Introduction
Since the passing of Title IX in 1972, female participation in sports at every level has increased tremendously and continues to grow. , With that, however, the prevalence of sports-related musculoskeletal injuries has also greatly increased in the female athlete. In particular, anterior cruciate ligament (ACL) injuries have become a critical issue for a variety of reasons, including the severity of the injury itself; time loss from participation in sports, recovery, and rehabilitation from surgery; and long-term consequences of injury, such as osteoarthritis. In addition to the substantial impacts an ACL injury has on an individual, the healthcare costs of reconstruction surgery and postoperative rehabilitation, as well as costs associated with nonsurgical treatment, are extremely high, surpassing $100 million per year. Female soccer players especially are a high-risk population, and among all athletes in general, they are some of the most frequent to be affected by ACL injuries. Female soccer players are at a significantly greater risk of sustaining a sprain or tear to the ligament compared with other female athletes participating in a variety of other sports, and even more so when compared with male soccer players.
The ACL functions as a knee stabilizer by preventing anterior translation of the tibia and providing rotational stability. The knee support provided by the ACL is key for multidirectional cutting, landing, and pivoting sports such as soccer. With a functional deficit of the ACL, performing these actions are challenging and it risks further injury, resulting in many ACL-injured athletes to be unable to return to their preinjury level of play, or even to quit playing. After reconstruction, athletes remain at high risk for reinjury of the index knee and for injury of the contralateral ACL, and female soccer players have been reported to be among athletes with the highest risk for these occurrences. Awareness of the prevalence and the severity of multiple factors related to ACL injury among female soccer players is an important step toward implementing prevention techniques to ultimately decrease the incidence of injury. Additionally, a better understanding of the mechanisms and multifaceted risk factors that cause this injury, as well as awareness of the additional risks and frequency of graft failure and second ACL injuries, are valuable information for proper treatment. Finally, knowledge of the efficacy of current prevention programs that exist and placing high importance on the need for intervention can provide athletes, coaches, trainers, and parents with methods in which they can minimize the risk of ACL injury.
Epidemiology
With increasing awareness of the severity of ACL injury in the athletic population, many epidemiology studies have been conducted on high-risk populations, such as female soccer players, particularly highlighting the difference in injury rates between males and females. In 2005, Agel et al. reported the ACL injury rate for basketball and soccer at the collegiate level in a 13-year review. Overall, there was a total of 1268 ACL injuries with 586 sustained while playing soccer, and within soccer, there were 394 ACL injuries in females versus 194 in males. It was also observed that soccer players sustained consistently more ACL injuries than basketball players. Throughout the study, the incidence of ACL injury in males reduced, whereas the rate in females remained constant, widening the magnitude of the disparity of injury rate between males and females. In an updated review of ACL injury rates across 15 different sports at the collegiate level over a 9-year period, Agel et al. observed a similar trend within soccer that female athletes sustained significantly more ACL injuries than male athletes at over double the rate. Overall, 71 females and 26 males experienced an ACL injury, signified by rates of 0.10 versus 0.04 per 1000 athletic exposures (AEs), respectively. Interestingly, there was a significant increase in injury rate per year over the course of this study, but these results also represented a 64% decrease in overall ACL injury rate in females and 56% decrease in males compared to the researchers’ original data from their previous study.
To further emphasize the consistency of the epidemiology of ACL injuries in female soccer players, a review published in 1995 found a higher prevalence of knee injuries in female soccer players than in males over the course of the 5-year study period, despite the fact that there were significantly more men’s teams included in the study (461 men’s teams vs. 278 women’s teams). The ACL injury rate was over double in the female athletes (0.31 vs. 0.13 per 1000 AEs) and 68% of females who sustained an ACL injury required surgery versus 59% of males. Notably, females also had a consistently higher rate of injury in any given year throughout the study period by at least double compared to males, in addition to a higher overall average rate.
Prodromos et al. also noted that the rate of ACL injury was three times higher in female soccer players than in male soccer players (5% vs. 1.7%, respectively) participating in year-round high-level competition. Hootman et al., who studied the epidemiology of all injuries at the collegiate level over a 16-year study period, reported that soccer players sustained 579 of the 4800 ACL injuries among 15 sports. Within soccer, 411 of the ACL injuries occurred in females, whereas just 168 were suffered by males, represented by a difference in injury rate of 0.28 versus 0.09 per 1000 AEs, respectively. At the high-school level, Joseph et al. also performed a multisport epidemiology study specifically comparing ACL injury patterns. The greatest prevalence of ACL injury was found in female soccer players, at a rate of 12.2 per 100,000 AEs, and among all the female athletes included in the study, 53.2% of ACL injuries occurred in soccer players. Overall, girls were twice as likely to sustain an ACL injury playing soccer compared to any other sport included in this population of high-school athletes. In a study, Larruskain et al. prospectively compared injuries between male and female elite soccer players over the course of eight seasons. A total of 50 males and 35 females participated and they all belonged to the same club and had the same medical staff. At this high level of play, ACL ruptures were 5 times more common in females, which resulted in over 40% of the absences from play that female athletes experienced. Females had a 21% greater amount of total days lost from participation, representing an injury burden twice as high compared to males, as well as a higher proportion of severe injuries overall.
