Jae Hoon Sim

Bone Conduction Thresholds and Skull Vibration Measured on the Teeth during Stimulation at Different Sites on the Human Head

Vibratory auditory stimulation or bone conduction (BC) reaches the inner ear through both osseous and non-osseous structures of the head, but the contribution of the different pathways of BC is still unclear. In this study, BC thresholds in response to stimulation at several different locations including the eye were assessed, while the magnitudes of skull bone vibrations were measured on the front teeth in human subjects with either normal hearing on both sides or unilateral deafness with normal hearing on the other side. The BC thresholds with stimulation at the ipsilateral mastoid and ipsilateral temporal region were lower than the BC thresholds with stimulation at the other sites, as reported by previous works. The lower thresholds with stimulation at the ipsilateral mastoid and ipsilateral temporal region matched higher amplitudes of skull bone vibrations measured on the teeth, but only at frequencies below 1 kHz. With stimulation at the eye, the thresholds were significantly higher than those with stimulation at the bony sites in the frequency range of 0.25–4 kHz. While skull bone vibrations as measured on the teeth during stimulation at the eye were low for low frequencies, significant bone vibrations were measured at 3 and 4 kHz, indicating different pathways for BC for either the soft tissue or bony site stimulation. This finding contradicts a straightforward relationship between vibrations of the skull bones and BC hearing thresholds.

The influence of prosthesis diameter in stapes surgery: a meta-analysis and systematic review of the literature.

Objective: To analyze the influence of stapes prosthesis diameter on postoperative hearing results after stapedotomy without interposition in otosclerotic patients. Data Sources: PubMed search from 1970 to 2009 using the key words stapedotomy or stapedectomy or otosclerosis or stapesplasty. Study Selection: Inclusion criteria to select articles and patient groups for meta-analysis and statistical analyses were as follows: otosclerosis as diagnosis, clear description of technique and prosthesis size, calibrated stapedotomy, and complete report of functional results. Data Extraction: Five controlled studies were found analyzing the influence of prosthesis diameter and reporting the results in a comparable way for meta-analysis (n = 590). Sixty-two studies not analyzing the influence of prosthesis diameter contained comparable subgroups with a total of 9,536 cases. These cases were pooled according to their diameter (0.3, 0.4, 0.5, 0.6,and 0.8 mm). The results of air conduction, bone conduction, air-bone gap (ABG), and success rate (closure of the ABG within 10 dB as percentage of the total cases) for all groups and frequency-specific ABG results were gathered. Furthermore, 12 clinical and experimental studies were reviewed that did not contribute to the statistical analysis. Data Synthesis: A meta-analysis performed for success rate of the 5 controlled studies showed favorable results for 0.6-mm over 0.4-mm prostheses (success rate, 67% versus 58%, p = 0.05). In the statistical analysis of the pooled data, the 0.6-mm prosthesis showed better results compared with 0.4 mm (p < 0.001) in the postoperative air conduction threshold (29 dB versus 35 dB), postoperative ABG (7 dB versus 11 dB), ABG improvement (25 dB versus 21 dB), and success rate (81.1% versus 75.1%). The frequency-specific analysis of the postoperative ABG showed no advantage for the small prosthesis in the high frequencies. There was no difference in postoperative change of bone conduction in the 0.6- and 0.4-mm groups. Statistically significant results could not be assessed for other prosthesis diameters because of the small number of cases reported. Conclusion: A 0.6-mm diameter piston prosthesis is associated with significantly better results than a 0.4-mm prosthesis and should be used if the surgical conditions allow it.

Objective assessment of stapedotomy surgery from round window motion measurement.

Objectives: As prostheses and techniques related to stapes surgery develop and improve, there is a need to assess the functional outcomes of the surgery objectively. This study provides a bench test method to assess the functional results of stapes surgery by measuring volume displacement at the round window (RW), which is closely related to pressure propagation of the travelling wave inside the cochlea and thus to hearing. Design: Motion of the RW membrane in fresh temporal bones was measured using a scanning laser Doppler interferometry system for normal and reconstructed conditions, and the performance of the reconstruction with stapes surgery was quantitatively assessed by comparison of the volume displacements at the RW between the two conditions. To obtain optimal measurements, reflectivity of the laser beam of the scanning laser Doppler interferometry system was improved by retroreflective beads coated onto the surface of the RW, and orientation of the RW membrane relative to the laser beam was obtained using micro-computed tomography imaging. Results: From measurements in 12 temporal bones, difference in the RW volume displacement between normal ears and ears reconstructed with stapes surgery was approximately 15 dB below 2 kHz and approximately 10 dB above 4 kHz, which was comparable with air–bone gaps in patients after stapes surgery. Two different sizes of the stapes prostheses were also tested (n = 3), and a tendency toward a better outcome with a larger diameter was found. Conclusion: The method developed in this study can be used to assess various prostheses and surgical conditions objectively in controlled laboratory environments. It may also have potential for providing ways to assess other middle- and inner-ear surgeries, and to study other aspects of hearing science.

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.

Correlation of Electrophysiological Properties and Hearing Preservation in Cochlear Implant Patients.

