Research On Improved Mass-Spring Models And Their Application In Virtual Surgery

Because of the continuously development of modern surgery techniques and instruments, especially the widespread application of endoscopic and interventional surgeries, more and more knowledge and technologies are needed to perform a surgery. As a result, surgeons need plentiful practices to master these new skills. Virtual surgery is an application of Virtual Reality (VR) in modern medicine. It provides a novel and effective mean for surgery education and training. Different from traditional training schemes, virtual surgery doesn't rely on substantial objects such as animals and cadavers. Moreover, virtual surgery is more suitable for beginners who lack surgery experiences because it hazards no true patient. Therefore, virtual surgery will play an important role in surgery training and the popularization of complex surgery techniques. Also, virtual surgery can be used in pre-surgery planning and rehearsal, and it can provide a way to predict and evaluate the result of the surgery which is about to perform in order to avoid potential medical malpractices.An applied virtual surgery system must provide realistic visual feedback for its user. There are lots of soft tissues in human body, and deformable instruments like sewing thread are needed in some surgery procedure, so the modeling of deformable objects is one of the key technologies in virtual surgery. There are three major requirements for deformable modeling in virtual surgery– speed, robustness and accuracy. Virtual surgery system must be a real-time interactive system, so the speed requirement is the most important one; the system must also keep stable and robust during the interaction with its user; the accuracy requirement can be relaxed because the soft tissues of different patients often have different physical attributes and their structure is so complex that it is very difficult to acquire their accurate physical parameters. According to these requirements, we use improved mass-spring models for the real-time deformable modeling in our virtual arthroscopic surgery system and the interventional cardiology simulation training system prototype.The deformable objects in our virtual surgery system have diverse geometrical and physical attributes, so three different improved mass-spring models are proposed for them. For surface mesh models of normal soft tissues in virtual surgery system, we proposed an improved mass-spring model using surface mesh model. In such a model, by adding a rigid core for surface mesh and updating the rigid core during simulation, we make the surface mesh model perform as volumetric models. By using inverse dynamics technique, this simple linear elastic model can provide some nonlinear deformation effect. For heartbeat in interventional cardiology, we proposed a real-time heartbeat modeling method based on a hybrid mass-spring model. In this method, instead of considering the microcosmic physiological and physical effects in heartbeat, we build our real-time heartbeat modeling method upon the basic rule of human heartbeat. By using a hybrid mass-spring model of human heart, our method can provide a visually realistic heartbeat effect. For some rope-like soft tissues and instruments, we proposed a mass-spring model combined with generalized cylinder. We first calculate the deformation result for the deformation calculation model for the object, and then combine it with the visual display model of the object by generalized cylinder method. These models all have good real-time performance and robustness, and can provide visually realistic deformation results for the relevant deformable objects which they are proposed for.Finally, all the three models have been applied in the virtual surgery systems which we developed. We use OOD method to merge these models into the software architecture of our systems. We also propose the file format for deformable models based on VRML standard so as to facilitate the data exchange between our system and other auxiliary software and the unification of deformable models and rigid models. The simulation result of our systems shows that all the improved mass-spring models proposed in this paper are fit for real-time deformable modeling in virtual surgery system.