When observing the incidence of ACL injuries in female soccer players compared to the risk of other injury types, ACL ruptures may always account for the majority, but they are still considered to be among the most severe type of injury with a high injury burden. , , Faude et al. prospectively analyzed injury incidences in female soccer players in the German National League over the course of a single season. A total of 165 players were included in the study, and on average, each player participated in 183 h of training and spent 31 h in matches. Overall, 241 injuries were sustained by 115 players (70% of the athletes) and 58% of the total injuries were severe injuries of the knee. Among them were 11 ACL ruptures sustained by 10 players (6% of all athletes), resulting in an incidence rate of 2.2 per 1000 match-hours. The average time lost from participation because of ACL injury was half of the year (178 days), which the authors noted that the severity of this injury and the high amount of time lost comes at a great disadvantage to the team and cost for the club. Not only is the team impacted but also the consequences for the athlete include risks of permanent disability of the injured knee, emphasizing the importance of finding effective prevention methods. Giza et al. also reported injuries in professional female soccer players and found that ACL injuries consisted of 4.6% of all injuries (8 of 173) and 14.6% of all knee injuries (8 of 55), and the incidence of ACL tears was 0.09 per 1000 player-hours. Le Gall et al. studied the incidence of injuries in young elite female soccer players prospectively over eight seasons, and out of the 119 players that participated in this study, 110 (92.4% of the athletes) experienced a total of 619 injuries. A majority of the injuries occurred in the youngest team (Under-15) and the least amount of injuries occurred in the oldest team (Under-19). Of the 619 injuries, a total of 12 ACL injuries were sustained by 11 players, with 7 of them occurring in the nondominant leg. Interestingly, no ACL injuries were reported in the last 3 years of the study, which the authors attributed to the introduction of a prophylactic conditioning program. Despite this, it was noted that the overall risk of ACL injury was comparable to adult elite female soccer players, which confirms the ongoing importance for intervention in the entire population of female soccer athletes and a further understanding of the many aspects associated with this specific injury.
Mechanisms and Risk Factors
Mechanisms of Injury
Understanding the most prominent mechanisms of ACL injury in female soccer players is crucial for minimizing injury risk. This necessary information can help develop targeted training programs toward avoiding the conditions that create the highest risk. Among female athletes in general, noncontact mechanisms are typically reported as the more common cause of injury compared with contact mechanisms. , , , , Agel et al. found that within 15 different collegiate-level sports, 60% of the ACL injuries sustained by female athletes were caused by noncontact mechanisms. Specifically with respect to soccer, a significantly higher incidence of noncontact ACL injuries has been reported compared with contact ACL injuries, although both mechanisms occurred more frequently in females than males (161 females vs. 66 males injured by noncontact; 115 females vs. 72 males injured by contact). In a separate study, females were two times more likely to sustain an ACL injury after contact with another player and three times more likely as a result of noncontact mechanisms when compared with males. Overall, more females were injured from noncontact methods than they were from player-to-player contact incidences. A 2-year retrospective study reported the mechanisms of injury in high-school athletes and found that overall, noncontact ACL injuries were more common in female soccer players than contact ACL injuries at this level. Researchers further divided noncontact injuries into three categories and found that female soccer players suffered significantly more ACL injuries related to stopping and cutting than those related to landing. Within contact injuries, it was also reported that females sustained significantly more indirect ACL injuries, defined as contact with other body parts before and at the moment of injury, than direct contact injuries, which involved direct contact to the knee. Faude et al. also supported these findings and reported that 7 of 11 ACL ruptures sustained by female soccer players in a season were caused by a change in direction, described as deceleration with a quick turn inducing high forces on the knee. The authors explained that their results reflected the observation that in quick stopping and cutting sports, such as soccer, increased incidence of ACL injury due to noncontact mechanisms is expected in female athletes. They also speculated that these patterns and higher rates of noncontact ACL injury could be attributed to the continuing progression in women’s soccer with respect to game dynamics and the athleticism of the players.
