Functional outcome of anterior cruciate ligament reconstruction in recreational and competitive athletes

The aim of this study was to compare the outcome after anterior cruciate ligament reconstruction in recreational and competitive athletes, with a minimum follow-up of two years. Forty-nine patients (24 males and 25 females) who, at the time of the index injury, were classified as recreational athletes (Tegner level 2-5) were compared with 226 patients (61 females and 165 males) who, at the time of the index injury, were classified as competitive athletes (Tegner level 9-10). At the follow-up, no significant differences were found between the study groups in terms of the Lysholm score, IKDC evaluation system, one-leg-hop test, KT-1000 laxity measurements, anterior knee pain and the patients' subjective evaluation of the results. However, the competitive athletes displayed a significantly higher reduction in Tegner activity level than the recreational athletes. The functional and objective results after anterior cruciate ligament reconstruction were comparable for the recreational and competitive athletes. We, therefore, conclude that anterior cruciate ligament reconstruction could be recommended for recreational athletes as well as competitive athletes.     

Subacute versus delayed reconstruction of the anterior cruciate ligament in the competitive athlete

The objective of this study was to compare the function and activity level in patients with anterior cruciate ligament injuries, who participated in competitive sports (Tegner activity level ≥ 7) and underwent a reconstruction of the anterior cruciate ligament, either subacute (2–12 weeks, group I) or late (12–24 months, group II) after the injury. The patients in group I (n = 97) were comparable with those in group II (n = 103) in terms of gender, age, pre-injury activity level, and the reconstruction technique. At the final follow-up (2–5.5 years after the operation), the Lysholm score, the IKDC evaluation system and the one-leg-hop test revealed no differences between the groups. There were also no differences between the groups in terms of the patients’ subjective evaluation or expectations. The Tegner activity level at follow-up was 8 (range 2–10) in group I and 6 (range 2–9) in group II (P = 0.0001). The same thing was found in terms of the desired Tegner activity level, which was 9 (range 4–9) in group I and 7 (range 3–10) in group II (P = 0.0002). The KT-1000 laxity meter revealed a total side-to-side difference of 1.5 mm (–3.5–8.5) in group I and 1.5 mm (–3.5–7) in group II (NS). Associated meniscal surgery between the index injury and the reconstruction, or during the reconstruction, was performed in 37/97 (38%) of the patients in group I and 59/103 (57%) of the patients in group II (P < 0.01). This study revealed that competitive athletes who underwent reconstruction at a subacute stage after the anterior cruciate ligament injury had a higher activity level 2–5.5 years after the index operation, as well as a higher desired level of activity compared to athletes who had the reconstruction delayed by 12–24 months. Furthermore, meniscal injuries were significantly more frequent if the reconstruction was delayed. Received: 10 May 1998 Accepted: 30 November 1998     

Intraarticular injuries associated with anterior cruciate ligament tear: findings at ligament reconstruction in high school and recreational athletes. An analysis of sex-based differences

BACKGROUND: Despite research on the increased risk of anterior cruciate ligament tears in female athletes, few studies have addressed sex differences in the incidence of associated intraarticular injuries. HYPOTHESIS: When patients are stratified by sport and competition level, no sex differences exist in either the mechanism of injury or pattern of intraarticular injuries observed at anterior cruciate ligament reconstruction. STUDY DESIGN: Prospective cohort study. METHODS: Two hundred twenty-one athletes undergoing anterior cruciate ligament reconstruction met our inclusion criteria of anterior cruciate ligament tear as a singular event without reinjury or history of prior injury or surgery in either knee. Data were collected on competition level (high school, amateur), sport (basketball, soccer, skiing), mechanism of injury, articular cartilage injuries, and meniscal tears. Data were statistically analyzed by sex with the chi-square test and Student's t-test. RESULTS: High school athletes had no significant sex differences in mechanism of injury. Female soccer athletes had fewer medial meniscal tears than did male athletes, and female basketball players had fewer medial femoral condyle injuries. At the amateur level, female basketball players had more contact injuries, an earlier onset of swelling, and fewer lateral meniscal tears than did male players. CONCLUSION: At the high school level, male and female athletes shared a common mechanism of injury, and yet the female athletes had fewer intraarticular injuries in basketball and soccer. If such intraarticular injuries prove to be a significant risk factor for poor long-term outcome, women may enjoy a better prognosis after reconstruction.     

