Design concept and technological considerations for the Cochlear

Cochlear™ Baha® 4 Attract System
Design concept and technological considerations
for the Cochlear Baha 4 Attract System.
Mark. C. Flynn, PhD
Abstract
The Cochlear™ Baha® 4 Attract System* is a bone conduction implant system that uses magnet retention to connect
the Cochlear Baha sound processor with the osseointegrated Cochlear Baha BI300 Implant. The BI300 Implant and the
implant magnet are placed entirely under the skin providing a more cosmetically appealing design than the current skinpenetrating offerings. The sound processor is attached with a single external sound processor magnet. Unique to the Baha
4 Attract System is the use of a single-point sound transmission that more effectively transmits sound to the cochlea than
multi-screw systems. To increase wearing comfort and to reduce the possibility of soft tissue complications, the Baha
SoftWear Pad™** was developed to increase the retention force without increasing the pressure against the soft tissue.
Clinical testing with a simulated full system demonstrates performance that is significantly (p<.001) better than obtained
using a Baha Softband. The Baha 4 Attract System provides a suitable alternative option for candidates with a conductive,
mixed, or single-sided sensorineural deafness.
Introduction
Bone conduction implant systems have been successfully used for
more than 35 years offering excellent hearing outcomes for patients
with conductive hearing loss, mixed hearing loss and single-sided
sensorineural deafness.1 Despite successful treatment of these patient
groups, the skin-penetrating abutment has some drawbacks, such as
the need for lifelong daily care, risk of implant trauma and patients
opting for no treatment due to perceived stigma.2 A non skinpenetrating solution could therefore provide a suitable alternative for
some patients because it leaves the skin intact thereby reducing the
issues related to the the abutment. The challenges reported in the
design of such a system can be summarised as the trade-off between
the benefits of a strong magnetic connection (namely retention
force and the efficiency of signal transmission) and the impact of
compression on the soft tissue between the magnets. As the surgery
for the Baha Connect System (with the DermaLock Abutment) is now
relatively straightforward,3,4 it is important that any such development
of a non skin penetrating system ensures that the surgery remains
straightforward and does not introduce additional complications.
The application of a magnetic connection is not novel, but it has proved
challenging to meet the expectations of the patients and clinicians
in terms of hearing performance, safety and retention. Significant
shortcomings in terms of hearing outcomes were reported a number
of years ago for the Audiant device, which were most likely due to the
design of the air gap needed for generating the signal output, including
the soft tissue. The reported critical issues were centred on soft tissue
complications, poor sound transmission and insufficient force for
retaining the sound processor.5-7 However, it should also be noted
that since the development of Audiant there have been significant
product developments in terms of transducer and sound processor
*The Baha 4 Attract System consists of:
Cochlear Baha BI300 Implant
Cochlear BIM400 Implant Magnet
Cochlear Baha SP Magnet
Cochlear Baha sound processor
** SP Magnet Softpad
technologies, perhaps making it possible to achieve improved hearing
outcomes for a modern magnetic bone conduction implant system.
Indeed, other systems have recently come to market that potentially
offer improved outcomes.8-11 While these developments are welcome,
critical examination of the available data leads to concerns regarding
generalisations about performance based on relatively small and
specifically selected subject samples in addition to limited information
about the technical performance and verification of the system. It does,
however, present a benchmark for the acceptable level of performance
that could provide further insights into expected performance.
When using a magnet on the skull, the soft tissue underneath the
magnet is affected by the force between the internal and external
magnets. If the soft tissue is subjected to excessive loads the cells are
not provided with a sufficient amount of oxygen and the risk of necrosis
arises.12,13 The literature suggests that excessive force will result in soft
tissue irritation (and in the worst case, tissue breakdown) and a magnet
that is too weak will result in retention difficulties.5 Attention to detail
with the design of the magnet connection and adequate magnet
strength would reduce the risk of skin irritations. Another important
aspect to consider in magnet design is the curvature of the skull.
