Features January 2010 Issue

New Materials, Improved Designs Mean Better Hips

Todayís hip implants are more durable, offer better range of motion, and may lessen the need for follow-up surgery.

For years, scientists have labored to produce more durable hip replacements. Their goal has been to find designs that not only provide the range of motion of a natural hip joint, but also are less susceptible to the wear and tear that can ruin a replacement hip.

Their efforts are paying off. More than four decades after the development of the first artificial hip, todayís prostheses employ the use of new designs and materials that researchers hope will make your first artificial hip the last one youíll ever need.

"Total hip replacement is a very effective operation. Done correctly, and as long as you donít abuse it, any hip replacement can last a very long time," says Peter Brooks, MD, a member of Cleveland Clinicís Department of Orthopaedic Surgery.

The key, he says, is to find an experienced surgeon who can recommend the type of prosthetic hip thatís best for you and implant it correctly.

Design and Fixation

A total hip prosthesis includes a hard, smooth ball that replaces the head of the thigh bone (femur) and bears against a smooth cup that fits into and replaces the damaged hip socket (acetabulum). The ball is connected via a thin neck to a metal stem driven into the shaft of the femur to add stability.

In earlier hip replacements, surgeons used bone cement to attach the cup into the hip socket and the stem to the femur. But researchers found the cement less than optimal, and the prostheses would sometimes loosen. Todayís total hips generally use no cement and instead rely on the surrounding bone to anchor the implant. "We learned that the best durability for fixation to the human skeleton is biological fixation, where we can encourage the bone to actually grow into the implant," Dr. Brooks says.

Design innovations also have improved total hip replacement. The stainless steel or cobalt chrome stems on older implants tended to crack, but new devices made of stronger alloys have made stem breakages rare. These stronger metals also mean that todayís implants can be made with thinner necks that allow for more range of motion.

Better Bearing

A standard hip implant employs a highly polished cobalt-chromium metal head that bears against a plastic (polyethylene) cup fitted into the socket. Friction between the bearing surfaces can gradually wear away the cup, releasing small plastic particles that collect inside the hip joint and trigger a biological response from your body. The result is destruction of the bone around the hip joint (osteolysis), which can lead to loosening of the prosthesis, pain, and fracture. To counter this effect, older implants used smaller metallic heads that reduce friction, but they were more prone to dislocation.

Advances in hip replacement have minimized the wear and tear of bearing surfaces, making them less likely to fail. New cross-linked plastics, irradiated for more durability, can decrease wear by up to 90 percent compared to older polyethylene models.

However, all plastic produces some wear debris, so researchers have investigated hard-on-hard bearing surfaces: metal-on-metal and ceramic-on-ceramic. The metal-on-metal implants were used in the 1960s and 1970s, but they fell out of favor after many failed due to technological limitations, Dr. Brooks explains.

"The fascinating thing was that occasionally these metal-on-metal total hips kept going, and some 25 to 30 years later, patients would walk into their doctorís office and the doctor would be astonished because it was still working after 25 years," he adds. "It seemed that metal-on-metal was worth looking at again, so these days, with the quality of manufacturing and the quality of the actual metal, you can get very long-lasting hips that are metal-on-metal."

The downside to having a metal ball bearing against a metal socket is that it releases metallic ions into the bloodstream, making these implants unsuitable for women who may become pregnant or people with poor kidney function or nickel allergies.

Another hard-on-hard alternative is a ceramic head bearing against a ceramic acetabular cup. Ceramic heads are harder and more scratch-resistant than metal heads and are easier to lubricate, Dr. Brooks says, and these implants avoid the problems of metal ions. One potential drawback is that ceramic-on-ceramic implants may cause loud squeaking, prompting some surgeons to opt for ceramic heads bearing on plastic sockets.

Overall, the hard-on-hard implants allow for larger heads, which provide better range of motion and less risk of dislocation. And if implanted properly, they allow for joint fluid to be sucked into the prostheses and provide even better lubrication.

"We donít have enough knowledge to say this to patients at this point, but from the 30-year results from metal-on-metal total hips, there is an inkling that these hard-on-hard hips might last indefinitely," Dr. Brooks says.

Hard-on-hard bearing surfaces generally are recommended for younger, more active patients who are expected to live longer and need more longevity from their implants, while metal-on-cross-linked-plastic prostheses are appropriate for most older patients.

Making Them Last

Dr. Brooks recommends no running, jumping or heavy lifting for anyone whose hip replacement has a plastic bearing surface, but non-impact sports such as skating or doubles tennis are acceptable. He advises patients with hard-on-hard implants to wait a year before returning to these more vigorous activities, although those with ceramic prostheses should avoid jumping and hard landings.

No matter what type of prosthesis you receive, getting the most out of your hip replacement starts with your choice of surgeon, Dr. Brooks says. As with other operations, surgical experience is vital to a successful hip replacement.

"Despite all of the technological advances, the most important variable is the surgeon," he says. "Take your hip to someone who does a lot of hip replacements. If you pick a surgeon you trust, heíll make the right decision on which hip to put in."