Toward robot-assisted vascular microsurgery in the retina - Colgate

699Center of Rotation Translates With Respect to Base/YFixed H"~r idEn tffal ic~° r ------%~]_ "" ~'- Translate ~ "(f)~ y eaBase I [ Base JCenter of Rotation Fixed With Respect to Base(e)~~ L~~///// ~'/ / ~MMicrornanipulator J///\\\Base]vBase ]Fig. 4 Schematic view of the end effector used for retinal vascularmicropuncture. A micropipette holder (a) provides a pressure-tightinterface to the glass micropipette. The micropipette assembly is attachedto a Narishige hydraulic actuator (b) which advances and retractsthe micropipette relative to the hypodermic needle (c). A siliconrubber boot (d) provides a seal between the micropipette andhypodermic needle to prevent the leakage of vitreous. A dovetailslide (e) guides the micropipette during initially insertion throughthe hypodermic needle. The hypodermic needle is held in place witha locking cam mechanism Q)Fig. 3a, b Splitting the pose variables into functional groups allowsan input device with fewer degrees of freedom than the manipulatorto be used intuitively for micropuncture, a The translational mode ofoperation translates the enter of rotation using cartesian coordinateswhile maintaining a fixed tool orientation. This is used to place thecenter of rotation of the tool coincident with the scleral incisionpoint prior to tool insertion, b The rotational mode of operation fixesthe center of rotation at the scleral incision point and pivots the toolabout that point or moves the tool through that pointto move the micropipette through the needle. The hypodermic needleprovides a passageway for the micropipette through the scleraand into the eye. A silicon rubber boot placed over the micropipetteand hypodermic needle prevents the leakage of vitreous.Experimental useThe manipulator has been used extensively in our laboratory tomake physiolocial measurements in the retinal circulation of thelive, anesthetized cat. All animal experiments were performed in accordancewith the Principles of laboratory animal care (NIH publicationNo. 86-23, revised 1985). Details on our experimental setupand micropuncture can be found elsewhere [2, 5, 6].To prepare the manipulator for use, a hypodermic needle wasloaded into the end effector such that the tip of the needle was atthe software-generated center of rotation. A microscope stand holdingthe manipulator was used to position the tip of the needle near thesclera and aim the needle in the general direction of the targeted vessels.Once approximate positioning was complete, the "translational"mode of operation was used to place the tip of the needle (and virtualcenter of rotation) at the desired scleral incision point. The center ofrotation was then fixed at the scleral incision point by changing themode of operation to "rotational," guaranteeing that the micropipeneassembly would always pass through this selected point.The needle was filled with normal saline to prevent the introductionof air into the vitreal chamber, rotated using the manipulator untilit was normal to the surface of the eye, and pushed through thesclera. With the hypodermic needle successfully inserted into theeye, a micropipette was filled with either a solution to be injectedor 2M saline for making pressure measurements. The micropipettewas placed in the holder and attached to the Narishige hydraulic cylinder.Using the dovetail slide as a guide while viewing the back of, the hypodermic needle with an operating microscope (Zeiss OPMI1;Carl Zeiss, Thornwood, N.Y.), the micropipene was carefully insertedinto the needle so as not to break the tip. After the initial insertion,the micropipette was advanced further while the operator observedthe tip of the hypodermic needle through the pupil usingthe operating microscope and a piano-concave lens over the cornea.The interior of the eye was illuminated through the pupil using awhite light source from the operating microscope. When the micropipettetip emerged from the tip of the hypodermic needle, the dovetailslide was locked which rigidly fixed the micropipette to the manipulator.While continuously observing the tip of the micropipette throughthe operating microscope, the operator directed the micropipette towarda target vessel using the hand-held trackball. Once the tip ofthe micropipette was centered over a retinal vessel and just touchingthe surface of the retina, the enable button was released, fixing themanipulator at that location. Using the Narishige hydraulic actuator,the micropipette was carefully advanced until it just penetrated thevessel wall [6], after which pressure measurements were taken ordrugs injected.ResultsA pressure measurement obtained from a 60-tam retinalvein using the manipulator, a micropipette and a servonulldevice (model 5A; IPM, LaMesa, Calif.) is shownin Fig. 5. The trace shows the pressure reading as themicropipette was inserted, held steady and then retractedfrom the lumen of the vessel. Over 50 retinalvessels in the live, anesthetized cat ranging in diameter

700Fig. 5 Pressure measurement ina retinal vein of a live anesthetizedcat acquired using the micromanipulatorand the micropuncturetechnique140120-I- 100EE80Micropipette placed withinlumen of retinal veinSystemic Blood PressureMicropipette withdrawnfrom retinal veint~o~20Intraocular Pressure0 I I I10 20 30 40 50Time (s)from 20 to 130 ~m have been successfully cannulatedfor pressure measurement and drug injection using thismanipulator-based micropuncture technique [6].The compact size of the robotic manipulator and thevirtual center of rotation made setting up the experimentin preparation for micropuncture significantly easier thanwith previous manipulators [6]. Since the robotic manipulatortakes up less space around the head than similarmechanical manipulators, we were also able to performexperiments requiring two instruments in the eye by holdingone instrument with the robotic manipulator and anotherwith an older mechanical manipulator.DiscussionWe have found the robotic manipulator described in thispaper to be easy to use and versatile while providing precise,steady placement of an instrument over large areasof the retina. One goal of this research was to develop amanipulator which is easy to set up for retinal microsurgeryand essentially transparent to an operator using it.The setup procedure has been simplified in that a virtualcenter of rotation at the scleral incision point has been establishedby touching the sclera with the tip of the instrument.Once the scleral incision point has been selected,the motion of the tool is constrained to always passthrough that point. Since the motion of the tool is constrained,the operator does not need to worry about damageto the instrument or the eye at the scleral incisionpoint due to an erroneous motion command.To move the micropipette tip within the eye, an operatorobserves the tip of the micropipette through an operatingmicroscope and guides it based on visual information.The operator is thus using the manipulator as a tooland always has direct control over how the tool is moving.To make the manipulator as intuitive to use as possible,the computer and robot residing between the operatorand the tool should be essentially transparent. This infersthat the operator should feel like they are holding the tooland that the tool moves in a manner which the operatorexpects. Transparent operation was addressed by carefullyselecting the mapping schemes which relate a rolling ofthe trackball to a corresponding notion of the manipulator.When the trackball is rolled in one direction, the back ofthe instrument follows with a reduction in scale makingthe operator feel as if this thumb is attached directly tothe back of the instrument.Using the manipulator in a teleoperated manner is onlyone of many possible applications. The computer controlleris also capable of following predefined motion profilesand placing the instrument at a specific location withinthe eye. This capability could be used to microscopicallyscan surfaces, move the instrument in a complicated preprogrammedmanner or repeatedly visit the same retinallocation. By registering the instrument to the eye, thecomputer controller could also be used to set up additionalmotion constraints based on eye geometry. An examplewould be to keep the instrument at least 10 pm from theretina or to allow controlled penetration of the instrumentinto the subretinal space for the injection of various substances.Acknowledgements The authors would like to thank Dr. SheldonBuzney and Dr. Charles Garcia for their discussions, suggestionsand many useful insights and also Daniel Laser, Mike Lohse, andPeter Ehman for their technical assistance and engineering expertise.This work was supported by The Margaret W. and Herbert HooverJr. Foundation and by NIH Grant EY09714.

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