Gert Hofmann

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.

Titanium clip prosthesis.

Objective: Prostheses for the reconstruction of a defective ossicular chain should be stable and firmly anchored to the ossicular remnants. This will prevent a defective connection from causing diminished sound transmission efficiency and will keep the prosthesis from tilting or even losing contact, which would result in a sound transmission block. Through temporal bone experimentation, we have consequently developed a very lightweight titanium prosthesis, which is fastened onto the stapes head with a clip mechanism. Methods: When temporal bone experiments using laser Doppler vibrometry confirmed that the prosthesis functioned well acoustically and when luxation experiments proved that it could be safely used without the risk of stapes dislocation, the prosthesis was used in a clinical application within an observational study. The University of Dresden Otorhinolaryngological Hospital as well as seven surgeons from five other hospitals participated in the study. Results: The results of 133 operations showed that, in over 90% of the cases, the prosthesis could be implemented without problems and with good mechanical stability. The first acoustical results obtained during the first year from 49 patients showed a sound transmission improvement range from 12 dB to 14 dB. Conclusion: With the clip prosthesis, it seems possible to further improve defective middle ear function, which would allow the patient to regain social hearing after middle ear reconstruction. The reliability of the fastening is an innovation. Revision operations showed a stable prosthesis-stapes complex in the middle of a recurring cholesteatoma and the prosthesis could always be easily pulled from the stapedial suprastructure.

Behavior of evoked otoacoustic emission under low-frequency tone exposure: Objective study of the bounce phenomenon in humans.

The bounce phenomenon has been investigated in humans, evaluating alterations of click evoked otoacoustic emission (EOAE) after presentation of 250-Hz frequency loud tones during 3 min. EOAE changes were manifested in initial augmentation followed by reduction, peaking at 1 and 3 min of post-exposure time, respectively. Recoveries took 5-7 min afterwards. Under linear and nonlinear EOAE acquisition modes both manifestations of bounce appeared similar. At lower exposure intensities, 65-75dB SPL, augmentations prevailed over reductions. At higher intensities, 80-95 dB SPL, augmentations and reductions were of similar magnitudes. At highest intensity, 100 dB SPL, an obvious EOAE drop has hardly been preceded by any augmentation. Based upon these data, the bounce is considered to be a compound of two opposite events, appearance of each being dependent upon the exposure level. Subjects with high bounce indices in one ear displayed comparable indices in other ear too. Low bounce magnitudes were accordingly typical for particular subjects irrespective of the ears tested. EOAE alterations were observed under ipsilateral, but not contralateral exposures of tones. It has been concluded therefore that the bounce involves peripheral receptor rather than central neural mechanisms. No EOAE shifts were seen under application of clicks without any low-frequency exposure tones. Correspondingly, the bounce is judged to reflect inner-ear processes triggered by low-frequency tones, but not by regular presentations of test-stimuli.

Unterschiede des auditiven Frequenzdiskriminationsvermögens bei Musikern verschiedener Fachbereiche

Das Frequenzdiskriminationsvermogen von 63 professionellen Musikern verschiedener Fachbereiche (Sanger, Streicher, Blaser, Pianisten und Rhythmiker) wurde mittels eines binauralen Frequenzdiskriminationstests untersucht. Dabei liessen sich statistisch signifikante Unterschiede zwischen den Musikergruppen sichern. Die Streicher zeigten vor den Blasern, Pianisten, Sangern und Rhythmikern die beste Einstellre-produzierbarkeit. Die Ergebnisse belegen, dass instrumenten- bzw. ausbildungsspezifische Phanomene das Frequenzdiskriminationsvermogenbeeinflussen.

Contribution of Paranasal Sinuses to the Acoustic Properties of the Nasal Tract

Background: The contribution of the nasal and paranasal cavities to the vocal tract resonator properties is unclear. Here we investigate these resonance phenomena of the sinonasal tract in isolation in a cadaver and compare the results with those gained in a simplified brass tube model. Methods: The resonance characteristics were measured as the response to sine sweep excitation from an earphone. In the brass model the earphone was placed at the closed end and in the cadaver in the epipharynx. The response was picked up by a microphone placed at the open end of the model and at the nostrils, respectively. A shunting cavity with varied volumes was connected to the model and the effects on the response curve were determined. In the cadaver, different conditions with blocked and unblocked middle meatus and sphenoidal ostium were tested. Additionally, infundibulotomy was performed allowing direct access to and selective occlusion of the maxillary ostium. Results: In both the brass model and the cadaver, a baseline condition with no cavities included produced response curves with clear resonance peaks separated by valleys. Marked dips occurred when shunting cavities were attached to the model. The frequencies of these dips decreased with increasing shunting volume. In the cadaver, a marked dip was observed after removing the unilateral occlusion of the middle meatus and the sphenoidal ostium. Another marked dip was detected at low frequency after removal of the occlusion of the maxillary ostium following infundibulotomy. Conclusion: Combining measurements on a simplified nasal model with measurements in a cadaveric sinonasal tract seems a promising method for shedding light on the acoustic properties of the nasal resonator.

