Anterior cruciate ligament (ACL) injury in the United States exceeds one in 3000 individuals, corresponding to an overall injury rate of approximately 100,000 injuries annually. The cost of healthcare associated with ACL injuries is considerable. More than half of individuals sustaining an ACL injury require surgery, with an estimated annual cost close to one billion dollars (Urochak, 2003 ). The cost of surgical treatment and rehabilitation for an athlete with an ACL injury in 1997 was approximately $17,000 (Huston, 2000).
This national estimate is not indicative of the problem this injury represents for women because ACL injury rates are four to eight times higher in women than in men participating in sports of soccer, basketball, and volleyball (Lephart, 2002). The NCAA reported during the 1996-1997 academic year that more than 130,000 women participated in college athletics, and there was an average knee injury rate of 1 per 1000 exposures (1 for every 10 female athletes). Given these figures, approximately 13,000 knee injuries will occur in females who participate in athletics at the collegiate level during any given year (Arendt, 1995).
Additionally, the National Federation of State High School Associations reported, in 1997, that more than 2.5 million females participated in a high school sports program. Although rates of injuries in high school athletes are as frequent as those of female collegiate athletes, the larger high school athletic population is expected to account for more than twice as many ACL injuries compared with athletes at the collegiate level. There could be as many as 25,000 knee injuries sustained by female athletes annually at the high school level alone (NFHS, 1998).
The anterior cruciate ligament is responsible for resisting anterior translation of the tibia onto the femur, specifically resisting 80-85% of anterior transitory loads (Lephart et al, 2002). It is one of the most important ligaments to athletes because of its main function, stabilization of the joint while decelerating. The ligament is primarily made up of two bands, the anteromedial and posterolateral, and an intermediate band sometimes present.
The ACL runs from the posteromedial portion of the lateral femoral condyle in an inferior, anterior, and medial orientation to an area just lateral to the medial tibial eminence. With the increasing number of women participating in athletics and the debilitating nature of ACL injuries, a better understanding of mechanisms of injury in women who sustain ACL injuries is essential. Published studies strongly support noncontact mechanisms as the primary cause of ACL tears in women. Therefore, close to 85% of all ACL injuries in females is non-contact in nature involving at least one of the following patterns of injury: ·
A sudden stop, twist, pivot or change in direction at the knee joint — With the foot planted on the ground, the knee is put into an extreme valgus position with tibial external rotation, thus stressing the ACL to the point of rupture ·
Extreme hyperextension of the knee – sometimes, during athletic jumps and landings, the knee straightens out more than it should and extends beyond its normal range of motion, causing an ACL tear. The other 15% of injuries may occur by a contact injury and usually involve an isolated mechanism with extreme force. The mechanisms of a contact ACL injury include: a force that drives the tibia anteriorly, a hyperextension force, or a valgus force. These mechanisms are usually isolated but can occur in combination (Gray et al, 2000). To date, several studies have examined ACL injuries at the high school, collegiate, and recreational levels, but no one has been able to determine conclusively why these injuries are more common in females. Researchers have examined intrinsic and extrinsic factors that may lead to injury, but the common conclusion is that these injuries are probably multifactorial in nature. Several extrinsic factors have been hypothesized to lead to the higher incidence of female ACL injuries including muscle strength, neuromuscular control, increased knee stiffness, improper landing techniques, and muscular activation patterns differing from those observed in males (Huston, 2000). Extrinsic also include the type of sports, playing surface, conditioning level, experience, skill, and equipment used (Arendt, 1999). Intrinsic factors are those factors that may not be able to be changed and include quadriceps femoris angle, femoral notch, joint laxity, and hormonal influences (Huston et al, 2000). Although one single factor has not been determined to be the cause of the higher incidence of ACL injuries in females, examining ACL injuries as a multifactoral cause is necessary. The combined role of intrinsic and extrinsic factors may lead to a higher injury rate in females, but studies have shown that there are ways to prevent injury in sports.
