## 1 Introduction  
Implantable hearing devices that directly stimulate middle ear structures (active middle ear implants; aMEI) or that provide direct bone conduction to transmit sound to the inner ear (active bone conducting hearing devices; aBCHD) are now routinely used within the United Kingdom and many countries around the world. These devices are used by individuals with hearing loss who cannot use conventional acoustic hearing aids for anatomical and/or medical reasons. Historically, BCHDs have had two parts: an external sound processor and a surgically implanted connector fixed in the bone behind the ear. The connector (abutment) protrudes through the skin and attaches to the external sound processor, which contains a microphone and hearing aid circuitry. These impart stimulation to the cochlea via bone conduction through the skull, bypassing the outer and middle ears. aMEIs are different from middle ear prostheses, which functionally replace damaged or absent anatomical structures without providing any driven component. MEIs provide mechanical energy to move the middle ear structures via an attached transducer. This transducer is controlled and driven by an externally worn component containing a microphone, signal processing chip, battery, and transmitter. Good coupling of the transducer to the remaining ossicular elements or the round window is key to providing good outcomes, and issues with coupling can lead to variability in results [1]. In the last two decades, there has been an increase in the use of active devices where the vibrating transducer is powered by and activated by radiofrequency (RF) coupling across intact skin. A surface induction coil surrounds a magnet and is retained by magnetic transcutaneous attraction to a matching subcutaneous coil. These magnets align the coils and maximize RF transmission efficiency. In contrast, devices with a passive percutaneous osseeointegrated component are directly connected to a vibrating external processor [2]. The two most commonly used active transcutaneous devices of this type are produced by MED-EL (www.medel.com, accessed on 25 June 2023). The MED-EL devices are an aMEI, the Vibrant Soundbridge (VSB; CE Mark 1997; FDA approval 2000), and an aBCHD, the Bonebridge (BB; CE Mark 2012; FDA clearance 2018). Without any physical connection through a permanent opening in the skin, these devices potentially reduce the opportunity for traumatic, infection-related, inflammatory, or other medical complications. However, the implantation is more complex than for percutaneous devices, which may result in greater surgical risk. [PERSON] et al. [3] in a systematic review of the safety of BCHDs compared the original percutaneous to the transcutaneous approach and found that fewer complications occurred with the transcutaneous approach, and the number of major events was significantly lower for active transcutaneous devices such as the BB. The BB is indicated for individuals with conductive or mixed losses, and the VSB for individuals with mild to severe sensor-neural losses, mixed losses, or conductive losses who cannot benefit from hearing aids. [PERSON] and colleagues [4] conducted a systematic review of the impact on hearing of aMEIs compared with conventional hearing aids. They concluded that many patients could benefit from aMEIs but that high-quality, long-term studies were not available at that time. The Medical Device Regulation ([[https://ec.europa.eu/health/md_sector/overview_en](https://ec.europa.eu/health/md_sector/overview_en)]([https://ec.europa.eu/health/md_sector/overview_en](https://ec.europa.eu/health/md_sector/overview_en)), accessed on 25 June 2023) states that follow-up for the lifetime of the device is necessary, which, together with the findings from [PERSON] et al. [4], provided the motivation for this research on the long-term follow-up of VSB and BB patients. To carry out a comparative review with sufficient cases for the VSB and BB, a multi-site study was conducted to overcome the relatively small number of implantations at individual sites.