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Saint Xavier University

A review of the anatomy and current research on the female predisposition to noncontact anterior cruciate ligament injuries

Honors Project

Submitted

In Partial Fulfillment of the

Requirements of HONOR 352/53

and for Graduation with Honors

Spring 2015

By:

Suzanne Broski

Mentor:

Dr. Rudyard Sadleir

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Thesis written by

Suzanne Broski

Approved by

______, Mentor

Accepted by

______, Honors Program Director

Abstract

Adolescent female athletes represent the population that is most likely to suffer from noncontact anterior cruciate ligament (ACL) injuries, with a 4-6 times greater risk than their male counterparts. Following an explanation of the normal anatomy and functioning of the ACL and the mechanism of ACL injury, this paper explores possible causes of the gender disparity surrounding ACL injury rates. Specifically, research studies regarding the differences between male and female anatomical structure, biomechanics, neuromuscular functioning, genetics, and hormones are considered. Despite attempts to conclude a singular cause of the increased risk of noncontact ACL injury amongst female athletes, the gender disparity is likely multifactorial. Therefore, prevention programs implemented to reduce the risk of female ACL injury should target the modifiable risk factors which include biomechanical and neuromuscular functioning. Ideally, these programs should be started as soon as a female begins participating in athletics in order to avoid developing maladaptive techniques that may put her at risk of ACL injury.

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INTRODUCTION

The anterior cruciate ligament (ACL) is a ligament in the knee that serves a stabilizing function. In the United States, approximately 1 of every 3,500 individuals injures their ACL annually, and an estimated 125,000-200,000 ACL reconstructions are performed each year. Worldwide, an estimated one million ACL injuries occur every year (Noyes & Barber-Westin, 2012). ACL injury is very commonly suffered during athletic participation as a result of indirect force to the knee, such as sudden changes in direction or speed. The consequences of injury to the ACL are severe, especially for competitive athletes; these athletes may potentially miss an entire season or more of their sport, lose scholarship funding or wages, suffer psychologically, and/or experience future osteoarthritis. In addition to painful physical costs, the financial costs of ACL reconstruction and rehabilitation are staggering for the patient and the broader community. The average cost for ACL surgical repair is $38,121, which is independent of the expenses incurred in physician evaluation, radiography of the knee, or postoperative rehabilitation (Peterson & Krabak, 2014).

Studies suggest that most noncontact ACL injuries, or injuries that are not a result of a direct external force to the knee such as a football tackle, occur between ages 16 to 18. Females have a 4-6 times greater risk of ACL injury than their male counterparts participating in the same sport (Hewett, Shultz, & Griffin, 2007). As the number of female athletes continues to rise, the statistics regarding female ACL tears are especially alarming. Noncontact ACL injuries have been reported to be as high as one out of every 100 high school female athletes and one out of every 10 collegiate female athletes (Hewett, Shultz, & Griffin, 2007).

ANATOMY OF THE ACL

To better understand the causes of ACL injury, it is important to establish a full picture of the anatomy of the human knee. The knee joint is the middle joint of the hind limb and is the largest joint in the human body. It joins the femur (thigh bone) to the tibia (shin bone). The patella, or knee cap, articulates with the femur and protects the knee joint in the front. The knee has the ability to move with six degrees of freedom- three translations and three rotations. The translations of the knee include proximal-distal (towards the body vs. away from the body), anterior-posterior (front vs. back), and medial-lateral (side to side), and the rotations include internal-external, varus-valgus (bow-legged vs. knock-kneed), and flexion-extension. The muscles of the body pull on the bones of the skeleton to provide movement, and the primary muscles that control the movement of the knee joint are the quadriceps, located on the anterior surface of the thigh, and the hamstrings, located on the posterior surface of the thigh. In addition to the bones and muscles of the knee, there are four ligaments that join the bones of the knee and stabilize the joint. The lateral collateral ligament (LCL) and the medial collateral ligament (MCL) run along the outside and inside margins of the joint, respectively. Named by their alignment, the anterior cruciate ligament (ACL) and the posterior cruciate ligament (PCL) cross deep within the center of the joint in the shape of an X.