Anatomic, Biomechanical, and Neuromuscular Risk Factors
The noteworthy occurrences of ACL injuries in female athletes, particularly soccer players, and the consequences that exist after suffering an injury have led researchers to attempt to understand the various intrinsic and extrinsic risk factors that cause such severe injuries. Various areas of study have been analyzed in an effort to explain the gender differences in ACL injury rates and mechanisms, including anatomic, biomechanical, muscle strength, and neuromuscular patterns, and to ultimately attempt to limit risk exposure to high-risk female athletes. Anatomic risk factors of ACL injury that have been mentioned, and notably reported, to occur more frequently in female athletes include smaller ligament size, decreased femoral notch width, increased posterior-inferior slope of the lateral tibia plateau, increased knee and generalized laxity, and higher body mass index. In both supine and standing positions, the quadriceps angle (Q-angle) has been shown to be greater in females than males, leading the quadriceps to be pulled more laterally at the knee, causing the ACL to be placed in a position where it is at a greater risk of rupture. Further related to the Q-angle, variances in pelvic structure and lower extremity alignment between males and females have also been argued as a potential explanation for the gender differences seen in ACL injury rates. Additional structural factors reviewed by Sutton and Bullock that potentially increase risk of ACL rupture include a narrower intercondylar notch, smaller ACL size, and an increased posterior tibial slope, although contradictory results have been reported regarding the role these factors have in injury. Hewett also published a review article addressing the multifaceted risk factors associated with ACL injuries in female athletes and described that the overarching evidence for anatomic theories continues to be contradictory. Additionally, given the lack of opportunity for intervention with respect to anatomy, other theories attempting to explain the high incidence of ACL injury in female athletes have the potential to provide viable solutions for prevention and may arguably deserve more focus, such as muscle strength deficits and neuromuscular theories.
Several studies have observed mechanisms in which alterations in muscle strength are related to ACL injury risk. Ryman Augustsson and Ageberg studied 225 high-school athletes and analyzed the role of lower extremity muscle strength in traumatic knee injury. The researchers categorized athletes into the weak or strong group based on the results of a one-repetition maximum barbell squat test. Within the population, a total 18 ACL injuries occurred and 14 of them were sustained by female athletes. Among these injuries, 12 were suffered by athletes categorized in the weak group compared to only 2 in the strong group. It was reported that the odds of experiencing an ACL injury was seven times greater for female athletes in the weak muscle strength group compared with the strong group ( P = .011). Notably, the ACL injury rate was also significantly higher for female athletes in the weak group compared with male athletes in the weak group, and the injury rate was similar between weak and strong males. A correlation between lower extremity muscle strength deficits and future ACL injury was seen only in the female athletes of this youth population, indicating that screening female athletes for muscle strength at a young age could be a key factor in injury prevention training. The authors also mentioned an additional concern for the young, talented female soccer players who play extra matches with other teams, especially those who play at a higher level with more physically matured and skilled opponents, which further increases the chance of injury in this already high-risk population.
In a study by Hannon et al. researchers analyzed hip and knee strength within two age groups of adolescent female soccer players to establish normative strength data for muscle groups where weakness has been previously associated with ACL injury risk. A total of 64 female soccer players aged 10–18 years were included in this study and were split up into two groups based on age. Group 1 included athletes between 10 and 14 years of age and group 2 included athletes between 15 and 18 years of age. No significant difference was found between the groups for quadriceps strength, hamstring strength, or hip external rotation strength when comparing dominant and nondominant limbs. However, a significant difference in hip abduction strength was observed between the two groups, measured by a handheld device as the subject pushed maximally into it for 3–5 s. On the dominant leg, group 1 had an average hip abduction strength, normalized to body mass, of 0.21 kg/mass compared to 0.18 kg/mass in group 2 ( P = .014). On the nondominant leg, the same difference in strength values was seen between the two groups (group 1: 0.21 kg/mass; group 2: 0.18 kg/mass) and was found to be statistically significant ( P = .019). Overall, the younger cohort had greater hip abduction strength and the authors mentioned that similar findings have been reported by other authors, which have observed a decrease in strength consistent with a change in pubertal status (from prepubertal to pubertal status). Furthermore, these results can be explained by the change in fat-free mass that females experience as they progress through puberty, and when combined with the skeletal growth that occurs during this period, which causes a change in location of center of mass, these factors have been shown to increase injury risk. , As stated by the authors, the importance of publishing normative strength values for this high-risk population is to improve clinical decision-making for return to sport after an ACL injury. In order to be cleared to return to sport, the strength of the injured limb should be equal to that of the healthy limb, but calculations of this symmetry can be altered by bilateral muscle strength changes that occur after ACL injury. In these circumstances, the symmetry exists, but the strength value has not been fully restored to preinjury levels, leading to athletes prematurely returning to play. Therefore the normative values taken from healthy individuals within this population can act as an accurate comparator for more appropriate clinical decision-making of when an athlete is fully ready to return to activity. Moreover, a hip strengthening program implemented during this transitional period might be advantageous for this population as they age because of the increase in ACL injury risk as hip abduction strength decreases.
