Effect of estrogen on cellular metabolism of the human anterior cruciate ligament

The intrinsic risk factors for ACL ruptures: an evidence-based review

This article aims to assess the evidence related to the intrinsic risk factors for anterior cruciate ligament (ACL) ruptures. The level of evidence (according to evidence-based guidelines) for each study and the level of certainty for each risk factor were determined. After a review of the evidence, femoral notch geometry was the only risk factor that was found to be associated with risk of ACL rupture with high certainty. This review concludes that more research, particularly high-quality, prospective studies, are required to provide accurate measures of risk. These data should be used to establish multifactorial risk factor models designed to identify individuals at high risk of sustaining an ACL injury.

Effect of estrogen on tissue elasticity of the ligament proper in rabbit anterior cruciate ligament: measurements using scanning acoustic microscopy

BACKGROUND

Previous epidemiological studies revealed that anterior cruciate ligament (ACL) injuries were more frequently seen in female athletes than in male athletes. To elucidate the pathogenetic roles of estrogen in ACL ruptures, the elasticity of ACL tissue was measured using a scanning acoustic microscope (SAM) in an estrogen-controlled animal model.

METHODS

A total of 40 ovariectomized Japanese white rabbits were randomly divided into four groups according to the administered dose of 17beta-estradiol (groups L, M, H, and C). Injection of 17beta-estradiol was performed 1, 2, 3, and 4 weeks after surgery, and doses in groups L, M, and H were 50, 100, and 500 microg/kg, respectively. Group C received no estradiol. Only groups L, M, and C were used for current analyses because their mean serum estrogen levels were within the physiological range (groups C, L, M, and H: 37, 50, 60, and 231 pg/ml, respectively). Five weeks after ovariectomy, the lateral portion of the ligament was harvested. Specimens were fixed with 10% neutralized formalin and embedded in paraffin. Then, 10 mum thick sections were cut perpendicular to the ligament fibers for routine histological staining and measurement with SAM.

RESULTS

The mean tissue sound speeds of groups C, L, and M were 1727 +/- 32, 1683 +/- 53, and 1665 +/- 63 m/s, respectively. Group M presented significantly lower tissue sound speed than group C (P = 0.021). Furthermore, a negative correlation was found between the mean serum estrogen level and mean tissue sound speed of the ACL among all animals in groups C, L, and M (r = 0.47, P = 0.016).

CONCLUSION

The results of the present study indicated that estrogen altered the tissue elasticity of rabbit ACL. Estrogen may constitute one of the pathogenetic factors in ACL rupture in female athletes.

Effect of selective estrogen receptor modulator/raloxifene analogue on proliferation and collagen metabolism of tendon fibroblast

The selective estrogen receptor modulator raloxifene is therapeutically beneficial for postmenopausal connective tissue degradation, such as osteoporosis, vascular sclerosis, and dermal degradation; however, the effects of raloxifene on postmenopausal tendon metabolism have not been clarified. In this study, we investigated the effects of raloxifene analogue (LY117018) on cell proliferation and collagen metabolism using cultured rat Achilles tendon fibroblasts. 17beta-Estradiol (E(2); 10(-11)-10(-9) M) and LY117018 (10(-9)-10(-7) M) had no significant effects on tendon fibroblast proliferation, based on a BrdU (5-bromo-2'-deoxyuridine) incorporation assay (24 hr) and a WST-8 colorimetric assay (2 or 6 days). Neither E(2) nor LY117018 significantly altered the expression of type I collagen, which is a main component of the tendon extracellular matrix (ECM), whereas both E(2) and LY117018 significantly increased the expression of matrix metalloproteinase (MMP)-13, which is responsible for tendon collagen degradation in rat. Also, both E(2) and LY117018 increased the expression of type III collagen and elastin, which are minor components of tendon ECM, but are considered to govern the elastic properties of tendons. These changes in collagen and MMP induced by either E(2) or LY117018 were attenuated by the estrogen receptor alpha blocker ICI 182,780. The results of this study suggest that postmenopausal estrogen deficiency might downregulate tendon collagen turnover and decrease tendon elasticity. Further, raloxifene treatment might restore these changes to premenopausal levels.

Prevention of non-contact anterior cruciate ligament injuries in soccer players. Part 1: Mechanisms of injury and underlying risk factors

Soccer is the most commonly played sport in the world, with an estimated 265 million active soccer players by 2006. Inherent to this sport is the higher risk of injury to the anterior cruciate ligament (ACL) relative to other sports. ACL injury causes the most time lost from competition in soccer which has influenced a strong research focus to determine the risk factors for injury. This research emphasis has afforded a rapid influx of literature defining potential modifiable and non-modifiable risk factors that increase the risk of injury. The purpose of the current review is to sequence the most recent literature that reports potential mechanisms and risk factors for non-contact ACL injury in soccer players. Most ACL tears in soccer players are non-contact in nature. Common playing situations precluding a non-contact ACL injury include: change of direction or cutting maneuvers combined with deceleration, landing from a jump in or near full extension, and pivoting with knee near full extension and a planted foot. The most common non-contact ACL injury mechanism include a deceleration task with high knee internal extension torque (with or without perturbation) combined with dynamic valgus rotation with the body weight shifted over the injured leg and the plantar surface of the foot fixed flat on the playing surface. Potential extrinsic non-contact ACL injury risk factors include: dry weather and surface, and artificial surface instead of natural grass. Commonly purported intrinsic risk factors include: generalized and specific knee joint laxity, small and narrow intercondylar notch width (ratio of notch width to the diameter and cross sectional area of the ACL), pre-ovulatory phase of menstrual cycle in females not using oral contraceptives, decreased relative (to quadriceps) hamstring strength and recruitment, muscular fatigue by altering neuromuscular control, decreased "core" strength and proprioception, low trunk, hip, and knee flexion angles, and high dorsiflexion of the ankle when performing sport tasks, lateral trunk displacement and hip adduction combined with increased knee abduction moments (dynamic knee valgus), and increased hip internal rotation and tibial external rotation with or without foot pronation. The identified mechanisms and risk factors for non-contact ACL injuries have been mainly studied in female soccer players; thus, further research in male players is warranted. Non-contact ACL injuries in soccer players likely has a multi-factorial etiology. The identification of those athletes at increased risk may be a salient first step before designing and implementing specific pre-season and in-season training programs aimed to modify the identified risk factors and to decrease ACL injury rates. Current evidence indicates that this crucial step to prevent ACL injury is the only option to effectively prevent the sequelae of osteoarthritis associated with this traumatic injury.