Analysis of subjective, objective and functional examination tests after anterior cruciate ligament reconstruction A follow-up of 527 patients

This study included 527 patients (178 female and 349 male) with unilateral anterior cruciate ligament (ACL) rupture who underwent arthroscopic ACL reconstruction using bone-patellar tendon-bone autograft and interference screw fixation. The follow-up examination was performed by independent observers at a median of 38 (21-68) months after the index operation. At the follow-up, the Lysholm score was 86 (14-100) points, the Lysholm instability subscore was 22 (0-25) points and the Lysholm pain subscore was 19 (0-25) points. The Tegner activity level was 6 (1-10). The one-leg-hop test was 91 (0-167)% of the non-injured knee. The difference in the anterior side-to-side laxity as measured with the KT-1000 arthrometer at 89 Newton (N) was 1.5 (-5-13) mm and the total KT-1000 side-to-side difference at 89 N was 2 (-7-11) mm. Using the International Knee Documentation Committee (IKDC) evaluation system, 177 (33.6%) patients were classified as normal (group A), 211 (40%) as nearly normal (group B), 109 (20.7%) as abnormal (group C) and 30 (5.7%) as severely abnormal (group D). The highest correlation coefficients were recorded between the IKDC evaluation system and the Lysholm score (p = 0.66), the patients' subjective evaluation (p = 0.53), the Tegner activity level (p = 0.34), all the laxity tests (p > or = 0.34) and the one-leg-hop test (p = 0.28). The resumption of sporting activities and work as evaluated by the Tegner activity level correlated with the patients' subjective evaluation (p = 0.34) but did not correlate with the laxity tests, i.e., the manual Lachman test (p = -0.06) and the total and anterior KT-1000 tests (p = -0.06). Furthermore, none of the laxity tests correlated with the functional tests or the patients' subjective evaluation. We conclude that the IKDC evaluation system is a reliable and useful tool for evaluating the post-operative outcome after an ACL reconstruction.     

Clinical comparison of intraarticular anterior cruciate ligament reconstruction using autogenous semitendinosus and gracilis tendons in men versus women

Reconstruction of the anterior cruciate ligament using a hamstring tendon autograft has often been recommended for female athletes. We compared the results of acute, isolated, intraarticular anterior cruciate ligament reconstructions using quadruple-looped hamstring autografts in 39 female and 26 male patients. All reconstructions were performed by the same surgeon using a similar surgical technique and the same postoperative management. In each case, patients had Endobutton femoral fixation and either post or button fixation for the tibial side. The average follow-up was 40.9 months for women and 39.0 months for men. Objective analysis of results included examination for the presence of effusion and crepitus, Lachman and pivot shift testing, and KT-1000 arthrometer testing for side-to-side differences. Subjective analysis consisted of a 15-item visual analog scale completed by patients postoperatively, and pre- and postoperative Tegner and Lysholm scores. The clinical failure rate was 23% (9 of 39) for the female patients and 4% (1 of 26) for the male patients, which was statistically significant. There was also a trend toward increased laxity in female patients. Subjectively, the women also reported a higher frequency and intensity of pain. Based on Tegner activity levels, more of the men returned to their preinjury level of activity than did the women. When compared with the male patients, female patients had a significantly higher failure rate after reconstruction.     

Five-year results of anterior cruciate ligament reconstruction with the Stryker Dacron high-strength ligament

Forty-two consecutive patients (27 male and 15 female, with a mean age of 26 years) suffering from uniflateral chronic anterior cruciate ligament insufficiency underwent surgical reconstruction with a Dacron highstrength ligament (Stryker). In 32 patients the synthetic legament was used as a reinforcement in an iliotibial band intra-articular procedure and in 10 patients as an intra-articular prosthesis without biological reinforcement. There were 24 concomitant injuries such as meniscal tears, status post—meniscectomy or medial collateral ligament insufficiency at the time of reconstruction. The initial injury occurred during soccer in 23 patients and other pivoting sports in 16 patients. Two and 5 years after reconstruction, the patients underwent clinical examination, including the instrumented knee laxity test (OSI), and performed subjective evaluation. The anterior tibial translation was 6.3±2.6mm grater than in the healthy knee, the mean Lysholm score was 78 points, and the Tegner activity score was lower than its pre-injury level. Radiography revealed that 31 of 37 ligaments had ruptured and another 2 ligaments showed more than 5 mm side-to-side increased laxity. The Stryker Dacron high-strength ligament ruptured in more than 80% of the cases and patients could not return to their pre-injury physical performance activities.Supported by grants from the Karolinska Institute     

