Key Points: 1. After summarising the evidence about "sport" as an extrinsic risk factor, the present review suggested potential differences between basketball and soccer female adolescent players in biomechanical risk factors predisposing to ACL injury risk while performing dynamic tasks. However, no good-quality evidence of differences in primary noncontact injury rate was reported between different sports for both male and female adolescents. 2. There was contrasting evidence about how the extrinsic risk factor "amount of exposure to sport" could be related to biomechanical and neuromuscular risk factors predisposing to noncontact ACL injury as well as primary noncontact ACL injury rate in both male and female adolescent players from different sports making it difficult to draw conclusions about this risk factor, while few good-quality articles suggested higher "sport level" as an extrinsic risk factor that might be associated with increased primary noncontact ACL injury rate in female adolescents playing basketball and floorball. 3. There was weak evidence suggesting that the extrinsic risk factors "environmental conditions" (in both male and female adolescent soccer players) and "season phase" (in male adolescent basketball players) might be associated with differences in biomechanical risk factors connected with ACL injury risk, while good-quality research investigating "equipment" as an extrinsic risk factor suggested that bracing might not prevent primary noncontact ACL injury in both male and female adolescent players from different sports. Abstract Background: Adolescents present a high incidence of ACL injury compared with other age groups. Examining the risk factors that predispose adolescents to primary noncontact ACL injury is a key strategy to decrease the number of injuries in this population. Objective: The aim of this systematic review was to summarise the existing literature investigating extrinsic risk factors that have been linked with primary noncontact ACL injury risk (identified either using ACL injury occurrence or using screening tests measuring biomechanical mechanisms for noncontact ACL injury) in adolescents including research investigating: (1) the association between extrinsic risk factors and primary noncontact ACL injury risk; and (2) whether primary noncontact ACL injury risk was different in populations or groups exposed to different extrinsic risk factors in adolescents. Methods: The same search strategy was used in MEDLINE, SPORTDiscus, CINAHL, PubMed and Embase. Articles were included if: written in English; published in peer-reviewed journals; investigating and discussing primary noncontact ACL injury risk associated with extrinsic risk factors; they were original research articles with an observational design; and participants presented a mean age ranging between 14 and 18 years. The Quality Assessment Tool for Observational Cohort and Cross-Sectional Studies (QATOCCS) was used to assess the quality and risk of bias of the articles included in this systematic review. Results: The systematic review included 16 eligible articles published up to August 2022 about extrinsic risk factors for primary noncontact ACL injury including: sport (8 studies); sport exposure amount (5); sport level (3); sport season (1); environment (2); equipment (1). Differences in biomechanical risk factors predisposing to ACL injury were reported by sport in female adolescents playing basketball and soccer; however, no good evidence of differences in primary noncontact ACL injury rate by sport was reported in both male and female adolescents. There was contrasting evidence about associations between sport exposure and biomechanical and neuromuscular risk factors predisposing to ACL injury or primary noncontact ACL injury rate in both male and female adolescent players from different sports. There was weak evidence of differences in biomechanical risk factors predisposing to ACL injury by environmental condition in both male and female adolescents playing soccer and season phase in male adolescents playing basketball. Lastly, few good-quality articles suggested that higher sport level might be associated with increased primary noncontact ACL injury rate in female adolescents playing basketball and floorball and that bracing might not prevent primary noncontact ACL injuries in both male and female adolescent players from different sports. Discussion: The findings emphasise the need for further research to clarify the evidence about extrinsic risk factors and primary noncontact ACL injury in adolescents to develop ACL injury prevention guidelines that would help practitioners and researchers identify adolescents at risk and design future interventions. Future epidemiological studies should collect data about extrinsic factors as well as data about primary noncontact injury separately from secondary injuries or contact injuries to better inform primary noncontact ACL injury prevention in adolescents.
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