Prevention of ACL injuries sees gains despite unknowns

The quest to prevent noncontact anterior cruciate ligament injuries dates back at least a decade, as evidenced by the cover story of the August 1998 issue of BioMechanics: “Women and the ACL: Equal Opportunity Injury.” At that time, researchers were only beginning to appreciate the extent to which female athletes were at greater risk of noncontact ACL injury than their male counterparts. Discussions about mechanisms of injury and methods of prevention were largely theoretical.

In 2008, it's unclear how far we've actually come. Research now suggests that a range of factors – biomechanical, neuromuscular, anatomical, hormonal – contribute to injury risk, but hasn't yet revealed just how and under what circumstances any particular combination of variables leads to an injury. In the meantime, despite not knowing the mechanism of the injury in question, researchers and clinicians have devised training programs designed to prevent noncontact ACL injuries from occurring at all.

Perhaps surprisingly, randomized clinical trials have shown that several of these training programs do in fact decrease incidence of knee injuries in general and noncontact ACL injuries in particular.^1-3 But data from the National Collegiate Athletic Association's Injury Surveillance System⁴ indicates that the incidence of noncontact ACL injury in the high-risk sports of basketball and soccer did not decrease from 1990 to 2002—suggesting that perhaps those prevention programs are not being implemented properly in the field, or perhaps that the programs are less effective outside a controlled study protocol.

“If current prevention methods carried reasonable efficacy, one would assume that a noticeable reduction in these rates would already be evident. It appears, therefore, that current strategies fail to counter key factors implicated within the injury mechanism,” said Scott G. McLean, PhD, assistant professor of athletic training and movement science and director of the Injury Biomechanics Laboratory at the University of Michigan. McLean was a keynote speaker in April at a consensus conference on gender and ACL injury hosted by the University of North Carolina-Greensboro.

Unfair assessment?

Upon closer examination, the NCAA study may not be the last word on whether prevention programs are succeeding or failing. Noting that the study, conducted at the University of Minnesota, only reviewed data through 2002, some researchers contend that the full effect of training programs developed in the late 1990s and early 2000s would not have been evident by then. Not surprisingly, researchers at Cincinnati Children's Hospital – whose neuromuscular training program(1) is one of the most well known – are among those who have pointed out this limitation of the NCAA study as a barometer of training intervention efficacy.⁵

However, even the Cincinnati researchers acknowledge that the NCAA data, as well as other studies that have failed to find an effect of training,^6-7 offer an important reminder that there is still a need for interventions that are both more effective and easier to implement in the field.

“The (NCAA) study is one of the best studies we have, so we have to look at that to some extent,” said Greg D. Myer, MS, a sports biomechanist in Cincinnati Children's Hospital's Sports Medicine Biodynamics Center. “Are we making a change? We do think we're making a difference. Do we think we've solved the problem? No. What we do as researchers is very labor intensive, and what people in the real world want is a quick fix. We need to make our programs more effective so that people will be more likely to implement them, and secondly we need to identify those who are most at risk.”

Knee mechanics

In Cincinnati, identifying athletes who are most at risk of noncontact ACL injury starts with knee abduction moment while landing from a jump—one of the biomechanical variables previously identified by the group as being a prospective predictor of injury.⁸ In that 2005 study, the Cincinnati team found that a knee abduction moment above 25.25 Nm predicted noncontact ACL injury with a sensitivity of 78% and specificity of 73%.

In a study published last summer in the online journal BMC Musculoskeletal Disorders,⁵ Myer and colleagues classified 18 female high school volleyball players as “high risk” or “low risk” based on the 25.25 Nm knee abduction moment cutoff point, then compared the effects of a seven-week neuromuscular training program on each group. Knee abduction moment was assessed during a drop vertical jump from a 31-cm box.