When muscle activity fails to stabilize the knee joint, there is an increase in the loads placed on the passive restraints of the knee leading to the eventual failure of the ACL. A study by Marotta et al. measured the activation time of muscles before contact with the ground in 20 professional soccer players. Surface electromyography was used to separately calculate the activation times of the rectus femoris, vastus medialis, biceps femoris, and semimembranosus muscles, while the athlete dropped from a platform and maintained a single-leg (the testing leg) landing for 5 s. They found that males had correct activation time in all the muscles examined, which related to a lower risk of ACL injury. On the contrary, females had delayed activation of the vastus medialis, which is related to an increase in anterior shear force, a known risk factor in sustaining an ACL injury. The clinical significance of these results, as noted by the authors, is that the testing protocol used in this study could be implemented in screening high-risk athletes and for developing interventions that target injury risks caused by specific muscle activation imbalances. Similarly, in his detailed review of neuromuscular factors associated with ACL injury, Hewett reported gender differences in muscle usage and recruitment patterns that provide theories as to why females are at a significantly higher risk of ACL injury. Males appear to be muscle-dominant in regard to joint control strategies, while females are ligament-dominant. Additionally, males primarily use hamstring and gastrocnemius musculature, which act as a protective method to counteract landing forces, and have been shown to contract their hamstrings first in response to anterior tibial translation. On the contrary, female athletes tend to contract their quadriceps first in response to this anteriorly directed force on the back of the calf. The combination of strong quadriceps contraction during deceleration and a low flexion angle below 45 degrees leads to increased strain on the ACL. Stronger contraction of the hamstrings, on the other hand, acts as an important knee stabilizer by restricting anterior tibial motion and ultimately decreasing anterior shear forces, thus significantly reducing the load placed on the ACL.
Multiple studies have supported the correlation between a lower hamstring-to-quadriceps strength ratio and ACL injury, also indicated by a deficiency in hamstring strength and recruitment. , , , On average, females experience this muscle imbalance more frequently than males, who have a significantly higher strength ratio as well as proper coactivation patterns of these muscles. Thus the knee joint in males is naturally more protected from ACL injury risks related to unstable positions, such as knee abduction and dynamic valgus. Additional risks of sustaining an ACL injury mentioned in Hewett’s review include off-balance, unstable landings in varus or valgus lower extremity alignment, and increased magnitude of adduction and abduction moments at the knee, which impact the hamstring and gastrocnemius functions in stabilizing the knee joint. Increased anterior knee laxity and lower knee flexor strength, also seen more commonly in females than males, have also been described as risk factors. The significance of observing these neuromuscular risk patterns is that conditioning of the muscles involved in ACL stabilization and developing proper training programs to correct imbalances have the ability to correct these imbalances in high-risk female athletes and ultimately decrease the risk of ACL injury.
Observing the role of improper neuromuscular control patterns in ACL injury risk provides a better understanding of the muscles involved in knee stabilization and their impact on the ACL. Developing proper training programs to condition these muscles and correct imbalances can improve strength deficits and modify activation patterns in high-risk female soccer athletes and ultimately reduce the risk of ACL injury. In one study, Hewett et al. used biomechanical measures of neuromuscular control and valgus loading to predict the risk of ACL injury in female athletes. Researchers prospectively screened 205 female soccer, basketball, and volleyball players prior to the start of their season during a reliable jump-landing task in which the subject dropped directly down off of a box and immediately performed a maximum vertical jump with arms raised. Neuromuscular control and joint loads were measured using three-dimensional kinematics (joint angles) and kinetics (joint moments) while the athletes performed this task, known as a drop vertical jump. By the end of their respective seasons, a total of nine ACL injuries were reported and seven of them occurred in soccer players. Notably, the nine athletes who suffered an ACL rupture had significantly different knee posture and loading compared to the 196 athletes who did not sustain an ACL rupture. More specifically, ACL-injured athletes had significantly greater dynamic lower extremity valgus and knee abduction than the uninjured athletes, indicating altered neuromuscular control characteristics. Additionally, the injured group had greater knee valgus motion and moments during ground impact of the jump-landing task compared with the uninjured athletes. This was determined to be a key component of the mechanism that led to ACL rupture and the main predictors of ACL injury risk in this study. In relation to the above-described findings, anterior tibial translation and loads on the ACL have been shown to be increased by valgus torques, and valgus loading is known to place a great amount of strain on the ACL. This was further demonstrated in the study by Hewett et al. by the significantly greater values of knee abduction angles and moments observed in the ACL-injured athletes, which are factors that contribute to dynamic knee valgus and joint loading. Interestingly, statistical analysis performed by the researchers showed that knee abduction moments had a sensitivity of 78% and specificity of 73%, indicating that screening for this component has a high probability of accuracy in predicting true positive results and true negative results of the ACL injury status. Based on these findings and as suggested by the authors, avoiding excessive dynamic valgus alignment and high abduction loads at landing, cutting, and/or decelerating can minimize athletes’ risk of knee injury. Notably, the results from this study can facilitate the development of training programs that aim to prevent the observed ACL injury predictors through modifications of neuromuscular control patterns, ultimately guiding female athletes toward more effective, targeted interventions.
