The impact of adherence on sports injury prevention effect estimates in randomised controlled trials: Looking beyond the CONSORT statement

Objective: To investigate estimated outcome effects of a sports injury prevention intervention when analysed by means of a per protocol (PP) analysis approach. Design: Randomised controlled trial (RCT) involving 522 athletes who sustained a lateral ankle sprain allocated to either an intervention (received a preventive programme in addition to usual care) or control group who were followed prospectively for one year. Methods: Secondary analysis of data relating to registered ankle sprain recurrences, exposure and adherence to the allocated intervention using a PP analysis approach. Results: Twenty-three percent of the RCT intervention group indicated to have fully adhered with the neuromuscular training programme. A per protocol analysis only considering fully adherent athletes and control athletes, showed a Hazard Ratio of 0.18 (95% CI: 0.07–0.43). Significantly fewer recurrent ankle sprains were found in the fully adherent group compared to the group that was not adherent (relative risk = 0.63; 95% CI: 0.43–0.99). Conclusions: A PP analysis on fully adherent athletes versus control group athletes showed that the established intervention effect was over threefold higher compared to an earlier intention-to-treat based analysis approach. This shows that outcomes of intervention studies are heavily biased by adherence to the allocated intervention.