Evaluation des Dresdner Tympanoplastik Modells (DTM)

The training of microsurgical motor skills is essentiell for surgical education if the interests of the patient are to be safeguarded. In otosurgery the complex anatomy of the temporal bone and variations necessitate a special training before performing surgery on a patient. We therefore developed and evaluated a simplified middle ear model for acquiring first microsurgical skills in tympanoplasty.The simplified tympanoplasty model consists of the outer ear canal and a tympanic cavity. A stapes model is placed in projection of the upper posterior tympanic membrane quadrant at the medial wall of the simulated tympanic cavity. To imitate the annular ligament flexibility the stapes is fixed on a soft plastic pad. 41 subjects evaluated the model´s anatomical analogy, the comparability to the real surgical situation and the general model properties the using a special questionnaire.The tympanoplasty model was very well evaluated by all participants. It is a reasonably priced model and a useful tool in microsurgical skills training. Thereby, it closes the gap between theoretical training and real operation conditions.

Biologic Fixation of the Electrode Cable of Cochlea Implants

Objectives: To verify the necessity for special surgical techniques or clips for fixation of the electrode cable of a cochlea implant against dislocation, and to test the stability of postoperative biologic cicatrization as the sole and solid anchoring of the cable. Material: Temporal bone experiments with a simulated connective tissue sheath around conventional (Med El Combi 40+) and prototype (profiled surface) electrode cables. Results and Conclusions: The electrode cable is anchored securely in a sheath of scar tissue, since unphysiologic loads are needed for pulling it out of its anchorage. The drag during one extraction trial with a profiled cable even resulted in the rupture of the cable. These results confirm our confidence in this biologic fixation of the electrode cable inside its postoperative cicatric tissue sheath. More than 80 cochlea implantations with the electrode simply imbedded in a drop of fibrin glue in the posterior tympanotomy never demonstrated a shift of the electrodes in the last 8 years. Therefore, special fixation of the electrode cable with clips or surgical techniques is not necessary.

Approach to evidence of middle ear occlusion effect by laser vibrometry

In the beginning of the last century German anatomist E. H. Weber first reported an audiological phenomenon. Up to now Webers test is widely used by the otologists for the differentiation of sound conduction and inner ear disorders. The sound of an excited tuning-fork on the top of the head will be lateralized into the ear with an occluded ear canal (occlusion effect). But so far there has been no objective criterion for this effect on humans or temporal bone preparations. We have performed various approaches for the measurement of the occlusion effect. The slow cortical acoustical evoked potentials (SAEP) on both sides after bone conduction stimulation of the vertex at several test persons. We succeeded in proving a decrease of latency during occlusion of ear canal. This has been an electrophysiological approach. However, the theory gives reasons for this effect by changed acoustical impedance conditions of the ear canal due to its occlusion. The related increased sound pressure level (SPL) inside the ear canal results in unilateral amplification effect and its transfer via the middle ear into the inner ear, and therefore performs a lateralization. Thus one should be able to measure this amplification in temporal bone preparations too. Laser vibrometry allows a non-contact access to the tympanic membrane and the middle ear apparatus. The tympanic membrane transforms the ear canals amplified sound pressure into increased mechanical vibration of the ossicular chain and eventually the stapes footplate affecting the inner ear liquid. That is why we positioned the beam of a laser vibrometer at the inner ear side of the footplate. The preparation was broadband-excited by a bone conduction vibrator. During occlusion of the ear canal an increased sound pressure level inside the ear canal was registered by a probe microphone. By assuming a transfer of this SPL increase onto the footplate we measured its displacement by laser vibrometry. Generally lower gains could have been proven only compared to subjective evaluation (audiometry) or SAEP derivation. Furthermore the effect was detected within limited excitation level and frequency ranges. This is explained by the changed impedance conditions at the foot plate termination due to the removed inner ear, combined with phase differences during osteophony into the ear canal and along the ossicular chain ligaments respectively. Unfortunately there are several problems regarding bone conduction stimulation of temporal bone preparations. Especially the strained fixation of the preparation in relation to the exciting vibrator had to be considered.

