Computer- and robot-based systems to support interventions become more and more important in modern surgery. In general these systems provide methods to plan an intervention pre-operatively [ 1,2,13] and to execute it with support from a autonomous robot-system [3,4]. Due to the principle restriction of a robot to comparatively simple work steps, there are some complex work steps which the surgeon may plan but which he/she has to execute manually. In craniofacial surgery osteotomised bone segments are deformed by hand to a shape given by the planning system. We support the execution of pre-planned deformation by comparison of the actual shape of an object with the target shape. The actual shape is obtained intra-operatively with a surface scanning device, the deviation from the target shape are visualised by projecting colour-coded error values directly on the object to be deformed. The surgeon uses these projections to adjust further deformation steps. The system is therefore able to validate the correct execution of planned deformations, especially of bony structures.