Alterations in knee joint laxity during the menstrual cycle in healthy women leads to increases in joint loads during selected athletic movements

BACKGROUND

It has been speculated that the hormonal cycle may be correlated with higher incidence of ACL injury in female athletes, but results have been very contradictory.

HYPOTHESIS

Knee joint loads are influenced by knee joint laxity (KJL) during the menstrual cycle.

STUDY DESIGN

Controlled laboratory study.

METHODS

Serum samples and KJL were assessed at the follicular, ovulation, and luteal phases in 26 women. Knee joint mechanics (angle, moment, and impulse) were measured and compared at the same intervals. Each of the 26 subjects had a value for knee laxity at each of the 3 phases of their cycle, and these were ordered and designated low, medium, and high for that subject. Knee joint mechanics were then compared between low, medium, and high laxity.

RESULTS

No significant differences in knee joint mechanics were found across the menstrual cycle (no phase effect). However, an increase in KJL was associated with higher knee joint loads during movement (laxity effect). A 1.3-mm increase in KJL resulted in an increase of approximately 30% in adduction impulse in a cutting maneuver, an increase of approximately 20% in knee adduction moment, and a 20% to 45% increase in external rotation loads during a jumping and stopping task (P < .05).

CONCLUSION

Changes in KJL during the menstrual cycle do change knee joint loading during movements. Clinical Relevance Our findings will be beneficial for researchers in the development of more effective ACL injury prevention programs.

Retrospective case evaluation of gender differences in sports injuries in a Japanese sports medicine clinic

BACKGROUND

Although both gender- and sports-specific injuries exist among athletes, gender differences in the types of injuries caused by sports activities, except for anterior cruciate ligament (ACL) injury and anterior knee pain, are not well established.

OBJECTIVE

An observational study with a retrospective case-series design was conducted to investigate gender-specific differences in the types of injuries sustained while engaging in sports activities common to both males and females.

METHODS

We analyzed injuries sustained during sports activities including basketball, volleyball, tennis, skiing, track and field, and swimming, using data on age, sex, sports activities, activity levels, and sports injuries that had been computerized at our sports medicine (orthopedics) clinic. Inclusion criteria were sports activities that had a record of >100 injuries in total and athletes aged <50 years who were engaging in only 1 sports activity. We determined the absolute number of patients in each category and their percentage (proportion) of our cohort. The proportions of common injuries caused by sports activities were investigated, and gender-specific differences in the types of common injuries caused by sports activities were clarified. The Fisher exact test was used to determine the significance (P < 0.01) of the gender-specific differences in the types of sports injuries.

RESULTS

According to our database, during the 14-year period between October 1992 and December 2006, a total of 2,989 athletes (1,624 males and 1,365 females) aged <50 years who engaged in 1 of the 6 sports activities described consulted our sports medicine clinic. The most common sports injuries were ACL injury (14.3%) and knee pain (13.7%), followed by ankle sprain (9.4%), lumbar disc disease (7.0%), meniscus injury (5.1%), stress fracture (2.9%), low back pain (2.5%), patellar tendinitis (2.1%), injury of the medial collateral ligament of the knee (2.0%), lumbar spondylolysis (1.7%), and muscle strain (1.5%). Among these 11 types of sports injuries, a significantly higher proportion of females who engaged in basketball (24.4% vs 10.5%), volleyball (20.5% vs 4.5%), or skiing (41.4% vs 26.5%) presented with an ACL injury, compared with their male counterparts (all, P < 0.001). There was also a significantly higher proportion of females than of males among the track and field athletes who presented with stress fractures (18.7% vs 3.9%; P < 0.001).

CONCLUSION

The findings of this retrospective study suggest that there are gender-specific differences in the types of injuries sustained during sports activities.

In vivo effects of ovarian steroid hormones on the expressions of estrogen receptors and the composition of extracellular matrix in the anterior cruciate ligament in rats

Female athletes have a significantly higher rate of anterior cruciate ligament (ACL) injury than their male counterparts. Sex steroid hormones are considered to have an influence as risk factors for female ACL injuries. We hypothesized that estrogen and progesterone have specific and synergistic influences on the composition of extracellular matrix in ACL. By ovariectomy (OVX) followed by subcutaneous estradiol (E2) and/or progesterone (P4) replacement, 40 female rats were divided into 5 groups: E2, P4, combined E2 and P4 (EP), OVX control, and sham group. After 30 days, using undecalcified sections of knee joints in conjunction with immunofluorescence staining of estrogen receptor alpha and beta (ERalpha and ERbeta), collagen types 1 and 3, and cartilage oligomeric matrix protein (COMP), the immunoreactivities of these proteins in two distinct parts of ACL, proximal and middle portions, were compared semiquantitatively among experimental groups. By E2 replacement, the expressions of ERalpha in ACL fibroblasts were elevated compared to the OVX group. At the proximal portion, the immunoreactivities of type 1 collagen by E2 replacement, type 3 collagen by P4 replacement, and COMP by E2 or P4 replacement were significantly reduced. At the middle portion, the immunoreactivity of type 3 collagen was significantly elevated by E2 replacement. However, no differences were observed between the sham and OVX groups. These findings suggest that ACL is ER-dependent and that ovarian hormones alter ligament tissue composition, especially at the proximal portion. Female hormonal influences are partly involved in the biological properties of ACL.

The interrelationships among sex hormone concentrations, motoneuron excitability, and anterior tibial displacement in women and men

CONTEXT

Sex hormone fluctuations have been implicated as a contributing factor to the high rates of noncontact injury to the anterior cruciate ligament in females.

OBJECTIVE

To determine the strength of the relationships among variables of sex hormone concentrations, motoneuron excitability, and anterior tibial displacement (ATD) in women and men and to determine if these relationships differ between the sexes.

DESIGN

Cohort study.

SETTING

Sports medicine laboratory.

PATIENTS OR OTHER PARTICIPANTS

Twenty-eight regularly menstruating women (age = 22.4 +/- 3.4 years) and 15 men (age = 22.3 +/- 3.7 years) participated in the study.

INTERVENTION(S)

Fluctuations in sex hormones were determined for the participants. Female participants were tested every other day of their menstrual cycles, whereas male participants were tested every fourth day during the 28-day period.

