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Research Papers

Novel Mechanical Actuation of a Modular Laparoscopic Surgical Tool

[+] Author and Article Information
David J. Miller

Department of Mechanical Engineering, University of Nebraska-Lincoln, N104 Walter Scott Engineering Center, P.O. Box 880656, Lincoln, NE 68588-0656

Carl A. Nelson1

Department of Mechanical Engineering, University of Nebraska-Lincoln, N106 Walter Scott Engineering Center, P.O. Box 880656, Lincoln, NE 68588-0656; Department of Surgery, Center for Advanced Surgical Technology (CAST), University of Nebraska Medical Center, 984075 Nebraska Medical Center, Omaha, NE 68198-4075cnelson5@unl.edu

1

Corresponding author.

J. Med. Devices 2(3), 031002 (Jul 17, 2008) (8 pages) doi:10.1115/1.2955974 History: Received October 17, 2007; Revised June 11, 2008; Published July 17, 2008

A functional analysis of current laparoscopic surgical technology prompted a redesign of the tools in order to provide multiple functionalities within a single tool. With this redesign came the need for a number of novel mechanisms to actuate and deploy functional tips to the surgical site from their storage locations. In this study we have adopted a multifaceted approach to biomedical device design: functional decomposition to determine problems with the current minimally invasive surgery paradigm, axiomatic design to ensure an efficient design, and quality function deployment to mathematically determine important design criteria. These methods were applied to the design of several novel mechanisms for achieving multifunctionality in a modular surgical instrument. The new actuation mechanism transfers squeezing motion from the hand through a gear train to the distal end of the tool where a pin-slot mechanism actuates the tool tip. The most pronounced change from current technology is the method for rotating the tool’s shaft: rather than a rotary∕rotary interaction using the index finger, a more ergonomic slider mechanism translates linear thumb motion into rotation of the tool’s shaft through a gear train. Finally, rather than locking or unlocking the jaws of the tool using multiple trigger∕ratchet interaction, the new tool uses a binary ratcheting mechanism (similar to a retractable ballpoint pen) to lock or unlock the tool with only one motion. In addition to the actuation mechanisms, the methods for indexing functional tips within the tool and interfacing the tips with a lead screw were redesigned for a modular tool. Rotary indexing of the tool cartridge is done using a Geneva-type mechanism and cam∕follower to provide positive locking once the tip is in place. Transferring the tool tip from the rotary chamber to contact the actuation∕shuttling screw is accomplished using a screw∕wedge assembly to ensure proper attachment. Each of these mechanisms is described and analyzed in detail to show the overall improvement in surgical performance of this novel tool. The benefits identified include multiple functionalities in a single tool, ergonomic benefits of an increased input∕output force scaling, decreased out-of-plane motion required to rotate the tool’s shaft, and decreased cognitive load required to lock and unlock the tool’s jaws.

FIGURES IN THIS ARTICLE
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Copyright © 2008 by American Society of Mechanical Engineers
Topics: Mechanisms , Design , Gears , Force
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References

Figures

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Figure 1

CAD model of the modular laparoscopic surgical instrument and photograph of tool prototype

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Figure 2

Tool actuation mechanism. (a) Input link and Plane 1 of mechanism. Link 1 is a Duraform handle attached to an internal gear, Links 2, 4, 6, and 7 are all compound gears, and Link 10 is a rubber pulley; all others are spur gears. The handle and Gear 3 are not meshed. (b) Output link and Plane 2 of mechanism, perpendicular to that in part (a). All links are spur gears. Numbers of teeth and gear ratios are shown in Table 1.

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Figure 3

Triggering mechanism to lock tool jaws in place

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Figure 4

Ratcheting mechanism used to activate and deactivate locking mechanism, which represents the ground and ratchet spinners from Fig. 3: (a) and (c) represent the mechanism in its extended state, and (b) and (d) represent the retracted state. Both (a) and (b) are on a transverse plane normal to the axis of the mechanism and (c) and (d) are a representation of the mechanism that has been “unrolled” radially about its axis. Not to scale.

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Figure 5

Mechanism used to rotate tool’s shaft about its axis. The slider is attached to a gear rack and Links 7 and 8 are bevel gears. All others are spur gears.

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Figure 6

(a) Mechanism used to index tool chamber and rotate to new tool tip. Link 1 is a spur gear that actuates the Geneva mechanism. (b) Positive lock mechanism used to ensure proper alignment of tool tips and lead screw. The same spur gear from part (a) actuates the cam and follower, which is held in place by the press pin. The tongue on the follower interfaces with grooves in the lock wheel to maintain the position of the chamber.

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Figure 7

Tool interface mechanism to transfer tools from cartridges to lead screw. The threaded shaft moves the two wedges to push the cartridge radially. The cartridge is returned to its normal position by a concentric spring, denoted by Fs.

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Figure 8

Free-body diagram used to calculate motor size required for indexing∕lock mechanism based on the cam and follower friction

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Figure 9

(a) Free-body diagram used to calculate motor size required for indexing∕lock mechanism based on tool chamber friction and (b) torque required to actuate Geneva mechanism

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Figure 10

Free-body diagrams used to calculate motor size required for tool interface mechanism: (a) initial position of cartridge and wedges (μs for nylon∕nylon), (b) transition between wedge and flats (μd for cellophane∕cellophane), and (c) final position of wedges (μs for nylon∕nylon). Coefficients of friction from Table 2.

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Figure 11

Free-body diagram of friction and screw force for tool interface mechanism

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Figure 12

Diagram of traditional laparoscopic tools: (a) input mechanism and (b) out-of-plane output mechanism (θp is the rotation about the long axis of the tool)

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Figure 13

(a) In order to rotate the shaft of a traditional laparoscopic tool, the surgeon must move his index finger drastically out of plane in order to affect the motion. (b) Using the current tool, the surgeon’s thumb is used to move a linear slider and the thumb remains in a comfortable plane, easing the strain on the surgeon.

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