A Scoping Review of Factors That Elevate the Risk of Anterior Cruciate Ligament Injury in Elite Male Field Team Sport Athletes

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Evidence-informed screening tests for ACLI risk play an important role in the decision-making process of athlete training in elite team sports. The ability to identify individual athletes’ risk to ACLI will promote athlete longevity via risk reduction strategies, maintain athlete availability, high-performance capacity, and team success.

Abstract

The primary aim of this scoping review was to identify practical risk factors associated with an elevated risk in anterior cruciate ligament injury (ACLI) in elite male field team athletes that can be applied meaningfully in screening tools by team support personnel. Five relevant databases were searched (SportsDISCUS, Medical Literature Analysis and Retrieval System Online, PsycINFO, Web of Science and Cumulative Index to Nursing and Allied Health Literature) following the PRISMA-ScR protocol using the criteria: (1) written in English and peer-reviewed; (2) full-text available; (3) discussed ACLI screening tests; (4) an elite athlete cohort; (5) males; (6) field team sport. The search identified 962 manuscripts, with nine manuscripts meeting the inclusion criteria. Field sports represented were soccer (n = 7), American football (n = 1), and a mixed-sport cohort of soccer, rugby, and field hockey (n = 1). Manuscripts reported modifiable risk factors (the joint range of motion n = 1, biomechanics n = 3, and strength n = 1) and non-modifiable (anatomical n = 2 and genetics n = 2). Whilst the joint range of motion screening indicated statistical significance to ACLI risk, there was little predictive value. Non-modifiable risk factors were significantly correlated to ACLI and reported a higher predictive capacity for ACLI risk. There is limited systematic research investigating and providing predictive insight for screening tests of ACLI risk in elite male team sport athletes. Future prospective investigations should consider the validity of ACLI screening tests in elite male field-based sport populations, and establish efficacy, so that sporting clubs can confidently implement screening tests of value into practice.

1. Introduction

Anterior cruciate ligament injury (ACLI) is one of the most debilitating injuries sustained by athletes. Over half of ACLI in team sport occur through non-contact mechanisms (changing direction, deceleration maneuverers, and landing) [1,2,3]. ACLI comes with significant physical and psychological burden on athletes, including reduced performance metrics, self-athletic identity, and career duration, as well as an increased re-injury and osteoarthritis risk [4,5,6,7,8]. For some athletes, ACLI is career-ending, with 21–26% of elite male team sport athletes not returning to play at the professional level [9,10,11]. This complex injury not only has consequences to the individual athlete but also for team members as the team’s season success can be negatively impacted [12]. The injury also affects team stakeholders as substantial costs are associated with surgical reconstruction of the anterior cruciate ligament (ACL) and rehabilitation [8].

Surgical reconstruction is frequently required following ACLI in elite athletes, and this jeopardizes return to competition in the same playing season [10,13]. Reported factors that can significantly affect the return to competition process include initiating weight bearing and motion immediately postoperatively, and avoiding bracing during the early postoperative period but using a functional brace during the actual return to sport [14]. During rehabilitation from surgical reconstruction for ACLI, muscle strength should be prioritized, followed by gains in proprioception [15]. However, O’Connor et al. [16] report that psychological recovery and physical recovery after ACL reconstruction are different constructs as they reported little or no relationship between measures of lower-limb strength and power with self-reported readiness to return to sport. People with ACLI have the longest absence from full sport participation out of all common injuries, with professional male soccer athletes absent for 198–218 days following ACLI (discontinued participation since injury to full participation agreed to by the medical team) [13]. Elite team sport populations have the highest non-contact ACLI proportion compared with intermediate and amateur level athletes, with elite male athletes having 59% compared with 48% in amateur-level male athletes [2]. Professional male athletes in team sports 3 years post ACLI were still to reach preinjury performance levels, and only 28.5–37% remained in their respective professional leagues [8,11,17]. Professional athlete populations have the greatest ACLI rates; thus, identifying factors for elevated ACLI risk in elite team sports is important to ensure team success, player availability, and athlete longevity.

To reduce the ACLI risk, firstly, we must understand the involved factors that can be classified as intrinsic and extrinsic. Intrinsic risk factors are inherent to the individual, and those that elevate the risk of ACLI can be screened for and further grouped as modifiable and non-modifiable [18]. The higher risk of ACLI in female athletes is reportedly due to a complex interaction of modifiable and non-modifiable factors. Modifiable risks identified for ACLI, in any sex, include strength, reactive strength, proprioception, biomechanics, neuromuscular control, sleep, psychosocial, psycho emotive, and neurocognition [19,20,21,22,23,24]. Thus, to a lesser or greater extent, each of these modifiable factors can be improved upon thorough training or behavior change. Changes in strength, reactive strength, and proprioception have been reported to have both have an association with ACLI and no association at all [19]. But considering that strength gains and, at the very least, the maintenance of lower-limb strength are accepted outcomes of athlete preparation strategies, it is possible that the precision or muscle function quality that is assessed is not always related to the population cohort’s ACLI mechanisms. Interestingly, Piskin and co-workers [23] in a previous systematic scoping review reported on the dearth of athlete specific data on the inclusion of neurocognitive challenges during rehabilitation, and yet the evidence clearly showed the link between cognitive skills and CNS functions to an increased injury risk and diminish postinjury performance in athletes. Non-modifiable ACLI risk factors, which have been previously reported, include joint morphology and anthropometrics among which include tibial and meniscal slopes, intercondylar notch width, ACL size, and hip morphology [19,25,26].

