The defibrillator leads pulled off the market this week by Medtronic Inc. may pose a higher risk of fracture in younger adults and children, a population for whom the devices were particularly popular in part because of their small diameter.

Children make up a relatively small share of patients who receive the implanted devices. But leads -- wires that attach defibrillators to patients' hearts -- tend to come under greater stress in more-active people, including kids, adolescents and younger adults.

Monday, Medtronic, based in Minneapolis, announced it would stop selling its Sprint Fidelis leads because of a risk of fractures that could erroneously dispatch jolts of electricity. The company said that of the 268,000 Sprint Fidelis leads implanted, 2,085 were in patients under the age of 21. About 235,000 patients still have the leads, and the company said the fractures may have contributed to five deaths. The deceased patients' ages haven't been made public.

Preliminary data from physicians at 32 institutions who specialize in treating pediatric patients and adults with congenital heart disease showed a fracture rate of 6.7% among 569 patients with Sprint Fidelis leads over 30 months. The "vast majority" of the patients are likely to be under the age of 21, given the physicians involved, says Wayne H. Franklin, an associate professor at Northwestern University's Feinberg School of Medicine, who gathered the physician reports.

Medtronic has reported a lower failure rate, 2.3% after 30 months, for all patients implanted with one particular Sprint Fidelis model. For that same model, Dr. Franklin saw a fracture rate of 4.9% among 304 patients in his survey. The real difference may be slightly greater, because the failure rate from Medtronic includes more than just fractures.