Risk and Gender Factors for Noncontact Anterior Cruciate Ligament Injury

Abstract

In the past several decades, there has been an increased emphasis on injury prevention in sports especially for non-contact anterior cruciate ligament (ACL) injuries. A significant difficulty with designing prevention programs for non-contact ACL injuries is our incomplete understanding of risk factors and mechanism of injury.

Initial analyses of injury risk factors focused primarily on the influence of isolated factors on noncontact ACL injury. More recent studies, using multivariate analysis, have begun to examine the influence of risk factor combinations. There has also been a recent trend to try to separate risk factor analysis for males and females. This body of work looks to identify significant gender differences in environmental, anatomical, hormonal, neuromuscular, and genetic risk factors for non-contact ACL injuries. With a better understanding of risk factors associated with ACL injuries, better prevention programs can be developed.

Despite the plethora of research and new approaches on gender-specific risk factors for ACL injuries over the past several decades, there remains no conclusive evidence of a direct cause-and-effect relationship. It is known that females have increased ACL injury rates. Perhaps this represents the interplay of several risk factors resulting in a cumulative effect. Further research is warranted.

Keywords

ACL, age, anatomy, anterior cruciate ligament, biomechanics, body mass index, brace, estrogen, epidemiology, gender, genetics, injury, ligament, neuromuscular, noncontact, notch width, physiology, risk factors, sex, tibia slope, tibia plateau depth

Introduction

In the past several decades, there has been an increased emphasis on injury prevention in sports. A significant difficulty with designing prevention programs for noncontact anterior cruciate ligament (ACL) injuries is our incomplete understanding of risk factors and mechanism of injury. It has been 8 years since the first edition of this book, but unfortunately, despite much research by excellent clinical and basic scientists in this field as summarized in multiple reviews on this subject, there is still much unknown regarding risk factors for ACL injury.

Initial analyses of injury risk factors focused primarily on the influence of isolated factors on noncontact ACL injury. More recent studies, using multivariate analysis, have begun to examine the influence of risk factor combinations. There has also been a recent trend to try to separate risk factor analysis for males and females, as a risk factor for one sex may not be a risk factor for the other.

Some categorize risk factors as intrinsic (meaning those unique to the individual such as anatomy, muscle strength, and balance) and extrinsic factors (those which are external influences on the body including such factors as shoe–surface interactions, braces, and weather conditions). Risk factors can also be categorized as environmental, anatomical, hormonal, neuromuscular, and genetic. The latter classification scheme will be used as the basis for this discussion.

Environmental Risk Factors

Many environmental risk factors specific to ACL injury have been studied, including weather and playing conditions, shoe–surface interaction, footwear, and bracing. These variables are important because they represent potentially modifiable risk factors.

The foot plant, the shoe, the surface, and the shoe–surface interaction are critical factors in noncontact ACL injuries. Energy is dissipated once the static frictional force is overcome, allowing movement. It is logical to assume that during foot plant, characteristics that increase static frictional force between the foot and ground will create higher-energy forces in the lower extremity.

Lambson et al., in a 3-year prospective study, looked at footwear to evaluate torsional resistance of modern football cleats. They compared four styles of football shoes and found that the edge design, a design having longer irregular cleats at the periphery and many smaller cleats in the interior of the shoe’s sole, was associated with higher ACL injury rates. A study by Wannop et al. on the effect of footwear type on injuries in high school football players, playing on either natural grass or a third-generation artificial turf (FieldTurf), found that increases in rotational traction, but not translational traction, resulted in a higher risk of noncontact injuries. They reported no significant differences in lower extremity noncontact injury rate between surfaces. Dragoo and Braun also reported the rate of injury on third-generation and natural grass surfaces appeared to be similar, although there were some differences in the types of injuries reported.

