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New knees, hips? Expect travel delays

New hips? They might help with the tango, but when it's time to fly, there will be no swiveling through the security monitors.

A new study by Boston doctors pitted metal detectors against patients with implants, testing to see what set the beepers off and what got through.

Since the attacks of Sept. 11 , many replacement joints have set off alarms and delayed passengers. But no one knew what kind of implants set off the machines.

"It's one of the most common questions patients ask after surgery -- 'Will I set off the metal detector?' " said Dr. Edward K. Rodriguez, chief of orthopedic trauma at Beth Israel Deaconess Medical Center and one of the study authors. "It's not a problem, it's really an inconvenience; you get screened twice, you get patted down, it adds time to the whole security thing."

The patting and the scanning became such an issue that in 2001, the American Academy of Orthopaedic Surgeons issued a statement telling physicians that they should consider writing notes for patients with metal implants. (Most do, and some doctors provide wallet-sized X-rays.)

But it turns out all that worry might be for naught -- at least for certain implants.

As part of the study, 129 patients with 149 various implants walked through an "M-scope three-zone" metal detector -- a common type at airports -- at high and low sensitivity ratings.

Half of the patients -- 52 percent -- sailed through without the implants being detected. They usually had small nails, plates, or screws, or they had implants in the upper half of their body, the study said. "It really surprised me that only half were detected," said Rodriguez, who believed that small implants and those encased by bone attributed to the misses.

Those with new hips or knees, though, need to plan for some extra time at the gate. All of the hip replacements and 90 percent of the total knee replacements were detected at the low-sensitivity settings. And cobalt-chromium and titanium implants were more likely to be detected than stainless-steel implants.

Dr. James V. Bono, clinical professor of orthopedic surgery at Tufts University, said the study will help doctors prepare their patients.

"People want to know if they're going to set off the alarm ahead of time so that they're not surprised," he said. "The study confirms that joint replacements will set off the alarms."

But at the end of the day, a letter might not be enough to avoid delays. At the time of his conversation with the Globe, Bono happened to be at Logan Airport, en route to Minneapolis.

"I interviewed the guard, and he said it doesn't matter whether they have a card or not," Bono said by cell phone. Guards are instructed by the federal Transportation Security Administration to pat down anyone who sets off the detector, even those with a doctor's note or an X-ray.