We present a deformable graphical model for interactive modeling of three-dimensional objects representing organs and tissues in surgery simulations. The proposed model is a physically-based model, generating real-time deformations using a high resolution triangular surface mesh with minimal pre-processing of mesh data. The surface mesh of an object is modeled by a spring network according to the connectivity of mesh nodes. Object deformations are simulated by computing nodal deformations based on a force equation at each mesh node. A deformed node index table (DNIT) is proposed to model the deformation propagation driven by the displacement at a surface contact point (SCP). Surface nodes that undergo a deformation are added to the current DNIT according to the triangular mesh topology. A PHANToM device is integrated in the simulator to provide haptic feedback. Coupled with the real-time visual feedback generated by the proposed model, the simulator allows users to interactively manipulate virtual organs utilizing the PHANToM device. Results of a virtual kidney biopsy simulation are presented to illustrate the deformations.