The effects of estrogen on material and mechanical properties of the intra- and extra-articular knee structures

BACKGROUND: Female athletes have a higher incidence of anterior cruciate ligament injuries than do male athletes in similar sports. Hormones, particularly estrogen, have been indicated by some studies to be correlated with the disproportionate number of anterior cruciate ligament injuries in female athletes. HYPOTHESIS: Estrogen does not affect the mechanical or material properties of primate anterior cruciate ligaments or patellar tendons. STUDY DESIGN: Controlled laboratory study. METHODS: Twenty-six cynomolgus monkeys (Macaca fascicularis) were divided into 2 groups: sham operated (estrogen maintained, n = 12) and ovariectomized (estrogen deficient, n = 14). After 2 years, the animals were sacrificed, and various properties of ligaments were tested. Two different ligaments were studied, the anterior cruciate ligament and patellar tendon. RESULTS: No differences were found between the sham-operated (sham) and ovariectomized group for failure load (anterior cruciate ligament: sham, 394 +/- 67 N; ovariectomized, 392 +/- 96 N; P > .74), stiffness, elongation at failure, ultimate stress (anterior cruciate ligament: sham, 115 +/- 30 MPa; ovariectomized, 128 +/- 36 MPa; P > .58), ultimate strain, modulus, failure sites, or energy to failure. CONCLUSION: Endogenous estrogen does not directly affect the mechanical or material properties of the anterior cruciate ligament or the patellar tendon of cynomolgus macaques. CLINICAL RELEVANCE: This study supports the idea that knee ligament injury in female athletes is not a direct result of estrogen's effect on ligament properties.     

Effectiveness of a neuromuscular and proprioceptive training program in preventing anterior cruciate ligament injuries in female athletes: 2-year follow-up

BACKGROUND: Among female athletes it has not been established whether a neuromuscular and proprioceptive sports-specific training program will consistently reduce the incidence of anterior cruciate ligament injuries. PURPOSE: To determine whether a neuromuscular and proprioceptive performance program was effective in decreasing the incidence of anterior cruciate ligament injury within a select population of competitive female youth soccer players. STUDY DESIGN: Cohort study; Level of evidence, 2. METHODS: In 2000, 1041 female subjects from 52 teams received a sports-specific training intervention in a prospective non-randomized trial. The control group consisted of the remaining 1905 female soccer players from 95 teams participating in the same league who were age and skill matched. In the 2001 season, 844 female athletes from 45 teams were enrolled in the study, with 1913 female athletes (from 112 teams) serving as the age- and skill-matched controls. All subjects were female soccer players between the ages of 14 and 18 and participated in either their traditional warm-up or a sports-specific training intervention before athletic activity over a 2-year period. The intervention consisted of education, stretching, strengthening, plyometrics, and sports-specific agility drills designed to replace the traditional warm-up. RESULTS: During the 2000 season, there was an 88% decrease in anterior cruciate ligament injury in the enrolled subjects compared to the control group. In year 2, during the 2001 season, there was a 74% reduction in anterior cruciate ligament tears in the intervention group compared to the age- and skill-matched controls. CONCLUSION: Using a neuromuscular training program may have a direct benefit in decreasing the number of anterior cruciate ligament injuries in female soccer players.     

Effect of fatigue on knee kinetics and kinematics in stop-jump tasks

BACKGROUND: Altered motor control strategies in landing and jumping maneuvers are a potential mechanism of noncontact anterior cruciate ligament injury. There are biomechanical differences between male and female athletes in the landing phase of stop-jump tasks. Fatigue is a risk factor in musculoskeletal injuries. HYPOTHESIS: Lower extremity muscle fatigue alters the knee kinetics and kinematics during the landing phase of 3 stop-jump tasks and increases an athlete's risk of anterior cruciate ligament injury. STUDY DESIGN: Controlled laboratory study. METHODS: Three-dimensional videography and force plate data were collected for 20 recreational athletes (10 male and 10 female athletes) performing 3 stop-jump tasks before and after completing a fatigue exercise. Knee joint angles and resultant forces and moments were calculated. RESULTS: Both male and female subjects had significantly increased peak proximal tibial anterior shear forces (P = .01), increased valgus moments (P = .03), and decreased knee flexion angles (P = .03) during landings of all 3 stop-jump tasks when fatigued. Fatigue did not significantly affect the peak knee extension moment for male or female athletes. CONCLUSION: Fatigued recreational athletes demonstrate altered motor control strategies, which may increase anterior tibial shear force, strain on the anterior cruciate ligament, and risk of injury for both female and male subjects. CLINIC RELEVANCE: Fatigued athletes may have an increased risk of noncontact anterior cruciate ligament injury.     