With this in mind Cochlear began technical feasibility studies and
development of a magnetic bone conduction implant system, the
Baha 4 Attract System. The Baha 4 Attract System has been specifically
designed to incorporate three key requirements (a) efficient sound
transmission through the skin to overcome the attenuation of the
signal, (b) sufficient magnetic retention for everyday activity and (c)
appropriate pressure against the skin for a full day of wear. This paper
describes how these issues were evaluated and addressed.
BI300 Implant
Baha sound processor
BIM400 Implant Magnet
Figure 1. Cochlear Baha 4 Attract System comprising the BI300 Implant, BIM400
Implant Magnet, sound processor magnet and Baha sound processor.
The Baha sound processor is available in different models to suit
different levels of hearing loss. Due to the universal snap connection
the Baha 4 Attract System is fully compatible with all current Baha
sound processors. Fitting is easily performed using the Baha Fitting
Software 4.0.
There are a number of key technical innovations in this system that
play an essential role in this development. These include single-point
sound transmission and the capability of achieving acceptable levels of
force and pressure. The next section describes some of the testing and
clinical investigations conducted to address the key technical issues,
including potential hearing performance.
Single-point transmission for efficient sound transmission
The foundation of the system is the osseointegrated BI300 Implant,
which has well documented stability in adults and children.14-19 Using
the BI300 Implant as the basis for both the Baha Connect System (with
an abutment) and Baha Attract System (with a magnetic connection)
results in similar expectations regarding implant stability and sound
transmission through a single point. In addition, using the same implant
for both systems also provides an opportunity for upgrading patients
to an abutment in the future if their hearing needs change. The 27mm
diameter implant magnet is attached to the 4mm or 3mm BI300
Implant by a centrally located screw. The fixation screw forms a tight
seal between the implant magnet and the BI300 implant.
The Baha 4 Attract System is designed to transmit sound more
efficiently by focusing the vibrations in a single point. When attempting
transmission through multiple screws, the vibration energy is spread
over several contact points resulting in lower transmission efficiency
(Figure 2)20.
In order to test the hypothesis that a single-point of sound transmission
is a more effective method of transmitting sound, a cadaver study20
was performed to evaluate sound transmission to the cochlea using
the Baha 4 Attract System, Baha sound processor on a Baha Softband
2
B
Figure 2. Conceptual drawing of the vibration transmission through a singlepoint system (A) versus a multi-screw system (B) demonstrating the potential
effectiveness of a single-point system.
and a multi-screw system. The test procedures were consistent with
methodology that has been well described in previous publications.21-27
The systems were compared by measuring the acceleration of the
cochlear promontory on the ipsilateral side on eight human cadaver
heads using a Laser Doppler Vibrometry (LDV) system. The sound
processor’s (Cochlear Baha BP110 Power) actuator was electrically
driven with one volt over a frequency range of 100 to 12000 Hz. The
statistical analyses of test results were performed by an independent
statistician.
The results indicated the Baha 4 Attract System showed superior
sound transmission compared to the multi-screw system (12.0 - 25.8
dB improvement across the frequency range) with very high statistical
confidence (p-values ranging from p<0.0001 to p=0.0058 across all
one-third octave bands) (Figure 3). The results also show that the Baha
4 Attract System is statistically superior to the Baha Softband at low
(315-800 Hz, p-value range p=0.0008-0.00365) and high frequencies
(2500 - 4000 Hz, p-value range p=0.0002-0.0420) by an almost
equally large amount (7.0-12.4 dB) (Figure 4). Given the link between
sound transmission and hearing performance, this strongly suggests
that the hearing performance for a patient using a single-point system
will be better than for a patient using a multi-screw system. Similarly,
the Baha 4 Attract System is expected to provide better hearing
performance than a Baha sound processor on a Baha Softband.
