Epidemiology of Athletic Injuries

Eric D. Zemper

Randall W. Dick

This chapter will introduce some fundamental concepts of epidemiology, the basic science of preventive medicine, and its application to sports medicine, specifically the epidemiology of athletic injuries. The word “epidemiology” is comprised of three Greek root terms: epi (meaning “upon”), demos (“people”), and logos (“study”). Therefore, epidemiology is the study of what is upon, or befalls, a people or population. A more formal definition is that provided by Duncan (1):

Epidemiology is the study of the distribution and determinants of the varying rates of diseases, injuries, or other health states in human populations.

The basic method of studying and determining these distributions and determinants is comparing groups within a population (the sick and the well; the injured and the noninjured). Doing an epidemiological study is a lot like being a detective, using logic to discover cause and effect relationships for illnesses or other medical conditions in a population. In many ways it is similar to diagnosing an illness, but it is done with a large population rather than with an individual patient.

Duncan (1) lists seven major uses for epidemiological data:

Identifying the causes of disease
Completing the clinical picture of a disease
Allowing identification of syndromes
Determining the effectiveness of therapeutic and preventive measures
Providing the means to monitor the health of a community or region, that is, input for rational health planning
Quantifying risks (health hazard appraisals)
Providing an overview of long-term disease trends

The initial development of the theory and methods of epidemiology focused on applications to communicable diseases, and guidelines for public health surveillance systems for communicable diseases have been published by the Centers for Disease Control and Prevention (CDC) (2). However, in recent years epidemiological theory and methodologies have been applied to a broader range of subject areas, including athletic injuries. One of the primary tools in applying epidemiological theory and methods to the study of athletic injuries is the use of the techniques of injury surveillance.

Sports injury surveillance applies the well-established principles of public health surveillance to the problem of athletic injury. Injury surveillance is not the same as injury research, although the two are similar. Injury research involves the slow and thorough accumulation of very precise data and can take years to come to fruition. By contrast, injury surveillance uses methods for the rapid collection and dissemination of data and evolves and develops to meet the ever-changing needs of the sports medicine community in general, and users of the data in particular (3). A thorough review of injury surveillance definitions and guidelines has been developed by the World Health Organization in conjunction with the CDC (4).

Meeuwisse and Love (5) suggest that researchers should take the following general recommendations into consideration when collecting and publishing injury data:

Maximize comparability of data between systems through clear indication of reporting system design and the methods used to collect data.
Clearly define what constitutes a reportable event.
Collect outcome information on each reportable event.
Acknowledge any potential source of error.

The same authors (5) suggest that an “ideal” system for assessing athletic risk would include the following:

Simplicity and ease of use
Flexibility to address changing patterns of injury
Collection of athletic exposure data
Standardized documentation of injury diagnosis, severity, treatment, and associated risk factors
Data collection by team athletic trainers who work with the team on a daily basis

A recent publication by Fuller et al. (6) contains recommendations for appropriate injury definitions and data collection procedures for injury surveillance, specifically for the sport of soccer, which involve many of the above concepts.

Epidemiological Rates

The basic tool of epidemiology is the calculation of rates of occurrence of medical cases of interest in a given population. The two most commonly used rates are prevalence and incidence. The prevalence rate includes all cases of the medical condition of interest that exist at the beginning of the study period and all new cases that develop during the study period. Incidence rates include only the newly developed cases. In sports medicine, the incidence rate is predominantly used to study athletic injuries, because it is assumed that all athletes are uninjured at the beginning of the season and it is the incidence of new injuries during the season that is of interest. Therefore, we will deal only with incidence rates here. The incidence rate is a measure of the rate at which new events (illnesses, injuries, etc.) occur during a specified time in a defined population:

Incidence Rate = (number of new events during specified period × k) ÷ number in the population at risk

