An anatomically unbiased foot template for inter-subject plantar pressure evaluation

Pedobarographic images reflect the dynamic interaction between the plantar foot and supporting surfaces during gait and postural activities. Since intra-foot and inter-subject contact geometry are grossly similar, images may be spatially registered and directly compared. Previously arbitrary subjects have been selected as registration templates, but this can conceivably introduce anatomical bias. The purposes of this study were: (i) to compute an unbiased pedobarographic template from a large sample of healthy young adult subjects, and (ii) to demonstrate how the resulting template may be used for practical clinical and scientific analyses. Images were obtained from N=104 subjects and were registered (10,712 pairs) using (i) an optimal linear scaling technique and (ii) a nonlinear, locally affine, globally smooth technique. The nonlinear technique was found to offer biomechanically non-trivial advantages over the linear technique, most likely due to non-proportional inter-subject geometry. Specifically, the nonlinear template was able to detect morphological signals in a hallux valgus sample with greater sensitivity than the linear template. Validity of the approach was confirmed by independently assessing left and right feet, through a statistical comparison of local maximal pressures, and also through examination of random subject subsets. The current template, representative of an average healthy foot, could be a valuable resource for automated clinical and scientific analyses of foot morphology and function.