The evidence for which risk factors and associated screening tests are relevant for elite male field-based team sport athletes is unknown as these have not yet been comprehensively reviewed. Further, ACLI research has anecdotally focused on female athletes due to the significantly greater relative risk of ACLI (3 to 8 times greater) compared to males [27,28], yet this presents as a knowledge gap in male athletes. This scoping review focuses on elite male athletes in part to overcome the paucity of reviewed research in this population. Thus, the question asked was as follows: What practical risk factors are associated with an elevated risk in anterior cruciate ligament injury (ACLI) in elite male field team athletes that can be applied meaningfully in screening tools by team support personnel? The primary aim of this scoping review was to first identify factors associated with an elevated risk of ACLI and then consider how the associated screening test in elite male field team sport athletes can be practically applied. The secondary aim of this scoping review was to identify gaps in the literature surrounding ACLI screening in elite field-based team sport athletes that can guide future research.

2. Materials and Methods

2.1. Protocol

This scoping review followed the Preferred Reporting Items for Systematic Reviews and Meta-analysis Protocols extension for Scoping Reviews (PRISMA-ScR) [29]. In conjunction with this, the methodological framework as proposed by Arksey and O’Malley was utilized [30].

The initial research questions were developed to explore primary and secondary prevention of ACLI in elite sports, given their burden on athletes’ careers and wellbeing [29,31]. The purpose was to develop a scoping review of the literature to help practitioners find cost-effective tools for identifying athletes at increased ACLI risk and to identify gaps for further research [29,31]. The research questions were further refined through a consumer engagement consultation process with a key research consumer (accredited sports physiotherapist) at an Australian Football League club to address literature gaps for elite male athletes [29,31]. Then, to broaden the literature search scope, institutional librarians were consulted to ensure the search strategy captured the primary and secondary aims [29,31].

2.2. Eligibility Criteria

The inclusion and exclusion criteria were guided by the Participants, Intervention/Exposure, Comparators, Outcomes, and Study Design (PICOS/PECOS) framework and is summarized in

Table 1. Eligibility criteria applied according to PICOS model.

PICOS Category	Inclusion Criteria	Exclusion Criteria
P (Population)	Healthy adult male athletes playing a field team sport at a professional, semi-professional, collegiate, or National Collegiate Athletic Association competitive level 1 and were 18 yrs or older	Female-only participants or the male-only data could not be extracted; the sport reported on was a court sport not a field team sport
I (Intervention)	Studies analyzing screening tools or assessments	Training study outcomes only with no use of screening tools or assessments
C (Comparators)	Optional	The injury was of the knee generally not ACLI specific, or it was a re-injury situation
O (Outcomes)	New data on any modifiable or non-modifiable risk factor	Review papers
S (Study designs)	No restrictions on the types of study designs eligible for inclusion	N/A

¹ An international framework defined this as “elite” [32].

. In more detail, studies were included if they met the following criteria: (1) written in English and peer-reviewed; (2) full-text available; (3) discussed ACLI screening tests; (4) an athlete cohort described as professional, semi-professional, collegiate, or National Collegiate Athletic Association (NCAA); (5) males; (6) field team sport [29,31]. A recent international framework defined “elite” athletes under professional, semi-professional, and university [32]. The definition of “professional athlete” was limited to an individual who generated their primary income through their sport and played in a professional league [21,23]. “Semi-professional athlete” was defined as an individual receiving moderate payment and/or commercial sponsorship whilst relying on supplementary income sources [32,33]. “University athlete” was defined as an individual competing in the NCAA Division 1 [32]. “Field team sport” was defined as invasion sports played on grass (natural or artificial) and not on hard court.

Studies were excluded if (1) the participants were female only; (2) the participants did not fall under the elite category as defined by the recent international framework that defined elite athletes (recreational/amateur/sub-elite/high school, ≤division 2 NCAA) [32]; (3) the study was of non-field-based team sport; (4) the participants were <18 years old; (5) we were unable to differentiate between sexes or sport type in results; (6) the injury examined was knee injury and not ACLI specific; (7) the study was of ACLI re-injury; (8) the study was a review paper [29,31]. No time filters were applied to allow for the broadest scope of the literature to be included in the screening process.

2.3. Data Sources and Searches

An online literature search was first conducted on 16 September 2022 by searching electronic databases including SportsDISCUS, Medical Literature Analysis and Retrieval System Online (MEDLINE), PsycINFO, Web of Science and Cumulative Index to Nursing and Allied Health Literature (CINAHL) [29,31]. A second updated search was conducted on 29 February 2024 to identify any new studies, which included a time filter to only include manuscripts published after the initial search.

Search strategy keywords and phrases (

Table 2. Search strategy presented with search concepts and applied using Boolean and Mesh search terms.

Keywords		Mesh Terms
Concept 1	((“ACL” OR “ACLI” OR “ACL injur*” OR “anterior cruciate ligament* injur*”) NOT (“reconstruction” OR “shoulder” OR “operation” OR “surg*” OR “return to play” OR “rtp” OR “return to sport” OR “rts” OR “reinjury”))	OR	MEDLINE: “Anterior Cruciate Ligament Injury/” PsycINFO: “injuries/” OR “physical disfigurement/” OR “physical disorders/” OR “sports medicine/” OR “knee/”
AND
Concept 2	(“risk*” OR “predict*” OR “screen*” OR “screening test” OR “screening tool” OR “deter*” OR “prevent*” OR “measur*” OR “monitor*” OR “odds ratio” OR “assess*” OR “intervention”)	OR	MEDLINE: “Screening Tool/” PsycINFO: “Test Reliability/” OR “Screening Tests/” OR “Screening/” OR “physical strength/” OR “physical therapists/” OR “physical therapy/”
AND
Concept 3	((“collegiate” OR “athlet*” OR “field sport*” OR “player*” OR “elite” OR “competitive” OR “football” OR “rugby” OR “AFL” OR “NFL” OR “american football” OR “professional” OR “australian football” OR “soccer”) NOT (“females” OR “female” OR “woman” OR “women” OR “women’s” OR “court” OR “basketball” OR “adolescent” OR “ski” OR “handball” OR “recreational” OR “high school” OR “amateur” OR “youth” OR “cricket” OR “cadaver” OR “volleyball”))	OR	MEDLINE: “Athletes/” PsycINFO: “College Athletes/” OR “Athletes/” OR “Professional Athletes/”
AND
Concept 4	(“male” OR “men”)	OR	MEDLINE: “Male/” PsycINFO: “Human Males/”

