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Citation: Lörscher T, Su N, Faggion CM Jr
(2025) The assessment of language restrictions
in abstracts of systematic reviews in dentistry:
A meta-research study. PLoS One 20(5):
e0323176. https://doi.org/10.1371/journal.
pone.0323176
Editor: Andrej M Kielbassa, Danube Private
University, AUSTRIA
Received: November 11, 2024
Accepted: April 3, 2025
Published: May 20, 2025
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Funding: The author(s) received no specific
funding for this work.
RESEARCH ARTICLE
The assessment of language restrictions in
abstracts of systematic reviews in dentistry: A
meta-research study
Tatjana Lörscher 1, Naichuan Su2, Clovis Mariano Faggion Jr.3*
1 Department of Periodontology and Operative Dentistry, University Hospital Münster, Münster, Germany,
2 Department of Oral Public Health, Academic Centre for Dentistry Amsterdam (ACTA), University
of Amsterdam and Vrije Universiteit Amsterdam, Amsterdam, The Netherlands, 3 Department of
Periodontology and Operative Dentistry, University Hospital Münster, Münster, Germany
* clovis.faggion@ukmuenster.de, clovisfaggion@yahoo.com
Abstract
Background
The adequate interpretation of findings in systematic reviews can be affected by the
lack of information on the language of the examined studies. The study sought to
assess the reported information on restrictions set on the language of primary studies
examined in systematic reviews published in dentistry. The study also investigated
associations between the characteristics of the systematic reviews and language
restrictions.
Methods
A comprehensive search was conducted in the Web of Science database for sys-
tematic reviews in the field of dentistry. Abstracts published from the inception of
the database up to 24 February 2023 were included and relevant information was
extracted. Only abstracts published in English were included. Logistic regression
analyses were performed to examine the association between the characteristics
of the systematic reviews and the presence of language restrictions. Additionally, a
random sample of 9.2% of the full texts was reviewed to identify differences in the
reporting of language restrictions between the abstract and the full texts.
Results
A total of 3922 abstracts were initially retrieved, and 3465 abstracts were included
in the analysis based on the eligibility criteria. Approximately 79% (2739) did not
report any language information. Only 7% (238) of the abstracts declared no lan-
guage restrictions in the primary studies selected. Meta-analysis conducted, journal
PLOS One | https://doi.org/10.1371/journal.pone.0323176 May 20, 2025 2 / 15
type, reporting of primary study design, actual number of words in abstracts and the
country and continent of first authors affiliation were statistically significantly associ-
ated with language restrictions of the systematic reviews. However, the absence of
information about language restriction appears to be a poor indicator of reporting or
not language restriction in the full-text of the article.
Conclusions
Abstracts of systematic reviews in dentistry frequently underreport language restric-
tions applied to the primary studies examined. Various characteristics of systematic
reviews are significantly associated with these restrictions, highlighting inconsisten-
cies in reporting practices.
Introduction
An abstract should receive the same attention (or even more) as the full text of a
scientific article regarding the completeness of the reporting. The relevance and
value of a complete abstract have likely risen with the increasingly large amount
of literature that is being regularly published. It has been estimated that nearly 80
systematic reviews are indexed in PubMed every day [1]. Hence, for busy readers,
such as clinicians, it is challenging to remain updated about a specific speciality by
reading the full text of all relevant articles. Some evidence suggests that many cli-
nicians, for example, likely only read the abstracts of many scientific articles to stay
current (with further reading being more selective). A study assessing the reading
habits of internists with and without epidemiological training reported that they read
only the abstracts of up to 63% of the articles from the nine medical journals they
read regularly [2].
Several guidelines have been published to help researchers report their studies,
and a guideline for reporting abstracts of systematic reviews has also been pub-
lished [3]. The PRISMA 2020 extension for Abstracts suggests 12 items that should
be reported in any abstract of systematic reviews. This does not include specific
guidance on how to report information on the language of the articles that are exam-
ined in a review or on how this has been applied as part of the search strategies or
eligibility criteria [4].
One can consider that some sort of language bias [5] may occur when there is a
restriction in the search regarding the number of languages chosen or the particu-
lar languages selected. It is important to the reader of the abstract of a systematic
review that the information provided is detailed enough to allow an adequate interpre-
tation of the methodology and results.
The aims of this meta-research study were twofold: (a) to investigate the reporting
of restrictions set in dental systematic review abstracts regarding the languages of
the examined primary studies and (b) to investigate potential associations between
the characteristics of the systematic reviews and language restrictions.
Competing interests: The authors have
declared that no competing interests exist.
PLOS One | https://doi.org/10.1371/journal.pone.0323176 May 20, 2025 3 / 15
Materials and methods
Study design
This was a cross-sectional study of systematic reviews. The abstracts were analyzed plus a random sample of 9.2% of the full
texts. Some passages in this paper use the terms “abstract” and “systematic review” interchangeably to facilitate understanding.
Eligibility criteria
Inclusion criteria. Abstracts of systematic reviews, with or without meta-analyses of primary studies, in the field of
dentistry were included in the analysis. To be included, a systematic review must have oral health as its primary focus,
excluding those centered on related or peripheral fields. Protocols of systematic reviews were also included. Other types
of reviews considered systematic such as, for example, scoping reviews were included if the key-word “systematic” was
reported in the title of the study. No restrictions were applied regarding the species examined or the type of research. Only
abstracts published in English were included.
Exclusion criteria. Abstracts of systematic reviews from other medical disciplines that did not focus on oral health
topics were excluded. Additionally, other study designs, including primary studies, comments, and statements, as well as
secondary research studies such as reviews of reviews (commonly referred to as umbrella reviews), were also excluded.
Studies that identified themselves as systematic reviews but included both reviews and primary studies were excluded.
Additionally, if it was unclear from the abstract whether primary studies were included, the study was also excluded to
maintain consistency in the selection process.
Search strategy
A search was conducted in the Web of Science (WoS) database from its inception up to 24 February 2023.
The search used the Medical Subject Headings (MeSH) terms from the United States National Library of Medicine,
together with Boolean operators (see supplementary file).
