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Section I: Symposium: Women's Musculoskeletal Health: Update for the New Millennium

Anterior Cruciate Ligament Injuries in the Female Athlete

Potential Risk Factors

Huston, Laura J. MS; Greenfield, Mary Lou V. H. MPH, MS; Wojtys, Edward M. MD

Editor(s): Griffin, Letha Y. MD, PhD; Garrick, James G. MD, PhD

Author Information
Clinical Orthopaedics and Related Research: March 2000 - Volume 372 - Issue - p 50-63
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Abstract

Rupture of the anterior cruciate ligament accounts for a significant number of knee injuries that occur during sporting activities. In the general population, an estimated one in 3000 individuals sustains an anterior cruciate ligament injury per year in the United States, corresponding to an overall injury rate of approximately 100,000 injuries annually.16,58 This national estimate is not indicative of the problem this injury represents for women because anterior cruciate ligament injury rates are two to eight times higher in women than in men participating in the same sports, presenting a sizable health problem.2,6,9,15,17,21,37,38,47,54,76,93,98

In 1997, the National Federation of State High School Associations reported that more than 2.5 million females participated in high school sports programs.68 Additionally, the National Collegiate Athletic Association reported that more than 130,000 women participated in varsity college sports during the 1996 to 1997 academic year.67 However, the National Collegiate Athletic Association also documented an average knee injury rate of greater than one per 1000 athletic exposures, or greater than one injury for every 10 female athletic participants.2 Given these figures, approximately 13,000 knee injuries will occur in females who participate in athletics at the collegiate level during any given year. Although rates of injuries in high school athletes are only 1/10 as frequent as those of female collegiate athletes,9 the larger high school athletic population is expected to account for more than twice as many anterior cruciate ligament 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.

Healthcare costs associated with anterior cruciate ligament injuries are considerable. The cost of surgical treatment and rehabilitation for an athlete with an anterior cruciate ligament injury in 1997 was approximately $17,000.10,29,43,53 Therefore, the total cost of anterior cruciate ligament injuries in female athletes alone could reach $646 million annually for high school and collegiate level athletes. There are more than financial costs for these athletes when one considers losses associated with missing an entire season or more of sports participation, the potential ensuing effects on the athlete's academic performance and mental health, and the potential for post-traumatic arthritis in the injured knee.18

With the growing participation of women in athletics and the debilitating nature of anterior cruciate ligament injuries, a better understanding of mechanisms of injury in women who sustain anterior cruciate ligament injuries is essential. Published studies strongly support noncontact mechanisms for anterior cruciate ligament tears in women,21,54,65,66,69,87,90 which make these injuries even more perplexing.

What causes female athletes to experience such a high rate of anterior cruciate ligament injuries? Speculation on the possible etiology of anterior cruciate ligament injuries in women have centered on anatomic differences, joint laxity, hormones, and training techniques. Investigators have not agreed on causal factors for this injury, but they have started to profile the type of athlete who is at risk. Thus, the goal of the current study is to review the most recent scientific studies of intrinsic and extrinsic risk factors thought to be contributing to the high rate of anterior cruciate ligament injuries in females, to highlight important differences between males and females, and where appropriate, to report recommendations proposed to alleviate or minimize these risk factors among female athletes.

INSTRINSIC RISK FACTORS

Structural Differences

The mechanical alignment of the lower extremity contributes to the overall stability of the athlete's knee. Differences in pelvic width and tibiofemoral angle between males and females affect the entire lower extremity and may pose as possible risk factors.78 The magnitude of the quadriceps femoris angle (Q angle) and the width of the femoral notch are thought to be anatomic factors that have contributed in the disparity of anterior cruciate ligament injury rates between males and females.

