Albrecht Eiber

Tight stapes prosthesis fixation leads to better functional results in otosclerosis surgery.

Tight fixation of stapes prostheses yields better functional results because sound transmission from the incus to the prosthesis is improved. Background: The optimal prosthesis to use for otosclerosis surgery is still a matter of debate. It has been proposed that using prostheses made of Nitinol, a shape-memory metal, produces better functional results with less variability and reduced risk for middle and inner ear damage. This is thought to be because heat activation rather than manual crimping of the prosthesis loop forms a tighter fixation. Methods: Functional results of two groups were compared 1 year after surgery. In one group were 75 cases of stapedotomy performed using Nitinol prostheses. Results were analyzed prospectively and compared with 75 retrospectively analyzed matched controls with conventional stapes prostheses. Crimping quality was measured in 23 patients by intraoperative laser Doppler interferometry (LDI). Causality was assessed by correlating results of intraoperative LDI and postoperative pure-tone thresholds. Results: Nitinol and conventional prostheses yielded postoperative air-bone gaps (ABGs) of 8.0 and 11.6 dB with 71 and 43% ABG closure within 10 dB, respectively. Intraoperatively, sound transmission was improved by 2.5 dB with the Nitinol prostheses as compared with conventional prostheses. These differences were statistically significant. Intraoperative fixation quality was positively correlated to functional outcome, but results were not statistically significant. Conclusion: Tight fixation, as provided by Nitinol prostheses leads to improved functional results because of better sound transmission properties at the incus-prosthesis interface. The improvement in ABG closure is in the range of 3 dB pure-tone average and more pronounced at higher frequencies. Nitinol prostheses provide an effective treatment option in otosclerosis surgery.

The effects of complex stapes motion on the response of the cochlea.

Hypothesis: The piston-like motion of the stapes footplate is the only effective stimulus to the cochlea, and rocking-like stapes motions have no effect on hearing. Background: Studies of the vibration of the stapes in response to acoustic stimulation of the normal ear have revealed a complex movement pattern of its footplate. At low frequencies, the vibrations are predominantly piston-like, but they become increasingly rocking-like at middle and high frequencies. These complex vibrations can be decomposed into a translational, piston-like displacement and 2 rotational movements around the long and short axes of the stapes. The rotational components produce no net volume displacement of the cochlear fluid at some distance from the footplate. Therefore, according to the classic theory of hearing, the rotational motion is not transformed into cochlear activity and a hearing sensation. It was the goal of this study to test this hypothesis experimentally in guinea pigs. Methods: A piezoelectric 3-axis device was used to vibrate the stapes in various desired directions while simultaneously monitoring the actual motion of the stapes by a 3-dimensional laser Doppler interferometer and the cochlear activity by recording the compound action potential. Results: The collected data of the presented study cannot be explained by the current theory of hearing. Conclusion: The qualitative results provide supportive evidence that complex movements of the stapes footplate may lead to cochlear activity. Further experiments are necessary to confirm and quantify these effects.

Mechanical Modeling and Dynamical Behavior of the Human Middle Ear

Very serious injuries may result from impulse noise applied to the human ear. To assess the hazard of a given impulse, its effects on the displacements and the velocities of the structures in the middle and inner ear have to be evaluated. Thus, it is necessary to consider the temporal pattern of applied pressure and the resulting temporal response of the ossicular displacements and velocities. These investigations have to be carried out in the time domain because the relations in the frequency domain known from steady-state motion do not hold. Mechanical models based on the finite-element approach and the multibody system method are presented to describe the spatial motions of the eardrum and the ossicles in the middle ear. The motion of all points of the ossicular chain can be calculated using these models. The free vibrations as well as the general solution of the excited system, consisting of a transient and a steady-state part, are analyzed. Three different sound pressure sources are considered and the dynamical response of the ossicular chain evaluated. It is not sufficient to assess a particular impulse only by its peak pressure and a characteristic time duration since the temporal response of the middle ear is strongly dependent on the waveform of sound pressure. In particular, it is shown that in most of the cases the first negative part of the pressure waveform is expected to cause the worst damage.

