Download idemic of Anterior Cruciate Ligament Injury in Female Athletes

The Epidemic of Anterior Cruciate Ligament Injury in Female Athletes
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The Epidemic of Anterior Cruciate Ligament Injury in Female Athletes:
Etiologies and Interventions
by
Katie L. Mitchell
Submitted in partial fulfillment of the requirements for graduation as an Honors Scholar at Point
Loma Nazarene University, San Diego, California, May 10, 2014
Approved by _______________________________________
Dr. Leon Kugler, Advisor
April 12, 2014
Committee Members:
________________________________________
Dr. Nicole Cosby
________________________________________
Dr. Rebecca Flietstra
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Abstract
Anterior cruciate ligament (ACL) injury in females is an epidemic in sports,
especially sports with cutting movements, such as basketball, soccer, and volleyball. This project
addresses risk factors for female ACL injury in a literature review of the most recent research
combined with cadaver dissection of the knee. Risk factors include structural, hormonal, and
mechanical factors specific to females. Structural differences in females, such as posterior tibial
slope and femoral notch size, are difficult to alter, and hormonal factor research is inconclusive.
The promise of intervention for prevention of female ACL injury resides in manipulating the
mechanical risk factors, such as female landing mechanics and neuromuscular deficiencies.
Developing prevention programs focused on correcting mechanical deficiencies is key in
lowering the number of ACL injuries in females.
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INTRODUCTION
Anterior cruciate ligament (ACL) injuries are responsible for 680 million dollars in medical costs
in the United States per year.1 In 2011 alone, there were 95,000 reported ACL injuries in the
United States. The majority of these injuries occurred in females, as females are 4 to 6 more
times likely to sustain an ACL injury than males.2 It has been reported that there are several risk
factors associated with female ACL injuries, therefore an understanding of those factors is
essential in the possible prevention of ACL injuries in females
The ACL serves to stabilize the knee, specifically against anterior translation of the tibia in
relation to the femur. Because it also reduces internal rotation at the tibia when anterior shear
forces are applied to the knee, it provides a restraint against excessive valgus or varus movement
at the knee.3 ACL injury, or rupture, typically occurs with rapid deceleration, hyperextension,
and excessive rotation.3 However, mechanisms of injury are still being researched in greater
depth. When the ACL ruptures, patients typically hear a pop and feel extremely unstable at the
knee joint.
While the consensus is that there are multiple risk factors associated with ACL injury in females,
there are still differing opinions on each risk factor specifically and the way they relate to each
other. The seminal research on this subject thus far has found that there are three main categories
of ACL injury etiologies in females: structural, hormonal, and mechanical risk factors. The
research on structural factors is based on the structural differences between females and males,
especially after puberty. These include differences in femoral notch width, ACL size, posterior
tibial slope, and quadriceps angle. Research on the hormonal factors influencing ACL injury in
females focuses on the role of hormones, such as estrogen, progesterone, and relaxin in
influencing the degree of joint laxity across the menstrual cycle, as well as the role of
contraceptives in possible reduction in injury risk. Mechanical risk factors contributing to ACL
injury in females include poor landing mechanics, quadriceps dominance, valgus collapse, and
proximal weakness and dysfunction. In addition to research on risk factors, there are multiple
studies confirming that neuromuscular training can be effective in lowering the risk for ACL
injuries in females.
Because of the greater risk of ACL injury in females, it is imperative to know the risk factors and
possible interventions to prevent injury. Educating coaches, athletic trainers, physical therapists,
athletes, and adolescents on the risk factors for ACL injuries in females can insure that the
proper interventions are made to reduce the risk as much as possible. Therefore, the purpose of
this review is focus on the seminal research on ACL injuries in females in order to provide a
profile of the population discussed above. In addition to reviewing the literature, cadaver
dissection and ligament stress testing were conducted in order to determine the role ligaments of
the knee play in the stability of the joint.
THE KNEE JOINT
The knee joint (Figure 1) is the largest joint in the body. The tibiofemoral joint has 2 degrees of
freedom; one in the sagittal plane allowing flexion and extension, and the other in the transverse
plane (internal and external rotation). In full extension, the rotational motion is resisted by
ligamentous support and the medial and lateral menisci. Because the knee is the articulation of
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the tibia to the femur, any injury to the knee joint will have negative effects on the entirety of the
lower extremities, in addition to causing kinetic chain problems in the trunk and upper
extremities.4
http://img.webmd.com/dtmcms/live/webmd/consumer_assets/site_images/
articles/image_article_collections/anatomy_pages/knee.jpg
Figure 1. Anterior knee joint
Bones
The bones of the knee joint consist of the patella, tibia, and femur. The tibia is inferior to the
femur, with the femoral condyles acting as a ball bearing and rolling over the tibial condyles to
allow flexion and extension at the knee. The extracapsular patella tracks in the femoral trochlear
groove.
Muscles
Knee joint stabilization is heavily reinforced by the musculature surrounding the joint. The
quadriceps femoris muscles as well as the hamstring muscles serve to provide dynamic
stabilization through movement and augment the stabilization provided by the ligaments of the
knee. The quadriceps primarily work as knee extensors, with the quadriceps tendon attaching to
the superior patella. The hamstrings are known as knee flexors, but it is important to note that
both the quadriceps and hamstring muscle groups participate in rotating movements and are not
uniplanar muscles. The popliteus muscle provides the “unlocking” mechanism for the knee by
rotating the femur laterally on the tibia to release tension in the ligaments and allow the knee to
move into flexion, which is essential upon jump landing to decrease strain on the ligaments and
to properly activate the posterior chain muscles.
Multiple muscles function to decelerate movement of the knee joint during jump landing or other
activities. The hamstrings concentrically flex the knee, but are extremely important in prevention
of ACL injury due to their ability to decelerate knee extension. Similarly, hip adductors and
external rotators greatly increase dynamic knee stabilization and prevent knee pain and injury
from occurring by decelerating excessive femoral motion.4
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Ligaments
Extraarticular ligaments include the medial collateral ligament and lateral collateral ligament
(MCL and LCL), which guard against valgus and varus movement at the knee as well as
excessive medal or lateral rotation. The posteriorly situated oblique popliteal ligament and
arcuate popliteal ligament reinforce posterior stability of the knee.
The intraarticular ligaments are the anterior cruciate ligament (ACL) and the posterior cruciate
ligament (PCL). These ligaments form the shape of an X, and are located between the femoral
condyles. The PCL is attached to the posterior tibia and traverses superiorly and medially to
attach on the medial condyle of the femur. The PCL prevents posterior translation of the tibia in
relation to the femur, or anterior translation of the femur in relation to the tibia. The ACL is
attached to the tibia and courses superiorly and laterally to attach on the lateral condyle of the
femur. The ACL is responsible for preventing anterior translation of the tibia in relation to the
femur, or posterior translation of the femur in relation to the femur. It also plays a role in
preventing hyper extension and assists the MCL and LCL in preventing excessive rotation at the
joint.
THE ANTERIOR CRUCIATE LIGAMENT
Anatomy and function
The average anterior cruciate ligament is 3 cm in length and 1 cm in diameter.3 The ACL crosses
with the PCL to create the shape of an X, providing a significant amount of stability to the knee
joint. The ACL has two bundles, anteromedial and posterolateral, which have slightly different
functions due to differences in anatomy.5 The anteromedial bundle is less substantial than the
posterolateral bundle, and it most effectively limits anterior translation of the tibia when the knee
is in flexion. The posterolateral bundle is more effective in rotation control because it lies in a
more oblique position, serving to prevent hyperextension, and rotation. This bundle is most at
risk for injury in positions of hyperextension and internal rotation. ACL is composed of 70%
collagen (90% type I collagen, 10% type II collagen). Because type I is more rigid, it is helpful
in stabilization, while type II allows for the degree of stretch that is needed in ligaments. Elastin
only makes up 1% of the dry weight of the ligament, but is vital to proper functioning of the
ligament due to its ability to store energy during activities, allowing the ACL to return to its
original length.3
The ACL functions as the primary restraint for anterior tibial translation (posterior femoral
translation), but also provides secondary restraint against frontal plane motion (varus and
valgus), along with the MCL and LCL. The amount of frontal plane motion restraint that it
provides is still debated. In the most current research, it is being found that the ACL provides
more restraint against this type of motion than previous research indicated.
