## 4 Discussion
### Limitations  
This analysis has different limitations. The search and methods were limited to articles extracted and analyzed from the WoS database. Although it includes all SCI- and SSCI-listed journals, this strategy may have excluded articles present in other datasets (e.g., Scopus). Moreover, a citation search in WOS is not exhaustive (e.g., some subject areas are poorly covered). However, this approach is widely adopted for bibliometric research.  
As [PERSON] et al. [36] recently suggested, the values of articles and metrics should be evaluated given a set of confounders including patterns of citation and variability related to study context and academic disciplines. The main purpose of our study was to present an overview of the topic to stimulate research and bridge scientific gaps. Detailed bibliometric and scientometric research will be needed to focus on the different indicators. Moreover, other software tools such as CiteSpace (which also removes duplicates or publications with an unclear year of publication) can be used for an in-depth visualization [37]. We adopted VOSviewer, as it focuses on visualizations at an aggregate level.  
Other analyses such as the Price's index [38], which is useful for accessing the trend in a specific country or a given discipline, as well as co-citation analysis (bibliographic coupling) were not performed. Since we presented a citation analysis (divided into four periods) and a descriptive analysis of the top-10-cited articles, the rate with which two articles are cited together by other documents is probably not useful for macroscopic description of the phenomenon. Supplementary aspects to be investigated were the impact of open access on the annual trend and quality of articles, the multidisciplinary background of researchers, and the involvement in collaborative networks of organizations.  
The string we used for bibliometric analysis (\"percutaneous electrical nerve stimulation (All Fields) and pain (All Fields)\") yielded a non-exhaustive number of articles with the exclusion of documents of particular interest. However, our analysis focused on the applications of PENS for pain management and other clinical use were intentionally excluded. Moreover, since the exported records from the WOS core collection included the \"full record\", we tried to intercept the keyword \"pain\" in all articles. These limitations have also been mentioned in other bibliometric investigations [13; 14; 29; 37; 38].  
Finally, the increase in publications for our research topic should be put in relation to the general increase of publications in WoS. It was estimated, for example, that the annual growth of publication production in the world is up 4% from 2010 to 2020 with a constant growth trend [39]. Consequently, our results concerning these aspects could be biased and probably not relevant at all because all publications increase during the same timeframe. Moreover, the last two years of the analysis must be carefully evaluated, because, due to publication procedures, not all articles being published in those years are already listed in WoS/accessible, this is especially true for 2022. We have chosen not to exclude the last few years a priori, to provide more extensive information in numerical terms. This implies that the analysis will have to be updated periodically.