It could be argued that since the sound processors were different on
the single-point versus the multi-point system, this observed difference
may be due to differences in sound processor output. To look into
this possibility, the investigation was repeated using the BP110 Power
Sound Processor actuator with both systems. Replacing the proprietary
sound processor with the more powerful BP110 Power Sound Processor
provided an increase in sound transmission of approximately 9 dB
(blue) (Figure 3). Importantly, the single-point sound transmission
through the implant (BI300) was found to increase efficiency even
further by adding another 15 dB (yellow) on average.
Similarly, the Baha 4 Attract System is expected to provide better
hearing performance than a Baha sound processor on a Baha Softband.
Compared with the Baha Softband, the BI300 Implant was found to
provide a 5 dB more efficient sound transmission on average (Figure
4). It was also important to compare with the performance provided
through a to a skin-penetrating abutment abutment (Baha Connect
System) because some loss of efficiency would be expected due to
attenuation of sound by the soft tissue. As expected, and consistent
with previous reports28 the sound was attenuated by the soft tissue,
this was most apparent in the high frequencies. Therefore, a singlepoint system, such as the Baha 4 Attract System, is expected to provide
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Baha Attract System with BP110 Power
Baha Connect System with BP110 Power
Multi-screw implant with BP110 Power
Baha Attract System with BP110 Power
Multi-screw system
Softband with BP110 Power
Figure 3. Comparison of the sound transmission of a single-point (Baha 4 Attract
System: blue) system compared with a multi-screw system with either the
proprietary (red) or BP110 Power (blue/red) Sound Processor.
Figure 4. Comparison of sound transmission through the Baha Connect System
(orange), Baha 4 Attract System (blue) and Baha Softband (grey).
improved sound transmission over Baha Softband and multi-screw
systems. As expected, direct transmission through the abutment will
always provide the most effective method of transmission.
to the compressive force by adapting to the underlying surface, thus
creating homogeneous pressure over the entire tissue contact area.
Design for optimal pressure distribution and comfort
Sound processor magnets of different strengths were developed
to provide adequate retention and to avoid irritation or damage to
the skin for a soft tissue thickness range from 3 to 6 mm. In order
to maintain a healthy soft tissue flap, good transfer of oxygen and
nutrients must be guaranteed (Figure 5). Therefore, the research
suggests13 the average pressure between the SP magnet and the skin
should not exceed the capillary blood pressure (~0.4 N/cm2) and the
peak pressure over an area greater than 2mm in diameter must not
exceed 0.6 N/cm2 (Figure 5). It should be noted that the key value is
not force (N) but pressure (N/cm2); in this way the diameters of the
internal and external magnets become crucial as a larger amount of
force can be exerted over a larger contact area, resulting in reduced
pressure when compared with smaller magnets.
Similarly, a rigid magnet plate should be avoided due to the fact that
peak pressure areas that exceeds the maximum pressure limits may
occur. These areas of peak pressure are likely because although the
rigid plate may be curved, the curvature of the skull and scalp is highly
variable.29 In order to maximise the contact area and minimise the risk
of peak pressure areas forming, the SP magnet was designed to evenly
distribute pressure on the skin surface by means of a soft material on
its skin-contact surface. For this purpose, slow-recovery foam lined with
polyurethane medical tape is used. The slow recovery foam responds
To investigate the impact of these soft materials, an investigation was
conducted using a highly sensitive pressure measuring tool (i-Scan®:
Tekscan, Boston, MA, USA) to measure the pressure distribution
exerted on skin by the Baha SoftWear™ Pad compared to a rigid
plate30. As seen in Figure 6, the use of the Baha SoftWear Pad results
in a better distribution of pressure over the SP magnet in addition to a
significant reduction in peak pressure. The test performed demonstrates
that the memory foam in the Baha SoftWear Pad distributes the
pressure exerted on the skin and reduces peak pressure better than a
conventional rigid plate.
