The variable stiffness orthosis: customizable mechanics for assistance and rehabilitation.
Nikko Van Crey, David Lam, Emily Bywater and 2 others
PMID 42642769WHAT IT FOUND
A prototype ankle-foot orthosis whose stiffness can be changed between and within steps matched its bench-test predictions.
In one woman with sciatic nerve injury it reduced foot drop and toe-striking. Two people walked in it, so it is not evidence it beats a standard orthosis.
Key findings
01The orthosis behaved close to what its design model predicted, and it returned about 91% of the energy put into it, against roughly 60% for articulated orthoses.
02Across its interchangeable cams, the device produced stiffness from -52.4 to 5.9 newton-metres per degree, spanning the softest to the stiffest commercially available orthoses and including negative stiffness, where the device drives movement rather than resisting it.
03In the one woman with sciatic nerve injury, walking in the orthosis reduced foot drop on her affected side (peak swing angles of -7 to +2 degrees at the stiffness she preferred, against up to about -20 degrees in shoes alone) and replaced toe-striking with a heel strike for level and downhill walking.
STILL TO COME
How it was doneWhat they foundWhat it means for PTs
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What it does not show
Only two people used the device, one woman with sciatic nerve injury and one able-bodied man. That is not enough to say how the orthosis behaves in the patients you see. There was no control group, no randomisation and no blinding. Each condition was compared with the same person's walking in shoes alone or in their existing orthosis, so nothing separates the device from practice, effort or expectation. The orthosis weighs 987 g before the shoes; the participant's own daily orthosis weighed 236 g. That extra mass falls hardest on smaller or weaker patients. Only one cam version was tested across all activities. The other cams were only tested on level ground at a self-selected speed, so the activity-by-stiffness picture is thin. The stiffness of the participant's own daily orthosis was not known, so the comparison between the two devices is not a like-for-like mechanical one. The authors state that the energy-exchange modules only work if the ankle moves far enough to reach the switching angles, and that adding stiffness reduces ankle movement and can eventually stop switching altogether. The authors state that the device has no battery or energy reservoir, so negative and extreme stiffness depend on energy harvested earlier in the gait cycle. Regions the model predicted would be infinitely stiff could not be tested on the dynamometer, which forces the angle to keep changing, so the measured stiffness there is likely underestimated.
Declared interests
The work was supported by the National Science Foundation Graduate Research Fellowship Program, a Michigan Translational Research and Commercialization Award from the MTRAC Life Sciences Hub, and the Eunice Kennedy Shriver National Institute of Child Health and Human Development. The text supplied names those funders and states no other competing interests or commercial relationship with an orthosis manufacturer.
The easy way to misread this
Do not read the reduced foot drop and faster stair descent as evidence this orthosis works better than a standard one. Those results come from a single woman with sciatic nerve injury compared against her own daily orthosis, with no randomisation or blinding, and the only other person tested had no impairment at all.
Summarised by AI from the full paper, without a clinician reviewing it. Check it against the source before it changes what you do. Read it on PubMed →