Hip replacements have transformed the lives of thousands of Canadians, but some patients are questioning the safety of one type of artificial hip, called a "metal-on-metal" implant.

A total hip replacement typically involves replacing the ball at the top of the femur bone, as well as "the socket" in the pelvis, called the acetabulum.

The implants are supposed to last a decade or longer. But evidence has been growing for years that metal-on-metal hip implants, which use both a metal ball and metal socket, fail at higher rates than systems that use other materials, such as plastic-on-metal or ceramic-on-metal.

British experts are now so concerned they are advising doctors there to stop using metal-on-metal implants altogether. And the U.S. Food and Drug Administration said it will assemble a panel of experts to review the implants.

Etobicoke, Ont. resident Eric Mets had a new hip implanted in 2008. But it didn't help his joint pain. In fact, the pain only grew worse. Mets learned that his implant had failed and he had to undergo another surgery to replace the defective joint. The surgeries led to a major infection of his hip and it took a total of six surgeries to repair it.

Even now, Mets, 52, is still in constant pain, can't work, and is facing the prospect of more surgeries to repair further damage to his hip joint.

"I loved my job. I loved the people I worked with. I loved my quality of life and it is all for naught now," he says.

Steven Aldred had two hip implants, but after just three years, both hips had failed and he, too, was suffering.

"The pain was unbearable. So basically, I went to see another specialist and their findings were to immediately come in and get this hip out of me," he says.

Studies are showing that while older-style ceramic hip replacements have up to a 2.5 per cent failure rate, metal-on-metal implants fail in 6 to 13 per cent of patients.

A spokesperson for Health Canada said the agency is aware of the concerns surrounding the implants and has recently notified orthopedic surgeons across the country about the potential risks.

"Doctors are being asked to monitor patients for symptoms (pain or swelling in the groin, hip or leg, and changes in joint movement) so that appropriate plans can be made promptly for patient care," Olivia Caron wrote in an email to CTV News, adding that Health Canada is "actively monitoring" the situation.

Dr. Nizar Mahomed, an orthopedic surgeon at Toronto Western Hospital, says he's seen patients who not only have failed implants, they also have bits of metal from the joint that have shorn off, causing inflammation.

"This is like an unknown ticking time bomb because we don't know what the long term effect is going to be," he told CTV News.

At one point, he says, about a quarter of the hips being replaced in Canada used metal-on-metal joints. Many Canadian doctors have stopped using the devices in recent years, but the problems may continue.

Dr. Ross Leighton, head of the Canadian Orthopedic Association, says surgeons across Canada received the letter from Health Canada last week. He said monitoring patients could involve yearly or even semi-annual exams that test in particular for metals in their blood.

In some patients, metal particles from the implants have been linked to soft tissue and muscle damage around the joints.

Dr. Leighton says that if patients don't know what kind of hip joint they have, they can get the information from their surgeons. He says that the devices were used primarily in younger patients because of expectations of increased longevity.

Older patients, who make up the majority of those who receive hip implants, traditionally receive ceramic or plastic components.

The situation, he says, raises questions about testing and approval systems for medical devices.

Mets and Aldred are now suing the makers of their implant as part of a class action lawsuit on behalf of the thousands who may be affected.

Joel Rochon, a lawyer at the Toronto law firm of Rochon Genova LLP which is leading one of the lawsuits, said many of his clients are out of work because of their health problems or have had recovery times that have gone on for months.

"In many case it has been a devastating situation for the clients," he said.

"The big problem here is once you have a medical device like this implanted and it is determined to be a defective device, it is really the ultimate catch-22. What do you do? Can you get this device removed without causing health problems?" asked Rochon.

With a report from CTV medical specialist Avis Favaro and producer Elizabeth St. Philip