Computer assisted procedures play a key role in the im-provement of surgical operations. The current techniques in simulationpotentially lead to more accuracy, more safety and more predictabilityin the surgical room. Despite the important number of algorithms pro-posed for interactively modelling deformable objects such as soft humantissues, very few methods have attempted to simulate complex anatomical configurations. In this paper, we present a new approach for softtissue modelling whose novelty is to integrate the interactions betweena given soft organ and its surrounding organs. The proposed discretemodel is compared to finite element method in order to quantify its per-formance and physical realism. The model is applied to the simulationof the prostate-bladder set.