On Direct Drive Bone Conduction Devices - Hearing Rehabilitation and Objective Assessment of Stimulation Methods

Abstract: Bone conduction devices (BCDs) rely on the transmission of sound in form of vibrations generated by a transducer to the inner ear via the skull and surrounding soft tissues. Direct drive BCDs stimulate the skull bone directly, either via a skin-penetrating abutment (BAHAs, Bone Anchored Hearing Aids), or with a transducer implanted under intact skin (active transcutaneous devices). In this thesis, several aspects related to direct drive stimulation were addressed with objective and subjective measurements. Vibrational measurements were performed to assess how the transducer to bone attachment affects the vibrations transmission to the cochleae. Three different attachments for active transcutaneous stimulation were compared to each other and to the BAHA screw. A comparative study was done also between the BAHA system and the novel active transcutaneous Bone Conduction Implant (BCI), where the transducer is attached to the skull bone via a flat surface contact. The BCI is currently on a clinical trial, and a comprehensive assessment of the rehabilitation after three years of device usage is included in this thesis, reporting on a number of audiometric tests, self-reported questionnaires, and objective measurements. Among the objective measures, a new method for intra and post operative verification of the implant functionality was evaluated, consisting in the measurement of the induced sound pressure in the nostril under bone conduction stimulation. In addition to the test battery from the clinical trial protocol, an exploratory study was conducted to investigate the effect of the BCI in a complex multi-talker listening environment. The results from the vibrational measurements were highly frequency-dependent, with a general trend of improved transmission when the contact area between transducer and bone was limited, especially for frequencies above 5 kHz. Sound field tone and speech tests, and user reported questionnaires show that the BCI provides considerable improvement from the unaided condition and contributes to a general increase of patients' life quality, with consistent outcomes over time. The implant verification method seems promising and showed stable properties of the implant to bone transmission. When compared to BAHAs, the BCI was found to be a viable alternative for indicated patients. In noisy and complex listening environments, the BCI users showed a lower ability to make use of the spatial cues when aided with their device, but an overall greater tolerance to interfering talkers.