MAIN OUTCOME MEASURE(S)

We measured Hoffmann reflexes (maximum Hoffmann reflex [H(max)] to maximum M-wave [M(max)] ratio in the soleus), ATD under a 134-N load, and saliva concentrations of estrogen and progesterone. The independent variable was sex. Pearson product moment correlation coefficients were calculated for each participant by pairing measurements made on the same day. Two-tailed independent-samples t tests were used to determine the difference between the male and female correlations for each variable.

RESULTS

Over the course of the study, the relationships between H(max)ratioM(max) and estrogen, H(max)ratioM(max) and progesterone, ATD and estrogen, and ATD and progesterone were not different between the sexes. However, the relationship between ATD and progesterone was different between the sexes (P = .036).

CONCLUSIONS

The observed correlations did not support our hypothesis that the relationships between sex hormone levels and reflex activity or between sex hormone levels and ATD would be different for women compared with men. If sex hormone concentrations significantly contribute to anterior cruciate ligament ruptures because of changes in laxity or in motoneuron excitability, their mechanism of action is likely multifactorial and complex.

Relationship of the menstrual cycle phase to anterior cruciate ligament injuries in teenaged female athletes

INTRODUCTION

Anterior cruciate ligament (ACL) injuries are more common among female athletes compared to male athletes. Several studies have been reported to explain the gender difference in ACL injury rates and several risk factors underlying gender disparity are believed to exist. Hormonal effects are considered to be one of the etiological factors for female non-contact ACL injuries. The objectives of this study were to determine if ACL injuries occurred randomly or correlated with a specific phase of the female menstrual cycle in teenaged female athletes and then to determine if pre-menstrual and menstrual dysfunctions influenced these ACL injuries.

MATERIALS AND METHODS

Eighteen of 37 consecutive female athletes, with ACL injuries, met the study criteria: teenage, with regular menstrual cycle, and non-contact injury. The menstrual history, athletic activity, and injury history were collected.

RESULTS

Seventy-two percent of the subjects had premenstrual symptoms and 83% had menstrual symptoms. The subjective activity level at the follicular phase was significantly lowest between the phases. A significant statistical association was found between the phase of the menstrual cycle and ACL injuries (P = 0.0002). There were more injuries in the ovulatory phase than expected, and fewer injuries occurred in the other phases.

CONCLUSION

The results showed a significant increase in non-contact ACL injuries in teenage female athletes during the ovulatory phase of the menstrual cycle and the subjective activity level and the premenstrual and menstrual symptoms might not affect the likelihood of the injuries. These findings suggest that sex hormones might play a role in the incidence of female non-contact ACL injuries.

Ethinyl oestradiol administration in women suppresses synthesis of collagen in tendon in response to exercise

Women are at greater risk than men of sustaining certain kinds of injury and diseases of collagen-rich tissues. To determine whether a high level of oestradiol has an acute influence on collagen synthesis in tendons at rest and in response to exercise, one-legged kicking exercise was performed for 60 min at 67% of maximum power by healthy, young oral contraceptive (OC) users when circulating synthetic (ethinyl) oestradiol was high (n = 11, HE-OC) and compared to similar women who had never used OCs when circulating endogenous oestrogen was low (n = 12, LE-NOC). Interstitial fluid was collected 24 h post-exercise through microdialysis catheters placed anterior to the patellar tendon in both legs and subsequently analysed for the amino-terminal propeptide of type I collagen (PINP), a marker of tendon collagen synthesis. To determine the long-term effect of OC usage, patellar tendon cross-sectional area (CSA) was measured by magnetic resonance imaging (MRI). A lower exercise-induced increase in tendon collagen synthesis was observed in HE-OC than in LE-NOC (DeltaPINP (mean +/- s.e.m.) 1.5 +/- 5.3 versus 24.2 +/- 9.4 ng ml(-1), P < 0.05). Furthermore, serum and the interstitial peritendinous tissue concentrations of insulin-like growth factor I (IGF-I) and IGF-binding proteins showed a reduced bioavailability in HE-OC compared with results in LE-NOC. No difference in patellar tendon CSA was observed between groups. In conclusion, the selective increase in tendon collagen synthesis in LE-NOC but not HE-OC 24 h post-exercise is consistent with the hypothesis that oestradiol inhibits exercise-induced collagen synthesis in human tendon. The mechanism behind this is either a direct effect of oestradiol, or an indirect effect via a reduction in levels of free IGF-I. However, the data did not indicate any long-term effect on tendon size associated with chronic OC use.

Ovariectomy delays alveolar wound healing after molar extractions in rats

PURPOSE

This study was conducted to investigate the morphological effects of the absence of estrogen on alveolar wound healing of young female rats after tooth extraction.

MATERIALS AND METHODS

A total of 60 4- to 6-week-old female rats underwent bilateral ovariectomy (OVX) or sham operations. Three weeks later, the first mandibular molars were extracted. Subsequently, the animals were killed by cervical dislocation 3, 5, 7, 14, 21, or 28 days after tooth extraction. The mandibles were removed, and serial transversal sections of mesial alveolus of the first mandibular molars were obtained for histometric analysis.

RESULTS

OVX sockets showed significant increases in fibroblasts and collagen content 3 and 5 days after the extractions, followed by significant decreases in these parameters in the subsequent periods. In accordance with the decreased collagen content in the latest period of healing, new bone formation was significantly reduced in the OVX animals.

CONCLUSION

These findings suggest that the initial molecular changes observed in the absence of estrogen lead to delayed alveolar wound healing.

ACL Tears in Female Athletes

With the growing number of female athletes, an increase is occurring in the number of sports-related injuries, which can cause physical, psychological, academic, and financial suffering. Female athletes are reported to be two to eight times more likely to sustain an anterior cruciate ligament (ACL) injury than male athletes. Further research on risk factors and preventative strategies for the female ACL is needed, because the cause of the disparity in injury rates remains equivocal and controversial. Individualized treatment for the injured knee is necessary and can include either conservative treatment or reconstructive surgery.

Prevention of anterior cruciate ligament injury in the female athlete

The relationships of gender, age and training to the incidence of anterior cruciate ligament (ACL) injury are pivotal to developing a comprehensive neuromuscular and proprioceptive training programme to decrease ACL injuries in female athletes. A prophylactic neuromuscular and proprioceptive training programme may have direct benefit in decreasing the number of ACL injuries in female athletes. This research foundation endorses further epidemiological and biomechanical studies to determine the exact mechanism of ACL injury and the most effective intervention for decreasing ACL injuries in this high-risk population.

