For patients with congenital bilateral conductive hearing loss (BCHL) who are not adequately treated by tympanoplasty surgery or conventional hearing aids, bone conduction devices (BCDs) offer a valuable rehabilitation option [1], [2]. The unilateral use of BCD, including the bone conduction hearing aid and bone conduction implant (BCI) have all been proven to be effective in improving the hearing and speech recognition abilities in patients with BCHL, with high subjective satisfaction [2], [3]. However, in challenging listening tasks, such as sound localization, the absence of binaural hearing may result in suboptimal performance for unilateral BCD users, as compared to individuals with normal hearing (NH) [4].

Binaural hearing is a crucial auditory function in daily life that allows individuals to achieve optimal speech recognition in complex “cocktail party” situations [5], horizontal spatial hearing [6], and better interactions with their social environment [7]. Binaural hearing is primarily dependent on the accurate processing of binaural hearing cues, interaural level differences (ILD) and interaural time differences (ITD). These cues were mathematically explained in a review by M. Risoud et al. [6], which concluded that ILD is associated with sound signals that have wavelengths shorter than the diameter of the head (>1500 Hz), with its effect becoming almost zero for sounds below 1500 Hz. Conversely, ITD plays a crucial role in sound source localization at frequencies below 1500 Hz, but its accuracy becomes blurred at higher frequencies. Sound source localization, an ability to auditory perceive sound in three dimensions, estimates the location of sound in the azimuth, elevation, and distance. The accuracy of sound localization in azimuth is known to be better than in elevation as well as in distance. Elevation sound and distance highly rely on spectral cues filtered by the pinna, shoulder, and body shape [8], while binaural hearing is fundamental for horizontal localization of sound [9].

However, it is still less clear how bilateral bone conduction (BC) stimulation affects the ability of binaural hearing. For patients with sensorineural hearing loss, conventional hearing aids and cochlea implants only stimulate the cochlea of the hearing-impaired side. Whereas the BCD relies on the skull as a medium for bone-conducted signal transmission and stimulates not only the cochlea with hearing impairment but also the contralateral cochlea, which is referred to as “cross hearing” [10], [11]. This cross-hearing may lead to interference with input from the contralateral ear and disturbances of binaural cues perceived by the bilateral cochlea. Thus, when bilateral BCDs are applicated in patients with BCHL, it is not fully known how the interaction of these signals affects their listening experience in the real world.

Due to concerns over uncertain hearing benefits and financial constraints, the bilateral use of BCDs is not the standard treatment approach for patients with BCHL so far [12]. Existing research on binaural hearing in bilateral BCD users has been limited by small sample sizes and heterogeneous study designs [13], [14], [15]. The current study evaluated binaural hearing of sound localization ability in 16 patients with congenital BCHL under the listening conditions of unaided, unilateral aided, and bilateral aided with a non-invasive adhesive bone conductive device (aBCD). The objective of this study was to demonstrate whether bilateral aBCD fitting can improve spatial hearing in patients with congenital BCHL, providing audiologists and surgeons with valuable insights into designing early rehabilitation strategies and conducting preoperative evaluations for BCIs in these patients.