When compared to standard otosurgeries, BCHI operations are simpler procedures from a technical point of view. However, it is crucial to have a thorough understanding and competence in each phase of the process. Due to the monolithic design of the Osia 2 system, the optimal positioning of the SP is directly linked to the placement of the BI300 implant. Therefore, preoperative planning of the BI300 position and OSI200 placement is an essential step and the procedure requires space for visualisation, especially for BI300 implantation and fixation of the OSI200. Although the learning curve of Osia surgery is short [11, 17], the procedure can be complicated and have a greater incidence of postoperative complications even for experienced surgeons due to the transducer size, bone thickness, or bone density issues in some cases. As per the manufacturer’s instructions, the incision line should be placed at a significant distance from the transducer borders to reduce tension on the suture line. Recommended C-shape, “lazy S”-shape, or J-shape incisions around the transducer leave long scars in the retroauricular space and usually bypass the transducer in the inferior region of the temporal area, where the risk of injury to the main regional arteries that provide blood supply to the soft tissue is high [18]. In addition, particularly in young children with underdeveloped mastoid processes, the lower section of the incision may be located too close to the neck muscles. A decrease in blood flow in this area jeopardise tissue perfusion and oxygenation, while increasing the likelihood of wound complications [21]. According to the Manufacturer and User Device Facility Experience (MAUDE) database, the most commonly reported problems with the Osia system are infection, pain, and extrusion of the device, which are predominantly treated conservatively, and only a few cases require surgical revision [14]. Previous studies on passive transcutaneous BCHI showed similar injury profiles, including discomfort, pressure injury, and wound formation, with subsequent infection or skin complications, though the majority of patients have good outcomes with these devices [14, 19]– [21]. Strong magnet compression contributes to some of these complications, while conservative treatment and magnet strength reduction typically resolve periimplant pain and erythema; in severe individual cases, skin necrosis necessitates system removal or conversion [20,21,22]. In contrast, active BCHIs needs much less magnetic strength as vibrations do not need to be transfered from an external source, but larger surgical approach is necessary due to their relatively big dimension compared to a passive percutaneous or transcutaneous system. An extended approach with large flap creation, particularly when the incision line crosses the proximal section of the primary arterial supply of the temporal region, might result in excessive bleeding and decreased perfusion of soft tissues, leading to difficulties in wound healing [18].

This observation is also supported by cochlear implantation (CI) studies, that described a correlation between large incisions, extended surgical approaches, and soft tissue complications such wound infections, seromas, and hematomas indicating that a reduced surgical approach minimised the occurrence of significant skin problems [23, 24]. Additionally in malformation cases, the presence of excessive scar tissue and impaired blood circulation is undesirable when ear reconstructive surgery is planned in the future. Using a different surgical approach above the transducer or between the coil-transducer area can decrease the length of the incision, reduce the size of the scar in the temporal region, and improve visibility for BI300 implantation [25,26,27]. However, there may still be difficulties in successfully implanting the BI300 if the bone density or thickness is inadequate. Our earlier morphometric study on the pediatric population revealed that the bone thickness in the age range of 5–6 years is around 3.5 mm at the BI300 level. Therefore, it is safe to use a 3 mm titanium implant [15]. Nevertheless, there is a growing necessity to lower the age restriction for surgery, considering the favorable audiological findings that have been reported by several groups [11, 12, 28]– [30]. Determining the bone thickness using CT is necessary for planning and safety reasons in much younger individuals or in cases of severe craniofacial malformations. In addition to irradiating the skull, the cooperation of youngsters can be a challenge and necessitates anesthesia, which raises the likelihood of complications and the overall expense of rehabilitation. The biggest advantage of our minimal invasive technique is that the incision is in the furthest possible position from the transducer, and no BI300 implantation is needed. The necessary incision length is the width of the transducer (3.5–4 cm) at the superior region of the temporal area, which leaves the retroauricular region scar-free and preserves the blood supply of the region. Due to the precise matching of the subperiosteal pocket and the OSI200 size, the flap preparation is minimal. Furthermore, the tight pocket securely holds the transducer in place and facilitates optimal contact with the bone surface which allows effective signal transmission even without the BI300, as evidenced by the vibroacoustic measurements as well as by audiological findings. Although emissary veins can lead to difficulties, the risk can be reduced by utilizing a preoperative CT scan and/or performing soft tissue flap elevation and bleeding control with the use of an endoscope. If bleeding cannot be managed, the endoscopic method can also assist in identifying the direction and required size of the further incision.

However, the duration of the follow-up in this study is limited, there is no evidence of any migration of the implant. Based on earlier research on CI, when the receiver-stimulator is placed in a tightly secured subperiosteal pocket without fixation, the likelihood of subsequent migration is minimal [31,32,33,34]. Remodelling and spontaneous development of bone surrounding the receiver-stimulator have also been described in these cases [31], which presumably will occur with the MSPT approach as well. Additionally, BI300 implantation and fixation of OSI200 is still possible later. The technique’s MRI compatibility raises questions; however, subperiosteally implanted non-fixed CI receiver-stimulators are also MRI compatible (for Nucleus 600 portfolio to 3Tesla) with an MRI kit or tight bandage, even with a magnet in its place. Therefore, MRI measurements might be non-problematic in compliance with the recommended safety rules (www.cochlear.com). In addition, it is important to note that the transducer is non-magnetic. Therefore, if removal is required, only the Osia magnet should be taken out. The entire OSI200 implant may be readily removed, even without the need for any specialized equipment, if deemed essential.

Prior research of Dolhen et al. on the Baha Attract system demonstrated that inserting the Attract internal magnet (BIM400 internal magnet) into a tight subperiosteal pocket without any fixation did not have a detrimental impact on the audiological results. In terms of PTA and SRT, the non-fixed Baha Attract yielded superior outcomes compared to the unaided or Baha 5 SP aided (on Soundarc) situations. Moreover, this minimally invasive pocket technique provided short surgical time, reduced incision, fast and uncomplicated wound healing with good esthetical outcome [35]. In our study, vibroacoustic measurements on head model also indicated, that omitting the fixation of the Osia system to BI300 do not substantially reduce the signal transmission in the level of the cochlea compared to the fixed scenario. The findings are further corroborated by our case series, as both PTA and suprathreshold testing shown a substantial improvement after the surgery using the non-fixated Osia system, moreover, in the case of the previously bilaterally rehabilitated atresia patient, the unilateral, non-fixed Osia system provided better audiological results than the bilateral Baha 5 SP on Softband, which was also supported by the subjective feedback of the patient and the family.

The Baha 5 SP on Softband demonstrated a significant change in PTA, with a threshold decrease of about ~ 25–30 dB, which aligns with the results of previous research [22, 35]. Furthermore, the Osia 2 without fixation showed an even higher improvement, with an additional threshold reduction of around 9 dB. When comparing the PTA results, it was seen that improvement of high and low frequency thresholds was more noticeable with the non-fixed Osia 2 system. Additionally, the greatest difference in average aided thresholds between adjacent frequencies did not exceed 5 dB, while in the case of the Baha 5 SP system, this difference reached 10 dB.

Previous studies have confirmed that improved performance at high frequencies enhances speech recognition and optimizes the identification and distinction of phonemes with high frequency energy [36,37,38]. However, insufficient high-frequency gain and inadequate maximum power output (MPO) can lead to distortion and feedback, which can negatively impact the BCHI performance, particularly in noisy environments. Our suprathreshold test results demonstrates, that MPO of the non-fixed Osia 2 system provides effective amplification, even in noise.