EXTRINSIC RISK FACTORS
Muscular Strength and Muscular Activation Pattern: Several researchers have documented that women have significantly less muscle strength in the quadriceps and hamstrings compared with men, even when muscle strength is normalized for body weight (Huston & Wojtys, 1996). Female athletes seem to have different muscle activation patterns compared with their male counterparts. In a study by Huston & Wojtys, they reported that female athletes preferred to contract their quadriceps first in response to anterior tibial translation, whereas the male athletes and male and female control subjects who were not athletes, responded to an anterior tibial translation by first contracting their hamstrings (1996). If the quadriceps fire without the hamstrings, the tibia may sublux anteriorly and significantly increase the load on the ACL. Conversely, if the hamstrings fire without a quadriceps contraction, anterior tibial translation is decreased.
Prevention:
Weight- training coupled with an endurance training program are only part of the necessary components to improve muscle function. The female athlete not only needs to be strong but her muscle reaction time needs to be as quick as possible. Therefore, plyometrics and agility-type exercises, such as running through cones, figure eights, and single-leg jumps are proven methods to improve muscle reaction time (Holm et al, 2004). In addition, since female athletes tend to become quadriceps dominant with sports, special emphasis must be placed on hamstring exercises for strength and functional limb control. Knee Stiffness: As muscles that span the knee contract, they act to increase joint force and decrease tibiofemoral displacements, dissipating potentially dangerous loads, and lowering the force carried by the ACL and other passive structures. Mechanoreceptors in the knee ligaments and the joint capsule regulate muscle stiffness by influencing muscle spindle afferents from agonist and antagonist muscles with excitatory and inhibitory activity (Huston, 2000). Muscle stiffness across the knee has intrinsic and extrinsic components. The intrinsic component is dependent on the number of active actin-myosin cross-bridges in the muscles at a specified point. The extrinsic component is dependent on the excitation provided by alpha and gamma motoneurons. Markolf et al. reported that patients who were not athletes could increase varus and valgus knee sniffness by two to fourfold with isometric co contraction of the hamstring and quadriceps, and that athletes who are well-conditioned were capable of increasing knee sniffness by a factor of 10. Prevention: Joint stiffness can be modulated over a certain limited range and regulated through antagonist muscle contractions. Therefore, muscle stiffness may improve with a functional training program that emphasizes the hamstring and gastrocnemius muscle groups.
Improper Landing Techniques and Jumping Characteristics: Gender differences do exist when athletes perform athletic maneuvers, such as cutting and landing from a jump. Women tend to land with the knee and hip in a more extended position, and therefore subject themselves to higher forces per body weight during the impact of landing. Females also tend to perform these maneuvers with more knee valgus and had greater quadriceps and lower hamstring activation than males, particularly at foot strike ( Lephart et al., 2002). Another study by Lafortune compared rebounding in basketball players who were healthy and basketball players who were injured previously. During landing, greater range of motion at the hip and knee was seen in athletes who were healthy. Athletes who were injured previously had less hip and knee joint motion (1985). Hewett et al, (1996), showed that introducing a jump training program to female athletes involved in high-risk sports decreased the overall incidence of serious knee injuries. They tested the effect of a jump training program on the mechanics of landing and on the strength of the lower extremity musculature in female athletes involved in sports that required jumping.
The program was designed to decrease landing forces by teaching neuro-muscular control of the lower limb during landing and increasing joint stability by maximizing the strength of knee musculature. Eleven female high school volleyball players participated in a 6-week jump training program that lasted 2 hours a day, 3 days a week. Three phases were implemented throughout the jump training program. The technique phase (Phase 1) included the first 2 weeks in which proper jump technique was shown and drilled. The fundamental phase (Phase 2) concentrated on the use of proper technique to build a base of strength, power, and agility. The performance phase (Phase 3) focused on achieving maximal vertical jump height. The program significantly increased hamstrings power and strength, and increased hamstrings to quadriceps peak torque ratios. Adduction and abduction moments of the knee decreased significantly at landing after the completion of the program. Prevention: A jump training program advocated by Hewett et al (1996) is recommended and should be incorporated into the training program for women who participate in sports that require jumping and pivoting. This exercise regimen currently is the only laboratory tested training protocol that concurrently documents a decreased rate of ACL injuries. INTRINSIC RISK FACTORS Quadricep Femoris Angle The Q-angle is defined as the acute angle between the line connecting the anterior superior iliac spine and the midpoint of the patella, and the line connecting the tibial tubercle with the same reference point on the patella (Huston, 2000).
