The Bone Conduction Implant (BCI) - Preclinical Studies, Technical Design and a Clinical Evaluation

University dissertation from Chalmers University of Technology

Abstract: The Bone Conduction Implant (BCI) system has been developed as an alternative to the percutaneous bone anchored hearing aid (BAHA) with the advantage that the skin is kept intact. The transducer is permanently implanted and attached to the skull via a flat surface contact to the temporal bone. By applying amplitude modulation, the sound signal is transmitted to the implanted transducer through the intact skin via an efficient wireless inductive link. The external audio processor includes digital and analog signal processing units, and an Application Specific Integrated Circuit driving the inductive link. Its retention is provided by permanent magnets. Preclinical investigations of the BCI system have been performed on a skull simulator, a dry skull, cadaver heads and in an animal model study. In an extensive technical evaluation using a skull simulator, it was found that the power output from the BCI system was very robust for skin flap thicknesses from 2 to 8 mm with variability of less than 1.5 dB. Moreover, the peak of the maximum power output was 107 dB relative to 1 μN at transducer resonance frequency and at 5 mm skin flap thickness. This means that the highest output forces were produced in the "normal" skin thickness range, which in fact was one important design goal. The BCI system drains approximately 7.5 mA of battery current at 1 kHz and at 65 dB input sound pressure level, which corresponds to a battery lifetime of 5-7 days under normal use. It was found that significantly higher gain setting can be used without feedback problems for the BCI as compared to the BAHA. In an animal model (sheep), it was found that the implant-to-bone attachment became firmer over time and that the vibration transmission was stable over this period of eight months. Finally, in a clinical study, the results of the first patient were reported at one month follow up. The surgical procedure for installing the implant was found to be easy and safe, and the BCI gave a significant improvement in hearing over the unaided condition. The functional power output of the BCI was similar to most powerful ear level BAHA devices on headband at lower frequencies and superior at higher frequencies. In summary, it was found that the new BCI system can be an attractive alternative to the present percutaneous BAHA system.