This model can be used in reconstruction of the link between the eardrum and the inner ear.
In conductive hearing loss cases, 59% involve defects of the incus. For efficient restoration of hearing, the defective incus is normally removed and the mechanical link between the eardrum and inner ear is reconstructed. In cases of incus defects, the reconstruction is normally of the form of a rod, positioned to connect the malleus handle or eardrum directly to the cochlea oval window. A more promising approach to prosthesis design is to reconstruct the chain along more physiologically relevant lines. It has been shown that excellent reconstruction of the ossicular chain can be achieved using a generic incus shape. In a series of in vitro studies, it was shown that secure attachment of the prosthesis to the stapes and malleus, with ionomeric cement, could restore hearing within 10 dB of the original frequency response. This study attempts to model these in vitro findings using a finite element computer model. The goal of the study is to produce a computer model that can be used to simulate different forms of attachment to the prosthesis.
The geometry of the model was derived from high-resolution magnetic resonance micro-imaging of human cadaver middle ear structures. The ossicular chain was immersed in a silicon oil and the oil imaged to produce volume outlines of the bones. Edge detection was used to trace the outlines, and the model was constructed using a bottom-up hierarchy from points to volumes. The eardrum was clamped at the annulus, and the stapes footplate was restricted to move along the line normal to its surface. The incus was constrained at its short process and the malleus constrained at the anterior process. A pressure load equivalent to 80 dB sound pressure level (SPL) was uniformly applied across the surface of the eardrum. The cochlear load was not modeled.
Although this is just a preliminary model, the form of the middle ear function is in approximate agreement with that found in middle ear studies in temporal bones. Differences are thought to be due to the modeling parameters used. This middle ear model did not consider inhomogeneity in the thickness of the eardrum or the flexible annulus modeled. Eardrum displacement is found to be higher than stapes displacement, as expected. The simulated stapes response matches the eardrum response quite well up to high frequency where the responses differ. This difference represents a loss of efficiency of the eardrum driving the malleus.
The model described here can be further used to predict changes that may occur through modification of the middle ear structures. The main parameters that may be investigated are mass, shape, stiffness, and position of the implant. With a better understanding of the effects of these parameters on sound transmission, implant designs could be optimized to produce transmission characteristics that are seen in the normal human ear.
This work was done by E. W. Abel and R. M. Lord of the Medical Engineering Research Institute, Department of Mechanical Engineering, at the University of Dundee, Scotland for the Army Research Laboratory. For more information, download the Technical Support Package (free white paper) at www.techbriefs.com/tsp under the Bio-Medical category. ARL-0067