Introduction

Although many studies have examined the incidence of anterior cruciate ligament (ACL) tears for given populations, an overall understanding of the real incidences is difficult to ascertain due to the breadth of the data and the disparate manner in which it is reported. The overall number of ACL tears appears to be increasing. This is caused in part by the increased participation of females in high-risk sports, as females clearly have an overall higher incidence of ACL tears than males. This realization has spawned the creation of training programs designed to decrease the incidence of ACL tears in females. The increase in ACL tears is also fueled by the increase in sports participation, from seasonal to yearlong, by athletes of both genders. This results in an increased number of exposures per year above that which was present for single-sport athletes in the past. Some sports appear to have higher risks of ACL tears than others, but these differences have not been well understood. Knowing the relative and absolute risks of ACL tear as a function of these parameters serves to focus attention on preventive strategies, where it is most needed. It also allows athletes and parents to understand the risks of participation in various sports.

Purpose

Our purpose was to acquire and review all of the relevant peer-reviewed published data on the incidence of ACL tears for the purpose of comparing the incidence of ACL tears in the following ways:

1 Among sports

2 Between females and males

3 Between those who have completed a program to decrease the incidence of ACL tears and those who have not

Methods

A computerized search of all papers in the peer-reviewed literature that had a possibility of dealing with the incidence of ACL tears was performed using a variety of indexing terms. Searches were then carried out by individual sports. This produced 793 articles that had some relation to knee or ACL injuries. These articles were reviewed, and bibliographies were cross-referenced for other papers, which were also reviewed for the purpose of identifying studies that had actual numerical incidences of complete ACL tears; this eliminated the overwhelming majority of papers. However, 33 papers were found that did have such quantitative data, and they form the basis for this chapter. Of these 33 papers, 25 had data that either used, or could be converted into, the preferred ACL injury incidence reporting method, namely: “ACL tears/1000 exposures.” An exposure is defined as either a practice or a game. These studies are listed in Table 4-1. They are divided by sport and then subdivided by level of competition, gender, and whether an ACL injury-reduction training program had been applied. The ratio of injury of females versus males is also listed for studies in which there were cohorts for both genders. These data form the basis for the analyses present in this chapter.

Table 4-1 ACL Tear Rate by Sport, Gender, and Injury Training*

Table 4-2 aggregates like subgroups from Table 4-1 and provides mean injury rates weighted according to the number of exposures. Table 4-3 lists the remaining studies, which use methods other than “tears/1000 exposures”¹^–⁸. Table 4-4 aggregates the like populations from studies that compared incidences by gender. Table 4-5 lists all the studies that involved training regimens designed to reduce ACL tear incidence.

Table 4-2 Weighted Means for Groups*

Table 4-3 Studies not Expressed in ACL Tears/1000 Exposures

Table 4-4 Ratios of Female to Male ACL Tear Rates*

Table 4-5 Effect of ACL Reduction Training Program on Tear Rate*

Exposures

In all of these studies it is important to remain cognizant of the number of exposures in the given study. The variance in the number of exposure between the studies is very large. The largest study has more than 8 million exposures and the smallest has only 600, a difference of more than 10,000 to 1 in the statistical power of the studies. These differences are so important that we have highlighted all the studies with exposures of more than 100,000 to make it easier for the reader to recognize those incidence studies of greatest statistical power.

Data Conversions

A number of studies report their incidence by dividing the number of ACL tears by hours of participation instead of practices. In these studies the hourly incidence was therefore used to calculate an incidence per 1000 exposures by converting hours to practices or games and adjusting the incidence accordingly. Doing so allowed these series to be used in the comparative analysis with the other studies, which used the tear per exposure methodology. Without this method, a large number of useful studies would have been lost from the analysis. If an exact practice length was not listed, we assumed a practice length of 2.25 hours. In the study by DeLee,²⁸ the data were presented in tears per hour, but exact data were given on number of practices and games and their lengths, so the data could be directly transformed into tears per 1000 exposures.

Individual Sports

Alpine Skiing

The alpine skiing data are notable for the large disparity in incidence between ski lodge employees, who are assumed to be expert skiers, and recreational skiers. The two studies of ski lodge employees by Oates and Viola show rates of 0.02 and 0.04, respectively.^9,¹⁰ The two general population studies by Warme and Deibert show tenfold higher rates of 0.63 and 0.40, respectively.^11,¹² Although the rate for the expert skiers is the lowest for any of the high-risk sports studied, the rate for the recreational skiers is overall one of the highest (P < 0.001). It is remarkable that this huge disparity is produced by two independent studies in each group, each with a very large number of exposures. These observations are thus of high reliability and statistical power.

The lower risk among the expert skiers is presumably a combination of increased skill and increased fitness in this group. The expert group is also remarkable for being one of only two cohorts for which the rate of injury is the same for males and females.

The lack of a gender difference in the large study by Viola ¹⁰ is also remarkable. Aside from lacrosse, alpine skiing is the only sport studied with a large enough number of exposures to generate reliable numbers to find this lack of a gender difference.