Speech perception in quiet.

Speech perception in quiet was tested by presenting the speech signal from the front (S0) in all studies. One study [15] reported no significant difference in performance between OTE (Kanso-1) and BTE (N5). Although there was no significant difference, a mean difference of 2.52% for OTE (Kanso-1) was reported in the Turkish CNC test compared with BTE (N6) [4]. Another study [18] reported no substantial difference in speech perception between the two processor designs (N6 and Kanso-1). Two studies [17, 21] indicated no significant differences in the FMT for OTE (Rondo-1) vs BTE (Opus-2) and in Australian CNC words for OTE (Kanso-1) vs BTE (N6), while comparable results in CUNY tests were found with BTE (Duet) vs OTE (Rondo-1) [20]. Two studies [19, 16] did not report performances in quiet conditions. Interestingly, one study [22] compared the first-generation OTE (Kanso-1) with OTE (Kanso-2) with ForwardFocus ON and found that speech understanding in quiet conditions was matched or improved with ForwardFocus ON in the Kanso-2 compared to the Kanso-1. This highlights the impact of processor generation on speech outcomes and provides caveats for comparisons. However, no comparisons were made between OTE and BTE in quiet conditions in this study. Across studies, findings suggest that OTE and BTE processors provide comparable performance for speech perception in quiet.

Speech perception in noise.

Speech testing in noisy conditions were varied among the studies. The presentation conditions reported on were as follows; S0N0(speech and noise from front), S0NIL(speech from front and noise form ipsilateral side of CI), S0NCL(speech from front and noise from contralateral side of CI), S0N180 (speech from front and noise from back), S180N180 (speech and noise from back).Further, S0N3(speech from the front and 4-talker babble noise from 3 speakers equal to 12-talker babble), S0Nrearhemi (speech from front and 4-independent talkers presented from four locations roving across 7-speakers in the rear hemispheric field).

In S0N0 conditions, where noise and speech were at fixed levels, a significant difference was observed, with OTE (Kanso-1) outperforming BTE (N5) by a mean improvement of 8.1% in sentence scores [15]. Two studies [4, 20] reported no significant mean difference for OTE (Kanso-1) in the Turkish CNC test compared with BTE (N6) and in BKB-SIN tests with BTE (Duet) vs OTE (Rondo-1). In adaptive speech test conditions, where noise is fixed and the speech signal varies, significant improvements for OTE (Kanso-1) over BTE (N5) were reported [15]. However, three studies [4, 16, 19] reported no significant differences in speech scores between OTE and BTE. Notably, in one study [17], the exact placement of the loudspeakers was not clearly described. However, based on the reported results, it is assumed that both the speech and noise were likely coming from the same direction (S0N0), with comparable performance reported for OTE and BTE.

In conditions such as S0NIL, S0NCL, S0N180, and S180N180, mixed responses were reported. One study [4] found that in S0NCL conditions, performance scores were better for both OTE and BTE, with a significant impact on S0N180 and S180N180 conditions for both processors. Another study [18] reported better performance for OTE when switched to adaptive directionality in S0NIL, S0NCL, and S0N180 conditions but did not report on S180N180 conditions. Additionally, one study [19] reported a significant deterioration in performance with OTE (Rondo-1) in S0N180 conditions with no significant effect on S0NIL or S0NCL conditions.

S0N3 and S0Nrearhemi conditions were reported in two studies [21, 22]. Both studies showed an advantage for BTE over OTE during roving test conditions. Additionally, with ForwardFocus enabled for OTE (Kanso-2) and BTE (N7), a statistically significant benefit of 2.3 dB was observed for BTE over OTE [22]. When comparing processor types within Zoom, Beam, and SCAN settings, BTE (N6) performed better than OTE (Kanso-1), with a mean difference in SRT of 1.4 dB, 2.0 dB, and 1.4 dB, respectively.

Only three studies reported sound field aided threshold (SFAT) measurements. Two studies [16, 18] found no significant effect of OTE or BTE speech processors on the perception of the quietest sound in SFAT measurements. However, one study [4] reported that BTE (N6) performed better than OTE (Kanso-1) in SFAT measurements with a difference of 3.3 dB HL. It should be noted that, of the nine studies included in this review, only three studies reported SFAT results, which limits the generalizability of these findings and interpretations.

The findings from these studies suggest that OTE processors exhibit comparable or superior performance in both quiet and noisy conditions.

Medical complications.

