Anterior Cruciate Ligament Injury in the Female Athlete

Introduction

Anterior cruciate ligament (ACL) injuries are one of the most common injuries seen by orthopedic surgeons and sports medicine specialists. Annually in the United States, there are 68.6 isolated ACL tears per 100,000 person-years. Worldwide there are roughly 1.4 million noncontact ACL tears annually. The ACL is subject to injury most commonly in sports that require movements such as cutting, pivoting, and jumping. These are significant injuries for both recreational and elite athletes alike, and these are associated with delayed recovery and/or inability to return to sport. In addition to the initial acute loss of function, even when the ACL is reconstructed the player then has a predisposition to early-onset osteoarthritis.

The highest incidence of ACL tears overall occurs in teenage female athletes. Age-specific patterns differ between males and females, with a peak incidence in males between 19 and 25 years of age (241.0 per 100,000) and the peak incidence in females between 14 and 18 years of age (227.6 per 100,000).

In the past 30 years, there has been a 10-fold increase in high-school and a 5-fold increase in collegiate sports participation by females. ^, As the number of females participating in high-level sports continues to increase since the passage of Title IX in 1972, the prevalence of ACL injuries is expected to increase as well. Prior to Title IX, fewer than 10,000 female athletes competed in collegiate sports. More recently, in the 2016–17 academic year, it was reported that there were a record-setting 494,992 collegiate athletes. In 2018, there were 10,586 women's teams and 9159 men's teams competing in the NCAA (National Collegiate Athletic Association) championship sports.

Although there is an overall higher number of ACL tears among male athletes than females, female athletes have a higher incidence rate of ACL injury. Multiple studies have shown that the relative risk of ACL injury in female athletes, compared to male athletes, is roughly 1.40–9.74. ^, Females in the military have been reported to have a relative risk of ACL injury of 2.44 when compared to males.

The increased incidence of ACL injuries in female athletes is most likely multivariate, stemming from mechanical, endocrinologic, and psychologic factors.

Mechanical Factors

Anatomically, the ACL is made up of two bands, the anteromedial and posterolateral, and extends from the region anterior to the tibial intercondylar eminence to the medial portion of the lateral femoral condyle. It works in conjunction with the surrounding muscles to stabilize the knee. With the knee in extension the posterolateral band is the tightest, and during knee flexion the anteromedial band is the tightest. During weight bearing, the ACL prevents the tibia from translating anteriorly. During flexion and extension moments, it works with the posterior cruciate ligament to control movement of the femur on the tibia. It also provides stabilization during internal rotation moments of the tibia and during varus and valgus stresses of the knee joint.

The miserable malalignment syndrome, consisting of a high quadriceps angle (Q-angle), increased pelvic width, anteverted femur, valgus knee, tibial external rotation, and pronated foot, is related to ACL injury. ( Fig. 2.1 ) Together, these individual factors create an environment that encourages extensor mechanism malalignment. This places strain on the patellofemoral joint and ultimately leads to pain. In 2003, Uhorchak et al. showed that other significant risk factors for ACL injury, aside from the miserable malalignment syndrome, include a small femoral notch width, generalized joint laxity, higher than normal body mass index, and anterior to posterior knee laxity values that were one standard deviation or more above the mean. This was further characterized in 2018 in a retrospective study that used magnetic resonance imaging (MRI) to look at anatomic risk factors associated with ACL injury. The group of ACL-injured patients was found to have a more narrow femoral intercondylar notch width index (<0.252), a larger β-angle (>38.5 degrees), and a larger lateral tibial slope (>7.5 degrees).

Femoral notch width and shape is related to the risk of ACL injury. van Eck et al. suggested that ACL injury is associated with the shape of the notch. Using arthroscopy, the authors defined three different notch shapes, which included A-shape, U-shape, and W-shape. Notches with an A-shape were found to be narrower in all width dimensions than the U-shaped variety. Patient height was correlated with notch shape, and there was a positive association with tall height and U-shaped and W-shaped notches. Females were found to have smaller notch widths at the base and in the middle.

