Study On Cutting Model Of Soft Tissue In Virtual Surgical Simulation System

Compared with the traditional surgical training,virtual surgery simulation system(VSSS)that builts on basis of virtual reality(VR)has many advantages such as being risk-free,high efficiency,reusable,and without restriction of time,space,and moral.It is becoming a hot research and developing trend under the social transition and the renovation of medical model.The VSSS presents a virtual surgery scene that is the same as the real world with the aid of computer technology.And the users interact with the virtual tissue and organ using a haptic device.The operation skill of junior surgeons will be improved quickly and efficiently by repeating the surgical operation such as palpating,cutting,tearing and sucking.In addition,the VSSS also is introduced as an auxiliary means for optimizing operation scheme,surgical navigation.In short,the VSSS is the enlargement and extension of VR in medicine,and opens up a new domain for training of surgeons and robot-operated surgery.As one of universal and most frequently used operations in actual surgery,realistic simulation of tissue cutting is an essential part of virtual surgery simulation.However,it is very difficult to establish an accurate and efficient model for soft tissue deformation and cutting because human tissue is a special elastomeric material with nonlinear,viscoelastic,anisotropic and incompressible properties.So in general,the surgeons adjust the cutting operation scheme in accordance with the feedback of vision and haptic during the interaction of surgical instruments and tissue.Therefore,from the perspective of vision feedback and haptic feedback,aiming for realism,this paper deeply studies the characteristic of soft tissue cutting to simulate a realistic cutting process for uses.The details of the main work are described as follows:We reconstructed the three-dimensional(3D)volumetric model of soft tissue that contained surface mesh and internal point cloud using two-dimensional(2D)computer tomography(CT)data source.Firstly,we utilized commercial software – Mimics to reconstruct 3D surface mesh model for soft tissue.The 2D gray image transform into 3D surface mesh after this step.Then,the Morping was introduced into our model to voxelize 3D surface mesh and thereby gaining internal point cloud model.Compared with typical subdividing method,the Morphing described in this article adopts Hermite interpolation functions,which is a nonlinear interpolation method to interpolate based on curvature of the liver model.Since the density of interpolation points is different on different curvature of corresponding position,the reconstruction model can fit the geometrical shape of the soft tissue.Meanwhile,the proper step length will be set according to the required model precision.While mesh-based approaches offer a rigorous mathematical framework for the geometric representation and are more natural for implementation,this still imposes significant challenges for reconstructing mesh efficiently in particular for complex surgical operations,such as cutting,tearing,splitting and suturing.Meanwhile,meshless approaches have been widely used for surgery simulation since they enable lightweight reconstruction of the models,but they are difficult to define boundary conditions in addition to their high computational complexity.To take advantage of the two above strategies while avoiding their respective disadvantages,a novel coupled soft tissue cutting model utilizing mesh and meshless approaches is proposed in this paper.Specifically,the reconstructed three-dimensional virtual model of human soft tissue consists of two essential components.One is associated with surface mesh that is convenient for surface rendering,and the other with internal meshless point elements that used to calculate the force feedback of soft tissue cutting.In order to combine two components in a seamless way,virtual points are introduced.During the simulation of cutting process,the Bézier curve is used to characterize smooth and vivid incision on the surface mesh.At the same time,the deformation of internal soft tissue caused by cutting can be treated as displacements of the internal point elements.Moreover,to improve the computational efficiency of the proposed scheme,the level set(LS)method is exploited to quickly track the variation of the cutting plane for calculating the displacements of the internal meshless point elements.Experimental results show that the proposed method offers high computational efficiency and good visual effect,enabling cutting of soft tissue with high stability.Unlike most existing methods that consider the cutting process of soft tissue in an over simplified manner,the presented model is able to simulate the complete cutting process that includes three stages: deformation before cutting open(DBCO),cutting open(CO),and deformation after cutting open(DACO).To characterize the complicated physical and mechanical properties of soft tissue,both nonlinearity and viscoelasticity were incorporated into the equations governing the motion of soft tissue.A line contact model was used for simulating the cutting process after analyzing the two major types of surgical instruments,i.e.,scalpel and electrostick.The cutting speed and angle were taken into account in order to improve haptic rendering.Biomechanical tests and simulation experiments verified the validity of the introduced model.Specifically,the displacement vs.cutting force curves can be divided into three segments corresponding to the three stages of the cutting process.The results were also applied in soft tissue cutting simulating system and satisfactory visual effect and haptic feedback were achieved.In view of the problems existed in haptic device are low back-drivability,high friction,low stiffness,and the actual feedback force may be covered by the noise,a novel hybrid gravity compensation method based on active compensation and passive compensation is put forward.In primary stage,the spring is applied to counteract most of the gravity.Subsequently,the remaining gravity entirely offset by active compensation.In this way,it could not weaken the net feedback force providing by the haptic,the control algorithm for the electrical machinery is also not complex.Furthermore,the proposed hybrid method will compensate the whole gravity of the haptic device under the premise of increasing less inertia.The comparison study of the doppler compensation and after the compensation feedback force show that the obtained feedback force using the compensation haptic device has a satisfying agreement with the experimental data.Finally,this paper designs a virtual surgery simulation platform with high immersion and interactivity.The platform can adaptively adjust the position based on the user's height and operating habits to feel more comfortable during training.Moreover,the platform has good scalability,it can configure different haptic device and virtual surgical soft system for different types of surgery.