Developing a Load Cycle Test Setup to Evaluate the Durability of Ankle-Foot Orthoses

Abstract

This graduation project is for an organisation that operates in the field of orthopaedic appliances, specialising in personalised hand braces based on 3D scans. They are now looking into developing personalized ankle-foot orthoses (AFOs) using 3D printing. However, it remains uncertain whether (partly) 3D-printed AFOs can provide two years of care without breaking or experiencing significant performance loss. Although static loading and impact testing should also be conducted, the project focuses on testing cyclic loading of the AFO, equivalent to two years of walking (2 million steps).

A proof-of-concept test setup was constructed. The design incorporates a dummy lower leg attached to a linear actuator, with the AFO strapped on the dummy leg, and a platform on which the dummy leg lands. The platform can be angled to separately test the three most characteristic phases of walking: heel, ankle, and forefoot rocker. A critical question involved determining the appropriate load on the AFO. This was established by correlating the bending of the AFO in the test setup with that occurring during the gait of a normal person. Displacement was measured through video analysis. Based on cycle testing, the test setup only the forefoot rocker could be simulated effectively. For the heel rocker, a dummy foot with an ankle joint is necessary to achieve the required plantar flexion. For the ankle rocker, the platform must rotate during the cycle. A trial run was performed to assess the reliability of the test setup; it was determined that a shoe is needed to properly secure the AFO within the test apparatus. A full-length dummy foot with a meta-phalangeal (MTP) joint is essential for testing with a shoe.

Based on these insights, a digital redesign of the AFO load cycle test setup was created. The redesign features a platform that rotates during a cycle and a dummy leg with an ankle and MTP joint. With this new design, all three rockers can be linked together to reproduce a complete representative step in one cycle. The combination of the linear actuator and the rotating platform can simulate the complex movement pattern of the stance phase of a step: the linear actuator replicates the vertical movement, while the platform models the angular orientation of the lower leg. Although time constraints in the project prevented the realization of the rotating platform, the dummy foot with ankle and MTP joint was achieved by adapting a prosthetic foot. Video analysis indicated that with this design, the heel rocker can now be simulated as well, and it is predicted that it will also permit testing for the ankle rocker if a rotating platform is included. The new foot also demonstrated promising results in maintaining the AFO and shoe in place.

The company's recommended next step following this project is to construct and test the redesign. If this redesign is successfully realised and optimised, AFO designs can be tested under cyclic loading due to walking to ensure they can provide two years of care.