Application enhances laparoscopic surgery

Photo: Demonstration of magnetic retractor lifting a liver

This is a demonstration of magnetic retractor lifting a liver. The application of magnetic force can make minimally invasive surgery easier and more effective; © Joe Howell/Vanderbilt University

"In 2007, a team of University of Texas researchers did some basic experiments using magnets in laparoscopic surgery," said Pietro Valdastri, assistant professor of mechanical engineering and director of Vanderbilt University's Science and Technology of Robotics in Medicine (STORM) Lab.

"Although their designs were very simple, mechanically speaking, they made me realize that small surgical devices guided and powered by external magnets have a number of potential advantages over placing tools on the end of a stick, which is the current approach. All that was required is a little sophisticated engineering!"

This realization led Valdastri and his graduate students - particularly Christian Di Natali and Nicolò Garbin - to develop a new approach to laparoscopic surgery that they call local magnetic actuation (LMA) and describe in an article titled "Closed-Loop Control of Local Magnetic Actuation for Robotic Surgical Instruments" published recently in the journal IEEE Transactions on Robotics.

The approach requires two components: an external unit that is placed on a patient's abdomen and an internal unit small enough to fit through the ¼ to ½ inch ports that are used in minimally invasive surgery. Each component carries a set of two strong permanent magnets. One pair anchors the internal unit to the inside of the abdominal wall, while the other pair provides the mechanical force that powers the device.

According to the researchers, their approach has two major benefits over the current "tool on a stick" approach. The most important is that it can deliver between 10 to 100 times more mechanical power than the electrical motors that are small enough to fit through the surgical ports can provide. The second is that it is easier to place the magnetic devices at optimal positions within the body than it is to position devices connected to sticks or wires.

The first instrument that embodies this principle is a fountain-pen-sized magnetic organ retractor that is described in an article appearing in the March issue of the ASME Journal of Medical Devices.
Di Natali came up with the innovative design, which Garbin then developed as a visiting master's student from the Polytechnic of Milan. After getting his master's degree, he joined STORM Lab as a doctoral student and continued working on the device.

The device is designed to move organs out of the way when needed to perform an operation. For instance, the liver lies on top of the gall bladder, so it must be moved aside before the gall bladder can be removed. That is not much of a problem in old-fashioned open surgery, but open surgery is increasingly being replaced by minimally invasive surgery where the operation is performed with special equipment through small incisions in the abdominal wall.

Although patients tend to have quicker recovery times and less discomfort with minimally invasive surgery than with conventional surgery, being forced to operate through small incisions (a procedure aptly nicknamed keyhole surgery) raises a new set of issues for the surgeon. One of these is organ retraction.

One of the device's main limitations, Valdastri acknowledged, is that it does not work if a patient's abdominal wall is too thick. The magnetic force between the inner and outer units weakens rapidly as the distance between them increases and, at about an inch, it becomes too weak to keep the inner unit firmly anchored. As a result, the magnetic retractor will not work with overweight adults, which means about a third of the population in the U.S.

To turn this limitation into a strength, Valdastri's group is adapting this approach for pediatric surgery and have begun developing a pediatric camera.

"Currently, no one has developed laparoscopic devices specifically for pediatric surgery," Valdastri said. "There is a real need that I think we can help fill."; Source: Vanderbilt University