Quadriceps 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 patella32(Fig 1). Researchers have reported average Q angles in adult men and women with no history of lower extremity abnormalities.25,30,97 These studies contain various normal values, ranging from 8° to 17°, with women consistently having greater Q angles than men.30,35 Woodland and Francis97 think that larger Q angles in women are attributable to their wider pelvic base and shorter femoral length, which results in a more lateral proximal reference point than in men. Quadriceps femoris angles greater than 15° for men and greater than 20° for women are thought to be clinically abnormal.35 Theoretically, larger Q angles increase the lateral pull of the quadriceps femoris muscle on the patella and put medial stress on the knee.78

Fig 1
Fig 1:
The static Q angle is determined by measuring the acute angle produced by the intersection of two lines. The first line is drawn through the anterior superior iliac spine and the midpoint of the patella. The second line is drawn through the midpoint of the patella and the tibial tubercle.

Shambaugh et al78 investigated the relationship between lower extremity alignment and injury in recreational basketball players. They studied 45 athletes after taking various structural measurements and found that the average Q angles of athletes sustaining knee injuries were significantly larger than the average Q angles for the players who were not injured (14° versus 10°).78

Although lower extremity alignment cannot be altered, several investigators have reported that the Q angle can change with an isolated quadriceps contraction.23,25,44 In support of this finding, Hahn and Foldspang25 theorized that through athletic training, the dynamic activity of the quadriceps could lower the Q angle.

Recommendations

Because lower extremity alignment cannot be altered, no recommendations can be given to help minimize the athlete's risk of anterior cruciate ligament rupture in regard to alignment. However, the dynamic position of the tibia can be improved with internal rotation exercise for the tibia (for example, medial hamstrings).

Femoral Notch

A narrow intercondylar notch may be a predictive factor for anterior cruciate ligament ruptures.64 As early as 1938, Palmer74 suggested that a pathologic relationship existed between the anterior cruciate ligament and the intercondylar notch in patients with anterior cruciate ligament injuries. He reported that the anterior cruciate ligament was in a vulnerable position because it could come into contact with the medial margin of the lateral femoral condyle when the knee is in flexion. In a cadaver study, Norwood and Cross71 showed that the anterior cruciate ligament impinges on the anterior intercondylar notch with the knee in full extension. Thus, they inferred that the relative confines of the bony intercondylar notch may be an important predictor of anterior cruciate ligament injuries in some knees.

Numerous studies have attempted to determine the relationship between the size and shape of the femoral notch and the incidence of anterior cruciate ligament injury.1,31,64,77,84,85 The dimensions of the intercondylar notch (height, width, ratio of height to width, and overall shape) often have been implicated as causes of anterior cruciate ligament failure in the newly injured and the anterior cruciate ligament-reconstructed knee3,5,14,31,46,52,64,77,79,84,85,89(Fig 2). In 1987, Houseworth et al31 measured the areas of the anterior notch opening, posterior arch, and distal femur from radiographs, and reported a significant difference in the ratios of the posterior arch area to the total area of the distal femur between patients with an anterior cruciate ligament injury (ratio = 0.063) and control subjects who did not have an anterior cruciate ligament injury (ratio = 0.069). They reported that a narrowed posterior notch might predispose a person to an anterior cruciate ligament injury. In 1994, Lund-Hanssen et al52 radiographically measured the intercondylar notch of the femur in female handball players who had a unilateral anterior cruciate ligament injury and compared them with a size matched control group of subjects who were not injured. They found that the anterior opening of the intercondylar notch was significantly narrower in the healthy knees of the subjects who were injured compared with the knees of control subjects. They suggested that females with 17 mm or less anterior notch width at the level of the popliteal groove were six times more susceptible to anterior cruciate ligament injury compared with players with wider notch widths. Shelbourne et al79 compared the measurements of the intercondylar notch width in men and women with intact anterior cruciate ligaments, with unilateral anterior cruciate ligament tears, and with bilateral anterior cruciate ligament tears. The mean radiographic notch width for women was 12.8 mm in the group with bilateral anterior cruciate ligament tears, 13.8 mm in the group with unilateral anterior cruciate ligament tears, and 14.5 mm in the control group (p < 0.05); for men, the mean radiographic notch width was 15.3 mm in the group with bilateral anterior cruciate ligament tears, 15.8 mm in the group with unilateral anterior cruciate ligament tears, and 16.9 mm in the control group (p < 0.05). They concluded that the intercondylar notch width of the femur was narrower in women than men, and that in men and women, the notch width was narrower in those patients who sustained anterior cruciate ligament tears when compared with controls.

