An essential part of understanding the increasing trend of pediatric ACL injuries is not simply understanding the rise of single-sport specialization but also the risk factors associated with specific sports that are played by pediatric athletes.

The National High School Sports-Related Injury Surveillance Study found the incidence of knee injuries to be 2.98 knee injuries per 10,000 athletic exposures, with an athlete exposure defined as a single practice or competition [22]. As may be expected, competition had a significantly higher risk of leading to a knee injury than practice (RR = 3.5) in nearly every sport. Boys football, girls soccer, and girls gymnastics had the highest rates of knee injuries: 6.3, 4.5, and 4.2 injuries per 10,000 athletic exposures, respectively. In competition, these numbers increased drastically for football, girls soccer, and girls gymnastics: 21.1, 10.8, and 9.4 injuries per 10,000 competitive exposures, respectively. ACL injuries comprised 25.4% of all knee injuries, and the highest numbers were seen with the same sports: boys football and girls soccer had 1.17 ACL injuries per 10,000 athletic exposures, and girls gymnastics had 1.14.

In addition, Gornitzky et al. performed a review of the literature to determine specific ACL injury rates in high school patients per 1000 athletic exposures (Table 4.1). A total of 700 ACL tears in over 11 million exposures were analyzed. Football and soccer were again found to be the sports with the highest incidence of ACL injury, with 0.089 and 0.099 injuries per 1000 athletic exposures, respectively. These values are similar to those quoted in the National High School Sports-Related Injury Surveillance Study. Other sports with increased rates of the ACL injury were basketball (0.055 ACL injuries per 1000 exposures), lacrosse (0.063 ACL injuries per 1000 exposures), and field hockey (0.048 ACL injuries per 1000 exposures) [25].

Hence, one could argue that special attention should be placed toward these sports in regard to injury prevention.

In addition to risk as it relates to the specific sport played, gender also represents a risk factor for ACL injury. The difference in risk for ACL tears based on gender has been quoted in numerous studies, ranging from two to nine times greater for women than men [25]. This same difference is seen throughout the literature for pediatric ACL injuries [15–18, 20, 22, 23, 25–32]. Database analyses have verified these differences as well. Comstock et al. in the National High School Sports-Related Injury Surveillance Study found that female ACL tears comprised 35.1% of knee injuries, whereas male ACL tears comprised only 19.0% of all injuries [17]. These differences hold true even when controlling for the sport played. A study analyzing insurance data from a company specializing in soccer injuries found that of all knee injuries, 37% of ACL tears were female and 24% were male [20].

There are a number of theories behind the causation of this gender differential risk which have been thoroughly examined in the adult and pediatric literature. The differences can be divided into intrinsic anatomic factors of the knee, extrinsic biomechanical factors of the surrounding muscles, and hormonal factors [33, 34]. The first intrinsic factor often cited is the increased Q angle in female [35]. A second risk factor is narrow notch width. A narrow notch is thought to result in impingement of the ACL, placing the athlete at risk for rupture. Notch width values of <17 mm have been found to be associated with increased ACL injury [36], and women have been found to have narrower notch width indices when compared to men. This has also been verified with CT imaging [37].

Extrinsic factors, namely, the biomechanical forces exerted on the knee joint by the quads and hamstrings, are also thought to contribute to the increased risk for ACL injuries seen in females. Female athletes preferentially fire their quads over their hamstrings when encountering anterior tibial translation. This is different from nonathletes and male athletes who compensate for this translation with hamstring activation [38, 39]. This differential neuromuscular activation also results in different jumping and landing mechanics. Studies have found that female athletes have a tendency of landing from a jump in increased extension, thus increasing the forces that are exerted across the knee and increasing the chances of an injury [40]. Laxity of the musculature and capsule of female knees relative to males has also been implicated in allowing for the rotational moment leading to ACL injury [41].

Fortunately, interventions exist to correct these extrinsic differences and can be particularly beneficial for young female athletes [42–46]. The most consistently successful intervention is the implementation of a neuromuscular training program, which includes strengthening, stretches, and warm-ups, focusing on increasing the control of certain muscle groups and improving the biomechanics of landing [44]. In a recent meta-analysis, Myer et al. found that the impact of these prevention programs was influenced by the age of the participant. Those who participated in the prevention program had half the probability of an ACL injury than those who did not, and participants in their mid teens (14–18 years) had a 72% reduction in their risk of ALC injury compared to late teens (18–20) who had a 52% reduction in their risk [46].

Once the culture in which these injuries take place is understood and the risk factors are identified, it is important to note that additional injuries can oftentimes be just as debilitating for the pediatric athlete who has many years of high-impact sporting activity ahead of them. These include medial and lateral meniscus tears as well as chondral injuries.

Werner et al. in their private insurance claims database analysis from 2007 to 2011 found the incidence of concomitant injuries/treatment increased over the same time period as ACL injuries. Increases in the meniscus repair and debridement from 2007 to 2011 in the 10–14-year and 15–19-year cohorts were significant when compared to adults. Approximately 50% of all patients from ages 15 to 19 with ACL tears who underwent intervention also had a partial meniscectomy, and 25% of patients from 10 to 19 had a meniscal repair [18].

The incidence of these secondary injuries increases with the amount of time the knee remains unstable [47–51]. Millett et al. demonstrated that delays in surgery as little as 6 weeks were associated with an increased incidence of medial meniscus tears [47]. Lawrence et al. found similar results in delays greater than 12 weeks as well as an increased incidence of lateral hemi-joint chondral injuries [48]. More recently, Newman et al. further analyzed the risk factors for these secondary injuries and found that in younger patients (<14 years of age), a delay greater than 3 months was most predictive of secondary chondral injuries [51]. To achieve more granularity in this timing, Anderson et al. classified repairs into acute (<6 weeks after injury), subacute (6–12 weeks), and chronic (>12 weeks) and found worsening damage of both the cartilage and meniscus with increased time [49]. Thus, when examining pediatric ACL injuries, care should be taken to recognize that there is a significant incidence of secondary injuries which are exaggerated by delays in care, perhaps reflecting the trend toward more surgical intervention in this population.