Arch indexes from ink footprints and pressure platforms are different

Background: Footprints from force sensitive resistor (FSR) type pressure platforms have been shown to differ from ink prints, leading to a relative distortion of the arch index (AI). Objective: To compare the accuracy of the AI derived from a capacitance pressure platform relative to ink footprints. Method: Area measurements (heel, midfoot, forefoot, hallux and lesser toes) were acquired simultaneously from ink footprints and a pressure platform. Difference between paired values revealed relative measurement error. Results: The platform overestimated heel area (2.2%), but underestimated midfoot area (10.2%), and lesser toe area (17.7 %). The mean AIs were 0.206 (platform) and 0.230 (ink prints). Conclusions: The pressure platform closely estimated contact areas for the heel and forefoot. Midfoot areas were underestimated sufficiently to bias the AI towards lower values. Therefore, the AI derived from a capacitance pressure platform is not the same as that from an ink footprint, and the two should not be used interchangeably. Feet classified according to AI from a pressure platform may be categorised differently than they would by using ink footprints. Pressure platform data cannot be used to determine AI with confidence until relative measurement error for isolated foot regions has been determined, or appropriate normative values established.