In a similar study, Ford et al. analyzed gender differences during landing of the drop vertical jump test and measured the maximum knee abduction angles and external moments in young athletes. Subjects participated in two testing sessions that were separated by 1 year and comparisons were made between year 1 and year 2 among pubertal females, postpubertal females, pubertal males, and postpubertal males. Researchers found that the knee abduction angle significantly increased in the pubertal female group between year 1 and year 2, during this period of rapid adolescent growth, but no change was seen in pubertal males. Postpubertal females also had a greater overall abduction angle than postpubertal males, but neither group saw a significant change between year 1 and year 2. In regard to landing with knee abduction motion and moment (torque), which are reported risk factors, both were significantly greater overall and across consecutive years in postpubertal females compared with postpubertal males following an adolescent growth spurt. The importance of these results is that they reflect an increase in ACL injury risk in female athletes during the early stages pubertal development, an important period for maturation. During this time, when sex hormonal profiles are noticeably different between males and females, there is also a divergence in ACL injury rates between genders, with a disproportionate increase in females, further exemplifying the multifaceted aspects involved in ACL injuries. ,
Hormonal Risk Factors
The effects of underlying hormonal influences on ACL injuries have been researched in an attempt to further understand the multifactorial nature of this significant injury. One study observed sex hormones in female athletes participating in a variety of sports and found a significant difference in hormonal profiles between athletes who suffered an ACL rupture and those who did not. Overall, ACL-injured athletes had significantly lower concentration of testosterone, 17β-estradiol, and progesterone than uninjured athletes, which led the authors to believe that females with these indications are predisposed to ACL rupture. The authors acknowledged that based on the significant difference in testosterone levels between the groups, it can be said that this hormone may act in a protective manner. Likewise, various direct and indirect influences of testosterone on the ACL have been previously reported, such as greater collagen synthesis and ligament resilience, and decreased joint laxity, among many others. , With respect to 17β-estradiol, results indicated a significant difference in hormone concentrations between the ACL-injured group and the control group, specifically during the luteal phase of the menstrual cycle. Compared to the follicular phase, the luteal phase is expected to have a 90% higher concentration of 17β-estradiol, which was seen in the control group, but only a 30% higher concentration during the luteal phase was observed in the injured group. Regarding progesterone levels, lower concentrations were also seen in the injured group during both the follicular and luteal phases, and although the magnitude of the concentration increase between phases was the same in both groups, the injured group had lower levels overall.
Based on the available data, hormone concentrations appear to be more crucial in determining ACL injury risk than the phase of the menstrual cycle, as suggested by previous studies, particularly due to the observation that differences in hormone concentrations can be misleading as to which phase of the menstrual cycle an individual is in. Furthermore, it has been reported by multiple studies that there is a greater risk of ACL injury during the preovulatory phase , ; however, Stijak et al. linked this generally accepted finding to the low concentrations of 17β-estradiol and progesterone seen during this phase, rather than the phase itself. Similarly, the researchers noted that individuals with even lower hormone levels than normal are at twice the risk of sustaining an ACL injury during the preovulatory phase because in general, females have naturally lower hormone concentrations during this stage. Hewett et al. further supported this theory in a systematic review based on the observation that a higher risk of ACL injury in females was correlated with times when hormone levels were most varied.
The mechanisms in which hormonal fluctuations appear to affect knee stability are mainly neuromuscular control and muscle strength, while the reported effects on the structure of ligaments is not well understood. For example, different levels of physical performance, neuromuscular firing patterns, motor skills, muscle function, and tendon and ligament strength have all been observed during different menstrual cycle phases. These influences have also been tied to estrogen effects, as it is a sex hormone that drastically changes throughout the duration of the menstrual cycle. Additionally, in support of the findings that fluctuating estrogen levels may increase the risk of ACL injury, Hewett and Hansen reported that female athletes who use oral contraceptives, which generally function to minimize surges of female sex hormones, have lower rates of ACL injury than those who do not use oral contraceptives. Researchers related the difference in injury incidence to increased overall knee stability through decreased knee laxity, increased hamstring-to-quadriceps strength ratios, and reduced valgus torques, among other stabilizing methods that were observed in oral contraceptive users.
