This study analyzes the researchers’ performance in the field of lichen planus (LP). The data were extracted from the PubMed database with the search parameters: relative growth rate, country-wise distribution, and prevailing vocabulary.

Lichen planus is a chronic inflammatory, immunodependent disease of the skin and mucous membranes [1]. In general structure of dermatological morbidity, LP is 1.5-2.4%, among all diseases of the oral mucosa — 30-35% [2]. Despite the prevalence, only one bibliometric analysis related to LP in the academic literature has been carried out.

Usually, bibliometrics studies focus on a single combination of words. Our approach is based on the study of terms using contextual analysis [3].

PubMed is MEDLINE's public search engine for biomedical literature [4]. PubMed has become a very popular resource among scientists interested in research publications in various biomedical fields [5]. One of the most valuable features of PubMed is users’ ability to restrict search through different settings. Users can target their search by restricting the query to publications that meet various requirements (including the time of publication, type of manuscript, type of research, language, authors’ affiliation, journal, etc.) [6].

Medical Subject Headings (MeSH) for scientometric analysis in the medical context have increased in the last few years. The MeSH terms have been used for identifying hot topics and research areas to map the dynamics of emerging technologies based on co-word analysis [7, 8].

The data for our study were extracted from the PubMed database. The following search keywords were used (search query fragment): “lichen planus” OR “lichen planus pigmentosus” OR “hypertrophic lichen planus” OR ” mucosal lichen planus” OR “annular lichen planus” OR “hypertrophic lichen planus” OR “atrophic lichen planus” OR “ulcerative lichen planus” OR “bullous lichen planus” OR “lichen planus pemphigoid” OR “inverse lichen planus” OR “actinic lichen planus.”

We analyzed articles published from 1880 to 2021. A total of 8173 records for LP and 4775 records for OLP were extracted from the PubMed database for this study (15.01.2021).

The papers from different countries were identified by using the Advanced Search Builder. The number of publications by country was determined by inserting the name/s of the countries in the «affiliation» field in the «advanced» section of the PubMed search engine, for example, (“lichen planus”(MeSH Terms) OR to (“lichen”(All Fields) AND “planus”(All Fields)) OR “lichen planus”(All Fields)) AND Andorra(Affiliation)), (“lichen planus”(MeSH Terms) OR (“lichen”(All Fields) AND “planus”(All Fields)) OR “lichen planus”(All Fields)) AND (USA(Affiliation) OR United States(Affiliation)). The search query for the oral lichen planus was the following: (“lichen planus, oral”(MeSH Terms) OR (“lichen”(All Fields) AND “planus”(All Fields) AND “oral”(All Fields)) OR “oral lichen planus”(All Fields) OR (“oral”(All Fields) AND “lichen”(All Fields) AND “planus”(All Fields))) AND (“0001/01/01”(PubDate): “2020/12/31”(PubDate)).

To calculate the number of publications per country, we searched for publications by authors' affiliation. Then we summarized the number of publications with known affiliation and expressed the number by country as a percentage of the total number of extracted publications. To find the number of publications per million persons or gross domestic product (GDP), we divided the number of publications per country's population (or per GDP).

A special search strategy was developed for the present study. To extract keywords from the titles and abstracts, we used MEDLINE/PubMed database for all years. A set of programs in the python language was developed. The programs extract articles with certain keywords/terms in titles and abstracts and perform statistical terms’ analysis of the MEDLINE/PubMed database records (titles, abstracts, authors, etc).

Based on the terminology statistics of thematic collections, we determined the words and collocations more often found together with the studied terms (the key terms of collection, for example, “lichen planus”) in titles and abstracts by periods.

From the entire database, we identified articles with the term “lichen planus,” breaking them down over the years. The process includes two steps. The first step uses a semantic selection of appropriate terms with a short context size (with the 3 words window). Next, semantic information is used to select the prevalent terms by year.

For further analysis, we chose three periods: 1960-1979s, 1980-1999s, and 2000-2020s. Therefore, we divided all terms into three groups. Each group was processed statistically, highlighting the main keywords and terms by occurrence frequency. Since the periods vary greatly in the total number of publications, the approach was as follows. For the period of 1960-1979, we selected words that occur at least 2 times a year for analysis. For the period of 1980-1999, we selected words occurring at least 3 times a year. For the period of 2000-2020, we selected words occurring at least 5 times a year. If the terms occurred less than five times in the year, they were ignored. In the resulting set of terms, thematic groups were identified, and the occurrence frequency of terms was analyzed.

When researching the trends of concomitant diseases, the search was built according to the scheme: (“lichen planus”(MeSH Terms) OR (“lichen”(All Fields) AND “planus”(All Fields)) OR “lichen planus”(All Fields)) AND (“leukoplakia”(MeSH Terms) OR “leukoplakia”(All Fields)).

Example of the Search query: (LICHEN PLANUS) AND LUPUS included to (“lichen planus”(MeSH Terms) OR (“lichen”(All Fields) AND “planus”(All Fields)) OR “lichen planus”(All Fields)) AND (“lupus vulgaris”(MeSH Terms) OR (“lupus”(All Fields) AND “vulgaris”(All Fields)) OR “lupus vulgaris”(All Fields) OR “lupus”(All Fields) OR “lupus erythematosus, systemic”(MeSH Terms) OR (“lupus”(All Fields) AND “erythematosus”(All Fields) AND “systemic”(All Fields)) OR “systemic lupus erythematosus”(All Fields)).

The research flow chart is presented in (Fig. 1).

Fig. (1). The research flow chart.

Analysis of trends was carried out using our programs.

