At Christ Medical Center in Oak Lawn, since mid-July, Dr. George Branovacki, an orthopedic surgeon, has performed more than a dozen knee-replacement operations with the iPod, which he said gives much more precise measurements in making incisions and placing the replacement knee. The result, he said, is less pain and swelling for patients, better range of motion and a longer life for the artificial knee. What he's using at Christ Medical Center is called Dash navigational software. The federal Food and Drug Administration approved the iPod-based technology in 2011, and Christ is the only U.S. hospital using the system, according to the company.