Data selection
The documents retrieved from the database were exported to an Excel worksheet for data selection and extraction. Titles
and abstracts were assessed for eligibility according to the predefined inclusion criteria. Abstracts that did not meet the
criteria were excluded, and the specific reason for exclusion was recorded.
Two assessors (TL and CMF) selected a sample of 10% of the eligible studies and made independent inclusion deci-
sions (achieving good agreement, 90 per cent), with the remainder selected by one reviewer (TL) [6].
Data extraction
The following data were extracted from the abstracts: (a) language restriction (no language restriction, restricted to some
languages, restricted to English only or no information about language); (b) topic of the review (based on the classification
of dental specialties by the National Commission on Recognition of Dental Specialties and Certifying Boards [NCRDSCB]
of the American Dental Association) [7]; (c) structure of abstract (structured or not structured); (d) type of systematic review
(intervention or non- intervention); (e) meta-analysis conducted (yes or no); (f) journal type (dental or other); (g) reporting of
primary study design (reported or not reported); (h) word limit of abstract (≤250 words or >250 words) based on the informa-
tion reported in the author guidelines of the journals; and (h`) actual number of words in the abstracts. Additionally, informa-
tion automatically retrieved from the Web of Science database was compiled, including: (i) author names; (j) article title; (k)
author affiliations; (l) number of citations (WoS Core); (m) publisher; (n) country of the affiliation of the first author (developing
or developed); and (o) continents (North America, South America, Europe, Asia, Africa or Oceania).
PLOS One | https://doi.org/10.1371/journal.pone.0323176 May 20, 2025 4 / 15
Two assessors (TL and CMF) extracted a sample of 10% of the eligible studies and achieved good agreement on the
information accuracy (85 per cent), with the remainder extracted by one reviewer (TL) [6].
Data analysis
Descriptive statistics were used to summarise the characteristics of the included systematic reviews (i.e., proportions were
used for categorical variables; median and interquartile range [IQR] were used for the continuous variables [i.e., actual
number of words in abstract and number of citations (WoS Core)], because those two variables were not normally distrib-
uted based on Kolmogorov-Smirnov test). To assess the association between the characteristics of the systematic reviews
(independent variables) and the language restrictions (outcome variable), multinominal logistic regression analysis was
used because the outcome variable included four categories (i.e., no language restriction [reference category], restricted
to some languages, restricted to English only and no information). To assess the association between the characteris-
tics of the systematic reviews (independent variables) and the number of languages included (outcome variable), binary
logistic regression analysis was used because the outcome variable was dichotomised (i.e., only one language [reference
category] and more than one language). In each logistic regression analysis, univariable analysis was first performed
to separately assess the association of each independent variable with the outcome variables. Second, a multivariable
regression analysis was performed for each outcome variable. In the multivariable regression analyses, only the indepen-
dent variables with a p-value < .05 in the univariable analyses were included. SPSS Statistics for Windows (version 28.0;
IBM, Armonk, NY, USA) was used to perform the statistical analyses.
Assessment of language restriction between abstracts and full texts
To understand potential differences in reporting of language restriction between abstracts and full texts, a random sample
of full texts of these abstracts (9.2%) were scrutinized for language restriction.
Results
Study selection
A total of 3922 abstracts were initially included. 8 duplicate records have been removed and one that wasn’t fully avail-
able. After assessing the abstracts, 448 were excluded because they did not meet the eligibility criteria. Therefore, infor-
mation from 3465 abstracts of systematic reviews was extracted and included in this study. Detailed information on the
selection process is reported in Fig 1.
From the 3465 entries in our database, a sample 347 was selected for full text assessment. This sample was selected
randomly using the online Research Randomizer Software (version 4.0; Lancester, Pennsylvania, USA) [8]. Of the 347
selected publications only 319 were accessible for analysis (9.2% of full dataset).
Study characteristics
Most abstracts (n = 2739, 79%) did not report any information about the language of the studies or related to the search
strategies, while the remaining 726 (21%) abstracts reported information about a language restriction. Among the 726
abstracts, the number of abstracts with no language restriction, restriction to some languages and restriction to English only
was 238, 68 and 420, respectively. For the group of abstracts that reported the number of included languages (n = 726), 422
(58%) focused on only one language. The main characteristics of the included systematic reviews are presented in Table 1.
Association between the characteristics of systematic reviews and language restrictions
Restricted to some languages versus no language restriction. In the univariable analysis, meta-analysis
conducted (P = .015) and journal type (P = .009) were significantly associated with systematic reviews restricted to some
PLOS One | https://doi.org/10.1371/journal.pone.0323176 May 20, 2025 5 / 15
languages in comparison with systematic reviews with no language restriction (Table 2). In the multivariable analysis,
meta-analysis conducted (P = .044), journal type (P = .040), countries (P = .044) and continents (P = .017 for South America)
were significantly associated with systematic reviews restricted to some languages in comparison with systematic reviews
with no language restriction (Table 3).
Restricted to English only versus no language restriction
In the univariable analysis, topics of reviews (P < .001) for orthodontics and dentofacial orthopaedics), meta-analysis
conducted (P < .001), reporting of primary study design (P = .027), publisher (P = .033 for Elsevier), countries (P = .001)
Fig 1. Flowchart of the search and selection process of abstracts of systematic reviews published in dentistry.
https://doi.org/10.1371/journal.pone.0323176.g001
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Table 1. General characteristics of the included systematic reviews and their distributions over language restrictions (N = 3465) and number
of included languages (N = 726) based on the abstracts.
