Determining fitting ranges of various bone conduction hearing aids

Accepted: 3 May 2017 DOI: 10.1111/coa.12901

ORIGINAL ARTICLE

Determining fitting ranges of various bone conduction hearing aids D.C.P.B.M. van Barneveld1,2

| H.J.W. Kok1 | J.F.P. Noten1 | A.J. Bosman1 |

A.F.M. Snik1 1

Department of Otolaryngology and Head and Neck Surgery, Donders Institute for Brain, Cognition and Behaviour, Radboud University Nijmegen Medical Centre, Nijmegen, The Netherlands 2 Department of ENT/Audiology, School for Mental Health and Neuroscience (MHENS), Maastricht University Medical Center, Maastricht, The Netherlands

Correspondence A.F.M. Snik, Department of Otolaryngology and Head and Neck Surgery, Donders Institute for Brain, Cognition and Behaviour, Radboud University Medical Centre, Nijmegen, The Netherlands. Email: [email protected]

Objectives: To define fitting ranges for nine bone conduction devices (BCDs) over different frequencies based on the device’s maximum power output (MPO) and to validate the assessment of MPO of BCDs in the ear canal. Background: Maximum power output (MPO) is an important characteristic when fitting BCDs. It is the highest output level a device can deliver and is one of the major determinants of a device’s fitting range. A skull simulator can be used to verify MPO of percutaneous BCDs. No such simulator is available for active and passive transcutaneous devices. Design: The MPO of nine different BCDs was assessed either by real-ear measurements and/or with skull simulator measurements. Main outcome measures: MPO and cross-validation of the methods using the Bland–Altman method. Results: Percutaneous BCDs have higher MPO levels compared to active and passive transcutaneous devices. This results in a wide dynamic range of hearing for percutaneous devices. Moreover, the assessment of MPO by real-ear measurements was validated. Conclusion: Based on MPO data, fitting ranges were defined for nine BCDs over seven frequencies.

1 | INTRODUCTION

device. However, information on how these ranges are determined is unclear.

Bone conduction devices (BCDs) can be used for conductive and

When a new BCD is introduced onto the market, studies usually

mixed hearing loss when reconstructive surgery or conventional

report on a device’s functional gain, which is defined as the differ-

hearing aids are not viable. Various types of implantable BCDs are

ence between aided and unaided hearing thresholds. This might not

available, and new devices are regularly introduced. There are direct-

be the best way to evaluate the performance of a BCD. For

drive and skin-drive devices.1 In direct-drive devices, the actuator

example, a functional gain of 30 dB for a patient with a conductive

produces vibrations, which are transmitted directly to the bone

hearing loss of 30 dB (ie, aided sound-field threshold and unaided

either via percutaneous coupling (eg, Cochlear BAHA Connect and

air-conduction threshold at 0 dBHL and 30 dBHL, respectively) is a

Oticon Ponto) or with an implanted transducer (eg, Med-EL Bone-

perfect result. If the patient, however, had a conductive hearing loss

bridge). Skin-drive devices can be divided into conventional devices,

of 60 dBHL, a functional gain of 30 dB still leaves a functional

like BCD on softband, and passive transcutaneous devices, such as

air–bone gap of 30 dB.

Cochlear BAHA Attract and Sophono Alpha that are magnetically coupled to an implant in the skull.

As BCDs directly stimulate the cochlea, bypassing the pathology that causes the conductive hearing loss component, it is more informa-

But how do we select the most appropriate BCD for an individ-

tive to consider aided thresholds relative to bone conduction thresh-

ual patient? The manufacturer usually provides the range of sen-

olds.2 This is referred to as effective gain, and in mixed hearing loss, this

sorineural hearing loss components that can be fitted with a given

reflects the available gain to deal with the sensorineural hearing loss

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© 2017 John Wiley & Sons Ltd

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Clinical Otolaryngology. 2018;43:68–75.

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component. A hearing device should compensate approximately half of the sensorineural hearing loss (eg, NAL or DSL fitting rule). An often-overlooked aspect when selecting an appropriate device is maximum power output (MPO). Maximum power output of a BCD is the maximum force level that the device can deliver to the cochlea. In other words, it is the loudest sound that can be perceived using that device. Given sufficient gain, a device with low MPO saturates at lower input levels than a device with high MPO. Maximum power 3-5

output can be used to determine the fitting range (eg, Ref.

Keypoints

• • • •

Determination of MPO of nine BCDs. MPO is used to determine fitting ranges. Validation of in-ear measurements to assess MPO. Percutaneous devices have higher MPO than passive and active devices, resulting in a wide dynamic range.

).

The MPO of percutaneous BCDs expressed in dBHL or dBSPL can be calculated from datasheets provided by the manufacturer and

was positioned in front of the subject at 1 m distance. We used

can be verified with a skull simulator. For other devices, however,

warble tone stimuli. In case of asymmetric hearing loss with better

such data are not always available and verification equipment is lack-

hearing in the non-implanted ear, we occluded that ear with a deeply

ing. An alternative to obtain MPO is to determine the input at out-

fitted plug (EARTM ClassicTM, 3M, Saint Paul, Minnesota, USA) and

put saturation by capturing the sound radiated from the BCD inside

earmuff. All equipment and sound-field stimulation were calibrated

6,7

an occluded cavity, such as the ear canal or nostril,

and add this

to the effective gain3,5 (see Methods). In the present article, we aim to validate the assessment of MPO of BCDs in the ear canal by comparing three different measure-

according to ISO-389, and the experiments were conducted in a soundproof double-walled booth. From these measurements, the effective gain is calculated by subtracting aided sound-field thresholds (T) from bone conduction thresholds (BC) (Figure 1C):

ments. In addition, we propose a model to determine fitting ranges for nine implantable BCDs based on MPO results.

