a sensitive pressure receptor. This implies that it not only transmits sound pressure waves with molecule-sized vibrational amplitudes, but that it also reacts to the million times larger variations of ambient air pressure. This pressure induces movements of the tympanic membrane and of the attached ossicular chain of up to 1,000 microm. Any artificial device that contacts these sound-transporting elements, be it a receiver for an implantable microphone or a transducer for an implantable hearing aid, has to respect these two different modes of biomechanical behavior. A hydroacoustical transmission system has therefore been developed consisting of a water-filled flexible tube, which contacts the ossicular chain with a balloon tip, and which is connected to a piezo-electric transducer at its other end. This soft contact prevents a localized pressure load and does not restrain the free movement of the underlying ossicle during ambient pressure variations. Temporal bone experiments showed that the device transmits vibration energy in good acoustical quality. The device can also be used in a reverse mode, working as a microphone, for example for a totally implantable hearing aid or a cochlear implant. This microphone concept has the advantage that it incorporates the biologic sound receiving components (the tympanic membrane and the ossicular chain micromechanics) into a technical device.

[Intraoperative measurement of stapes mobility using a hand-guided electromagnetic probe]. / Intraoperative Messung der Steigbügelbeweglichkeit mittels einer handgeführten, elektromagnetischen Sonde.

BACKGROUND: The hearing results of otosurgery are still unsatisfactory. Even after successful implantation of middle ear prostheses there often remains an air bone gap of 30 dB or more. As possible reasons dislocation of the prostheses due to scar growth, changes in prostheses' attachment or ventilation disorders are being discussed. Decreased stapes mobility, which has been judged only manually up to now, is supposed to be a further reason. METHOD: We are introducing a new electromagnetic probe. The output signal of this device is proportional to the impedance of the stapes-annular ligament cochlear fluid system at the sensor's resonance frequency (2.4 kHz). The advantage of this system is characterised by its hand-guidance. Injury of the sensitive stapes-annular ligament due to tremor movements of the surgeon can be excluded using a special construction of the sensor head. The maximum force of the sensor's tip onto the stapes during measurement is limited to below 5 mN. RESULTS: Preliminary measurement results of 20 patients are presented with normal and abnormal stapes mobility. These results are compared to the subjective impression of the surgeon, who usually tested the stapes mobility by hand. As a result of our investigations probe measurements can detect more exactly decreased mobility of the stapes than the surgeon. CONCLUSIONS: Our device may help to detect latent stapes fixation caused by chronic inflammation of the middle ear. The intraoperative measurement of stapes mobility may influence the strategy of the surgeon. Furthermore it would be helpful in patient consulting prior to a revision-tympanoplasty with predicting the potential hearing improvement.

Experimental investigations of the use of cartilage in tympanic membrane reconstruction.

BACKGROUND: Temporalis fascia, perichondrium, and cartilage are commonly used for reconstruction of the tympanic membrane in middle ear surgery. Cartilage grafts offer the advantage of higher mechanical stability, particularly in cases of chronic tubal dysfunction, adhesive processes, or total defects of the tympanic membrane, in contrast to fascia and perichondrium, which presumably offer better acoustic quality. HYPOTHESIS: The purpose of this study was to determine the acoustic transfer characteristics of cartilage of varying thickness and its mechanical deformation when exposed to fluctuations in atmospheric pressure. METHOD: Ten pairs of cartilage specimens from the cavum conchae and the tragus were obtained from fresh human cadavers. Young's modulus was determined by mechanical tension tests and statistically evaluated using the t test. The acoustic transfer characteristics of an additional 10 specimens were measured by a laser Doppler Interferometer after stimulation with white noise in an external auditory canal--tympanic membrane model. Mechanical stability was determined by measuring displacement of the cartilage using static pressure loads of < or = 4 kPa. RESULTS: Young's modulus determinations for conchal and tragal cartilage were 3.4 N/mm2 and 2.8 N/mm2, respectively, but the difference was not significant. Acoustic testing showed a 5-dB higher vibration amplitude in the midfrequency range for conchal compared with tragal cartilage, but the difference was not significant. Reducing cartilage thickness led to an improvement of its acoustic transfer qualities, with a thickness < or = 500 microm resulting in an acceptable acoustic transfer loss compared with the tympanic membrane. CONCLUSION: Both conchal and tragal cartilage are useful for reconstruction of the tympanic membrane from the perspective of their acoustic properties. The acoustic transfer loss of cartilage can be reduced by decreasing its thickness. A thickness of 500 microm is regarded as a good compromise between sufficient mechanical stability and low acoustic transfer loss.

Identification of parameters for the middle ear model.

This paper presents a method of parameter identification for a finite-element model of the human middle ear. The parameter values are estimated using a characterization of the difference in natural frequencies and mode shapes of the tympanic membrane between the model and the specimens. Experimental results were obtained from temporal bone specimens under sound excitation (300-3,000 Hz). The first 3 modes of the tympanic membrane could be observed with a laser scanning vibrometer and were used to estimate the stiffness parameters for the orthotropic finite-element model of the eardrum. A further point of discussion is the parameter sensitivity and its implication for the identification process.

Modelling of components of the human middle ear and simulation of their dynamic behaviour.

In order to get a better insight into the function of the human middle ear it is necessary to simulate its dynamic behaviour by means of the finite-element method. Three-dimensional measurements of the surfaces of the tympanic membrane and of the auditory ossicles malleus, incus and stapes are carried out and geometrical models are created. On the basis of these data, finite-element models are constructed and the dynamic behaviour of the combinations tympanic membrane with malleus in its elastic suspensions and stapes with annular ligament is simulated. Natural frequencies and mode shapes are computed by modal analysis. These investigations showed that the ossicles can be treated as rigid bodies only in a restricted frequency range from 0 to 3.5 kHz.

[Acoustic and mechanical properties of tympanic membrane transplants]. / Akustische und mechanische Eigenschaften von Trommelfelltransplantaten.

BACKGROUND: The human tympanic membrane has reasonably good sound transmission properties and withstands high static pressure loads. Destruction of the tympanic membrane resulting from middle ear diseases or trauma may be repaired by different types of grafts. Middle ear surgery mostly uses autologous temporal fascia, cartilage, or cartilage perichondrium transplants which differ in their acoustical characteristics and mechanical strength. METHODS: We have investigated the acoustical and mechanical properties of these materials and compared them with human tympanic membranes by constructing an ear canal-tympanic membrane model. Fresh human tympanic membrane, fascia, perichondrium, and cartilage preparations were exposed to static pressures up to 4 kPa and white noise sound pressure levels of 70 dB. The vibrational amplitudes and displacements due to static pressure were measured by laser Doppler vibrometry. RESULTS: The temporal fascia and perichondrium show similar amplitude frequency responses compared to the tympanic membrane for dynamic excitation. The displacement of these materials at static pressures above 4 kPa indicates a higher compliance than the tympanic membrane. The acoustical and mechanical properties of cartilage transplants are determined by the thickness of the slices. Thin cartilage slices are less stable although their frequency response is comparable to the intact tympanic membrane. Layer thickness above 500 microns result in a decrease of vibration amplitudes. CONCLUSIONS: Cartilage is an excellent transplant material which provides a better prognosis than soft materials in cases of ventilation disorders with long-term middle ear pressure problems. Large cartilage slice transplants should not exceed layer thickness of 500 microns in order to minimize transmission loss.