A large Q-angle may contribute to increased incidence in female ACL ruptures, although some studies have found no correlation (Gray, 1985). Quadriceps femoris angles greater than 150 for men and greater than 200 for women are thought to be clinically abnormal (Huston, 2000). Theoretically, larger Q-angles increase the lateral pull of the quadriceps femoris muscle on the patella and put medial stress on the knee (Gray, 1985). Prevention: Although lower extremity alignment cannot be altered, several investigators have reported that the Q-angle can change with an isolated quadriceps contraction. In support of this finding, Hahn and Foldspang theorized that through athletic training, the dynamic activity of the quadriceps could lower the Q-angle (1997). However, more studies are needed to investigate this relationship.
Femoral Notch:
The size of the femoral notch and the size of the ACL are two other factors that may predispose female athletes to ACL rupture, however the association between a small notch size and incidence of injury is not necessarily related (Huston, 2000). When the knee is in flexion, the ACL comes in contact with the medial margin of the lateral femoral condyle and in extension the ACL comes in contact with the anterior intercondylar notch. It is thought that a small notch increases the chances for impingement during knee motion if the ACL is normal size and shape (Norwood, 1977). In contrast, several authors have reported no correlations between notch width and ACL injury rates. It was once thought that a female with a small notch probably has a proportionally small ACL, however this has been recently refuted. Malinzak et al. reported that ACL size does not change regardless of the size of the notch (2001). Cadavers with a small or normal intercondylar notch width had similar sized ACL’s. Anderson et al. confirmed these findings as he conducted that notch characteristics measured in male and female high school basketball players do not contribute to the gender differences in ACL injury ( 2001). In both males and females, ACL dimensions did not change with the size of the notch. It was concluded that if notch size contributes to ACL injury, the normal sized ACL in a stenotic notch is probably the cause (Anderson et al, 2001).
Prevention:
Currently, surgeons may choose notchplasty to widen the femoral notch when reconstructing the ACL. However, scientific justification is lacking at the present time to conduct this procedure for prevention of ACL injury.
Joint Laxity:
Several studies have shown that joint laxity tends to be greater in women than in men. However, the relationships between ligamentous laxity and injury is not clear (Huston, 2000). Urochak et al. measured knee laxity and generalized joint laxity in 859 West Point cadets. Knee laxity was measured using a KT2000 knee arthrometer and generalized joint laxity was determined by a score of 5 or more on the following measures: 5th finger MCP hyperextension, elbow hypextension, knee hyperextension, and thumb hyperextension and abduction to the volar aspect of the arm. Females had greater joint laxity and generalized laxity. Moreover, non contact ACL injured subjects had greater joint laxity and generalized laxity than non-injured ACL subjects (2000).
Several authors have tried to link a ligamentously lax knee with increased incidence of injury. For example, in 1970 Nicholas reported that football players classified as having loose joints suffered more knee injuries than players classified as having tight joints.
This study was completed in 1970 however and the methods for determining “looseness” are questionably subjective. Although it has been hypothesized that females may exhibit more joint laxity than males, it is not known that increased joint laxity leads to injury. Some authors have reported no correlation between joint laxity and injury in athletes.
Measurement technique must be questioned when looking at the results of the previous studies as some studies used methods or machines that have not been previously validated.