Knee function after anterior cruciate ligament injury in elite collegiate athletes

BACKGROUND: Anterior cruciate ligament injuries are common in athletes, but there are few studies of long-term outcomes. HYPOTHESIS: Long-term knee function of anterior cruciate ligament-injured athletes is inferior to that of their uninjured teammates. STUDY DESIGN: Retrospective cohort study. METHODS: Thirty-three Division I-A athletes who had sustained an anterior cruciate ligament injury during their college career completed a series of questionnaires that assessed knee function and quality of life 2 to 14 years after injury. Their responses were compared with those of a matched cohort of their uninjured teammates. RESULTS: There were no differences in the mean Tegner scores, modified Lysholm scores, or in the scores of the SF-36 between groups. Sixteen anterior cruciate ligament-injured athletes scored A or B in the subjective portion of the International Knee Documentation Committee score and 17 scored C or D, whereas 24 control subjects scored A or B and 9 scored C or D, a statistically significant difference between groups. Five injured and 14 control athletes had participated at a professional or national team level after college. CONCLUSIONS: Quality of life of elite collegiate athletes who sustained an anterior cruciate ligament injury was not significantly different from that of their uninjured teammates, but knee function differed between groups.     

A prospective four- to seven-year follow-up after arthroscopic anterior cruciate ligament reconstruction

The aim of this study was to evaluate the results after arthroscopic anterior cruciate ligament reconstruction using a bone-patellar tendon-bone graft in 99 patients, who were followed up prospectively for four to seven years. The pre-injury Tegner activity level was 7 compared with 5 at the four- to seven-year follow-up (P<0.0001). The preoperative Lysholm score was 74.5 points. At the two-year follow-up, the Lysholm score was 95 points, while it was 90 points at the four- to seven-year follow-up (P<0.0001 preoperative vs two years and preoperative vs four to seven years and P<0.0005 two years vs four to seven years). Using the IKDC evaluation system, 80% of the patients were classified as normal or nearly normal and 20% as abnormal or severely abnormal at the final follow-up. The KT-1000 laxity measurements revealed a side-to-side difference of 2.9 mm preoperatively, 0.6 at two years and 1.0 mm at four to seven years. Twenty-six patients underwent additional surgery during the follow-up period. The results after arthroscopic reconstruction of the anterior cruciate ligament appear to be satisfactory both at the short- and the medium-term follow-ups, but there appears to be some deterioration between the two-year and the four- to seven-year follow-up.     

Biomechanical measures of neuromuscular control and valgus loading of the knee predict anterior cruciate ligament injury risk in female athletes: a prospective study

BACKGROUND: Female athletes participating in high-risk sports suffer anterior cruciate ligament injury at a 4- to 6-fold greater rate than do male athletes. HYPOTHESIS: Prescreened female athletes with subsequent anterior cruciate ligament injury will demonstrate decreased neuromuscular control and increased valgus joint loading, predicting anterior cruciate ligament injury risk. STUDY DESIGN: Cohort study; Level of evidence, 2. METHODS: There were 205 female athletes in the high-risk sports of soccer, basketball, and volleyball prospectively measured for neuromuscular control using 3-dimensional kinematics (joint angles) and joint loads using kinetics (joint moments) during a jump-landing task. Analysis of variance as well as linear and logistic regression were used to isolate predictors of risk in athletes who subsequently ruptured the anterior cruciate ligament. RESULTS: Nine athletes had a confirmed anterior cruciate ligament rupture; these 9 had significantly different knee posture and loading compared to the 196 who did not have anterior cruciate ligament rupture. Knee abduction angle (P<.05) at landing was 8 degrees greater in anterior cruciate ligament-injured than in uninjured athletes. Anterior cruciate ligament-injured athletes had a 2.5 times greater knee abduction moment (P<.001) and 20% higher ground reaction force (P<.05), whereas stance time was 16% shorter; hence, increased motion, force, and moments occurred more quickly. Knee abduction moment predicted anterior cruciate ligament injury status with 73% specificity and 78% sensitivity; dynamic valgus measures showed a predictive r2 of 0.88. CONCLUSION: Knee motion and knee loading during a landing task are predictors of anterior cruciate ligament injury risk in female athletes. CLINICAL RELEVANCE: Female athletes with increased dynamic valgus and high abduction loads are at increased risk of anterior cruciate ligament injury. The methods developed may be used to monitor neuromuscular control of the knee joint and may help develop simpler measures of neuromuscular control that can be used to direct female athletes to more effective, targeted interventions.     