Language restrictions (N = 3465) Number of included languages (N = 726)
Total
(N = 3465)
No language
restriction
(N = 238)
Restricted
to some
languages
(N = 68)
Restricted to
English only
(N = 420)
No informa-
tion (N = 2739)
Total
(N = 726)
Only 1
language
(N = 422)
>1 language
(N = 304)
Topics of the reviews
Dental public health 326 (9.4%) 15 (4.6%) 9 (2.8%) 41 (12.6%) 261 (80.1%) 65 (9.0%) 41 (63.1%) 24 (36.9%)
Endodontics 171 (4.9%) 12 (7.0%) 2 (1.2%) 22 (12.9%) 135 (78.9%) 36 (5.0%) 22(61.1%) 14 (38.9%)
Oral and maxillofacial
radiology
27 (0.8%) 1 (3.7%) 0 (0.0%) 6 (22.2%) 20 (74.1%) 7 (1.0%) 6 (85.7%) 1 (14.3%)
Oral and maxillofacial
surgery
332 (9.6%) 12 (3.6%) 4 (1.2%) 32 (9.6%) 284 (85.5%) 48 (6.6%) 32 (66.7%) 16 (33.3%)
Oral medicine 61 (1.8%) 5 (8.2%) 3 (4.9%) 7 (11.5%) 46 (75.4%) 15 (2.1%) 7 (46.7%) 8 (53.3%)
Orofacial pain 138 (4.0%) 13 (9.4%) 5 (3.6%) 23 (16.7%) 97 (70.3%) 41 (5.6%) 24 (58.5%) 17 (41.5%)
Orthodontics and den-
tofacial orthopaedics
349 (10.1%) 49 (14.0%) 6 (1.7%) 32 (9.2%) 262 (75.1%) 87 (12.0%) 32 (36.8%) 55 (63.2%)
Pediatric dentistry 122 (3.5%) 10 (8.2%) 1 (0.8%) 14 (11.5%) 97 (79.5%) 25 (3.4%) 14 (56.0%) 11 (44.0%)
Periodontics 461 (13.3%) 23 (5.0%) 7 (1.5%) 46 (10.0%) 385 (83.5%) 76 (10.5%) 46 (60.5%) 30 (39.5%)
Prosthodontics 1165 (33.6%) 78 (6.7%) 24 (2.1%) 155 (13.3%) 908 (77.9%) 257 (35.4%) 156 (60.7%) 101 (39.3%)
Others 313 (9.0%) 20 (6.4%) 7 (2.2%) 42 (13.4%) 244 (78.0%) 69 (9.5%) 42 (60.9%) 27 (39.1%)
Structure of abstract
Structured 2449 (70.7%) 183 (7.5%) 48 (2.0%) 328 (13.4%) 1890 (77.2%) 559 (77.0%) 330 (59.0%) 229 (41.0%)
Not structured 1016 (29.3%) 55 (5.4%) 20 (2.0%) 92 (9.1%) 849 (83.6%) 167 (23.0%) 92 (55.1%) 75 (44.9%)
Types of systematic
reviews
Intervention 2082 (60.1%)
155 (7.4%) 38 (1.8%) 251 (12.1%) 1638 (78.7%) 444 (61.2%) 252 (56.8%) 192 (43.2%)
Non-intervention
1383 (39.9%) 83 (6.0%) 30 (2.2%) 169 (12.2%)
1101 (79.6%) 282 (38.8%) 170 (60.3%) 112 (39.7%)
Meta-analysis
conducted
Yes 1323 (38.2%) 95 (7.2%) 16 (1.2%) 105 (7.9%) 1107 (83.7%) 216 (29.8%) 106 (49.1%) 110 (50.9%)
No 2142 (61.8%) 143 (6.7%) 52 (2.4%) 315 (14.7%) 1632 (76.2%) 510 (70.2%) 316 (62.0%) 194 (38.0%)
Journal type
Dental 2393 (69.1%) 182 (7.6%) 41 (1.7%) 300 (12.5%) 1870 (78.1%) 523 (72.0%) 302 (57.7%) 221 (42.3%)
Other 1072 (30.9%) 56 (5.2%) 27 (2.5%) 120 (11.2%) 869 (81.1%) 203 (28.0%) 120 (59.1%) 83 (40.9%)
Reporting of primary
study design
Reported 2061 (59.5%) 183 (8.9%) 46 (2.2%) 289 (14.0%) 1543 (74.9%) 518 (71.3%) 290 (56.0%) 228 (44.0%)
(Continued)
PLOS One | https://doi.org/10.1371/journal.pone.0323176 May 20, 2025 7 / 15
and continents (P = .037 for North America, P < .001 for South America, P < .001 for Asia and P = .007 for Oceania) were
significantly associated with systematic reviews restricted to English only in comparison with systematic reviews with no
language restriction (Table 2).
In the multivariable analysis, meta-analysis conducted (P < .001) and continents (P = .032 for North America and
P = .007 for Oceania) were significantly associated with systematic reviews restricted to English only in comparison with
those with no language restriction (Table 3).