A valgus knee is often implicated in knee injuries. Three-dimensional kinematic analyses have shown that while jumping, female athletes have a higher amount of knee valgus. A prospective study assessed 291 female high-school athletes newly enrolled in basketball and handball. They analyzed dynamic knee valgus during single-leg drop jumps. The participants were then followed up for 3 years specifically looking for ACL injury. In the injured group, there was a significantly greater amount of dynamic knee valgus. They concluded that dynamic knee valgus is a risk factor for noncontact ACL injuries in female high-school athletes.

A person's Q-angle is the angle that is formed from the combined vectors for the pull of the quadriceps muscle and the patellar tendon. The average Q-angle in uninjured males is 12.1 degrees, and 16.7 degrees in uninjured females. This higher Q-angle in females increases the lateral pull of the quadriceps on the patella and potentiates disorders of the knee. Q-angles exceeding 15 degrees in males and 20 degrees in females are considered to be abnormal. An increased Q-angle may contribute to an increased risk for ACL injury by increasing the obliquity of the femur, increasing the knee valgus, and thus increasing the contact pressure applied to the patellofemoral joint. The ligament is under varying degrees of tension throughout all movements. There have been multiple studies that sought to establish an association between knee injury and Q-angle. Two of them showed that there was not a significant association. ^, However, there have been a couple of other studies that did note an association between Q-angle and increased risk for knee injury. ^, The opposing outcomes of these studies raise the question of how much a Q-angle measured in a static position correlates to dynamic motion such as in sports. When looking at collegiate basketball players, there was a significant difference in Q-angles between males and females, especially when measuring with the knee in 30 degrees in flexion. This is clinically relevant, because Xerogeanes et al. have shown that the greatest magnitudes of force on the ACL are incurred with the knee in 30 degrees of flexion. Thus having a larger Q-angle in this position is not favorable. This is likely multifactorial considering that Emami et al. also found that 16% of the males and 20% of the females who had an abnormally high Q-angle did not present with a knee injury.

Q-angle has also been associated with a larger moment of tibial internal rotation. Internal rotation of the tibia affects tibiofemoral contact forces and the force seen by the ACL during impact. In model simulations, it has been shown that ACL forces were highly correlated with contact forces on the anterior component of the tibiofemoral joint during impacts with larger knee abduction moments, internal tibial rotation, and larger contact forces.

In animal models, hamstring (HS) contraction helps resist anterior tibial shear force at 30 degrees in flexion, which reduces the amount of force on the ACL. This has also been shown in human subjects. ^, Activation of the quadriceps had the most significant effect on ACL strain. The HSs are activated independently from the quadriceps, and they act as a protagonist force to the ACL. When the HSs contract, they decrease the amount of anterior tibial translation and internal tibial rotation. They also reduce tension on the ACL, with the knee between 15 and 45 degrees of flexion.

When the pelvis is examined, females have a relatively wider and/or different shaped pelvis compared with males. Several authors have found a significant association between pelvic width and the risk of knee injuries in females. The structural differences of the wider pelvis in females are thought to increase the risk of knee injury by creating a larger coxa vara/genu valgum alignment, with a simultaneous increase in tibiofemoral rotation forces in the transverse plane, which ultimately places a greater force onto the ACL. ^, With the hip being the most proximal link in the lower extremity, excessive hip adduction and internal rotation while weight bearing affects the kinematics of the entire extremity below it. Hip adduction and internal rotation can cause the mechanical axis of the knee to move medially and results in dynamic knee valgus.

