Effect of coupler tip size on round window stimulation with the floating mass transducer under controlled preload

OBJECTIVE: Mechanical stimulation of the cochlear round window (RW) extends the indication of active middle-ear implants to mixed and conductive hearing loss, yet outcomes vary considerably due to transducer-RW size mismatch and uncontrolled preload. The Hannover Coupler version 2 (HC2) addresses these issues with a small tip and a force indicator. This study further evaluates the effect of HC2 tip size on RW stimulation efficiency under controlled preload force. METHODS: Stapes displacements were measured in six human temporal bones using laser Doppler vibrometry during acoustic and RW stimulation. The RW was stimulated by a floating mass transducer (FMT) with three HC2 tip diameters: small (0.5 mm), medium (0.7 mm), and large (1.0 mm). RW preload ranged from 4 to 60 mN. RESULTS: The large HC2 tip produced the highest transfer function magnitude and average equivalent sound pressure level across speech-relevant frequencies (0.5-4 kHz), peaking at a preload of 30 mN-approximately 11 dB higher than the small tip. Differences between large and small tips were significant at nearly all preload, whereas medium and small tips showed no significant differences. Optimal preload for FMT depended on coupler tip size: 15 mN (small), 25 mN (medium), and 30 mN (large). CONCLUSION: RW stimulation efficiency increases with HC2 tip size. The large tip provides the highest output and lowest RW membrane stress, reducing penetration risk. FMT stimulation with different HC2 tip sizes has distinct optimal preload. These findings support the use of a larger HC2 tip to improve clinical outcomes in RW stimulation.