A Prototype Haptic Suturing Simulator - Computer Science

567 Medicine Meets Virtual Reality 2001 J.D. Westwood et al. (Eds) IOS Press, 2001

A Prototype Haptic Suturing Simulator Roger W. Webster Ph.D.1, Dean I. Zimmerman1, Betty J. Mohler1, Michael G. Melkonian M.D.2, Randy S. Haluck M.D.2 1

Department of Computer Science, School of Science and Mathematics Millersville University of Pennsylvania, Millersville, PA 17551 2 Department of Surgery, Penn State College of Medicine The Pennsylvania State University, MC H070, PO Box 850, Hershey, PA 17033 Abstract. A new haptic simulation designed to teach basic suturing for simple wound closure is described. Needle holders are attached to the haptic device as the graphics of the needle holders, needle, sutures and virtual skin are displayed and updated in real time. The simulator incorporates several interesting components such as real-time modeling of deformable skin, tissue and suture material and realtime recording of state of activity during the task using a finite state model.

1. Background/Problem. Simple wound closure by suturing is a fundamental procedure utilized by a number of types of healthcare providers worldwide. Poor techniques can result in sub-optimal outcomes in terms of healing, infection, and cosmetics. Using a haptic suturing simulator, students can practice the virtual procedure at any time to potentially improve their technique. Researchers and developers have attempted to solve the problems that arise in the development of a generalized anatomical force feedback surgical simulator, however, a complete system is still unavailable [1-4]. By limiting their training application, i.e. bronchoscopy and lumbar puncture, developers have produced a few commercial haptic surgical simulators that are currently available [5,6]. Our intent was to develop a haptic suturing simulator, limited to suturing, that is realistic, simple to operate, economical (runs on a single personal computer), and available for widespread use. 2. Methods & Tools. The development computer is a Windows™ NT workstation with dual 600 mHz pentium processors and the Wildcat™ Open GL graphics accelerator. The Sensable Technologies Phantom™ 1.5 Desktop unit serves as the haptic interface which provides force feedback. A ‘Reachin Display’™ unit is used to provide the user with visuo-motor alignment and association. Crystal Eyes™ are used to provide three dimensional stereographic images. The three dimensional (3D) models of the virtual needle and needle holders were built in 3D Studio Max and stored as 3ds files. The haptics software modules make calls to the General Haptic Software Toolkit (GHOST) development kit from Sensable Technologies. The graphics modules make calls to EAI/Sense8's WorldToolkit API of OpenGL calls.

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The skin or soft tissue is modeled as a mesh of mass-springs (dynamic vertices) with a wound texture map wrapped onto the geometry. The system is a physically based particle model utilizing a mass-springs-damper connectivity with an implicit predictor to speed up calculations during each time step. This consists of a set of point masses (nodes) connected to each other with a network of springs and dampers. Internal and external forces act upon the springs. Each vertex in the skin geometry has a mass and is connected to every other vertex with springs and dampers. Solving mass-springs systems involves repeated application of Newton’s second law of motion, F = ma. An implicit solver for numerically solving the first order differential equations was developed similar to a technique described by Baraff and Witkin [7] in modeling cloth animation systems. The fundamental problem with implicit solvers is that they need to solve a linear system at each time-step, which is not practical with today's computers. To circumvent this computational expense, an approximate solver based upon the work of Desbrun et. al. [8] that pre-computes the solution to a linear system was incorporated. There are four basic forces (gravity, collision, spring forces and damping) which are manipulated within this system. The gravitational forces accelerate all masses in the y-axis at 10.0 m/s/s. The perfectly elastic collision forces transfer all momentum back to the colliding mass points, for example, prodding the skin with the needle. The springs, which are stretched or compressed away from their initial resting length, are subject to the conventional Hooke's Law restoring force (F = -kx). To prevent numerical instabilities due to the approximations, a damping coefficient in which moving masses experience a nominal force opposite to the direction of their movement was calculated. 3. Results. The simulation software calculates contact forces and generates tissue displacements. The resistant force calculations vary depending upon the depth of insertion and the insertion angle of the curved needle. The forces also change when the needle punctures the virtual skin. As the user ‘sutures’, the software pulls the stitches together utilizing the mass-springs deformation. The user is constrained (force effect) when the needle has penetrated the skin but the forces ease off if the user pulls the needle out in the same path as it was inserted. The user is constrained from sliding the needle along the plane once it has penetrated the skin. The user can, of course, continue penetration of the needle into the soft tissue. Forces are exerted if the user deviates from the natural penetration of the projected needle path and when pulling on the suture or drawing the suture tight (Figure 1).

Figure 1: Screen shot of haptic suturing simulator. The software records the positions and orientation of the Phantom encoders and all 3D graphics objects (needle, sutures, needle driver). Therefore, the 3D suturing technique may be replayed showing the user what they did during the training session. Rosen et al [9] developed a model of surgical maneuver states and their transitions which included idle, grasping the needle with the needle driver, puncturing the skin, pushing the needle through

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the skin, opening the needle driver, closing the needle driver and pulling the needle through the skin. Software analyzing surgical motions, by using a modification of Rosen’s finite state model for scoring purposes, is in development. 4. Discussion/Conclusions. Ongoing work is directed toward the development of imbedded instructional materials such as demonstrative video clips, higher fidelity force feedback and a scoring mechanism. Future work includes using a true 6 DOF haptic device to provide more accurate twist constraints and torques. The long-term goal is to measure skills in both haptic virtual surgery and in vivo surgery. The end result is to develop a training system that promotes development of skills in basic suturing techniques are transferable to the real world. 5.

Acknowledgments.

This project was funded, in part, by the National Science Foundation under grant numbers DUE-9950742 and DUE-9651237, and a Penn State University College of Medicine Department of Surgery Feasibility Grant, the Eberly Virtual Hospital Project, the Millersville University Neimeyer-Hodgson Grants Program and by the Faculty Grants Committee of Millersville University. Thanks to Cindy Miller for her editorial expertise.

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