Fig 2
Fig 2:
The notch width index is the width of the intercondylar notch (A) divided by the width of the distal femur at the level of the popliteal groove (B).

LaPrade and Burnett46 conducted a 2-year prospective study of 213 male and female Division I collegiate athletes. Radiographs of the bilateral intercondylar notch were taken of all athletes enrolled in the study, and the notch width index (the width of the anterior outlet of the notch divided by the total notch width at the level of the popliteal groove) was calculated (Fig 2). After 2 years, there were seven anterior cruciate ligament tears. Statistical analysis showed a correlation between femoral intercondylar notch stenosis and anterior cruciate ligament injuries. No statistical difference was found between gender and notch width indices and rate of anterior cruciate ligament tears. The authors reported that athletes with intercondylar notch stenosis seemed to be at increased risk for noncontact anterior cruciate ligament injuries. Souryal and Freeman84 have confirmed these findings.

Anderson et al1 used computed tomography (CT) measurements to compare the intercondylar notch dimensions of subjects with no history of anterior cruciate ligament injury (control group), unilateral anterior cruciate ligament injuries, and bilateral anterior cruciate ligament injuries. They found a significant degree of notch stenosis, specifically at the anterior outlet, in subjects with an anterior cruciate ligament injury compared with subjects who did not have an anterior cruciate ligament injury. This finding suggested that notch stenosis may predispose an athlete to anterior cruciate ligament injury. In 1988, Souryal et al85 evaluated standard radiographic measurements of intercondylar notch dimensions and the notch width index. They reported no significant differences in the notch width index between subjects with unilateral anterior cruciate ligament injuries and subjects who did not have anterior cruciate ligament injuries. However, subjects with bilateral anterior cruciate ligament injuries had a significant difference in the notch width index when compared with subjects with unilateral anterior cruciate ligament injuries. Similarly, Harner et al27 retrospectively studied 31 patients with noncontact, bilateral anterior cruciate ligament injuries and compared them with 23 control subjects. The patients with bilateral anterior cruciate ligament injuries had a significantly wider lateral femoral condyle compared with the subjects in the control group.

In contrast, several authors have reported no correlation between notch width and anterior cruciate ligament injury rate.77,89 Schickendantz and Weiker77 used standard radiographic measurements of intercondylar notch and femoral condyle dimension to compare three groups of subjects (subjects with bilateral anterior cruciate ligament injuries, subjects with unilateral anterior cruciate ligament injuries, and subjects who did not have anterior cruciate ligament injuries). There were no statistically significant differences in the radiographic measurements between the three groups. Thus, it was concluded that intercondylar notch measurements obtained from standard radiographs should not be used to predict potential for injury to the anterior cruciate ligament. Similarly, Teitz et al89 compared the notch width index in 40 male and female patients who had sustained a unilateral anterior cruciate ligament rupture with the notch width index in 40 patients who did not have an anterior cruciate ligament tear. Their findings suggested that notch width index alone was not the critical etiologic factor in patients with anterior cruciate ligament tears.

The actual shape of the notch also may vary with gender and may contribute to the incidence of anterior cruciate ligament injury36(Fig 3). A small, A-shaped notch may not be actually pinching a normal-size anterior cruciate ligament, but may be a sign of a congenitally smaller anterior cruciate ligament.36 Radiographically evident decreased notch to width ratios and an A-shaped notch may predispose female athletes to increased risk for a noncontact anterior cruciate ligament injury.

