Motorized Endoscopic Grasper (MEG)

Abstract

Background

Accurate biomechanical characteristics of tissues is essential for developing realistic virtual reality surgical simulators using haptic devices. Surgical simulation technology has progressed rapidly but without a large database of soft tissue mechanical properties with which to incorporate. In addition, the majority of the research that has been done on measuring mechanical properties of abdominal soft tissues was performed in vitro, on animals and cadavers. As simulation technologies continue to be capable of modeling more complex behavior, a tissue property database needs to be developed to fill this gap. This problem has been addressed in recent work with a variety of tools and techniques.[1-4]

Methods & Tools

We have adapted our previous design for the force-reflecting endoscopic grasper (FREG) [1-2] to a motorized endoscopic grasper (MEG) that uses a brush DC motor instead of a voice-coil actuator. The motor is attached to a capstan that drives a cable and partial pulley. The pulley is attached to a ball joint that converts the rotational motion of the motor and pulley to a linear translation. The motor is capable of producing 29 mNm of continuous torque, but it is coupled with a 19:1 planetary gearhead and partial pulley that increase the torque to 3.98 Nm. This torque is equivalent to 52 N of grasping force applied by a surgeon on an endoscopic grasper's finger loops, close to the maximum value applied by surgeons in our previous work. Standard laparoscopic instruments can be attached to the base plate mount and inserted into the ball joint. Two strain gage force sensors are embedded in the pulley to provide accurate grasping force measurement. A digital encoder, attached to the motor, measures position. Computer control is provided real-time via a PC using a PD controller implemented in Simulink and custom dSPACE user interface. The MEG is a hand-held device that weighs 0.7 kg (including grasper) and can be inserted into the body through regular endoscopic ports to perform computer-controlled dynamic and static uniaxial compressive displacements (position-controlled) of soft tissues.

Results

The MEG was used to measure biomechanical characteristics of porcine tissues in vivo of the following organs: liver, spleen, lung, stomach, small bowel, and colon through a series of static and dynamic grasping tests. The resulting force-deformation data were transformed to stress-strain, which were further processed to obtain elastic modulus, creep time constant, stress relaxation time constant. Tissue types exhibited significant differences in properties. Tissue property inhomogeneity was also studied.

Conclusion

The MEG will help provide realistic data for surgical simulation and corroborate the results of other researchers. Future work will be to compare in vivo MEG data with in vitro MEG and universal testing machine data to observe the changes in tissue mechanical properties postmortem.

MEG - Rendered CAD drawing

MEG - Close-up photograph of the hand-held mechanism

MEG -- Close-up photograph of Babcock grasper

MEG -- Data Acquisition: Large Bowel, 3 Hz
MPG Video Clip (677Kb)

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[156]
J.D. Brown, J. Rosen, M. Moreyra, M. Sinanan, B. Hannaford, 'Computer-Controlled Motorized Endoscopic Grasper for In Vivo Measurements of Soft Tissue Biomechanical Characteristics,' Studies in Health Technology and Informatics - Medicine Meets Virtual Reality, vol. 85, pp. 71-73, IOS Press, Newport Beach, CA, January 2002.