Objective To monitor changes in cochlear function during cochlear implantation using electrocochleography (ECoG) and to correlate changes to postoperative hearing preservation. Methods ECoG responses to acoustic stimuli of 250, 500, and 1000 Hz were recorded during cochlear implantation. The recording electrode was placed on the promontory and stabilized to fix the position during cochlear implantation. Baseline recordings were obtained after completion of the posterior tympanotomy. Changes of the ongoing ECoG response at suprathreshold intensities were analyzed after full insertion of the cochlear implant electrode array. Audiometric tests were conducted before and 4 weeks after surgery and correlated with electrophysiological findings. Results Ninety-five percent (18/19) of cochlear implant subjects had measurable ECoG responses. Under unchanged conditions, recordings showed a high repeatability without significant differences between 2 recordings (p ⩽ 0.01). Ninety-four percent (17/18) of subjects showed no relevant changes in ECoG recordings after insertion of the cochlear implant electrode array. One subject showed decreases in responses at all frequencies indicative of cochlear trauma. This was associated with a complete hearing loss 4 weeks after surgery compared with mean presurgical low-frequency hearing of 78 dB HL. Conclusion Extracochlear ECoG is a reliable tool to assess cochlear function during cochlear implantation. Moderate threshold shifts could be caused by postoperative mechanisms or minor cochlear trauma. Detectable changes in extracochlear ECoG recordings, indicating gross cochlear trauma, are probably predictive of complete loss of residual acoustic hearing.

The effect of rocking stapes motions on the cochlear fluid flow and on the basilar membrane motion

The basilar membrane (BM) and perilymph motion in the cochlea due to rocking stapes motion (RSM) and piston-like stapes motion (PSM) is modeled by numerical simulations. The full Navier-Stokes equations are solved in a two-dimensional box geometry. The BM motion is modeled by independent oscillators using an immersed boundary technique. The traveling waves generated by both stimulation modes are studied. A comparison of the peak amplitudes of the BM motion is presented and their dependence on the frequency and on the model geometry (stapes position and cochlear channel height) is investigated. It is found that the peak amplitudes for the RSM are lower and decrease as frequency decreases whereas those for the PSM increase as frequency decreases. This scaling behavior can be explained by the different mechanisms that excite the membrane oscillation. Stimulation with both modes at the same time leads to either a slight increase or a slight decrease of the peak amplitudes compared to the pure PSM, depending on the phase shift between the two modes. While the BM motion is dominated by the PSM mode under normal conditions, the RSM may lead to hearing if no PSM is present or possible, e.g., due to round window atresia.

Extra- and Intracochlear Electrocochleography in Cochlear Implant Recipients.

Objective: To monitor cochlear function by extra- and intracochlear electrocochleography (ECoG) during and after cochlear implantation and thereby to enhance the understanding of changes in cochlear function following cochlear implantation surgery. Methods: ECoG responses to acoustic stimuli of 250, 500 and 1,000 Hz were recorded in 9 cochlear implant recipients with presurgical residual hearing. During surgery extracochlear ECoG recordings were performed before and after insertion of the cochlear implant electrode array. After insertion of the electrode array, intracochlear ECoG recordings were conducted using intracochlear electrode contacts as recording electrodes. Intracochlear ECoG recordings were performed up to 6 months after implantation. ECoG findings were correlated with findings from audiometric tests. Results: Extra- and intracochlear ECoG responses could be recorded in all subjects. Extracochlear ECoG recordings during surgery showed moderate changes. Loss or reduction of the ECoG signal at all three frequencies did not occur during cochlear implantation. During the first week following surgery, conductive hearing loss, due to middle ear effusion, led to a decrease in intracochlear ECoG signal amplitudes. This was not attributable to changes of cochlear function. All persistent reductions in ECoG response magnitude after normalization of the tympanogram occurred during the first week following implantation. Thresholds of ECoG signals were at or below hearing thresholds in all cases. Conclusion: Gross intracochlear trauma during surgery appears to be rare. In the early postoperative phase the ability to assess cochlear status by ECoG recordings was limited due to the regular occurrence of middle ear effusion. Still, intracochlear ECoG along with tympanogram recordings suggests that any changes of low-frequency cochlear function occur mainly during the first week after cochlear implantation. ECoG seems to be a promising tool to objectively assess changes in cochlear function in cochlear implant recipients and may allow further insight into the mechanisms underlying the loss of residual hearing.

An artificial temporal bone as a training tool for cochlear implantation.

Objective Description and evaluation of a newly developed artificial temporal bone (TB) model suitable for surgical training for cochlear implantation. Subject Based on micro–computed tomographic images, a TB model was designed with material properties as similar to bone as possible. The bony anatomic details were rebuilt as closely as possible with preservation of the endocochlear lumen. Intervention The TB model was compared with a human cadaveric TB by 8 otologists experienced in cochlear implantation. Main Outcome Measure The otologists were asked to respond to a semiquantitative questionnaire with scales from 1 (strongly disagree) to 5 (strongly agree). Anatomic details were compared macroscopically and microscopically. The surgical steps of mastoidectomy, posterior tympanotomy, cochleostomy, and insertion of a cochlear electrode were assessed. Results The material properties and anatomic details of the TB model were generally comparable to the human TB. One exception was the round window membrane, which was not modeled appropriately. The surgical steps, including the insertion of the electrode, were rated as comparable. Conclusion The TB model is suitable for surgical training for interventions such as cochlear implantation. It cannot replace cadaveric human temporal bones completely, but it provides an easily available alternative to train and develop surgical skills. A wider variety of anatomic models, such as an infant’s TB or malformations, will increase the value of TB models.

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.