Hundreds of patients have become guinea pigs in an unfolding medical mystery. Their artificial hips are made of ceramic materials that were promoted as being much more durable than older models. But for reasons not yet fully understood, their hips started to squeak, raising questions about whether the noises herald more serious malfunctions. More than 250,000 Americans get total hip implants each year, a procedure that generally costs close to $45,000. Hip replacements have a success rate of more than 90 percent, based on patients’ achieving relatively pain-free mobility after recovery periods that range from a few months to a year.

Any artificial hip can occasionally make a variety of noises. But until Stryker, a medical products company, began marketing highly durable ceramic hips in the United States in 2003, squeaking was extremely rare. Now, tens of thousands of ceramic hips later — from Stryker and other makers that entered the field — many patients say their squeaking hips are interfering with daily life. One study in the Journal of Arthroplasty found that 10 patients of 143 who received ceramic hips from 2003 to 2005, or 7 percent, developed squeaking. Meanwhile, no squeaks occurred among a control group of 48 patients who received hips made of metal and plastic.

Beyond annoyance and embarrassment, many patients and their surgeons fear that the squeaky ceramic hips may signal that the joints are wearing out prematurely. That could force patients to undergo the very operation — a second replacement of the same hip joint — they had hoped to avoid by choosing ceramics. Already, dozens of patients have elected to endure subsequent surgeries to replace the noisy hips. Some have sued Stryker, the pioneer and market leader, which some doctors say has been slow to take their patients’ concerns seriously.