Effects of the menstrual cycle on anterior cruciate ligament injury risk: a systematic review

Several theories have been proposed to explain the 3- to 6-fold gender difference in the incidence of anterior cruciate ligament injuries. One potential theory for the increased incidence is based on gender-related hormonal differences between men and women, especially after puberty and the onset of menses in the female athlete. The purpose of this systematic review was to compile and systematically analyze the published literature to determine if the menstrual cycle is associated with anterior cruciate ligament injury risk and to provide an objective comparison of the published results. Investigations were included in the systematic review if the report included associations between the menstrual cycle and noncontact anterior cruciate ligament injuries in female athletes. Abstracts and unpublished studies were excluded. Seven articles were identified that met the systematic review inclusion criteria. The 7 reviewed studies favored an effect of the first half, or preovulatory phase, of the menstrual cycle for increased anterior cruciate ligament injuries. The 6 studies that separated the non-oral contraceptive and oral contraceptive data also favored an effect of the first half of the menstrual cycle for increased anterior cruciate ligament injuries. The clinical relevance of this finding is that female athletes may be more predisposed to anterior cruciate ligament injuries during the preovulatory phase of the menstrual cycle. These findings may lead to potential interventions targeted toward this phase of the menstrual cycle to reduce the incidence of anterior cruciate ligament injury.     

Correlation of anthropometric measurements, strength, anterior cruciate ligament size, and intercondylar notch characteristics to sex differences in anterior cruciate ligament tear rates

We performed a prospective study based on the hypothesis that physiologic differences exist between men and women in strength after adjustments for body weight; that the size of the anterior cruciate ligament is proportionate to the strength of its antagonists, the quadriceps muscles; and that women have a relatively small anterior cruciate ligament, thus predisposing them to a disproportionate number of anterior cruciate ligament injuries. One hundred matched high school basketball players, 50 male and 50 female, were evaluated with anthropometric measurements, body fat analysis, muscle strength evaluation, and magnetic resonance imaging measurements of the intercondylar notch and cross-sectional area of the anterior cruciate ligament at the outlet. The male players were taller and heavier than their female counterparts, although they had 11% less body fat. Male players had statistically greater quadriceps and hamstring muscle strength than female players, even when adjustments were made for body weight. With adjustments for body weight, the size of the anterior cruciate ligament in girls was found to be statistically smaller than in boys. There was no statistically significant difference in the notch width index between the sexes. The study data support our hypothesis that sex differences in anterior cruciate ligament tear rates are caused primarily by several interrelated intrinsic factors. Most importantly, stiffness and muscular strength increase stress on the anterior cruciate ligament in female athletes. The anterior cruciate ligament, when adjustments have been made for body weight, is smaller in female athletes, and therefore, probably does not compensate for the lack of stiffness and strength.     

Anterior cruciate ligament injuries in female athletes: Part 2, a meta-analysis of neuromuscular interventions aimed at injury prevention

Female athletes have a 4 to 6 times higher incidence of anterior cruciate ligament injury than do male athletes participating in the same landing and pivoting sports. This greater risk of anterior cruciate ligament injury, coupled with a geometric increase in participation (doubling each decade), has led to a significant rise in anterior cruciate ligament injuries in female athletes. The gender gap in anterior cruciate ligament injury, combined with evidence that the underpinnings of this serious health problem are neuromuscular in nature, leads to the development of neuromuscular interventions designed to prevent injury. A systematic review of the published literature yielded 6 published interventions targeted toward anterior cruciate ligament injury prevention in female athletes. Four of 6 significantly reduced knee injury incidence, and 3 of 6 significantly reduced anterior cruciate ligament injury incidence in female athletes. A meta-analysis of these 6 studies demonstrates a significant effect of neuromuscular training programs on anterior cruciate ligament injury incidence in female athletes (test for overall effect, Z = 4.31, P < .0001). Examination of the similarities and differences between the training regimens gives insight into the development of more effective and efficient interventions. The purpose of this "Current Concepts" review is to highlight the relative effectiveness of these interventions in reducing anterior cruciate ligament injury rates and to evaluate the common training components between the training studies. In addition, the level of rigor of these interventions, the costs and the difficulty of implementation, the compliance with these interventions, and the performance benefits are discussed. This review summarizes conclusions based on evidence from the common components of the various interventions to discuss their potential to reduce anterior cruciate ligament injury risk and assess their potential for combined use in more effective and efficient intervention protocols.     