Figure 1. Knee Anatomy: This illustration depicts an anterior perspective of the right knee. The ACL is shown coursing anteriorly, medially, and distally from femur to tibia. Note the alignment in relation to the PCL (American Academy of Orthopedic Surgeons, 2009).

The ACL, the primary focus of this paper, is a band of dense regular connective tissue that connects the femur to the tibia. Specifically, the ligament originates at the medial side of the lateral femoral condyle and crosses obliquely through the intercondylar fossa to the medial tibial eminence where it is inserted. The tibial attachment is somewhat broader than the femoral attachment, with the ACL beginning to “fan out” in the proximal one-third of the ligament. As the ACL courses anteriorly, medially, and distally from femur to tibia, it turns in a slight lateral spiral giving it a twisting appearance (Jackson, 1993).

The ACL primarily resists anterior displacement of the tibia with respect to the femur and secondarily resists tibial rotation. However, the ACL does not function as a singular cord, but rather as two major fiber bundles, named according to the insertion sites on the tibia: the anteromedial (AM) bundle and the posterolateral (PL) bundle. The AM bundle acts as the primary tissue restraining anterior tibial translation; in other words, the AM bundle prevents the tibia from displacing anterior to the femur. The PL bundle stabilizes the knee at full extension to prevent internal and external rotation of the tibia. The difference in functionality between the two bundles results from varied tensions throughout the knee joint’s range of motion. With the knee extended, the AM bundle is moderately loose, and the PL bundle is tight. When the knee is flexed, the femoral attachment of the ACL has a horizontal orientation that causes the AM bundle to tighten and the PL bundle to relax (Petersen & Zantop, 2006).

BIOMECHANICS OF THE ACL: the pathway to INJURY

The ACL limits the anterior tibial translation, axial tibial rotation, and varus or valgus knee angulation. The ACL provides 87% of the total restraining force to anterior tibial translation when the knee is flexed at a 30° angle and 85% at a 90° angle (Noyes & Barber Westin, 2012). Without an intact ACL, stabilization of the knee is greatly reduced. Varying angles of knee flexion, knee constraint, and the magnitude and direction of the applied load can produce different forces in the ACL. Studies done by Berns et al. (1992), Markolf et al. (1995), and Fleming et al. (2001) all demonstrate that anterior shear force on the proximal end of the tibia increases ACL loading. Additionally, the studies show that knee valgus angulation and internal rotation of the tibia, in combination with anterior shear force at the proximal end of the tibia, increase the strain on the ACL. The quadriceps muscles are the primary contributors of anterior shear force on the proximal end of the tibia, while the hamstrings reduce ACL loading during contraction. Finally, the studies show that a decreasing knee flexion angle increases ACL loading (Hewett, Shultz, & Griffin, 2007).

An ACL injury can be categorized as a partial tear, a complete tear, or a bone avulsion. Partial tears are characterized as such when only a portion of the ACL fibers are damaged. A complete tear is defined by both the AM and PL bundles being completely severed. A bone avulsion occurs when the bony attachment area of the ACL gets pulled away from the rest of the bone. 70% of ACL tears are a result of noncontact forces such as cutting, pivoting, accelerating, decelerating, or landing from a jump. The forces that cause noncontact ACL injuries are a product of ground reaction forces and internal soft-tissue and muscles forces. The other 30% of ACL injuries are termed contact injuries, which result from a direct blow to the knee joint. Contact ACL injuries are commonly seen in football. Kiapour et al. (2014) identified a multi-planar mechanism of non-contact ACL injuries. Using human cadaveric tissue, the study indicated that a combination of anterior tibial translation, knee abduction (valgus moment), and internal tibial rotation leads to greater strain on ACL, and therefore, increases the risk for injury. The results also emphasized the significant role of anterior tibial translation and knee abduction as primary contributors and internal tibial rotation as a secondary contributor to the risk of ACL injury.

THE GENDER DISPARITY IN ACL INJURY RATES

The female athlete has a 4-6 times greater risk of injuring her ACL in comparison to the male athlete playing a sport with similar landing and cutting movements. The gender disparity associated with ACL injury is speculated to be multifactorial in nature. Proposed causes of gender differences in ACL injury rates include the effects of anatomical structure, biomechanics, neuromuscular functioning, genetics, and hormones. While many studies have attempted to look at these categories as separate entities, it is important to note the possible interplay between risk factors.

