Surrogate lower limb design for ankle-foot orthosis mechanical evaluation

Abstract

Purpose This study designs and provides a pilot evaluation of a novel surrogate lower limb (SLL) that provides anatomically realistic three-dimensional (3D) foot motion, based on a literature consensus of passive lower limb motion. This SLL is intended to replace single axis surrogates currently used in mechanical testing of ankle-foot orthoses (AFO). Material and Methods The SLL design is inspired by the Rizzoli foot model, with shank, hindfoot, midfoot, forefoot, and toe sections. Ball and socket joints were used between hindfoot-midfoot (HM)-forefoot sections. Forefoot-toes used a hinge joint. Three-dimensional printed nylon, thermoplastic polyurethane (TPU) and polylactic acid (PLA), as well as casted silicone rubber were used to re-create foot components. After fabrication, motion capture was performed to measure rotation using fiducial markers. The SLL was then loaded under both static and cyclic loads representing a 100 kg person walking for 500,000 cycles. Results Most joints were within 5° of target angles. The SLL survived static loads representing 1.5 times body weight for both static and cyclical loading. Conclusions This SLL moved as designed and survived testing loads, warranting further investigation towards enabling essential mechanical testing for AFO currently on the market, and helping to guide device prescription.

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

“…The silicone was thinned at 40% to better replicate the stiffness of human soft tissue, and poured into a mold the size of a male leg roughly within the 50th percentile of male U.S. Army Service members according to knee height and calf circumference [24]. A model tibia, fused foot bones, and distal femur section were printed from Rigid 10K resin (Formlabs Inc., Somerville, MA, USA) with a Young's modulus (10 GPa) Surrogates have been devised to evaluate AFOs [23] but, to our knowledge, surrogates for the assessment of tibial strain under replicated anatomical loads have yet to be developed and implemented. We developed a robotic lower-extremity surrogate with the capability of applying anatomical plantarflexor muscle action/reaction forces and a vertical external force caused by the mass of the body (Figure 1b).…”

“…Surrogates have been devised to evaluate AFOs [23] but, to our knowledge, surrogates for the assessment of tibial strain under replicated anatomical loads have yet to be developed and implemented. We developed a robotic lower-extremity surrogate with the capability of applying anatomical plantarflexor muscle action/reaction forces and a vertical external force caused by the mass of the body (Figure 1b).…”