Which threshold do we trust? A comparison of threshold measurements in adult bone-conduction device users and normal hearing adults

Abstract

ObjectivesAdvancements in prescriptive formulae for bone-conduction hearing devices (BCDs) have highlighted the importance of measuring in-situ bone-conduction hearing thresholds. In-situ measurements are performed within the BCD manufacturer's software, using the patient's BCD as a transducer. While in-situ testing is a different approach than standard diagnostic bone-conduction testing, both approaches are in fact measuring the same inner ear hearing. Despite this, each approach may result in different thresholds for the same individual. This study aimed to answer the following question: In adults with normal hearing and adults who currently wear a bone-conduction device, are there differences and, if so, how large are these differences when thresholds are measured using different approaches? DesignBone-conduction hearing thresholds were measured for a group 32 normal hearing participants and a group 15 percutaneous BCD users. The normal hearing participants were tested in two conditions: (1) in-situ, with a BCD worn on a soft-band and (2) with the B71 bone-conduction diagnostic transducer. The BCD users were tested in these two conditions and (3) in-situ with a BCD attached to their abutment. In-situ hearing thresholds were measured with BCDs from two manufacturers. The mean intra-subject differences between these conditions were calculated. ResultsFor the normal hearing participants, BCD softband thresholds were poorer than the thresholds obtained with the diagnostic transducer across all tested frequencies. The average differences between the BCD softband thresholds and the diagnostic transducer were particularly large in the high frequencies (13 to 35 dB from 3 to 6 kHz). Similar differences were observed for the BCD user participants when comparing their BCD softband thresholds to the diagnostic transducer thresholds. The in-situ percutaneous thresholds were on average better than the diagnostic bone-conduction thresholds, although the differences were not statistically significant. Skin attenuation, calibration differences, and BCD characteristics were contributing factors to these differences. ConclusionsThe intra-subject differences measured in this study confirmed that using different bone-conduction transducers results in the measurement (i.e., recording) of different hearing threshold levels. These results support the necessity for a measurement tool able to measure audibility at threshold independent of the various coupling configurations used in bone-conduction amplification. An effective measurement tool would be required to take into account, or bypass, the factors contributing to the differences measured in this study. Until such a tool is commercially available, clinicians will continue to face uncertainty when fitting and attempting to assess the audibility of passive transcutaneous BCDs.