June 2007 | Back to Table of Contents
Clinical and Health Affairs
Musculoskeletal Injuries of the Knee
Are Females at Greater Risk?
By Elizabeth Anne Arendt, M.D.
Abstract
Research has shown that the incidence of anterior cruciate ligament injuries is higher in female athletes than in male athletes. And anecdotal evidence points to a higher incidence of patellofemoral disorders in females versus males. This article reviews what is known about the incidence of these injuries, the reasons for sex differences in injury rates, the mechanisms behind the injuries, and strategies to prevent them.
The difference between the sexes in medical disease states is the subject of much study. Researchers are finding differences between males and females not only in their risk for diseases such as heart disease and cancer but also in their response to treatment and their perception and experience of pain. One of the areas where sex differences have been noted is knee injuries, particularly anterior cruciate ligament (ACL) injuries and patellofemoral (PF) disorders.
At the University of Minnesota, we have been involved in clinical musculoskeletal research related to dimorphism or the difference in the presentation of disease by sex, including studies on bone health, stress injury to bone, and risk factors for knee injuries. Specifically, we have demonstrated that noncontact ACL injuries, those caused by the individual’s own body mechanics as opposed to external force, continue to occur at a higher rate among female collegiate athletes who play soccer and basketball than male collegiate athletes who participate in those sports.1,2 This article reviews our recent research as well as that of others on the rate of knee injuries among males and females and provides an overview of what we know about the reasons for the differences.
ACL Injuries and PF Disorders
The anterior cruciate ligament is a main stabilizer of the knee, preventing anterior translation of the tibia on the femur and anterior lateral rotation of the tibia beneath the femur. Rupture of this ligament accounts for a significant number of knee injuries during sporting activities in both males and females. The typical mechanism of injury involves sudden deceleration, often a quick change in direction during a plant-and-pivot maneuver. Sometimes, the injury happens when an athlete lands on 1 foot.
A number of studies have shown that the rate of ACL injury is 2 times to 8 times higher in females than males who participate in the same sports.1-3 The sports most studied are soccer and basketball in the United States, team handball in Europe, and netball in Australia. Because of the growing participation of females in athletics and the often debilitating consequences of ACL injuries, a better understanding of the mechanisms behind these injuries is an essential first step toward preventing them.
Patellofemoral disorders are injuries and diseases of the kneecap. They can be divided into 3 categories: patellofemoral pain, patellofemoral instability, and patellofemoral arthritis. Current epidemiological studies do not answer the question of whether patellofemoral pain is more prevalent in females. However, clinicians have observed that anterior knee pain is more common among such patients. Patellofemoral dislocations have long been thought to be more prevalent in females than males. The literature indicates that first-time patellar dislocation injuries are nearly equal in males and females, with recurrent patellar dislocations occurring more frequently in females.4,5 Patellofemoral arthritis, as well as any form of arthritis in the knee, is more prevalent in females. In one French study of 578 patients with isolated patellofemoral arthritis, 72% were female.6
Population studies of musculoskeletal disorders and injuries by sex are scant. To date, the most researched area relates to the incidence of noncontact ACL injuries in females. Those studies and others emphasize the role of noncontact mechanisms as the most frequent cause of both ACL injuries in females and PF dislocations in both sexes.7,8 If we had a better understanding of the mechanism behind these injuries, intervention strategies could be designed to reduce their incidence.
Risk Factors in Females
Risk factors for any disease typically fall into 3 categories: anatomic, hormonal, and neuromuscular. The most discussed risk factor for injuries of the female knee is the anatomic alignment of a woman’s lower limbs. The width of the pelvis, the angle of the femoral neck in the femoral shaft, and the angles of the thigh bone and leg bone differ in males and females.9 Many have theorized that the wider angles of a female’s lower legs place more stress on the joints—and the knees in particular—putting them at greater risk for injury. However, no one has found a direct relationship between anatomic alignment and the risk of ACL injury.
Another feature associated with ACL injury is the shape of the intracondylar notch (ICN) of the femur, the space in the middle of the knee through which the ACL traverses. Recently, researchers have speculated that the relatively smaller notch in females houses an ACL that is too small and too weak to hold up to the rigors of some sports with moves that involve planting and pivoting.8,9 There is some evidence that suggests that the dimensions of the ACL of females is smaller than that of males, thus supporting the theory that the ACL of females is more prone to injury because it is smaller. More research needs to be done to determine whether the size of the ACL is related to its strength and whether a female’s smaller knee and smaller ligament are proportioned for a smaller female body. Although there is an association between ICN stenosis and ACL injury in both males and females, the ICN has not been found to actually cause ACL injury and, thus, does not explain the large number of ACL injuries in women. Several studies have shown that joint laxity—joint looseness such as being able to hyperextend one’s elbows and knees—tends to be more common among females than males.7-10 However, the relationship between ligamentous laxity and ligament injury is speculative. Study findings conflict with one another: some report an increased injury rate associated with joint laxity, some show no change in injury rate with joint laxity, and others have suggested that joint laxity has a protective effect.11
Hormonal influence is the issue most discussed with regard to risk factors for knee injury in females. Hormones can have a direct influence (ligament function or strength is weakened by circulating hormones) or an indirect influence (hormones affect the action of nerves or muscles that then affect knee joint function). The most studied hormone is estrogen, followed by relaxin.
