In this Chapter, we describe the Tissue Retraction (TR) task, which has been selected as representative task to validate DEFRAS (Chapter 9), and we detail its implementation within our framework. The TR task allows to assess the ability of the framework to deal with uncertainty, since it involves interaction with the soft anatomical tissues, whose mechanical properties are generally only approximately known before the intervention. While an initial Patient-specific Biomechanical Model (PBM) can be generated from each patient pre-operative images and data, it may not accurately describe the actual behavior of the tissue, thus requiring intra-operative update. Section 1 describes the implementation of the tissue retraction task, and Sect. 2 presents the simulation and experimental setup that were used. Section 3 discusses the results of the simulations of TR and Sect. 4 presents the experimental validation on the laboratory experimental setup. Section 5 concludes this Chapter.