Computer-integrated finite element modeling of human middle ear.

The objective of this study was to produce an improved finite element (FE) model of the human middle ear and to compare the model with human data. We began with a systematic and accurate geometric modeling technique for reconstructing the middle ear from serial sections of a freshly frozen temporal bone. A geometric model of a human middle ear was constructed in a computer-aided design (CAD) environment with particular attention to geometry and microanatomy. Using the geometric model, a working FE model of the human middle ear was created using previously published material properties of middle ear components. This working FE model was finalized by a cross-calibration technique, comparing its predicted stapes footplate displacements with laser Doppler interferometry measurements from fresh temporal bones. The final FE model was shown to be reasonable in predicting the ossicular mechanics of the human middle ear.

Biomaterials for implantable middle ear hearing devices.

The use of biomaterials to ameliorate the effects of diseases dates back centuries to the ancient Greeks and Chinese. In the twentieth century, the use of synthetic and natural materials rapidly increased as clinicians developed problem-solving strategies. From replacement prostheses, to structural polymers, to the controlled release of engineered proteins into the body, to the present-day tissue engineering, biomaterials have transformed medicine. The ear is no exception. Various alloplastic and homograft biomaterials have been used to restore hearing with ossicular replacement prostheses, cochlear implants, and most recently, implantable middle ear devices. This article updates an earlier review of biomaterials used in implantable middle ear devices and focuses on biomaterials used in implantable middle ear devices currently under development or in clinical trials.

Mass loading on the ossicles and middle ear function.

The middle ear as a levered vibrating system for sound transmission from the external to the inner ear is affected by changes in ossicular chain mass. Mass loading of the ossicles may impair ossicular dynamics and sound transmission to the inner ear. It is incumbent on otologic surgeons and researchers of middle ear mechanics to consider the mass loading effect on middle ear function in clinical and physiological applications. The residual hearing and frequency response can change after surgery or implantation of middle ear prostheses. We conducted experiments on mass loading effects on the middle ear transfer functions by using laser Doppler interferometry and a human temporal bone model. Two implant mass loading conditions were tested on 17 fresh or fresh-frozen temporal bones and compared with the unloaded condition for the frequencies 250 to 8,000 Hz. The results show that the linearity of the middle ear function did not change, although displacement of the stapes footplate decreased after the increased masses were placed on the incudostapedial joint. The greater the mass of the implant, the less displacement was measured at the stapes footplate. We conclude that there is a quantitative limit to increased mass on the ossicular chain above which the mass will remarkably impair hearing thresholds.

Implantable hearing device performance measured by laser Doppler interferometry.

Recent application of the Doppler principle laser interferometry to audiology, acoustics and otology has facilitated the development of implantable hearing devices (IHDs). During the design and testing of two different electromagnetic middle ear implants for sensorineural hearing loss, we used single-point laser Doppler interferometry (LDI). A commercially available interferometer, internally calibrated and validated against a National Institute for Standards and Technology (NIST) standard, was used with both mechanical fixtures and fresh temporal bones to evaluate implant mass, shape and orientation, attachment, electromagnetic coupling and acoustic properties. At both Hough Ear Institute and Symphonix Devices, Inc., we have shown that high fidelity and amplitudes can be recorded in vitro over a frequency range of 500 Hz to 10 kHz. These data can provide greater assurance of safety and efficacy to regulatory agencies before entering clinical trials. We propose that LDI be considered as an international standard for accurate, consistent comparison of performances of all IHDs during development. Furthermore, the future availability of human IHD data will allow for the extrapolation of a mechanical bench model of the middle ear transfer function for use in quality control during manufacturing and diagnosis of failure in IHDs.

Vascular impedance analysis in dog lung with detailed morphometric and elasticity data.

On the basis of experimentally measured morphometric and elasticity data and model-derived mean pressure-flow conditions, we attempt a theoretical modeling of pulsatile flow in the whole lung. In the model we use the "elastic tube" for arteries and veins, and the vascular impedance in arteries and veins follows Womersley's theory and electric analogue. We employ the "sheet-flow" theory to describe the flow in the capillaries and to obtain the microvascular impedance matrix. The characteristic impedance of each order along the vascular tree, the input impedance at the capillary entrance and exit, and the pulmonary arterial input impedance at the main pulmonary artery are computed under certain physiological conditions. Using the pulsatile flow model, we investigate the effects of arterial vascular obstruction on pulmonary vascular impedance. The model-derived data are compared with the available experimental results in the literature.

Effect of chronic thromboembolism on the pulmonary artery pressure-flow relationship in dogs.

To understand the hemodynamic alterations associated with chronic thromboembolic pulmonary hypertension, the large pulmonary arteries of mongrel dogs were chronically obstructed with lysis-resistant thrombi. Pulmonary hemodynamics were experimentally measured and described by multipoint pulmonary arterial pressure (PAP) vs. flow plots. In nine anesthetized chronically embolized dogs, but not in six control dogs, the PAP-flow line shifted significantly upward in a parallel fashion by 4.2 +/- 0.7 mmHg. The postembolic pulmonary circulation was further characterized by predictions from a morphometric-based elastic tube and sheet flow model of the canine pulmonary circulation. After model validation with the preembolic PAP-flow data, the derived postembolic PAP matched the in vivo results to within 1 mmHg. A detailed analysis of the model-derived PAP drop revealed that the PAP-flow line shift can be accounted for by a novel fixed resistor in the largest obstructed pulmonary artery.

Morphometry of the dog pulmonary venous tree.

The biophysical approach to the study of blood flow in the pulmonary vasculature requires a detailed description of vascular geometry and branching pattern. The description of the pulmonary venous morphometry in the dog is the focus of this paper. Silicone elastomer casts of a dog lung were made and were used to measure the diameters, lengths, and branching pattern of the pulmonary venous vasculature. The anatomic data are presented statistically with a diameter-defined Strahler ordering scheme, a rule for assigning the order numbers of the vessels on the basis of a diameter criterion. The asymmetric branching pattern of the pulmonary venous vasculature is described with a connectivity matrix. Results show that for the dog's right pulmonary venous tree 1) a total of 11 orders of vessels lay between the left atrium and the capillary bed; 2) the average ratios of the diameter, length, and number of branches of successive orders of veins were 1.701, 1.556, and 3.762, respectively; and 3) a fractal description of the tree geometry resulted in diameter and length fractal dimensions of 2.49 and 2.99, respectively. The morphometric data were used to compute the cross-sectional area, vascular volume, and Poiseuillean resistance in the venous vessels.