Reproductive hormone effects on strength of the rat anterior cruciate ligament

The material properties of the anterior cruciate ligament (ACL) in female rats with normal estrous cycles were compared to those regulated by oral contraceptive steroids. Forty female Sprague-Dawley rats were divided into two groups: an experimental group received daily ethinylestradiol and levonorgestrel in a dosing model designed to simulate a typical oral contraception regime in humans, while a control group received daily oral placebo. After eight cycles, six rats from each group underwent daily phlebotomy to measure serum estradiol and progesterone levels over the course of a single 5-day estrous cycle. Significant differences between groups were found for the area under the curve of blood progesterone levels versus time over the length of the estrous cycle (P=0.02). After 12 cycles, the rats were euthanized and one femur-ACL-tibia complex from each animal was dynamically loaded to failure. The ACLs from the rats in the experimental group had significantly decreased average and tangent stiffness, (P=0.002 and 0.0001, respectively), and significantly increased elongation (P=0.002) and total energy absorbed (P=0.03), or greater toughness than controls. In rats, it appears that the administration of reproductive hormones designed to simulate typical oral contraception in humans alters the mechanical properties of the rat ACL.

The effects of the menstrual cycle on anterior knee laxity: a systematic review

Female athletes are at a 4- to 6-fold increased risk of anterior cruciate ligament (ACL) injury compared with male athletes. There are several medical, emotional and financial burdens associated with these injuries. Sex hormones may be involved in the ACL injury disparity, with potential associations reported between phases of the menstrual cycle and ACL injury rates. The reported relationships between ACL injury and menstrual status may be related to associated changes in ligament mechanical properties from cyclic fluctuations of female sex hormones. A PubMed electronic database literature search, including MEDLINE (1966-2005) and CINAHL (1982-2005), with the search terms 'menstrual cycle' and 'knee laxity' was used for this systematic review. Studies were included in this systematic review if they were prospective cohort studies and investigated the association between the menstrual cycle and anterior knee laxity in females. Nine prospective cohort studies, published as 11 articles, were included in the systematic review. Six of nine studies reported no significant effect of the menstrual cycle on anterior knee laxity in women. Three studies observed significant associations between the menstrual cycle and anterior knee laxity. These studies all reported the finding that laxity increased during the ovulatory or post-ovulatory phases of the cycle. A meta-analysis, which included data from all nine reviewed studies, corroborated this significant effect of cycle phase on knee laxity (F-value = 56.59, p = 0.0001). In the analyses, the knee laxity data measured at 10-14 days was >15-28 days which was >1-9 days. Future studies testing the relationship between the menstrual cycle and potentially associated parameters should consider the limitations outlined in this article and control for potential biases and confounders. Power analyses should be utilised. Subjects should be randomly entered into the studies at alternate points in the cycle, and standard and consistent data acquisition and reporting methods should be utilised. Future studies should clearly define what constitutes a 'normal' cycle and appropriate control subjects should be utilised. Furthermore, there is a need to define cycle phase (and timing within cycle phase) with actual hormone levels rather than a day of the cycle. Although hormone confirmations were provided in many of the studies that selected specific days to depict a particular cycle for all women, it is unknown from these data if they truly captured times of peak hormone values in all women. A combined systematic review and meta-analysis of the literature indicate that the menstrual cycle may have an effect on anterior-posterior laxity of the knee; however, further investigation is needed to confirm or reject this hypothesis.

Does estrogen alter the mechanical properties of the anterior cruciate ligament? An experimental study in rabbits

BACKGROUND

It is well known that anterior cruciate ligament (ACL) injuries are commoner in female athletes. Accordingly, we hypothesized that serum estrogen may play some role in this sex difference. We evaluated the relationship between serum estrogen levels and the mechanical properties of the ACL in rabbits.

ANIMALS AND METHODS

In 40 ovariectomized rabbits, the serum estrogen levels (SEL) were controlled by intramuscular injection of 17beta-estradiol. The mean SEL in each rabbit was defined as the average of 5 determinations done at 1, 2, 3, 4 and 5 weeks after ovariectomy. The animals were divided into 4 groups according to the dose of estradiol administered (low, medium, high and control: L, M, H and C, respectively) and into 2 groups according to the mean SEL (high-SEL group and low-SEL group). The medial portions of ACL attached to both femur and tibia harvested at 5 weeks after ovariectomy were examined mechanically.

RESULTS

The mean serum estrogen levels in groups C, L, M and H were 37, 50, 60 and 231 pg/mL. Statistically significant differences in the mean serum estrogen levels were seen among the 4 groups, except between groups L and M. Statistically significant differences were found between groups M and H in both the ultimate tensile stress and linear stiffness. In the comparison between 2 groups using the mean SEL value, both ultimate tensile stress and linear stiffness were lower in the high-SEL group. In all animals, a positive correlation was found between ultimate tensile stress and linear stiffness.

INTERPRETATION

Our findings suggest that high SEL might be one of the factors in the multifactorial pathogenesis of ACL rupture.

Quantitation of estrogen receptors and relaxin binding in human anterior cruciate ligament fibroblasts

The significantly higher incidence of anterior cruciate ligament (ACL) injuries in collegiate women compared with men may result from relative ligament laxity. Differences in estrogen and relaxin activity, similar to that seen in pregnancy, may account for this. We quantified estrogen receptors by flow cytometry and relaxin receptors by radioligand binding assay in human ACL cells and compared the presence of these receptors in males and females. ACL stumps were harvested from seven males and eight females with acute ACL injuries. The tissue was placed in M199 cell culture medium. Outgrowth cultures were obtained, and passage 2 cells were used for all studies. Estrogen receptor determination was performed using flow cytometry. Relaxin binding was performed in ACL cells derived from five female and male patients using I(125)-labeled relaxin. Estrogen receptors were identified by flow cytometry in 4 to 10% of ACL cells. Mean fluorescence of cells expressing estrogen receptors was approximately twice that of controls, with no significant differences between males and females. Relaxin studies showed low-level binding of I(125)-relaxin-labeled ACL cells. Relaxin binding was present in four out of five female ACL cells versus one out of five male ACL cells.

