Imagine plopping six cups of coffee grounds on the heads of patients just before they are wheeled into the operating room to have nose or throat surgery.
In essence, that is what a team of Vanderbilt University engineers are proposing in an effort to improve the reliability of the sophisticated “GPS” system that surgeons use for these delicate operations. They have designed a “granular jamming cap” filled with coffee grounds that does a better job of tracking patient head movements than current methods.
The coffee grounds form a thin layer inside a stretchy silicone headpiece, which looks like a black latex swim cap decorated with reflective dots. After the cap is placed on the patient’s head, it is attached to a vacuum pump that sucks the air out of the cap, jamming the tiny grounds together to form a rigid layer that conforms closely to the shape of the patient’s head.
Before surgery, a special scanner is used to map the location of the dots relative to key features on the patient’s head: a process called registration. Then, during surgery an overhead camera observes the position of the dots, allowing the navigation system to accurately track the position of the patient’s head when the surgeon repositions it. The computer uses this information to combine a CT scan, which provides a detailed 3-D view of the bone and soft tissue hidden inside the patient’s head, with the position of the instruments the surgeon is using and displays them together in real time on a monitor in the operating room.
“These are very delicate operations and a sophisticated image guidance system has been developed to help the surgeons, but they don’t trust the system because sometimes it is spot on and other times it is off the mark,” said Robert Webster, Ph.D., associate professor of Mechanical Engineering and Otolaryngology, who is developing a surgical robot designed specifically for endonasal surgery.
Webster and his research team discovered it wasn’t the hardware or the software in the guidance system that was causing the problem. It was the way the reflective markers were attached to the patient’s head that was at fault. Typically, these “fiducial markers” are attached by an elastic headband and double-backed tape and are subject to jarring and slipping, which produced large tracking errors.
“The basic assumption is that, after registration, the spatial relationships between the patient’s head and the fiducial markers remains constant,” said Patrick Wellborn, the graduate student who is making the presentation. “Unfortunately, that is not the case.”
Previous research has found that when everything goes well, the guidance system produces targeting errors of about 2 millimeters, but in about one operation out of seven, the target error is much larger, forcing the surgeon to redo the registration process.
“Actually, we do have a solution to this problem but it involves drilling and attaching the markers directly to the skull, which we don’t like to do because it is painful and it’s a step backward from the majority of what we are doing,” said associate professor of Otolaryngology Paul Russell, M.D., who is collaborating with the engineers on the project.
So the team began thinking up alternative, non-invasive methods to attach these critical markers. In the last three years, they have gone through a number of designs. They began with headbands that had coffee ground-filled bags over the temples. Their tests showed that these models could reduce the targeting error by about 50 percent. But the engineers still weren’t satisfied.
Then, Wellborn had a brainstorming session with fellow graduate student Richard Hendrick. Among the materials that his predecessor had left behind was a latex bald cap. “That sparked the idea of caps in general,” Wellborn recalled. “We wanted something elastic that was form fitting, which led to the idea of a swim cap.”
In addition to fitting extremely tightly to the head, the new design had another advantage. The headband system has only three fiducial markers attached on the ends of three thin rods to form a triangle. The new design allowed them to attach several dozen markers directly to the surface of the cap, which the researchers believe will also contribute to improving the guidance system’s accuracy.
They designed three tests to determine how well this “granular jamming cap” performed relative to the current headband in reducing targeting error.
On the strength of these results, Vanderbilt University has applied for a patent on the design and the technology is available for licensing. (Interested parties should contact the Vanderbilt Center for Technology Transfer and Commercialization.)