Language restrictions (N = 3465) Number of included languages (N = 726)
Total
(N = 3465)
No language
restriction
(N = 238)
Restricted
to some
languages
(N = 68)
Restricted to
English only
(N = 420)
No informa-
tion (N = 2739)
Total
(N = 726)
Only 1
language
(N = 422)
>1 language
(N = 304)
Not reported 1404 (40.5%) 55 (3.9%) 22 (1.6%) 131 (9.3%) 1196 (85.2%) 208 (28.7%) 132 (63.5%) 76 (36.5%)
Word limit of
abstracts
<or = 250 2086 (60.2%) 137 (6.6%) 35 (1.7%) 230 (11.0%) 1684 (80.7%) 402 (55.4%) 232 (57.7%) 170 (42.3%)
> 250 1379 (39.8%) 101 (7.3%) 33 (2.4%) 190 (13.8%) 1055 (76.5%) 324 (44.6%) 190 (58.6%) 134 (41.4%)
Actual number of
words in abstracts*
242 (206-278) 248 (215-298) 251 (227-289) 249 (221-294) 239 (203-274) 249 (218-294) 249 (220-294) 248 (217-295)
Number of citations
(WoS Core)*
10 (2-30) 11 (3-32) 10 (3-27) 11 (2-35) 10 (2-30) 11 (3-32) 11 (2-34) 11 (3-31)
Publisher
Elsevier 381 (11.0%) 36 (9.4%) 11 (2.9%) 35 (9.2%) 299 (78.5%) 82 (11.3%) 35 (42.7%) 47 (57.3%)
Springer 268 (7.7%) 11 (4.1%) 7 (2.6%) 24 (9.0%) 226 (84.3%) 42 (5.8%) 24 (57.1%) 18 (42.9%)
Wiley 785 (22.7%) 49 (6.2%) 10 (1.3%) 104 (13.2%) 622 (79.2%) 163 (22.5%) 104 (63.8%) 59 (36.2%)
Sage 72 (2.1%) 4 (5.6%) 3 (4.2%) 5 (6.9%) 60 (83.3%) 12 (1.7%) 5 (41.7%) 7 (58.3%)
MDPI 324 (9.4%) 12 (3.7%) 5 (1.5%) 34 (10.5%) 273 (84.3%) 51 (7.0%) 34 (66.7%) 17 (33.3%)
Taylor & Francis 79 (2.3%) 2 (2.5%) 0 (0.0%) 7 (8.9%) 70 (88.6%) 9 (1.2%) 7 (77.8%) 2 (22.2%)
Others 1556 (44.9%) 124 (8.0%) 32 (2.1%) 211 (13.6%) 1189 (76.4%) 367 (50.6%) 213 (58.0%) 154 (42.0%)
Countries
Developing 1504 (43.4%) 132 (8.8%) 35 (2.3%) 178 (11.8%) 1159 (77.1%) 345 (47.6%) 180 (52.2%) 165 (47.8%)
Developed 1957 (56.6%) 106 (5.4%) 32 (1.6%) 242 (12.4%) 1577 (80.6%) 380 (52.4%) 242 (63.7%) 138 (36.3%)
Continents
North America 368 (10.6%) 20 (5.4%) 4 (1.1%) 63 (17.1%) 281 (76.4%) 87 (12.0%) 63 (72.4%) 24 (27.6%)
South America 565 (16.3%) 89 (15.8%) 16 (2.8%) 28 (5.0%) 432 (76.5%) 133 (18.3%) 28 (21.1%) 105 (78.9%)
Europe 1404 (40.5%) 80 (5.7%) 27 (1.9%) 137 (9.8%) 1160 (82.6%) 244 (33.7%) 137 (56.1%) 107 (43.9%)
Asia 880 (25.4%) 37 (4.2%) 13 (1.5%) 148 (16.8%) 682 (77.5%) 198 (27.3%) 150 (75.8%) 48 (24.2%)
Africa 58 (1.7%) 6 (10.3%) 4 (6.9%) 8 (13.8%) 40 (69.0%) 18 (2.5%) 8 (44.4%) 10 (55.6%)
Oceania 186 (5.4%) 6 (3.2%) 3 (1.6%) 36 (19.4%) 141 (75.8%) 45 (6.2%) 36 (80.0%) 9 (20.0%)
*median and interquartile range was used
https://doi.org/10.1371/journal.pone.0323176.t001
Table 1. (Continued)
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Table 2. Univariable multinominal logistic regression analysis for the association between characteristics of included systematic reviews and
language restrictions (N = 3465) based on the abstracts.
Restricted to some languages vs.
No language restriction (ref.)
Restricted to English only vs. No
language restriction (ref.)
No information vs. No language
restriction (ref.)
Variables B OR (95%CI) P B OR (95%CI) P B OR (95%CI) P
Topic of the reviewA
Dental public health 0.54 1.71 (0.52 5.65) 0.376 0.26 1.30 (0.59 2.89) 0.516 0.36 1.43 (0.71 2.85) 0.315
Endodontics -0.74 0.48 (0.09 2.68) 0.400 -0.14 0.87 (0.36 2.11) 0.763 -0.08 0.92 (0.44 1.94) 0.831
Oral and maxillofacial radiology
Oral and maxillofacial surgery
-17.66
-0.05
-
0.95 (0.23 3.95)
-
0.946
1.05
0.24
2.86 (0.32 25.35)
1.27 (0.54 2.97)
0.346
0.582
0.49
0.66
1.64 (0.21 12.86)
1.94 (0.93 4.05)
0.638
0.078
Oral medicine 0.54 1.71 (0.32 9.11) 0.527 -0.41 0.67 (0.19 2.36) 0.530 -0.28 0.75 (0.27 2.11) 0.591
Orofacial pain 0.09 1.10 (0.29 4.21) 0.891 -0.17 0.84 (0.36 2.00) 0.697 -0.49 0.61 (0.29 1.28) 0.191
Orthodontics and dentofacial -1.05 0.35 (0.11 1.17) 0.088 -1.17 0.31 (0.16 0.62) <0.001*-0.83 0.44 (0.25 0.76) 0.003*
orthopaedics
Pediatric dentistry -1.25 0.29 (0.03 2.65) 0.271 -0.41 0.67 (0.25 1.76) 0.413 -0.23 0.80 (0.36 1.76) 0.795
Periodontics -0.14 0.87 (0.26 2.91) 0.820 -0.05 0.95 (0.46 1.98) 0.896 0.32 1.37 (0.74 2.55) 0.318
Prosthodontics -0.13 0.88 (0.33 2.33) 0.796 -0.06 0.95 (0.52 1.72) 0.856 -0.05 0.95 (0.57 1.59) 0.857
Others Ref. Ref. Ref.
Structure of abstract
Structured -0.33 0.72 (0.40 1.32) 0.288 0.07 1.07 (0.73 1.57) 0.722 -0.40 0.67 (0.49 0.91) 0.012*
Not structured Ref. Ref. Ref.
Type of systematic review
Intervention
Non-intervention
-0.39
Ref.
0.68 (0.39 1.17) 0.165 -0.23
Ref.
0.80 (0.57 1.11) 0.174 -0.23
Ref.
0.80 (0.60 1.05) 0.108
Meta-analysis conducted
Yes -0.77 0.46 (0.25 0.86) 0.015*-0.69 0.50 (0.36 0.71) <0.001*0.02 1.02 (0.78 1.34) 0.880
No Ref. Ref. Ref.
Journal type
Dental -0.76 0.47 (0.26 0.83) 0.009*-0.26 0.77 (0.53 1.11) 0.161 -0.41 0.66 (0.49 0.90) 0.009*
Other Ref. Ref. Ref.
