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Letters to the Editor

Dear Editor: The recently published study of Hewett et al (April 2005, pages 492-501) presents important findings that have the potential to reduce the risk of noncontact anterior cruciate ligament (ACL) injuries. Any association found in a study has the potential to have been affected by chance, confounding, or bias, and it is the authors’ responsibility to try to account for other possible factors that may have caused the observed findings. In this article, Hewett et al present a red flag for a highly plausible confounder but then do not go on to assess whether this confounder may have affected the data. They report that, from a mixed population of 3 sports, 7 soccer, 2 basketball, and 0 volleyball players tore their ACL while under study. Although I cannot find the injury incidence rates for the sports in the article, these frequencies suggest, for example, that the soccer players were at much higher risk than the volleyball players. The major findings of the study were that female athletes with greater knee abduction moments (plus other associated factors) on landing were at higher risk of ACL injury. Is it not possible that soccer players land with greater knee abduction moments than basketball and volleyball players (because of the different requirements of their sport)? Greater knee abduction moment on landing may possibly be a marker for being a soccer player, which itself may be the true risk factor. Given that soccer players participate on grass, whereas basketball and volleyball players use hard surfaces, it may also be that the surface is a confounder. I couldn’t expect authors to have analyzed for possible confounders that weren’t measured, for example, surface characteristics, but why weren’t the mean knee abduction moments for each sport presented, and why wasn’t “sport played” entered into the logistic regression analysis along with the biomechanical variables? The authors may also possibly uncover, if they rerun their logistic regression, a risk factor that they appear to have discarded, namely, athlete height. They presented a univariate trend toward taller athletes being more likely to injure their ACL (P = .08). However, those playing the relatively lower risk sports (basketball and volleyball) were likely to have been significantly taller than those playing the high-risk sport (soccer). I would hypothesize that when paired together, “soccer participation” and “taller athlete” may both be significant risk factors for ACL injury in a multivariate analysis of this cohort. A previous study from the same first author in the same journal had a similar feel to it, although at least in this study, the authors assessed and discussed the effects of both recruitment bias and confounding. For example, they found that a trained group (of which 50% were volleyball players) had a significantly lower incidence of knee injury than an untrained group (of which only 20% were volleyball players), but when the volleyball players were discarded from both groups, the results were no longer significant. My criticism of this previous publication (which is of the reviewers and editors as much as the authors) is that the abstract simply reported the association as being statistically significant (without mentioning confounding by sport), and it was only in the fine print that it was reported that confounding by sport was responsible for the results achieving statistical significance. Hewett et al should be encouraged to continue their work, focusing on soccer players, as the yield from future prospective studies will be higher in the sport of highest risk, and the issue of confounding by sport will disappear. I hope for the sake of ACL injury prevention that their findings are replicated when they either repeat the study (or analysis) in a single sport or account for sport in future multivariate analysis. In the worst case scenario, which would be that confounding variables such as height and sport completely account for the observed biomechanical associations, their work will still have given us important information (eg, that tall girls could be encouraged to play volleyball rather than soccer to minimize ACL risk). The best case scenario in which they have found a valid biomechanical predictor for ACL injuries is obviously the major reason why this article received such an important award.