Importantly, investigations demonstrated that the soft material will
adapt over time.30 Figure 7 shows that over a short time period the area
in contact with the skin will increase, thereby also increasing the force.
A
B
P=F/A < 0.4 N/cm2
Figure 5. The implant magnet and external magnet surface area have been
designed to be similar in size in order to distribute the load evenly across the skin.
Increase in peak pressure
SP Magnet combined
with Baha SoftWear Pad
A
dB [rel. 1 μN]
Cochlear Baha 4 Attract System (Figure 1) uses magnet retention to
connect the Baha sound processor with the BI300 Implant. The internal
BIM400 Implant Magnet (placed underneath the skin) is attached
to the BI300 implant and the Baha sound processor snaps onto the
external SP magnet placed on the skin (Figure 1). The SP Magnet and
the BIM400 Implant Magnet are attached via magnetic retention.
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Cochlear Baha 4 Attract System design
Figure 6. Comparison of i-Scan pressure measurements taken without (A) and with (B)
the Baha SoftWear Pad, demonstrating that a significantly greater surface area is in
contact with the skin and peak pressure is lower when the Baha SoftWear Pad is used.
3
Baha sound processors come in a series of weights and are at times
used with attachable accessories (e.g. audio cable, FM and telecoil),
which will increase the weight of the sound processor and required
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Figure 7. Changes in force, area and pressure over time as the Baha SoftWear Pad
adapts to the curvature of the scalp and skull.
retention force. However, even with a significant increase in force from
1 to 2N, the corresponding pressure increase was found to be minor and
the absolute mean pressure was always below 0.4N/cm2 (Figure 8).
One crucial issue that had to be addressed was ensuring that the Baha
SoftWear Pad did not attenuate the sound being transmitted from the
sound processor. An investigation of the potential impact of the Baha
SoftWear Pad on sound transmission was performed on 20 subjects
with normal hearing.31 BC Direct measurements with and without a
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The relationship between contact force and hearing performance is
interesting. One could readily assume that the higher the force the
better the sound transmission, which would lead to improved hearing
performance. In order to investigate this, a number of investigations
were conducted.
Firstly, the potential impact of contact force on sound transmission
for a conventional Baha Softband compared with the Baha 4 Attract
System using a sound processor magnet attached to a modified Baha
Softband was investigated on 20 test subjects with normal hearing.33
The ears of the test subjects were blocked to create an artificial
conductive hearing loss. A Baha BP100 Sound Processor on an SP
magnet with a Baha SoftWear Pad was attached to the head by means
of a modified softband and BC Direct thresholds were measured for
contact forces of 1 N, 2 N and 3 N and compared to a BP100 on a Baha
Softband at 2 N. The investigation showed that higher contact force
results in less transmission loss. Importantly, at the same contact force
level, similar or better results were obtained for the Baha 4 Attract
System at all frequencies compared to the Baha Softband reference
(statistically significant difference at 750, 1,000, 4,000 and 8,000Hz).
Therefore, the results of that investigation appeared to imply the logical
conclusion that the higher the force the better the sound transmission.
The second investigation involved a more advanced model of the
system where an innovative simulation of the Baha 4 Attract System
was created to be able to simulate performance in patients who had
Pressure (N/cm2)
Figure 8. Average pressure in N/cm2 (green lines) recorded for different forces in
Newton (blue lines) with the Attract system.
Baha SoftWear Pad on the pressure plate were performed. A statistically
significant difference between the two configurations in terms of
sound transmission was only noted in one frequency (3 kHz), where
the patients actually performed better with the Baha SoftWear Pad
attached. Therefore, the results confirm that the Baha SoftWear Pad
does not negatively affect sound transmission.
Sound processor
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One potential explanation for the difference between Baha Softband
fitting versus a Baha Attract System fitting could be the nature of the
attachment. For a Baha Softband, a certain force is required to both
retain the Baha Softband and to create compression against the skull to
aid sound transmission. Since the Baha 4 Attract System uses a singlepoint of sound transmission in terms of an osseointegrated implant,
additional compression against the skull over a certain level (e.g. 1N)
may not be required. This data is preliminary and further testing is
required.
