Laparoscopic Multifunctional Instruments: Design and Testing of Initial Prototypes
The concept of multifunctionality and increased end-effector dexterity is achievable as demonstrated by the prototypes presented. Further work is required to refine, simplify, and improve the multifunctional instruments to a point where they may be useful as surgical tools.
Advances in minimally invasive surgical techniques will require new types of instrument end-effectors for smaller, longer, and flexible instruments. These include a new class of multifunctional instruments capable of performing more than 1 task with a single set of working jaws. Furthermore, it is desired that multifunctional instruments be designed to provide improved dexterity compared with that in currently commercially available instruments.
Laparoscopic instruments are often used to perform a variety of functions in addition to their primary design function. Based on a prior study, patterns of usage and instrument exchanges in common laparoscopic procedures were identified. 2 With this background, we focused on developing dexterous instruments for MIS that are capable of performing more than 1 task with a single set of working jaws.
Several instrument designs that could be considered multifunctional are reported in patent filings. These devices include a grasping device with an extending articulation feature. 7 This device, however, placed the articulation pivot far off the tool's axis making it difficult to administer precise movement. Other patents include a compliant grasping tool design with transversely retractable scissor blades, 8 a surgical instrument comprising a grasping end-effector with a transversely extendable blade along side the end-effector used for cutting in the knee cavity joint operation, 9 and a handle for a medical instrument that could be used to control a 2-function instrument. 10
In 1996, Melzer 1 provided an extensive review of advanced concepts for “intelligent” endoscopic instruments and described several devices that could be considered multifunctional. These included an instrument that could perform blunt dissection as well as suction and irrigation and a tool that combined a high-frequency hook with an ultrasonic dissection probe. Cohn et al 3 and Sastry et al 4 have developed hand-like end-effectors that provide a high level of dexterity, but have limited force output capability. Pietrabissa et al 5 presented a multifunctional instrument for hand-assisted laparoscopic surgery that exhibits grasping and dissecting capabilities. Other tools include an elastic jaw grasping forceps that utilizes 2 elastic beams in place of a mechanical hinge joint. 6
Most instruments used for minimally invasive surgery (MIS) are single-function by design and are continually exchanged during endoscopic procedures. Instrument exchanges comprise 10% to 30% of total time thus adding to procedure time, 1 disrupting the surgeon's train of thought, and possibly compromising the patient's safety. 2 Surgeons stand to benefit from the availability of dexterous multifunctional instruments, ie, instruments that are capable of performing more than 1 task with a single set of working jaws.
METHODS
Three prototype instruments were designed to perform multiple functions at the tool tip using a single set of working jaws with a single handle input to control each function. The combination scissors and grasper was selected as a candidate for multifunctional design, and a 5.0-mm diameter effector and shaft platform was used for the laparoscopic instrument development. One important design consideration applicable to all prototypes is the surface of the working jaws. A compromise was needed between conflicting design requirements of rough grasping surfaces with smooth cutting surfaces.
Prototype #1: Mechanical Scissor-Grasper ( and )
It shows that the intermediate portion of the jaws has a smooth surface with a sharp edge for cutting, and a small portion of the tip and the proximal portion of the working jaws have texture for grasping. The end-effector mechanism is actuated by 2 half-circular cross-section pushrods. The grasping motion of the end-effector is accomplished through a pin and slot mechanism ( ). As the pushrod is moved forward or backward, the pin slides in the slot, causing the grasping jaw to open and close, while the second jaw remains stationary. Cutting is controlled by the second pushrod, linked to the second jaw ( ). As the pushrod is moved, the connecting link causes a rotational motion of the end-effector cutting scissor blade producing a scissors shearing action, while the first jaw remains stationary.
A toggle switch on the top of the handle is moved side to side to switch between cutting and grasping functions ( ). The 2-position toggle switch engages one of the pushrods to the input handle while the other pushrod is locked in position. A finger knob on the shaft allows rotation of the instrument shaft 360° about its axis.
