Clever application of magnetic forceenhances laparoscopic surgery2 March 2015

Demonstration of magnetic retractor lifting a liver. Credit:Joe Howell, Vanderbilt University

Pietro Valdastri is convinced that the cleverapplication of magnetic force can make minimallyinvasive surgery easier and more effective.

"In 2007, a team of University of Texasresearchers did some basic experiments usingmagnets in laparoscopic surgery," said Valdastri,assistant professor of mechanical engineering anddirector of Vanderbilt University's Science andTechnology of Robotics in Medicine (STORM) Lab.

"Although their designs were very simple,mechanically speaking, they made me realize thatsmall surgical devices guided and powered byexternal magnets have a number of potentialadvantages over placing tools on the end of astick, which is the current approach. All that wasrequired is a little sophisticated engineering!"

This realization led Valdastri and his graduatestudents - particularly Christian Di Natali andNicolò Garbin - to develop a new approach tolaparoscopic surgery that they call local magneticactuation (LMA) and describe in an article titled

"Closed-Loop Control of Local Magnetic Actuationfor Robotic Surgical Instruments" published recentlyin the journal IEEE Transactions on Robotics.

The approach requires two components: anexternal unit that is placed on a patient's abdomenand an internal unit small enough to fit through the¼ to ½ inch ports that are used in minimallyinvasive surgery. Each component carries a set oftwo strong permanent magnets. One pair anchorsthe internal unit to the inside of the abdominal wall,while the other pair provides the mechanical forcethat powers the device.

According to the researchers, their approach hastwo major benefits over the current "tool on a stick"approach. The most important is that it can deliverbetween 10 to 100 times more mechanical powerthan the electrical motors that are small enough tofit through the surgical ports can provide. Thesecond is that it is easier to place the magneticdevices at optimal positions within the body than itis to position devices connected to sticks or wires.

The first instrument that embodies this principle is afountain-pen-sized magnetic organ retractor that isdescribed in an article appearing in the March issueof the ASME Journal of Medical Devices.

Di Natali came up with the innovative design, whichGarbin then developed as a visiting master'sstudent from the Polytechnic of Milan. After gettinghis master's degree, he joined STORM Lab as adoctoral student and continued working on thedevice.

The device is designed to move organs out of theway when needed to perform an operation. Forinstance, the liver lies on top of the gall bladder, soit must be moved aside before the gall bladder canbe removed. That isn't much of a problem in old-fashioned open surgery, but open surgery isincreasingly being replaced by minimally invasivesurgery where the operation is performed with

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special equipment through small incisions in theabdominal wall.

Although patients tend to have quicker recoverytimes and less discomfort with minimally invasivesurgery than with conventional surgery, beingforced to operate through small incisions (aprocedure aptly nicknamed keyhole surgery) raisesa new set of issues for the surgeon. One of these isorgan retraction.

As it is currently practiced, minimally invasiveabdominal surgery frequently requires making threeto five incisions in order to insert all of theequipment needed. When an organ must berepositioned, an extra incision is needed to insertthe organ retractor. One type of retractor looks likea miniature garden rake. The tines are collapsedtogether so they will fit through the incision andthen spread apart so they can grip an organ withenough force to shift its position. The range ofmovement of the retractor is limited by the locationof the incision through which it is inserted.

The STORM device looks much different than aconventional retractor. It consists of two parts: afountain-pen-sized unit that is inserted into the bodyand a larger external unit that is placed on thepatient's stomach. (There is no need to make anadditional incision because the device can beinserted through a port used by anotherinstrument.)

Both units contain powerful permanent magnetsthat are oriented so they attract one another. Themagnetic force is strong enough to firmly anchorthe internal unit against the inside of the abdominalwall directly below the external unit. The magneticattraction between the two allows the user toaccurately position the inside unit by rotating andsliding the external unit.

Both units also contain a second set of magnetsthat transmit the power. The magnet in the externalunit is attached to the shaft of a powerful electricmotor that causes it to spin. The magnet in theinternal unit is also attached to a shaft, but one thatdrives a two-inch lever. When the electric motor onthe external unit twirls its magnet, it generates arotating magnetic field that forces the magnet on

the shaft in the inner unit to spin at the same speed.When it spins in one direction, the lever opens upand when it spins in the opposite direction, thelever closes.

To retract an organ, the surgeon picks up theinternal unit with a laparoscopic grasper and insertsit through the port into the body. When hemaneuvers the internal unit close enough to theexternal unit, it snaps into position against the innersurface of the abdominal wall. The motor on theexternal unit is engaged, lowering the lever. Usingstandard laparoscopic instruments, the surgeonattaches one end of a line to the tip of the lever andthe other end to a clip or suction cup fastened tothe organ that must be moved.

The electric motor is run in reverse and the leverretracts, pulling the organ into the desired position.

Valdastri and his colleagues have tested the devicein the lab and found that it can lift a 500-gram (1.1pound) weight without any problem. The internalunit remained anchored and the lever opened andclosed smoothly. They also tried it out with piglivers, which are comparable in size to humanlivers. Although the livers typically weigh three tofour pounds, the device was able to lift and hold upthe portion that typically covers the stomach and gall bladder with power to spare. The device hassuccessfully undergone large animal testing.

"This device demonstrates for the first time thatcontrollable mechanical power can be transferredacross the abdominal wall via an intelligentmagnetic link to power a robotic instrument," saidValdastri.

In the future, the engineer intends to apply thissame principle to creating more complexinstruments, such as laser and radio-frequencyscalpels.

Besides the ability to deliver a lot of power, themagnetic actuation approach has some otherimportant advantages: the internal units do notcontain any expensive and delicate electronics sothey can be easily sterilized and, if manufactured inbulk, could be made inexpensively enough to bedisposable, Valdastri said.

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One of the device's main limitations, Valdastriacknowledged, is that it does not work if a patient'sabdominal wall is too thick. The magnetic forcebetween the inner and outer units weakens rapidlyas the distance between them increases and, atabout an inch, it becomes too weak to keep theinner unit firmly anchored. As a result, the magneticretractor will not work with overweight adults, whichmeans about a third of the population in the U.S.

To turn this limitation into a strength, Valdastri'sgroup is adapting this approach for pediatricsurgery and have begun developing a pediatriccamera.

"Currently, no one has developed laparoscopicdevices specifically for pediatric surgery," Valdastrisaid. "There is a real need that I think we can helpfill."

Provided by Vanderbilt UniversityAPA citation: Clever application of magnetic force enhances laparoscopic surgery (2015, March 2)retrieved 22 February 2019 from https://phys.org/news/2015-03-clever-application-magnetic-laparoscopic-surgery.html

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