Reporting of primary study design
Reported -0.47 0.63 (0.35 1.13) 0.123 -0.41 0.66 (0.46 0.96) 0.027*-0.95 0.39 (0.28 0.53) <0.001*
Not reported Ref. Ref. Ref.
Word limit of abstract
< or = 250 -0.25 0.78 (0.46 1.34) 0.372 -0.11 0.89 (0.65 1.23) 0.487 0.16 1.18 (0.90 1.54) 0.234
> 250 Ref. Ref. Ref.
Actual number of words in
abstract
0.000 1.000
(0.996 1.004)
0.864 0.000 1.000
(0.998 1.003)
0.710 -0.004 0.996
(0.994 0.998)
<0.001*
Number of citations (WoS Core) -0.002 0.998
(0.992 1.005)
0.559 0.001 1.001
(0.998 1.004)
0.444 0.001 1.001
(0.998 1.003)
0.545
PublisherA
Elsevier 0.17 1.18 (0.54 2.58) 0.671 -0.56 0.57 (0.34 0.96) 0.033*-0.14 0.87 (0.59 1.28) 0.473
Springer 0.90 2.47 (0.89 6.87) 0.084 0.25 1.28 (0.61 2.71) 0.514 0.76 2.14 (1.14 4.03) 0.018*
Wiley -0.24 0.79 (0.36 1.73) 0.557 0.22 1.25 (0.83 1.87) 0.286 0.28 1.32 (0.94 1.87) 0 .111
Sage 1.07 2.91 (0.62 13.65) 0.176 -0.31 0.74 (0.19 2.79) 0.650 0.45 1.56 (0.56 4.38) 0.394
MDPI 0.48 1.62 (0.53 4.92) 0.399 0.51 1.67 (0.83 3.34) 0.150 0.86 2.37 (1.29 4.35) 0.005*
Taylor & Francis -17.93 - - 0.72 2.06 (0.42 10.06) 0.373 1.30 3.65 (0.88 15.07) 0.073
Others Ref. Ref. Ref.
(Continued)
PLOS One | https://doi.org/10.1371/journal.pone.0323176 May 20, 2025 9 / 15
No information about language versus no language restriction
In the univariable analysis, topics of reviews (P = .003 for orthodontics and dentofacial orthopaedics), structure of abstracts
(P = .012), journal type (P = .009), reporting of primary study design (P < .001), actual number of words in abstracts
(P < .001), publisher (P = .018 for Springer and P = .005 for MDPI), countries (P < .001) and continents (P < .001 for South
America) were significantly associated with systematic reviews with no information in comparison with those with no lan-
guage restriction (Table 2).
In the multivariable analysis, reporting of primary study design (P < .001) and actual number of words in abstracts
(P < .001) were significantly associated with systematic reviews with no information of the language used in examined
articles in comparison with systematic reviews with no language restriction (Table 3).
An explanation of these associations is provided in the supplementary file.
Association between the characteristics of systematic reviews and the number of included languages
In the univariable analysis, topics of the review (P = .019), meta-analysis (P = .001), publisher (P = .029), countries
(P = .002) and continents (P < .001) were significantly associated with the number of included languages (Table 4).
In the multivariable analysis, topics of the reviews (P = .002), meta-analysis (P = .004), publisher (P = .010) and conti-
nents (P < .001) remained statistically significant (Table 4).
An explanation of these associations is provided in the supplementary file.
Differences in reporting language restrictions between the abstract and the full text
Based on Table 5, the proportions of the full texts reporting no language restriction, restriction to some languages, restric-
tion to English only, and no information about language restriction are 27%, 13%, 53%, and 7%, respectively.
Among the 23 studies reporting no language restrictions in the abstracts, 18 (78%) of them also reported the same in the
full texts. Among the 9 studies reporting restriction to some languages in the abstracts, all (100%) also reported the same in
the full texts. Among the 37 studies reporting restriction to English only in the abstracts, 36 (97%) also reported the same in
the full texts. This is contrasted with the finding that among the 250 studies reporting no information about language restric-
tion in the abstracts, only 23 studies (9%) reported the same in the full texts, while 67 (27%), 30 (12%), and 130 (52%) stud-
ies reported no language restriction, restriction to some languages, and restriction to English only, respectively (Table 5).
Restricted to some languages vs.
No language restriction (ref.)
Restricted to English only vs. No
language restriction (ref.)
No information vs. No language
restriction (ref.)
Variables B OR (95%CI) P B OR (95%CI) P B OR (95%CI) P
Countries
Developing -0.13 0.88 (0.51 1.51) 0.640 -0.53 0.59 (0.43 0.81) 0.001*-0.53 0.59 (0.45 0.77) <0.001*
Developed Ref. Ref. Ref.
Continents
North America -0.52 0.59 (0.19 1.89) 0.376 0.61 1.84 (1.04 3.27) 0.037*-0.03 0.97 (0.58 1.61) 0.903
South America -0.63 0.53 (0.27 1.06) 0.073 -1.69 0.18 (0.11 0.31) <0.001*-1.09 0.34 (0.24 0.46) <0.001*
Asia 0.04 1.04 (0.48 2.24) 0.918 0.85 2.34 (1.48 3.68) <0.001*0.24 1.27 (0.85 1.90) 0.241
Africa 0.68 1.98 (0.52 7.53) 0.319 -0.25 0.78 (0.26 2.33) 0.654 -0.78 0.46 (0.19 1.12) 0.086
Oceania 0.39 1.48 (0.35 6.33) 0.596 1.25 3.50 (1.41 8.68) 0.007*0.48 1.62 (0.69 3.78) 0.264
Europe Ref. Ref. Ref.
*P < 0.05; A: the variables were not included in the multivariable analysis because extremely small sample size in some categories which may influence
the validity of the model; OR: odds ratio; CI: confidence interval; Ref.: reference category
https://doi.org/10.1371/journal.pone.0323176.t002
Table 2. (Continued)
PLOS One | https://doi.org/10.1371/journal.pone.0323176 May 20, 2025 10 / 15
Discussion
Main findings
This meta-research assessing the reporting of languages in abstracts of systematic reviews in the dental field found that
the great majority (79%) of the abstracts did not report any information about language restriction. Meta-analysis con-
ducted, journal type, reporting of primary study design, actual number of words in abstract, country and continent of first
authors´ affiliation were significantly associated with the reporting of language restrictions in the abstracts of systematic
reviews. In addition, the topic of the review, meta-analysis conducted, publisher and continents were significantly associ-
ated with the number of languages reported.