The numerator is simply a count of the number of new cases that occur during the study period. The denominator is the total number of people in the population under study who are “at risk” or exposed to the possibility of infection, injury, and so on. To provide reasonable numbers that are neither extremely large nor extremely small, and to make comparisons easier, this ratio is transformed to a common metric by multiplying by a convenient multiple of 10 (represented by the constant k in the preceding equation). If k = 1,000 the result would be a rate per 1,000 in the population; if k = 100,000 the result would be a rate per 100,000. For example, suppose 24 cases of measles were reported on a college campus of 34,000 students. A moment’s thought will show that stating a rate of 24/34,000 is not the most informative way of presenting this information. The probability of an individual having the disease is not readily apparent, and it is not easy to compare the rate with the five cases that occurred in the population of 630 student-athletes on that campus. The base ratio of 24/34,000 is 0.000706, which is the probability that any one individual has measles. But obviously this is not an easy number to work with. Using k = 100,000 we transform this rate to 70.6 cases per 100,000, which is a little more manageable. If we make the same calculation for student-athletes, we get a case rate of 793.7 cases per 100,000. Now it is easier to see that student-athletes had a much higher rate of measles, so immediate preventive measures might be in order for this special population.

Determining the numerator of the case rate equation is usually relatively easy. The most critical part of the calculation is determining the denominator, or the “population at risk.” This should include everyone in the population who could be affected by the disease or condition of interest, and should exclude those who could not be affected or are not really a member of the population of interest. For instance, in calculating a case rate for pregnancy, males, females past menopause, and females who have not reached menarche should not be used in the denominator. In calculating a case rate for football injuries during games, only those who actually played and were exposed to the possibility of injury, not the whole team, should be included in the denominator.

In sports medicine, case rates are generally used to present epidemiological information about athletic injuries. In the past, these rates have been presented most often as injuries per 100 athletes, which is analogous to the rate per 100,000 population used for reporting disease rates. However, there is a difference between the continuous exposure of a population to a disease and the discrete exposure of an athlete to injury, which occurs only during practices or games. The number of practices and games varies considerably from one sport to another, and often varies from one team to another, or even from one year to another in a given sport. In addition, not every player participates in every practice and every game, and the number of participants on a team may change considerably as the season progresses. Therefore, the common practice of reporting athletic injuries as a rate per 100 participants can lead to questionable conclusions, particularly when results from different sports, or even from different studies of the same sport, are compared. A more precise method is to report case rates per 1,000 athlete-exposures (A-Es). An A-E is defined as one athlete participating in one practice or game where there is the possibility of sustaining an athletic injury. If a football team of 100 players has five practices during the week, there are 500 A-Es to the possibility of being injured in practice during that week. If 40 players get into the game on Saturday, the team has 40 A-Es in the game, and the weekly total is 540 A-Es to the possibility of being injured.

Using A-Es as the denominator allows more accurate and precise comparisons of injury rates between sports and in different years (7). Case rates per 1,000 A-Es are currently used by the National Collegiate Athletic
Association (NCAA) Injury Surveillance System (ISS) (8) and the Athletic Injury Monitoring System (AIMS) (9). An even more precise approach would base the exposure rate on the amount of time actually spent in practices or games. Case rates per 1,000 hours of participation exposure might be possible in small local studies, and should be done at this level if possible. But, in most cases, the amount of record keeping required for a national-scale surveillance system would be prohibitive and impractical for those doing the on-site data recording. Case rates per 1,000 A-Es are therefore believed to be a reasonable compromise that gives a more accurate picture of the epidemiology of athletic injuries than the use of simple rates per 100 athletes.

Concerns Regarding Published Literature on Sports Injury Rates

A major weakness commonly seen in the published literature on athletic injury rates is that the denominator data for the incidence rate equation is poorly defined or has not been determined. This reduces these articles to simple case series reports that have little or no epidemiological value (10). Unless the calculation of rates is based on the population at risk, it is impossible to generalize the results beyond the specific population used in the study. This highlights a major problem in much of the earlier research literature on athletic injury rates, and even some of the current literature: most authors have little or no training in epidemiology, so these articles often are not of any great use on a broader scale in that the information cannot be generalized to other places and situations. For example, several years ago Powell et al. (11) did a thorough review of the literature on running injuries and found only two published articles and one meeting presentation that met minimal criteria for factors such as definition of injury, selection of subjects, and use of proper denominator data (“population at risk”) in calculating injury rates.

