Anterior cruciate ligament (ACL) injuries are common in athletes participating in running sports requiring jumping and pivoting, and are significantly more common in female athletes participating in such sports. The annual incidence of ACL injury is between 80,000 and 250,000, with women displaying a four-to sixfold increased risk for ACL injury compared with men participating in similar sports (1). As with any injury, understanding the mechanisms for ACL injury has helped determine potential risk factors for injury and, thus, identify areas for intervention and development of prevention strategies. Because of increased incidence of ACL injury in female athletes, many of the prevention efforts have focused directly upon this population.
Mechanisms of ACL Injury
The majority of ACL injuries are sustained via a non-contact mechanism, with approximately 70% of ACL injuries occurring in a non-contact situation (2). Interviews with athletes after injury and evaluation of injury footage have demonstrated that most non-contact ACL injuries occur during landing from a jump, deceleration, or pivoting/cutting. Analysis has revealed that the athlete's lower extremity is in a position of valgus with foot pronation and relative knee and hip extension at the time of injury (3).
These observations suggest that landing with the knee and hip in minimal flexion and the lower extremity in valgus may increase the risk for ACL injury. Indeed, in vivo analysis of ACL strain has confirmed high rates of ACL strain when the knee is in nearly full extension, validating the idea that landing or pivoting with the knee in near full extension places the ACL at risk (4,5). Furthermore, female athletes, who have a greater incidence of ACL injuries than men, have been found to land with the knee in greater dynamic valgus and with the knee in less flexion than men (6,7). And, a prospective analysis by Hewitt et al. (8) found that female athletes who tend to land with increased knee valgus were more likely to sustain an ACL injury, further supporting the notion that increased knee valgus on landing increases ACL injury risk.
Thus it stands to reason that neuromuscular patterns that lead the athlete to land or pivot with the lower extremity in greater valgus or near full extension may increase the risk for ACL injury. Great efforts have been made to determine what these neuromuscular factors may be and to develop interventions to prevent injury by addressing these patterns. Specifically, hamstring and quadriceps strength, landing patterns, proprioception, and muscle endurance have been evaluated as potential contributors to ACL injury risk.
HAMSTRING AND QUADRICEPS STRENGTH
Because the hamstring works to flex the knee, which will place the knee in a less risky position during landing and pivoting, investigators have sought to determine whether hamstring muscle activation might play a role in ACL injury. In vivo analysis of ACL strain has demonstrated that high rates of ACL strain occur during quadriceps contraction and lower rates of strain occur during hamstring contraction, suggesting that hamstring activation during landing and pivoting maneuvers may be ACL protective [4,5)] Solomnow et al. (9) further demonstrate the importance of hamstring strength in maintaining knee joint stability, showing that direct stress upon the ACL leads to inhibition of quadriceps activity and increased hamstring activity. And both Markolf (10) and Sell (11) found that increased quadriceps activity in proportion to hamstring activity in female athletes during a stop-jump activity leads to increased anterior tibial loads. These studies suggest that hamstring activation is likely an important means of ACL protection. Another group demonstrates increased anterior shear force in athletes who activate the lateral quadriceps musculature more than the medial quadriceps musculature on landing from a jump, indicating that poor medial quadriceps activation may increase ACL injury risk (11,12). Several investigators demonstrate that female athletes tend to land from jumps and pivot with decreased hamstring and increased quadriceps activity compared with men and suggest that the lack of hamstring recruitment may, in part, account for the increased risk for ACL injury in women (10,11,13-15). In a prospective study by Hewett et al. (8), hamstring recruitment with knee joint loading was evaluated before the athletic season, and female athletes who had poor hamstring recruitment were found to be more likely to experience ACL injury in the ensuing season. These studies in the athletic setting further support the notion that hamstring activation is an important means of ACL protection. In addition, women have been found to have an increased tendency to activate the lateral quadriceps musculature upon landing compared with men, leading the investigators to propose that poor medial quadriceps activation may increase the risk for ACL injury in women (11,12).
BALANCE AND PROPRIOCEPTION
Poor proprioception may contribute to increased risk for ACL injury by contributing to poor neuromuscular control of the hip and knee leading to increased lower extremity valgus on landing from jumps and pivoting (8). Myer et al. (16) demonstrated that proprioceptive training can decrease lower extremity valgus upon landing from a jump. A prospective study of college athletes demonstrates that female athletes with poor core proprioception are at increased risk for ACL injury (17).