Interpretation and relevance of the findings
To the best of current knowledge, this is the first study to comprehensively assess the reporting of language in the
abstracts of systematic reviews in any biomedical field. The findings emphasize the importance of more detailed and con-
sistent reporting of the language in the abstracts of systematic reviews.
Table 3. Multivariable multinominal logistic regression analysis for the association between characteristics of included systematic reviews
and language restrictions (N = 3465) based on the abstracts.
Restricted to some languages vs.
No language restriction (ref.)
Restricted to English only vs. No
language restriction (ref.)
No information vs. No language
restriction (ref.)
Variables B OR (95%CI) P B OR (95%CI) P B OR (95%CI) P
Structure of abstract
Structured -0.18 0.84 (0.44 1.58) 0.582 0.09 1.10 (0.73 1.64) 0.655 -0.22 0.80 (0.58 1.12) 0.191
Not structured Ref. Ref. Ref.
Meta-analysis conducted
Yes -0.65 0.52 (0.28 0.98) 0.044*-0.63 0.54 (0.38 0.76) <0.001*0.28 1.33 (1.00 1.76) 0.052
No Ref. Ref. Ref.
Journal type
Dental -0.63 0.54 (0.30 0.97) 0.040*-0.06 0.95 (0.64 1.39) 0.780 -0.20 0.82 (0.59 1.13) 0.227
Other Ref. Ref. Ref.
Reporting of primary study
design
Reported -0.39 0.68 (0.37 1.24) 0.209 -0.35 0.71 (0.49 1.03) 0.072 -0.85 0.43 (0.31 0.59) <0.001*
Not reported Ref. Ref. Ref.
Actual number of words in
abstract
0.001 1.001
(0.997 1.005)
0.759 0.000 1.000
(0.998 1.002)
0.993 -0.004 0.996
(0.994 0.998)
<0.001*
Countries
Developing 2.15 8.59 (1.07 69.28) 0.044*-0.63 0.53 (0.11 2.67) 0.443 0.21 1.23 (0.28 5.51) 0.784
Developed Ref. Ref. Ref.
Continents
North America -0.70 0.50 (0.15 1.67) 0.258 0.63 1.88 (1.06 3.34) 0.032*-0.03 0.97 (0.58 1.63) 0.916
South America -2.64 0.07 (0.01 0.62) 0.017*-0.98 0.38 (0.07 2.03) 0.255 -1.42 0.24 (0.05 1.11) 0.068
Asia -2.05 0.13 (0.02 1.13) 0.064 1.54 4.66 (0.92 23.62) 0.063 -0.07 0.94 (0.21 4.23) 0.931
Africa -1.47 0.23 (0.02 2.69) 0.241 0.44 1.55 (0.22 10.84) 0.661 -1.02 0.36 (0.06 2.07) 0.253
Oceania 0.32 1.38 (0.32 5.94) 0.665 1.24 3.46 (1.40 8.59) 0.007*0.43 1.53 (0.65 3.60) 0.326
Europe Ref. Ref.
*P < 0.05; OR: odds ratio; CI: confidence interval; ref.: reference category
https://doi.org/10.1371/journal.pone.0323176.t003
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Table 4. Univariable and multivariable binary logistic regression analysis for the association between characteristics of included systematic
reviews and number of included languages (N = 726) based on the abstracts.
Univariable Multivariable
Variables B OR (95%CI) (only 1 lan-
guage as the reference)
P value B OR (95%CI) (only 1 lan-
guage as the reference)
P value
Topic of the review 0.019*0.002*
Dental public health -0.09 0.91 (0.45 1.83) 0.793 0.16 1.17 (0.52 2.67) 0.703
Endodontics -0.01 0.99 (0.43 2.26) 0.981 0.34 1.41 (0.55 3.62) 0.477
Oral and maxillofacial radiology -1.35 0.26 (0.03 2.27) 0.223 -2.26 0.11 (0.01 1.13) 0.063
Oral and maxillofacial surgery -0.25 0.78 (0.36 1.68) 0.523 -0.35 0.71 (0.29 1.71) 0.444
Oral medicine 0.58 1.78 (0.58 5.47) 0.316 0.86 2.37 (0.63 8.88) 0.199
Orofacial pain 0.10 1.10 (0.50 2.42) 0.809 0.00 1.00 (0.40 2.48) 0.997
Orthodontics and dentofacial orthopaedics 0.98 2.67 (1.40 5.13) 0.003*1.21 3.35 (1.58 7.08) 0.002*
Pediatric dentistry 0.20 1.22 (0.48 3.09) 0.671 0.40 1.49 (0.51 4.33) 0.462
Periodontics 0.01 1.01 (0.52 1.98) 0.966 0.09 1.10 (0.50 2.39) 0.814
Prosthodontics 0.01 1.01 (0.58 1.74) 0.980 0.03 1.03 (0.54 1.97) 0.933
Others Ref. Ref.
Structure of abstract
Structured -0.16 0.85 (0.60 1.21) 0.365
Not structured Ref.
Type of systematic reviews
Intervention 0.15 1.16 (0.85 1.57) 0.348
Non-intervention Ref.
Meta-analysis conducted
Yes 0.53 1.69 (1.23 2.33) 0.001*0.55 1.73 (1.19 2.53) 0.004*
No Ref. Ref.
Journal type
Dental 0.06 1.06 (0.76 1.47) 0.737
Other Ref.
Reporting of primary study design
Reported 0.31 1.37 (0.98 1.90) 0.065
Not reported Ref.
Word limit of abstracts
< or = 250 0.04 1.04 (0.77 1.40) 0.801
> 250 Ref.