Neuromuscular performance and knee laxity do not change across the menstrual cycle in female athletes

Female athletes incur anterior cruciate ligament ruptures at a rate at least twice that of male athletes. Hypothesized factors for the increased injury risk in females include biomechanical, neuromuscular, and hormonal differences between genders. A wealth of literature exists examining these potential predispositions individually, but the interactions between these factors have not been examined extensively. Our purpose was to investigate changes in neuromuscular control and laxity at the knee across the menstrual cycle of healthy females. Fourteen female collegiate athletes with normal, documented ovulatory menstrual cycles, confirmed ovulation, and no history of serious knee injury participated. The presence and timing of ovulation was determined during a screening cycle with ovulation detection kits and during an experimental cycle with collection of daily urine samples and subsequent analysis of urinary estrone-3-glucuronide (E3G) and pregnanediol-3-glucoronide (PdG), which correlate with circulating estrogen and progesterone. Each subject had measures of knee neuromuscular performance and laxity once during the mid-follicular, ovulatory, and mid-luteal stages of her menstrual cycle. The test battery included assessments of knee flexion and extension peak torque, passive knee joint position sense, and postural control in single leg stance. Knee joint laxity was measured with an arthrometer. Analyses of variance revealed that E3G and PdG levels were significantly different across the three testing sessions, but there were no significant differences in the measures of strength, joint position sense, postural control, or laxity. No significant correlations were found between changes in E3G or PdG levels and changes in the performance and laxity measures between sessions. These results suggest that neuromuscular control and knee joint laxity do not change substantially across the menstrual cycle of females despite varying estrogen and progesterone levels.

Understanding and preventing noncontact anterior cruciate ligament injuries: a review of the Hunt Valley II meeting, January 2005

The incidence of noncontact anterior cruciate ligament injuries in young to middle-aged athletes remains high. Despite early diagnosis and appropriate operative and nonoperative treatments, posttraumatic degenerative arthritis may develop. In a meeting in Atlanta, Georgia (January 2005), sponsored by the American Orthopaedic Society for Sports Medicine, a group of physicians, physical therapists, athletic trainers, biomechanists, epidemiologists, and other scientists interested in this area of research met to review current knowledge on risk factors associated with noncontact anterior cruciate ligament injuries, anterior cruciate ligament injury biomechanics, and existing anterior cruciate ligament prevention programs. This article reports on the presentations, discussions, and recommendations of this group.

The relationship between menstrual cycle phase and anterior cruciate ligament injury: a case-control study of recreational alpine skiers

BACKGROUND

Female athletes suffer a greater incidence of anterior cruciate ligament tears compared with male athletes when participating in common sports; however, very little is known about the factors that explain this disparity.

STUDY DESIGN

Case-control study; Level of evidence, 3.

METHODS

Female recreational alpine skiers with an anterior cruciate ligament rupture and age-matched control skiers provided a serum sample and self-reported menstrual history data immediately after injury. Both serum concentrations of progesterone and menstrual history were then used to group subjects into either preovulatory or postovulatory phases of the menstrual cycle.

RESULTS

Analysis of serum concentrations of progesterone revealed that alpine skiers in the preovulatory phase of the menstrual cycle were significantly more likely to tear their anterior cruciate ligaments than were skiers in the postovulatory phase (odds ratio, 3.22; 95% confidence interval, 1.09-9.52; P = .027). Analysis of menstrual history data found similar results, but the difference was not statistically significant (odds ratio, 2.38; 95% confidence interval, 0.86-6.54; P = .086).

CONCLUSION

The likelihood of sustaining an anterior cruciate ligament injury does not remain constant during the menstrual cycle; instead, the risk of suffering an anterior cruciate ligament disruption is significantly greater during the preovulatory phase of the menstrual cycle compared with the postovulatory phase.

CLINICAL RELEVANCE

Phase of menstrual cycle may be one of the risk factors that influence knee ligament injury among female alpine skiers. The findings from this study should be considered in subsequent studies designed to identify persons at risk for anterior cruciate ligament injury and to develop intervention strategies.

Knee ligament mechanical properties are not influenced by estrogen or its receptors

Women are at greater risk of tearing their knee anterior cruciate ligament (ACL) than men participating in similar athletic activities. There is currently no conclusive explanation for this disparity; however, as ACL injuries in women have been linked with estrogen fluctuations during the menstrual cycle, one hypothesis is that estrogen has a direct detrimental effect on knee ligament mechanical properties. This study investigated the influence of estrogen and its receptors (ER alpha and ER beta) on knee ligament mechanical properties. This was achieved by testing the viscoelastic and tensile mechanical properties of knee medial collateral ligaments (MCL) and ACLs from: 1) male Sprague-Dawley rats treated with either estrogen (17alpha-ethynylestradiol; 0.03 mg/kg) or an ER alpha-specific agonist (propyl pyrazole triol; 2 mg/kg), and 2) female mice with a null mutation of the gene encoding for ER beta. Estrogen treatment had no significant effects on the viscoelastic or tensile mechanical properties of the rat MCL or ACL. Similarly, pharmacological stimulation of ER alpha using a selective agonist in rats and genetic modulation of ER beta by null mutation of its gene in mice did not influence MCL or ACL properties. These data indicate that estrogen does not have a major direct effect on ligament mechanical properties. Energies for the prevention of the disproportionately high rate of knee ligament injuries in women may be better spent focusing on more established and modifiable risk factors, such as abnormalities in neuromuscular control about the knee.

Absolute serum hormone levels predict the magnitude of change in anterior knee laxity across the menstrual cycle

This study aimed to determine whether absolute sex hormone concentrations predict the magnitude of knee joint laxity changes across the menstrual cycle. Twenty-two females (18-30 years, body mass index <or=30), who reported normal menstrual cycles for the previous 6 months were tested daily across one complete menstrual cycle for serum levels of estradiol (E=pg/mL), progesterone (P=ng/mL), and testosterone (T=ng/dL), and knee joint laxity (K(Lax)=mm displacement at 134N) measured with a standard knee arthrometer. The change in K(Lax) across the cycle (maximum-minimum), and minimum (early follicular) and peak (postovulatory) hormone concentrations were recorded for each subject. A stepwise linear regression determined if the minimum, peak, or absolute change in hormone concentrations would predict the magnitude of change in K(Lax) across the cycle. K(Lax) changed on average 3.2+/-1.1 mm across the menstrual cycle (range, 1.5-5.3 mm). Minimum levels of E (39.9+/-11.8 pg/mL) and P (0.61+/-0.27 ng/mL), coupled with peak concentrations of E (199.6+/-54.9 pg/mL) and T (22.5+/-10.5 ng/dL) explained 57.6% of the change in K(Lax) across the cycle. Greater absolute changes in K(Lax) were observed in response to peak E and T levels when minimum E concentrations were lower and minimum P concentrations were higher in the early follicular phase. The absolute minimum concentrations of E and P in the early follicular phase appear to be important factors in determining the sensitivity of the knee joint's response to changing hormone levels.