FATIGUE AND FITNESS LEVEL
Several authors have demonstrated that knee injuries are more common in the later portions of games than in earlier portions, suggesting that fatigue may play a role in knee injury (18,19). It is suggested that fatigue and poor fitness may alter neuromuscular control and, thus, lower extremity mechanics during landing and pivoting. When fatigued, both male and female athletes have been found to land and pivot with decreased knee flexion angles, increased knee valgus, and increased proximal tibial anterior shear force (20).
NEUROMUSCULAR INTERVENTION PROGRAMS
Many investigators have developed training programs aimed at improving proprioception, hamstring strength, and muscle endurance in order to change landing and pivoting patterns and decrease ACL injury risk in both male and female athletes.
In the 1990s, Hewett et al. demonstrated that introducing a plyometric training program to female athletes could improve hamstring strength, decrease adduction and abduction moments at the knee on landing and pivoting, and decrease peak landing forces (21). The athletes were able to land from jumps and pivot with the knee in less valgus and in more flexion after training. They proposed that the program could potentially decrease the risk for ACL injuries (21).
Hewett et al. (22) then performed a prospective, controlled study with 1263 high school-aged female soccer, basketball, and volleyball players to evaluate the effects of this program on the incidence of knee injury. The athletes performed three 90-min training sessions per week over a 6-wk period before the start of the athletic season. Plyometrics, flexibility, balance, and weight training were incorporated in the sessions, with particular emphasis upon proper form in pivoting and landing activities. The plyometric exercises became progressively more difficult over the training period. In the season after the training, they found significantly fewer ACL injuries in the trained group than in the untrained group.
Several other groups of investigators have prospectively investigated the effects of balance, plyometric, and landing technique training on ACL injury incidence. These programs have encountered varied success; however, the results generally have been encouraging.
Mandelbaum et al. (23) performed a 2-yr prospective, non-randomized, controlled, in-season prevention program consisting of 15-min training sessions performed three times per week as a warm-up. A total of 1041 high-school aged female soccer players were included in the study. Like the program performed by Hewett et al., plyometric, balance, and agility training were all included as part of the training. The trained group had 74% fewer ACL injuries than the untrained group, a statistically significant finding.
Olsen et al. (24) performed a randomized, controlled trial of balance, hamstring specific strength, agility, and landing technique training in a large group of adolescent handball players. The 15-20-min program was performed as the warm-up at 15 consecutive training sessions and then once a week thereafter. The intervention group had significantly fewer total lower extremity injuries. Knee ligament injuries with ACL, PCL, and MCL injuries combined, were significantly decreased by 80% in the intervention group.
Myklebust et al. (25) performed a prospective study in handball players with a combination of preseason and in-season intervention in female team handball players. The intervention program consisted of 15 min of balance and landing technique training. In elite level players, non-contact ACL injuries were reduced significantly during the two seasons that the intervention was performed, in comparison with the season before intervention.
Heidt et al. (26) performed a randomized, controlled, 7-wk preseason intervention with 300 high school-aged female soccer players that included three training sessions each week, two of which were treadmill running sessions, and one was a plyometric training session. The players were followed for the next soccer season, and lower extremity injuries decreased significantly in the intervention group. While ACL injuries decreased in the intervention group, the difference did not reach a level of statistical significance. The lack of a significant reduction in ACL injuries may have been due to inadequate sample size or to the relatively lower intensity and frequency of plyometric training activities compared with the investigators who did note decreased ACL injury risk after training.
Petersen et al. (27) used a plyometric, agility, and balance training intervention in team handball players and failed to find a significant difference in ACL injuries in the trained group. However a non-significant reduction in ACL and ankle injuries was found. The program consisted of 10-min training sessions performed three times per week for the 8 wk before the season and once a week during the competitive. Compared with studies that demonstrate a significant effect, Petersen et al. had shorter and fewer training sessions, indicating that longer or more frequent training sessions may be needed. In addition, the statistical power was low, and the number of ACL injuries was relatively low with one injury in the intervention group and five in the control group.
Wedderkopp et al. (28) performed a randomized trial comparing agility and plyometric training alone to agility, plyometric and balance board training. In the group where balance board training was included, the total number of traumatic injuries was significantly decreased, but knee injuries, alone, were not significantly reduced, possibly due to low power.
