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Published 4/1/2016
Terry Stanton

20 Years of Study into Mechanisms of ACL Injury Honored

Timothy E. Hewett, PhD, receives OREF Clinical Research Award
Timothy E. Hewett, PhD, and his research associates have spent more than 2 decades contemplating and confronting the challenges presented by anterior cruciate ligament (ACL) injury, with the goal of better understanding the causes and mechanism of ACL injury and developing techniques and regimens for its prevention.

Dr. Hewett, the director of the Mayo Biomechanics Laboratories and of Sports Medicine Research at the Mayo Clinic, received the 2016 Orthopaedic Research and Education Foundation (OREF) Clinical Research Award. His manuscript summarizes more than 80 reports published by his research team from 1996 to 2015. Coauthors are Gregory D. Myer, PhD; Kevin R. Ford, PhD; Mark V. Paterno, PhD; and Carmen E. Quatman, MD, PhD.

Two decades of work
Dr. Hewett divides his research objectives into four sequential categories:

  • Establish the extent of the ACL injury problem
  • Establish the etiology and mechanisms of injury—including risk factors, mechanisms, prediction of risk, and clinical assessment tools to identify at-risk athletes
  • Introduce prevention measure—including overview of prevention techniques and timing of intervention
  • Assess the effectiveness of intervention

He notes that, since 1980, an estimated 1.5 million ACL reconstructions have been performed, at an annual cost of more than $3 billion, and profound physical, emotional, and financial consequences for patients and families. Although surgical techniques and rehabilitation programs have improved, restoration of knee joint stability may not correlate with clinical outcomes. Specifically, patients with ACL injury exhibit osteoarthritis symptoms and radiographic evidence of knee joint degeneration, irrespective of surgical intervention. For this reason, Dr. Hewett argues that prevention of injury may be the most effective intervention.

In plotting out their work, Dr. Hewett and his colleagues followed a coupled biomechanical-epidemiologic approach.

"Within this model of injury prevention," Dr. Hewett writes, "our work over the past 20 years has focused on utilization of this model to target the most modifiable risk factors (biomechanical and neuromuscular) related to ACL injuries with the goal to provide the most effective interventions for the orthopaedic community. Specifically, we have focused on defining what are the underlying risk factors for ACL injury, who are the athletes at risk for ACL injury, and determination of when those athletes become most vulnerable to injury. In addition, we have targeted how to prevent ACL injury by identifying which prevention strategies are most effective and when implementation of these strategies offers the greatest benefit to young athletes."

Etiology, risk factors, and the sex difference
Dr. Hewett notes that female athletes have a 2- to 8-fold greater rate of ACL injury compared with male athletes, and that about 5 percent of female high school varsity athletes sustain an ACL injury each year. Prior to puberty, ACL injuries are somewhat rare and no sex-related incidence difference is observed.

"Females and males demonstrate important anatomical, hormonal, and neuromuscular differences after the onset of puberty that potentially influence the divergence in ACL injury rates between the sexes after puberty," he observes. "It is theorized that males and females may even have different mechanisms of ACL injury."

Dr. Hewett's group noted that postpubertal females exhibit greater landing forces and force loading rates, lower hamstring-to-quadriceps torque ratios at high angular velocities, and altered quadriceps and hamstrings activation strategies compared to males.

In light of this, he suggests that "females may preferentially rely on higher activations of quadriceps muscles relative to hamstrings muscles with incremental increases in landing intensities."

Dr. Hewett notes that previous ACL injury increases the odds of sustaining an additional injury, with athletes with previous injury having a 15 to 20 times greater chance of reinjury or contralateral injury. An important risk factor for reinjury is asymmetric loading of the contralateral limb, as occurs when athletes favor the limb after the original injury.

Dr. Hewett's team developed a research paradigm (in sim) that integrates in vivo, in vitro–cadaver, and in silico (computer modeling) approaches. His laboratory used this "in sim" approach to evaluate injury mechanisms and compare tibiofemoral cartilage pressure distributions with in vivo bruise patterns.

"The results supported a valgus collapse mechanism with potential multiplanar loads of tibial abduction combined with anterior tibial translation or external or internal tibial rotations," he writes. "Many ACL injury mechanism studies demonstrate lower extremity biomechanical loading patterns that may be avoided with neuromuscular training programs. Modification of landing techniques and education of dangerous knee postures may be important injury prevention strategies in the future."

Prevention techniques, timing of intervention
Dr. Hewett observes that comprehensive ACL injury prevention programs can consume large amounts of time and involvement by players and coaches. His team focused on creating more efficient and effective training protocols to address neuromuscular deficits and high-risk landing biomechanics. His group conducted comparative studies of plyometric training versus dynamic stabilization and evaluated targeted training of the trunk, hip, and knee on power, strength, coordination, limb asymmetries, ground reaction forces at landing, and knee abduction measures.

They found that plyometric and balance training both appear to reduce high-risk knee abduction and flexion measures.

"Targeted core muscular (trunk) training can increase hip abduction strength and potentially improve neuromuscular control of lower limb alignment during sports activities," Dr. Hewett writes. "Studies from our lab show that targeted core training and improvements in lower extremity strength can improve single-limb postural stability and neuromuscular control."

He notes that the optimal length of such training is undetermined, and "whether it can be incorporated with in-season sports training remains unclear."

"Collectively," he concludes, "our research indicates that comprehensive protocols that incorporate plyometric, strength, and balance exercises may provide the most effective ACL injury prevention strategies."

Dr. Hewett writes that "there is strong, nearly unequivocal evidence that neuromuscular training reduces biomechanical risk factors for ACL injury and decreases ACL injury incidence in athletes.

"Assessment of relative injury risk using widespread neuromuscular screening techniques should be further employed," he continues. "Pre- or early adolescent athletes may have the greatest potential for both the development of optimal biomechanical movements and decreased injury risk during sports participation. Neuromuscular training is currently the only effective tool for prevention of osteoarthritis in the injured knee, with or without surgical reconstruction. We have made significant strides toward these goals, but we must continue until strong epidemiological evidence shows that ACL injury risk is unequivocally decreased in young athletes."

Dr. Hewett's disclosure information can be accessed at www.aaos.org/disclosure

Terry Stanton is the senior science writer for AAOS Now. He can be reached at tstanton@aaos.org