Treatment of anterior cruciate ligament injuries, part 2

Anterior cruciate ligament tears, common among athletes, are functionally disabling; they predispose the knee to subsequent injuries and the early onset of osteoarthritis. A total of 3810 studies published between January 1994 and the present were identified and reviewed to determine the current state of knowledge regarding the treatment of anterior cruciate ligament injuries. Part 1 of this article focused on studies pertaining to the biomechanical behavior of the anterior cruciate ligament, the prevalence of and risk factors for injuries related to it, the natural history of the ligament-deficient knee, injuries associated with anterior cruciate ligament disruption, indications for the treatment of anterior cruciate ligament injuries, as well as nonoperative and operative treatments. Part 2 includes technical aspects of anterior cruciate ligament surgery, bone tunnel widening, graft healing, rehabilitation after anterior cruciate ligament reconstruction, and the effects of sex, age, and activity level on the outcome of such reconstructive surgery.     

Jump landing strategies in male and female college athletes and the implications of such strategies for anterior cruciate ligament injury

BACKGROUND: Female athletes are more likely than male athletes to injure the anterior cruciate ligament. Causes of this increased injury incidence in female athletes remain unclear, despite numerous investigations. HYPOTHESIS: Female athletes will exhibit lower hamstring muscle activation and smaller knee flexion angles than male athletes during jump landings, especially when the knee muscles are fatigued. STUDY DESIGN: Controlled laboratory study. METHODS: Eight female and six male varsity college basketball athletes with no history of knee ligament injury performed jump landings on the dominant leg from a maximum height jump and from 25.4 cm and 50.8 cm high platforms under nonfatigued and fatigued conditions. Knee joint angle and surface electromyographic signals from the quadriceps, hamstring, and gastrocnemius muscles were recorded. RESULTS: Women landed with greater knee flexion angles and greater knee flexion accelerations than men. Knee muscle activation patterns were generally similar in men and women. CONCLUSION: As compared with male college basketball players, female college basketball players did not exhibit altered knee muscle coordination characteristics that would predispose them to anterior cruciate ligament injury when landing from jumps. This conclusion is made within the parameters of this study and based on the observation that hamstring muscle activation was similar for both groups. The greater knee flexion we observed in the female subjects would be expected to decrease their risk of injury. CLINICAL RELEVANCE: Factors other than those evaluated in this study need to be considered when attempting to determine the reasons underlying the increased incidence of anterior cruciate ligament injuries consistently observed in elite female athletes.     

Lack of hormonal influences on mechanical properties of sheep knee ligaments

BACKGROUND: The number of anterior cruciate ligament injuries in female athletes exceeds that in male athletes at similar competitive levels. This difference has been attributed by some authors to hormone-mediated alteration in knee laxity in women. HYPOTHESIS: Sheep anterior cruciate and medial collateral ligament strength and stiffness are not altered by administration for 6 months of estrogen or a selective estrogen receptor agonist (raloxifene). STUDY DESIGN: Controlled laboratory study. METHODS: Thirty-eight mature ewes were divided into five groups: sham operation (N = 6), ovariectomy (N = 9), ovariectomy and estradiol implant (N = 7), low-dose raloxifene (N = 9), and high-dose raloxifene (N = 7). After 6 months, the animals were sacrificed and ligaments were tested along with those from five rams' knees. RESULTS: No differences were found between treatment groups for maximum force, stiffness, energy to failure, or failure site. The ultimate stress of the rams' anterior cruciate ligaments was significantly higher than that of the ewes. CONCLUSIONS: Estrogen and estrogen receptor agonists at physiologic levels do not lead to decreased knee ligament strength. CLINICAL RELEVANCE: The female hormonal milieu may not be responsible for the increased incidence of anterior cruciate ligament injury in female athletes compared with their male counterparts.