Preclinical methods to determine output of the floating mass transducer in forward and reverse stimulation

Our experimental study evaluated the efficiency of sound transmission with the Floating Mass Transducer (FMT) in forward (incus short process, SP) and reverse (round window, RW) stimulation modes using different coupling configurations.

Using laser Doppler vibrometry (LDV) at the stapes and intracochlear pressure difference (ICPD) the equivalent sound pressure level output was determined according to ASTM standard. Coupling configurations included the Vibroplasty-SP-Coupler, the Vibroplasty-RW-Coupler, and the research RW-Precision-Coupler. Reverse stimulation was studied with both intact and disrupted ossicular chains.

In forward stimulation with SP of the incus coupling, output levels increased from 200 Hz to ∼1.26 kHz and remained constant up to 8 kHz. LDV and ICPD yielded similar results in forward stimulation. In reverse stimulation, both RW coupling methods showed resonance peaks at 1.5-2 kHz, though output amplitudes were ∼10-12 dB lower than in forward stimulation. The RW-Precision-Coupler produced higher output with less variability than the Vibroplasty-RW-Coupler. In mid-frequency range (0.6 - 1.5 kHz) the output levels measured with LDV and ICPD were similar for forward and reverse stimulation, but ICPD indicated higher outputs at low and high frequencies. Variability was greater in reverse stimulation while intact versus disrupted ossicular chains showed no significant differences in reverse stimulation.

LDV is well-established for assessing forward stimulation, while ICPD is more accurate in reverse stimulation. Despite lower overall output, RW stimulation frequency characteristics are preserved, supporting its clinical relevance when forward stimulation is not feasible.