Fig 3
Fig 3:
Schematic drawing showing the three types of intercondylar notch shapes.

Recommendations

Currently, surgeons may choose notchplasty to widen the femoral notch when reconstructing the anterior cruciate ligament. However, most orthopaedists would not prophylactically widen the femoral notch of an uninjured knee in an athlete with an anterior cruciate ligament injury on the contralateral side. Although there may by an association between notch dimensions and anterior cruciate ligament injury, this association has not been shown to be causal. The authors recommend that this prophylactic surgical procedure be avoided because scientific justification is lacking at the present time.

Joint Laxity and Flexibility

Several studies have shown that joint laxity tends to be greater in women than in men.20,33,34 However, the relationship between ligamentous laxity and injury is not clear. Several authors have tried to relate a ligamentously lax knee with increased incidence of injury.20,21,33,70 For example, Nicholas70 reported that football players classified as having loose joints suffered more knee injuries than players classified as having tight joints. In subsequent analyses, other investigators have disputed this claim.19,20,41,63,92

Recommendations

Joint laxity is inherent within an individual. Given the controversial literature regarding the association of joint laxity and the incidence of anterior cruciate ligament injury, no recommendations can be given at this time. Again, the association does not imply causality. However, it may be possible to counter the deleterious effects of excessive ligamentous laxity by strengthening the knee muscles around the lax joint (for example, quadriceps, hamstrings, and gastrocnemius muscles).

Hormonal Influence

The role of hormones in predisposing female athletes to injury of the anterior cruciate ligament recently has received attention. Estrogen and relaxin may be involved indirectly in increased anterior cruciate ligament injury in females.50,82 In 1996, Liu et al49 reported that estrogen and progesterone receptor sites exist in human anterior cruciate ligament cells, suggesting that female sex hormones may play a role in the anterior cruciate ligament's structure and composition. They also found that physiologic levels of estradiol have a significant dose-dependent effect on the fibroblasts of the anterior cruciate ligament.49 Fibroblast proliferation and rate of collagen synthesis were reduced significantly with increasing estradiol concentration.49 Slauterbeck et al82 reported that the administration of estrogen significantly reduced the tensile properties of rabbit anterior cruciate ligament. Collagen, produced by fibroblasts, is known to perform the major load-bearing function of the anterior cruciate ligament and perhaps alterations in the metabolism of these fibroblasts influence the quantity, type, and stability of the collagen in the anterior cruciate ligament. The resulting structural and compositional changes in these fibroblasts could result in reduced strength of the anterior cruciate ligament, predisposing female athletes to ligamentous injury.82 Recently, Levine et al compared the mechanical behavior of estrogen primed and estrogen plus relaxin primed connective tissues in a mouse model. (Levine RE, Steinetz BG, Hannafin JA, Granda SM, Wright TM: Mechanical response of the interpubic ligament and knee joint in mice to relaxin. Presented at the 45th Orthopaedic Research Society Meeting, Anaheim, CA, February 1-4, 1999). Anteroposterior (AP) laxity was significantly less in the ligaments in mice that received estrogen plus relaxin compared with ligaments in mice exposed only to estrogen. Similarly, knee stiffness also was higher in the mice that received estrogen plus relaxin compared with the mice that received only estrogen.