Mechanisms of injury
The majority of ACL injuries are non-contact injuries which occur as a result of rapid
deceleration and rotation at the knee joint.3 Patients will most often describe feeling a pop
followed by intense pain and swelling. While there is consensus that anterior tibial translation,
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frontal plane motion, and rotation all play some role in ACL injury, it is unknown which
mechanism is most prevalent and how these force vectors summate. It is known that cadaver
ACLs show higher strain when placed in a position of compression, tibial valgus, tibial internal
rotation, and combined valgus and internal rotation.6
“Valgus collapse” is a multiplanar movement in which there is anterior tibial shear as well as
valgus movement at the knee.7,8 Quatman et al.8 used the location of bone bruises in ACL injured
patients to demonstrate the likelihood that valgus collapse is the primary mechanism of injury.
The relative etiological influence of anterior tibial translation and shear forces compared to
extreme valgus motion at the knee in ACL injury is debatable. Since valgus collapse is so often
the mechanism of injury, it is unclear why only 30% of ACL injuries are accompanied by MCL
strain.9 One possible explanation is that MCL injuries are more often misdiagnosed or ignored
when ACL injury is also present. Additional mechanisms are discussed in the mechanical factors
section of this literature review.
Morbidity of injury
Patients with a previous ACL injury are more likely to reinjure the ACL or rupture their
contralateral ACL.6 Osteoarthritis is more likely to occur in patients with previous ACL injury
compared with patients having no previous ACL injury.8 The reported incidence of osteoarthritis
is difficulty to accurately report, but may be as high as 87%.9 Since females are 4 to 6 more times
likely than males to sustain an ACL injury, it is medically and monetarily imperative to continue
research efforts to prevent an epidemic of females presenting with recurring ACL injuries and
conditions such as osteoarthritis.
STRUCTURAL FACTORS
Males and females differ greatly in the structure of the knee joint and surrounding tissue. Since
puberty correlates with an increased risk of ACL injury in women, it is important to examine
structural differences present before and after puberty as possible risk factors for ACL injury.
Femoral notch size
The femoral notch (intercondylar notch) is located between the condyles of the femur (Figure 2).
The femoral notch is narrower in populations sustaining ACL injury, which leads to the belief
that the narrower femoral notch may be a predisposition to injury.6,7 Abate et al.7 utilized 60
research articles in a cumulative narrative review on ACL risk factors. Evidence for their claim
that femoral notch is an indicator of injury risk in females included a study in which 714 patients
undergoing patellar tendon graft for ACL construction were measured for femoral notch size
differences. Females had significantly narrower femoral notches than men, and ACL patients
who have narrower notches are more likely to tear their contralateral ACL than those with wider
femoral notches. Shultz et al.6 cited 198 articles in a research update on ACL injury risk and
prevention. This research update was the result of discussion and presentations by 70 clinicians
and researchers. Although Shultz et al.6 are in consensus with Abate et al.7 in finding smaller
femoral notches in the ACL injured population, they assert that it may be a better predictor for
injury in males than females. As both articles cite several other sources as evidence and provide
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an overview of current information on ACL injury, it is likely that they are correct in finding
evidence to support femoral notch width as an ACL injury risk factor, but it is unclear whether
this risk factor is a good predictor in females.
Stijak et al.11 conducted a study on 33 ACL injured patients in which they studied 32 anatomical
components as possible risk factors for ACL injury. In addition to finding that there was no
significant evidence that women are at greater risk for injury due to smaller femoral notches,
they found that precise diagnosis for women based on anatomical factors is 75.7%, compared to
88.9% in men, which is consistent with the findings above that femoral notch width may be a
better ACL injury predictor in men. Out of the 13 anatomical parameters used to measure the
femoral notch, none were found to be significant in predicting ACL injury in females, while 3
were found to be significant in predicting injury in males. Statistical analysis on a mixed
population in some studies may demonstrate that femoral notch was of significance in predicting
ACL injury, but Stijak et al.11 states that the method of measurement used in many studies is less
reliable than the one used in their study. The results of their study indicated that although smaller
femoral notches are found in the ACL injured population, this anatomical factor may be a better
predictor of injury in males.
Because female knee anatomy tends to be smaller, and small femoral notches are linked to
increased risk for ACL injury, it is still a possibility that femoral notch size is a risk factor for
ACL injury in women. However, research indicates that this is not an accurate predictor of injury
in females, and may be better used as a predictor for male ACL injuries. More studies using only
female subjects would be useful in further exploration of the effect of femoral notch size on
female ACL injury risk.
http://www.hughston.com/hha/b_11_3_2a.jpg
Figure 2. Femoral (intercondylar) notch
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ACL size
ACL size may be related to ACL injury risk, although this is more debated than femoral notch
size. Patients who sustain an ACL injury generally been found to have smaller ACLs.6,7 Because
females tend to have smaller ACLs, ACL size may predispose females to higher risk for injury.6
Evidence presented by Abate et al.7 and Shultz et al.6 was normalized by BMI to account for size
differences between men and women. Shultz et al.6 specifically found that women have smaller
ACLs relative to length, cross-sectional area, and volume after adjusting for BMI. They also
found that female ACLs generally have less collagen fiber density and decreased mechanical
properties, which may make it difficult to isolate ACL size alone as a significant risk factor.
Composition may also play a key role.
Wolters et al.12 determined that there is a positive correlation between femoral notch size and
ACL insertion size, though the correlation is weak, being between 0.222 and 0.379. However,
the p-value determined in this study was < 0.05, indicating significant evidence for differences
between notch size and ACL insertion size in men and women, with women having smaller
structures than men. These findings were found in a study of 82 patients (41 men and 41
women). These patients underwent ACL reconstruction, and measurements were taken at the
base, middle, and top of the femoral notch, with the height being measured at the highest point of
the notch. In order to interpret the relationship between femoral notch size and ACL insertion
size, more research is required. Additionally, as femoral notch size was found to be a somewhat
inaccurate indicator of ACL injury risk in females, the relationship between femoral notch size
and ACL insertion size may not be very significant in determining risk, and there are other
factors that demonstrate a greater likelihood of increasing ACL injury in females.
Posterior Tibial Slope
A large posterior tibial slope angle is a possible anatomical risk factor for ACL injury (Figure 3).
Tibial slope is defined as the line representing the posterior inclination of the tibia and the line
perpendicular to the diaphysis of the tibia.13 Research concerning the posterior tibial slope in
relation to ACL injury in females is a recent research question, but while large gaps remain in the
research, the probability indicates ACL ruptured subjects are more likely to have a larger
posterior tibial slope.
Simon et al.14 studied 54 subjects separated into two groups to determine differences in tibial
slope between injured and non-injured subjects. These subjects were assigned to an injured group
and a non-injured group. Each injured subject was matched with a subject in the non-injured
group by weight, height, gender, and age. MRI was used to measure the tibial slopes of the
contralateral knee in each of the injured subjects, and a random knee was chosen to measure in
each of the uninjured subjects. The study yielded significant evidence that the tibial slopes were
greater in the injured group than the non-injured group, with a p-value of 0.02. One limitation of
this study was the use of the contralateral knee in the injured population, as variability of tibial
slopes between a subject’s two knees is unknown.
Studies have found that the lateral slope is significant in determining amount of risk, while the
medial slope is not.11,13,14 Stijak et al.11 measured the tibial proximal anatomic axis in order to
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determine the angle of the posterior tibial slope. The lateral posterior tibial slope in both men and
women was found to be higher than the medial slope in ACL injured subjects (p < 0.01).
