BOSTON -- Doctors have detected a pattern of problems in a small percentage of patients with metal-on-metal joint replacements.

"I just woke up one day and it was really hard to walk with a hip," said Wandra Harmsen, a teacher who had her first hip replacement at age 54.

But within a year, the implant that she and her doctors expected would last at least a decade started causing her trouble.

"In the summer time, I'd become very active when I'm not working," Harmsen said. "Swimming, walking and biking and kayaking. I noticed at the height of the summer that with every step, I'd hear a squeak when I was active."

Her doctors eventually determined the noise and discomfort was caused by her implant. The metal ball was rubbing against its metal socket, leaking potentially toxic metal debris into her bloodstream.

"In a small percentage of patients that have metal-on-metal hips ... there have been recent reports of unexpected damage to tendon and muscles in patients with this replacement," said Dr. Young-Min Kwon, an orthopaedic surgeon at Massachusetts General Hospital.

Complicating Harmsen's case was the fact she consulted more than one doctor about the noises and pain. They performed X-rays and bone density tests without reaching a successful diagnosis.

Kwon said that's not uncommon. "Because this is soft tissue or muscle and tendons, problems do not show up on X-ray," he said. "For us to diagnose this problem, we need to perform ultrasound or MRI scan to detect these damages."

Last March, Harmsen had her hip replaced for a second time.

Doctors insist that problems like Harmsen's are not an indictment of replacements in general, or even of metal-on-metal joint replacements.

"In terms of proportion," Kwon said, "it's been reported in a recent article that up to one-third, or 36 percent, of all hip replacements performed in the United States use metal on metal bearing surfaces."

Kwon said they tend to last longer than other materials and the risk of complications is relatively low.

"The most important message is, if you have a metal-on-metal hip and you experience any significant pain around the hip, then you need to be assessed by an orthopedic surgeon," said Kwon.