Currently, the primary concern in cochlear reimplantation is the lack of an internationally unified and normative definition. In Jackler et al. (1989) defined cochlear reimplantation as the removal of an indwelling CI electrodes followed by reinsertion of a new device. They noted that reimplantation is a maneuver of uncertain consequences to the cochlea and its surviving nerve (Robert et al., 1989).

However, according to a literature review, most researchers have not clearly defined cochlear reimplantation (Orús et al., 2010; Ciorba et al., 2012). Some articles did not attempt to define cochlear reimplantation at all (Bhadania et al., 2018; Batuk et al., 2019). “Reimplantation/re-implantation” are often used to mean CI reimplantation, and “reinsertion/replacement” are also used in the same context (Desoyer and Burian, 1985; Parisier et al., 1991; Holcomb et al., 2018; Lane et al., 2019). However, there existed confusion and genericity between “revision” and “reimplantation” in many studies (Lassig et al., 2005; Rivas et al., 2008; Hwang et al., 2019). For example, Marlowe et al. proposed that revision surgery is a means to deal with abnormal implantation sites or internal problems with the implanted device (Marlowe et al., 2010). Revision surgery has been defined as the removal of the old device and replacement with a prosthesis, which is similar to the definition of cochlear reimplantation.

However, the procedures performed to treat CI complications were classified as follows: (1) cochlear reimplantation, (2) other revision surgery, and (3) medical treatment (Lescanne et al., 2011; Tarkan et al., 2013). Therefore, we speculate that cochlear reimplantation is characterized by replacing the initial electrodes with brand-new devices, and that revision surgery comprises surgical operations performed to address CI complications. Thus, cochlear reimplantation is a part of revision surgery.

Overall, the first requirement for future studies is to formulate a scientific and rigorous definition of reimplantation to standardize relevant studies and enhance comparability. In this regard, it is imperative to establish a global committee comprising cochlear implant manufacturers, FDA authorities, and clinicians/academicians from a variety of settings to propose an international consensus on cochlear reimplantation to standardize its definition.

The second important issue is the unclear range of cochlear reimplantation. In Jackler et al. (1989) were the first to systematically summarize the following reasons for reimplantation: device failure, flap infections, electrodes mis-insertion or compression, hematoma or trauma at the receiver site, and accidental displacement (Robert et al., 1989). The European Consensus Statement published in 2005 classified the reasons as device failure, medical reason, characteristic decrement, and performance decrement (No authors listed, 2005). Device failure can be divided into hard and soft failures based on whether the failure can be proven with in vivo tests (Balkany et al., 2005). This was the first unified document regarding cochlear reimplantation. However, the 2005 consensus did not clearly propose a definition and scope of cochlear reimplantation and lacked procedures and tools for screening and the classification of the reasons.

In Battmer et al. (2010) published the International Classification of Reliability for Implanted Cochlear Implant Receiver Stimulators, which adopted a similar framework as the 2005 European Consensus. This framework illustrated some details, such as the CI survival time, (reduced) clinical benefit, and specification (Battmer et al., 2010). In 2017, the Association for the Advancement of Medical Instrumentation (AAMI) updated and standardized the classification of explanted CIs (Zwolan and Verhof, 2017) and proposed the following four categories: medical reasons, non-medical reasons, inconclusive/no faults found, and combined reasons. Most studies have adopted these three classification categories (Hermann et al., 2020; Layfield et al., 2021). However, there is no unified research range for cochlear reimplantation.

According to a literature review, most studies regarded the following situations as reimplantation: (1) removal of the initial electrodes for various reasons and implantation of a new device on the ipsilateral or contralateral side, which was the most common scenario defined as reimplantation (Lassig et al., 2005); (2) replacement of the failed hybrid CI with short electroacoustic stimulation (EAS) electrodes or full-length electrodes (Kamat et al., 2011; Jayawardena et al., 2012; Dunn et al., 2015); (3) reinsertion of the initial electrodes into the cochlea in cases of device migration (Luo et al., 2020); (4) reinsertion of the initial electrodes into the cochlea on the operation day or within a few days of the operation due to the electrodes in incorrect insertion places, such as the internal auditory canal, eustachian tube, vestibul. (Gözen et al., 2019); and (5) simultaneous implantation on both sides when patients with unilateral CI accept reimplantation surgery (Tang et al., 2019).