Several researchers have attempted to link hormone fluctuations during the menstrual cycle to rate of anterior cruciate ligament injuries.60,61,65,66,95 In 1998, Wojtys et al95 found an association between the menstrual cycle phase and the incidence of an anterior cruciate ligament injury. Using self-reported questionnaires after an acute noncontact anterior cruciate ligament injury, women reported more injuries than expected in the ovulatory phase of the cycle (Days 10-14, when estrogen levels surge). In contrast, fewer injuries occurred in the follicular phase (Days 1-9, where estrogen and progesterone levels are low). The authors concluded that there may be an association between the menstrual cycle and anterior cruciate ligament injuries in women, but cautioned that additional studies were needed to confirm this relationship. Concurrently, Myklebust et al66 also found an association between menstrual cycle and incidence of anterior cruciate ligament injuries in Norwegian team handball players. However, in their group of 17 women, significantly fewer injuries occurred during the midcycle estrogen surge (Days 8-14). The contrast between these two studies may be attributable to the patient population differences: Wojtys et al96 primarily reported on women with regular menstrual cycles, whereas 1/2 of the subjects in the study of Myklebust et al66 were taking oral contraceptives. In the largest study to date, Möller-Nielsen and Hammar60 prospectively evaluated 86 women soccer players from the Swedish Football Association during a 12-month period (1008 menstrual cycles). They observed that women using oral contraceptives had significantly lower injury rates than those who were not taking oral contraceptives. Additionally, Möller-Nielsen and Hammar reported that the women were more susceptible to traumatic injuries during the premenstrual and menstrual phases compared with the rest of the menstrual cycle. However, they examined traumatic injuries in general, and did not include specific data on knee injuries or anterior cruciate ligament injuries.

Recommendations

Although results of studies to date are compelling, findings are too preliminary for treatment or prevention recommendations to be made.

EXTRINSIC RISK FACTORS

Extrinsic risk factors include muscle strength, neuromuscular control, knee stiffness, landing characteristics, and posture control. Several investigators think these factors may be modifiable with appropriate training and conditioning.8,28,29,96 Athletic training, therefore, holds great promise in reducing some potential risk factors and perhaps decreasing the incidence of anterior cruciate ligament injury injuries.

Muscular Strength and Muscular Activation Patterns

Muscle strength, muscle coordination, and the timely ability to recruit muscles (muscle reaction time) are needed to maintain knee stability. 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.22,26,34,42,56,57 This lack of quadriceps and hamstring muscle strength may place the female athlete at a significant disadvantage because the muscles surrounding the knee protect the joint from deleterious loads. Additionally, some female athletes seem to have different muscle activation patterns compared with their male counterparts. In 1996, Huston and Wojtys34 reported differences in muscle recruitment patterns between elite male and female athletes. Female athletes preferred to contract their quadriceps first in response to anterior tibial translation (quadriceps dominant), 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 (hamstrings dominant). Unfortunately, adequate strength and reaction time of the hamstrings is critical in knee stability. If the quadriceps fire without the hamstrings, the tibia may sublux anteriorly and significantly increase the load on the anterior cruciate ligament.48 Conversely, if the hamstrings fire without a quadriceps contraction, anterior tibial translation is decreased, and loads on the anterior cruciate ligament are decreased significantly.62 This quadriceps dominant pattern observed in female athletes is thought to place significantly more strain on the anterior cruciate ligament compared with an individual who simultaneously cocontracts his or her quadriceps and hamstring muscles, or contracts his or her hamstring muscles before contracting the quadriceps muscles.

Gender differences in coactivation patterns in young subjects have been investigated by Baratta et al.4 They quantified the coactivation patterns of the knee flexor and extensor muscles of subjects who were nonathletic, subjects involved in recreational athletics, and subjects involved in highly competitive athletics. Electromyographic data were collected during an isokinetic strength evaluation that produced maximal muscle contractions. High performance athletes with hypertrophied quadriceps had strong inhibitory effects on the coactivation of the hamstrings compared with a group of healthy subjects who were not involved in athletics. Athletes who routinely exercised their hamstrings, however, had a coactivation response similar to the response of the subjects who were not athletic. Baratta et al4 concluded that the coactivation of the hamstrings with quadriceps was necessary to aid the dynamic component of joint stability, to equalize articular surface pressure distribution, and to regulate the joint's mechanical impedance. They suggested that high performance athletes with a muscular imbalance could reduce their risk of knee injuries by performing complementary resistive exercises of the hamstring muscles.