Anatomical differences

Naturally, an area of initial interest regarding the gender disparity of the rate of ACL injuries is the anatomical structure of the female ACL compared to the male ACL. The ACL passes through the intercondylar femoral notch and moves within it during motion of the knee joint. According to research by Shelbourne et al (1998) the rate of ACL injury is higher in people with more narrow notches (defined as <15 mm), regardless of gender. However, they also found that women, on average, have more narrow notches compared to men, which may account for the greater incidence of ACL injuries. Further, the study showed that following ACL reconstruction in which the new ACL size is standardized for both sexes, the higher rate of ACL tears in patients with narrower notches is eliminated. As a result, Shelbourne et al concluded that the width of the notch is not a causative factor of ACL tears, but a narrow notch does reflect a smaller ACL housed within which may affect the susceptibility to injury. This study did not go as far as to determine if a larger ACL (in terms of volume) represents a stronger ligament. Although Shelbourne et al. and other authors have found that, on average, females have smaller intercondylar notches even when accounting for smaller stature, various other studies prove this finding to be controversial. Lombardo et al. (2005) did not find a significant correlation between intercondylar notch width and ACL injuries, while LaPrade et al. (1994) found that athletes with a narrower notch were at increased risk of ACL injury but found no difference in notch width between genders. The controversies between studies seem to result from differences in study designs and methodologies, such as imaging and measurement techniques. Regardless, the equivocal results concerning the width of the intercondylar notch width with regards to female ACL injuries highlight that fact that anatomy is not the sole determinate in ACL tears.

Hypotheses relating to different anatomical structure are not limited to the knee joint specifically. Researchers have also investigated the role of the entire lower extremity in ACL injury. The femur meets the tibia at an angle termed the quadriceps femoris angle or “Q angle.” Clinically, it is defined as the angle in the frontal plane that is formed by the intersection of the line that connects the center of the patella to the anterior superior iliac spine and the line that connects the center of the patella to the tibial tubercle. In comparison to men, women not only have an increased pelvic width, but they also have a shorter femoral length; these anatomical differences result in females having an increased Q angle. According to studies by Zelisko et al. (1982) and Haycock and Gillette (1976), a larger Q angle results in greater force placed on the medial aspect of the knee, and essentially greater risk of ACL injury. Yet, studies by Gray et al. (1985) and Endsley et al. (2003) showed no correlation between Q angle measurements and injury rate. Thus, the results reveal that static anatomical measurements may not be the best predictors of ACL injury in females.

According to Boden et al. (2000), the female athlete has increased joint laxity compared to the male athlete. They reported that patients who suffered from ACL injuries demonstrated excessive extension at the knee joint and increased ability to touch the palms of their hands to the floor. Uhorchak et al. (2003) found that women with generalized joint laxity were at a 2.7 times greater risk of ACL injury than females without joint laxity. Joint laxity has been shown to increase knee hyperextension and knee valgus, which can put strain on the ACL. Boden et al. also reported that ACL injured athletes displayed more lax hamstring muscles. While males demonstrate decreased flexibility with age after puberty, females demonstrate increased flexibility after puberty. Studies by Hewett et al. (2006) and Huston and Wojtys (1996) suggest that a decrease in dynamic control of the knee in females could be partially caused by increased hamstring flexibility during and after puberty. However, the co-contraction of the quadriceps and the hamstring muscles needs to be considered to add more strength to this argument.

Biomechanical differences

Biomechanical risk factors of female ACL injuries have been areas of in-depth research because they are modifiable, as opposed to anatomical risk factors. The knee, hip, and ankle have been focused on during research in order to determine the contribution of each joint to ACL injury. According to research done by Hutchinson and Ireland (1995), planting and cutting (29%), straight knee landing (28%), and one step-stop landing with the knee hyperextended (26%) are the most commonly observed movements involved in female noncontact ACL injuries. The posture and lower extremity alignment of females during these movements may put the athlete at increased risk of ACL injury. In general, women change direction in a more erect position than men, which can result in decreased flexion in the knee and hip, increased valgus in the knee, and greater activation of the quadriceps muscles- all of which can add strain to the ACL.