Actual number of words in abstracts -0.001 0.999
(0.997 1.002)
0.651
Number of citations (WoS Core) -0.002 0.998
(0.995 1.001)
0.243
Publisher 0.029*0.010*
Elsevier 0.62 1.86 (1.14 3.01) 0.012*0.62 1.86 (1.07 3.24) 0.028*
Springer 0.04 1.04 (0.54 1.98) 0.911 -0.22 0.81 (0.37 1.75) 0.583
Wiley -0.24 0.79 (0.54 1.15) 0.212 -0.43 0.65 (0.41 1.02) 0.062
Sage 0.66 1.94 (0.60 6.22) 0.267 1.15 3.15 (0.89 11.15) 0.076
MDPI -0.37 0.69 (0.37 1.28) 0.242 -0.50 0.61 (0.30 1.22) 0.160
Taylor & Francis -0.93 0.40 (0.08 1.93) 0.251 -0.35 0.71 (0.12 4.22) 0.701
Others Ref. Ref.
Countries
(Continued)
PLOS One | https://doi.org/10.1371/journal.pone.0323176 May 20, 2025 12 / 15
More detailed information would allow a more accurate interpretation of the findings by the readers, for example, when
the effect size of the meta-analytic estimates is small and there is no information in the abstract about whether there was
any limitation set in the search strategies regarding languages of the examined articles. In such cases, one cannot be
sure whether potential language bias could in fact influence the size of these estimates.
Some evidence suggests that not including primary studies in languages other than English will not alter the conclu-
sions of a systematic review [9,10]. However, the scope of the discussion of potential language bias should be broader.
Some authors suggest that primary studies published in some languages can provide more positive results [11]. This
assumption is corroborated by evidence of language and indexing bias in Chinese-sponsored clinical trials that suggest
positive trials are more likely to be published in English than negative trials [12].
In the cited study [12], Chinese-sponsored trials with positive results and registered in the Chinese Clinical Trial Regis-
try were 3.92 (95% CI, 2.20–7.00) times more likely to be published in English than Chinese when compared with nega-
tive Chinese-sponsored trials. Similarly, Chinese-sponsored trials with positive results and registered in English-language
registries were 3.22 (95% CI, 1.34–7.78) times more likely to be published in English than Chinese when compared with
negative Chinese-sponsored trials.
Furthermore, various authors reported issues other than language bias when limiting the search in systematic reviews.
For example, a language limitation in systematic reviews of pharmacogenomics and pharmacogenetics may hinder the
generalisability of findings to non-native English-language populations [13].
In fact, constructing a comprehensive search and including studies without language restriction minimizes the chance
of publication bias [14]. A comprehensive search means a detailed search strategy, applied in multiple databases and grey
Univariable Multivariable
Variables B OR (95%CI) (only 1 lan-
guage as the reference)
P value B OR (95%CI) (only 1 lan-
guage as the reference)
P value
Developing 0.48 1.61 (1.19 2.16) 0.002*2.04 7.71 (0.80 74.66) 0.078
Developed Ref. Ref.
Continents < 0.001*<0.001*
North America -0.72 0.49 (0.29 0.83) 0.008*-0.83 0.44 (0.24 0.79) 0.006*
South America 1.57 4.80 (2.95 7.81) <0.001*-0.29 0.75 (0.07 7.71) 0.809
Asia -0.89 0.41 (0.27 0.62) <0.001*-3.04 0.05 (0.01 0.48) 0.010*
Africa 0.47 1.60 (0.61 4.19) 0.339 -1.67 0.19 (0.02 2.27) 0.189
Oceania -1.14 0.32 (0.15 0.69) 0.004*-1.08 0.34 (0.15 0.77) 0.010*
Europe Ref.
*P < 0.05; OR: odds ratio; CI: confidence inter; ref.: reference category
https://doi.org/10.1371/journal.pone.0323176.t004
Table 4. (Continued)
Table 5. Language restrictions reported in the abstracts and full texts.
Abstracts
Full texts No language restriction Restricted to some languages Restricted to English only No information Total
No language restriction 18 0 0 67 85
Restricted to some languages 1 90 30 40
Restricted to English only 3 0 36 130 169
No information 1 0 1 23 25
Total 23 9 37 250 319
https://doi.org/10.1371/journal.pone.0323176.t005
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literature without language and publication date restrictions. Such a strategy contributes to a more realistic effect estimate
of a specific intervention. Language restrictions, on the other hand, may yield to an effect overestimation.
In the regression analyses, several variables were found to be significantly associated with language restrictions in
the abstracts. For example, systematic reviews with meta-analyses may be more likely to include more comprehensive
literature in all available languages to generate more precise meta-analytic estimates than systematic reviews without
meta-analyses. This could be why systematic reviews with a meta-analysis were likely to have fewer language restrictions
than those without a meta-analysis. Furthermore, it was observed that systematic reviews originating from North America
or Oceania were more likely to impose language restrictions, whereas those from South America were less likely to do so.
This may be because English is the main official language on those continents (e.g., Canada and USA in North Amer-
ica) and Australia and New Zealand in Oceania.
Therefore, systematic reviews from those two continents are prone to be restricted to English alone. In contrast, coun-
tries in South America have a greater variety of official languages, such as Spanish, Portuguese, French, English and
Dutch. In addition, dental journals are more likely to have language restrictions than non-dental journals. This may indicate
that systematic reviews in dentistry need to include more comprehensive literature to improve the external validity of the
results in the future. Most of the other significant associations remain to be explained.
A random sample of 9.2% of the full texts was reviewed and it appears that there is a poor consistency in reporting no
information about language restriction between abstracts and full texts. When no information about language was reported
in the abstract, 91% of the full texts of these abstracts reported information about language restriction. On the contrary
there is good consistency in reporting no language restrictions, restriction to some languages and restriction to English
only between abstracts and full texts.
Strengths and limitations
The study has several strengths. It is the first study in dentistry to assess language restrictions in the abstracts of sys-
tematic reviews. The sample size was substantial and may be representative of the dental field. Furthermore, this large
sample of abstracts rendered a robust statistical assessment with several variables.