) remained the same in each database for consistency of results. Boolean operators were utilized. The four concepts were combined with “AND” and keywords within each concept combined with “OR”. The use of “NOT” was applied in each concept to exclude studies that did not meet eligibility criteria. Mesh terms in MEDLINE and PsycINFO were applied to the search using “OR” in the respective concept. Subject headings were explored in each database, so Mesh terms were specific to the subject headings used in the database. Keywords were truncated to generate the maximum number of results. All concepts were screened against title and abstract. This allowed for concise study selection of relevant studies. All records retrieved by the search strategy were imported into Endnote 20 (Clarivate, Philadelphia, PA, USA) [29,31].

2.4. Study Selection and Methodological Quality

Records from each database were imported electronically and stored using a structure based on the database searched, while a master folder was used to combine all records [29,31]. A backward search of the bibliography of systematic reviews identified during the search and relating to the aims were screened to identify relevant studies missed through the database search. Duplicates were then removed, with 962 studies remaining, of which two authors (M.G. and J.V.) completed a title and abstract review of 500 studies, and three authors (S.M., M.J., and A.F.) completed this process on 462 studies [29,31].

Following title and abstract screening, all remaining studies were analyzed and discussed by five authors (M.G., M.J., S.M., J.V., and A.F.) and a consensus reached on whether the study met the strict inclusion criteria [29,31]. Agreement/disagreement was documented, resulting in 9 studies included in the data charting process (Figure 1). Studies at all levels of evidence were eligible for inclusion with the intent to synthesize information regardless of any assessment of quality as per the reported PRISMA-Scr guidelines [34]. Consequently, no formal assessment of bias was conducted. No new eligible studies were identified following the second updated database search.

2.5. Data Extraction, Synthesis, and Analysis

The included studies were descriptively analyzed using a customized electronic data sheet (

Table 3. Screening tests for ACLI risk factor screening.

