Most participants (88%) wanted feed back of the DAMASK trial results which is comparable with findings from a literature review that a median of 90% (range 20%-100%) of participants wished to receive study results . This range of patients wanting feedback is, however, very wide. Interestingly the review showed that for several studies of patients being treated for cancer, 90% of participants or more wanted to be informed of the study results [16–19]. In contrast, for studies of pregnant women only 20%  and 40%  of woman wanted study results. In our trial we asked participants about being fed back the study results at the final postal questionnaire follow-up, but it might be more appropriate to do this when enrolling patients to take part and include it in the patient information leaflet and consent of patients.
The majority of participants (55%) preferred the longer leaflet and the main reasons for this were the technical information provided and use of diagrams to explain trial results. In contrast, every participant who preferred the one-page leaflet gave length of the leaflet as the reason for this. In the free text responses it was clarified that the leaflet being short and simple was the reason for preferring the one-page leaflet although some suggested the leaflet would have benefited from the use of diagrams. Graphical formats are increasingly being used to present risk information to patients [21, 22] and has been shown to improve their understanding of quantitative information over textual or written formats [23, 24]. Our findings support the use of a longer leaflet with visual aids to explain trial results which in particular might benefit participants with reading difficulties or low numeracy skills. We did, however, only compare participants' preference for a one-page leaflet compared with a four-page leaflet. It is possible that a shorter leaflet such as two-pages with the use of diagrams would have been preferred.
The findings from the regression analyses suggest that gender can significantly influence preference for type of leaflet (one-page) or no preference (P = 0.030); although this was the only significant finding from the several explanatory analyses that were performed and thus possibly a false-positive result that should be interpreted with caution. Whilst taking this into consideration: 71% of participants who had no preference for a leaflet were male (29% female); 50% and 58% of males respectively preferred a one-page or four-page leaflet. These findings indicate that females were more interested in how the information about trial results was presented to them which is consistent with women using health services more than men and seeking more health information . Therefore it might be appropriate in research that has a female population such as treatments for breast cancer or during pregnancy to consider feeding back results differently than for research including males such as the treatment for prostate cancer. Research about communicating results to participants of a cardiac rehabilitation trial has shown that gender can be important with women (97%) compared to men (76%) significantly more likely to prefer receiving study results by letter (P = 0.009); and no women (0%) compared to men (15%) preferring communication by email or the web (P = 0.024) . The use of qualitative methods could help to further explore the communication of trial results to males and females. None of the other characteristics of the participants appeared to be important for influencing their preference for a type of leaflet or not. Including ethnicity as a variable in the analyses would have been desirable, but in our survey sample there were only two participants who were non-white (who both preferred the four-page leaflet). Therefore, it was not appropriate to include ethnicity in the model.
A limitation of our survey was the low response rate, although participants' characteristics were mostly similar between those who did and did not respond. Younger patients, however, were significantly less likely to respond, though there was only a small difference in years and, from the analyses, age was not important for explaining participants' preferences. Participants who scored lower on the emotional functioning sub-scale of the KQoL-26 questionnaire were also significantly less likely to respond. This might possibly be a chance finding although the emotional functioning sub-scale is concerned with participants feeling downhearted and low, angry and annoyed, or worried about their knee worsening. The trial findings indicated to participants that there were only small benefits from GP access to MRI in terms of the physical functioning of their knee. It is conceivable that participants who were downhearted and low or worried about their knee were dissatisfied with the limited benefits of the intervention and therefore were less willing to complete the survey. Alternatively, they could have been dissatisfied with the methods offered for feeding back the trial results. Research into communicating clinical trial results to participants in a study about Huntington's disease found that participants reported high or complete satisfaction with an investigator of the trial contacting individual participants by telephone to explain the results (89%) or participants' attendance to a conference call with study investigators (82%) but relatively low satisfaction with the sponsor's press release (50%) . It is possible that the participants did not respond to our survey because they valued more personalised approaches to communicating the trial results. There was also a considerable delay from December 2006 which was when the last participant was followed-up in the trial to May 2009 when we informed the participants of the results of the trial. This was because we wanted to communicate the most accurate results possible to participants that had been peer-reviewed and accepted for publication in scientific journals. This delay might have contributed to participants' lack of interest in the survey.
Despite several studies commenting on the negative psychological impact of feeding back results of trials to participants [7, 9, 10], the trial co-ordinator has not had any contact with participants about the trial results. Nor did participants in the survey express any anxieties about the information conveyed. This may partly be due to the limited generalisability of this study, in that it is a single randomised trial including participants with a musculoskeletal condition which was not particularly acute or life-threatening and for which there was minimal difference in patient outcomes between the two groups. In a trial with a more vulnerable population (e.g. cancer patients) or with serious adverse events, participants could potentially have very different experiences which could affect their attitude towards the method of feeding back trial results. They might prefer a short leaflet that is plain and simple with clear messages such as "what these results mean to you" or "further steps you should take". It might also be important to present the findings separately for those who had a positive result compared with those who have had negative results, with the latter requiring a more personalised delivery of results through a telephone call or visit to the clinic . Indeed, it might be appropriate to guide the participant back to their main care provider who can offer the necessary clinical advice in the context of the trial results. Our study also used a randomised trial design and was evaluating the effectiveness of a diagnostic test. Participants that have a different experience of research which use alternative study designs (e.g. surveys, cohorts) or an evaluation of different treatments might like the results of the study to be communicated to them differently. This study might also have benefited from the involvement of trial participants in the design and piloting of the leaflets which has been recommended for future research . In addition to postal methods of feeding back results, other communication methods have been suggested including the telephone or a face-to-face consultation . Using the postal method is likely to be less time consuming and costly than the additional salary required for a suitably trained and knowledgeable person to spend time explaining trial results to individual participants. In our study, it only cost £105 and £252 to print the one-page and four-page leaflets respectively and the cost of posting these materials was estimated to be £150. The cost per trial participant for sending out both leaflets was therefore £1.22. Finally randomised trials are increasingly making use of the internet and email to recruit participants, to maintain contact during the trial, to deliver the intervention and to collect data [29, 30]. Participants could be asked when enrolled into a trial whether an electronic method could be used to feed back the results.