Investigating the reliability of a 3D foot measurement application

Background: The measurement of foot size is important for appropriate shoe fit. Ill-fitting footwear has been associated with tissue injury, discomfort and dissatisfaction. Despite this consumers regularly buy footwear without having their feet measured, particularly online. Recently mobile applications generate a 3D model of the foot from photographs to provide dimensions for consumers purchasing online.Aims: This work determined the repeatability and validity of one such application, which provides length and width measures.Methods: A two day repeated measures approach was implemented. Six participants had both feet measured three times on two days with both the application and 3D laser scanner. Participants stood bilaterally for the scans. The mobile application required at least ten photographs of the foot to be taken on a calibration background. Some scans were ineffective, therefore ten feet were analysed. Width and length measures were compared through t-tests and standard error of the measurements were calculated (SEM). The repeatability of the two devices was calculated using intra-class correlation coefficient (ICC) within and between days.Results: The application was less repeatable within (ICC = .678, .437) and between (ICC = .654, .532) day for length and width respectively than the 3D scanner (ICC = .590–.973). Mean differences between the two methods were larger for width (5.2 ± 3.1 mm, p < .001) than length (2.0 ± 3.9 mm, p = .146).Summary/Conclusions: The SEM for the application was approximately equal to half a UK shoe size for length (3.2 mm) and width (2.6 mm), more than the 3D scanner (