Hearing performance
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Figure 10. Mean BC Direct thresholds from all subjects at different magnet
retention forces and when using a Baha Softband.
an existing percutaneous abutment. The abutment adapter (Figure
9) was created to simulate the Baha 4 Attract System on patients
with an abutment.34 This investigation was performed with ten Baha
users with mixed or conductive hearing loss. A BP110 Power Sound
Processor was attached to an abutment adapter and artificial skin to
simulate the Baha 4 Attract System. BC Direct measurements were
performed using SP magnets of three different strengths. The results
from this investigation indicate that the impact of contact force on
sound transmission is minor (Figure 10). Importantly, the results
of this comparison with the Baha Softband, in terms of degree and
frequencies (Hz) of improvement, are very similar to those obtained in
the previously mentioned cadaver studies.20
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Tests of hearing performance with simulated skin attenuation were
performed on ten patients with a Baha system using a special adapter
and artificial skin (Figure 9)35 to simulate the Baha 4 Attract System’s
magnetic connection. BC Direct measurements were taken and freefield audibility, speech in quiet and adaptive speech in noise was tested
using the simulated Baha Attract System and compared with outcomes
where the Baha Connect System and the Baha Softband had been used.
The tests showed that the subjects performed as well or better with
the abutment adapter compared with the Baha Softband in terms
of audibility and speech recognition (Figures 11 & 12). There was a
difference in average BC Direct thresholds in the low and mid frequency
area in favour of the abutment adapter (simulated Baha Attract
System). The test subjects performed best in the high frequencies when
the sound processor was fitted on the abutment, as is expected in the
absence of transmission loss through the soft tissue.28 However, in
adaptive speech in noise tests, no difference was found between the
simulated Baha Attract System and Baha Connect system when high
frequency attenuation had been compensated for using the fitting
software. The test subjects performed worse with the Baha Softband
compared with the other connection types. Overall, the study indicates
that patients will have as good or better performance regarding
audibility and speech recognition with the Baha 4 Attract System
compared with a Baha Softband.
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Figure 9. Illustration of the model used to simulate the Baha 4 Attract System
performance in patients with a skin-penetrating abutment.
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The SP magnets are designed for a soft tissue flap thickness between 3
and 6 mm. In addition to the soft tissue thickness, other factors such
as hair thickness and skin compression will impact the required magnet
strength. The range of SP magnets was designed with that in mind,
offering multiple magnet options for each soft tissue thickness. Lower
retention force may be sufficient for some patients while others with
the same soft tissue thickness may require a stronger magnet due to
having a lot of hair or very short “spikey” hair.
In order to maximise attachment force, while minimising the peak soft
pressures, the Baha SoftWear Pad enables a higher total force, leading
to significantly better retention of the sound processor. Tests performed
demonstrate that the Baha sound processor remained attached at G
forces of up to 6.5.32 In everyday life, this would correspond to activities
such as running down the stairs.
3
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Optimising the force (N) to retain the sound
processor
Hearing
Hearinglevel
level(dBHL)
(dBHL)
However, there is no significant increase in pressure over time. Since
the system becomes stable after only 5-10 minutes, this is important to
take into account when fitting and evaluating performance in the clinic.
Baha
Softband
Baha Softband
Percutaneous
Baha
Baha Connect System
Baha
Attract
System
Baha Attract
System
Figure 11. BC Direct measurements for the Baha 4 Attract System with a BP110
Power Sound Processor compared with a BP110 Power Sound Processor on a Baha
Softband and an abutment.
Baha
Softband
Softband
Baha Connect System
Percutaneous
Baha
Baha Attract
System
Baha
Attract
Unaided
Unaided
Figure 12. Free-field audiometry measurements for the Baha 4 Attract System
with a BP110 Power Sound Processor compared with the unaided condition, BP110
Power Sound Processor on a Baha Softband and abutment.
