Modeling surgery:

SPRING employs 3-D models of deformable tissue that include spring-based force computations to model the physical characteristics of real tissues. Users have full 3-D interaction with tissue models through 3-D interfaces, including 3-D digitizers and specialized devices that look and feel like laparoscopic tools. The simulation of tissue-tool interaction also includes force computation, providing realistic haptic feedback to the user. Through the provided tool interfaces. users manipulate the surgical world using familiar surgical methods including palpating, grabbing, cutting, dissection, ablating, and suturing.

SPRING simulates deformable tissues constructed from 3-D nodes, edges, faces, and tetrahedra. Edges include stiffness parameters which determine the ease of tissue deformation. Laparoscopic tools including probes, graspers, forceps, scissors, ablators, and suture needles, provide realistic interaction with tissue models. SPRING's numerical methods are optimized to provide real-time interaction in the 3-D world.

SPRING's models employ force computations from physical laws and apply these forces to the 3-D model components. The computations modeled include:

Tissue deformation and relaxation
External forces such as gravity
3-D collision detection with force feedback

The set of 3-D drawable objects comprises SPRING's universe, which is implemented as an array of 3-D drawable objects that interact when the simulation is running.

SPRING and Tissue Models

SPRING may be used directly with user-supplied tissue models for free-form interaction and surgical simulation. In addition, SPRING is a customizable development platform for surgical scenarios and skill development games. Using standard software development tools, structured surgical scenarios and exercises may be constructed within the SPRING architecture, fully employing SPRING's physical simulation and sensor interface capabilities.

SPRING accepts 3-D models of tissues and surgical tools in standard formats, including but not limited to:

Simple Model Format (SMF)
3-D Object (OBJ)
Virtual Reality (WRL)

These 3-D formats can be created by many programs that are commercially available. In addition, articulated tools with hinges and other moving components are defined in the MESH format, an extension of SMF which explicitly defines sub-objects.

See SPRING documentation for more details on these and other technical topics.

Tools Models in SPRING

A number of tool models are included in the SPRING distributing, including forcepts, laparascopes, dilators, and others. The shapes of tools are defined in 3-D format files, including SMF and OBJ. However, articulated tools with subparts that move in response to user actions, such as graspers, scissors, etc., require that nodes and faces are labeled with the subpart identity. This is provided in the MESH format, similar to SMF and OBJ, but including an subpart identifier for each node.

Tools are usually connected to interactive controllers, such as a mouse, trackball, 3-D positioning device, haptic interface, or other user-controlled manipulator. SPRING uses the sensor construction to link the real input/output device to the graphical depiction of the tool in the simulator's universe. Motions of the device are converted to rotations and translations of the graphical tool, and forces computed by SPRING's physics simulation are transmitted to the haptics interface via the sensor abstration.

Sensors in SPRING

The sensor concept is an abstration of a 3-d positioning device that controls virtual tools in the SPRING simulation. A sensor connects a 3-D graphical objects,typically a representation of a surgical tool, to an actual 3-D positioning device. The operations of a sensor are simple in concept:

The sensor receives 3-D information from an external interface, such as a haptic device, a 3-D positining tool, or a mouse. The connection may be through SPRING's direct link with the interface, fore example by obtaining the mouse's movements directly and converting the 2-D position into motions in a selected plane.
Some interfaces are made through network connections to specialized device servers, which are simple programs that continually read the 3-D position and orientation of a device such as the arm of a Sensable Phantom OMNI haptic interface. This simple haptic server program formats the data and sends network messages to the sensor object in SPRING, which stores the 3-D information for access by the linked graphical object.
When the simulation is actively running, SPRING reads the 3-D information from each sensor object and then applies the transformation to the object's nodes, edges, and faces. The updated positions are used for drawing the 3-D object and for computing collisions of the tool with tissues, tools, and other objects in th4e universe. SPRING's physics simulation is driven by the resolution of each collision, with the resulting forces resulting in movements of nodes, deformation of soft tissues, and behaviors that are dependent on the type of tool, e.g., probing, grasping, cutting, ablating, etc.
When collisions result in forces on the tool, SPRING sends the resulting force vector to the sensor object.
In each simulation cycle, each sensor that is connected to a haptic output device sends the force information to the device for rendering the force feedback to the user.

The rate of 3-D position update and haptic feedback is determined by the main loop processing time within SPRING. For models of moderate size running on a powerful computer system, SPRING can provide feedback of up to 1000 times a second.