INTRODUCTION: In plastic and reconstructive surgery, self-activating hydrogel expanders are used to augment soft-tissue space. The purpose of this study was to investigate the morphological response of underlying bone to the constant pressure exerted by a hydrogel expander. METHODS: Eighteen Lewis rats were randomly divided into three groups. In group 1, a hydrogel expander was placed subperiosteally directly onto the calvaria of the rats. In group 2, the expander and the underlying bone were separated by a polydioxanone (PDS) foil. Group 3 animals served as controls. Before and 14 days after the insertion of the expanders, micro-computed tomography (CT) images were obtained and fused. We analysed hydroxyapatite density beneath and at the periphery of the expander and performed a histomorphometric bone analysis. RESULTS: Whereas there were no significant differences (p<0.05) (groups 1 and 2) in bone density at the periphery of the expanders between the study groups, a significant decrease in hydroxyapatite density beneath the expanders was observed in those animals in which the devices were placed directly onto the calvaria (group 1). Whereas bone thickness was unaffected at the periphery of the expanders in all groups, it was significantly decreased beneath the expanders in all implanted animals. A morphological examination revealed resorption lacunae with a diameter of 218.4+/-56 mICROm in those rats in which the expanders had been placed directly onto the calvaria. CONCLUSION: This study shows the direct influence of hydrogel expanders on underlying bone. Whereas bone resorption and connective tissue formation also occur underneath hydrogel expanders, these effects can be avoided if the expander and the underlying bone are separated by PDS foil. The key to success is to ensure the appropriate placement of expanders and thus to avoid bone resorption.