In a prospective controlled study, Caraffa et al8 showed that proprioceptive training of semiprofessional male soccer players significantly decreased the incidence of anterior cruciate ligament injury. After a progressive five-phase training program on balance boards, incidence of anterior cruciate ligament injury decreased more than sevenfold in these male athletes. These results suggests that perhaps these neuromuscular patterns can be changed and optimized with the corrected training regimen.

Recommendations

Most important, female athletes must attain good physical condition before participating in any sport. 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. Plyometrics and agility-type exercises, such as running through cones, figure eights, and single-leg jumps are proven methods to significantly improve muscle reaction time.96

The hamstring muscles have been shown to protect the anterior cruciate ligament from excessive strain.11 Because 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

Muscle stiffness across the knee represents an important component to knee stability and injury prevention. As muscles that span the knee contract, they act to increase joint contact force and decrease tibiofemoral displacements, dissipating potentially dangerous loads, and lowering the force carried by the anterior cruciate ligament 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.39,40,81,83

Muscle stiffness across the knee has intrinsic and extrinsic components. The intrinsic component is largely 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 the alpha and gamma motoneurons. A muscle's intrinsic muscle stiffness is probably the knee's first line of protection. However, the potential of the extrinsic component's protection is greater, and it can be modified with training.96 Markolf et al55 reported that patients who were not athletes could increase varus and valgus knee stiffness by two- to fourfold with isometric cocontraction of the hamstrings and quadriceps, and that athletes who are well-conditioned were capable of increasing knee stiffness by a factor of 10.

Additionally, gender differences may exist in the ability to produce adequate muscle stiffness.7,88 In 1988, Bryant and Cook7 observed the varus and valgus stiffness of 17 female and 24 male subjects. Knees in females rotated 66% more than knees in males and were 35% less stiff. In 1998, Wojtys et al investigated passive and active components of knee stiffness (in the AP plane) in a group of healthy men and women. (Wojtys EM, Ashton-Miller JA, Huston LJ: Active knee stiffness differs in young men and women. Submitted to the American Journal of Sports Medicine, April 15, 1999). Although results were normalized for body weight and height, a significant gender difference was observed in the ability to voluntarily stiffen the knee. Men were able to increase their knee stiffness by an average of fourfold, whereas women were only able to double their joint stiffness with a voluntary muscle contraction. Wojtys et al concluded that the ability to increase knee stiffness may not be determined primarily by a person's muscle strength or body height to weight ratio, but primarily by unknown factors relating to their gender. (Wojtys EM, Ashton-Miller JA, Huston LJ: Active knee stiffness differs in young men and women. Submitted to the American Journal of Sports Medicine, April 15, 1999). This report contradicts the work of Such et al,88 who tested 70 knees in males and females and showed that although knees in women had lower values of stiffness than knees in men, lower extremity muscle mass had the largest influence on the stiffness properties of the knee.

Apparent gender differences in the generation of muscle stiffness also may be associated with differences in musculotendinous elasticity properties. For example, Winter and Brookes94 compared the delay in performance attributable to muscle elasticity in men and women. A group of 22 men and women performed an ankle plantar flexion movement in the dominant leg as quickly as possible on hearing an acoustic signal delivered via headphones. Significant gender differences were observed in the electromechanical delay time (defined as the time interval between the onset of muscle tension and movement) and the elastic charge time (defined as the time interval between the onset of muscle tension and movement).

Recommendations

Fortunately, joint stiffness can be modulated over a certain limited range and regulated through antagonist muscle contractions.24,51 Therefore, muscle stiffness may improve with a functional training program that emphasizes the hamstring and gastrocnemius muscle groups.