Study Name (Author Year)	Population	Sport and Competition Level	Risk Factor Domain	Screening Test (Methods)	Major Findings (Results)
Restriction in hip internal rotation is associated with an increased risk of ACL injury (Bedi et al., 2016) [35]	234 football athletes at 2012 NFL National Invitational Camp (Scouting Combine)	American football—semi-professional	Joint range of motion	1 of 3 orthopedic surgeons with fellowship training in sports medicine assessed the following prospectively: Hip range of motion bilaterally (hip flexion, hip internal rotation at 90 degrees flexion) whilst stabilizing the pelvis for optimum contact of the anterior femoral head–neck junction with the acetabular rim Lachman’s test Anterior drawer Pivot-shift	Prevalence of ACLI = 10.5%. Logistic regression of hip IR predicts ACLI: - Left hip IR restriction significantly increased OR of ACLI in the ipsilateral knee (p = 0.0001, 95% CI 0.93, 0.98, OR = 0.95) and contralateral knee (p <0.0001, 95% CI 0.93, 0.97, OR = 0.95). - Right hip IR restriction was associated with increased OR (n.s p > 0.05) of ACLI. A post-estimation calculation of OR for ACLI for left and right hip IR restriction of 10–20–30–40 degrees. The greater the hip IR restriction, the greater the odds of ACLI in the ipsilateral and contralateral knee. e.g., a 30° reduction in left hip IR was associated with 4.06 times greater odds of left ACLI and 2.71 times greater odds of right ACLI, and a 30° reduction in right hip IR was associated with 5.29 greater odds of left ACLI and 5.19 times greater odds of right ACLI.
The effect of angle on change of direction biomechanics: Comparison and inter-task relationships (Dos’Santos, Thomas, and Jones. 2021) [36]	27 males from multiple field sports (soccer = 19, rugby = 7, field hockey = 1).	Soccer, rugby and field hockey players—semi professional	Biomechanical	Lower-limb and trunk kinematics and kinetics using. 3D motion and GRF analysis whilst performing COD tasks at different angles (45°, 90°, 180°).	Largest peak KIRMs and KAMs occurred during the 90° COD task (p < 0.001, d = 0.88–1.81), inferring that this may the riskiest COD angle and that biomechanics of non-contact ACLI could be angle-dependent. No correlation between participants with greater knee IR moments and knee abduction moments going on to sustain an ACLI.
ACL biomechanical risk factors on single leg drop-jump: a cohort study comparing football players with and without history of lower-limb injury (Daoukas et al., 2019) [37]	21 males split into 2 groups (age 19–30 years, height 1.77 ± 0.05 m, weight 75.6 ± 5.4 kg, BMI 23.2 ± 0.8): Group A: 10 males with soft tissue lower-limb injury last 12 months. Group B: 11 males with no lower-limb injury last 12 months.	Soccer—professional	Biomechanical	2D video analysis of single-leg vertical drop-jump task from 30 cm box (frontal and sagittal view).	Significant differences in KVA and knee flexion angle KFA between the groups. Players with lower-limb injury in the previous 12 months landed with increased KVA (6.20 ± 5.80°) and decreased KFA (35.20 ± 7.8°) at initial contact compared to those with no injury in the last 12 months. The altered biomechanics could, therefore, effect ACL tensile strength and length. Strong positive correlation between least contact time KVA and mean KVA (r = 895, p = 0.000) and least contact time KFA and mean KFA (r = 699, p = 0.000).
Coordination and variability during anticipated and unanticipated sidestepping (Weir et al., 2019) [38]	22 male players	Soccer—collegiate	Biomechanical	Vector coding of an 11-camera system that quantifies coordination and coordination variability in the trunk and pelvic and in the hip and knee.	Variability of coordination increases during unanticipated sporting tasks. COD angle was greatest in unanticipated side-stepping compared to anticipated (40.3° vs. 35.1° respectively) (p < 0.001). Externally applied knee flexion moments were greater in unanticipated side-stepping versus anticipated (p = 0.049). Knee valgus moments were greater in unanticipated sidestepping compared to anticipated side stepping (p = 0.001). Trunk–pelvis coordination demonstrates the importance of implementing complex tasks into athletes training programs.
The Hamstring and ACL Injury Incidence during a Season is not Directly Related to Preseason Knee Strength Ratios in Elite Male Soccer Players (Ižovská et al., 2022) [39]	The 134 male players from 7 teams in the highest soccer league in Czech Republic	Soccer—professional	Strength	Isokinetic strength of knee extensors and flexors during concentric muscle contraction at a 60°⋅s−1.	No significant differences in peak extensor torque, peak flexor torque, or unilateral or bilateral strength ratios between players who suffered an ACLI during the season and those who did not. No significant association between concentric muscle contraction at 60°⋅s−1 and ACLI, with a 1.5% difference of peak torque of knee extensors [ACLI group 3.04 ± 0.45 N/m/kg and non-injured group 3.08 ± 0.22 N/m/kg (p = 0.806)] and a 2.6% difference of peak torque of knee flexors [ACLI group 1.85 ± 0.43 N/m/kg and non-injured group 1.90 ± 0.54 N/m/kg (p = 0.098)].
Relationship between anterior cruciate ligament rupture and the posterior tibial and meniscal slopes in professional soccer athletes (Ikawa et al., 2021) [40]	120 male athletes in Brazilian soccer split into 2 groups: ACL Tear Group: 59 athletes with ACL tear (average age 21.9 years) Control Group: 61 athletes with no history of knee injury (average 23 years)	Soccer—professional	Anatomical	MRI for evaluation of LTS MTS MPMS LPMS delta-TS delta-MS	For every 1° increase in LMS, the risk of ACL rupture increases by 1.42-fold. For every 1° increase in MMS, the risk of injury increases by 1.40-fold.
Increased Lateral Tibial Plateau Slope Predisposes Male College Football Players to Anterior Cruciate Ligament Injury (Rahnemai-Azar et al., 2016) [41]	90 male U.S. National Collegiate Athletic Association (NCAA) Division-I college football players	American Football—Collegiate Division 1	Anatomical	MRI for evaluation of Medial and lateral femoral condylar widths Medial and lateral tibial plateau widths Femoral intercondylar notch width Femoral bicondylar width Medial and lateral tibial slopes	Increased lateral tibial slope (OR= 1.32, 95% CI, 1.03 to 1.70) was the sole independent risk of ACLI.
Gene variants within the COL1A1 gene are associated with reduced anterior cruciate ligament injury in professional soccer players (Ficek et al., 2013) [42]	Intervention Group: 91 males (age 23 ± 3 years) who had non-contact ACLI confirmed surgically. Overall training time 14–18 h per week (7–9 training sessions, 2 h each). Control Group: 143 males (age 25.2 ± 2.6 years), no self-reported ligament or tendon injury.	Soccer—Division 1 professional league	Genetics	Genomic DNA extracted from oral epithelial cells using GenElute Mammalian Genomic DNA Miniprep Kit. Allelic discrimination of COL1A1 Sp1 +1245G/T (rs1800012) and −1997G/T (rs1107946) polymorphic sites was performed using TaqMan Pre-Designed SNP Genotyping Assays. Samples were genotyped using Rotor-Gene real-time polymerase chain reaction.	Higher frequency of COL1A1 G-T haplotype was significantly associated with reduced ACLI (p = 0.048). Carrying 2 copies of G-T haplotype may be protective against ACLI. Trend towards underrepresentation of TT genotype in ACLI group, with no players with ACLI having this genotype. Result is not statistically significant (p = 0.084). No significant differences between the two groups in genotype distribution and allele frequencies for COL1A1 Sp1 +1245G/T polymorphism (OR: 0.73, p = 0.232) and the 1997G/T polymorphism (OR: 1.35, p = 2.46).
Interactions Between COL5A1 Gene and Risk of the Anterior Cruciate Ligament Rupture (Lulińska-Kuklik et al., 2018) [43]	Intervention Group: 134 males (age 23.4 ± 3.1 years), non-contact primary ACLI (surgically diagnosed). Control Group: 211 males (age 25.3 ± 3.4 years) no self-reported ligament or tendon injury. All participants were from the same soccer teams and ethnicity (Polish, Eastern European >3 generations) and had same exposure to ACLI risk (training and match play same).	Soccer—Division 1 professional league	Genetics	DNA Swab (Copan, Murrieta, CA USA) was used to collect oral epithelial cells. Genomic DNA was extracted from the oral epithelial cells using a Gen Elute Mammalian Genomic DNA Mini prep Kit. Allelic discrimination of COL5A1 rs12722 and COL5A1 rs13946 polymorphic sites was performed using TaqMan Pre-Designed SNP Genotyping Assays. Samples were genotyped using Rotor-Gene polymerase chain reaction. Genotyping results were reviewed by 2 independent and blinded assessors.	No significant differences between the two groups in the genotype or allele frequency distribution for COL5A1 rs12722 and COL5A1 rs13946 (p values for T-T, C-C, and C allele p > 0.05). Statistically significant differences were found in the genotype frequency distribution for COL5A1 rs13946 C-T when dominant mode of inheritance was tested, with 48% in control group and 37% in ACLI group (p = 0.039). The COL5A1 rs13946 C-T haplotype was underrepresented in ACLI group compared with healthy controls. COL5A1 rs12722-rs13946 C-C haplotype was overrepresented in the control group (31% compared to 26% in ACLI group) and may be protective against ACLI (−2.06, p = 0.038).

