Copyright © 2011 Wolters Kluwer Health | Lippincott Williams & Wilkins Chapter 14 Screening and Prevention of Illnesses and Injuries: Research Methods and Data Analysis

Copyright © 2011 Wolters Kluwer Health | Lippincott Williams & Wilkins Chapter Overview The concept of injury prevention. Principles of injury risk identification and prevention research design. Risk identification and injury prevention are concepts often linked to each other. Prospective versus retrospective study designs: more robust information about injury risk factors. Randomized controlled trials are the key to injury prevention research design. Randomized controlled trials provide higher levels of evidence. Limitations must be emphasized when discussing the results of case control studies. Risk factor studies and screening studies share similar characteristics. Injury definition is more problematic in retrospective studies.

Copyright © 2011 Wolters Kluwer Health | Lippincott Williams & Wilkins The Steps in the Bahr et al. Paradigm 1.Establish the extent of the injury problem. –Done through prospective injury surveillance and anecdotal observations. –Injury Surveillance System utilized by the National Collegiate Athletic Association. 2.Establish the etiology and mechanisms of the sports-related injuries. –The cause of a sports injury may be traumatic or atraumatic. –Identifying the mechanism of injury: observational research. Identifying risk factors that may be involved in the etiology of a specific injury: collection of baseline data to assess potential risk factors followed by a prolonged period of injury surveillance.

Copyright © 2011 Wolters Kluwer Health | Lippincott Williams & Wilkins The Steps in the Bahr et al. Paradigm (continued) 3. Introduce a preventative measure. –The intervention should be based on information gained in the first two steps of the paradigm. –Avoid the use of the “shotgun approach” of injury prevention. 4. Reassess the extent of the injury problem. –If the incidence and severity of injuries have been substantially reduced, the permanent implementation of the intervention is likely warranted. Repeating these steps may show that particular athletes are at greater risk of certain injuries and would benefit most from targeted injury prevention programs.

Copyright © 2011 Wolters Kluwer Health | Lippincott Williams & Wilkins Prospective Design The design of studies of injury risk factors may be of either a prospective or a retrospective nature; prospective designs are preferred. A prospective cohort design is the gold standard for studies of injury risk factors. A large group of participants potentially “at risk” for injury of interest are baseline tested. Participants are followed over time to determine if they go on to suffer the injury. Statistical comparisons of the potential risk factors are made between participants who were injured and those who were not. The preferred method is to compare the baseline measures of the injured to the uninjured group. Some researchers “match” each subject in the injured cohort with a comparable subject from the uninjured cohort.

Copyright © 2011 Wolters Kluwer Health | Lippincott Williams & Wilkins Screening Studies and Retrospective Designs Screening Studies Prospective studies for risk factors are also known as “screening studies.” Risk factor studies and screening studies share similar characteristics. Retrospective Designs Case control studies are retrospective in nature and are sometimes called ex post facto (after the fact) designs. Individuals who have already suffered the injury are compared to a control group that has not suffered the injury.

Copyright © 2011 Wolters Kluwer Health | Lippincott Williams & Wilkins Flow chart of a case control study. By definition, all case control studies utilize a retrospective design.

Copyright © 2011 Wolters Kluwer Health | Lippincott Williams & Wilkins Limitations of Retrospective Designs Case control designs require a smaller number of subjects for testing (compared to prospective cohort designs). Limitation of the retrospective design: measures of “risk factors” are taken after the injury of interest has occurred, so there is no way to know if the measures were present before the injury occurred. It is recommended that the term “injury risk factor” not be used when describing the results a case control design study. Injury definition can be more problematic in retrospective studies than in prospective studies. Ideally, injury confirmation can be obtained using imaging methods or strict diagnostic criteria derived from participants’ medical records. However, many case control studies rely on patient self-report for their injury history.

Copyright © 2011 Wolters Kluwer Health | Lippincott Williams & Wilkins Estimating Injury Rates and Risks The estimation of injury rates and risks is accomplished through injury surveillance systems. This is not as simple as just counting the number of injured people or injuries that occur. One needs to understand the difference between injury incidence and prevalence, injury rates and risks, and specifications of how injuries are defined. The prevalence of an injury or illness refers to the proportion of a sample that has a given injury or illness at a single time point. It is pPresented as a proportion or a percentage.