Relaxin, a hormone with collagenolytic effects, has also been studied for its potential role in ACL tears. Interestingly, introducing relaxin to an animal model resulted in a significant weakening of the ACL, suggesting that relaxin concentration has damaging effects on the structure of collagen. Similarly, it has been shown that females with higher serum relaxin concentration (SRC) have greater incidences of ACL tears. In a study consisting of female athletes during the course of their college athletic career, there was a 21.9% incidence rate of complete ACL tears, and among the athletes who suffered an ACL injury, the mean SRC was 6.0 pg/mL compared to a mean SRC of 1.8 pg/mL in female athletes without a tear. Overall, it was found that individuals with SRC levels greater than 6.0 pg/mL had over 4 times the risk of sustaining an ACL tear compared with individuals with lower levels of relaxin. Furthermore, researchers found relaxin receptors located on the ACL of nonpregnant females, but no receptors were found on the ACL of males. Although the mechanism of the risk of high levels of relaxin on ACL tears remains unclear, it is suggested that the integrity of the ACL in females is altered over time due to long-term exposure effects. With this knowledge, it may be possible to develop medications to block relaxin receptors as an alternative to the unreasonable suggestion that individuals with predisposed risks should avoid participating in sports.
Overall, understanding the roles that various hormones may play in ACL injury risk is an additional key factor in developing all-encompassing prevention programs that better target each aspect of ACL injuries. Being aware of all areas capable of intervention can help athletes, trainers, and physicians more efficiently reduce risk, which is especially important for the vulnerable population of female soccer players.
Other Novel and Noteworthy Observations
In addition to the neuromuscular and hormonal influences on ACL injury risk in female soccer players, other modifiable risk factors, including fatigue related to playing volume, have been studied in this population. A study by Snyder et al. analyzed the effects of two consecutive soccer matches on landing mechanisms in Division I collegiate female athletes. The first match was played on a Friday evening and the second match was played 43 h later on a Sunday afternoon, and results were based off of physical performance and biomechanical measurements during a sidestep cutting movement. It was observed that biomechanical risk factors related to ACL injury significantly increased after playing a single soccer match and intensified further after playing in consecutive matches with less than 48 h of rest. In particular, there was a significant increase in ground reaction force leading to greater tibial compression and in lateral tibial shear joint force after the first match. After the second match, there was an even greater increase in each measurement, in addition to an increase in anterior tibial shear force, a known risk factor of ACL injury. , Significant findings of changes in physical performance parameters were also seen 12 h after the first match, including reduced knee extension strength and countermovement jump (an indication of muscle function). After the second match, countermovement jump was further reduced, and increased knee flexion and decreased knee flexion strength were also observed. Reduced knee flexion strength has been previously related to serious knee injury and observed to be more prominent in females than in males after intermittent high-intensity exercises, as commonly experienced in soccer. The authors of this study also mentioned that playing in a second consecutive match with this imbalanced strength between knee flexors and extensors significantly impacts knee mechanisms. Collectively, each of these risk factors, caused by excessive musculoskeletal stress and fatigue without optimal recovery time for neuromuscular and strength improvements, has the potential to greatly increase the incidence of ACL injury. , A similar finding has also been observed by Ryman Augustsson and Ageberg, who reported that playing two matches in a week with a maximum of 4 days of recovery in between significantly increases injury risk in youth athletes compared to playing only one match per week. In the same sense, greater training per game hours is beneficial for reducing injury risk based on the potential for heightened performance and ideal physical conditioning.
Another study observed the effect of season-long participation on ACL volume in the bilateral knees of 17 female soccer players using MRI scans. Overall, there was a significant increase in average ACL volume from preseason to postseason (1426 vs. 1556 cc, respectively; P = .006), with a greater increase observed in the right knee than the left knee. All the athletes included in the study were right-leg dominant, and the difference in volume between limbs was attributed to the different types forces that act on each leg. The left leg is the planting leg, which causes stress at the knee during a kick, and the right leg is the kicking leg, which has additional external forces on it during contact with the ball. The authors proposed that the increase in ACL volume could be explained by repetitive trauma creating microscopic tears over the course of the season, resulting in inflammation and edema of the ligament. The results of this study reflect the process of ligament healing, which occurs in three stages: the acute inflammatory stage, followed by the proliferative repair stage (rebuilding of tissue matrix), and finally the remodeling phase. The researchers did not report any ACL injuries in this population and therefore were unable to determine the effects of ACL volume on injury risk. However, these findings provide clinically relevant data on ligament healing and the functional changes that result from remodeling after injury, encouraging further research to analyze these effects on injury.
