Metal Detector Locates Misplaced Orthopedic Screws

Inspired by the device used to find lost coins in the sand, Johns Hopkins undergraduates have invented a small handheld metal detector to help doctors locate hidden orthopedic screws that need to be removed from patients’ bodies. The device emits a tone that rises in pitch as the surgeon moves closer to the metal screw. It also serves as a surgical tool to guide the removal of the hardware.

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 battery-powered device blends electronic technology from traditional treasure finders with design features from modern medical instruments that are used in minimally invasive procedures. Like many full-size metal detectors, the students’ device features two coils of wire, one in the search probe and the other in a control box. When electric current flows through the two coils, each produces an oscillating magnetic field. Circuitry in the control box then compares the frequency of the oscillations in the two coils and translates the difference in frequencies of these oscillations into an audible sound.

“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.”

Hoi added, “I was extremely proud of our team and how we were able to pull this together. We got the device to work exactly how we wanted it to work, exactly how we imagined it would work.”

Hoi and Luong received their bachelor’s degrees in May, shortly after the prototype was unveiled. Several younger members of the team will continue refining and testing it. Surgical Transformations, the sponsor of project, plans to perform market research with orthopedic surgeons to gauge their interest and determine whether enhancements are needed.

Along with Hoi and Luong, the metal detector design team members were seniors Laura Rupprecht of St. Paul, Minn., and Christine Medina of Pasadena, Calif., and freshmen David Huberdeau of Woodbridge, Va.; Joe Chao of San Dimas, Calif.; Michelle Harran of Neptune City, N.J.; and Xia (Katie) Bi of Wilimington, Del.

Robert H. Allen, an associate research professor in the Department of Biomedical Engineering, was technical advisor for this project and director of the design course.

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