Anterior translation of the tibia during knee flexion was greater on the lateral tibial plateau, and
patients with an intact ACL had greater medial posterior tibial slopes than lateral posterior tibial
slopes (14.8 ̊ compared to 11.8 ̊). A greater lateral posterior tibial slope could result in the femur
slipping posteriorly in relation to the tibia as a result of anterior tibial translation while using the
medial plateau as an axis of rotation, resulting in injury.14
MRI studies used by Shultz et al.6 showed greater posterior-inferior tibial slopes on the lateral
side of the knee, with reduced condylar depth of the medial plateau. The increased posterior
tibial slope on the lateral side is thought to cause greater anterior joint reaction forces and greater
anterior translation of the tibia, causing increased ACL strain. They found that females have a
greater posterior slope than males, a finding that is disagreed upon in a study using all
noncontact ACL injuries that were treated operatively at the U.S. Military Academy from 2004
to 2007.13 To determine the significance of posterior tibial slope in predicting ACL injury, Todd
et al.13 digitally measured the posterior tibial slope of 140 patients with noncontact ACL injuries
and compared them to a control group made up of 179 people. With genders combined into one
group, the injured population had a significantly higher posterior tibial slope of 9.39 ̊ ± 2.6 ̊ when
compared to the control group, which had a measurement of 8.50 ̊ ± 2.67 ̊. The trend was also
observed in each gender independently, but the data was only significant in the female group,
with a posterior tibial slope of 9.8 ̊ ± 2.6 ̊ in the injured group compared with 8.20 ̊ ± 2.4 ̊. No
statistical differences were found when male and female slopes were compared. Todd et al.13
acknowledges that this study had limitations, including a small female cohort size. Conflict
remains in the literature over the anatomical differences in posterior tibial slope between males
and females. Even with evidence that females have greater posterior tibial slopes than males, it is
uncertain if this isolated structural difference results in increased risk of injury.
http://www.ijoonline.com/articles/2013/47/1/images/IndianJ
Orthop_2013_47_1_67_106910_f6.jpg
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Figure 3. Posterior tibial slope
Q-angle
The quadriceps angle is derived by the intersection of a line from the center of the patella to the
anterior-superior iliac spine of the pelvis and the line from the patella to the tibial tubercle.
Following puberty, women have several proximal lower extremity structural differences
including greater pelvic tilt and quadriceps angles.6,15 The assumption is that increased Q-angle
results in greater frontal plane forces acting on the patellofemoral joint, resulting in increased
knee valgus and greater pressure on the lateral knee.15 This is thought to increase the risk of knee
injury in females. However, Shultz et al.6 do not believe that there is enough significant evidence
showing an increase in risk of ACL injury due to differences such as the quadriceps angle.
Quadriceps angle is particularly interesting because prior to puberty, females do not demonstrate
greater Q-angles than males. Therefore, an increase in Q-angle is most likely a result of the
widening of the female pelvis due to hormonal differences in women after puberty. While there
is not enough convincing evidence to identify Q-angle size as a significant risk factor for women,
it does bring attention to the interplay between structural, hormonal, and mechanical differences
between women and men. Several structural differences may be due to hormonal changes in
women, and what seems to be a structural issue may actually be a mechanical deficiency. The
greater knee valgus potentially caused by Q-angle may be explained by weak muscles
surrounding the hip or the physics of female landing patterns. When studying structural factors, it
is beneficial to understand the difficulties in altering these structural risk factors. While an
understanding of structural differences is important, hormones and mechanics must be
researched in depth to determine which risk factors can be altered.
HORMONAL FACTORS
Evidence substantiates that males and females display similar mechanics until puberty, after
which females have an increase in knee abduction angles, frontal plane motion at the knee, joint
laxity, and active joint stiffness.6,7 The timing of these changes correlates with the increase in
occurrence of ACL injury among females. Because of this, hormones may influence an increase
in ACL injury risk during certain phases of the menstrual cycle due to a variation in hormone
levels.16
Hormones
Receptors for sex hormones such as estrogen have been found in the human ACL and some
skeletal muscle.6,17,18 Therefore, hormones could have a direct effect on the composition of the
ligament and surrounding muscles. Estrogen, relaxin, and progesterone, have been studied to
examine the effects of hormones on the ACL.
Estrogen has been found to depress collagen synthesis and fibroblast proliferation in certain
ligaments in the human body.3,7 Since estrogen receptors have been found in the fibroblasts of
the human ACL,18 it is very likely that during the menstrual cycle, estrogen levels directly affect
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the composition of the ACL. This includes direct regulatory effects on fibroblast function,
collagen remodeling, and alteration of the structural and mechanical structures of the ACL.18 The
decrease in fibroblast functioning and collagen production causes a greater degree of knee joint
laxity, which may contribute to injury and instability. While the evidence for estrogen’s effects
on the ACL is well-supported, it is important to note that Abate et al.7 states that many studies on
estrogen use tissue cultures that do not take into account environmental factors that may override
the effects of estrogen. More research is needed to determine if estrogen’s effects on ligament
composition are enough to override the protective mechanical movements of the human body.
There may be a negative relationship between estrogen levels and muscle stiffness.16,19 In a study
to determine the effects of estrogen on muscle stiffness in the hamstrings, males and females
were tested using isomeric contractions that were not weight bearing.19 Female participants were
required to be free from oral contraceptives during the 6 months preceding the study, have no
history of pregnancy, and have a self-reported consistent menstrual cycle for at least 3 months.
The male and female groups were both required to have no history of musculoskeletal injury in
the lower extremities within 6 months of the study. The female subjects were tested during the
follicular phase of the menstrual cycle because the pre-ovulatory phase (i.e. end of follicular
phase) is thought to be the phase in which there is highest risk of ACL injury in females. The
males and females were tested using isometric contractions that were non-weight bearing.
Females were found to have a negative correlation between estrogen and muscle stiffness (r = 0.52, p = 0.05), but men had no significant correlation between estrogen levels and muscle
stiffness. While this study adequately measured the level of muscle stiffness in the subjects,
further studies using weight-bearing positions are warranted to imitate realistic situations in
which injury would be more likely to occur.
Relaxin changes the mechanical properties of connective tissues by lowering collagen levels.7
Specific receptors for relaxin have been found in female tissues, but not male tissues. In a study
reviewed by Abate et al.,7 guinea pigs had weaker ACLs during load to failure after 21 days of
treatment to increase relaxin levels. However, there was selectivity in how relaxin affected
tissues, and although relaxin concentration was greatest during the luteal phase, the differences
in levels across the menstrual cycle were relatively small. It also seems unlikely that changes in
the composition of the ACL were able to keep pace with monthly relaxin variations.7 It was more
likely that prolonged high relaxin serum levels in certain females contributed to increased joint
laxity.
Progesterone may cause an increase in muscle stiffness.16 Park et al.16 hypothesized that
progesterone would influence knee laxity and stiffness. They tested 26 female subjects who
regularly participated in sports and had regular menstrual cycles for 24 months leading up to the
study. The subjects also had not used oral contraceptives for at least 6 months prior to the study.
Each subject was tested for hormone levels and joint laxity during the follicular, ovulatory, and
luteal phases using blood draws and a knee arthrometer. The study found a correlation between
increased muscle stiffness and progesterone levels (p < .042). Bell et al.19 only tested their
subjects during one phase of the menstrual cycle, and the menstrual cycles were self-reported by
the subjects. This study acknowledged that further research is needed to examine the effects of
progesterone on muscle stiffness, as any increase in muscle stiffness observed could be the result
of other hormones or factors.