The literatures regarding this subject is still controversial and more studies are needed. Therefore, no recommendations were given at this time
Hormonal Influences:
The role of hormones in predisposing female athletes to injury of the ACL recently has received attention. In 1996, Liu et al. reported that estrogen and progesterone receptor sites exist in human ACL cells, suggesting that female sex hormones may play a role in the ACL’s structure and composition. They also found that physiologic levels of estradiol have a significant dose-dependent effect on the fibroblasts of the ACL. Fibroblast proliferation and rate of collagen synthesis were reduced significantly with increasing estradiol concentration. It was also reported that the administration of estrogen significantly reduced the tensile properties of rabbit ACL. Collagen, produced by fibroblasts, is known to perform the major load bearing function of the ACL and perhaps alteration in the metabolism of these fibroblasts influence the quantity, type, and stability of the collagen in the ACL. (Liu et al., 1996).
Several researchers have attempted to link hormone fluctuation during the menstrual cycle to rate of ACL injuries. Wojtys et al. found an association between the menstrual cycle phase and the incidence of ACL injury. Using a self reported questionnaire after an acute noncontact ACL injury, women reported more injuries than expected in the ovulatory phase of the cycle (Days 10-14, when estrogen level surge). In contrast, fewer injuries occurred in the follicular phase (Day 1-9, where estrogen and progesterone levels are low). The authors cautioned that additional studies are needed to confirm this relationship.
Findings are too preliminary for treatment or prevention recommendations to be made.
FUTURE RECOMMENDATIONS
It is thought that efforts to decrease the incidence of anterior cruciate ligament injuries in women should be directed toward the extrinsic factors. For years, conditioning programs have mainly stressed increasing muscle strength, flexibility, endurance, and power (isolated joint mechanics) to be the basics necessary for injury prevention in sports. More recently, there has been an increased recognition that to prevent injury one also must improve functional joint stability through enhancement of neuromuscular control mechanisms (Griffin, 2003). A series of studies over 4 years suggests that intercollegiate female athletes have significantly different proprioceptive characteristics, muscle firing patterns, and landing strategies compared with their male counterparts (Lephart et al, 2002). Therefore, prevention programs for ACL injuries must emphasize the importance of agility training, proprioception and balance training. Improvement of hamstring/quadriceps strength and hamstring recruitment should also be considered. A plyometric training program that incorporates an eccentric preload (a quick eccentric stretch) followed by a forceful concentric contraction, a combination of events used frequently in sports (the jump and the throw) also are being investigated.
Many programs are being developed and used to decrease the incidence of ACL injuries. The program of Caraffa et al. is a five phase proprioceptive program based on increasingly difficult skills done initially without a balance board and progressing through a series of balance boards of various design. Athletes participate in the program 20 minutes a day beginning 30 days before the beginning of the season (1996). Another program developed by Sportsmetric in Cincinnati is a three-part prevention program consisting of stretch, plyometrics, and strengthening drills designed to address potential deficits in the neuromuscular strength and coordination of the stabilizing muscles of the knee (Hewett, 1999).
A study by Griffin et at found that teaching women a modified style of planting, cutting, and pivoting with the knees in a flexed position resulted in an 89% reduction of ACL injuries. As a result, drills were performed in which athletes practiced neuromuscular control when stopping quickly, cutting, or landing in a position of hip and knee flexion with the body balanced over the lower extremity. Athletes do these drills changing three components: the “plant and cut” was changed to an accelerated rounded turn in a bent knee position, straight knee landings to landings with the knee bent, and one step stops with the knee extended to multiple step stops with knees bent (Griffin, 2003).
A variety of exercises can be implemented into a program to prevent ACL injuries but more data are needed to ascertain the essential elements necessary for a reliable, easy to implement, and effective anterior cruciate ligament prevention program. Most prevention programs should aim at increasing injury awareness and enhancing neuromuscular control through agility and proprioceptive drills. Prevention of anterior cruciate ligament injuries is possible with the use of neuromuscular training in female athletes, but successful prevention depends on good compliance among the players. References Anderson, A.F., Dome, D.C., Gautam, S., Awh, M.H. & Rennirt, G.W. (2001).
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