Laser interferometry for ear ossicular motion detection

Abstract

Middle ear reconstruction for hearing improvement is a commonly performed procedure. However, there are no means available to evaluate objectively the reconstruction intra-operatively. Intra-operative assessment is made only on the basis of the subjective impression of the surgeon as to the fit, tension, size, and position of the middle ear prosthesis. The inaccuracy of such assessment is documented by the fact that fifty percent of the reconstructions are unsuccessful. The characterization of the frequency and amplitude response of the middle ear during surgery will provide factual information on the quality of the reconstruction.The emphasis of this project has been to initially detect the displacements of the eardrum by using a laser interferometer. Future work will include measurements on middle ear components. A small cobalt-samarium magnet (typically weighing less than 4 mg) was placed on human cadaver eardrums and positioned inside a solenoid coil. AnAC modulated signal, having a frequency range of 500-2500 Hz, was passed through the coil. This signal created an AC magnetic field inside the coil that caused the magnet to vibrate at the modulation frequency. A commercial (TSI) laser interferometer as well as a laboratory-based fiber-optic interferometer, both operating at 632 nm, were used to examine the displacements initially. For measurements made with the fiber-optic interferometer at a frequency of 1500 Hz, the displacement of the eardrum was approximately 68 nm. The response decreased linearly at a rate of 0.02 nm/Hz for frequencies greater than 1500 Hz. This paper will report on the experimental apparatus as well as the experimental results. Details on the design of the driving circuit and the interferometers will also be presented.