Acoustic and mechanical properties of tympanic membrane transplants

Abstract

The human tympanic membrane has reasonably good sound transmission properties and withstands high static pressure loads. Destruction of the tympanic membrane resulting from middle ear diseases or trauma may be repaired by different types of grafts. Middle ear surgery mostly uses autologous temporal fascia, cartilage, or cartilage perichondrium transplants which differ in their acoustical characteristics and mechanical strength. We have investigated the acoustical and mechanical properties of these materials and compared them with human tympanic membranes by constructing an ear canal-tympanic membrane model. Fresh human tympanic membrane, fascia, perichondrium, and cartilage preparations were exposed to static pressures up to 4 kPa and white noise sound pressure levels of 70 dB. The vibrational amplitudes and displacements due to static pressure were measured by laser Doppler vibrometry. The temporal fascia and perichondrium show similar amplitude frequency responses compared to the tympanic membrane for dynamic excitation. The displacement of these materials at static pressures above 4 kPa indicates a higher compliance than the tympanic membrane. The acoustical and mechanical properties of cartilage transplants are determined by the thickness of the slices. Thin cartilage slices are less stable although their frequency response is comparable to the intact tympanic membrane. Layer thickness above 500 microns result in a decrease of vibration amplitudes. Cartilage is an excellent transplant material which provides a better prognosis than soft materials in cases of ventilation disorders with long-term middle ear pressure problems. Large cartilage slice transplants should not exceed layer thickness of 500 microns in order to minimize transmission loss.