Three-dimensional force analysis of surgical manipulations at the long process of the incus.

PURPOSE: Surgical manipulation with application of inappropriate force may damage middle ear structures leading to hearing loss. This work analyzes the forces applied in simulated otosurgical exercises in a laboratory set-up by measuring the spatial components of applied forces with objective assessment criteria. With these criteria, the individual force characteristics applied by the surgeon can be quantified and an objective feedback can be given about their surgical maneuvers. METHODS: A natural size model of the human incus was mounted on a load cell to measure the spatial forces in all three directions during different manipulation tasks performed under the microscope by ten surgeons from our department having different levels of experience in otosurgery. The motions of the incus model and the instrument tip were recorded simultaneously with a video camera. RESULTS: Independent of surgical experience, a three-dimensional force pattern could be detected with components transverse to the desired force directions. The measured forces applied by trainees showed larger variations in magnitude, in spatial distribution and in temporal course than those applied by experienced surgeons. A better repeatability of identical tasks, constancy of force patterns and low peak force values could be seen in the group of experienced surgeons. CONCLUSIONS: The laboratory system presented in this study using simultaneous video and 3-D force registration allows the objective assessment of surgical manipulations, e.g., at the long process of the incus. Training with video and force feedback provides information about surgical techniques and skill development of surgeons and has the potential to shorten the learning curve and to diminish intra-operative risks to patients.

Sound wave propagation on the human skull surface with bone conduction stimulation.

BACKGROUND: Bone conduction (BC) is an alternative to air conduction to stimulate the inner ear. In general, the stimulation for BC occurs on a specific location directly on the skull bone or through the skin covering the skull bone. The stimulation propagates to the ipsilateral and contralateral cochlea, mainly via the skull bone and possibly via other skull contents. This study aims to investigate the wave propagation on the surface of the skull bone during BC stimulation at the forehead and at ipsilateral mastoid. METHODS: Measurements were performed in five human cadaveric whole heads. The electro-magnetic transducer from a BCHA (bone conducting hearing aid), a Baha® Cordelle II transducer in particular, was attached to a percutaneously implanted screw or positioned with a 5-Newton steel headband at the mastoid and forehead. The Baha transducer was driven directly with single tone signals in the frequency range of 0.25-8 kHz, while skull bone vibrations were measured at multiple points on the skull using a scanning laser Doppler vibrometer (SLDV) system and a 3D LDV system. The 3D velocity components, defined by the 3D LDV measurement coordinate system, have been transformed into tangent (in-plane) and normal (out-of-plane) components in a local intrinsic coordinate system at each measurement point, which is based on the cadaver head's shape, estimated by the spatial locations of all measurement points. RESULTS: Rigid-body-like motion was dominant at low frequencies below 1 kHz, and clear transverse traveling waves were observed at high frequencies above 2 kHz for both measurement systems. The surface waves propagation speeds were approximately 450 m/s at 8 kHz, corresponding trans-cranial time interval of 0.4 ms. The 3D velocity measurements confirmed the complex space and frequency dependent response of the cadaver heads indicated by the 1D data from the SLDV system. Comparison between the tangent and normal motion components, extracted by transforming the 3D velocity components into a local coordinate system, indicates that the normal component, with spatially varying phase, is dominant above 2 kHz, consistent with local bending vibration modes and traveling surface waves. CONCLUSION: Both SLDV and 3D LDV data indicate that sound transmission in the skull bone causes rigid-body-like motion at low frequencies whereas transverse deformations and travelling waves were observed above 2 kHz, with propagation speeds of approximately of 450 m/s at 8 kHz.

A method to measure sound transmission via the malleus-incus complex.

BACKGROUND: The malleus-incus complex (MIC) plays a crucial role in the hearing process as it transforms and transmits acoustically-induced motion of the tympanic membrane, through the stapes, into the inner-ear. However, the transfer function of the MIC under physiologically-relevant acoustic stimulation is still under debate, especially due to insufficient quantitative data of the vibrational behavior of the MIC. This study focuses on the investigation of the sound transformation through the MIC, based on measurements of three-dimensional motions of the malleus and incus with a full six degrees of freedom (6 DOF). METHODS: The motion of the MIC was measured in two cadaveric human temporal bones with intact middle-ear structures excited via a loudspeaker embedded in an artificial ear canal, in the frequency range of 0.5-5 kHz. Three-dimensional (3D) shapes of the middle-ear ossicles were obtained by sequent micro-CT imaging, and an intrinsic frame based on the middle-ear anatomy was defined. All data were registered into the intrinsic frame, and rigid body motions of the malleus and incus were calculated with full six degrees of freedom. Then, the transfer function of the MIC, defined as velocity of the incus lenticular process relative to velocity of the malleus umbo, was obtained and analyzed. RESULTS: Based on the transfer function of the MIC, the motion of the lenticularis relative to the umbo reduces with frequency, particularly in the 2-5 kHz range. Analysis of the individual motion components of the transfer function indicates a predominant medial-lateral component at frequencies below 1 kHz, with low but considerable anterior-posterior and superior-inferior components that become prominent in the 2-5 kHz range. CONCLUSION: The transfer function of the human MIC, based on motion of the umbo and lenticularis, has been visualized and analyzed. While the magnitude of the transfer function decreases with frequency, its spatio-temporal complexity increases significantly.

