Analysis of stiffness reduction in varying curvature ankle foot orthoses.

Ankle foot orthoses (AFO) are often used for patients who cannot generate a strong enough extension moment at the knee to allow functional gait. Orthotists often cut out portions of the AFO around the malleoli in order to improve comfort. There has been some question as to how this affects the stress distribution around the orthosis, the fatigue performance of the device, and the AFOs stiffness. To examine this, three orthoses were constructed with differing curvatures cut out of the malleolar regions. Photoelastic coatings were placed on the most stiff and least stiff orthoses, and the stress distributions while wearing the device were examined. A fixture was created to test the orthosis, and the stress distribution while loaded in the fixture closely matched the distribution with actual wear. These orthoses were then tested in fatigue for 500,000 cycles at 5 Hz in displacement control. Initial displacements were set to provide maximum loads of 45 lbs. The displacement settings for the stiffest orthosis were 0.4 to 0.6 inches of deflection; the load decreased from 44 lbs to 28 lbs after the final cycle. The least stiff displacement varied from 1.3 to 1.5 inches, and the load value changed from 46 lbs to 35 lbs. The data will be useful in guiding orthotists in building AFOs, particularly when shaving portions of the AFO for comfort. Excessive shaving may seriously degrade the performance of the device, especially after longer life cycles.

Structural testing of a juvenile prosthetic foot pylon.

Clinicians at the Shriner's Hospitals for Children have noticed unexpected catastrophic failures in the 2024-T3 aluminum prosthetic pylons of their juvenile patients. They postulated that small "dings", caused by sharp impacts with playground equipment or other obstacles, might be causing stress concentrations in the pylons. To investigate this, students at the United States Air Force Academy performed a series of tests to determine potential failure modes for the pylon. Initially, a technique for introducing small reproducible "dings" was established. These damaged specimens were then subjected to compressive axial tests to failure, axial fatigue tests (5-100 lbs at 5 Hz), and finally four point bending fatigue tests (370 in-lbs at 5 Hz). The maximum compressive load of the damaged specimen was virtually identical to that of the control specimen (14 kips), and the axial fatigue test was halted with no damage after one million cycles. Two bending fatigue tests resulted in cracks and eventual failure at the load application points of the fixture on the pylon. This has caused us to speculate that failure of the pylons is not due to the dings, but may be due to over-tightening of the pylon attachment points to the foot and the deficient limb. Further testing will be conducted to examine this possibility.