However, these classifications are not entirely appropriate. We consider that the first kind falls in the range of cochlear reimplantation, which replaced the initial device with a brand new one. And the second one can be regarded as a sort of reimplantation as well, which is relatively rare. For the third and fourth definitions, no new devices were inserted into the cochlea; thus, they cannot be regarded as reimplantation. For the fifth definition, ipsilateral implantation with a new device can be considered as reimplantation; however, contralateral implantation was the first CI, not the reimplanted CI. In conclusion, future studies should propose a more precise scope and classification of cochlear reimplantation.

The third important issue is the calculation formula of CI reimplantation rate, which varies greatly across studies. In Battmer et al. (2010) proposed the definition and calculation of the cumulative survival rate (CSR), which was in accordance with the methodology outlined in ISO standard 5841-2:2000 and targeted device failure without accounting for the medical reasons. In 2017, AAMI published the definition and calculation of the cumulative removal percentage (CRP), which covered all explanted CIs (Zwolan and Verhof, 2017). However, the formulation of CRP is not applicable to cochlear reimplantation rate. Cochlear removal mainly refers to the explantation of existing devices due to device failure, medical reasons, and inconclusive defects. In contrast, cochlear reimplantation is defined as the explantation of the initial device, followed by implantation of new electrodes. However, not all patients undergoing explantation of ordinary devices received new devices, and in some cases, the failed device was left in situ, while the new device was implanted on the contralateral side. These issues highlight the importance of standardizing the calculation formulation for the CI rate based on the CRP.

According to a literature review, some studies have used the number of patients as the unit of measurement, regardless of whether they had undergone unilateral and bilateral implantation (Sterkers et al., 2015), Dotú et al. (2010), whereas others have used the CI number as the unit of measurement Googe and Carron (2016); Karamert et al. (2019). Regarding the formula used for calculation, some studies adopted either of the following formula: “rate = no. of reimplanted CIs/no. of total CIs” or “rate = no. of reimplanted CIs/no. of primary CIs,” in which primary CIs refer to devices inserted into the cochlea for the first time. Thus, the reimplantation rate may be different even for the same batch of patients.

Based on published studies, the CI reimplantation rate ranged from 0.5 to 30% (Beadle et al., 2005; Qiu et al., 2010), and a few studies have reported reimplantation rates higher than 20%. This high reimplantation rate may be attributed to the early publishing time in which the CI surgeries and devices were immature, small sample size, data bias, and immature surgical technology (Hamzavi et al., 2003; Beadle et al., 2005; Kanchanalarp et al., 2005). Patients of different ages with CIs showed different reimplantation rates. The reimplantation rate in children ranged from 0.7 to 30.0% (Kanchanalarp et al., 2005; Sun, 2019), whereas that in adults ranged from 0.4 to 27.3% (Hamzavi et al., 2003; Dağkıran et al., 2020). Some studies have reported that the reimplantation rate in children was significantly higher than that in adults (Sunde et al., 2013; Dağkıran et al., 2020). Children are prone to falls, resulting in head trauma during rapid growth and development (Weise et al., 2005; Wang et al., 2014). The skull and mastoid of children are immature, and rapid growth of the skull can lead to electrode array migration. In addition, the high prevalence of various types of otitis media in children increases the risk of CI failure (Manrique-Huarte et al., 2015). However, some studies have demonstrated no significant differences in the revision rates due to infection complications and device failure rates between adults and children (Sunde et al., 2013; Distinguin et al., 2017). The number of studies that separately calculated the reimplantation rates for children and adults is relatively low; therefore, larger longitudinal cohort studies are required.