As expected, knee abduction moment in the high-risk group decreased significantly following training, and did not change significantly in the low-risk group or in control subjects. However, the decreases (about 5 Nm on average) brought the KAM levels in the high-risk group down to 34.6 ± 9.6 Nm on the right side and 32.4 ± 10.7 Nm on the left side, still well above the 25.25 Nm threshold level and the values measured in the low-risk group (which averaged less than 15 Nm on each side prior to training).

In addition, the 13% decrease observed in the high-risk group was a less remarkable change than the 21% decrease associated with neuromuscular training in a 2005 study that did not stratify athletes by risk;⁹ theoretically, the improvement seen specifically in high-risk athletes in that earlier study can be assumed to have been even greater. Two key differences between the two studies suggest that, although training in general is more effective when targeted to high-risk athletes, not all high-risk athletes respond similarly to all training interventions.

First, Myer said, the intervention protocol used in the earlier study involved almost twice as much plyometric and balance training as that used in the 2007 study. The slimmed-down version had been developed in the interest of creating a training protocol that would be easier to implement in the field.

In addition, the 2005 study involved athletes who participated in soccer, basketball and volleyball, while the 2007 study included volleyball players only, for logistical reasons.

“Volleyball players may not be as receptive to these types of activities because they're already trained jumpers, and basketball and soccer appear to be the higher risk sports,” Myer said.

For that matter, the 25.25 Nm cutoff value also came from a study that involved a multi-sport population, suggesting that the definition of a “high risk” knee abduction moment may vary depending on the athletes in question.

“We have to continue to validate that number with different populations,” Myer said. “And people who try to reproduce this study need to consider that they may have a different number for their study population.”

< b>PEP girls

Research from the University of Southern California, which also examined the effect of training on knee valgus moment, suggests that this effect may be determined in part by the maturation level of the athletes being trained.

The investigators studied 48 female soccer players whose ages ranged from nine to 17, specifically looking at knee valgus moment during a side-step cutting maneuver. All subjects were tested before and after participating in a 10-week “Prevent injury and Enhance Performance” (PEP) training program developed at the Santa Monica Orthopaedic and Sports Medicine Research Foundation.³

On average, the athletes demonstrated a 14% decrease in knee valgus moment after training, but this change was not statistically significant. When the researchers classified the athletes into pre-pubertal (average age 10 years), pubertal (12.6 years), and post-pubertal (15 years) groups, however, they found that the decrease in knee valgus moment in the post-pubertal group was in fact statistically significant. More than three quarters (78%) of the athletes in the post-pubertal group responded positively to the training, compared with 63% in the pubertal group and 45% in the pre-pubertal group.

“Athletes across levels of maturation did not respond similarly to the training program,” said Susan M. Sigward, PhD, PT, ATC, assistant professor of research physical therapy, who presented the group's findings in Greensboro. Sigward noted that the results were perhaps not surprising, given that the PEP program was originally studied in athletes between the ages of 14 and 18.

The different responses across maturation groups may be related to postural differences between those groups, only some of which appear to be gender-specific. A UNC-Greensboro study of 178 athletes aged nine to 18, presented at the consensus conference and published in March,¹⁰ found that more mature subjects exhibited more relative straightening and external rotation at the knee, along with a more supinated foot position. Female subjects were more likely to demonstrate hip internal rotation and knee valgus than male subjects, particularly in later maturation groups.

LESS is more

A study of youth soccer players from UNC-Chapel Hill, also presented in Greensboro, took a different approach to assessing training efficacy and found that response differed between genders. The investigators used their Landing Error Scoring System to assess landing mechanics in 72 players before and after a nine-month intervention involving a customized 10-minute warm-up (including static stretching, strengthening, agility and plyometric exercises) prior to each soccer practice.

Three months after completion of the intervention, the 50 girls in the study demonstrated significantly greater LESS score improvement than the 22 boys; the 43 subjects (32 girls) who had improved during the course of the intervention were more likely to demonstrate improvement three months later than the 29 (18 girls) who had not. Because some boys did improve with training and some girls did not, the results suggest that gender is only one of many contributing factors.