In addition to laxity of the knee, females also demonstrate increased joint laxity of the foot. Excessive pronation in the subtalar joint has been found to be common in the American population. This increase in ligamentous laxity of the foot has been brought up as a potential cause for increased navicular drop in females. Patients with an increased amount of pronation have been found to have an increased amount of knee rotation when the knee is flexed to 5 degrees. In the stance phase of the gait cycle, subtalar pronation and internal rotation of the tibia occur simultaneously and the ACL becomes taut as the tibia rotates. Navicular drop has been reported as a significant predictor of tibial translation by Trimble et al. They also suggested there was a relationship between increased subtalar joint pronation and increased anterior translation of the tibia. When increased navicular drop causes the tibia to move forward, this would ultimately place an increased amount of strain on the ACL. A couple groups have in fact shown that there was an association between increased subtalar joint pronation and ACL injury. They measured the navicular drop height from seated to standing in 22 athletes with an injured ACL and 22 control subjects with an uninjured ACL. What they found was that the subjects with an uninjured ACL dropped an average of 5.9 mm and the ACL-injured subjects dropped an average of 8.4 mm, which was statistically significant.

As early as 1982, differences in the transverse plane, in the form of femoral anteversion, have been discussed as a factor in ACL injuries. Greater femoral anteversion (as seen in females compared to males) as well as a smaller pelvic angle have been found to be a predictor of greater hip internal rotation and knee excursion. It is these motions, as described in this chapter, that play a significant role in the higher incidence of ACL injuries in female athletes.

There are likely multiple mechanical variables that factor into the increased ACL injury risk for females, as discussed earlier. One of the functions of the ACL is to prevent internal rotation of the tibia. When the knee is at 30 degrees of flexion, there is an increase in tibial internal rotation. At this position, there is also a decrease in the HSs-to-eccentric quadriceps strength ratio. This means that during deceleration with the knee at 30 degrees of flexion the joint is taking on two simultaneous forces that compromise the ACL.

Many of the forces on the knee and the ACL are easiest to describe in a static state. However, it is more realistic to think about forces during a dynamic movement. Motion perturbations (contact with another player) also likely have a role in altering the biomechanics associated with ACL injuries. During videographic and biomechanical analysis, it has been shown that motion perturbation changes an athlete's coordination and movement. When looking at a group of injured basketball players, they all sustained their injuries while handling the ball and within the first 0–3 steps. During side-cutting in the presence of a nearby opponent, females were found to have greater amount of knee valgus, greater variability in knee valgus, an increase in foot pronation angles, and increased tibial internal rotation. Looking at hip biomechanics, females also had a decreased amount of hip and knee flexion as well as hip abduction during cutting. These results combined suggest that the dynamic lower extremity biomechanical changes at the hips, knees, and ankles play a significant role in the higher rate of ACL injuries seen in female athletes.

Biomechanical differences between genders, as described earlier, are further exacerbated by muscular fatigue. This increases the alterations in the trunk, pelvis, and lower extremity kinematics involved in injuries to the ACL. As shown in a study, when landing from a single-leg drop after being fatigued, males had a greater amount of trunk flexion than females. Males also had a decrease in peak knee flexion and a higher amount of gluteus maximus and biceps femoris activation than their female counterparts.

Neuromuscular Factors

There are many systems within the human body that operate automatically and subconsciously to maintain the body in its homeostatic state. One of these systems is the sensorimotor system, which incorporates all the afferent, efferent, and central integrating and processing components that are involved in providing functional joint stability during motion. A prior study showed that mechanoreceptor density in the ACL is highest at its most proximal and distal osseous attachments. About 1% of the ligament's dry weight is made up of neural tissue.

A neurologic link between the cerebral cortex and the ACL has been documented using electroencephalographic signals by stimulating the ACL during arthroscopy. Following an injury to the ACL, multiple sensorimotor impairments may occur. These include proprioceptive deficits, decreased strength of the stabilizing muscles of the knee, and alterations in muscle activation onset patterns.

One proposed method to reduce ACL injuries is to implement a neuromuscular training regimen into adolescent female athlete training programs. In a large study looking at 23,554 female athletes, four variables were found to reduce ACL injury risk. The variables included a younger participant age, neuromuscular training performed for at least 20 min and at least twice per week, a greater number of exercise variations, and more usage of verbal feedback.