Design and Development of Instrumented Foot Form for Testing of Metatarsal Protective Footwear

Abstract

Protective footwear with metatarsal guards can play a critical role in preventing high energy impact injuries to the foot. Currently, there is a wide variety of different metatarsal guard types, each of which must pass impact testing standards before being used. These current impact testing standards to assess the efficacy of protective footwear, such as ASTM F2412, often use deformation of an internal material like wax as the performance indicator for metatarsal guards. However, previous studies have shown that deformation is not a great predictor of metatarsal fracture injury risk. Therefore, the overall goal of this work was to develop and validate a biofidelic instrumented foot surrogate for evaluating the impact performance of metatarsal protective equipment. A combination of computational and experimental methods were first used to identify a surrogate material that demonstrates a similar force-deformation response to the human foot. Next, design for additive manufacturing parameters, such as lattice structures, infill density, and unit cell size, were used to produce a foot form with a biofidelic impact response. This design process used an iterative methodology to develop three prototypes of additively manufactured surrogates using engineered hyperelastic material and embedded load cells. A series of drop tower tests were conducted using ASTM methodology, and the force and deformation for each prototype was compared to cadaveric data reported in the literature. Three types of metatarsal guards were drop tested with the prototypes, and transmitted forces were recorded through an embedded force measurement system. Results from impact testing showed that all developed prototypes provided a closer match to cadaveric force-deformation behaviour, with Prototypes I and III performing slightly better. Metatarsal guard performance results were limited as the load-sensing equipment was overloaded. Prototype I’s load-sensing method was unreliable. Prototype II reported the worst performance for soft metatarsal guard footwear. Prototype III demonstrated that soft and hard metatarsal guards offer better protection than boots without guards. In conclusion, this study presented a novel foot surrogate for metatarsal impact resistance testing. Further studies are required to refine the design to improve the force measurement system and ensure a better fit of the foot surrogate inside protective footwear.