The researchers did note that high school-aged subjects tended to demonstrate more of a response to the training than the younger children, but also saw other trends that suggested maturation alone did not account for the improved scores.

“If the changes we saw were only due to maturation, then the 10-year-olds' post-test should look like the 11-year-olds' pre-test, but in fact we found that 10-year-olds' post-test was significantly lower than 11 year olds' pre-test, and so on,” said Lindsay J. DiStefano, ATC, a doctoral student in the human movement science program and a research assistant in the Sports Medicine Research Laboratory, who presented the findings in Greensboro.

Either gender or maturation could have played a role in a University of North Florida study, which found that a customized exercise program was associated with significant decreases in peak vertical ground reaction force during a jump landing task – another variable identified by the Cincinnati group as being predictive of noncontact ACL injury.⁸

Forty eight high school basketball players were assigned either to a generalized exercise program (an adaptation of the PEP protocol emphasizing lower extremity flexibility and sagittal plane lower extremity strength and flexion during jumping) or a customized program in which instruction in soft landing techniques was supplemented according to each athlete's performance on a standardized double leg squat test. In the customized group, athletes who landed in a toe out position were prescribed exercises that emphasized dorsiflexion/plantar flexion muscle balance; those who landed with extreme knee valgus were given a program that emphasized hip abduction/adduction muscle balance.

After 12 weeks of intervention, the 29 athletes in the customized group demonstrated significantly lower peak vGRF than the 19 athletes in the generalized group. Anterior-posterior GRF and medial-lateral GRF decreased significantly in both groups, but more so in the customized group.

However, for logistical reasons, the two groups were not matched in terms of age or gender; the customized group consisted of 10 male varsity players and 19 female varsity and junior varsity players, while the generalized group consisted of 19 male JV and freshman players. For this reason, a gender effect or a maturation effect cannot be ruled out, according to Christopher J. Joyce, PhD, ATC, CSCS, an associate professor of athletic training at UNF, who presented the findings in Greensboro.

All told, researchers are identifying more ways in which training programs affect variables associated with injury risk. Whether those training programs actually prevent injury, and in which athletes, remains to be seen.

Ultimately, however, the key to effective prevention of noncontact ACL injuries may go back to researchers' unending pursuit of the mechanism of injury, said Sandra J. Shultz, PhD, ATC, an assistant professor of exercise and sport science and co-director of the Applied Neuromechanics Research Laboratory at UNC-Greensboro.

“Knowing the underlying causes of ACL injury,” Shultz said, “Is critical to injury prevention.”

Jordana Bieze Foster is a freelance medical writer in Massachusetts and the former editor of BioMechanics.

References

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2.Myklebust G, Engebretsen L, Braekken IH, et al. Prevention of anterior cruciate ligament injuries in female team handball players: a prospective intervention study over three seasons. Clin J Sport Med 2003;13:71-78.

3.Mandelbaum BR, Silvers HJ, Watanabe D, et al. Effectiveness of a neuromuscular and proprioceptive training program in preventing anterior cruciate ligament injuries in female athletes: two-year follow up. Am J Sports Med 2005;22:1003-1010.

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6.Pfeiffer RP, Shea KG, Roberts D, et al. Lack of effect of a knee ligament injury prevention program on the incidence of noncontact anterior cruciate ligament injury. J Bone Joint Surg Am 2006;88(8):1769-1774.

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8.Hewett TE, Myer GD, Ford KR, et al. Biomechanical measures of neuromuscular control and valgus loading of the knee predict anterior cruciate ligament injury risk in female athletes: a prospective study. Am J Sports Med 2005;33(4):492-501.

9.Myer GD, Ford KR, Palumbo JP, Hewett TE. Neuromuscular training improves performance and lower-extremity biomechanics in female athletes. J Strength Cond Res 2005;19(1):51-60.

10.Shultz SJ, Nguyen AD, Schmitz RJ. Differences in lower extremity anatomical and postural characteristics in males and females between maturation groups. J Orthop Sports Phys Ther 2008;38(3):137-149.