Head-mounted display simplifies surgery

Minneapolis--Operating rooms can be crowded, chaotic places. A surgeon is often surrounded by nurses, resident doctors, interns, and an anesthesiologist. Assistants also enter the room from outside to deliver supplies, thus adding more bodies to an already crowded situation.

Amid this crush of activity, some surgeons rely on television monitors, often located five or six feet away, to gauge the movement of their instruments inside the patient. To view the monitor, they must often cock their heads and look over one shoulder, then contend with the movement of bodies in front it.

To provide surgeons with a better method of functioning in an operating room, engineers from Honeywell's Technology Center have developed a video display system that fits on the surgeon's head. Working with designers and engineers from Polivka Logan Design Inc., they have created a medical head-mounted display that delivers the high-resolution image from a video borescope directly to a projection display just inches from the surgeon's eye.

This new system not only provides the surgeon with a clear line of sight to the display, it places him or her at the center of the procedure. During a knee operation, for example, instead of looking over one shoulder to see a monitor, surgeons can now orient themselves as if they were looking straight into the knee.

The result is less confusion about which direction is up and down, left and right. "Instead of turning my head to look at a TV screen, I now look in the same direction that I'm looking with the arthroscope," notes Dr. Patrick St. Pierre, an orthopedic surgeon at Madigan Army Medical Center, Tacoma, WA, who has used the new headset. "And that's a more natural way of doing things for any human being." The headset simplifies the surgical procedure because surgeons don't have to perform mental reorientation and triangulation as they move their instruments, St. Pierre says.

Equally important, the new head-mounted display has allowed U.S. Army surgeons to perform the first-ever field-deployed arthroscopic knee surgeries. Until now, army surgeons could not perform arthroscopy in the field because conventional video monitors and associated equipment were simply too large. As a result, during the Gulf War soldiers with minor cartilage tears needed to be flown back to the U.S. for surgery. With the availability of head-mounted displays, however, that would no longer be necessary. "Using these headsets, we don't have to bring television monitors and a lot of fragile equipment to the site," St. Pierre says.

Swords to plowshares. That portability issue was one of the driving forces behind the system's development. Funded by the Defense Advanced Research Projects Agency (DARPA), Honeywell engineers began working on the technology about two years ago. Key challenges for the group included miniaturization of the ocular system, development of drive electronics, and building a head-mounted unit that would be easy to use and wear.

For the ocular system, Honeywell engineers employed a pair of miniature image sources that use active-matrix liquid-crystal display (LCD) technology. Each display measures 34 mm diagonally--about the same size as those used in conventional camcorders. Engineers first talked to surgeons about a single video screen as a means of cutting costs, but surgeons universally preferred the binocular display. The ocular system is contained in a thermoplastic optical housing that includes backlighting, a mirror, lens, and a mechanical clutching system that enables the surgeon to rotate the display housing, or move it up and down or in and out.

Key to the success of the unit was the development of the drive electronics. They include a scan converter, which converts the video signal for use by the devices that drive the two LCDs. In early iterations, the drive electronics were contained in an enclosure measuring about 9 x 7 x 1.5 inches and requiring about 25W for operation. By shifting from discrete components to multi-layer surface-mount boards, however, engineers ultimately reduced it to approximately 3 x 2 x 1 inch and 6W. "We asked ourselves what we absolutely needed and didn't need," says Scott Nelson, staff scientist for information processing systems at Honeywell. "Then we brought the size and weight down as much as we possibly could." The smaller system is now contained in a deck-of-cards-sized pack that clips to the surgeon's smock, Nelson says.

To make the overall system less obtrusive, Honeywell engineers teamed with engineers and designers from Polivka Logan Design Inc. to change the wiring that connects the ocular system to the drive electronics and the camera. Although ordinary LCD screens more commonly em-ploy high-speed (20-MHz) flex tape, they switched to round, bundled coaxial cable. The round coaxial cable, they say, integrated more easily with the headset. "It tracks more efficiently over the user's head and down the back," notes Brett Johnson, an industrial designer with Polivka Logan. "It would have been more difficult to do that with flat tape."

To improve usability, the design team devised an adjustment mechanism that enables users to easily position the display. A plastic paddle on the upper surface of the optical housing engages a small clutch, which then allows the housing to be tilted, or moved up and down or in and out. "It's a one-touch system," notes Dan Cunagin, vice president of engineering for Polivka Logan. "You grab it and release it to move the system anywhere you want." The clutch-paddle mechanism was a more elegant solution than individual knobs that would move it in x, y, and z directions, Cunagin says.

Usability critical. Because the system is used in operations that can sometimes last as long as five or six hours, the design team focused on comfort in the overall design. To efficiently support the system's weight, they placed all of the optics in front of the headset and all of the electronics at the rear. A high-density polyethylene headband carries the coaxial wires and combines sufficient tensile strength with pliability. A conforming foam from EARSpecialty Composites (Indianapolis) on the interior of the headset eliminates potential pressure points on the surgeon's head. "We've learned from pilots that if you wear a helmet for 10 to 12 hours, you can get 'hot spots,' which can be very painful," Nelson says.

To accomplish all their goals, Cunagin says, the team designed and refined the unit on PRO Engineer CAD software, then used on-site rapid prototyping to build models of the parts. When feedback required changes, they went back to the database, made the changes, and prototyped new parts. Ultimately, they built two non-functioning prototypes and two functioning versions before settling on the current design.

The current system, which weighs less than 28 oz and measures 12 x 8 x 7 inches, has been used in tests at Madigan Army Medical Center in Tacoma, WA, but has not been put into production. Its developers say it can be used in arthroscopic, laproscopic, and endoscopic procedures. It could also be used in conjunction with stereo microscopes, which opens up many other applications.