2 | MATERIALS AND METHODS

G ¼ BC T

1

2.3 | MPO

2.1 | Patients and BCDs

2.3.1 | Method 1

Nineteen patients with conductive or mixed hearing loss who

In method 1, we assessed MPO at 500, 1000, 1500, 2000, 3000

received BCDs at our department participated in the experiments

and 4000 Hz by assessing input–output behaviour of the percuta-

(methods 2 and 3). In addition, the MPO of these and other devices

neous BCDs to warble tone stimuli using a skull simulator (SKS10,

was assessed using a skull simulator instead of patients (method 1).

Interacoustics). See Table 1 for an overview of the devices used per

Table 1 gives an overview of the different devices and number of

method. Increasing the input while measuring the output allowed us

measurements of the present study.

to determine the MPO that a device can deliver in dBFL rel 1 lN.

Maximum power output is a device property that in principle is

This value was converted into dBHL using RETFLdbc,8 so it could be

not dependent on patient characteristics. We used test devices of

related to hearing thresholds and loudness discomfort levels. Fig-

the type similar to the patient’s own devices that were not individu-

ure 1A illustrates this schematically; in this example, the MPO at

ally programmed. The devices were set to deliver maximum amplifi-

1000 Hz is 118 dB FL rel 1 lN, which is equivalent to 72.5 dBHL.

cation without generating feedback and were unlimited in output and in linear amplification mode with omnidirectional microphone settings. All advanced processing algorithms were turned off.

2.3.2 | Method 2

All patients participating in this study provided informed consent,

In method 2, we assessed MPO at 500, 750, 1000, 1500, 2000, 3000

and the study has been performed according to the Declaration of

and 4000 Hz by assessing input–output behaviour of the BCDs to

Helsinki. The local medical ethical committee refrained from evaluat-

warble tone stimuli using a skull simulator (SKS10, Interacoustics) com-

ing the study protocol as most of the measurements were part of

bined with the patients’ effective gain. See Table 1 for an overview of

standard operating procedures.

the devices used. Method 2 (and 3) is based on the definition of maximum output: MPO is the sum of the input level at which the device

2.2 | Measurements 2.2.1 | Audiometry: effective gain

saturates (Isaturation) plus the (linear) effective gain (G): MPO ¼ Isaturation þ G

2

Bone conduction thresholds were acquired with standard procedures

The “input level at output saturation” (Isaturation) was obtained by

and equipment using the B71 bone vibrator with a clinical audiome-

intersecting the diagonal of the input–output curve and horizontal

ter (Affinity or Equinox, Interacoustics, Middelfart, Denmark). Aided

asymptote at maximum output (Figure 1A,B). The effective gain was

sound-field thresholds were assessed with the same audiometer. The

obtained using equation 1. Maximum power output was subse-

loudspeaker (Control 1x, JBL, Harman, Stamford, Connecticut, USA)

quently converted from dBSPL to dBHL according to reference.9

Direct-drive with percutaneous coupling

Oticon Medical, Askim, Sweden

Cochlear, Sydney, Australia


Ponto Plus

BAHA BP110 Connect



BAHA BP110 Connect

BAHA BP110 Attract


Med-EL, Innsbruck, Austria

Medtronic, Boulder, USA

Bonebridge

Sophono Alpha2


BAHA BP110 Connect

BAHA BP110 Attract


Oticon Ponto Plus Power

Method 3


Oticon Ponto Plus Power

Method 2

BAHA4



BAHA5

Skin-drive with passive transcutaneous

Direct-drive with active transcutaneous coupling

Skin-drive with passive transcutaneous coupling



Skin-drive with passive transcutaneous coupling









Type of device [1]

Ponto Plus Power

Company

BAHA Cordelle II

Method 1

T A B L E 1 Number of devices tested per frequency and per measurement method

1

3

4

4

3

4

5

3

5

5

4

3

500 Hz

3

2

3

4

4

3

4

750 Hz

4

3

4

3

4

4

3

4

5

3

5

5

4

3

1000 Hz

1

2

4

3

4

4

3

4

5

3

5

5

4

3

1500 Hz

2

2

3

3

4

4

3

4

5

3

5

5

4

3

2000 Hz

2

1

3

4

4

3

4

5

3

5

5

4

3

3000 Hz

1

3

4

4

3

4

5

3

5

5

4

3

4000 Hz

70

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(A)

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Skull simulator

(B)

0.25 0.5 1

3: input at output saturation

1: maximum output 118

Frequency (Hz)

(C)

In ear

71

2

4

8

0

Input (dB HL)

2: input at output saturation

Output

Output

(dB rel 1 µN)

20

60 80

78

Input (dB SPL)

40

100

79

Input (dB SPL)

120

F I G U R E 1 Schematic representation of the methods. (A) Schematic representation of an input–output curve measured using the skull simulator. Method 1 determines the maximum power output (MPO) by reading off the maximum value of the input–output curve in dBFL rel. 1 lN. Method 2 uses the input at output saturation, which is determined by intersecting the diagonal of the input–output curve and horizontal asymptote at maximum output, denoted by the vertical dashed line. (B) Schematic representation of an input–output curve measured in the ear canal with device on (black line) and device tuned off (grey line) measuring direct sound. Method 3 uses the input at output saturation measured in the ear canal, which is obtained by intersecting the first diagonal of the input–output curve and horizontal asymptote at output saturation. Note that the main outcome is the input at output saturation and that output is in not specified as it depends on variables such as distance of the probe microphone to the bone conduction device. (C) Example audiogram. In method 2 and method 3, MPO is obtained by adding the “input at output saturation” to the effective gain. The effective gain is defined as the aided sound-field threshold (T) subtracted from the bone conduction threshold (