Limited data on medical complications associated with BTE and OTE was reported. One study [17] reported instances of redness, irritation, itching, and pain with OTE devices. No other significant medical complications were highlighted in the available data.

Device handling.

Across multiple studies [16, 17, 20–22], recipients consistently favoured OTE processors in terms of device handling. Four studies [4, 15, 18, 19] did not comment on device handling in their outcome measures.

Retention.

Only two studies documented the retention aspect of OTE processors through subjective ratings or user feedback questionnaires. One study [21] indicated that their cohort did not report any retention concerns. In contrast, another study [16] reported that 44% of their recipients experienced significant retention concerns at least once per day. However, none of the studies provided data logging reports regarding the number of unlock events or instances of the processor falling from the head.

Comfort.

User comfort was a recurring finding favouring OTE over BTE across the five studies reported [16, 17, 20–22].

Self-assessment and overall satisfaction.

Overall satisfaction, including a holistic assessment of device performance and user experience, leaned toward favouring OTE processors. Three studies [16, 20, 21] consistently reported recipients expressing higher satisfaction levels with OTE devices, reflecting a wider preference for this design across various measures. However, one study [17] reported that recipients were satisfied with OTE but did not dislike BTE processors. Another study [22] reported that some recipients felt negatively impacted by OTE and preferred to return to their BTE processors.

Studies reported self-assessment of perceived quality of life changes with several questionnaires, including HISQUI (hearing implant sound quality index), SSQ (speech, spatial, and qualities of hearing scale), and SHQ (spatial hearing questionnaire). Three studies reported these measures [16, 17, 20]. HISQUI [17] and SSQ [20] measured separately indicated similarity between OTE and BTE processors. However, one study [16] measured a collective assessment of HISQUI, SSQ, and SHQ and revealed superior outcomes with OTE over BTE recipients.

Quality of studies.

Eight studies were of good quality with one being fair (high risk). The main concern was the acclimatization period used for the studies between the two processors. Most of the studies engaged the use of both processors, either in the same session or being relatively short period of usage. Three studies did not report on details, such as whether follow-up was long enough for outcomes to occur or how outcomes were assessed [15, 17, 19], while two studies [16, 20] lacked information on the selection of a non-exposed group. This again limits the effectiveness of follow up. Selection bias was apparent in two studies due to their limitation of sample and control groups. The quality assessment of all studies is summarized in Table 3.

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Table 3. Quality and risk of bias assessment (Newcastle–Ottawa Scale) criteria.

https://doi.org/10.1371/journal.pone.0318218.t003

Speech perception in quiet.

Included studies indicate that performance in quiet conditions appears relatively comparable between OTE and BTE processors for CI recipients. However, understanding their efficacy in different listening environments is crucial. The majority of studies indicate no significant difference between OTE and BTE processors in quiet settings [4, 17, 18, 21]. Insights from additional studies consistently indicate comparable speech performance in quiet conditions, suggesting that changes in processor placement, from the pinna to the side of the skull, do not significantly impact speech perception in quiet conditions. The comparable performance also highlights improved microphone technology, enabling these devices to detect low-level input signals with fidelity. One study [22] further highlighted that with the latest OTE (Kanso-2) processors, even disabling certain user-controlled features, such as ForwardFocus options, does not degrade speech perception in quiet. It should be noted that the first generation OTE devices did show degraded performance. While performance in quiet conditions is comparable, these observations may not hold true in noisy or other challenging situations [21].

Speech perception in noise.

It is well recognized that CI recipients find it hard to hear in noisy and challenging conditions. Background noise renders speech understanding even more difficult [22]. Regarding speech processor placement and performance in noisy situations, the included studies showed mixed results. One study [18] explored OTE processors with adaptive microphone directionality, providing important insights by indicating improved performance in noise with adaptive features turned on.

Similarly, one study [17] compared OTE (Rondo-1) and BTE (Opus-2) processors, challenging assumptions favouring BTE devices in noisy environments and demonstrating no loss in speech understanding with OTE processor placement. However, these findings should be interpreted cautiously considering the test environment setup and the specific research questions addressed. In contrast, one study [19] examined speech intelligibility in noise with OTE (Rondo-1) and reported that the processor positioned further behind the ear may affect speech perception in challenging acoustic conditions. Furthermore, another study [16] upgraded BTE to OTE for subjects with single-sided deafness and reported that the OTE processor position did not influence outcome measures. In fact, after a period of usage, the majority of the recipients accepted continued use of the OTE device.