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Joint Laxity across the Menstrual Cycle
Park et al.16 acknowledged that greater joint laxity at the knee in combination with a quadriceps
dominant landing pattern is most likely related to the degree of ACL injury risk, and may also
explain why some females have greater injury risk than others. Variations in sex hormones
across the cycle cause changes in collagen metabolism and production, knee joint laxity, and
muscle stiffness.6 There are many opinions on the exact effects that hormones have on joint
laxity, and it is further complicated by the variability of the menstrual cycle in individual
women. Shultz et al.6 stated that women generally have a greater magnitude of joint laxity,
which may be explained by hormones. Some women experience up to 50-77% decrease in knee
stiffness during certain phases of the menstrual cycle.16
Joint laxity appears to be greater during the pre-ovulatory or ovulatory phase of the menstrual
cycle. Some studies state that it is the pre-ovulatory phase in which joint laxity is the greatest,
while others assert that it is the ovulatory phase.2,6,16 There are variations in the amount of
hormones found during each phase of the menstrual cycle depending on the individual and that
more research needs to be conducted. Some investigators hypothesized that knee joint laxity is
greatest during the pre-ovulatory phase, but found that the majority of articles reviewed stated
that it was the ovulatory phase.2 They only found 13 articles that met the criteria to be used in
their review, and 8 out 13 found significant evidence for laxity changes across the menstrual
cycle. This is a small amount of research on the subject, and more may be needed to make an
accurate consensus statement. They suggest that future studies should examine subjects over a
longer period of time, as many studies that are currently available only examine a subject 1 to 3
times during 1 menstrual cycle. Park et al.16 conducted blood draws during their study to test
hormone levels, which increases the validity of their results. However, they acknowledge a
possible delay of hormones’ effects on the knee of about 4 to 5 days in addition to a large
amount of individual variability of the degrees of change in knee laxity in response to hormone
changes. Although there is discrepancy, the general belief is that higher estrogen levels
contribute to joint laxity because of its effects on collagen production and fibroblast
proliferation. Estrogen is thought to be the most influential sex hormone in causing changes in
joint laxity.16 Progesterone levels have been found to peak during the luteal phase of the
menstrual cycle, therefore increasing muscle stiffness during that phase.16 More research is
needed in order to determine if hormones are capable of influencing muscle stiffness enough to
significantly affect joint stability.19
Most researchers agree that more studies are needed to pinpoint a specific menstrual cycle phase
as the greatest time of risk for injury, but some researchers are of the opinion that providing
conclusive data on the subject of joint laxity across the menstrual cycle is nearly impossible due
to large individual differences in cycle and phase length.7,18 Vescovi et al.18 suggest that
menstrual cycle variability is actually quite common, with one in five women having cycle
variability of at least 14 days. In addition, when prospective cycles are compared with past
cycles reported, it seems that more women report having regular cycles than actually have them.
This makes it difficult to accurately pinpoint which phase of the menstrual cycle a woman is in at
any particular time, and also explains one possible reason for several studies not finding
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significant results. Studies must further consider the amount of menstrual cycle variability in
each subject. Methods of measurement are faulty and the populations in most studies are
generally uncontrolled due to cycle variations.16 Vescovi et al.18 articulate that it is very possible
that there is a delay in the effect that hormones have on knee joint laxity, so determining how
hormone levels effect joint laxity is further complicated, as it may take up to a few days to
observe changes. For each individual subject, body composition, menstrual history, genetic
factors, and activity levels must be taken into consideration in addition to the possible menstrual
cycle variations.16 The immense amount of variability found in most females complicates the
research to a large extent. More accurate methods for determining phases of the menstrual cycle
and hormone levels are needed before clarity can be gained. These methods include testing
hormone levels with blood tests and determining exact menstrual cycles throughout the study
instead of relying on self-reporting of past menstrual cycles. Even if consistent results are found,
there is little that can be changed in order to minimize hormonal risk factors. However, it is
helpful for women to be aware of how their menstrual cycle affects joint laxity so that they are
self-aware and mindful of precautions.
Contraceptives
Women taking contraceptives may experience less of estrogen’s effects on knee joint laxity due
to different progestins that counteract estrogen.6 Hormones of women taking contraceptives are
also more stable throughout the menstrual cycle than those of women not taking contraceptives
due to constant levels of estrogen and progesterone in the blood provided by the contraceptives.
While contraceptives logically should decrease joint laxity, there is little evidence that
contraceptives are protective.6,7 Some studies reported a lower rate of ACL lesions in women
who use contraceptives when compared to those who do not use contraceptives, while other
studies found no significant evidence.7 It is difficult to determine the effectiveness of
contraceptives in preventing ACL injury due to the variety of contraceptives available.6,7 The
potency of progestin in each contraceptive is important in determining the effectiveness. A better
understanding of how different progestins influence the ACL and surrounding tissues is needed.
While Belanger et al.2 found that contraceptive use combined with neuromuscular training
decreased joint instability, these results may be solely due to the neuromuscular training.
It is difficult to predict how contraceptive use could be integrated into ACL injury prevention. It
is unlikely that physicians would prescribe contraceptives to young female athletes for the
purpose of injury prevention, so the protective effect would most likely be limited to females
already taking contraceptives for other purposes. However, it is necessary to research
contraceptive use further, as it is one of the only ways to possibly lower the risk of ACL injury
risk in females due to hormonal factors.
MECHANICAL FACTORS
Females display several mechanical deficits that may contribute to ACL injury. In order for the
knee to have proper mechanics, the surrounding muscles and joints, such as the hip and ankle,
must be functioning properly to decrease strain on the knee joint and ligaments. Females interact
with their environment in a way that predisposes them to greater risk of injury. While the
The Epidemic of Anterior Cruciate Ligament Injury in Female Athletes
14
following risk factors are not exclusive to females, they experience significantly more
mechanical dysfunction than males, putting them at greater risk for injury.
Neuromuscular Control Deficits
Because research supports that neuromuscular training can decrease the risk of ACL injury,
neuromuscular control deficits must be a primary risk factor for ACL injury.1 Neuromuscular
deficiencies more frequently manifested in women than men include decreased dynamic
stabilization, decreased ability to absorb large forces, and slower muscle recruitment.7 Men are
also able to voluntarily increase muscle stiffness to a greater degree than women.3 Most research
conducted on neuromuscular control in relation to ACL injuries has been focused on ligament
dominance and quadriceps dominance found in women. These imbalances in combination with
landing in greater extension than men are possible risk factors for injury.
Ligament Dominance
Ligament dominance is characterized by use of anatomic stabilizers, such as bony configuration
and articular cartilage, and static stabilizers (ligaments) to absorb ground reaction forces.20
Females tend to rely too much on ligamentous support because of delayed recruitment of
muscles surrounding the knee joint.21 Ligament dominance is due to momentary neuromuscular
delay which is not conscious nor voluntary.22 Because the active restraint that muscles provide is
absent or insufficient, the joint must rely on the passive restraints. Pollard et al.21 hypothesized
that females also have poor strength in sagittal plane musculature. Ligament dominance is a
result of this weak musculature, as the weak muscles cause the athlete to rely more on passive
restraints. During plant-and-cut maneuvers or jump landing, females put excessive stress on
ligaments prior to muscular activation.1 Ligament dominance is an ineffective, inefficient and
hazardous means by which to cope with ground reaction forces as compared to utilizing
muscular control.1
One possible cause of ligament dominance is the tendency of females to limit the amount of hip
and knee flexion during jump landing.21 This faulty mechanic results in improper stress
management at the joint and leads to increased valgus motion at the knee, increased force on the
knee, and high torque at the knee and ACL. There is an inability to control frontal plane motion
at the knee during jump landing and cutting movements.23 High amounts of force in a short
period of time may cause ligament rupture.20 Hewett et al.20 state that the most important muscles
in the prevention of ligament dominance are the gluteal muscles, hamstrings, gastrocnemius, and
soleus. If activated properly, these large muscles can absorb ground reaction forces through
eccentric decelerating contractions to prevent excess strain on the ACL and other ligaments of
the knee.
Quadriceps Dominance
Women land with more leg extension and greater quadriceps activation, causing lower hamstring
to quadriceps torque ratios, high angular velocities, increased knee abduction, and greater
movement in the frontal plane.7 Hewett et al.1 have found that females generally prefer to
activate knee extensors faster than knee flexors, resulting in a more extended landing position.