Acoustomechanical properties of open TTP® titanium middle ear prostheses

OBJECTIVE The purpose of the study was to identify acoustcomechanical properties of various biostable and biocompatible materials to create a middle ear prosthesis with the following properties: (i) improved handling including a good view of the head of the stapes or footplate and adjustable length, (ii) improved acoustical characteristics that are adequate for ossiculoplastic. The identified material should serve to build CE and FDA approved prostheses for clinical use in patients. METHODS Test models made of Teflon, polyetheretherketone, polyethylenterephtalate, polysulfone, gold, Al2O3 ceramics, carbon and titanium were investigated for their potential to fulfill the requirements. Acoustical properties were investigated by laser Doppler velocimetry (LDV) in mechanical middle ear models (MMM). Measured data were fed in to a recently created computer model of the middle ear (multibody systems approach, MBS). Using computer-aided design (CAD) measured and computed data allowed creation and fine precision of titanium prostheses (Tübingen Titanium Protheses, TTP). Their handling was tested in temporal bones. Acoustomechanical properties were investigated using the MBS and mechanical middle ear models. MAIN OUTCOME MEASURES Input impedance, mass, stiffness, and geometry of test models and prostheses were determined. Furthermore, their influence on the intraprosthetic transfer functions and on coupling to either tympanic membrane or stapes was investigated. RESULTS Final results were FDA- and CE-approved filigreed titanium prostheses with an open head that fulfilled the four requirements detailed above. The prostheses (TTP) were developed in defined lengths of between 1.75 and 3.5 mm (partial) and 3.0 and 6.5 mm (total) as well as in adjustable lengths (TTP-Vario). CONCLUSIONS The results suggest acoustomechanical advantages of TTPs because they combine a significantly low mass with high stiffness. In contrast to closed prostheses, the open head and filigreed design allow an excellent view of the prosthesis foot during coupling to the head or footplate of stapes, contributing to an improved intraoperative reliability of prosthesis coupling.

Dynamics of Middle Ear Prostheses – Simulations and Measurements

The efficient and systematic development of a middle ear prosthesis necessitates the use of computer models for the prosthesis itself and the reconstructed middle ear. The structure and parameters of the computer model have to be verified by specific measurements of the implant and the reconstructed ear. To obtain a realistic model of a reconstructed ear, three steps of modeling and measurements have been carried out. To get a first approach of the coupling elements a mechanical test rig representing a simplified reconstructed middle ear was built. The velocity of the stapedial footplate was measured with a laser Doppler vibrometer. The corresponding computer model was formulated, and the respective parameters were determined using the measured dynamical transfer functions. In the second step, a prosthesis was implanted into a human temporal bone without inner ear. Exciting this system with noise, the velocity of the stapes footplate was measured with the laser Doppler vibrometer. Based on the multibody system approach, a mechanical computer model was generated to describe the spatial motions of the reconstructed ossicular chain. Varying some significant parameters, simulations have been carried out. To describe the dynamical behavior of the system consisting of middle and inner ear, the computer model used in the second step has been enlarged by adding a simplified structure of the inner ear. The results were compared with in situ measurements taken from living humans.

Nonlinear stiffness characteristics of the annular ligament

The annular ligament provides a compliant connection of the stapes to the oval window. To estimate the stiffness characteristics of the annular ligament, human temporal bone measurements were conducted. A force was applied sequentially at several points on the stapes footplate leading to different patterns of displacement with different amounts of translational and rotational components. The spatial displacement of the stapes footplate was measured using a laser vibrometer. The experiments were performed on several stapes with dissected chain and the force was increased stepwise, resulting in load-deflection curves for each force application point. The annular ligament exhibited a progressive stiffening characteristic in combination with an inhomogeneous stiffness distribution. When a centric force, orientated in the lateral direction, was applied to the stapes footplate, the stapes head moved laterally and in the posterior-inferior direction. Based on the load-deflection curves, a mechanical model of the annular ligament was derived. The mathematical representation of the compliance of the annular ligament results in a stiffness matrix with a nonlinear dependence on stapes displacement. This description of the nonlinear stiffness allows simulations of the sound transfer behavior of the middle ear for different preloads.

