"Hip Action," Feature Article, December 2004

hip action

Better ways to make ends meet aim to give the artificial ball and socket a new lease on life.

by John DeGaspari, Associate Editor

For people who have been stricken with arthritis or who have fractured bones in accidents and falls, artificial joints are a godsend. They have kept millions from a life of chronic pain or of confinement to a wheelchair. Today, doctors replace some 168,000 hips in the United States each year, according to the American Academy of Orthopaedic Surgeons. And the replacements have performed well. Many patients can reasonably expect artificial hip joints to last 20 to 25 years. For many elderly recipients, that means pain-free joints for the rest of their lives.

The success of hip implants with elderly recipients has encouraged surgeons to increasingly perform hip replacement surgery on younger, more active patients as well. Younger patients, however, are more likely to resume physical labor or a few rounds of golf, and the increased activity may lead to eventual loosening of the joint replacement and may require follow-up surgery.

Nanoscale debris (above) from a replacement hip contributes to eventual loosening of the hip prosthesis. Prosthetic manufacturers say that highly crosslinked polyethylene cups (below) stand up to wear better than conventional polyethylene and greatly reduce damaging wear particles.

The condition that causes hip prostheses to loosen is known as osteolysis. It is a biological reaction that is triggered by tiny particles of debris from the bearing surfaces of the implant as they rub against one another.

While the problem affects only a relatively small set of recipients now, it may well grow as hip replacement surgery encompasses a wider range of eligible patients. As hip joint replacement has extended its reach, researchers have been busily looking for materials that better stand up to wear. Their efforts have produced new wear-resistant polymer, metal, and ceramic combinations. A research group at the University of Leeds in the United Kingdom says it has patented a ceramic-on-metal hip prosthesis that produces one-tenth the wear particles of currently available hip replacement joints. The prosthesis has been licensed to a prosthetic manufacturer and is about to enter clinical trials in Europe.

NANO-SCALE WEAR

Timothy Wright, a biomedical engineer, is a senior scientist at the Hospital for Special Surgery and a professor at the Weill Medical College of Cornell University, both in New York. He explained that a healthy person's skeleton is constantly being remodeled in a delicate balancing act between cells that form bone and other cells that destroy it. If something happens to upset that balance, the cells that eat bone can outperform their bone-forming counterparts, resulting in a loss of bone mass. This is roughly what happens during osteolysis.

As debris particles from the rubbing surfaces of the prosthesis accumulate around a joint, the body signals its macrophages, or garbage men, to try to ingest the foreign matter. A biological cascade ensues, eventually resulting in recruitment of cells that eat bone. The phenomenon upsets the balance between cells that destroy bone and cells that replace it, and eventually loosens the prosthetic stem, which is anchored in bone. Size is key, said Wright. Submicron debris bits from the wear surfaces of the joints trigger the body's reaction.

Wright said that osteolysis tends to show up sometime during the second decade of a replacement. While its causes are not well understood, osteolysis appears to be linked to the level of a patient's activity. Older patients or those with restricted mobility may never experience the problem. But patients in their 60s or younger, who may go back to a relatively active lifestyle, are more prone to the condition, he said.

A MATERIAL WORLD

Hip replacements date from the early 1960s, when Sir John Charnley, a British surgeon, devised an artificial hip joint consisting of a cobalt-chrome alloy shaft and head resting in a high-molecular-weight polyethylene cup. The metal shaft was inserted and cemented in the patient's femur and the cup was affixed to the hipbone, replicating the body's natural ball-and-socket joint formed by the femoral head resting in the acetabulum, a cup-shaped hollow in the hip. Versions of Charnley's metal-polyethylene prosthesis still account for the vast majority of implants performed today.

Over the past 20 years, researchers have experimented with a number of bearing surfaces in an attempt to minimize wear. The ceramic-on-metal joint developed by the Leeds group, led by John Fisher, a professor of mechanical engineering at the university, is one of the latest in a line of surface material combinations. Other groups have worked with metal-on-metal and ceramic-on-ceramic heads and sockets, crosslinked polyethylene, and various types of wear-resistant alloys and ceramics. Many of these combinations have now been commercialized; all have attempted to improve wear resistance.

