"You Know The Drill," Feature Article, May 2005

you know the drill

Magnetic sensors in a bone implant give trauma surgeons the green light.

by John DeGaspari, Associate Editor

Not only do bones provide the body's scaffolding, but they are natural healers as well. When we suffer a fracture, blood vessels, cartilage, and bone-building cells begin the work of knitting broken parts back together. Trauma surgeons help the healing process by realigning the broken parts into the bone's correct shape and length, using devices such as screws, rods, and nails to hold the fragments in place.

In a long bone such as the femur, surgeons may use a device called an intramedullary nail, which consists of a hollow metal rod that is inserted into the canal of the bone marrow. After inserting the rod, the surgeon secures it with screws. The screws pierce the outer layer, or cortex, of the bone, pass through predrilled holes in the metal rod, and then through the cortex on the other side of the bone.

For the surgeon, the tricky part of the process is placing the screws accurately so that they line up exactly with the predrilled holes of the intramedullary nail. Traditionally, surgeons drill freehand, through the bone's cortex and into the predrilled hole of the implant. It's a task somewhat akin to locating a stud hidden behind sheetrock, but considerably more difficult. The surgeon must not only match the location of the tube's predrilled hole, but must also drill at the correct angle, so the screw passes cleanly through both predrilled holes in the tube and out the other side.

To accomplish this, the surgeon uses a real-time video X-ray of the bone as a guide. This has its drawbacks—the most serious is that it exposes everyone in the room to intense X-rays for several minutes. A skilled technician, who may not be readily available in the middle of the night when many trauma surgeries take place, must set up the X-ray gear.

Alfred Durham, an orthopedic surgeon practicing in Roanoke, Va., noted that it is hard to correct mistakes when drilling the hole through the bone. "If you start wrong, you stay wrong and if you start right, you stay right," he said. X-ray equipment must be aligned with the hole between the patient and surgeon, exposing his hand to direct X-rays while drilling.

X-ray shows screws that secure an intramedullary nail inside a broken bone.

Durham had an idea of using a magnetic field and sensors to locate the predrilled hole in the metal tube, thus eliminating the need for X-rays. He sought the help of mechanical engineering students at Virginia Polytechnic Institute and State University in Blacksburg to work out the details. "One thing that wasn't clear was the pattern of the magnetic field and the arrangement of the sensors that would make this work," he said.

Durham stopped by the office of Alfred Wicks, a professor of mechanical engineering at Virginia Tech, and explained his idea. Wicks invited Durham to present his problem to Wicks's graduate class in instrumentation. Durham took up the offer and lectured the class on femoral nails. "It was a great experience for my students, all mechanical engineers, because they were seeing something that they would normally not see," Wicks said. During his lecture, Durham explained to the students that he needed a way to place the screws properly in the nail's hollow tube. That became the focus of a three-week class project, after which Durham would evaluate the proposals.

One of the student groups suggested using an array of magneto-resistive sensors to measure the magnetic field from a magnet inserted in the center of the hollow intramedullary nail. The shape of the magnet is important, Wicks said, because it emits a well-understood magnetic field. He noted that the quality of the magnet was also critical in eliminating flaws that could result in distortions in the field. The field is symmetrical around the center of the magnet, radiating in a three-dimensional pattern.

The concept uses an array of eight sensors that are arranged symmetrically in an elliptical pattern around the center of the magnet. The sensors are arranged in opposing pairs. Readings that are the same for each pair indicate an equal distance from the target, or center of the magnet. Working together, the sensors indicate whether the target is balanced or unbalanced between each pair.

One of the students in the group that came up with the concept, David Szakelyhidi, created a working device, which was funded by the Virginia Tech Applied Biosciences Center and Carilion Biomedical Institute in Roanoke, Va.

A magnetic targeting device uses sensors and LED lights to guide the surgeon while drilling through the bone, thus eliminating the need for X-rays.

The working prototype consists of a fixed magnet that is placed inside the intramedullary nail and a handheld unit housing the sensors, electronics, and drill sleeve. The target magnet is located on the end of a wand inside the hollow nail. The wand is secured to the top of the handle that holds the nail in place. In the handheld unit, each sensor is represented by a light-emitting diode, glowing either red or green to indicate whether the sensor pairs are centered over the target magnet.

A drill sleeve, just forward of the light-emitting diodes, is located directly over the predrilled holes in the intramedullary nail. When the LEDs glow green, the sensors are centered over the target magnet and the drill holes are aligned. The surgeon is then able to drill through the bone.

Carilion partnered with local companies to develop a working prototype. Triad Semiconductor Inc. of Winston-Salem, N.C., is providing the electronics. It plans to combine the microcontroller, instrumentation amplifiers, and other electronic components into a customized integrated circuit chip that could process the voltage output signals of each sensor. Dan Wrappe, CEO of Triad, said the integrated circuit would reduce the size of the device and lower its manufacturing cost.

Plastics One Inc., an injection molder in Roanoke, milled prototype devices out of nylon. The biggest challenge was working to tight tolerances so that sensors and holes in the handle would be properly lined up with the predrilled holes in the nail, said vice president John Richardson. Production volumes of the housing will be injection molded.

Andre Muelenaer, medical director of Carilion Biomedical Institute, said the device has been demonstrated on artificial femurs as well as on a cadaver for an orthopedic company. Carilion is currently negotiating to license the technology, he said.