A team of Johns Hopkins biomedical engineering students have developed a handheld metal detector to help surgeons ‘see through’ muscle and scar tissue, to find screws which need to be removed.
Orthopedic screws, usually made of a stainless steel or titanium alloy, are produced in varying lengths and can have screwheads that range from roughly 3 to 7 millimeters in diameter. Orthopedic surgeons often use these screws and related hardware to hold broken bone fragments together for proper healing. These doctors often need to remove orthopedic screws that shift position, trigger an infection or cause pain, but skin and scar tissue can make it difficult to find the troublesome hardware, even with the aid of real-time X-ray technology. The small handheld detector is designed to zero in on the hardware and steer the doctor’s screwdriver into position for prompt removal.
The prototype, devised and built over the past school year by eight Johns Hopkins biomedical engineering majors, was unveiled recently at the university’s BME Design Day event. Surgical Transformations, a Manhattan-based company that sponsored the project, has applied for a provisional patent covering the invention and is moving it toward further refinement, clinical testing and possible sale to doctors in the coming years.
“When orthopedic screws are difficult to find, removing them can require an expensive operation,” said Malcolm M. Lloyd, a physician and chief executive officer of Surgical Transformations. “Orthopedic surgeons told us it would be great to have a metal detector to locate these small, sometimes isolated screws. These surgeons felt that expensive, time-consuming and more invasive surgical procedures could be avoided if such a metal detection tool was readily available. We presented that challenge to the Johns Hopkins students last summer and gave them a set of requirements for this tool. They nailed every one of those goals. Their solution is very simple, elegant and more advanced than I expected. I thought their prototype was fantastic.”
The prototype detector emits a tone that rises in pitch as the surgeon moves closer to a metal screw.
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“A piece of metal will interfere with the search coil’s magnetic field and change its frequency,” said Eli Luong of Garden Grove, Calif., one of the two leaders of the student design team. “Our device is set up so that if no metal is near the search coil, there is no sound. But as the detector comes closer and closer to a piece of metal, like a screw, it sends out a tone that rises higher and higher in pitch.”
The search coil is located inside the slender non-metallic needle-shaped portion of the probe, which sits inside a hollow tube. These two parts of the probe together form a device that the doctor can insert during a minimally invasive procedure. First, a small incision is made near the expected site of a tiny orthopedic screw that needs removal. The probe is then inserted to help the doctor home in on the head of the screw. Its movements can be observed by the C-arm fluoroscope imaging equipment often used in these operations.
When the screwhead is found, the coil detector is carefully removed from the probe’s hollow tube. This two-part design resembles the cannula and trocar tools commonly used in biopsies. After the coil detector segment is taken out, the doctor inserts a screwdriver through the tube section, which remains perfectly positioned for removal of the screw.
“During our research, many of the orthopedic surgeons we talked to showed interest in our device. They said they usually have to remove 5 to 10 percent of the screws they’ve implanted,” said Jennifer Hoi of Belmont, Calif., the other team leader. “That’s why we believe this device would be a useful addition to every orthopedic operating room and every surgeon’s toolbox.”
