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Objective To compare the reporting of essential applicability data from randomised controlled trials and non-randomised studies evaluating four new orthopaedic surgical procedures. Data sources Medline and the Cochrane central register of controlled trials. Study selection All articles of comparative studies assessing total hip or knee arthroplasty carried out by a minimally invasive approach or computer assisted navigation system. Data extraction Items judged to be essential for interpreting the applicability of findings about such procedures were identified by a survey of a sample of orthopaedic surgeons (77 of 512 completed the survey). Reports were evaluated for data describing these "essential" items and the number of centres and surgeons involved in the trials. When data on the number of centres and surgeons were not reported, the corresponding author of the selected trials was contacted. Results 84 articles were identified (38 randomised controlled trials, 46 non-randomised studies). The median percentage (interquartile range) of essential items reported for non-randomised studies compared with randomised controlled trials was 38% (25-63%) versus 44% (38-45%) for items about patients, 71% (43-86%) versus 71% (57-86%) for items considered essential for all interventions, and 38% (25-50%) versus 50% (25-50%) for items about the context of care. More than 80% of both study types were single centre studies, with one or two participating surgeons. Conclusion The reporting of data related to the applicability of results was poor in published articles of both non-randomised studies and randomised controlled trials and did not differ by study design. The applicability of results from the trials and studies was similar in terms of number of centres and surgeons involved and the reproducibility of the intervention. INTRODUCTIONRandomised controlled trials provide the most reliable evidence for quantifying treatment effect sizes.1 2 In the specialty of surgery, however, results of such trials are often criticised for being poorly applicable. The results of non-randomised studies are believed to have better applicability. [3][4][5][6][7][8] Applicability (also called external validity or generalisability)9 concerns a multidimensional concept depending on the extent to which participants, the context of care, and the interventions (and comparators) evaluated in studies are representative of, or can be reproduced in, usual care. The applicability of a trial's results could be limited if patients represent only a small proportion of those being treated in normal practice. 10 The participation of centres with different resources and surgeons with different skills may mean that treatment effects observed in research may not be applicable, or at worst are irrelevant, to nonresearch settings. [11][12][13][14][15] Surgical procedures are complex interventions that can be difficult to describe, standardise, and reproduce consistently in clinical practice. 16 Appraising the applicability of the results of a study is in...