October 2011

AFO strap mapping

In the moment: O&P

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Study locates optimal position, angle

When researchers from the Georgia Institute of Technology were trying to determine the ideal position for a dorsal strap to prevent pistoning within an ankle foot orthosis, they approached the problem from multiple angles—literally. What they found was that the optimal angle for a dorsal strap position isn’t close to the angle most often used in practice.

By Jordana Bieze Foster

“If they are acting on bony anatomy, these straps should be placed as proximally as possible and perpendicular to the line connecting the metatarsophalangeal joint and the axis of the ankle,” said Tyler Manee, MSPO, who developed the study for his thesis project in Georgia Tech’s Master of Science in Prosthetics and Orthotics program.

The objective of a dorsal AFO strap is to limit pistoning, or vertical motion of the heel within the heel cup of the device, which can lead to friction and skin ulceration. In practice, dorsal straps are typically positioned proximally on the foot, but tend to be angled at 45° posteriorly and inferiorly. Manee was not able to find any scientific evidence in the medical literature supporting this conventional approach, however.

The objective was to identify a strap position that would require the least amount of applied force to maximize foot stability within the AFO. Applying principles of physics suggested that the ideal strap position should be proximal and the ideal strap angle should be 90° from an imaginary line along the bony dorsum of the foot.

“Think about the top of the foot as a wrench, and the ankle joint as the bolt. If you have a wrench, you want to grab it as far down the handle as possible and push at an angle 90° to the handle to maximize your force,” Manee said. “So if you want to hold the top of the foot down and only apply a force to the top of the foot, we want to apply it as proximally as possible and perpendicular to that line connecting the axis of the ankle and the metatarsophalangeal joint.”

Putting this theory to the test, Manee and colleagues assessed three different strap positions along the dorsal foot and three different angles for each position, using an experimental device designed to simulate third rocker in a cadaveric foot. Approximately 35 pounds of weight pulled up on the leg to simulate heel rise during each trial, with a metal bar holding down the toes. Each strap position was tested at angles of 80°, 90°, and 100° from the dorsal line.

As expected, the most proximal location was associated with the least amount of force on the foot, and for each location, the 90° angle was the one most often associated with the greatest stabilizing force on the strap. The findings suggest that the conventional 45° strap angle provides a much lower than ideal stabilizing force.

“If you’re 90 degrees to how the foot has to pivot, that’s the best you can do. Any other direction is basically wasted,” Manee said.

Interestingly, however, for the most proximal location tested, the largest strap angle was actually associated with more force on the strap than the perpendicular angle. This may be because, at that angle, the strap is positioned over the ankle dorsiflexor muscles rather than directly over bone, distorting the way forces are attenuated, Manee theorized.

The study, presented as a poster at the annual meeting of the American Orthotic & Prosthetic Association in September, earned Manee the inaugural Otto and Lucille Becker Award for best orthotic poster abstract. The award included the costs of travel to Las Vegas for the annual meeting, registration fees, and $500.

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