The first time John L. Johnson's artificial hip squeaked, he was bending down to pick up a pine cone in his yard in Thomasville, Ga. Johnson looked up, expecting to find an animal nearby.

Susan O'Toole, a nutritionist at Montefiore Medical Center in the Bronx, who first squeaked going up stairs after getting home from her hip-replacement surgery in 2005, said she thought the banister she was gripping needed repair.

Once they pinpointed the source of the noises, both patients -- and hundreds of others -- discovered they had 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 potentially more serious malfunctions.

"There is something amiss here," said Douglas E. Padgett, chief of adult reconstructive and joint replacement service at the Hospital for Special Surgery in New York.

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.

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 U.S. in 2003, squeaking was rare.

Now, tens of thousands of ceramic hips later, from Stryker and other makers, 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.

Last fall, the Food and Drug Administration issued a warning to Stryker, saying it had failed to take the steps needed to prevent squeaking and other problems. Clouding things further, Stryker last year recalled ceramic hip parts made at its factory in Cork, Ireland, after determining that some did not meet its sterility specifications.

Stryker says none of the problems underlying the recall or the warning letter from the FDA reflect problems that cause squeaking, which it contends occurs in less than 1 percent of implants.

Beyond annoyance and embarrassment, many patients and their surgeons fear that the squeaky hips may signal that the joints are wearing out prematurely.

That could force patients to undergo a second replacement of the same hip joint, the operation they had hoped to avoid by choosing ceramics.