Two to three decades ago the research literature on the epidemiology of athletic injuries was very sparse, but since the mid 1960s there has been a slow growth in sports injury rate research as the need for this type of data has become more apparent. Even so, many studies cover only one year (or season), occasionally two. Nearly all studies have limitations imposed by sample size, covering one school, one city, or one geographical area. Some studies are limited to injuries of one anatomical site, such as the knee, or one type of injury, such as fatalities or ankle sprains. Getting a clear national perspective by combining results from different studies are greatly hindered by differences in methodologies, such as dissimilar definitions of a reportable injury or different means of collecting and reporting data. Combining study results would be ill advised in any case, because it would be highly unlikely that the various data sources that are combined are truly representative of athletes and teams across the whole country.

Still another problem with many studies is the source used to obtain injury data. Some rely on insurance claim forms, which has the disadvantage of not representing the true injury rate as not all athletic injuries result in insurance claims. Also, these records seldom contain much detail on the circumstances and mechanisms of injury or even the diagnosis itself. Some studies rely on a coach’s assessment or recognition of an injury although it has been shown that, unless coaches have received specific training, they do a poor job of recognizing most treatable injuries (12). Studies that depend on recall of injuries at the end of a season have the obvious problems of inaccuracy and incompleteness of recall. One recent study (13) compared weekly recording of soccer injuries with a retrospective questionnaire completed by the athletes at the end of the season, and showed that although more than 80% of the players were injured during the season, less than half the players reported being injured. Only 73% of serious injuries, approximately one third of the moderate injuries, and less than 10% of the mild injuries recorded by medical staff during the season were reported by the athletes in the retrospective questionnaire at the end of the season.

Another example where the source of information has limitations is the National Electronic Injury Surveillance System (NEISS) which compiles injury information related to consumer products. The Consumer Product Safety Commission (CPSC) initiated the system in 1971 utilizing a national stratified probability sample of 100 U.S. hospital emergency departments. In 2000, the CPSC in collaboration with the CDC’s National Center for Injury Prevention and Control expanded NEISS to include all injuries, regardless of being consumer product related. The expanded system called the National Electronic Injury Surveillance System All Injury Program (NEISS-AIP), utilizes a national stratified probability sample of 66 of the original 100 NEISS emergency departments to track injuries, including athletic injuries (14).

The hospitals selected for NEISS-AIP were chosen as a nationally representative sample of emergency departments, and therefore the system should be sensitive to those injuries that are seen by an emergency department (15). However, many, if not most, athletic injuries never make it to the emergency department and so are not captured by the system. A second limitation is that no true exposure values of individuals at risk for athletic injuries are associated with the system. Although this system can outline the relative proportion of more serious injuries in certain activities, it still falls short of a sport-specific surveillance system with associated appropriate exposure information.

It has become evident over the past 25 years that there is a need for accurate, reliable data on injury rates for various sports and exercise activities. With the increase in participation in organized sports and in fitness activities, participation that is encouraged by the medical community as a public health intervention, it is often not realized that even today there still is little or no dependable risk data available for these activities. A great deal of effort is
focused on defining the benefits of participation of sports and fitness activities, but little is done to assess risk (16). This information is needed to make informed decisions about the value of taking part in a particular activity, and to provide information on how injury rates can be reduced. Therefore, it is desirable to collect data for all types of exercise and fitness activities as well as all levels of sports participation. Unfortunately, although there has been improvement in recent years in the data available for some sports activities, little or no data are available at this time for anything other than college and high school sports. A comprehensive compilation and review of the literature on sports injury epidemiology can be found in the book by Caine et al. (17).

Model Sports Injury Data Collection Systems

Several sports injury data collection systems have been developed using the concept of rates discussed earlier. The following examples meet most of the criteria noted earlier for an “ideal” system, and the published reports from these systems avoid the problems noted in the previous section.

National Center for Catastrophic Sports Injury Research

In 1931, the American Football Coaches Association initiated the First Annual Survey of Football Fatalities and this research has been conducted at the University of North Carolina, Chapel Hill, since 1965. In 1977, the NCAA initiated a National Survey of Catastrophic Football Injuries, which is also conducted at the University of North Carolina. As a result of these research projects important contributions to the safety of the sport of football have been made. Most notable have been the 1976 rule changes, prohibiting spearing and initial contact with the head and face when tackling and blocking, the implementation of the football helmet standard, improved medical care for the participants, and better coaching techniques.

