Characterizing the Mechanical Stiffness of Passive-Dynamic Ankle-Foot Orthosis Struts

The effect of carbon fiber custom dynamic orthosis use and design on center of pressure progression and perceived smoothness in individuals with lower limb trauma

BACKGROUND

Carbon-fiber custom dynamic orthoses are used to improve gait and limb function following lower limb trauma in specialty centers. However, the effects of commercially available orthoses on center of pressure progression and patient perception of orthosis smoothness during walking are poorly understood.

METHODS

In total, 16 participants with a unilateral lower extremity traumatic injury underwent gait analysis when walking without an orthosis, and while wearing monolithic and modular devices, in a randomized order. Device alignment, stiffness, participant rating of perceived device smoothness, center of pressure velocity, and ankle zero moment crossing were assessed.

FINDINGS

The modular device was approximately twice as stiff as the monolithic device. Alignment, smoothness ratings, peak magnitude of center of pressure velocity, and zero moment crossing were not different between study devices. The time to peak center of pressure velocity occurred significantly later for the modular device compared to the monolithic and no orthosis conditions, with large effect sizes observed.

INTERPRETATION

Commercially available orthoses commonly used to treat limb trauma affect the timing of center of pressure progression relative to walking without an orthosis. Despite multiple design differences, monolithic and modular orthoses included in this study did not differ with respect to other measures of center of pressure progression. Perceived smoothness ratings were approximately 40% greater with the study orthoses as compared to previous studies in specialty centers, which may be due to a more gradual center of pressure progression, as indicted by lower peak magnitude of center of pressure velocity with both study orthoses.

Advancing Exoskeleton Development: Validation of a Robotic Surrogate to Measure Tibial Strain

Bone stress injuries are prevalent among athletes and military recruits and can significantly compromise training schedules. The development of an ankle-foot orthosis to reduce tibial load and enable a faster return to activity will require new device testing methodologies capable of capturing the contribution of muscular force on tibial strain. Thus, an actuated robotic surrogate leg was developed to explore how tibial strain changes with different ankle-foot orthosis conditions. The purpose of this work was to assess the reliability, scalability, and behavior of the surrogate. A dual actuation system consisting of a Bowden cable and a vertical load applied to the femur via a material testing system, replicated the action-reaction of the Achilles-soleus complex. Maximum and minimum principal strain, maximum shear strain, and axial strain were measured by instrumented strain gauges at five locations on the tibia. Strains were highly repeatable across tests but did not consistently match in vivo data when scaled. However, the stiffness of the ankle-foot orthosis strut did not systematically affect tibial load, which is consistent with in vivo findings. Future work will involve improving the scalability of the results to match in vivo data and using the surrogate to inform exoskeletal designs for bone stress injuries.