The effects of proprioceptive training, alone, on ACL injury have been mixed. Caraffa et al. (29) demonstrated decreased incidence of non-contact ACL injuries with the addition of balance board training over a three-year period of time in male soccer players. However, in a study by Soderman et al. (30), balance training alone, did not significantly decrease ACL or other lower extremity injuries in professional female soccer players. In this study, 10-15 min of balance board training was performed daily for 1 month and then three times per week for 6 months. However, there was a significant dropout rate of 37%, and the study was not powered sufficiently statistically to determine a difference in injury rate.
While the results of studies evaluating proprioceptive and plyometric training do not universally support a decreased ACL injury risk, they generally are encouraging. It is not known yet whether a preseason or in-season program is most effective in decreasing injury risk. Likely, a combination of the two would be best. From a practical standpoint, the programs with 60-90-min sessions, such as those performed by Heidt et al. (26) and Hewett et al. (22), may be performed most easily in the preseason time period when more time can be devoted to general training and the 15-20-min programs intended to replace the usual warm-up, like that by Mandelbaum et al. (23), may be incorporated most easily during the in-season period when teams have less time for non-sport-specific training.
Some of these neuromuscular training programs have been evaluated for performance effects, as well. Hewett et al. (22) demonstrates significant improvements in vertical jump, hamstring strength, and controlled dynamic loading of the knee. However, Holm (31) did not find significant improvement in muscle strength or proprioception after wobble board training. If these neuromuscular training programs can improve performance indices, compliance may be improved, as coaches may be more willing to use valuable training time to perform these programs. Athletes also may be more compliant with training programs if they know that performance may be enhanced.
HORMONES AND ACL INJURY
The increased risk for ACL injury in women has led investigators to study the effects of hormones upon the ACL. Laboratory studies demonstrate that the ACL has receptors for estrogen, progesterone, and relaxin, indicating that these hormones may have an effect upon the ACL (32).
In vitro studies on cells harvested from human ACL tissue demonstrate that both estrogen and progesterone may influence metabolism and collagen synthesis in the ACL. In one study, increasing concentrations of estrogen led to decreased fibroblast proliferation and type 1 procollagen synthesis in the ACL (33). However, when increasing doses of progesterone were added to the protocol, the effects of estrogen were mitigated (34). When estrogen was given in steady doses, increasing progesterone doses actually led to dose-dependent increases in fibroblast proliferation and type 1 collagen synthesis. These findings indicate that the changing hormonal milieu during the menstrual cycle may influence ACL metabolism and strength in women.
Animal studies evaluating the effect of estrogen upon the biomechanical properties of sheep, goat, mice, and rabbit ACLs have yielded conflicting results, with some finding lower load to failure with estrogen treatment and some finding no difference (35-37). In monkeys, endogenous estrogen was not found to have any direct effect upon mechanical properties of the ACL (38). Recent animal studies in monkeys and rats did not identify any influence of estrogen upon viscoelastic or tensile mechanical properties of the ACL (37,38).
Unfortunately, it has been difficult to evaluate the effects of estrogen upon the ACL in women because of difficulties in accurately determining menstrual cycle phase and hormone concentrations. In addition, while ACL laxity can be assessed non-invasively, it is not possible to accurately measure ACL strength in vivo. Studies attempting to determine the effects of the varying hormonal milieu during the menstrual cycle on ACL laxity in women have been very inconsistent, as investigators have used differing methods of determining menstrual cycle phase and hormone levels. In one study, where ACL laxity was measured daily using KT-1000 and serum hormone levels also were measured daily to confirm menstrual phase, the greatest measures of laxity were noted during the early luteal phase(39).
Attempts to evaluate the actual risk for ACL injury based upon menstrual phase have been fraught with even more methodological inconsistency. The majority of studies attempting to define ACL risk during the menstrual cycle have determined menstrual cycle phase retrospectively based upon the injured athletes' recall. In studies where hormone levels were measured at the time of ACL injury, either by urine or saliva measures, the follicular and ovulatory portions of the cycle have implicated (25,40). If hormonal variations during the menstrual cycle alter ACL strength and injury risk, it is possible that use of monophasic birth control pills may decrease risk. Hewett et al. (41) demonstrates that female athletes taking oral contraceptive pills (OCP) have increased passive and dynamic knee stability compared with athletes not taking OCP. Another group of investigators found that athletes taking OCP were less likely to suffer injuries, in general, but there are currently no data demonstrating that female athletes taking OCP are less likely to suffer ACL injury (42). Further investigation of menstrual cycle phase and OCP use with large sample sizes and reliable hormonal assays to confirm menstrual cycle phase are needed to definitively determine the role of hormonal variation and ACL injury risk.