The Epidemic of Anterior Cruciate Ligament Injury in Female Athletes
15
This is due to slower neuromuscular signaling to the hamstrings. Since the hamstrings partly
function to resist anterior tibial translation, quadriceps dominance may increase anterior tibial
translation, which is a likely mechanism of ACL injury.17 Predominantly using the quadriceps to
stabilize the joint induces an anterior shear stress to the tibia and ACL.20 In response to anterior
tibial translation, women often contract the quadriceps disproportionately to the hamstrings.17,20
It is necessary for anterior tibial shear force to be countered by enough posterior tibial shear
force, which is provided by the hamstrings.17 Disproportionate activation of the quadriceps and
hamstrings allows more frontal plane motion than adequate hamstring contraction. One reason
for the increased frontal plane motion is the anatomy of the quadriceps muscles. The quadriceps
insert at a single anterior site at the knee joint, which does not allow for adequate control over
frontal plane movement.20 In comparison, the hamstrings are more adequate for reducing frontal
plane motion, as they have medial and lateral insertions at the knee. Powers et al.15 found that
landing from a jump with the trunk flexed results in 28% less quadriceps activation when
compared to landing with the trunk more erect.
In a systematic review, Benjaminse et al.24 did not find quadriceps dominance in females when
compared to men. They determined that there is questionable clinical relevance for many
biomechanical factors, including quadriceps dominance. Specifically, they determined that
quadriceps dominance was not found in women during plant and cutting maneuvers. However,
they acknowledge that lack of clinical relevance found in their analysis could be due to the
heterogeneous nature of articles as well as the low sample sizes used in the studies. The studies
used different motion analysis systems and different tasks to determine risk factors, which makes
it difficult to quantify data from these studies into significant statistics. Small differences were
found between genders, but the differences were not statistically significant. However, some of
the articles are 20 years old, and due to this lack of current research, the results found by
Benjaminse et al.24 are most likely not valid.
Quadriceps dominance is closely related to ligament dominance. The hamstrings aid the ACL in
pulling the tibia posteriorly, therefore decreasing strain on the ACL.20 When the quadriceps are
activated disproportionally to the hamstrings, an increase in ligament dominance will occur. It is
beneficial to primarily activate the hamstrings in order to reduce strain on the ACL.
Females exhibit landing in a greater degree of extension than males. Pollard et al.21 conducted a
study on 58 healthy female club soccer players with no history of ACL injury. Markers were
placed on multiple anatomical structures to measure movement, specifically hip and knee
flexion. Three trials of drop landing tasks were conducted to determine the amount of knee
flexion and resulting movements upon landing. Subjects with combined hip and knee flexion
angles greater than 170 ̊ were assigned to the high flexion group, and subjects with hip and knee
flexion angles lower than 170 ̊ were assigned to the low flexion group. The low flexion group
had average knee adductor moments that were 2.2 times greater than subjects in the high flexion
landing group. They also demonstrated greater peak knee valgus angles and greater frontal plane
loading, with increased knee extensor moments and decreased hip extensor moments. It was
found that hip and knee extensors are capable of absorbing 50% more energy during a soft
(flexed) land than during a stiff land. Since the hip and knee extensors do not absorb as much
force in an extended landing, the strain on the ACL and other ligaments is greater than when the
knee is in flexion upon landing.
The Epidemic of Anterior Cruciate Ligament Injury in Female Athletes
16
Valgus Collapse
Valgus collapse, a commonly proposed mechanism of ACL injury, is the combination of valgus
movement at the knee and anterior shear force on the ACL. It takes into account the multiplanar
nature of most ACL injuries. Females are known to exhibit higher knee valgus angles than males
in stop jump and drop landing tasks, making the likelihood of valgus collapse higher than in
males.25
Torry et al.25 conducted a study in which they observed bone movement directly. Contrary to
expected results, they did not find that high knee valgus caused peak anterior tibial translation,
lateral tibial translation, or medial tibial translation during drop landing tasks. One possible
reason for this is the ability of soft tissues to restrain tibial translation.25 It is also possible that
knee valgus observed visually may be different than the amount of knee valgus happening at the
joint level. The investigators theorize that it is unlikely that healthy individuals can reach the
high valgus angles found in laboratories.25 Benjaminse et al.24 agree, stating that 94-Nm valgus
load is required to tear the ACL, and the highest possible number found in women is 47.94-Nm.
However, this needs to be tested in vivo, as these numbers were gained from testing in vitro. Due
to the lack of tibial translation found, it is possible that knee valgus may be a secondary indicator
of risk.25
Ground Reaction Forces
Newton’s third law states that, for every force, there is an equal and opposite force. When one
performs a stop jump or landing, the force on the ground by the body is opposed by a force on
the body by the ground. Females have a greater tendency to allow ground reaction forces to
control the direction of the lower extremities, especially at the knee joint.1 This is due to the
many mechanical risk factors that females display. Lack of neuromuscular control, hip weakness,
and excessive trunk movement can all cause a change in the ground reaction force vector.
Ground reaction forces are generated in the vertical and posterior direction during jump landing,
which tends to produce knee flexion upon landing.26 When the female athlete lands in too much
extension, ground reaction forces are more vertical.6 The most common cause of ground reaction
forces controlling female movement is weak muscles surrounding the knee joint and hip.20 When
the muscles do not sufficiently absorb ground reaction forces, the joints and ligaments must
dissipate the ground reaction forces instead. The ligaments are forced to absorb large amounts of
force over a very brief period of time, which increases the amount of strain on the ligaments.
Because the ligaments are not as substantial as the surrounding muscles, it is more difficult to
control movement using only these restraints. Therefore, muscles not strong enough to overcome
the mechanical disadvantage so often seen in landing females allow for increased frontal plane
motion at the knee.
When the trunk has too much movement, the center of mass moves away from being directly
above the lower extremity limbs. This results in a greater distance between the ground reaction
forces and the knee joint center, increasing frontal plane movement at the knee.15 In addition to
The Epidemic of Anterior Cruciate Ligament Injury in Female Athletes
17
frontal plane trunk movement, the amount of flexion at the trunk during a drop landing may
significantly influence the magnitude of ground reaction forces and quadriceps activation at the
knee. Blackburn et al.26 performed a study in which they tested the effects of trunk flexion on
quadriceps activation and ground reaction forces in 20 females and 20 male. Each subject
performed two drop landings, the first in a preferred landing pattern and the second in an actively
flexed landing pattern. Lower vertical ground reaction force magnitude and quadriceps activation
were found during the landing with the trunk flexed. There were also changes in posterior ground
reaction forces, but they were not statistically significant. Trunk flexion may improve the ability
of the lower extremities to absorb landing forces using the musculature of the legs. More
research is needed to determine the minimal change in trunk flexion necessary to significantly
decrease the effect of ground reaction force on the knee joint so that it may be implemented into
prevention. Ground reaction forces may play a significant role in increasing quadriceps
activation and other risk factors associated with increased ACL injury in women.
Proximal Dysfunction
An important risk factor for injury is proximal dysfunction, in which the hips or trunk have
deficiencies that affect the knee joint. Females tend to have greater movement at the trunk and
weaker muscles at the hip that may contribute to ACL injury. Ignoring proximal risk factors and
isolating the knee joint causes researchers to have an incomplete understanding of the way the
knee joint and other parts of the body interact with each other. A complete understanding that
includes assessing the hip and trunk for weakness is more useful for prevention of the injury.
Diminished hip muscle strength has been associated with high valgus moments at the knee.