In a separate study, Kosaka et al. prospectively studied a potential psychologic aspect of ACL injury with respect to competitiveness in female high-school athletes. Among the 300, 15-year-old athletes studied over the course of the 3-year study, 25 ACL injuries occurred through noncontact mechanisms. It was found that the athletes who sustained ACL injuries had significantly higher scores in competitiveness based on a Diagnostic Inventory of Psychological Competitive Ability (DIPCA.3) questionnaire that was completed at the beginning of the study. More specifically, these athletes scored higher in aggressiveness, volition for self-realization, volition for winning, judgment, and cooperation. Overall, it was observed that a higher confidence in ability was related to a higher risk of sustaining a noncontact ACL injury. The authors proposed that an explanation for this could be that the more competitive players pursued a higher level of their own performance and therefore placed themselves in higher risk situations by participating in training and competitive matches in a more aggressive manner. In addition, a noteworthy correlation between the history of concussion incidences and lower extremity injuries in collegiate athletes has been observed by multiple studies. Each study found that athletes who reported a history of concussions were more likely to also report a history of a knee injury and were at a greater risk for knee injury than those who did not have a concussion history. Further investigation is necessary to understand the underlying mechanisms of this observation, but it is worth mentioning that a consistent relationship has been established by several studies. The importance of investigating the various mechanisms of ACL injury that have been analyzed by countless studies is to emphasize that there is no single factor that is solely responsible for this severe, unpredictable injury. It has been shown that it is not possible to visually predict which female soccer athletes may be at a greater risk of ACL injury during an activity. This further indicates the need for screening these athletes to determine the specific risk factors an individual may have in order to better predict injury risk and more effectively prevent ACL injury.
Postreconstruction Outcomes and Reinjury Risks
Aside from the high incidence of a primary ACL injury in female soccer players, they also have an increased risk of secondary ACL injuries, such as graft failure or contralateral ACL injury. A 2020 study reported a higher risk of further ACL injury of ACL-reconstructed soccer players in a 10-year follow-up. Among the 684 respondents of a questionnaire that was used to obtain results about return to play and additional knee injuries at 10 years after subjects’ primary ACL injury, 51% reported that they had returned to soccer while 49% did not return. Within the players who did not return, 65.4% reported that it was due to causes related to knee. Among the players who did return to playing soccer, it was found that they had a significantly higher risk of graft failure and rerupture, contralateral ACL injury, and new injury to the affected knee. In this population, 28.7% sustained a new injury overall, where 9.7% had a rerupture and 20.6% suffered a contralateral ACL tear. Notably, further ACL injury was more prominent in females of this study, but this was not found to be statistically significant, and no significant difference in risk of rerupture was found between the types of graft used (bone-patellar tendon-bone vs. hamstring). In a similar study of a 5-year follow-up post-ACL injury, 5.0% of patients underwent a contralateral ACL reconstruction surgery and 4.1% had a revision surgery of the affected knee overall. Particularly within the 15- to 18-year-old group of soccer players in this study, 22.0% of females had either a contralateral ACL reconstruction (10.2%) or a revision surgery (11.8%) compared to only 9.8% of males (4.4% contralateral and 5.4% revision). At the collegiate level, female soccer athletes also had significantly higher rates of recurrent ACL ruptures than male soccer players (5.2 vs. 1.4 per 10,000 AEs, respectively) over a 10-year study period. Additionally, Fältström et al. compared 117 active soccer players after ACL reconstruction to 119 knee-healthy matched controls on the same team (average age of 20 years old). The ACL-reconstructed group experienced new ACL injury at five times the rate of the control group and other traumatic and nontraumatic knee injuries at two to four times the rate, but no rate difference between the groups was observed for injuries unrelated to the knee. Throughout the 2-year study period, only 38% of the ACL-reconstructed group continued to play soccer compared to 64% of the control group, and the ACL-reconstructed group also had a greater reduction of activity level as indicated by a greater decrease in average Tegner Activity Scale score. Among the 62% of the players who quit after their ACL reconstruction, the main reasons were because they had suffered a new injury or because they had a lack of trust in their affected knee and feared a new injury. Based on this information, the researchers noted the importance of individualized rehabilitation and peak functional performance before returning to play, as well as allowing a proper amount of time for recovery after an ACL reconstruction. Interestingly, the optimal recovery time for minimizing risk of further injury in soccer is returning to play after at least 9 months, and 1 out of 4 players in this study returned to soccer within 9 months of their ACL reconstruction surgery.