Notes: NFL: National Football League; KIRMs: Knee Internal Rotation Moments; KAMs: Knee Abduction Moments; OR: Odds Ratio; IR: Internal Rotation; ER: External Rotation; GRF: Ground Reaction Force; COD: Change of Direction; KVA: Knee Valgus Angle; KFA: Knee Flexion Angle; LTS: Lateral Tibial Slope; MTS: Medial Tibial Slope; MPMS: Medial Posterior Meniscal Slope; LPMS: Lateral Posterior Meniscal Slope; delta-TS: Posterior Tibial delta Slope; delta-MS: Posterior Meniscal delta Slope.

) using the following headings: title, author and year of publication, population, sport, biopsychosocial domain, screening test, and major findings [29,31].

3. Results

3.1. Study Characteristics

Of the 962 citations identified through the search strategy, nine manuscripts met the inclusion criteria and were reviewed [35,36,37,38,39,40,41,42,43]. Of those included, none required results to be differentiated by sex, the level of professionalism, or age. The use of the field-based team sport term resulted in the inclusion of a variety of field-based team sport disciplines (n = 7 soccer [37,38,39,40,41,42,43], n = 1 American football [35], and n = 1 mixture of soccer, rugby, and field hockey [36]). The risk factor domains reported by the included manuscripts were modifiable (the joint range of motion n = 1 [35], biomechanics n = 3 [36,37,38], and strength n = 1 [39]) and non-modifiable (anatomical n = 2 [38,39] and genetics n = 2 [40,41,42,43]) (Table 2).

3.2. Modifiable Risk Factors

3.2.1. Biomechanical—Hip Range of Motion

The hip internal rotation range of motion in 90° of hip flexion was predictive of ACLI in 324 semi-professional American football athletes at the 2012 Scouting Combine [35]. Of the 324 athletes, 34 players sustained an ACLI (16 left knee and 18 right knee). Estimated odds ratios comparing an athlete with no hip internal rotation reduction to an athlete with a 10–20–30–40° reduction had been calculated. It was found that the greater the reduction in hip internal rotation, the greater the risk of ACLI. Through logistic regression, there was a significant association between reduced hip internal rotation and increased odds of sustaining a left ACLI (left hip internal rotation reduction OR: 0.95, 95%CI: 0.93–0.98, p = 0.0001 and right hip internal rotation reduction OR: 0.95, 95%CI: 0.93–0.97, p < 0.0001) [35]. Although there were increased odds of right ACLI with hip internal rotation reduction, the results did not reach statistical significance (left hip internal rotation reduction OR: 0.97, 95%CI: 0.92–1.02, p = “not significant” and right hip internal rotation reduction OR: 0.95, 95%CI: 0.89–1.01, p = “not significant”) [35].

3.2.2. Biomechanical—Kinematics and Kinetics

The reported biomechanical testing consisted of a two-dimensional (2D) analysis of single-leg drop jump (DJ) [37] and a three-dimensional (3D) analysis of change of direction (COD) and unplanned side stepping [36,38]. Values for knee flexion and knee valgus angles were evaluated from DJ in 21 professional male soccer athletes [37]. DJ testing identified mechanistic patterns indicative of ACLI risk factors [37]. Vertical ground reaction forces and peak knee abduction and internal rotation moments were analyzed in the COD task of 27 semi-professional athletes (soccer n = 19, rugby n = 7, and field hockey n = 1) and in the unplanned side-stepping task of 22 collegiate soccer athletes [36,38]. Both studies reported 90° COD and unplanned side-stepping produced the largest knee valgus moment, potentially increasing the risk of ACLI [36,38].

3.2.3. Muscular Strength

Isokinetic strength of the knee extensors and flexors during a voluntary concentric muscle contraction at 60°·s⁻¹ was recorded from 134 professional soccer athletes [39]. Values of peak muscle torque of knee extensors, flexors, and quadriceps–hamstring (Q–H), quadriceps–quadriceps (Q–Q), and hamstring–hamstring (H–H) ratios from the isokinetic dynamometry were analyzed retrospectively for ACLI risk by grouping athletes into injury sustained over two seasons (ACLI n = 10, grade-3 hamstring strain n = 10, and randomly selected non-injured n = 20) [39]. No significant association was identified between concentric muscle contraction at 60°·s⁻¹ and ACLI, with a 1.5% difference of peak torque of knee extensors [ACLI group 3.04 ± 0.45 N·m·kg and non-injured group 3.08 ± 0.22 N·m·kg (p = 0.806)] and a 2.6% difference of peak torque of knee flexors [ACLI group 1.85 ± 0.43 N·m·kg and non-injured group 1.90 ± 0.54 N·m·kg (p = 0.098)] [39]. The bilateral and unilateral strength ratio showed no significance between all groups.