Specifically, higher knee valgus angles and moments may be a result of insufficient use of the
hip extensors to function as controllers of motion to decelerate the body’s center of mass.21 In the
study performed by Pollard et al.,21 measuring hip and knee flexion during a drop landing task
resulted in a correlation between low hip flexion and decreased energy absorption at the knee
and hip, as well as a decrease in hip extensor moments. Hip extensor weakness may be
contributing to a low flexion landing strategy. Weakness of the hip extensors also resulted in
increased use of the knee extensors (quadriceps). In addition, individuals who are unable to
adequately use their hip extensors eccentrically experience an increase in frontal plane motion at
the knee. This causes an increase in knee valgus angles and knee adductor moments, which are
known to increase risk for ACL injury. The connection between weak hip extensors and many of
the risk factors associated with female ACL injury makes it essential to understand the role the
hip plays in protection of the knee.
An additional study hypothesized that females who relied more on hip extensors than knee
extensors in order to decelerate movement lowered their risk of ACL injury.15 Powers15 found
that increased use of hip extensors to absorb shock resulted in lower knee valgus angles and a
53% reduction in the average knee valgus moment during landing. While this study focuses on
hip extensors instead of knee and hip flexion, the focus of the study is to increase use of
proximal muscle control to lower strain on the ACL. Both studies addressed the need for hip and
trunk muscle stability in prevention of ACL injury.1,15
The Epidemic of Anterior Cruciate Ligament Injury in Female Athletes
18
It is established that abnormal hip functioning has a direct effect on the knee joint.15 Abnormal
motion at the femur affects kinematics at the knee joint and strains soft tissues, such as
ligaments, that surround the joint. The knee joint center will move medially relative to the foot
when an athlete displays excessive femoral adduction and internal rotation. Knee valgus and
diminished hip strength have been shown to be related. Insufficient utilization of hip extensors to
decelerate the center of mass results in higher knee valgus.15,21 Females who better rely on their
hip extensors during jump landing and other impact forces had lower knee valgus angles and a
reduction in average knee valgus moment by 53%.15 While stronger hip extensors allowed for
limiting frontal plane motion at the knee, there was not an increase in hamstring activation.21 Due
to this finding, it is implied that the gluteus maximus may have a greater role in contributing to
high flexion landing, as activating the gluteus maximus does allow an increase in hamstring
activation.21 The gluteus maximus is the best muscle to provide triplanar stability to the hip. It
may help unload the knee by decreasing the need for excessive quadriceps contraction.15 Because
weak hip abductors also place additional strain on the knee joint, improving hip abductor
strength, including the gluteus medius and gluteus minimus, would contribute to optimal
alignment of the pelvis and protect the knee from excessive frontal plane movement.
Hashemi et al.26 suggest an alternative mechanism for ACL injury, citing simultaneous knee
flexion and hip extension as a possible mechanism. They assert that this mechanism explains
many confounding issues surrounding the mechanics of ACL injury. Ideally, the knee and hip
flex together and produce low anterior tibial translation, keeping the load on the ACL low.
However, with delayed co-activation of the quadriceps and hamstrings, the trunk may be upright
upon jump landing, causing the center of mass to be positioned posterior to the knee. This will
increase ground reaction forces, leading to further knee flexion while the hips are extended. The
hips may be influenced into more extension depending on the location of ground reaction forces.
This combination of knee flexion and hip extension may lead to large amounts of anterior tibial
translation and shear forces, increasing the likelihood of ACL injury.
The proposed mechanism above is different than existing theories in multiple ways. First,
Hashemi et al.26 state that ground reaction forces are the actual cause of injury, not excessive
muscle forces or torques. They believe that their theory connects external variables to the
structure of the knee. A clear initial event causing the injury is found in a delay in co-activation
of the quadriceps and hamstrings. The mechanism of injury only requires the time of delay in
muscular activation, so it fits within the typical time for ACL injury to occur after landing. They
state that the contribution of passive knee laxity to injury is explained because it causes the
athlete to be more susceptible to anterior tibial translation. They also believe that it explains why
valgus collapse occurs, citing valgus collapse as merely a result of injury. While the mechanism
proposed is plausible, the investigators did not account for triplanar movement.
Anterior tibial translation is highly unlikely to be sufficient on its own to cause ACL rupture. It
would be beneficial for the authors to expand on how this mechanism would contribute to
rotational torque at the knee. Ground reaction forces contribute to injury, but excessive muscle
forces and torques must be taken into account. The authors present a provocative challenge to
conventional thinking, but the mechanism should be researched further.
The Epidemic of Anterior Cruciate Ligament Injury in Female Athletes
19
Another cause of proximal dysfunction is excessive trunk movement. Coordination imbalances
and lateral trunk displacements have been found to be greater in females than males.7 The term
trunk dominance refers to the trunk displaying excessive motion that is directed by inertia rather
than controlled by core muscles.1 The orientation of the trunk in the sagittal plane may influence
the muscular function of the lower extremity.15 Medial and lateral movements at the trunk will
influence frontal plane movement at the knee due to the movement of the center of mass and
ground reaction forces.15 A posterior trunk lean minimizes the demand on hip extensors by
reducing the hip flexion moment, but this would increase the demand on the quadriceps, placing
strain on the ACL.15 The overwhelming evidence suggests that the hips and trunk have
significant influence on knee kinematics. Focusing on proximal risk factors may be very
effective in preventing and treating ACL injury.
INTERVENTIONS
Although structural and hormonal risk factors are difficult to alter, some mechanical risk factors
can be successfully mitigated through pre-habilitation programs. Research thus far has confirmed
that most neuromuscular training programs are successful at decreasing biomechanical
deficiencies associated with increased risk of ACL injury in females. Benefits gained from
neuromuscular training include increased strength, power, and coordination.27 This evidence
suggests that there will be a decrease in ACL injury rates among female athletes, but more longterm studies are needed to determine the actual amount of risk reduction in female subjects.
Assessing for Risk
A complication of assessing for ACL injury risk in females is that most current methods require
expensive laboratories and advanced equipment to make measurements.23 Most studies
conducted assess risk using these methods. While these methods are valuable for furthering basic
science findings, they are not practical for coaches or trainers who do not have access to the
same labs and equipment.20 Therefore, the tuck jump assessment has become a simple,
inexpensive method to assess multiple mechanical risk factors associated with ACL injury.20,23
Myer et al.23 state that the most effective way to use the tuck jump is to have the athlete perform
consecutive tuck jumps for 10 seconds. The athlete jumps repeatedly as high as she can with the
hips and knees flexing toward the trunk during the jump (Figure 4). The landing assessment can
be used to determine if the athlete displays ligament dominance, quadriceps dominance, leg
dominance, or core dysfunction.23 Ligament dominance is evident when there is lack of motion
control in the frontal plane (Figure 5). The athlete will display excessive frontal plane motion
when landing in knee valgus. Quadriceps dominance will result in the athlete landing in
excessive extension, demonstrating large recruitment of the quadriceps muscles (Figure 6). Leg
dominance is indicated by a difference in knee flexion angles, maximum knee valgus angles, or
feet landing in a non-parallel pattern (Figure 7). Pausing between jumps or not landing in the
same footprint each time may be indicative of core dysfunction. In order to quantify results,
Myer et al.23 suggested taking still photographs at several points during the jump and
determining whether the athlete is deficient in the different components (Table 1). Athletes
exhibiting one or more of the criteria for a dysfunction may be at risk due to that mechanical risk
factor.
The Epidemic of Anterior Cruciate Ligament Injury in Female Athletes
20
The Tuck Jump Assessment (TJA) is an economical assessment tool for coaches, athletic
trainers, fitness specialists, and healthcare professionals to use in determining possible risk for
ACL injury. The tuck jump is an important tool in assessing several risk factors for injury at
once, and the simplicity of the exercise allows for practical use in multiple athletic environments.
The standardization of the assessment is useful in providing a simple tool for determining risk,
but standardization may also oversimplify the determination of risk factors. The tuck jump
should be used as a test and be incorporated in a comprehensive ACL injury prevention program.