The fourth important issue is that the current research mainly focuses on cause analysis and treatment complications. Thus, studies on postoperative effect assessment and the related methods remain limited. Hochmair-Desoyer and Burian (1985) used Bekesy threshold tracking, loudness scaling, open lists of unknown single-syllable words and everyday sentences, and other tests to evaluate the effects of reimplantation in their initial report (Desoyer and Burian, 1985). Since then, almost half of the studies related to reimplantation have mentioned effect evaluations using assessments, such as the Bamford–Kowal–Bench (BKB) test, phonetically balanced kindergarten (PBK) test, Northwestern University number 6 (NU#6) test, common phrases test (CPT), lexical neighborhood test (LNT), categories of auditory performance (CAP), and the speech intelligibility rating (SIR) (Saeed et al., 1995; Beadle et al., 2005; Marlowe et al., 2010; Bhadania et al., 2018). However, these studies have primarily focused on hearing thresholds and speech recognition, and no new assessment methods for neuro-electrophysiological monitoring and evaluation of neurofunctional characteristics have been proposed.

Nevertheless, some new technologies have been demonstrated to be applicable to patients with hearing loss or CIs, such as functional near-infrared optical brain imaging (fNIRS) and electroencephalography (EEG). In Sevy et al. (2010) used NIRS and fMRI to examine the cortical activity in response to auditory stimuli in five children with CIs and five children with normal hearing in Boston Children’s Hospital. In Wang et al. (2020) adopted EEG to evaluate the effect of CIs at three time points after surgery and showed that multiple EEG indices could be used to assess speech perception ability. Many studies have demonstrated that neurological imaging is a safe and feasible approach for the examination of children with CIs, and that it could be an effective method for assessing the effects of reimplanted CIs. Therefore, if these advanced tools could be adopted for the general evaluation of both the first and reimplanted CIs, a better comparison and evaluation of the effect and the regularity of the rehabilitation of cochlear reimplantation can be achieved. In addition, results can be compared between patients with reimplanted CIs and the normal-hearing population, and differences can be used to evaluate the rehabilitation effect.

Regarding the actual effects of reimplantation, Chung et al. (2010) reported that the pure tone audiometry (PTA) and speech recognition scores of all patients who underwent reimplantation for soft failure decreased after reimplantation. Manrique-Huarte et al. (2015) used aided free-field auditory tests. Aided PTA findings after reimplantation improved in 44.4% of patients, deteriorated in 44.4%, and showed no significant difference in 11.1%, whereas speech recognition scores improved in 63.6%, showed no significant change in 9.1%, and worsened in 27.3%. In a study on the performance of 56 children 18 months after cochlear reimplantation, 87% showed better speech perception, 10% reported similar results, and 3% showed worse speech perception after surgery (Marlowe et al., 2010). Younger children were more likely to achieve or exceed their previous best performance than older children. The age at reimplantation, interval between the initial implantation and reimplantation, auditory input during the interval, depth of the electrode array, device activation, and device upgrade also influenced the postoperative effects (Marlowe et al., 2010; Lenarz, 2017; Roßberg et al., 2021). However, no single study has systematically discussed these factors. In addition, only a few effect evaluations have been carried out, with effect evaluation analysis not performed in some large-scale studies. Therefore, future studies should utilize more advanced assessment tools, such as functional near-infrared spectroscopy and electroencephalography, and should take more factors into consideration to systematically assess the rehabilitation effect and explore the internal patterns.

Most studies on cochlear reimplantation were single-center studies with a relatively small sample size, and multicenter studies only accounted for 6% (Chung et al., 2010; Hermann et al., 2020). Therefore, the use of rigorous and advanced statistical analysis methods to obtain more robust findings is difficult. Moreover, in the absence of a standard criterion for the selection of the reimplantation side, most studies selected the side based on infection, cochlear ossification, and deformity (Chung et al., 2010; Lu and Cao, 2014; Manrique-Huarte et al., 2015), without considering other influencing factors, such as the duration between the first implantation and reimplantation and continuous auditory input during the period. Thus, integration of the patient resources of several hospitals and multicenter studies should be performed to elevate the level of evidence on this topic.