Contribution of the incudo-malleolar joint to middle-ear sound transmission.

The malleus and incus in the human middle ear are linked by the incudo-malleolar joint (IMJ). The mobility of the human IMJ under physiologically relevant acoustic stimulation and its functional role in middle-ear sound transmission are still debated. In this study, spatial stapes motions were measured during acoustic stimulation (0.25-8 kHz) in six fresh human temporal bones for two conditions of the IMJ: (1) normal IMJ and (2) IMJ with experimentally-reduced mobility. Stapes velocity was measured at multiple points on the footplate using a scanning laser Doppler vibrometry (SLDV) system, and the 3D motion components were calculated under both conditions of the IMJ. The artificial reduction of the IMJ mobility was confirmed by measuring the relative motion between the malleus and the incus. The magnitudes of the piston-like motion of the stapes increased with the reduced IMJ mobility above 2 kHz. The increase was frequency dependent and was prominent from 2 to 4 kHz and at 5.5 kHz. The magnitude ratios of the rocking-like motions to the piston-like motion were similar for both IMJ conditions. The frequency-dependent change of the piston-like motion after the reduction of the IMJ mobility suggests that the IMJ is mobile under physiologically relevant levels of acoustic stimulation, especially at frequencies above 2 kHz.

Can an incomplete ossicular discontinuity be predicted by audiometric and clinical findings?

OBJECTIVE: To investigate a pathology of conductive hearing loss caused by an incomplete ossicular discontinuity. It can manifest as a triad of the following: 1) conductive hearing loss most prominent in the high frequencies (hfCHL), defined as [ABG for 4 kHz] > [mean ABG for 0.25-0.5 kHz] + 10 dB or more; 2) fluctuating hearing loss; and 3) short-lasting improvement of hearing after Valsalva maneuver. STUDY DESIGN: Retrospective clinical trial. SETTING: Tertiary referral center. PATIENTS: Fourteen patients with an incomplete ossicular discontinuity who underwent incus interposition were included. INTERVENTION: Incus interposition, mathematical model. MAIN OUTCOME MEASURES: First, the prevalence of the triad was documented. Second, the hypothesis that mechanical ossicular compliance was responsible for the triad of symptoms was evaluated and simulated in a mathematical model. Finally, the postoperative hearing results with a follow-up of 12 months were analyzed and compared with those reported in the literature. RESULTS: The presence of the triad of symptoms is a strong indicator for detecting patients with an incomplete ossicular discontinuity. High frequency conductive hearing loss was present in 93% (13/14 patients). Ten (71%) of the 14 patients presented with fluctuating hearing loss and improvement of hearing after Valsalva maneuver. The hfCHL could be simulated adequately in the mathematical model. Success rate for surgical intervention (ABG < 20 dB; 0.5, 1, 2, and 3 kHz) was 93% and was comparable to the results reported in the literature. CONCLUSION: Patients with hfCHL, fluctuating hearing loss, and improvement of hearing after Valsalva maneuver are likely to have an incomplete ossicular discontinuity. A favorable postoperative hearing recovery by incus interposition can be expected.

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.

Experimental study on admissible forces at the incudomalleolar joint.