Because of the success of these two football projects, the research was expanded to all sports for both men and women, and a National Center for Catastrophic Sports Injury Research (NCCSIR) was established in 1982. In 1987, a joint endeavor was initiated with the section on sports medicine of the American Association of Neurological Surgeons by involving a physician to assist in the collection of medical data. Since 1982, the Center has attempted to collect information on all catastrophic high school and collegiate sports-related injuries (18). Although a primary goal of this system is to understand the cause of these events, participation data can also be used as a denominator to arrive at rates for risk comparison across activities.

The decision to expand the research was based on the following factors:

Research based on reliable data is essential if progress is to be made in sports safety
The paucity of information on injuries in all sports
The rapid expansion and lack of injury information in women’s sports

Definitions

The Center uses consistent, clear injury definitions to create a catastrophic injury database with significant application. Injury definitions include:

Catastrophic injury. Any severe injury incurred during participation in a school/college sponsored sport involving a fatality, permanent severe functional disability, or severe head or neck trauma.
Direct injury. Those injuries resulting directly from participation in the skills of the sport, such as paralysis resulting from a football tackle.
Indirect injury. Those injuries caused by systemic failure as a result of exertion while participating in a sport activity or by a complication that was secondary to a nonfatal injury. Catastrophic heat illness or cardiac problems would fit into this category.

Data Collection

As the absolute number of catastrophic injuries is small, the goal is to collect information on every event. Data are compiled with the assistance of coaches, athletic trainers, athletic directors, executive officers of state and national athletic organizations, a national newspaper clipping service, and professional associates of the researchers. On receiving information concerning a possible catastrophic sports injury, contact by telephone, personal letter, and questionnaire is made with the injured player’s coach or athletic director. Data collected include background information on the athlete (age, height, weight, experience, previous injury, etc.), accident information, immediate and postaccident medical care, type of injury, and equipment involved. Autopsy reports are used when available.

Applications

Specific data may be obtained by accessing the Center’s most recent annual report at www.unc.edu/depts/nccsi. It provides data on a variety of sports and is one of the few resources on catastrophic injuries in the activity of cheerleading. Examples of sport-specific applications of this data collection as noted in the annual report are presented later in this chapter.

The NCAA Injury Surveillance System

Established in 1982, the NCAA ISS (8) is the largest continuous collegiate injury surveillance system in North America and possibly the world. Its mission is to provide current and reliable data on injury trends in intercollegiate athletics in order to optimize student-athlete health and safety. From 1988 to 2003, injury data for 15 intercollegiate sports were collected annually from a representative sample
of NCAA member institutions through the volunteer efforts of certified athletic trainers. Since 2003, the ISS has been updated from a paper-based to a web-based format that has the capability of collecting data on all NCAA championship sports as well as club sport activities. This web-based system allows the athletic trainer at an individual school to have a real-time electronic record of athletic training room activities while also contributing to an aggregate national database. The NCAA ISS includes many of the qualities noted previously by Meeuwisse and Love (5).

Sports monitored. All NCAA championship sports and a variety of club activities.
Seasons. The traditional sports seasons are used for data collection and are divided into the following three subcategories:
- Preseason—full team practices/competitions before first regular season contest.
- In-season—all practice and games from the first regular season contest through last regular season contest.
- Postseason—all practice and games following the last regular season contest through the last postseason contest.

Definitions

Injury. A reportable injury according to the NCAA ISS is defined as one that:
- Occurs as a result of participation in an organized intercollegiate practice or game
- Requires medical attention by a team athletic trainer or physician
- Results in any restriction of the student-athlete’s participation or performance for one or more days beyond the day of injury
Exposures. An A-E, the unit of risk in the ISS, is defined as one athlete participating in one practice or game where he or she is exposed to the possibility of athletic injury.
Injury rate. An injury rate is a ratio of the number of injuries in a particular category to the number of A-Es in that category. In the ISS, this value is expressed as injuries per 1,000 A-Es. For example, six reportable injuries during 563 practice exposures result in an injury rate of (6/563) × 1,000 or 10.7 injuries/1,000 A-Es. In this example, one would anticipate 10.7 injuries if one athlete participated in 1,000 practices, if 50 athletes participated in 20 practices, or if 100 athletes participated in 10 practices.