This study has certain limitations. Some information regarding language restrictions that was not reported in the
abstracts may have been included in the full texts of the articles. An analysis of the full texts from a 9.2% sample of
all included studies supports this assumption. However, restricting our analysis to abstracts can also be considered a
strength in terms of relevance, as many readers rely solely on abstracts when assessing scientific articles. Therefore,
comprehensive reporting within abstracts is crucial.
Additionally, the sample size used in the multinomial regression analysis may be insufficient. According to the
rule of events per variable (EPV > 10), at least 300 samples per category of the dependent variable (language
restriction) are required, given that 12 independent variables with 30 categories were included. However, some cat-
egories within the dependent variable contained fewer than 300 samples, potentially reducing the statistical power
of the analysis.
Further developments
Reporting checklists, such as PRISMA for Abstracts, could be updated to include more detailed information on guiding
researchers to report on the language of examined articles in the abstracts of systematic reviews. For example, by guiding
authors to declare whether language restrictions were imposed in the search strategy phase or whether language restric-
tion was applied as an eligibility criterion [4].
Furthermore, a similar assessment should be conducted in other medical disciplines to understand whether the prob-
lem of this type of underreporting is widespread in the biomedical field.
PLOS One | https://doi.org/10.1371/journal.pone.0323176 May 20, 2025 14 / 15
Conclusion
This meta-research study demonstrates that abstracts of systematic reviews in dentistry frequently underreport language
restrictions applied to the primary studies examined. Several characteristics of systematic reviews are significantly asso-
ciated with these restrictions, underscoring inconsistencies in reporting practices. This study emphasizes the necessity of
reevaluating reporting practices to enhance transparency and standardization.
Supporting information
S1 Data.
(XLSX)
S1 File. Supplementary file.
(DOCX)
Author contributions
Conceptualization: Clovis Mariano Faggion Jr.
Formal analysis: Tatjana Lörscher, Naichuan Su, Clovis Mariano Faggion Jr.
Investigation: Tatjana Lörscher, Clovis Mariano Faggion Jr.
Methodology: Clovis Mariano Faggion Jr.
Project administration: Clovis Mariano Faggion Jr.
Supervision: Clovis Mariano Faggion Jr.
Validation: Tatjana Lörscher.
Writing – original draft: Tatjana Lörscher, Clovis Mariano Faggion Jr.
Writing – review & editing: Tatjana Lörscher, Naichuan Su, Clovis Mariano Faggion Jr.
References
1. Hoffmann F, Allers K, Rombey T, Helbach J, Hoffmann A, Mathes T, et al. Nearly 80 systematic reviews were published each day: Observa-
tional study on trends in epidemiology and reporting over the years 2000-2019. J Clin Epidemiol. 2021;138:1–11. https://doi.org/10.1016/j.
jclinepi.2021.05.022 PMID: 34091022
2. Saint S, Christakis D, Saha S, Elmore J, Welsh D, Baker P, et al. Journal reading habits of internists. J Gen Intern Med. 2000;15:881–4. https://doi.
org/10.1046/j.1525-1497.2000.00202.x PMID: 11119185
3. Page M, McKenzie J, Bossuyt P, Boutron I, Hoffmann T, Mulrow C, et al. The PRISMA 2020 statement: an updated guideline for reporting system-
atic reviews. BMJ. 2021;372:n71.
4. Pieper D, Puljak L. Language restrictions in systematic reviews should not be imposed in the search strategy but in the eligibility criteria if neces-
sary. J Clin Epidemiol. 2021;132:146–7. https://doi.org/10.1016/j.jclinepi.2020.12.027 PMID: 33383129
5. Catalogue of Bias Collaboration, Brassey J, Spencer EA, Heneghan C. Language bias. In: Catalogue Of Bias 2017 [cited 2024 Jan 12]. Available
from: https://catalogofbias.org/biases/language-bias/
6. Shea B, Reeves B, Wells G, Thuku M, Hamel C, Moran J, et al. Amstar 2: a critical appraisal tool for systematic reviews that include randomised or
non-randomised studies of healthcare interventions, or both. BMJ. 2017;358(j4008).
7. ncrdscb.ada.org [Internet]. Chicago: American Dental Association; [cited 2024 Jan 12]. Available from: https://ncrdscb.ada.org/
recognized-dental-specialties
8. Urbaniak GC, Plous S. Research Randomizer. Version 4.0 [software].1997 [cited 2024 June 20]. Available from: http://www.randomizer.org/
9. Jüni P, Holenstein F, Sterne J, Bartlett C, Egger M. Direction and impact of language bias in meta-analyses of controlled trials: empirical study. Int J
Epidemiol. 2002;31(1):115–23. https://doi.org/10.1093/ije/31.1.115 PMID: 11914306
10. Morrison A, Polisena J, Husereau D, Moulton K, Clark M, Fiander M, et al. The effect of English-language restriction on systematic review-
based meta-analyses: a systematic review of empirical studies. Int J Technol Assess Health Care. 2012;28(2):138–44. https://doi.org/10.1017/
S0266462312000086 PMID: 22559755
PLOS One | https://doi.org/10.1371/journal.pone.0323176 May 20, 2025 15 / 15
11. Egger M, Zellweger-Zähner T, Schneider M, Junker C, Lengeler C, Antes G. Language bias in randomised controlled trials published in English and
German. Lancet. 1997;350(9074):326–9. https://doi.org/10.1016/S0140-6736(97)02419-7 PMID: 9251637
12. Jia Y, Huang D, Wen J, Wang Y, Rosman L, Chen Q, et al. Assessment of Language and Indexing Biases Among Chinese-Sponsored Randomized
Clinical Trials. JAMA Netw Open. 2020;3(5):e205894. https://doi.org/10.1001/jamanetworkopen.2020.5894 PMID: 32463469
13. Aali G, Shokraneh F. No limitations to language, date, publication type, and publication status in search step of systematic reviews. J Clin Epide-
miol. 2021;133:165–7. https://doi.org/10.1016/j.jclinepi.2021.02.002 PMID: 33571633
14. Boutron I, Page M, Higgins J, Altman D, Lundh A, Hróbjartsson A. Considering bias and conflicts of interest among the included studies. The
Cochrane Handbook for Systematic Reviews of Interventions. London: The Cochrane Collaboration. 2024.
