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. 2018 Jul 6;18:725. doi: 10.1186/s12885-018-4581-5

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© The Author(s). 2018

Open AccessThis article is distributed under the terms of the Creative Commons Attribution 4.0 International License (http://creativecommons.org/licenses/by/4.0/), which permits unrestricted use, distribution, and reproduction in any medium, provided you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made. The Creative Commons Public Domain Dedication waiver (http://creativecommons.org/publicdomain/zero/1.0/) applies to the data made available in this article, unless otherwise stated.

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Fig. 1 — Screening and identification of included studies. a A total of 18 articles that met the inclusion criteria were included in meta-analysis, which were searched from the database of Medline/PubMed, EMBASE, Cochrance Library, Web of Science, Chinese Journal Full-text Database and Chinese Sci-Tech Journals Database. b, c Through a comprehensive analysis according to the Cochrane Handbook of Systematic Review, we found that except for a study that was low risk, the other studies did not have obvious biases