Other studies have evaluated the influence of surface conditions on ACL injury rates. Olsen et al. and Torg et al. independently studied team handball and found an increased risk of ACL injury while playing on synthetic floors versus traditional parquet floors. Both believed that the increased friction of synthetic flooring was the cause. Balazs et al., in their systematic review of risk of ACL injury in athletes on synthetic playing surfaces, reported an increased rate of ACL injury on synthetic playing surfaces in football but not soccer. Confounding variables such as weather conditions, footwear, and player position pose limitations for these types of studies.

Orchard et al., Heidt et al., and Scranton et al. all reported higher rainfall and cooler temperatures were related to a decrease in ACL injuries and theorized that dry, hot weather conditions promote increased frictional forces on the playing field, thus in turn resulting in increased injury rates.

Bracing Pros and Cons

Prophylactic and functional (postreconstructive) knee bracing has long been a controversial subject. Over the past 20 years, attitudes have fluctuated regarding the effectiveness of braces in preventing knee injury in the uninjured athlete, the ACL-deficient athlete, and the ACL-reconstructed athlete. A study by Decoster and Vailas on brace prescriptions patterns noted that there has been a decreasing tendency for orthopaedic surgeons to prescribe ACL braces. The authors also noted that a primary factor influencing brace prescription by orthopaedists was the activity level of the patient.

Early studies on prophylactic brace wear by Teitz et al. and Rovere et al. indicated no benefit to brace wear. These authors cited increased rates of knee injury in some athletes using prophylactic knee braces. In contrast, two other studies, the West Point study by Sitler et al. involving 1396 cadets at the US Military Academy who played intramural tackle football and the Big Ten Conference study by Albright et al. involving 987 NCAA football players, concluded that prophylactic knee braces were effective in reducing injury. Since these studies, there has been a paucity of data to support prophylactic bracing for ACL injury protection in athletes with intact ligaments, but it is believed that braces may provide some advantage in prophylaxis against medial collateral ligament (MCL) injury.

Braces are commonly prescribed following ACL injury or reconstruction; however, little evidence supports their physiological or biomechanical efficacy. In a prospective randomized clinical trial of functional bracing for ACL-deficient athletes, Swirtun et al. found that, subjectively, patients had an initial sense of increased stability, but these investigators were unable to find objective benefits. In contrast, Kocher et al. studied the use of braces to prevent reinjury in 180 ACL-deficient alpine skiers and found reinjury occurred in 2% of the braced skiers compared with 13% of the unbraced skiers.

Risberg et al. investigated the effect of knee bracing during the first 2 years following an ACL reconstruction in a prospective clinical trial of 60 patients randomized postoperatively (30 braced and 30 without brace). They found no evidence that bracing affected knee joint laxity, range of motion, muscle strength, functional knee tests, patient satisfaction, or pain in braced athletes compared with athletes who did not use a brace following ACL reconstruction. Furthermore, they found prolonged bracing, which they defined as brace wear for 1 to 2 years postoperatively, produced decreases in quadriceps muscle strength. McDevitt et al. in a prospective, randomized multicenter study of 100 subjects, likewise found no significant differences between braced and nonbraced subjects following ACL reconstruction.

It has been theorized that damage to the ACL can disrupt mechanoreceptors in the knee leading to decreased proprioception. Birmingham et al. has suggested that brace wear may help correct this deficit somewhat, but benefits do not carry over to more demanding tasks. To examine knee proprioception, researchers have studied the threshold for detection of passive knee motion and found that brace application to the ACL-deficient limb does not improve the threshold to detect passive range of motion. Wojtys et al. reported that the combination of muscle activation and functional knee bracing in ACL-deficient knees reduced anterior tibial translation between 70% and 85%. A brace with a constraint to knee extension was found by Yu et al. to increase the likelihood of landing on a bent rather than an extended knee, a position thought to be less likely to result in an ACL injury.

Although the preponderance of evidence would suggest that braces are ineffective in protecting the ACL-deficient or ACL-reconstructed athletic knee, many patients, initially following their injury or ACL reconstruction, still wish for a brace because they subjectively report that braces increase their confidence during sports participation.

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