Anatomic factors, such as Q-angle, navicular drop, and muscle laxity, that may lead the athlete to land with increased knee valgus or extension also have been evaluated for effects upon ACL injury risk.
It has been suggested that increased Q-angle may predispose athletes to ACL injury by contributing to lower extremity valgus (43). Shambaugh et al. (44) did find that basketball players with increased Q-angle were more likely to sustain knee injuries; however, only one athlete in the study sustained an injury severe enough to miss a game. Other investigators have failed to demonstrate a correlation between Q-angle and ACL injury risk (7). Because anatomic alignment of the lower extremities can be modified only with large surgical procedures, there is little potential for intervention in this area.
Because excess navicular drop contributes to lower extremity valgus, it has been identified as a potential risk factor for ACL injury. Several investigators have measured navicular drop in athletes with ACL injury compared with controls and found greater degrees of navicular drop in the injured group (45-47). Smith et al. (48), however, failed to demonstrate excess navicular drop in athletes with ACL injury. No prospective studies evaluating the use of orthotic devices to correct the pronation associated with excess navicular drop upon ACL injury risk have been reported.
JOINT LAXITY AND FLEXIBILITY
Increased hamstring laxity has been implicated as a possible risk factor for ACL injury in a case-control study (49). It is proposed that athletes with excessive hamstring laxity may have delayed hamstring muscle activation during landing and pivoting activities (7). As women tend to have significantly greater hamstring flexibility than men, this has been proposed as a potential contributing factor in the increased in ACL injury risk in women. There are no prospective data confirming increased risk for ACL injury in athletes with excessive hamstring laxity, nor have any intervention programs been developed to address this issue.
PLAYING SURFACE AND SHOES
Some investigators also have attempted to determine whether playing surface, bracing, or hormonal influences may play a role in ACL injury.
Because the foot is planted on the playing surface during landing and pivoting maneuvers, shoe-surface interactions may contribute to ACL injury risk. Early studies demonstrate an increased risk for injury in athletes playing on Astro Turf® compared with grass surfaces (50). A recent prospective study of high school aged football players found that athletes playing on Fieldturf®, a newer version of artificial turf, were not at increased risk for injury compared with those playing on grass fields (51).
Another prospective study in Australian footballers found that harder, drier fields present an increased risk for ACL injury because of increased shoe-surface traction. Thus good field care may help decrease injury risk (52).
Finally, in indoor sports, a retrospective investigation of hard surface floor type indicates that hardwood floors may pose less risk for ACL injury than artificial floors, due to the decreased friction on hardwood floors (53).
While athletes participating in running sports that require cutting and jumping may prefer shoes that provide increased friction in order to improve performance, increased friction may increase risk for ACL injury (54). Specifically, cleats with longer irregular cleats on the periphery and smaller pointed cleats on the interior seem to increase risk for ACL injury compared with shoes with flat cleats on the forefoot that are of the same height, shape, and diameter (54).
Little information is available regarding the use of prophylactic bracing for ACL injury. A large trial performed at the U.S. Military Academy found significantly fewer ACL injuries in athletes wearing knee braces compared with unbraced athletes (55). Some smaller studies have not replicated these findings, and the American Academy of Orthopeadic Surgeons Position Statement on Knee Braces does not recommend the use of bracing to prevent ACL injury, because of insufficient data to support such a recommendation (56).
Recent research has attempted to determine risk factors for ACL injury and to study prevention strategies. Neuromuscular components of ACL injury risk have received much of this research attention. Based upon these studies, it seems clear that landing and pivoting with the lower extremity in a valgus position increases the risk for ACL injury. Poor hamstring strength, poor proprioception, and poor muscle endurance may all play a role in contributing to poor lower extremity landing mechanics. While the results of neuromuscular training programs aimed at improving landing technique, hamstring strength and activation, and balance have not been universally successful, the results to date are encouraging.
Evidence regarding the effects of the menstrual cycle and OCP use upon ACL injury risk is inconsistent, and further investigation is warranted in this area. Finally, although the data are limited, shoes, field and floor types, and field conditions that increase friction between the shoe and field surface may also increase ACL injury risk.
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