Table 1. Tuck Jump Diagnostic for Kinetic Chain Dysfunction
DYSFUNCTION
Ligament dominance
Quadriceps dominance
Leg Dominance
Trunk dominance (core dysfunction)
CRITERIA
Lower extremity valgus at landing
Foot placement not shoulder width apart
Excessive landing contact noise
Landing in extension
Femur alignment not equal during flight
Foot placement not parallel
Unequal foot contact timing
Thighs not parallel at peak of jump
Pause between jumps
Does not land in same footprint (drifts)
Figure 4. Tuck Jump Assessment (TJA)
The Epidemic of Anterior Cruciate Ligament Injury in Female Athletes
Figure 5. TJA – ligament dominance
Figure 6. TJA – quadriceps dominance
21
The Epidemic of Anterior Cruciate Ligament Injury in Female Athletes
22
Figure 7. TJA – leg dominance
Components of Prevention Programs
Many studies have shown the value of prevention movement training as prophylaxis specific to
ACL sprain prevention and which incorporate multiple components.1,15, 20,21,28,29 Neuromuscular
training is a common component in most programs, and has been proven to be very effective in
decreasing evidence of risk factors in individual athletes.
Hewett and Johnson1 address several components believed to be essential to successful
prevention programs for ACL injury. The first component is a dynamic warm-up period, which
is followed by plyometrics and jump training focusing on posture and control by strengthening
the hip and trunk muscles. Stronger hip and trunk muscles result in an increased ability to
attenuate large forces without placing excessive strain on the knee joint. Third, strength training
is conducted, focusing on the trunk and lower extremity. Sports specific skill components are
needed to implement proper technique learned during plyometrics, jump training, and strength
training into the sport that the athlete plays. Pre-season and in-season training programs are both
needed in order to have more effective prevention throughout the season. This program is
thought to prevent ligament, quadriceps, leg, and trunk dominance.1
Plyometric exercises that put the athlete in a position of 90 ̊ flexion at the knee and hip are
especially effective to decrease the amount of quadriceps dominance and ligament dominance
experienced by the athlete. Russian hamstring curls play a similar role in decreasing the need for
quadriceps compensation by strengthening the hamstring muscles. Exercises that enhance
recruitment of the posterior chain muscles while requiring core activation are most effective in
addressing multiple deficits at once.20 Single leg activities could be used to restore symmetry
between lower limbs. The strength of this program is that it used multiple components to ensure
well-rounded training for each athlete. The challenge would be to implement this program into
practices so that it does not require excessive outside time for the athletes.
In a study conducted by Lim et al.,28 a similar program was used in a controlled laboratory study
with 22 subjects. The 22 high school female basketball players were divided into 2 equal groups,
The Epidemic of Anterior Cruciate Ligament Injury in Female Athletes
23
with one group performing a prevention training program for the first 20 minutes of practice for
8 weeks. The other group participated in practice without the training program. A jumping task
with force plate landing was performed by both groups before and after the 8 weeks of training
to assess the effects of the prevention training program. The program consisted of 6 parts, similar
to the components listed by Hewett and Johnson.1 The 6 parts were warm-up, stretching,
strengthening, plyometrics, agilities, and warm-down. During pre-testing and post-testing,
reflective markers were placed at several anatomical structures on each athlete to measure
specific components of the jump. These reflective markers, in addition to measurements taken
with the force plate, revealed that significant training effects were found on all strength
parameters. The trained group had higher knee flexion angles, greater interknee distances, lower
hamstring-quadriceps ratios, and lower maximum knee extension torques during the posttest than
during the pretest.28 While there were no significant differences between the trained and
untrained groups before the prevention training, there were significant differences between the
groups after training due to the above changes that the trained group displayed. The limitation of
this study is the small sample size. This study addresses the concern about using excessive time
outside of practice, as this program only required 20 minutes at the beginning of each practice to
complete. More research is needed to determine the correct length of the program. Eight weeks
seemed to be sufficient to create changes in mechanics, but it remains to be seen how long the
effects of training last.
Some prevention programs focus on increasing the use of hip musculature. This approach was
studied by Stearn and Powers29 in a study using 21 recreationally active women between the ages
of 18 and 25 years old. During pre-training assessments, the strength of knee extensors, hip
extensors, and hip abductors were measured. In addition, force plates were used during a doubleleg drop landing task in order to perform biomechanical testing. After the assessments, the
subjects underwent 12 training sessions in 4 weeks. They performed multiple plyometric
exercises along with balance exercises using a BOSU ball. There were 3 levels of exercises, and
the athlete had to complete one level with proficient technique before moving on to the next
level. During post training assessments, it was found that the subjects had significantly greater
peak knee flexion (p < .001) and greater hip flexion (p = .017). No significant change was found
in knee extensor moments, but the knee/hip extensor ratio was significantly lowered. This
indicates that hip extensor moments increased. While this program resulted in changes in
biomechanics consistent with a decrease in injury risk, it remains to be seen whether the actual
rate of injury decreases with the use of this prevention program. The study did not track ACL
injuries in the subjects after completion of the study. It was also noted that the improvements in
strength were most likely a result of neural change, as actual changes in muscle do not occur
until at least 6 to 8 weeks after beginning a program. Changes may have been more profound if
the program was extended to at least 8 weeks.
Pollard et al.21 studied the effects of limited hip and knee flexion in 58 female soccer players,
and found that individuals with low flexion landing patterns exhibit greater frontal plane loading
at the knee. It was concluded that these athletes would benefit from a prevention program
focusing on sagittal plane shock absorption through hip and knee flexion to prevent excessive
frontal plane movement. More research is needed to test this specific type of prevention program.
The Epidemic of Anterior Cruciate Ligament Injury in Female Athletes
24
Pelvis and trunk stability along with dynamic hip joint control are two aspects of prevention that
need to be incorporated into training program.15 Focus on strengthening the hip abductors may
result in more correct alignment of the pelvis, protecting the knee joint from excessive frontal
plane movements. Neuromuscular control of lower limb alignment, allowing for decreased knee
loads, may also occur as a result of hip abductor strengthening.27 Because the gluteus maximus
works in 3 planes and is a large muscle at the hip joint, strengthening the gluteus maximus would
also allow for unloading at the knee and decreased stress on the ligaments, as well as a decreased
need for quadriceps compensation.15
The above research has been implemented into a comprehensive prevention program meant to be
performed by individuals, physical education classes, and athletic teams. The program is brief
enough to easily implement into class or practice time, and it focuses on multiple mechanical
deficiencies found in female athletes. Ligament dominance, quadriceps dominance, leg
dominance, and core dysfunction are addressed in this program (Appendix A). A video of this
program is accessible at: https://www.youtube.com/watch?v=heLvgpKaBZ0&feature=youtu.be
Efficacy of Prevention Programs
Little research has been done in comparing the effectiveness of different prevention programs,
and the optimal length of a prevention program is unknown. However, there is a large body of
evidence for the effectiveness of consistent neuromuscular training in decreasing mechanical
deficiencies associated with increased risk of female ACL injury. Neuromuscular training is
believed to have a 50% efficacy rate in altering active knee joint stabilization in the laboratory.1
The program used by Hewett and Johnson1 effectively decreases quadriceps, ligament, leg, and
trunk dominance in females. A study conducted to determine which prevention programs are
most effective yielded inconclusive results.30 This is due to the complexities of each training
program, which makes isolating individual components of the programs difficult. In general,
protective effects were stronger in programs that required more training time per week than other
similar programs. There was no specific amount of time found to be most effective, but more
training time was generally better. Gagnier et al.30 concluded that many different types of
programs containing neuromuscular training are effective in prevention, and may reduce
incidence rate by an average of 50%.
Age at the initiation of prevention programs has been studied. The general belief is that early
implementation of prevention training is beneficial to the athlete.27 Myer et al.31 assessed several
articles that used different ages as subjects in order to determine which age responds to
prevention programs the best. The study found that subjects in their mid-teens showed the
greatest risk reduction (72%) when compared with late teens, who showed a 52% reduction in
injury risk. However, age effectiveness is complicated to measure because there are other
variables separating mid-teens from late teens. Younger adolescents tend to demonstrate greater
variability in skill level and amount of involvement in the sport they play than older adolescents.