Copyright © 2011 Wolters Kluwer Health | Lippincott Williams & Wilkins Incidence The incidence of an injury or illness: the number of new cases of the pathology in a given period of time. –To establish incidence of pathology: a surveillance system needs to record the number of new cases. This requires a prospective study design. Incidence proportion: the number of newly injured individuals in a defined population over a given period of time. Incidence rate: the number of new cases that occur per unit of person-time at risk. –To calculate incidence rate, the amount of time that each individual is exposed to injury/illness risk must be calculated.

Copyright © 2011 Wolters Kluwer Health | Lippincott Williams & Wilkins Injury Risk and Rate Injury risk refers specifically to the probability of new injury per individual. The incidence proportion is an example of injury risk. Injury rate specifically refers to the number of new injuries per unit of exposure time. The incidence rate described above is an example of injury rate. A time loss injury refers to an injury that forces a worker to miss work or an athlete to not participate in his or her sport. A non–time loss injury refers to an injury requiring medical care but the worker or athlete is able to continue with his or her regular participation in work or sport.

Copyright © 2011 Wolters Kluwer Health | Lippincott Williams & Wilkins Statistical Analysis The simplest comparison between two measures of injury incidence is to calculate the ratio of the injury incidence between two groups. Prevalence ratio: If the ratio of the injury prevalence estimates between two groups is taken. Risk ratio: If the ratio of injury risk estimates between two groups is calculated. Rate ratio: If the ratio of the injury rate estimates between two groups is determined.

Copyright © 2011 Wolters Kluwer Health | Lippincott Williams & Wilkins Relative Risk When assessing injury risks or rates between two groups, the method of comparison is the calculation of the relative risk of injury between the two groups. Relative risk (RR): provides a proportion of injury incidence between two groups and is identical to the calculation of risk ratio (or rate ratio). Relative risk reduction (RRR): –A more easily understood statistic. –Represents the percentage that the experimental condition reduces injury risk compared to the control condition. –Calculated by taking 1 minus the relative risk and multiplying by 100. Relative risk increase (RRI): If the experimental condition is found to lead to heightened risk of injury, the sign is changed to positive and termed RRI (rather than expressing the relative risk reduction as a negative number).

Copyright © 2011 Wolters Kluwer Health | Lippincott Williams & Wilkins Absolute Risk Reduction and Increase Absolute risk reduction (ARR): the difference between the injury rate/risk in the intervention group and the injury rate/risk in the control group. –The control group incidence is always subtracted from the intervention group incidence. –As a percentage, ARR indicates the reduction in number of injuries per 100 people who received the intervention. Absolute risk increase (ARI): If the experimental condition is found to lead to heightened risk of injury, the sign is changed to positive and termed “ARI” (rather than expressing the absolute risk reduction as a negative number). The absolute risk reduction is dependent on the magnitude of the injury incidence.

Copyright © 2011 Wolters Kluwer Health | Lippincott Williams & Wilkins Numbers Needed to Treat Numbers needed to treat (NNT): represents the number of patients that need to be treated with the experimental treatment to prevent one injury compared to receiving the control condition. –A statistic that builds on the absolute risk reduction. –Calculated by taking the inverse of the absolute risk reduction (1/ARR) comparing two groups. –The ideal NNT is 1. –The worst NNT is infinity (∞). Numbers needed to treat to benefit (NNTB): When NNT refers to a treatment that benefits patients. Numbers needed to treat to harm (NNTH): When NNT refers to a treatment that is deleterious to patients (i.e., causing adverse events).

Copyright © 2011 Wolters Kluwer Health | Lippincott Williams & Wilkins Odds The last statistic discussed in this chapter involves the odds of being injured. The odds of any given population becoming injured is calculated by dividing the injury risk by 1 minus the injury risk. –Remember that odds must be in comparison to another number. By calculating an odds ratio between the two groups, a clearer estimation of the treatment effect of the intervention becomes evident.

Copyright © 2011 Wolters Kluwer Health | Lippincott Williams & Wilkins Chapter Summary and Key Points Determining an appropriate sample size is a critical step in designing a prospective study of injury risk factors. Due to the limitations of case control designs, results of case control studies must be put into their proper context. Care must be taken to avoid any bias when determining subject selection. The simplest comparison between two measures of injury incidence is to calculate the ratio of the injury incidence between two groups. Many case control studies rely on patient self-report for their injury history. An athlete-exposure refers to one athlete participating in one practice or game. The ideal NNT is 1, meaning that for every patient treated with the experimental treatment, an injury is prevented. The worst NNT is infinity (∞), meaning that an infinite number of patients would need to be treated to prevent one injury.