Figure 1

Flow of studies through the systematic review process. The figure describes the flow of studies through the review, according to PRISMA guidelines. The search of the literature databases yielded 9542 separate references. Following the screening of abstracts and full-text publications against the inclusion/exclusion criteria a total of six RCTs (reported in 13 publications) and 94 non-randomised studies (reported in 105 publications) met the inclusion criteria for the review.