June 2010

In the moment: Sports medicine

by Jordana Bieze Foster

ACL injury: Plane talk

Experts concede complexities of risk

After more than a decade of discord and sometimes contentious debate, biomechanists appear moving toward a consensus that the mechanism of anterior cruciate ligament injury cannot be wholly attributed to either the frontal or sagittal planes but is in fact multiplanar.

At the annual meeting of the American College of Sports Medicine in June, a thematic poster session featured several presentations that explored the multiplanar nature of ACL loading. Although researchers acknowledged that their work thus far stops short of actually predicting injury, the findings underscore the need to focus on multiple planes of motion in developing screening tools and exercise interventions to reduce the risk of ACL injury, especially in young female athletes.

“We need a multi-directional approach,” said Carmen Quatman, PhD, a research fellow at Cincinnati Children’s Hospital and a medical student at the University of Toledo.

Despite the Cincinnati group’s having long been associated with the frontal-plane camp, Quatman presented the results of a modeling study that suggest a certain percentage of ACL injuries are very unlikely to result from abduction loading alone.

Sagittal-plane advocates have suggested for years that if abduction was the primary mechanism of injury, ACL injuries would almost always occur in conjunction with medial collateral ligament injury. In fact, however, only about one-third of ACL injuries also involve the MCL.

Quatman and colleagues designed a computerized finite element model to replicate a young female athlete’s knee, then tested the effects of adding abduction loads, anterior tibial shear loads in the sagittal plane, and a combination of the two.

When abduction loads were applied, ACL strain increased as expected, but MCL strain also increased almost twice as much—suggesting that an isolated ACL tear would be very unlikely under such conditions. When a combined abduction/anterior shear force was applied, the increase in ACL shear was 1.4 times higher than the increase in MCL shear.

“For the last 20 years, we’ve lived in this world where we have to put things into an anterior shear load category or a valgus load category, when in fact it’s probably not that simple,” Quatman said.

It may be worth noting that most of the neuromuscular interventions designed to prevent ACL injury do include exercises that emphasize knee flexion angles as well as abduction angles. In another Cincinnati study presented at the same session, five clinical variables designed specifically to predict knee abduction moment included knee flexion range of motion as well as knee valgus (the other variables were tibial length, mass and quadriceps-hamstrings ratio).

Complicating the issue, as often happens in discussions of ACL biomechanics, is the possibility that the sagittal plane component of the mechanism may stem from a variable that cannot be un-taught or otherwise altered through an exercise intervention.

In a 12-specimen cadaver study presented at the same ACSM session, researchers from the University of Michigan found that peak anterior tibial acceleration was positively correlated with peak ACL strain during five loading trials simulating a single-limb landing. Anterior tibial acceleration also correlated strongly with mean peak tibial slope, which has previously been shown to change significantly with maturation but is difficult to measure clinically.

“The only way to measure tibial slope is with CT or MRI. I would suggest that acceleration is easier to measure with an accelerometer,” said Scott G. McLean, PhD, assistant professor of athletic training and movement science at the University of Michigan, who presented his group’s findings.

He noted, however, that tibial slope is unlikely to change in response to an exercise intervention.

McLean, also a longtime proponent of the frontal-plane theory, at the ACSM meeting espoused a philosophy that was more consistent with the sagittal-plane focus of his presentation.

“I’m a firm believer,” he said, “that the process is multiplanar.”

Motion control shoes may help offload lateral compartment knee osteoarthritis

Motion control running shoes may provide the same off-loading effect as a medial wedge in patients with lateral compartment knee osteoarthritis, according to research from the University of Evansville in Indiana.

Investigators analyzed 16 male and 16 female volunteers as they walked at a self-selected speed in both a motion-control shoe and a cushioned shoe. They found that peak knee abduction moment, a marker of lateral compartment loading, was 11% lower during the motion control condition than the cushioned shoe condition in the female subjects. In the male subjects, the knee abduction moment was 5% lower in the motion control shoes, but the difference was not significant.

“The motion control did what we thought it was going to do,” said Melissa A. Elpers, a graduate student in the department of physical therapy, who presented the findings in June at the annual ACSM meeting.

In a December 2009 study published in Physical Medicine & Rehabilitation, University of Virginia researchers found that motion control shoes increased knee varus torque during running by 38% compared to barefoot in healthy young adult runners.

Shorter running stride could change foot strike pattern, reduce loading

Athletes who run with a natural midfoot strike pattern have lower loading rates than habitual rearfoot strikers, but the key difference between the two styles may be stride length, according to research from the University of Delaware.

Comparing 12 rearfoot strikers and nine midfoot strikers as they ran on an instrumented platform, the investigators found that the midfoot strikers had significantly lower transient vertical impact peaks, average vertical loading rates, and instantaneous vertical loading rates.

Stride length was also about 50% shorter in the midfoot strikers – which was not the case in a previous study in which rearfoot and midfoot strike patterns were compared in runners who were all natural rearfoot strikers.

Although telling runners to consciously try to run with a MFS pattern does not seem to reduce loading rates, telling them to shorten their stride length might be more successful.

“If they can shorten their stride length, we think the midfoot strike pattern will come more naturally,” said Allison R. Altman, a graduate student in the department of physical therapy, who presented the findings in June at the ACSM meeting.

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