Prototype #2: Mechanical Scissors-Grasper-Articulator ( and )
The multifunctional scissors-grasper-articulator design is similar to the scissors-grasper design, but a third function of articulation is added. The function of articulation permits the jaws to rotate 80° off axis approximately 12 mm from the distal instrument tip ( , ). Articulation is actuated via 4 cables routed through the shaft from the handle to the end-effector. The working grasper jaws articulate by rotating in a plane perpendicular to the plane of opening and closing of the jaws. This articulation provides nonlinear access at the surgical site. When the instrument is in scissors mode, one of the jaws is held stationary. When the moving handle link is rotated, 2 steel cables connected to the upper jaw are actuated producing a scissor motion ( ). The grasping motion is activated via a pushrod and pin and slot joints ( ).
shows the articulation switch in the handle, which by sliding forward or backward moves the corresponding internal links connected to the concentric shafts and articulation cables to control both grasper and scissor motions. The mechanical connection between the sliding links and the concentric shafts uncouples the rotational motion of the shaft and the translational motion of the sliding internal links, thus allowing the cables to be actuated at any angular position of the shaft. When moved transversely into and out of the side of the handle, the toggle switch switches the instrument between scissors and grasper modes.
Prototype #3: Compliant Scissors-Grasper
This prototype incorporates a compliant mechanism that is a single-piece flexible structure that exploits elastic deformation to achieve motion transmission. It can be thought of as a mechanism without hinge joints. The topology optimization software used to design and analyze the compliant mechanism structural elements has been previously discussed by Frecker and Dziedzic.11,12
illustrates the prototype compliant end-effector in its 3 primary positions, where the grasping and cutting occur in perpendicular planes. The prototype end-effector was fabricated from stainless spring steel using wire electro-discharge machining and micro-milling. The working portion of the jaws is 12.5-mm long, but the compliant mechanism is 36-mm long to allow the jaws to open completely without overstressing the compliant members.
The distal portions of the jaws have texture to provide friction during grasping, as shown in .
The prototype instrument shown in has a compliant tool tip actuated by pushrods that connect the handle to the distal end of the instrument. One rod forces the grasping jaws closed by pulling on the outer portion of the tip, and a second rod forces the scissor blades closed by pulling on the inner portion. The compliant mechanism does not have any hinges and works by elastic deformation; therefore, it requires an increasing amount of input force to increase the deflection of the tool tip. A static balance mechanism was added to the handle to counteract this effect and reduce the “stiffness” felt at the handle. The static balance mechanism together with the compliant mechanism has equal potential energy in any position, thus the force required to actuate the system is constant.
The 3 multifunctional prototypes underwent several analyses:
Kinematic and Finite Element Analysis
Kinematic and finite element analyses were performed on the 2 mechanical linkage designs (Prototypes 1 and 2), using Working Model (Knowledge Revolution, San Mateo, CA) software. Finite element analysis (FEA) was performed on the compliant scissors-grasper end-effector design (Prototype 3), using Pro/Mechanica (PTC, Needham, MA).
Bench-top Testing
Simple bench-top tests, grasping force at the jaws, cutting force at the jaws, and pull-off force were performed on the prototypes to quantify and compare force-deflection performance. For prototypes 1 and 2, grasping force was measured by closing the grasping jaws against a 500g Omega load cell, and for prototype 3, a Chatillon Model DFM 10-lb digital force meter was used. Cutting force in all cases was measured by closing the cutting jaws against a digital force meter. The measured cutting force was simply the closing force of the scissor jaw, and not a measure of the shear force between the scissor blades. Pull-off force was measured by gripping a 0.24 in section of red rubber tubing in the grasper jaws and measuring the force required to pull the tube from the jaws with a digital force meter. For comparison purposes, force measurements were taken for the Auto Suture Endo Dissect disposable instrument (US Surgical, Norwalk, CT). shows the test set-up used to measure the cutting and grasping force of the mechanical linkage prototypes (left) and compliant prototype (right).
Rosser Station Evaluation/Dexterity Testing
The prototypes were tested in a basic bench-top laparoscopic training box. The Bean Drop and Cobra Rope Rosser station tasks, paper cutting, and “fuzzy ball” tests were used to assess dexterity ( ).13 The fuzzy ball test was used to determine fine grasping accuracy of the “fuzz” with the tips of the instrument only. This test consists of picking up and passing several small soft plush balls back-and-forth between the prototypes and a disposable grasping tool for comparison (Endo Dissect, US Surgical, Norwalk, CT). For the paper-cutting test, the disposable grasping tool held a small piece of paper while the multifunctional scissors was used ( ). The ease of toggling between instrument functions was also assessed during this procedure.
