This is the first study looking at audiological outcomes between these two devices in the pediatric population. Our results demonstrated that indeed while both devices did well improving outcomes in the low and mid frequencies, the BB resulted in significantly better functional gain in the higher frequencies.

Many studies have noted high rates of soft tissue complications in children implanted with percutaneous BAHDs [7, 8]. This has translated to higher needs for revision surgeries and abutment changes often under repeat general anesthetic, as well as frequent and often unplanned physician visits [8,9,10,11]. Modifications to the surgical technique over the years has undoubtedly lowered these complication rates. However, aspects unique to the developing pediatric skull that can affect osseointegration [12], as well as confounding variables including higher rates of behavioural issues and the child’s reliance on a care provider, has meant that these challenges nevertheless remain a concern.

The main appeal of transcutaneous alternatives is the absence of a skin-penetrating abutment. Potentially, this could mean a lower risk of soft tissue complications and device loss from trauma, improved wear comfort, higher cosmetic acceptance by the patient, and elimination of the need for regular daily maintenance. Although there are more technical and surgical considerations for placement of transcutaneous BAHDs, studies over the past few years have shown these devices to be safe with favourable short term clinical outcomes [13,14,15]. In general, transcutaneous devices can be categorized as either passive or active. In the former, vibrations to the skull are driven by an external mechanical transducer, and the bone vibrations are sent via a magnetic connection to an internally implanted component, resulting in skull bone stimulation. With an active device on the other hand, the mechanical transducer is surgically implanted in the skull, and the externally worn component acts as a receiver transmitter. Given this design difference, there could be an audiological advantage to an active design in that it avoids the soft tissue attenuation of sound vibrations, with a 5–15 dB HL improvement in sensitivity at 1 kHz and above expected compared to a passive BAHD [16, 17]. It is also possible that the closer distance of the transducer to the cochlea could present an audiological advantage [18,19,20]. Furthermore, since the transducer is on the outside with a passive implant, a stronger magnetic connection may be required to improve audiological gain, and this may affect patient comfort.

The AT was introduced in 2013, and studies have since demonstrated this device’s safety and efficacy in the pediatric population, with significant audiological gain in the pure tone average at 0.5, 1, 2 and 4 kHz (PTA4) compared to the unaided condition. In a cohort of 10 children that received the AT system, Giannantonio et al. found a mean PTA4 functional gain of 23.70 dB [21]. Similarly, Powell et al. reviewed 12 patients (ten children and two adults) who received the AT, and noted a PTA4 functional gain of 30.2 dB. These studies however did not evaluate the FG in frequencies higher than 4 kHz. The current study likewise has demonstrated significant improvement in the low and mid frequencies, with a PTA4 functional gain of 33.2 dB.

From an audiological standpoint, the main potential advantage of the BB is avoiding soft tissue attenuation and hence a more favourable outcome in the higher frequencies. Studies thus have shown a mean functional gain ranging between 26.1 and 36.5 dB [22,23,24,25,26]. This functional gain is evident across all frequencies, although prior studies have suggested that it is more significant in the higher than in the lower frequencies [21]. This is in contradistinction to percutaneous BAHDs, where the functional gain appears to be more significant in the lower frequencies. Huber et al. [27] found that in human subjects, the thresholds improvement of transcranial attenuation (which is stronger at frequencies above 3 kHz [28]) was less in BB than in percutaneous BAHD recipients, suggesting better separation between ipsilateral and contralateral sides. In the current study, the BB demonstrated a PTA4 functional gain of 35.2 dB. When evaluating the gain at 6 and 8 kHz, the BB performed better than the AT (28.9 and 29.0 dB versus 16.7 and 17.3 dB respectively).

Zernotti et al. compared the BB to the Sophono (Medtronic; Boulder, Colo.), a passive transcutaneous BAHD, and found that both devices reduced air conduction thresholds at 1 and 2 kHz (p < 0.01), however only the BB device reduced the air conduction thresholds at 0.5 (p = 0.0140) and 4 kHz (p < 0.0001) post-implantation [29]. In this study, assessment of improvement at higher frequencies was unavailable. More recently, Han et al. evaluated audiological outcomes between three groups of adult patients who received either a percutaneous BAHD, the AT or the BB for MHL or SSD. In cases of MHL, there were no differences between the three groups in FG outcomes. However, the authors only measured hearing frequencies between 250 and 4000 Hz [30].

While the PTA4 has served as an important and standardized audiological outcome measure in data reporting, it disregards the higher frequencies from comparative analyses. In children, higher frequencies may play an important role in various aspects of speech and psychosocial development. The term minimal hearing loss has had various definitions in the pediatric population, and generally includes bilateral mild hearing loss, as well as high frequency hearing loss, in which the air-conduction thresholds are ≥ 25 dB HL at two or more frequencies above 2 kHz in both ears [31]. Historically, these children were not identified until they entered school, and even after that they often received little or no intervention. However, evidence gathered over time has demonstrated that children with minimal hearing loss face challenges in various domains including speech recognition, language development and competence, academic performance, psychosocial and emotional wellbeing, listening effort, and localization [32]. In the auditory rehabilitation of children with CHL and MHL, aiding the higher frequencies is an important component that is often under-evaluated in the assessment of the efficacy of implantable devices. The current study demonstrates the advantage of an active transcutaneous BAHD over a passive one in the higher frequencies in a consistent fashion in pediatric patients. Given what we know about the impact of high frequency hearing loss on child development, an active transcutaneous BAHD may provide long-term advantages in the pediatric population.

The BB does portray technical challenges given the relatively large dimensions of the implantable FMT. The general recommendation has been for surgical placement within the sinudural angle, although there are often limitations to complete fitting in this location within the pediatric population. While lifts are available ranging in size from one to four millimetres, a degree of dural decompression may be necessary. Alternatively, a retrosigmoid or supratemporal placement is an option [33, 34]. Rohani et al. have studied the potential impact of screw type, lift thickness, and FMT placement on sound transmission in BB surgery [35]. They found no significant differences in these factors to affect the vibartion of sound transmitted to the cochlea using Doppler vibrometry technique. You et al. have reviewed their experience with the supratemporal approach with BB surgery in forty patients and found the technique to be sae and effective [34]. Based on published data, we felt it was appropriate to include the patient who underwent supratemporal placement in our pooled data. The newly introduced BB 602 has addressed this technical limitation to some extent as its FMT is only 4.5 mm deep, which is approximately 50% that of the original BB 601 system used in this study. Early studies on the BB 602 have shown favourable clinical and audiological outcomes [36].

A limitation of our study is that primary outcomes of audiometric measures do not capture the broad utility of BAHDs. Quality of life (QOL) outcomes is not addressed and are a focus of future research.