3.3. Non-Modifiable Risk Factors

3.3.1. Morphological

Two studies investigated morphological characteristics of the knee joint via Magnetic Resonance Imaging (MRI) and their relationship with ACLI [40,41]. Osseous morphological characteristics including medial and lateral condylar width, medial and lateral plateau width, notch width, and medial and lateral tibial slopes were investigated in 90 collegiate American football athletes [41]. Multivariate statistical analysis revealed that for every 1° increase in lateral tibial plateau slope, a corresponding 32% (OR: 1.32, 95%CI: 1.03–1.70) increase in ACLI risk occurred [41]. Lateral and medial posterior tibial slopes, lateral and medial posterior meniscal slopes, and posterior tibial meniscal delta slopes were investigated in 61 professional soccer athletes [40]. Authors reported that for every 1° increase in lateral posterior meniscal slope (LMS), the risk of ACLI increases by 42% (OR: 1.42, 95%CI: 1.12–1.74). Similarly, for every 1° increase in medial posterior meniscal slope (MMS) (OR: 1.40, 95%CI: 1.09–1.82), the risk of ACLI increased by 40% [40].

3.3.2. Genetics

Two studies investigated genomic DNA extracted from oral epithelial cells to determine if there was an association of polymorphisms with ACLI in professional male soccer athletes [42,43]. No statistical differences between healthy versus ACLI groups in genotype distribution and allele frequencies for COL1A1 Sp1 +1245G/T polymorphism (OR: 0.73, p = 0.232) and the 1997G/T polymorphism (OR: 1.35, p = 2.46) existed in 234 athletes [42]. However, a higher frequency of the COL1A1 G-T haplotype was significantly associated with reduced ACLI (p = 0.048) [42]. No statistically significant differences between healthy versus ACLI groups in genotype distribution or allele frequencies for COL5A1 rs12722 and COL5A1 rs13946 (T-T, C-C and C allele p values > 0.05) were identified in 134 athletes [43]. COL5A1 rs12722-rs13946 C-C haplotype was overrepresented in the control group (31% compared to 26% in ACLI group) and was significantly associated with reduced ACLI risk (OR: −2.06, p = 0.038) [43].

4. Discussion

The primary aim of this scoping review was to identify factors that associate with an elevated risk of ACLI in elite field-based team sport athletes, which may then be used to inform appropriate athlete risk monitoring practices in elite sporting clubs and programs. Through a defined literature search, nine studies were identified that examined risk factors and ACLI incidence in male athletes. One study that reported hip internal rotation reduction as a risk factor for ACLI in semi-professional American football athletes suggested causative factors for ACLI risk through a prospective study design. Eight studies demonstrated associative risk through retrospective study designs; however, these are considered of lower-level evidence. It should be noted that there is a chance of type 1 error amongst all studies, with some studies not providing confidence interval ranges [36,37,38,39,42,43], while others reported confidence intervals close to 0 [35,40,41].

4.1. Modifiable Risk Factors

4.1.1. Biomechanical—Hip Range of Motion

The current search identified the hip range of motion as the only modifiable risk factor to have prospectively investigated ACLI risk in semi-professional American football. Hip internal rotation reduction on either side demonstrated significant association to left ACLI (left hip internal rotation reduction OR: 0.95, 95%CI: 0.93–0.98, p = 0.0001 and right hip internal rotation reduction OR: 0.95, 95%CI: 0.93–0.97, p < 0.0001) [35]. While the authors reported significantly increased odds of left ACLI with greater reductions in hip internal rotation motion, an odds ratio of <1 contradicts this notion [44]. Further to this distinction is the 5% difference in odds represents a small effect size, and a reported confidence interval close to 0 limits a clear interpretation. It is intriguing that no significant associations were found between hip internal rotation reduction and right ACLI, but significant associations existed for left ACLI. Whether this outcome is an artefact of limb dominance bias or of statistical relevance requires further investigation.

A retrospective study not meeting inclusion criteria for this review reported that non-professional athletes (including American footballers) with hip internal rotation reduction and >60° alpha angle (measurement defining hip cam deformity) were at greater risk of ACLI (OR: 2.7, 95%CI: 1.4–5.2, p = 0.001) [45]. It is important to consider that hip internal rotation reduction can also be classified as a non-modifiable risk factor given the potential for hip morphology, such as a cam deformity, to cause reduction in hip internal rotation motion [35,45].

Careful interpretation of these results for hip internal rotation reduction and ACLI risk are required as the primary cause of hip internal rotation reduction is uncertain and may stem from other risk factors. The potential for modifying hip internal rotation and its associated benefits is questionable and highly dependent on the individual athlete’s primary cause of reduced hip internal rotation. For instance, repetitive end-range hip internal rotation mobility work in attempt to increase hip internal rotation motion in an athlete with underlying anterior femoroacetabular impingement syndrome may not be appropriate as this may consequently, over time, lead to posterior capsular laxity through a fulcrum effect [46]. A more thorough examination of the role hip range plays in ACLI risk is warranted given the findings from this review.

Future prospective randomized control trials would be required to determine a causal relationship between hip internal rotation reduction and ACLI. An intervention group performing exercises aiming to improve hip internal rotation motion would be plausible whilst monitoring for various confounding variables such as fatigue, game time, footwear, etc. The hip range of motion testing is a cost and time-effective screening test that can be used in the elite field-based team sports. However, the ability of coaching and training staff to modify the outcomes may be limited.