Similar findings were also supported by an earlier study that compared the risk of a second ACL injury in female soccer players to nonsoccer athletes matched by age, activity level, and graft type over a 2-year follow-up period. Each group contained 90 female subjects and the average age was 20 years, and the nonsoccer group included athletes participating in basketball, downhill skiing, volleyball, and track. Compared to the nonsoccer group, soccer players had a significantly higher rate of graft failures (11% vs. 1%; P < .01) and contralateral ACL tears (17% vs. 4%; P < .01). With respect to survival rates at 6, 12, 24, 60, and 120 months, the graft survival rate for soccer players at each time point saw a significantly greater and consistent decline compared with that for nonsoccer players ( P < .01). Similarly, the difference in survival rates of the contralateral ACL at each time point had the same pattern, with the most notable difference observed at the 120-month time point, where the contralateral survival rate for soccer players was merely 53% compared to 93% in nonsoccer players. Overall, 28% of the soccer athletes suffered a second ACL injury compared to only 5% of the nonsoccer athletes. Interestingly, soccer players also sustained significantly more lateral meniscal injuries alongside their primary ACL rupture compared with nonsoccer athletes (40% vs. 23%; P = .02). Particularly within the soccer group, players who returned to soccer after their primary ACL reconstruction surgery (67/90) had significantly more graft failures than the initial soccer players who did not return (15.0% vs. 0%; P = .04). A difference in contralateral tears was also observed, but it was not found to be statistically significant (19% vs. 9%; P = .34). Overall, 34% of the soccer players who returned suffered a second ACL injury compared to 9% of those who did not. Regarding graft survival rates from 6 to 120 months, the players who returned to soccer had a consistent decline at each time point, whereas players who did not return remained at a 100% graft survival rate throughout the study period. Interestingly, almost half of the graft failures that occurred within this group happened within 12 months of returning to soccer, whereas contralateral ACL tears tended to occur later and the survival rates between returners and nonreturners were similar. An additional finding that was reported only within the soccer group was older age as a risk factor for graft failure. The risk increased by a factor of 1.5 with every year of age, keeping in mind that the entire cohort was under 25 years old, which is generally considered young in age.
In a separate study, Bourne et al. reported knee flexor weakness in 84 elite female soccer players who underwent an ACL reconstruction procedure in the previous 10 years. Within this group, 12 subjects had a history of a unilateral ACL reconstruction and their peak eccentric knee flexor force was compared to 72 subjects who did not have a history of an ACL reconstruction in the past 10 years. Values were measured during a Nordic hamstring exercise, where the participants knelt on a pad with their ankles secured and as they gradually leaned forward toward the ground, the force generated by their knee flexors was measured. It was found that those with a history of ACL reconstruction were weaker in their affected limb than their uninjured contralateral limb, represented by a 20% between-limb imbalance. In addition, the two-limb average peak eccentric knee flexor strength was weaker in the reconstructed group than the control group, and when hamstring-injured players were removed from analysis, the observed differences in strength were even greater. Notably, no difference was observed between the uninjured contralateral limbs of the ACL-reconstructed group and the healthy limbs of the control group. Additionally, among the ACL-reconstructed players, no correlation was found between the time since ACL reconstruction and a change in knee flexor strength or imbalance between limbs. Based on their study, the authors proposed that the postreconstruction knee flexor weakness may be explained by medial hamstring atrophy and reduced activation, which are known risk factors of primary ACL injury, although the mechanisms remain unclear. An additional noteworthy finding in this study was that 25% of the ACL-reconstructed players suffered a hamstring strain injury after reconstruction compared to only 2.8% of the uninjured players. Therefore it can be said that the further consequences of ACL injury and reconstruction and the strength imbalances that occur are an increased risk of sustaining a hamstring strain. Worth mentioning as well is the consequence of developing osteoarthritis in the injured knee, where the likelihood has been reported to be as high as 80% within 15 years of an ACL injury.
Despite the consistent findings of the high ACL reinjury risk in female soccer players, there is little understanding of the mechanisms and the reasons for the difference in rates compared to males. One possible mechanism is a further decrease in quadriceps and hamstring strength and inferior performance, particularly with jumping, of ACL-reconstructed athletes compared to healthy controls. Additionally, increased risk in female soccer players compared with males may be explained by altered kinetics and neuromuscular control, a decrease in lower extremity agility, and consequent stiffening, which are all found after reconstruction predominantly in females. Also potentially affecting reinjury risk are influences of whether the injured limb is the player’s dominant or nondominant leg, the increased demand placed on the contralateral limb after an injury, and the incomplete development of the athlete’s original biomechanics, among other intrinsic factors specific to the individual. Awareness of the epidemiologic data and risk factors associated with both primary and secondary ACL injuries further signifies the importance of implementing effective prevention programs to reduce injury in female soccer players.
Conclusions
With the continually increasing involvement and enhancement of athleticism in female soccer players, it is of utmost importance that athletes, coaches, and those involved in their healthcare are aware of the predominance of ACL injuries in this population. Additionally, knowledge of the severity of this injury and its consequences should not be overlooked when considering the high risks associated with sustaining an ACL injury. A better understanding of the specific risk factors and mechanisms involved is crucial for the development of effective injury prevention programs and for athletes to be able to protect themselves. Finally, great emphasis should be placed on the importance of implementing these programs in order to minimize risk exposure and reduce injury rates in this high-risk population.
References
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