The variability in performance observed across studies suggests that the effectiveness of OTE versus BTE processors can be influenced by several factors, including the directionality of noise and specific processor features. For instance, studies [4, 15, 16, 19] that evaluated fixed conditions with speech and noise from the front (S0N0) found mixed results. One study [15] reported a notable advantage for OTE (Kanso-1) over BTE (N5) in sentence scores. As mentioned before, caution must be applied in interpreting these effects based on the generation of processors being compared. For example, one study [15] compared two different generations of speech processor technologies: OTE (Kanso-1) vs BTE (N5). Kanso-1, with SCAN technology and automatic microphone directionality, offered better performance in various conditions compared to N5, which has two omnidirectional microphones.

In more dynamic and challenging noise conditions, such as those where noise is presented from different angles relative to speech (S0NIL, S0NCL, S0N180, and S180N180), the results are less consistent [4, 18]. This variability may reflect the differing abilities of OTE and BTE processors to handle complex noise environments. For instance, adaptive directionality features in OTE processors showed improved performance in certain conditions, suggesting that these features can enhance speech perception in noise for this type of processor.

However, other studies highlighted that BTE devices outperform OTE processors in specific noise configurations, such as S0N180 [19], as well as in roving conditions [21, 22]. The advantage of BTE devices in these conditions suggests they might offer superior performance in highly challenging listening scenarios.

Speech test set-up and performance outcomes.

It must be noted that test set-up and processor technology can impact speech performance scores. As mentioned above, OTEs with dual microphones have shown improved performance scores compared to older generations of speech processing technology [4, 15, 22]. Furthermore, the dual microphone effect is thought to improve the SNR with OTE [4, 15]. Poor performance by OTE and to an extent with BTE in roving conditions still reinforces the need for continuous technological advances in segregating speech from competing environments.

Technologies such as ForwardFocus further improve speech intelligibility by emphasizing sounds coming from the front and reducing noise from other directions. This directional processing can be particularly beneficial in situations where the listener is facing the speaker, helping to enhance the clarity of the target speech [22].

Overall, these findings underscore the complexity of speech perception in noisy environments and suggest that while OTE devices offer competitive performance, BTE processors may have advantages in certain conditions, depending on the experimental set-up. Adaptive features in newer OTE processors (Kanso-2, Rondo-3) provide benefits but do not universally perform as well as BTE devices in all noisy scenarios. Continued research is essential to further understand these relationships and guide optimal CI device selection based on specific listening environments.

Despite this, improved processor technology in newer generations of OTE devices may offer comparable performance, which needs to be further investigated in future studies. For example, the current review did not include any studies comparing the latest BTE devices (Sonnet-2) with newer OTE processors (Rondo-3). It is possible that these newer OTE devices may perform comparably or even be superior to BTE devices, but this requires empirical validation. A comparison of BTE (N8) with OTE (Kanso-2) may already be outdated as technology marches forward.

Sound Field Aided Threshold (SFAT).

The limited number of studies reporting SFAT measurements indicates a need for further research. While some studies found no significant difference in SFAT performance between OTE and BTE, others reported better performance for BTE. This discrepancy highlights the necessity for more comprehensive research to understand the impact of processor type on perception not only in the speech testing condition but also for aided thresholds.

Effect of microphone placement.

The positioning of microphones varies between the two sound processor designs, which is thought to impact speech understanding with OTEs. Findings from the included studies showed comparable speech understanding performance for both sound processor types across different positions in noise [4, 16, 18, 22]. Despite distinct microphone locations for BTE and OTE, the incorporation of dual-microphone features in both types of processors significantly reduced differences, yielding similar outcomes in various noisy conditions [18, 22]. Additionally, the uniformity in speech processing technology, microphone design, and background noise reduction algorithms across both sound processor types may contribute to the similar findings [21, 22].

Evidence from these studies indicate that speech perception in quiet and noise is highly influenced by factors including type of speech processor, microphone technology used, and other noise reduction features incorporated into the processors.

Device handling.

Device handling is a critical non-audiological factor that influences the daily experience of CI recipients. Studies [16, 17, 20–22] comparing different speech processors provide insights into how variations in device design and functionality impact user interactions. Particularly in ease of use for telephone calls and wearing glasses, OTE was preferred over BTE [17], but in cases of SSD, subjects preferred to use the contralateral normal hearing ear for telephone usage [16]. Other studies did not report recipient feedback on telephone usage with OTE versus BTE. It must be noted that OTEs reported in this review did not offer the Bluetooth streaming option for telephones, although newer OTE models are now available with Bluetooth streaming capabilities. Understanding the ergonomics and user-friendliness of CI processors, as revealed by these studies, is essential for optimizing the handling experience and ensuring ease of use for individuals with CI.

