Study designs and potential biases in sports injury research. The case-control study

Several different epidemiological study designs can be used for aetiological investigations of potential risk factors for the occurrence of sports injuries. The case-control study is an example of a retrospective design in which the investigator starts with the classification of injury status (case or control) and obtains information regarding prior exposure to risk factors. Several decisions need to be made when designing case-control studies. Firstly, the source of the study participants needs to be considered. Cases and controls need to be identified from the same source, i.e. same sport or clinic. Secondly, the same eligibility criteria need to be applied to potential cases and controls. Thirdly, when an injury occurred must be established. The fourth issue concerns the status of cases (incident or prevalent cases). Finally, the number and size of the control groups needs to be determined. Strengths of the case-control study design are the high level of information obtained, the relatively low cost and its usefulness for studying rare sports injuries. The higher susceptibility to bias is one of the limitations of case-control studies. Bias in a case-control study can lead to over or underestimation of the true association between an alleged risk factor and the occurrence of sports injuries. Three types of bias have been distinguished: (i) selection bias; (ii) information bias; and (iii) confounding. Furthermore, the applicability of this type of design is limited to risk factors that remain relatively stable after the occurrence of an injury. The effect of changeable risk factors, such as quadriceps strength and range of motion, is difficult to assess since in many cases data at the time of injury are unavailable.