A Risk-Factor Model for Anterior Cruciate Ligament Injury

The incidence of non-contact anterior cruciate ligament (ACL) injury is reported to be 6-8 times greater in females than males competing in the same activities. Injury to the ACL occurs as a result of insufficient stability of the tibiofemoral joint, which fails to prevent posterior dislocation of the femur on the tibia. The stability of the tibiofemoral joint is maintained by passive (non-contractile) and dynamic (contractile) mechanisms. The passive mechanisms include the shape of the articular surfaces, the menisci, the ligaments and the joint capsule. The dynamic mechanisms consist of the muscle-tendon units that cross the joint, in particular, the quadriceps and hamstrings. The relative significance of the various passive and dynamic mechanisms in maintaining the stability of the tibiofemoral joint is not clear. A number of intrinsic and extrinsic risk factors have been proposed to account for the sex difference in the incidence of ACL injuries. However, most of the proposed risk factors have arisen from univariate correlation studies based on relatively small samples.

No effect of menstrual cycle on myofibrillar and connective tissue protein synthesis in contracting skeletal muscle

We tested the hypothesis that acute exercise would stimulate synthesis of myofibrillar protein and intramuscular collagen in women and that the phase of the menstrual cycle at which the exercise took place would influence the extent of the change. Fifteen young, healthy female subjects were studied in the follicular (FP, n=8) or the luteal phase (LP, n=7, n=1 out of phase) 24 h after an acute bout of one-legged exercise (60 min of kicking at 67% W(max)), samples being taken from the vastus lateralis in both the exercised and resting legs. Rates of synthesis of myofibrillar and muscle collagen proteins were measured by incorporation of [(13)C]leucine. Myofibrillar protein synthesis (means+/-SD; rest FP: 0.053+/-0.009%/h, LP: 0.055+/-0.013%/h) was increased at 24-h postexercise (FP: 0.131+/-0.018%/h, P<0.05, LP: 0.134+/-0.018%/h, P< 0.05) with no differences between phases. Similarly, muscle collagen synthesis (rest FP: 0.024+/- 0.017%/h, LP: 0.021+/- 0.006%/h) was elevated at 24-h postexercise (FP: 0.073+/- 0.016%/h, P<0.05, LP: 0.072+/- 0.015%/h, P<0.05), but the responses did not differ between menstrual phases. Therefore, there is no effect of menstrual cycle phase, at rest or in response to an acute bout of exercise, on myofibrillar protein synthesis and muscle collagen synthesis in women.

17β-Estradiol suppresses proliferation of fibroblasts derived from cardinal ligaments in patients with or without pelvic organ prolapse

BACKGROUND

Estrogen replacement therapy (ERT) has been used in the treatment of pelvic organ prolapse (POP) but clinical results are inconclusive. The purpose of this study was to investigate the effect of 17beta-estradiol (E(2)) on the proliferation of fibroblasts derived from cardinal ligaments in women with or without POP.

METHODS

Fibroblasts were derived from seven patients with POP and seven age-matched controls. The growth rate of POP fibroblasts was compared with that of control by 3-(4,5,-dimethyl thiazolyl-2)-2,5-diphenyl tetrazolium bromide (MTT) assay. Four cell strains from each patient and control group were treated with different concentrations of E2 (10(-4), 10(-8), 10(-9) and 10(-10) mol/l). The effect of E2 on cell proliferation was then measured by MTT assay.

RESULTS

The overall growth rate of POP fibroblasts was significantly slower than that of controls under normal culture conditions. Addition of E2 suppressed cell proliferation of all the fibroblasts, especially in POP fibroblasts. POP fibroblasts showed a significantly lower proliferative rate than that of controls at all E2 concentrations, with the most prominent inhibitory effect at physiological concentration (10.83 34.41% versus 81.56 48.10% at 10(-8) mol/l).

CONCLUSIONS

Our results suggest that decreased fibroblast turnover may contribute to the development of POP; and ERT may not be an effective POP treatment.

The effect of estradiol and progesterone on knee and ankle joint laxity

BACKGROUND

Female athletes suffer a higher incidence of anterior cruciate ligament injuries compared to their male counterparts, and they appear to be at increased risk for these injuries when they have increased anterior-posterior knee laxity and at specific phases of the menstrual cycle. Although the mechanism by which these factors combine to increase injury risk is unclear, studies suggest that cyclic variations in joint laxity produced by hormone fluctuation during the menstrual cycle predispose an athlete to increased risk of ligamentous injury. Little is known about whether joint laxity varies cyclically during the menstrual cycle and if so, whether it is modulated by cyclic variations of estradiol (E2) and progesterone (P4).

HYPOTHESIS

Increased serum estradiol (E2) and progesterone (P4) levels are associated with increased ankle and knee joint laxity.

STUDY DESIGN

Cohort study. Level of evidence, 2.

METHODS

Ankle laxity, anterior-posterior knee laxity, and serum concentrations of estradiol (E2) and progesterone (P4) were measured during the menstrual cycle in women and at corresponding time intervals in men (controls). Ankle laxity was measured from stress radiographs and included anterior talar translation relative to the tibia and talar tilt relative to the tibia; anterior-posterior knee laxity was measured with the KT-1000 arthrometer.

RESULTS

Women had greater knee and ankle laxity values compared to men. There was, however, no change in knee and ankle laxity over the normal menstrual cycle in women and no change over time in men. There was no relationship between estradiol and progesterone fluctuation and ankle and knee joint laxity.

CONCLUSIONS

Knee and ankle joint laxities are greater for women compared to men; however, the cyclic estradiol and progesterone fluctuations that occur during the menstrual cycle do not produce cyclic fluctuations of joint laxity. Studies using joint laxity to identify a subject at risk for ligamentous injury need only consider making measurements at a specific point in time, such as during a preseason screening evaluation.