Even with these other variables, it would be beneficial for parents, physical education teachers
and youth coaches to implement neuromuscular training into classes and practices as a tool of
prevention of injury. Teaching adolescents proper exercises and techniques may prevent injury
as the teen grows older and more involved in their sport. Instruction of proper technique and
The Epidemic of Anterior Cruciate Ligament Injury in Female Athletes
25
close supervision is key in educating adolescents to ensure that improper technique during
exercise is not hindering the effectiveness of the training.
Interpretation of the current research is confounded such that it is not clear which ACL injury
preventive program is most effective. Many programs reduce mechanical deficiencies believed to
increase ACL injury risk, but there are very few long-term studies examining the actual change
in injury rate over time.32 Transferring lab success to actual athletic practices and competition is
needed in order to benefit women. Future studies are needed to examine long-term effects of
training and to find more practical ways of implementing training into physical education and
sports practices. With younger adolescents being taught proper technique in sports combined
with neuromuscular training, it is extremely likely that decreases in female ACL injury risk may
be seen as the adolescents mature physically and in their sporting activities.
CADAVER DISSECTION
Lower limb cadaver dissection was performed in order to enhance learning of the knee anatomy
and the roles of the ligaments in knee joint stabilization. Skin incisions were made to remove the
skin from the thigh and leg to expose the superficial and deep fascia and muscles of the thigh and
leg. Before dissecting the knee joint, the anterior, medial, and posterior compartments of the
thigh were cleaned in order to appreciate the quadriceps muscles, sartorius, gracilis, and
hamstrings. The gastrocnemius in the posterior compartment of the leg and the tibialis anterior,
extensor hallucis longus, and extensor digitorum longus in the anterior compartment of the leg
were identified.
The rectus femoris, hamstrings, sartorius, and gracilis were transected to allow motion at the
joint. The amount of motion allowed at the knee once the quadriceps tendon was transected
indicated the considerable stability afforded the knee by this muscle group. The medial collateral
ligament and lateral collateral ligament were identified and the patella was reflected to reveal the
ACL, which had sutures, indicating a previous injury (Figure 8). Stress tests were performed to
determine the stability of the knee before any ligaments were transected. The lateral collateral
ligament was harvested, and stress tests were performed to determine stability loss in the sagittal,
frontal, and transverse planes. This process was repeated with the medial collateral ligament.
While there was an increase in frontal plane motion when each ligament was transected, the ACL
controlled anterior translation of the tibia. Surprisingly, there was little stability loss in terms of
rotation at the knee, indicating the importance of the ACL in preventing excessive rotation. The
stress tests performed allowed insight into the incredible capabilities of the ACL to provide
stabilization in the sagittal and transverse planes when other ligaments have been sacrificed.
To gain access to the PCL and popliteus, dissection on the arcuate complex was attempted. This
specimen had an especially thick arcuate complex, and little progress was made. The popliteus
was never revealed, but there was enough flexibility to allow cleaning of the PCL. When the
PCL was revealed, it was transected and stress tests were performed again to determine the
stabilizing force of the ACL. Without the PCL, the knee was much more unstable, but the intact
ACL still provided stabilization in the sagittal plane as tested by the Anterior Drawer Test.
The Epidemic of Anterior Cruciate Ligament Injury in Female Athletes
26
Figure 8. Cadaver dissection with ACL exposed
CONCLUSION
While ACL injury in females is an epidemic in its rate and morbidity, there are identifiable risk
factors and interventions that can be made to prevent female ACL injuries. Femoral notch size
and ACL size may not be significant structural risk factors in determining ACL injury risk.
Femoral notch size was shown to be a better predictor of injury in men than women. Since ACL
size may be related to femoral notch size, the lack of statistical significance in using femoral
notch size as a predictor indicates that ACL size may not be a reliable predictor either.
Regardless of notch and ligament size implications, they are not modifiable risk factors; thus,
research should be focused elsewhere. Structural risk factors that seem to be the most significant
in females are lateral posterior tibial slope angles and Q-angle, although Q-angle is due to a
change in hormones, which may actually be causing the increased risk of injury. The increased
lateral tibial slope angle may cause the femur to translate posteriorly in relationship to the tibia
(anterior tibial translation) and use the medial posterior tibial slope as an axis of rotation,
creating a rotational strain on the ACL. While structural factors cannot be prevented or changed,
they may be important identifiers for risk if an inexpensive measuring technique could be found.
Hormonal factors seem to be significant in predicting ACL injury, but determining which phase
of the menstrual cycle is associated with the most risk is difficult. It has been found that estrogen
increases joint laxity, but it remains to be seen if these increases significantly affect movement.
Relaxin also increases joint laxity, while progesterone decreases it by countering the effects of
estrogen. Although the roles of hormones have been determined, the phase of the menstrual cycle
that places females at the highest risk has not been determined. Due to inaccurate self-reporting
of menstrual cycles by the subject and inconsistent methods of measuring hormones, the preovulatory and ovulatory phases have both been cited as the phase of most risk. Contraceptives
theoretically should decrease joint laxity because of the constant hormone levels, but there is
little evidence to conclude that contraceptives effectively prevent injury. Hormonal risk factor
research should focus on accurate measurement of hormone levels and length of menstrual
The Epidemic of Anterior Cruciate Ligament Injury in Female Athletes
27
cycles in subjects. Studies should be performed in order to record hormonal fluctuations through
many menstrual cycles of each subject. However, even if the timing of the greatest hormonal risk
of injury could be determined, it is unlikely that the knowledge would prevent a significant
amount of injuries. Athletes practice sports throughout all phases of their menstrual cycles, and
contraceptives, even if proven effective in preventing injury, most likely will not be prescribed to
young adolescents for that reason alone.
The amount of mechanical risk factors and the complexity of each is daunting in interventional
terms, but determining the most significant modifiable risk factors in this category is the most
promising in terms of prevention. There is significant evidence suggesting that many of the
mechanical deficiencies found in women, such as ligament dominance, quadriceps dominance,
leg dominance, and trunk dominance can be mitigated through exercises focused on
strengthening the hips, trunk, and hamstrings. Developing prevention programs with a variety of
exercises designed to target these areas as well as neuromuscular training will be effective in
prevention of ACL injuries. There is considerable evidence that prevention programs rooted in
neuromuscular training with emphasis on plyometrics and agility development, jump training,
and strength training of the quadriceps, hamstrings, and hip and trunk musculature decrease the
incidence of several known mechanical risk factors. It is important to introduce these training
programs to adolescent females in order to train them with proper form before puberty to reduce
their exposure to injury across their active lives. If physical education teachers and coaches could
invest time into training young women to perform jump landings and plant-and-cut maneuvers
with proper technique while strengthening the muscles that are activated during these
movements, there would most likely be a decrease in ACL injuries in female athletes. While
females may always have a higher risk of ACL injury than males due to structures that cannot be
altered and hormones that cannot be changed, focusing on mechanical factors will likely bring
about a large improvement in the way female athletes perform and avoid anterior cruciate
ligament injury.
The Epidemic of Anterior Cruciate Ligament Injury in Female Athletes
28
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The Epidemic of Anterior Cruciate Ligament Injury in Female Athletes
Appendix A. Prevention Program for Female ACL injury
Figure 9. Component of dynamic warm-up – walking lunges
Figure 10. Plyometrics – agility ladder
31
The Epidemic of Anterior Cruciate Ligament Injury in Female Athletes
Figure 11. Jump training – box jump
Figure 12. Jump training – drop jump
32
The Epidemic of Anterior Cruciate Ligament Injury in Female Athletes
Figure 13. Plyometrics – balance training
Figure 14. Strength training – calf raises
33
The Epidemic of Anterior Cruciate Ligament Injury in Female Athletes
Figure 15. Strength training – hamstring curl with core stabilization
Figure 16. Strength training – squats
Figure 17. Strength training – hip abductor exercise with resistance
34