Letter to the editor regarding "efficacy and safety of platelet-rich plasma combined with hyaluronic acid versus platelet-rich plasma alone for knee osteoarthritis: a systematic review and meta-analysis".

In this letter to the editor, we discuss the article by Zhang et al., published recently in the Journal of Orthopaedic Surgery and Research. The authors reviewed the efficacy and safety of platelet-rich plasma combined with hyaluronic acid versus platelet-rich plasma alone for knee osteoarthritis. Whether the authors' purpose in grouping was to investigate the role of hyaluronic acid in the treatment of knee osteoarthritis is a question we would like to raise. In terms of the methodology of the study, combining randomized controlled trials with cohort studies in this meta-analysis is a methodological error. Secondly, the study methodology of the four included randomized controlled trial studies lacked a clear method of randomization. In addition to this, the completeness of the search needs to be taken into consideration. Some of the results of this study were highly heterogeneous, and no sensitivity analysis or meta-regression was performed to further analyse the sources of heterogeneity. The above issues will affect the conclusions of the article, and we believe this needs further improvement and discussion.