The combined regulation of estrogen and cyclic tension on fibroblast biosynthesis derived from anterior cruciate ligament

Female athletes are two to eight times more likely to suffer a knee or ankle ligament injury than male athletes, and sex hormones have been considered to play an important role in the injury. Because ligaments are always under mechanical loading during sports, mechanical force is also a critical factor in ligament injuries. In this study, the effects of estrogen and mechanical loading on the gene expression of three major components of ligament--collagen type I, type III, and biglycan--in primary cultured porcine anterior cruciate ligament (ACL) fibroblasts were investigated individually and collectively using reverse transcript-polymerase chain reaction (RT-PCR). The results revealed that cyclic tensile loading alone increased the messenger RNA expression of collagen I but did not affect that of collagen III and biglycan, and estrogen alone increased the gene expression of collagen I and III but not of biglycan. However, combined administration of estrogen and cyclic loading inhibited the mRNA expression of all the three genes. These results suggested that the inhibition of the gene expression of major extracellular matrix component molecules caused by the combined effects of estrogen and mechanical loading, unique to females, might be responsible for the increased incidence of ligaments injury in female athletes.

Effect of testosterone on the female anterior cruciate ligament

Injuries to the anterior cruciate ligament (ACL) result in immediate and long-term morbidity and expense. Young women are more likely to sustain ACL injuries than men who participate in similar athletic and military activities. Although significant attention has focused on the role that female sex hormones may play in this disparity, it is still unclear whether the female ACL also responds to androgens. The purpose of this study was to determine whether the female ACL was an androgen-responsive tissue. To identify and localize androgen receptors in the female ACL, we used Western blotting and immunofluorescent labeling, respectively, of ACL tissue harvested during surgery from young women (n = 3). We then measured ACL stiffness and assessed total testosterone (T) and free [free androgen index (FAI)] testosterone concentrations, as well as relative estradiol to testosterone ratios (E(2)/T and E(2)/FAI) at three consecutive menstrual stages (n = 20). There were significant rank-order correlations between T (0.48, P = 0.031), FAI (0.44, P = 0.053), E(2)/T (-0.71, P < 0.001), E(2)/FAI (-0.63, P = 0.003), and ACL stiffness near ovulation. With the influences of the other variables controlled, there were significant negative partial rank-order correlations between ACL stiffness and E(2)/T (-0.72, P < 0.001) and E(2)/FAI (-0.59, P = 0.012). The partial order residuals for T and FAI were not significant. These findings suggest that the female ACL is an androgen-responsive tissue but that T and FAI are not independent predictors of ACL stiffness near ovulation. Instead, the relationship between T, FAI, and ACL stiffness was likely influenced by another hormone or sex hormone binding globulin.

Are unilateral and bilateral patellar tendinopathy distinguished by differences in anthropometry, body composition, or muscle strength in elite female basketball players?

BACKGROUND

Overuse injury to the patellar tendon (patellar tendinopathy) is a major reason for interrupted training and competition for elite athletes. In both sexes, the prevalence of unilateral and bilateral tendinopathy has been shown to differ. It has been proposed that bilateral pathology may have a different aetiology from unilateral pathology. Investigation of risk factors that may be unique to unilateral and bilateral patellar tendinopathy in female athletes may reveal insights into the aetiology of this condition.

OBJECTIVES

To examine whether anthropometry, body composition, or muscle strength distinguished elite female basketball players with unilateral or bilateral patellar tendinopathy.

METHODS

Body composition, anthropometry, and muscle strength were compared in elite female basketball players with unilateral (n = 8), bilateral (n = 7), or no (n = 24) patellar tendinopathy. Body composition was analysed using a dual energy x ray absorptiometer. Anthropometric measures were assessed using standard techniques. Knee extensor strength was measured at 180 degrees /s using an isokinetic dynamometer. z scores were calculated for the unilateral and bilateral groups (using the no tendinopathy group as controls). z scores were tested against zero.

RESULTS

The tibia length to stature ratio was approximately 1.3 (1.3) SDs above zero in both the affected and non-affected legs in the unilateral group (p<0.05). The waist to hip ratio was 0.66 (0.78) SD above zero in the unilateral group (p<0.05). In the unilateral group, leg lean to total lean ratio was 0.42 (0.55) SD above zero (p<0.07), the trunk lean to total lean ratio was 0.63 (0.68) SD below zero (p<0.05), and leg fat relative to total fat was 0.47 (0.65) SD below zero (p<0.09). In the unilateral group, the leg with pathology was 0.78 (1.03) SD weaker during eccentric contractions (p<0.07).

CONCLUSIONS

Unilateral patellar tendinopathy has identifiable risk factors whereas bilateral patellar tendinopathy may not. This suggests that the aetiology of these conditions may be different. However, interpretation must respect the limitation of small subject numbers.

Anterior and posterior cruciate ligament injuries

Knee pain is a common complaint in the primary care setting, and ACL and PCL injuries are common causes of knee pain. Therefore, it is important for the primary care physician to be skilled in the diagnosis and initial management of these injuries and to be aware of potential associated knee injuries. By understanding the history, mechanism of injury, physical examination maneuvers, and imaging modalities related to ACL and PCL injuries, primary care physicians can prepare themselves to make an accurate diagnosis. Then they can counsel the patients on the available treatments, initiate conservative treatment if appropriate, and facilitate an effective referral to a primary care sports medicine physician or orthopedic surgeon.

Relationship between sex hormones and anterior knee laxity across the menstrual cycle

PURPOSE

To comprehensively quantify through daily, serial measures changes in knee laxity as a function of changing sex-hormone levels across one complete menstrual cycle.

METHODS

Twenty-five females, 18 - 30 yr, body mass index < or = 30, who reported normal menstrual cycles (28-32 d) over the past 6 months participated. Participants were tested daily across one complete menstrual cycle; 5-7 cc of venous blood were withdrawn to assay serum levels of estradiol, progesterone, and testosterone. Knee laxity was measured as the amount of anterior tibial displacement at 133 N, using a standard knee arthrometer. To evaluate the relationship of knee laxity to changes in sex hormone concentrations, a multiple linear regression model with the possibility of a time delay was performed on each individual subject and the group as a whole.

RESULTS

Individual regression equations revealed an average of 63% of the variance in knee laxity was explained by the three hormones and their interactions. All three hormones significantly contributed to the prediction equation, and the amount of variance explained was substantially greater when a time delay was considered. On average, knee laxity changed approximately 3, 4, and 4.5 d after changes in estradiol, progesterone, and testosterone, respectively. When females were analyzed as a group, only 8% of the variance in knee laxity was explained by sex-hormones levels.

CONCLUSION

Changes in sex hormones mediate changes in knee laxity across the menstrual cycle. However, the strength of this relationship, the relative contribution of each hormone, and the associated time delay are highly variable between women. This individual variability is consistent with the variability in menstrual cycle characteristics among women.