Sampling

Participation in the NCAA ISS is voluntary and limited to the 1,026 NCAA member institutions (as of September 2005). Before the web-based system was established in 2003, ISS participants were selected from the population of institutions sponsoring a given sport. A convenience sample of those schools indicating a willingness to participate was generated with a goal of establishing an appropriate weighted sample of each NCAA division (I, II, and III) and at least 10% of all schools sponsoring the particular sport. With the introduction of the web-based system, all schools have the ability to participate and contribute data to as many sports as they wish. Schools submitting an appropriate amount of exposure weeks and completed injury forms are included in the national sample with appropriate divisional weighting. It is important to emphasize that this system does not identify every injury that occurs at NCAA institutions in a particular sport. Rather, it collects a sampling that is representative of a national cross-section of NCAA institutions.

Data Reporting

Injury and exposure data are recorded in the web-based system on an ongoing basis by certified athletic trainers and student athletic trainers from participating institutions. Information is collected from the first official day of preseason practice to the final tournament contest.

Applications

One of the unique features of the ISS is the annual mechanism for data review and policy development that benefits not only collegiate athletics but also the general sports medicine community. The primary audience for this system was originally envisioned to be the NCAA Committee on Competitive Safeguards and Medical Aspects of Sports, and relevant NCAA sport rules committees, who used the information as a resource upon which to base recommendations, rules, and policies impacting student-athlete health and welfare. However, over the last decade the audiences interested in this information have expanded to include individual colleges and universities, sports medicine researchers, other administrative and sports medicine organizations, media, and the general public. The system’s data have been applied to development or modification of sports rules, policies, and issues by organizations such as the American Medical Society for Sports Medicine (AMSSM), American College of Sports Medicine (ACSM), American Orthopaedic Society for Sports Medicine (AOSSM), CDC, and the National Athletic Trainers’ Association (NATA). Such efforts have benefited not only collegiate athletics but also the entire sports medicine community. Some specific examples are noted later. Data collection through the ISS followed by annual review through the NCAA sport rules and sports medicine committee structure is a unique mechanism that has led to significant advances in health and safety policy within and beyond college athletics. Basic comparative injury information across sports and general information about the ISS is available on the NCAA Web site (www.ncaa.org/ISS).

Athletic Injury Monitoring System

The AIMS was established in 1986 as a national sports injury data collection system capable of doing injury surveillance
on a variety of sports. The structure and methodology of AIMS is very similar to the NCAA ISS. It was designed to complement the NCAA ISS in that it would cover noncollegiate sports as well as collegiate sports, and cover other levels of competition, from youth and high school to elite and older recreational athletes. AIMS meets the major criteria for reliable studies of sports injury rates outlined in 1987 by the AOSSM (19). Data collected by AIMS has been used for published reports on a number of issues in sports medicine, including general injury rates (9,20,21,22,23,24,25), concussion rates (26,27,28,29,30), prophylactic knee braces (31,32,33), and football helmets (34,35).

National Athletic Injury/Illness Reporting System

A predecessor to both ISS and AIMS was the National Athletic Injury/Illness Reporting System (NAIRS), developed by Kenneth S. Clarke at Pennsylvania State University in the mid-1970s. This first major attempt at a national sports injury data collection system incorporated many important new design features such as longitudinal data collection from a much larger sample than previously had been attempted, standardized definitions and procedures, and the use of case rates per 1,000 A-Es. However, there were concerns about the number and complexity of the data collection forms and the lack of a truly representative national sample. NAIRS stopped collecting high school and college data in 1983, but it produced by far the best and most comprehensive sports injury data available up to that time. As NAIRS was completing its data collection efforts, the NCAA instituted its own ISS. The NCAA ISS was designed and implemented in 1982 by Eric D. Zemper, as a member of the NCAA staff. Since 1985, ISS has been under the direction of Randall W. Dick. ISS essentially is a direct descendent of NAIRS, being similar in many ways to NAIRS, but utilizing two simplified basic data collection forms, and using a representative national sample of NCAA member schools (8,9,10,11,12,13,14,15,16,17,18,19,20,21,22,23,24,25,26,27,28,29,30,31,32,33,34,35,36). The AIMS was also designed and implemented by Zemper, while at the University of Oregon. AIMS was developed in 1986, utilizing the same basic format as NAIRS and ISS, with the intent of covering a wider variety of sports at all levels of participation (20,21,22,23,24,25,26,27,28,29,30,31

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