Measurement of the vibration of the middle ear ossicles with removed eardrum: A method for quantification of ossicular fixation

Chronic inflammation of the middle ear is a common disease in which the mobility of the middle ear ossicles may be reduced; resulting in hearing impairment. Knowledge of the degree of ossicular mobility is useful in helping a surgeon determine how to proceed with treatment. In advanced cases, mobility can be assessed by manually pressing on the ossicles, but in less advanced cases manual assessment can provide limited useful information. Ossicular vibration can be measured with a laser vibrometer, but only the manubrium of the malleus is optically visible without removing the eardrum. Since the eardrum is the means by which acoustic energy is translated into the mechanical motion of the ossicles, removing it renders any subsequent measurements of ossicular motion meaningless. We therefore devised a technique in which the ossicles are vibrated magnetically. After measuring the response of the umbo to acoustic stimulation, we removed the eardrum and attached a small magnet to the manubrium. An electromagnetic excitation coil was then used to vibrate the magnet, and the signal to the coil was adjusted until the vibration of the ossicles matches that achieved acoustically. In this paper we explain the method and describe some test measurements on a vinyl membrane, and some preliminary results obtained on a fresh-frozen human temporal bone before and after artificial fixation of the ossicles.