HYPOTHESIS: The forces that cause rupture of the incudomalleolar joint during the fixation of stapedial prostheses can be determined by means of load-deflection measurements at the long process of the incus. As in other tissues, 3 ranges of forces can be defined: micro rupture, rupture, and short-term maximum. BACKGROUND: A crucial step in stapes surgery is the attachment of the stapedial prosthesis to the long process of the incus. It is unknown which forces occur during the crimping process that increase the risk of damage to the incudomalleolar joint or incus luxation. The goal of this study was to assess the admissible range of forces at the long process of the incus that would be tolerable before damaging the structures and to compare them with the forces occurring during surgery. METHODS: Load-deflection curves in the lateral-medial and anterior-posterior direction were measured in 9 freshly frozen or fresh temporal bones. The force was measured with a load cell, and displacement was taken from the encoder information of the electrically driven translation stage on which the load cell was mounted. The long process of the incus was coupled to the load cell via a customized needle. We also monitored with video recordings for visual confirmation of findings. RESULTS: The rupture force at which the middle ear was found to be severely injured was 894 (724-1018) mN in the anterior-posterior direction and 695 (574-771) mN in the lateral-medial direction. Micro-ruptures occurred at forces around 568 (469-686) mN in the anterior-posterior direction and in the lateral-medial direction at 406 (254-514) mN. Short-term maximum forces of 1,321 (1,051-1,533) mN were measured in the anterior-posterior direction and 939 (726-1,132) mN in the lateral-medial direction. CONCLUSION: Rupture forces of the incudomalleolar joint could be defined with high accuracy. These results were used to calculate risks of incus luxation or subluxation during stapes surgery. Compared with the use of clip and SMA prostheses, the risk of damage from a crimping procedure is significantly higher.

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.

Errors in measurement of three-dimensional motions of the stapes using a laser Doppler vibrometer system.

Previous studies have suggested complex modes of physiological stapes motions based upon various measurements. The goal of this study was to analyze the detailed errors in measurement of the complex stapes motions using laser Doppler vibrometer (LDV) systems, which are highly sensitive to the stimulation intensity and the exact angulations of the stapes. Stapes motions were measured with acoustic stimuli as well as mechanical stimuli using a custom-made three-axis piezoelectric actuator, and errors in the motion components were analyzed. The ratio of error in each motion component was reduced by increasing the magnitude of the stimuli, but the improvement was limited when the motion component was small relative to other components. This problem was solved with an improved reflectivity on the measurement surface. Errors in estimating the position of the stapes also caused errors on the coordinates of the measurement points and the laser beam direction relative to the stapes footplate, thus producing errors in the 3-D motion components. This effect was small when the position error of the stapes footplate did not exceed 5 degrees.

Complex stapes motions in human ears.

It has been reported that the physiological motion of the stapes in human and several animals in response to acoustic stimulation is mainly piston-like at low frequencies. At higher frequencies, the pattern includes rocking motions around the long and short axes of the footplate in human and animal ears. Measurements of such extended stapes motions are highly sensitive to the exact angulation of the stapes in relation to the measurement devices and to measurement errors. In this study, velocity in a specific direction was measured at multiple points on the footplates of human temporal bones using a Scanning Laser Doppler Vibrometer (SLDV) system, and the elementary components of the stapes motions, which were the piston-like motion and the rocking motions about the short and long axes of the footplate, were calculated from the measurements. The angular position of a laser beam with respect to the stapes and coordinates of the measurement points on the footplate plane were calculated by correlation between the SLDV measurement frame and the footplate-fixed frame, which was obtained from micro-CT images. The ratios of the rocking motions relative to the piston-like motion increased with frequency and reached a maximum around 7 kHz.A novel method for quantitatively assessing measurements of complex stapes motions and error boundaries of the motion components is presented. In the frequency range of 0.5 to 8 kHz, the magnitudes of the piston-like and two rocking motions were larger than estimated values of the corresponding upper error bounds.

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

HYPOTHESIS: 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 problems in human hearing.

A description of the hearing process is given using three-dimensional mechanical models. By means of simulation, normal, pathological and reconstructed situations can be investigated. The development of new concepts and prototypes as well as the optimization and the way of insertion of passive and active implants is facilitated by carrying out virtual tests. Mechanical models of spatial structures of the middle ear and its adjacent regions are established by applying multibody systems and finite element modeling approach. In particular, the nonlinear behavior of the elements is taken into account. For the determination of parameters such as coupling parameters in reconstructed ears, measurements using Laser Doppler Vibrometry (LDV) were carried out. The governing differential equations of motion allow the investigation of transient and steady state behavior by time integration and frequency domain methods. Optimization methods can be applied for determination of design parameters such as coupling stiffness and damping, the characteristics of actuator, the position of attachment and direction of actuation. Mechanical models enable non-invasive interpretation of dynamical behavior based on measurements such as LDV from umbo or multifrequency tympanometry. It is shown: The transfer behavior is depending on static pressures in the ear canal, tympanic cavity or cochlea. For reconstructed ears, the coupling conditions are governing the sound transfer substantially. Due to restricted coupling forces, the excitation of inner ear is limited and the sound transfer is distorted. Other sources of distortion are nonlinear coupling mechanisms. In reconstructions with active implants, the actuator excites the microphone whereby feedback effects may occur.