Book Reviews · Recensions · Buchbesprechungen

Mirko Tos legt mit diesem Buch die logische Folge seiner drei vorherigen «Manuals of Middle Ear Surgery» vor. Dieser Band widmet sich ausschliesslich den Fixationen der Ossikel, was allerdings dem Titel nicht zu entnehmen ist. Dabei liegt das Hauptaugenmerk auf den verschiedenen operativen Verfahren zur Behebung dieser Pathologie, wobei wiederum eine umfangreiche Darstellung praktisch sämtlicher in der Literatur angegebener Techniken mit den Erfahrungen dieses exzellenten Operateurs kombiniert wird. Der Leser erhält so nicht nur einen Überblick über alle möglichen und denkbaren operativen Techniken, sondern gleichzeitig – und dies ist besonders wichtig – Informationen über die Vorund Nachteile dieser verschiedenen Techniken. Das Buch ist entsprechend den Ursachen der ossikulären Fixationen in vier Sektionen unterteilt: Der erste Abschnitt behandelt die Tympanosklerose, wobei hier viel Wissenswertes zur Ätiologie und Pathogenese dieser immer noch rätselhaften Erkrankung, auch durch die eigenen Forschungen von Mirko Tos, aufgeführt ist. Im zweiten Abschnitt werden postentzündliche, posttraumatische und postoperative Fixationen der drei Ossikel besprochen. Dem grössten Abschnitt widmet sich das dritte Kapitel, die Otosklerose. Die modernen Techniken werden vor dem Hintergrund einer ausführlichen Beschreibung der vielen Techniken in der Geschichte der Otosklerosechirurgie dargestellt. Besonders ausführlich sind die unterschiedlichen Komplikationen während und nach der Stapedektomie und Stapedotomie aufgeführt. Auch kleine, aber in der klinischen Praxis ungemein wichtige Details, wie der Hörverlust spät nach der Operation oder «host reactions» werden in dieser umfassenden Besprechung aufgeführt. Der Rezensent vermisst hier nur ein kleines Detail: die in der deutschen Literatur viel diskutierten Flusen der Baumwollabdeckung, die als Ursache des Fremdkörpergranuloms bei unerklärbaren Innenohrausfällen nach Stapedektomie verantwortlich gemacht werden, sind nicht erwähnt. Der Abschluss dieses Kapitels behandelt das seltene, aber schwierige gleichzeitige Vorhandensein einer Otosklerose mit einer chronischen Otitis media. Im letzten Kapitel werden die kongenitalen Fixationen mit der Betonung auf die Stapesfixation besprochen. Hier findet sich auch ein ausführlicher Abschnitt über die Embryologie der Stapesentwicklung sowie die verschiedenen Klassifizierungen dieser Entität. Es ist kaum verwunderlich, dass auch hier Mirko Tos eine eigene Klassifikation, aufbauend auf den bisherigen Überlegungen anderer Autoren und seinen eigenen Erfahrungen, entwickelt hat. Hervorzuheben ist bei diesem Buch die wiederum exzellente graphische Bearbeitung, die für eine Darstellung operativer Verfahren unumgänglich ist. Die vielen plastisch sehr instruktiven Abbildungen (576 z.T. mehrfach untergliederte Illustrationen) verdeutlichen jedes beschriebene Operationsverfahren bzw. einzelne Operationsschritte. Gerade dies stellt neben der fast lückenlosen Darstellung des Themas den besonderen Wert dieses Buches dar. Es ist zu hoffen, dass Mirko Tos seine Ankündigung im Vorwort wahr macht, auch die in seinen vier Büchern noch nicht beschriebenen Probleme der Mittelohrchirurgie in den folgenden Monografien zu Papier bringen zu wollen. Der weltweiten Gemeinde der Otochirurgen sind diese Bücher als Referenz und «state of the art» unverzichtbar geworden. K.-B. Hüttenbrink, Dresden

Laser vibrometry for investigation of tympanic membrane implant materials

The human tympanic membrane has reasonably good sound sensing properties. A destroyed tympanic membrane due to middle ear diseases or traumata may be repaired by different types of grafts. Middle ear surgery mostly uses autologous temporal fascia, cartilage, or cartilage perichondrium transplants. We have investigated the acoustical and mechanical properties of these materials and compared them with human tympanic membrane by constructing an ear canal model completed by an artificial tympanic membrane. Circular stretched human 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 of the graft material were measured by laser Doppler vibrometry and compared. The thin materials 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 yields a higher compliance than tympanic membrane. The acoustical and mechanical properties of cartilage transplants change with the thickness of the slices. However, the thinner the cartilage slice combined with lower stability, the more similar is the frequency response with the intact tympanic membrane. The vibration amplitudes decrease more and more for layer thicknesses above 500 micrometers. Cartilage acts as an excellent transplant material which provides a better prognosis than different materials in cases of ventilation disorders with long-term middle ear pressure changes. Large cartilage slice transplants should not exceed layer thicknesses of 500 micrometer in order to prevent drawbacks to the transfer characteristics of the tympanic membrane.