Magnetically driven middle ear ossicles for optical measurement of vibrations in an ear with opened eardrum

Vibrations of the middle ear ossicles are easily measured by means of laser vibrometry. However, exposing the ossicles requires the removal of the eardrum, with the result that the ossicles can no longer be stimulated acoustically. To overcome this we devised a new set up in which the ossicles can be driven magnetically. After measuring the response of the eardrum to an acoustic signal, we then remove the eardrum and attach a small magnet to the exposed manubrium (the part of the first auditory ossicle, the malleus, which is normally attached to the eardrum). An electromagnetic excitation coil is then used to drive the magnet, and the output to the coil adjusted until the vibration of the manubrium, as measured by the vibrometer, matches that measured in response to the acoustic signal. Such a set-up has uses in research on middle ear mechanics, such as the measurement of non-linearities in their response, as well as applications in the diagnosis of middle ear conditions such as the fixation of the ossicles by otosclerosis, or in chronic otitis media. We describe our set up in which the vibrometer unit is attached to a surgical microscope, offering accurate positioning of the laser beam. We discuss the viability of our method and its future potential by presenting some measurements on artificially fixated ears.