Jumping and Landing Characteristics

A high percentage of anterior cruciate ligament injuries occur when the athlete lands from a jump.13,17 Landing imposes forces on the body that must be absorbed primarily by the lower extremity. If loads become too great for the body to accommodate or if impact absorption fails, an injury will occur.80 Ground reaction forces reaching three to 14 times body weight have been measured for landing activities (for example, basketball rebounding, landing from a block in volleyball, back somersaults in gymnastics), suggesting that tremendous loads normally are absorbed by the body during these activities.12,13,59,72,73,75,80,86,91

In their study of landing in volleyball players, Stacoff et al86 proposed that the knee angle at touchdown was extremely important in determining the maximum force at the knee. More knee extension produced greater maximum impact forces.86 Lafortune45 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.45 Recently, Malinzak and colleagues obtained kinematic data on young healthy men and women during controlled running and cutting maneuvers. (Malinzak RA, Colby SM, Kirkendall DT, Garrett WE: Electromyographic and three-dimensional kinematic analysis of cutting maneuvers in men and women: Implications for anterior cruciate ligament injury. Presented at American Orthopaedic Society for Sports Medicine Specialty Day, Anaheim, CA, February 7, 1999). Females performed these maneuvers with significantly less knee flexion (25° versus 29°), more knee valgus, and less hip flexion than males. Females also had greater quadriceps and lower hamstring activation levels than males, particularly at foot-strike.

In 1996, Hewett et al29 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. These parameters were compared in a study group before and after training with a group of untrained males. 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.29 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. A list of exercises used in the program is shown in Table 1. Before training, the female athletes had marked imbalances between hamstrings and quadriceps strength. After 6 weeks of training, the female subjects decreased landing peak force by 22%. Maximal knee flexion at landing increased from 69° ± 14° to 72° ± 9°. Adduction and abduction moments at the knee decreased significantly at landing after the athletes completed the training program. The program significantly increased hamstrings power and strength (power, 33% increase; strength, 20% increase), increased hamstrings to quadriceps peak torque ratios (20%), and eliminated side-to-side hamstring strength imbalances. No significant differences were observed in hip flexion and extension angles on landing. In a followup study, Hewett et al28 also reported a decreased incidence of serious knee injury rates after this training program in a population of female athletes who are at high risk for having a knee injury.

TABLE 1
TABLE 1:
Jump Training Program According to Hewett et al29

Recommendations

The jump training program advocated by Hewett et al29 is strongly 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 anterior cruciate ligament injuries.

SUMMARY

An overview has been given of intrinsic and extrinsic factors thought to predispose a female athlete to a greater risk of anterior cruciate ligament injury; structural differences, joint laxity, hormonal influence, muscular strength and imbalance, joint stiffness, and jumping and landing characteristics. However, it is important to emphasize that each of these factors is not solely responsible for the high rate of anterior cruciate ligament injuries; it is suspected that etiology of anterior cruciate ligament injuries is multifactorial. In general, intrinsic risk factors are primarily anatomic differences that cannot be manipulated or changed.

However, to minimize these devastating injuries, the greatest promise is in the study of extrinsic risk factors. Currently, neuromuscular factors seem to be the most urgent elements to research and improve.

FUTURE DIRECTIONS

What can be done? How can anterior cruciate ligament injuries be prevented? Theoretically, a female athlete with an anterior cruciate injury may be an individual whose Q angle is significantly larger than average, has a notch width significantly smaller than most, has joints classified as ligamentously lax, and is weaker than her male counterparts. It is thought that efforts to decrease the incidence of anterior cruciate ligament injuries in women should be directed toward the extrinsic factors, including physical conditioning, body movement, and muscular strength. Research has shown that injuries associated with athletics are reduced with proper conditioning.8,28 Consequently, good conditioning before sports participation likely will decrease the incidence of anterior cruciate ligament injury, despite gender. More specifically, female athletes tend to be quadriceps dominant, so a training and conditioning program that emphasizes the hamstrings and gastrocnemius muscles is critical. In addition, female athletes who participate in sports that require jumping, pivoting, or both should be coached to avoid vulnerable injury-prone positions (for example, little or no knee flexion). Landing and pivoting techniques that stress increased knee flexion should be emphasized.

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