5
Unlike a multi-screw system, the conclusion is that the single-point
system described here will provide significantly better hearing
performance than Baha Softband. This is crucial to pre-operative
guidance and counselling as this means that the patient who performs
well and is satisfied with performance on a Baha Softband will continue
to have excellent hearing performance outcomes post operatively.
Conclusion
Significant technical and research efforts have gone into the design
of the Baha 4 Attract System in recent years. A key focus in these
efforts was to ensure excellent sound transmission and good wearing
comfort with sufficient force to retain the sound processor. Clearly, a
single-point system of sound transmission combined with a consistent
contact area is a more effective pathway for vibrational energy to the
cochlea than one where energy is dissipated across a number of points.
This system, combined with the ability to attach a more powerful
sound processor, enables natural soft tissue attenuation to be largely
overcome. Therefore, providing hearing performance that for many
people will be similar to the abutment solution is better than the Baha
Softband solution. Previous attempts at magnetic bone conduction
implant systems have been hampered by the challenge of providing
sufficient force for sound transmission and retention without leading to
soft tissue problems. The use of the unique Baha Softwear Pad enables
the system to adapt to the contours of the individual wearer’s scalp.
This provides a greater contact area for the sound processor magnet
to be in contact with the soft tissue, thereby enabling sufficient force
without a concomitant increase in pressure.
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In terms of retention and hearing performance, the investigations
performed highlight the fact that once a sufficient amount of force
(approximately 1N) has been applied, this is sufficient for both effective
sound transmission and sound processor retention. Interestingly, the
effect of increased force does not appear to lead to significant gains
in hearing performance. This is an area that requires some further
investigation to be fully understood. In conclusion, the data presented
show that the Baha 4 Attract System is a well designed system.
Combined with a straightforward surgical approach, this solution
should add to the possibilities for patients requiring bone conduction
hearing. Importantly, since the BI300 Implant is the foundation of both
the Baha 4 Attract and Baha 4 Connect Systems, it is conceivable that
a patient may be able to benefit from upgrades from one system to the
other as they become available in the future.
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27. Stenfelt S, Goode RL. Transmission properties of bone conducted sound:
measurements in cadaver heads. J Acoust Soc Am 2005;118:2373-91.
28. Verstraeten N, Zarowski AJ, Somers T, Riff D, Offeciers EF. Comparison of the
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31. Andersson J, Flynn M. Investigation of Supersoft material for TBaha pressure
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32. Nyström D. G3.5 rel.1 - Verification Retention with acc. Mölnlycke, Sweden:
Cochlear Bone Anchored Solutions AB; 2013. Report No.: 617728.
33. Andersson J, Flynn M. Investigation of the potential impact of contact force
regarding BC Direct thresholds for T-Baha passive. Mölnlycke, Sweden:
Cochlear Bone Anchored Solutions AB; 2013. Report No.: 612822.
34. Hedin A, Flynn M. Investigation of the potential impact of magnet force
regarding sound transmission. Mölnlycke, Sweden: Cochlear Bone Anchored
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Solutions AB; 2013. Report No.: 615077.
In the United States and Canada, the placement of a bone-anchored implant
is contraindicated in children below the age of 5.
6
7
Manufacturer:
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Tel: +46 31 792 44 00 Fax: +46 31 792 46 95
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Tel: +41 61 205 0404 Fax: +41 61 205 0405
www.cochlear.com
Ardium, Baha, Baha Divino, Baha Intenso, Baha PureSound, Baha SoftWear, DermaLock,
Vistafix and WindShield are either trademarks or registered trademarks of Cochlear Bone
Anchored Solutions AB. Cochlear, Hear Now. And always and the elliptical logo are either
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Solutions AB 2013. All rights reserved. NOV13. E82744