Nonlinear modelling of the middle ear as an elastic multibody system - Applying model order reduction to acousto-structural coupled systems

In this study, modelling of the human hearing is considered. Due to the nonlinearity of the middle ear, the sound transfer changes as the equilibrium position of the middle ear structure varies. For the description of the middle ear a nonlinear elastic multibody system is derived. The tympanic membrane and the air in the ear canal as well as in the tympanic cavity are considered as elastic bodies. They are first modelled using the finite element method. The large number of degrees of freedom makes a following reduction step of the acousto-structural finite element model inevitable. The second-order structure of the system matrices is preserved by applying reduction techniques based on Petrov-Galerkin projection. The nonlinearity of the tympanic membrane is included following the approach of parametric model order reduction by matrix interpolation assuming that the nonlinearity can be represented by the relative pressure between the ear canal and the tympanic cavity. Finally the static and dynamic behaviour of the simulation model is reviewed for different static pressure loads of the middle ear.

Clinical evaluation of the NiTiBOND stapes prosthesis, an optimized shape memory alloy design.

Objective To prospectively analyze short-term (3 mo) results in patients with otosclerosis who underwent stapedotomy with the newly designed NiTiBOND prosthesis and compare them with patients that underwent SMart piston stapedotomy. We aimed to assess “noninferiority” for the new prosthesis. Study Design Prospective controlled trial. Setting Tertiary referral center. Patients Thirty-eight patients were included in the NiTiBOND group (41 ears), and 74 patients were included in the SMart Piston group (75 ears). Intervention(s) Stapedotomy. Main Outcome Measure(s) Pure-tone audiometry 3 months after surgery, intraoperative prosthesis handling as assessed using a questionnaire, and complications were analyzed. Results Pure-tone audiometry showed postoperative air-bone gap means (standard deviation) of 8.1 (8.3) and 9.9 (5.4) dB; air-bone gap closure within 10 dB was achieved in 71% and 72% and within 20 dB in 93% and 96% for the NiTiBOND and the SMart piston prosthesis, respectively. Noninferiority was shown at all frequencies and in the pure-tone average. The NiTiBOND prosthesis provides excellent intraoperative handling, and no adverse reactions were reported. Conclusion Preliminary short-term results suggest safety and reliability for the new NiTiBOND stapes prosthesis.

Contribution of complex stapes motion to cochlea activation.

Classic theories of hearing have considered only a translational component (piston-like component) of the stapes motion as being the effective stimulus for cochlear activation and thus the sensation of hearing. Our previous study (Huber et al., 2008) qualitatively showed that rotational components around the long and short axes of the footplate (rocking-like components) lead to cochlear activation as well. In this study, the contribution of the piston-like and rocking-like components of the stapes motion to cochlea activation was quantitatively investigated with measurements in live guinea pigs and a related mathematical description. The isolated stapes in anesthetized guinea pigs was stimulated by a three-axis piezoelectric actuator, and 3-D motions of the stapes and compound action potential (CAP) of the cochlea were measured simultaneously. The measured values were used to fit a hypothesis of the CAP as a linear combination of the logarithms of the piston-like and rocking-like components. Both the piston-like and rocking-like components activate cochlear responses when they exceed certain thresholds. These thresholds as well as the relation between CAP and intensity of the motion component were different for piston-like and rocking-like components. The threshold was found to be higher and the sensitivity lower for the rocking-like component than the corresponding values for the piston-like component. The influence of the rocking-like component was secondary in cases of piston-dominant motions of the stapes although it may become significant for low amplitudes of the piston-like component.

Characterization of Stapes Anatomy: Investigation of Human and Guinea Pig

The accuracy of any stapes model relies on the accuracy of the anatomical information upon which it is based. In many previous models and measurements of the stapes, the shape of the stapes has been considered as symmetric with respect to the long and short axes of the footplate. Therefore, the reference frame has been built based upon this assumption. This study aimed to provide detailed anatomical information on the dimensions of the stapes, including its asymmetries. High-resolution microcomputed tomography data from 53 human stapes and 11 guinea pig stapes were collected, and their anatomical features were analyzed. Global dimensions of the stapes, such as the size of the footplate, height, and volume, were compared between human and guinea pig specimens, and asymmetric features of the stapes were quantitatively examined. Further, dependence of the stapes dimensions on demographic characteristics of the subjects was explored. The height of the stapes relative to the footplate size in the human stapes was found to be larger than the corresponding value in guinea pig. The stapes showed asymmetry of the footplate with respect to the long axis and offset of the stapes head from the centroid of the medial surface of the footplate for both humans and guinea pigs. The medial surface of the footplate was curved, and the longitudinal arches of the medial surface along the long axis of the footplate were shaped differently between humans and guinea pigs. The dimension of the footplate was gender-dependent, with the size greater in men than in women.