Nasal sound pressure as objective verification of implant in active transcutaneous bone conduction devices

Abstract

Objective: Active transcutaneous bone conduction devices consist of an external audio processor and an internal implant under intact skin. During the surgical procedure, it is important to verify the functionality of the implant before the surgical wound is closed. In a clinical study with the new bone conduction implant (BCI), the functionality of the implant was tested with an electric transmission test, where the output was the nasal sound pressure (NSP) recorded in the ipsilateral nostril. The same measurement was performed in all follow-up visits to monitor the implant's functionality and transmission to bone over time. The objective of this study was to investigate the validity of the NSP method as a tool to objectively verify the implant's performance intraoperatively, as well as to follow-up the implant's performance over time. Design: Thirteen patients with the BCI were included, and the NSP measurement was part of the clinical study protocol. The implant was electrically stimulated with an amplitude-modulated signal generator using a swept sine 0.1-10 kHz. The NSP was measured with a probe tube microphone in the ipsilateral nostril. Results: The NSP during surgery was above the noise floor for most patients within the frequency interval 0.4-5 kHz, showing NSP values for expected normal transmission of a functioning implant. Inter-subject comparison showed large variability, but follow-up results showed only minor variability within each subject. Further investigation showed that the NSP was stable over time. Conclusion: The NSP method is considered applicable to verify the implant's functionality during and after surgery. Such a method is important for implantable devices, but should be simplified and clinically adapted. Large variations between subjects were found, as well as smaller variability in intra-subject comparisons. As the NSP was found to not change significantly over time, stable transmission to bone, and implant functionality, were indicated.

Keywords: bone conduction; bone conduction implant; ear-canal sound pressure; nasal sound pressure; objective intraoperative verification.

Figure 1

Measurement setup for the nasal sound pressure method. The amplitude-modulated driver stage, including the Agilent 35670A, the Agilent 33220A, and the transmitter inductive link, is driving the implant. The nasal sound pressure is measured by a microphone with a pre-amplifier and is analyzed by the Agilent 35670A.

Figure 2

The nasal sound pressure for patient 12 at surgery, fitting, and follow-up visits at 1, 3, 6, and 12 months after fitting. The noise floor from surgery is also presented.

Figure 3

Average nasal sound pressure level based on follow-up data for each patient (in colors) together with the overall mean (black solid line) and the average noise floor (black dotted line).

Figure 4

Boxplot of the nasal sound pressure of all patients at surgery at frequencies 0.5, 1.0, 2.0, and 4.0 kHz, showing average (*), median (red line), 25 and 75 percentiles (blue box), and minimum and maximum values (whiskers).

Figure 5

Linear model of the nasal sound pressure over time relative to baseline (fitting) at four frequencies with the respective equation showing slope and intercept.

Figure 6

Random effects visualization: for each patient included in the analysis, the individual k-value is estimated with its 95% CI. Intervals not including 0 indicate that the patient has a significant deviation from the overall slope (group average), marked with a red asterisk.

Figure 7

Slope value (k) as a function of frequency. Asterisks (*) mark the values at the audiometric frequencies.