4.1.2. Biomechanical—Kinematics and Kinetics

Three studies included report greater knee valgus angles and knee abduction moments during 90° COD, unplanned side-stepping, and single-leg DVJ as inferred risk factors for ACLI in elite male field-based team sports based on the existing literature on ACL loading mechanisms [36,37,38]. Findings should be carefully interpreted as these retrospective studies do not establish causation for greater knee valgus angles and knee abduction moments to ACLI risk by prospectively investigating ACLI incidence. Particularly, findings from Daoukas and co-workers warrant caution for three reasons: (1) none of the athletes recruited sustained an ACLI, rather soft tissue injuries or ankle sprains [37]; (2) the cohort (n = 21) was stratified into two groups, resulting in an achieved power (1-β) of 0.49 and 0.68 for knee valgus angle and knee flexion angle, respectively, which is substantially less than a recommended 0.8 [37]; (3) 2D motion analysis has reduced precision and ability to identify poor knee positions compared to 3D motion analysis [47,48].

More recent studies investigating correlation between double-leg DJ and ACLI risk in female sporting populations report knee valgus angle and knee abduction moment did not show any significant association to increased ACLI risk when using 2D or 3D motion analysis [7,49]. A recent investigation in professional female soccer and handball players concluded that 2D motion analysis of DJ should not be used to screen for ACLI risk because it could not identify individuals with an increased risk of non-contact ACLI [7]. In contrast, Hewett et al. [50] reported knee valgus angles and a knee abduction moment during DJ to predict ACLI, and Corban et al. [51] in their prospective study with mixed sex collegiate athletes reported knee valgus and knee flexion angles at peak contact during double-leg DJ to be excellent predictors of ACLI [51].

The inconsistency in findings and techniques makes determining the validity of DJ as an ACLI screening test difficult. No research investigating the correlation between DJ and ACLI risk in elite male field-based team sport was identified. Recent findings of the prospective study in professional female athletes indicate future research should place less emphasis on investigating DJ and ACLI risk [7]. This would appear reasonable, given ACL rupture occurs prior to adopting excessive knee valgus and tibial external rotation positions [52,53]. This gap in the literature is surprising given that the DJ is considered to challenge fundamental jumping and landing patterns, with potential to clearly identify deficient movement strategies as a precursor to injury [54]. Further investigation in elite male athletes is necessary before clubs could confidently utilize the DJ to screen for ACLI risk. Whilst the feasibility of 3D motion analysis can be considered challenging due to the high costs, substantial time required [20], and marker placement errors [55], Corban et al. demonstrated it can be practical, inexpensive, and markerless with good–excellent prognostic ability [51].

4.1.3. Muscular Strength

There was no significant correlation identified between the variables assessed via isokinetic dynamometry and ACLI in the elite soccer population [39]. In contrast, Myer et al. [56] assessed similar isokinetic strength data in female youth basketball and soccer populations, reporting a Q–H ratio of 1.78 (95%CI: 1.06–2.70) predicted high knee abduction moment in laboratory testing, inferring increased ACLI risk [56]. However, Steffen et al. [57] in their prospective study concluded that isokinetic quadricep and hamstring strength and the H–Q ratio were not associated with increased ACLI risk in elite female handball and soccer athletes and should not be used for ACLI risk screening in this cohort [57]. Stating that, the isokinetic dynamometry of the quadriceps and hamstrings as a screening test for ACLI offers little predictive value for clubs in elite field-based team sport [57].

Alternative strength testing methods, using handheld dynamometry to measure isometric hip strength has been reported to be useful in other prospective studies [57,58]. Khayambashi et al. [58] established that increased ACLI risk is significantly associated with reduced hip external rotation strength (OR: 1.23, 95%CI: 1.08–1.39, p = 0.001) and hip abduction strength (OR: 1.12, 95%CI: 1.05–1.20, p = 0.001) in male and female non-professional athletes from various sports [58]. In contrast, Steffen et al. [57] reported no association with isometric hip abduction strength and ACLI risk in elite female handball and soccer athletes.

The conflicting results amongst studies, although notably primarily in female athlete cohorts, do provide some direction for further research on screening hip strength versus knee strength variables with ACLI risk in elite male field-based team sport athletes. Comparing isokinetic dynamometry or a force frame with handheld dynamometry to assess hip strength variables could be useful to determine if the tool is a differentiating confounding factor impacting results rather than the muscles assessed. The cost-effectiveness, time-efficiency, and reproducibility of handheld dynamometry could deem this a useful screening tool and testing methodology for clubs, although internal validity needs close consideration if there are multiple assessors performing the test. However, it must be highlighted to the reader that the authors are in no way diminishing the role that muscular strength has as a significant tool for injury prevention [59]; it is simply that isolated joint assessments of torque production are not generally significant factors for determining ACLI risk. Developing strength as a multi-joint compound movement parameter remains important to incorporate through resistance training programs in the training of all athletes.

4.1.4. Other Modifiable Risk Factors

Associations in neurocognitive function and ACLI risk have been investigated, with significant outcomes reported between the ImPACT score (typically used to assess neurocognitive function and concussive symptoms) and non-contact ACLI in male and female collegiate athletes [60]. Slower reaction times and processing speeds, coupled with poor visual and verbal memory in preseason testing, were significantly associated with ACLI risk [60]. Whilst the authors did not differentiate between sexes for ACLI risk, the ImPACT Score would be feasible for elite clubs to implement. However, the correlation of these tests to ACLI risk needs to be established to determine validity in the elite field-based team sport population [60].

