Search strategy

To identify all possible articles that meet the predefined inclusion criteria, an electronic search of PubMed (1966–September 2017), Embase (1980–September 2017), ScienceDirect (1985–September 2017), and Cochrane library (1966–September 2017) was performed according to the Cochrane Collaboration guidelines. Additionally, manual searches of the reference lists of all included studies were conducted to identify trials that the electronic search may have missed. All searches were conducted based on the following terms “osteoarthritis”, “knee”, “hyaluronic acid or HA”, and “platelet-rich plasma or PRP” with the Boolean operators “AND or”. Two reviewers independently assessed the titles and abstracts of reports identified by the electronic and manual searches. Full text articles were retrieved to facilitate identification based on the abstract, when necessary. Any disagreements were resolved through discussion.