The first mention of the term “Lichen planus” was noted in 1880. A graphical representation of publication activity revealed the exponential nature of the trend. The magnitude of the accuracy of the approximation (R²) is 0.8388. R² is close to 1, so the selected function accurately approximates the data. A comparative analysis of publication activity over the decades showed that more than 62% of the total publications were published in less than 20 years of the 21st century.

Of the most productive 20 countries in LP research, India stands out in the first place with 17,11% of the total number of publications, followed by the USA with 11,79%, China with 7,86%, Italy with 6,27%, and Iran with 4,64%. Of these countries, four are located in Asia, nine in Europe, three in Near East, two in North America, one in South America, and one in Oceania. So LP is a topic of interest in several regions of the world. LP has a worldwide distribution with no overt racial predisposition.

We distinguish groups of countries according to the number of publications:

• more than 300 – India, United States of America, China, Italy, Iran,
• from 100 to 300 – Brazil, United Kingdom, Japan, Germany, Turkey, Spain, Poland, Canada, Australia, France, Saudi Arabia,
• from 50 to 100 – Taiwan, Romania, Switzerland, Greece, Thailand, Israel, Austria, Belgium, Egypt,
• from 10 to 50 – Finland, Sweden, Portugal, Malaysia, Croatia, Georgia, Mexico, Denmark, Pakistan, South Africa, Singapore, Norway, Hong Kong, Chile, Lebanon, Argentina, Russia, Hungary, Nigeria, Colombia, Ireland, Nepal, Morocco, Iraq, South Korea, Tunisia, Kuwait, New Zealand, Serbia, Sri Lanka, Qatar,
• less than 10 – Bulgaria, Jordan, Indonesia, Sudan, Benin, Oman, Syria, United Arab Emirates, Vietnam, Bangladesh, Bosnia and Herzegovina, Estonia, Tanzania, Yemen, Côte d'Ivoire, Ethiopia, Ghana, Guatemala, Kenya, Mali, Slovenia, Venezuela, Cameroon, Libya, Malta, Philippines, Uruguay etc.

The absolute number of scientific publications does not fully reflect the «scientific situation» in the country since it strongly depends on the size of the population. Barbados ranked first with 34,84 publications per million persons, followed by Switzerland (9,87) and Croatia (9,42).

We also investigated whether the Gross Domestic Product (GDP) influences LP research productivity in all countries by economic groups according to the classification of the International Monetary Fund.

The undisputed leaders among the economically developed countries are Italy, the USA, the UK, Spain, Japan, Germany, and Greece, the countries that entered the top 20 in the total number of publications. Among the countries with economies in transition, China is the undisputed leader. Among the emerging economies, the leaders are India, Iran and Brazil.

In general, we can conclude that not all developed countries conduct research in the field of dermatology. Therefore, the Czech Republic and Iceland had a single publication, 2 articles are from Slovakia and Cyprus, 3 are from Malta, and 4 are from Slovenia.

Countries with economies in transition and countries of the former USSR either do not have publications at all or have very few. There is no publication from Turkmenistan, Belarus, Moldova, Azerbaijan, Tajikistan, or Kyrgyzstan. There are few publications from Kazakhstan, Armenia, Uzbekistan (1 publication); Latvia, Lithuania, Ukraine (2 publications); Estonia (5 publications), Russia (21 publications), and Georgia (33 publications). Mongolia, Albania, and Laos also have no publications. There are few publications from Cambodia (1), North Macedonia (2), Bosnia and Herzegovina (5), and Vietnam (6 publications).

Our approach permits more detailed information about the terms, their characteristics, related factors, and processes. A statistical analysis of the headings and abstracts of articles related to LP made it possible to distinguish the following lexical and semantic groups: “Medical terms related to disease,” “General medical terms,” “General scientific terms,” and “Other vocabulary.” We analyzed the highlighted groups of terms for the periods 1960-1979, 1980-1999, and 2000-2020 (see the methods section). The total number of words selected in 1960-1979 was 664, in 1980-1999 - 4413, 2000-2020 - 11168.

A change in the ratio of terms in titles over time has been found. In each reviewed period, the titles were dominated by specific medical terms (related to the disease). In each period, general scientific terms were in the second place in use, and general medical terms were in the third place. However, the dynamics of the correlation of terms are noted. From the period 1960-1979 to the modern period, there is a clear tendency to reduce the frequency of use of specific terms (by 8.86%), while the number of general medical (by 2.48%) and general scientific terms, especially the latter (by 7.78%), is growing. There are examples of general scientific terms: association, study, case, expression, report, review, analysis, and evaluation. The terms “case” and “study” are consistently popular among this group. Over the past 21 years, the term “review” has been used frequently.

Among the general medical terms over the past 60 years, five common terms remain popular: treatment, patient, clinical, therapy, and disease.

The bibliometric analysis of publications in the field of lichen planus was performed from the period 1880–2020. It is shown that publication activity has increased significantly in the 21st century. At the same time, the growth of publications related to OLP is faster. We have identified the most active 20 countries by the number of publications per 1 million people. The analysis of scientific publication activity on the GDP of countries was carried out. The article contains a lexical analysis of key terms, statistical processing of various terms, and the definition of thematic groups of terms. Based on the analysis of publication activity, terminological trends were calculated for the period from 1960-2020. The correlation between the most popular areas of research in the lichen planus was determined. Recently, special attention has been paid to the problem associated with lichen planus and other diseases such as carcinoma and leukoplakia, which may be a topic for further deeper studies. The growth is noted in publications related to carcinoma and LP, notably for carcinoma and OLP.