Background: When flatfoot is acquired during adulthood, the shape of the foot changes. In addition to a decreased arch, there may be valgus angulation of the hindfoot or abduction of the forefoot, or both. However, there is little objective information to provide a better understanding of the anatomical or morphological changes that occur in acquired adult flatfoot. We wondered if such an understanding of the three-dimensional anatomy might shed light on the pathway by which these changes occur. We designed this study to measure the three-dimensional position of the talocalcaneal joint in patients who have painful flatfoot.Methods: Computed tomography scans of the feet of eight patients who had symptomatic flatfoot were used to construct a model of the talocalcaneal articulation. The scans were performed on a custom loading frame developed to simulate weight-bearing with the foot in a neutral position while a seventy-five-newton axial compressive load was applied. The digital data from the scans were used to make three-dimensional computer models of the articular surfaces of the talus and calcaneus of each foot. These models then were used to calculate the percentage of the articular surface that was in contact and, conversely, the percentage that was subluxated. Two surfaces were modeled for each bone; the posterior facet formed one surface, and the anterior and middle facets were combined to form the second surface. The data were compared, with use of Mann-Whitney nonparametric U analysis, with those derived from scans of the feet of four patients without a deformity of the hindfoot who served as controls.Results: A mean (and standard deviation) of 68 ± 9 percent of the posterior facet of the calcaneus was in contact with the talus in the patients who had flatfoot compared with 92 ± 2 percent in the controls, and a mean of 51 ± 23 percent of the anterior and middle facets of the calcaneus was in contact with the talus in the patients who had flatfoot compared with 95 ± 6 percent in the controls. These differences were significant (p = 0.0066 for both).Conclusions: Marked subluxation of the talocalcaneal joint occurs in some patients who have symptomatic planoabductovalgus deformity.