Retention.

Retention of CI processors is crucial for user confidence and uninterrupted auditory experiences, especially with the OTE lacking surrounding structures for easy anchoring [23]. One study [16] reported that OTE processors unexpectedly fall off in daily use. Contrary to this, another study [21] found no significant difference in retention between OTE and BTE. Interestingly, some recipients’ initial reliance on retention accessories for OTE decreased over time as users gained confidence. Some recipients felt superior retention with OTE processors in specific situations, such as lying down or bending over, which may provide potential advantages in certain real-world scenarios [4, 21, 22]. These insights into device retention are valuable, guiding considerations for recipients and clinicians concerning device stability and security during daily activities.

Comfort.

Comfort plays a significant role in the long-term acceptance and usability of CI devices. Comfort and retention of OTE processors depend on selected magnet strength, which is influenced by factors such as skin flap thickness and specific retention requirements [16, 21]. To improve comfort, manufacturers provide soft pads to attach beneath the processor, distributing pressure across the processor area and increasing coil-implant separation. Clinical outcomes with OTE processors combined with user comfort help explain how device design and form factor impact the overall wearing experience [21]. Wearing comfort for OTE was further enhanced when used with spectacles compared to BTEs, as well as offering improved comfort over tight-fitting ear moulds with BTEs [16, 17, 20]. These findings reinforce the principle that processor comfort preferences are vital for long-term satisfaction.

Medical complications.

Although not an explicitly stated outcome in the primary studies, only one study reported complications related to OTE devices, such as infections, redness, irritation, itching, and pain over the implant site [17]. Often, these issues could be related to the increased magnet strength chosen for recipients, which clinicians could mitigate. As previously mentioned, skin flap thickness can impact the risk of skin-related irritations with OTE. These issues are also observed in BTE processors, although to a lesser extent.

Overall satisfaction.

Overall satisfaction with OTE compared to BTE encompasses varied aspects of the CI experience. One study [17] reported that more than 70% of participants would highly recommend OTE over BTE after six months of using the processors. Another study [16] reported that 80% of participants favoured the OTE over their previous BTE device after 28 days of usage. Therefore, overall satisfaction depends on a holistic approach to CI assessments, considering not only functional outcomes but also user preferences and satisfaction [20–22].

Effect of device configurations.

The head-shadow and binaural squelch effects are particularly relevant in noisy environments, where spatial separation between target speech and competing sounds significantly enhances speech understanding. Evidence from the included studies indicates that bilateral CIs are superior, especially in noisy and context-dependent situations, due to the substantial binaural benefits they offer [15]. Bilateral cues enhance speech intelligibility and daily life performance more effectively than unilateral use, which often shows poorer results due to the lack of stimulus redundancy. There were no or limited comparisons reported between BTE and OTE under bimodal configurations, limiting the inferences that could be drawn from the available evidence. Only one study [20] reported on the Electroacoustic System (EAS), comparing BTE and OTE with ITE, and found a preference among subjects for OTE over BTE.

Though there is no evidence-based guidance on the use of OTE or BTE from this review, it is essential to tailor recommendations based on the individual needs and circumstances of each recipient when considering different device configurations, such as OTE or BTE in bilateral, unilateral, and bimodal setups. These configurations can significantly impact auditory outcomes and user experiences.

Sound processor technology.

The sound processors included in the review were both BTE and OTE processors from MED-EL® and Cochlear® Ltd, including models like BTE-N7, N6, N5, Opus2, Duet, and OTE-Kanso-1, Kanso-2, Rondo-1. These processors lack some technological advances available to the CI recipients at the time of writing this review, such as hands-free options, enhanced usage in challenging environments, and improved telephone usability, although we have discussed ForwardFocus options for the Kanso-2 OTE.

The authors note that no studies have compared the latest processor technology, such as N8 (BTE) with Kanso-2 (OTE) as well as Sonnet-2 (BTE) with that of Rondo-3, and Rondo-2 (OTE). These newer OTE and BTE models offer significant technological advances, including improved speech processing algorithms and non-audiological factors like lighter weight processors, better retention and Bluetooth streaming options. Empirical evidence in these areas would enhance clinical knowledge and improve recipient satisfaction.