Dynamic properties of round window membrane in guinea pig otitis media model measured with electromagnetic stimulation.

The round window, one of two openings into the cochlea from the middle ear, plays an important role in hearing and is known to be structurally altered during otitis media. However, there have been no published studies systematically describing the changes in biomechanical properties of the round window membrane (RWM) that accompany bacterial otitis media. Here we describe the occurrence of significant changes in the dynamic properties of the RWM between normal guinea pigs and those with acute otitis media (AOM) that are detectable by electromagnetic force stimulation and laser Doppler vibrometry (LDV) measurements. AOM was induced by transbullar injection of streptococcus pneumoniae into the middle ear, and RWM specimens were prepared three days after challenge. Vibration of the RWM induced by coil-magnet coupling was measured by LDV over frequencies of 0.2-40 kHz. The experiment was then simulated in a finite element model, and the inverse-problem solving method was used to determine the complex modulus in the frequency domain and the relaxation modulus in the time domain. Results from 18 ears (9 control ears and 9 AOM ears) established that both the storage modulus and loss modulus of the RWM from ears with AOM were significantly lower than those of RWM from uninfected ears. The average decrease of the storage modulus in AOM ears ranged from 1.5 to 2.2 MPa and the average decrease of the loss modulus was 0.025-0.48 MPa. Our findings suggest that middle ear infection primarily affects the stiffness of the RWM due to the morphological changes that occur in AOM ears. We also conclude that the coil-magnet coupling method for assessment of RWM function may provide a valuable new approach to characterizing the mechanical response of the RWM when reverse driving is selected for middle ear implantable devices. This article is part of a special issue entitled "MEMRO 2012".