Minimally invasive surgery offers many benefits: less damage to healthy tissue, shorter hospital stays, and faster recovery. But these so-called arthroscopic procedures, based on keyhole incisions that limit access to the body, prevent surgeons from directly touching internal tissue.
Conventional thoracic surgery to remove a lung nodule, for example, involves an incision long enough to allow spreading the ribs and reaching into the patient's chest. By squeezing the spongy lung tissue by hand, a surgeon can easily locate and determine the size of the hard nodule. In contrast, the minimally invasive method typically means three incisions, each roughly half an inch in diameter, through which the surgeon inserts a videocamera and long-handled instruments. The tip of a probe resembling a stainless steel chopstick 10 inches long is used to stroke the lung to find the small lump. This can be a frustrating and time-consuming task. "We've seen surgeons take 20 or 30 minutes to locate a lump while the rest of the surgical team is just standing by," says McKay professor of engineering Robert Howe, who, along with his former student William Peine, Ph.D. '99, has conducted palpation studies in real operating rooms. "Surgeons think they have the best sense of touch in the world," Howe adds. "They spend a long time developing that sense and they really want to feel things inside a patient's body."
To address some of these problems, Howe and Peine have developed remote palpation instruments that convey contact information to the surgeon's fingertips from inside the patient's body. These systems involve tactile sensors, which measure pressure distribution as the surgeon probes tissue, and signal-processing algorithms, which determine what the finger would feel if it were directly touching the tissue. Tactile display devices, which contain tiny pins under a rubber touchpad, recreate these sensations on the finger.
The complexity of the human sense of touch made development of these devices very difficult. For instance, the pins must be able to go up and down very quickly so that the tactile displays produce the correct sensations as the finger moves over a surface. And ideally, says Howe, you'd like to have lots of these pins in a small space, noting that his current prototypes have only 10 pins in a line 20 millimeters long. "That's enough to find lumps," he says, "but not enough to do anything very sophisticated, like learn about tissue compliance" (i.e., its stiffness). Another design challenge involved the mechanism that connects the sensor to the surgeon's hand. Traditional surgery allows reaching into the body and moving one's fingers around structures to get needed information. But minimally invasive procedures dictate coming straight in from the point of the incision. As a result, Howe explains, "We had to build a very flexible instrument tip that would track over surfaces without requiring the surgeon to orient it very carefully."
The sense of touch has two main purposes: exploration and manipulation. Howe's existing prototypes--which have been tested in surgical procedures performed on pigs--address only exploration. In the future, he hopes to make the devices even more useful by building them into manipulation tools. "For instance, if you have a surgical grasper, you might like to have the tactile sensor on the back part so surgeons can just flip the grasper over and use that to feel," he says. "Eventually, you'd like to build it into the gripping surfaces so that they can feel as they grip."
Locating lung tumors is just the first application for remote palpation systems in minimally invasive surgery. In the future, surgeons might use these instruments for such tasks as locating kidney tumors and colonic polyps or assessing blood vessel or liver health.
"In classic abdominal surgery, where they go in with an open incision, surgeons not only fix what they want to fix--take out your gallbladder or whatever--but they'll also give you a 50,000-mile check-up," says Howe. "They'll run your bowel and make sure there's nothing wrong there, they'll poke at your liver and make sure there's no cirrhosis, those sorts of things. Now, in the minimally invasive case, where they don't have the sense of touch, all they can do is look around, and that doesn't reveal a lot of the information they would get if they were in there with the entire hand. By building these palpation devices, we hope we can allow some of these opportunities for diagnosis to be transferred over to the minimally invasive world."
~ Kathleen Koman