Studies to date favor the idea that there is no direct change in ligament composition or function secondary to the presence or absence of estrogen.12,13 Specifically, estrogen does not affect ligament structural properties negatively. However, hormones may mediate other changes such as a change in neuromuscular mechanisms. An indirect link between hormones and neuromuscular activity has been studied, but no causal link has been identified. Although many studies have attempted to find a potential negative effect of estrogen on ligament function and health, few have focused on whether male hormones may have a positive effect.
Neuromuscular mechanisms are the third major category of risk factors for knee injuries in females. Because there is ample evidence showing that injuries can be reduced when neuromuscular strategies are implemented, and that this risk factor is modifiable, there has been more effort directed at reducing neuromuscular risk factors than other categories of risk factors.14-17
By studying videos of athletes in action, mechanisms for noncontact ACL injuries have been identified.8 The most frequent mechanisms that lead to injury involve sudden deceleration or a quick plant-and-pivot maneuver. Females tend to do these maneuvers in a more upright position and have less flexion at both the hip and the knee. Males tend to have more flexion in their knees and hips, allowing the hamstrings to stabilize the knee. Because females are more upright when they land and/or do cutting moves, the mechanical advantage goes to the quadriceps. A strong quadriceps pull may be detrimental to the anterior forces on the tibia, leading to an ACL tear. In addition, females are more likely to internally rotate their femur during knee flexion; this can cause an inward collapse of the knee when landing a jump or pivoting. The buttock muscles mainly control the hip and femoral rotation underneath the pelvis.
Females have less control over these muscles than males. Their upright stance, combined with their lesser core strength, leads to a lack of central control of limb rotation and makes them favor knee positions that make them susceptible to both ACL ruptures and PF dislocations. These problems are not exclusive to females, but researchers find females performing these riskier neuromuscular mechanisms more often than males when doing similar sporting moves.
It is impossible to say whether any single mechanism is responsible for these serious knee injuries. A multitude of factors come into play during activity, and to focus on one variable is difficult. However, the fact that most ACL injuries and patellofemoral dislocations in females result from a noncontact mechanism suggests that there are predictable movements that could be modified. This is the rationale for many injury-prevention programs directed at women.
Interventional Strategies to Reduce Injury Risk
With proper training, athletes can learn to modify certain neuromuscular actions to reduce their risk of injury. Injury-prevention programs have concentrated on teaching females to 1) decelerate in a more controlled fashion by taking several small steps rather than a sudden single step; 2) round off turns when pivoting, keeping one’s knee inside an imaginary cylinder of one’s body and not taking any long steps that place the knee outside the cylinder of the upper torso; and 3) concentrate on core-strength-building exercises that focus on the muscles, including the gluteal and abdominal muscles, that attach the legs to the pelvis and control leg rotation.
When these precautions are in place, there is a reduction in the incidence of serious injury to the knee. And it is thought that the younger the athlete, the more likely they are to incorporate these habits into their muscle memory.
Yet many questions about these prevention strategies remain unanswered: At what age should such training be implemented so that athletes can permanently incorporate new neuromuscular patterns? What should the length and intensity of this training be, and what kinds of activities should be incorporated? Can one screen athletes at an early age for motor patterns that put them at risk for knee injuries and train only those who display such patterns? What is the best feedback mechanism for proper learning? What is the rate of recidivism at various ages and how fast does the unlearning of habits occur? How much of the training will translate to activity on the court or field of play?
Conclusion
The risk of injury is inherent in sports. Understanding injury mechanisms and modifying those that put one at risk has led to fewer occurrences of certain injuries. These include serious neck injuries with the elimination of “spearing” in football and shallow-end diving in swimming pools. The current campaign by youth hockey against checking from the back is also aimed at injury reduction.
To date, the goal of sport scientists has been to better understand the mechanisms of noncontact ACL injuries in order to learn which elements might be modifiable. This will aid in reducing other serious knee injuries such as PF dislocations. Such research is happening both in Minnesota and around the world. The hope is to provide a safer environment for all athletes in regard to reducing the risk of serious knee injury. MM
Elizabeth Anne Arendt is vice chair of the University of Minnesota’s department of orthopedic surgery and medical director of the university’s men’s and women’s intercollegiate athletics department.
References
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