The effect of estrogen on ovine anterior cruciate ligament fibroblasts: cell proliferation and collagen synthesis

Estrogen has been implicated as a causal factor for anterior cruciate ligament injuries in women. Studies have demonstrated a decrease in anterior cruciate ligament fibroblast proliferation and collagen synthesis at supraphysiologic levels of estrogen in a rabbit model.

HYPOTHESIS

The authors hypothesized that physiologic levels of estrogen would have no significant effect on anterior cruciate ligament fibroblast proliferation and collagen synthesis in an ovine model.

METHODS

Anterior cruciate ligament fibroblasts were isolated from sheep knees using routine cell culture methods. The cells were exposed to 17beta-estradiol at physiologic concentrations of 2.2, 5, 15, 25, 250, and 2500 pg/ml. Cell proliferation was determined by cell counts on days 4 and 6. Collagen synthesis was determined by (3)H-proline incorporation on day 4. Immunohistochemistry was performed to detect estrogen receptors.

RESULTS

Immunohistochemistry demonstrated the presence of estrogen receptors in ovine anterior cruciate ligament fibroblasts. There was no significant difference in anterior cruciate ligament fibroblast proliferation or collagen synthesis regardless of 17beta-estradiol concentration.

CONCLUSIONS

Based on results of this study, and given the low turnover of collagen in ligaments, it is unlikely that a 2- to 3-day per month increase in circulating estrogen would result in rapid, clinically significant alterations in material properties of the anterior cruciate ligament in vivo. The etiology of noncontact anterior cruciate ligament injuries is complex and multifactorial in nature, meriting further investigation.

Tensile forces attenuate estrogen-stimulated collagen synthesis in the ACL

The purpose of this study was to examine whether mechanical tensile forces affect estrogen regulation of collagen synthesis of anterior cruciate ligament fibroblasts at the mRNA level. Estrogen was studied at three physiologic levels, 10(-11), 10(-10), and 10(-9)M. The results revealed that estrogen alone stimulated Type I and III collagen synthesis at the mRNA level, and application of mechanical force decreased the expression of collagen Type I and III genes at all tested estrogen levels. These findings suggest that estrogen may directly regulate ligament structure and function by alteration of Type I and III collagen synthesis. This regulation is dependent on mechanical loading.

Immunohistological detection of relaxin binding to anterior cruciate ligaments

Relaxin, a member of the insulin-like growth factor family, alters collagen metabolism in fibroblasts. It was hypothesized that relaxin interacts with the anterior cruciate ligament (ACL), contributing to its elasticity. Twelve ACL specimens were collected from reconstruction surgeries, sectioned, rinsed, and exposed to rh-relaxin overnight. Polyclonal antirelaxin antibodies, in conjunction with HRP-AEC, localized areas of tissue binding. Controls were used to infer binding specificity. Staining was present in the positive control and all 12 ACL specimens; little or no staining occurred in the negative controls. These data suggest that relaxin binding to the ACL is specific, indicative of a receptor-mediated event.

The correlations between estradiol, estrone, estriol, progesterone, and sex hormone-binding globulin and anterior cruciate ligament stiffness in healthy, active females

BACKGROUND

Injury to the anterior cruciate ligament (ACL) often requires surgery and extensive rehabilitation. Women who participate in collegiate sports and military drills are more likely to injure their ACL than are men participating in similar activities. The influence of the normal fluctuation of sex hormones on the physical properties of the ACL is one potential cause for this disparity. The purpose of this study was to report the correlation between estradiol, estrone, estriol, progesterone, and sex hormone binding globulin (SHBG) and ACL stiffness during three phases of the menstrual cycle in normally cycling, healthy females.

METHODS

We tested ACL stiffness and collected blood from 20 female subjects who were not using oral contraception during three phases of their menstrual cycle. Ligament stiffness was tested with the KT-2000 trade mark knee arthrometer (MEDmetric, San Diego, CA). Concentrations of estradiol and SHBG were assessed via radioimmunoassay (RIA). Progesterone, estriol, and estrone concentrations were determined via enzyme-linked immunoassay.

RESULTS

Spearman rank correlation analysis indicated a significant correlation between estradiol concentration and ACL stiffness (-0.70, p < 0.001) and estrone concentration and ACL stiffness near ovulation (0.46, p = 0.040). With the effects of the other variables controlled, there was a significant partial correlation between estradiol (-0.80, p < 0.001), estriol (0.70, p = 0.003), and progesterone (0.66, p = 0.005) and ACL stiffness near ovulation.

CONCLUSIONS

Our results indicate that there is a significant correlation between estradiol, estriol, and progesterone and ACL stiffness suggesting that fluctuating levels of sex hormones may influence the stiffness of the ACL near ovulation. Future studies that examine the relationship between sex hormones and the physical properties of the ACL should be focused near the ovulation phase of the menstrual cycle.

Combined effects of estrogen and progesterone on the anterior cruciate ligament

Previous studies from the authors' laboratory have established the presence of estrogen and progesterone receptors in the human anterior cruciate ligament. The purpose of the current study was to investigate the combined effects of 1beta-estradiol and progesterone on cell proliferation and procollagen synthesis of the human anterior cruciate ligament fibroblasts. Fibroblast proliferation and procollagen synthesis in response to logarithmic concentrations of 17beta-estradiol (0.0025 ng/mL, 0.025 ng/mL, 0.25 ng/mL) and progesterone (1 ng/mL, 10 ng/mL, 100 ng/mL) were assessed with the measurement of 3H-thymidine incorporation and Types I and III procollagen specific equilibrium radioimmunoassays. On Days 1, 3, and 5 there was a dose dependent decrease in the fibroblast proliferation and procollagen Type I synthesis with increasing estradiol concentrations. The effect was attenuated with increasing progesterone concentrations. Controlling for estrogen levels, a dose dependent increase in fibroblast proliferation and procollagen Type I synthesis was observed with increasing progesterone concentrations. The effect was more pronounced at lower concentrations of estrogen, suggesting estrogen levels were the dominant factor. The effects of estrogen and progesterone became less apparent by Day 7. No significant differences in Type III procollagen synthesis were seen with varying estradiol concentrations at any of the designated times. These early physiologic changes in fibroblast proliferation and Type I procollagen synthesis may provide a biologic explanation for the increased anterior cruciate ligament injury rate observed in female athletes, suggesting the acute cyclical hormonal variations in the female athlete during menstruation predispose her to ligamentous injury.