## 3 Results  
### Bibliometric Analysis of Publication Output  
A total of 1353 articles on PENS therapy for pain management were published from 1990 to April 2022 in WOS. After the exclusion of 35 papers, 1318 articles were included in the knowledge visualization process [18]. The annual trend of the retrieved articles is shown in Figure 2.  
Over the whole study period (1990-2022), a linear upward trend for the number of annual new publications was observed (Dickey-Fuller test, \(p\) = 0.73). The annual trend investigation (join-point analysis) found two segments (time periods) with different slopes. In the interval 1990-2020, there was a significant increment in the annual percent change (APC) (APC = 6.05: 95%CI 7.4-9.5; * \(p<0\). 01). Concerning the 2021-2022 (April 2022) period, the curve was not significant (APC = \(-35.1\); \(p=0.34\)). (Figure 3). Figure 1: Flowchart of the study. An amount of 795 articles was published in Q-ranked journals (Q1 to Q4). The quartile analysis demonstrated that 45% of the documents (n = 358) were published in Q1 journals (Figure 4). Figure 3: Analysis of the annual trend (join-point analysis). From 1990 to 2022, a linear upward trend was observed (Dickey–Fuller test, \(p\) = 0.73). There was a significant increase in the annual percent change (APC) (APC = 9.39: 95% CI 6.8–12.0; * \(p\) < 0.001) for the interval 1990–2020. In the 2021–2022 (April 2022) period, the APC was not significant (APC = \(-\)6.44; \(p\) = 0.088). Figure 2: Scientific output and trendline on PENS therapy for pain management from 1990 to April 2022 (total n = 864, we did not consider articles without assigned publication years). The linear (stronger line and shadowed bounds) and cubic (lighter dotted line and bounds) trends are shown. Regarding the most productive journals, the journals \"_Pain_\" (IF 7.926; Q1) and \"_Neuro-modulation_\" (IF 3.025; Q3) published 52 and 49 articles, respectively (Table 1). With respect to article types, 307 were clinical investigation articles (186 observational studies, 121 randomized controlled trials, RCTs), 229 were reviews (182 narrative reviews and 47 systematic review/metanalysis) and 53 were case reports. An amount of 131 documents concerned preclinical research (Figure 5). \begin{table}
\begin{tabular}{l c c c} \hline \hline
**Journal** & **Impact Factor**\({}^{\circ}\) & **Best Quartile**\({}^{\circ}\) & **Articles (n)** \\ \hline _Pain_ & 7.926 & Q1 & 52 \\ \hline _Neuromodulation_ & 3.025 & Q3 & 49 \\ \hline _Neuroscience Letters_ & 3.197 & Q3 & 18 \\ \hline _Pain Physician_ & 4.396 & Q1 & 17 \\ \hline _Regional Anesthesia and Pain_ & 5.571 & Q2 & 15 \\ _Medicine_ & 3.708 & Q3 & 15 \\ \hline _Current Pain and Headache Reports_ & 3.904 & Q2 & 14 \\ \hline _Pain Practice_ & 3.079 & Q2 & 14 \\ \hline _Experimental Brain Research_ & 2.064 & Q4 & 11 \\ \hline _European Journal of Pain_ & 3.651 & Q2 & 11 \\ \hline Legend: \({}^{\circ}\) source—Journal Citation ReportsTM 2021. & & \\ \end{tabular}
\end{table} 
Table 1: Metrics of the 10 most productive journals. Figure 4: Quartile analysis (n = 795): 45% (n = 358) were published in Q1 journals; 75% (n = 596) in the first two quartiles (Q1 + Q2). For journals included in different categories (and different quartiles), the best quartile was considered. Abbreviation: Q—quartile. The most relevant topics of the selected papers were \"efficacy\" (n = 212), \"TENS\" (n = 210), and \"mechanisms of action\" (n = 145) (Figure 6). Articles were cited 36,610 times with a mean of 42.4 (SD 83.2) citations per article. Considering four different time intervals (1990-99; 2000-10; 2011-15; >2015), we found a solid heavy left tail for each period. In fact, the median of citations was 23 articles (IQR = [7, 50]) and one-half of the articles were cited less than 23 times in a range of 21.3 years (Figure 7). Figure 5: Article types (n = 1327, one or more types were recorded for each article). Concerning clinical research, we found 186 observational studies (14%) and 121 RCTs (9.1%). Other: meeting abstracts, book chapters, corrections.