Magnetic Targeting Device for IM Bone Implants

Mechanical Engineering magazine describes some interesting research coming out of Carilion Biomedical Institute, that one day may make the job of fixing long bone fractures easier and safer:

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…
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.
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 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.
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.

The project page at Carilion Biomedical Institute…