Image: Granular jamming cap (L) compared to current elastic headband (R) (Photo courtesy of Joseph Howell / Vanderbilt).
A granular jamming cap filled with coffee grounds conforms closely to the shape of a patient’s head, helping surgeons to better track movements.
Developed at Vanderbilt University (Nashville, TN, USA), the cap is based on a flexible silicone bladder headpiece that is filled with coffee grounds. Once the cap, which is covered with pattern of reflective dots, has been placed on the patient’s head, it is attached to a vacuum pump that evacuates the air in the headpiece to jam the coffee grounds together, forming a rigid layer. Prior to surgery, a video scanner registers the location of the reflective dots--which serve as fiducial markers--relative to key features on the patient’s head.
During the surgery itself, an overhead camera observes the position of the dots, allowing the surgical navigation system to accurately track the position of the patient’s head even when the surgeon moves and repositions it. The computer uses this information to combine a computerized tomography (CT) scan, which provides a detailed three-dimensional (3D) view of the bone and tissue with the position of the surgical instruments the surgeon is using, displaying them together in real time on a monitor in the operating room.
While current fiducial markers, usually attached by an elastic headband and double-backed tape, are subject to jarring and slipping, the granular jamming cap conforms to the patients head and locks firmly into place. In tests designed to determine how well the cap performed relative to current headbands, they found that the cap reduced targeting errors by 83%, improved resistance to external displacement forces by 76%, and reduced head repositioning error rates by 66%. The new tracking cap was presented at the International Conference on Information Processing in Computer-Assisted Interventions, held during June 2017 in Barcelona (Spain).
“It’s a very clever way that doesn’t involve drilling holes in patients’ skulls to greatly improve the accuracy of the guidance system when we are operating in the middle of a person’s skull: a zone where the accuracy of the current system is inadequate,” said associate professor of otolaryngology Paul Russell, MD, who is collaborating with the engineers on the project.