4.2. Non-Modifiable Risk Factors

Anatomical and genetic screening tests were classified in our search as non-modifiable risk factors for ACLI. Screening tests included MRI and genomic DNA testing [40,41,42].

4.2.1. Morphological

Joint morphology assessed via MRI was reported to be a significant risk factor to ACLI in elite male field-based sport athletes [40,41]. However, while others replicated the outcome in female high school and college athletes, they reported no a significant correlation in the male athletes tested [61]. Performing an MRI to determine the knee morphology of a club’s entire team would come at a significant financial cost. The feasibility and practicality of morphological identification are questionable given the non-modifiable nature of this risk factor. Further systematic work is required to consider whether individuals with increased ACLI risk due to morphological risk factors would allow injury prevention strategies to be implemented.

4.2.2. Genetics

Researchers are increasingly investigating a genetic link to injury and whether there is potential for a genetic screening tool. A higher frequency of COL1A1 G-T and COL5A1 rs13946 C-C haplotypes were significantly associated with reduced ACLI in professional male soccer athletes; however, these results may be attributed to chance [42,43]. Kim et al. explored genetic markers with ACLI risk in a mixed-sex cohort, also reporting a 21% chance of false positive results [62]. They extended the analysis to identify that no single nucleotide polymorphisms were associated with ACLI at a genomic significance level [62]. Genetic DNA testing may not be suitable for clubs due to high financial costs, the violation of athlete ethics and privacy, and the rate of false positive results [42,43,62].

4.2.3. Other Non-Modifiable Risk Factors

There is limited systematic evidence on other non-modifiable ACLI risk factors that met our inclusion criteria. Environmental factors such as weather and grass type have been captured in our search as influencing ACLI risk. While we acknowledge that high rainfall, low water evaporation, and rye grass were associated with lower ACLI risk [63,64], we excluded this literature from our results as we considered these external non-modifiable risk factors. We note that there is a broad cohort of research considering factors such as footwear, in particular, cleat design, as a contributor to ACLI risk, with bladed studded and edge cleats said to potentially result in injury due to excessive traction [65,66,67] and round-studded cleats being preferrable for natural grass and turf [65].

The secondary aim of this scoping review was to identify gaps in the literature surrounding ACLI screening in elite field-based team sport athletes that can guide future research. ACLI screening in this cohort is significantly lacking, with current screening tests seeming impractical and unfit for purpose. Given the burden of ACLI on elite athletes, their teams, and stakeholders, there is need for research to be undertaken specific to this population.

Psychosocial and emotional factors and their importance to ACLI risk remain unexplored in elite male team sports in isolation. Negative-life-event stress and daily hassle have been demonstrated as significant predictors of injury among professional male and female soccer players, accounting for 24% of the variance in injury, but it was not reported whether ACLI was among the injuries registered [68]. Similarly, adolescent athletes who experienced negative life events such as parental divorce and other trauma (identified using the Life Event Scale for Adolescents) were more likely to sustain injury [69]. These findings demonstrate that these emotional and psychological predictors of injury may be significant cogs in a complex subsystem of ACLI risk determinants. Psychosocial factors are potentially modifiable and manageable risk factors, making them appropriate to be considered in future ACLI risk research. Addressing psychosocial factors in managing injuries is common, given its importance and contribution to individual pain experiences. It is not unreasonable to hypothesize psychosocial and emotional influences potentially having significant contributions to a significant initial injury such as ACLI.

5. Limitations

The several limitations we have identified when developing this scoping review are as follows: (1) no analysis of study quality was conducted as per the limitations of scoping reviews; (2) findings are limited to the strict criteria that was imposed, meaning extrapolating findings to female, recreational, or other sport athletes may be challenging; (3) the conjecture around and not including the risk scenario of contact between athletes as either a non-modifiable or modifiable factor; and (4) included manuscripts were limited to publications in written English. To expand on the first limitation, it should be noted by readers that scoping reviews are intentionally broad, and the current scoping review does not focus purely on RCT’s; hence, using a methodological quality assessment tool such as the PEDRO score is not suitable. Furthermore, this scoping review did not seek to exclude studies based on quality, and we purposefully included non-RCTs. Appreciating the role that methodological quality can have on outcome interpretation, we sought not to provide commentary on the methodological rigor or quality of the included studies. The fourth limitation to the scoping review was a delimiter imposed by the authors to the search used in this scoping review as only manuscripts written in English were included. By having the search delimited by language may have resulted in the search omitting suitable manuscripts published in alternative languages.

6. Conclusions

ACLI remains problematic for elite male team sport athletes. No ACLI screening tests for modifiable risk factors have been identified in our results to be of value to the elite male field-based team sport populations. A significant proportion of the literature in this area understandably focus on female athletes given the prevalence and substantially greater risk of ACLI among this population. Many have conducted research with a mixed population in either sex, sport, or competition level, resulting in an inability to extract applicable data to our population. Screening tests for non-modifiable risk factors do demonstrate some predictive value, however, they may be impractical for clubs to utilize due to high cost, low value, and time. No screening tests exist for psychosocial risk factors and ACLI risk in isolation.

Given the significant burden on athletes, teams, and stakeholders, it is surprising to note a lack of research surrounding screening tests. The predictive value of ACLI risk screening tests for modifiable risk factors need to be established in elite male field-based team sport populations before elite clubs can consider implementing these into practice. In future, more prospective cohort studies are required to determine the validity of ACLI screening tests.