Fisher said the ceramic-on-metal joint is part of a trend toward the use of hard surfaces that rub against each other. Metal-on-metal and ceramic-on-ceramic joints are commercialized in both the United States and Europe. His group's implant design, which uses an alumina ceramic head in a cobalt-chrome alloy cup, builds on the idea of using hard surfaces, but with different materials. Surfaces that are alike tend to want to mate with each other. According to Fisher, "Tribology tells us that we should be using materials that are different. We simply applied that same principle to hard bearings."

Fisher began studying ceramic-on-metal in the late 1990s and patented the principle in 2001. The original head consisted of widely used alumina ceramic. Fisher has refined the joint head, using a ceramic of alumina and a small amount of zirconia, which he said gives greater fracture toughness. The cobalt-chrome alloy cup, the softer of the two bearing surfaces, generates metal debris, but far less than occurs with metal-on-metal joints, he said.



Ball and joint components (directly above) of wear-resistant contact materials could extend the life of hip replacements in active patients. Stryker Orthopaedics claims that alumina-on-alumina ceramic bearings of its Trident Ceramic hip system (top) wears less than other materials.

Fisher expects the European clinical trials that are about to start to confirm results of his laboratory tests. He said the ceramic-on-metal joint should produce one-tenth the metal debris of metal-on-metal joints, another alternative to traditional metal-on-polyethylene joints. Fisher has tested the ceramic-on-metal joint in his lab, on a mechanical hip simulator that runs day and night to replicate in one year about 10 years of actual use. Fisher believes that the better performance results from contact between different materials and the smooth surface of ceramics, which reduces friction.

The joint's synovial fluid and water help lubricate natural joints and replacements, according to other experts. Wright said there is a need to better understand how the surfaces and geometries of hip replacements affect lubrication dynamics of the joint.

According to Wright, attempts to use new materials to limit the wear of bearing surfaces have met with varying degrees of success, and often result in tradeoffs in design objectives. Alternative bearing materials may change how a joint is affixed to the bone or structural consequences in terms of stiffness and load transfer to the bone.

Today there are three major types of hip replacement joints. Each has advantages and drawbacks that illustrate the difficulties of developing new designs, which are often compromises between competing objectives. "You end up with a lot of different solutions, and that is why there is more than one company and lots of different designs out there in joint replacements," Wright said.

The femoral head of a degenerative hip joint removed during surgery.

Starting in the mid-1980s, researchers realized that gamma radiation—an effective way to sterilize implants—also cuts polymer chains in ultrahigh-molecular-weight polyethylene, making the component more susceptible to wear. Researchers found that several factors were at work. Radiation causes chains to break, but also causes some of the chains to reattach or crosslink, and strengthen the material.

Oxygen causes the material to become brittle, reducing toughness and strength, so manufacturers now avoid any sterilizing in air. Some also avoid radiation, and sterilize with ethylene oxide instead. This avoids breaking the polymer chains, but does not derive the benefit of additional crosslinking by irradiation. Others irradiate the prostheses in an inert environment.

Some companies produce highly crosslinked polyethylene, either by thermal treatment or by radiation. Wright said that laboratory tests and follow-up studies of patients have shown that highly irradiated metal-on-polyethylene joints have reduced wear significantly. Doses of between six and 10 mega-rads are typical today among commercial implants.

Other attempts to deal with the wear problem have focused on getting away from polyethylene altogether. Metal-on-metal bearings have had mixed results, according to Wright. Some studies suggested that metal-on-metal joints produce small particles that trigger osteolysis.

Cells interact with debris (above) around a joint replacement. They can destroy bone and loosen the implant. A hip simulator (below) at the University of Leeds tested ceramic-on-metal hip joints.

According to Aiguo Wang, director of bearings and advanced implant design at Stryker Orthopaedics in Mahwah, N.J., the company's laboratory tests of metal-on-metal implants have encountered "runaway wear," or unexplained increases of wear debris. The phenomenon does not occur in every test, and researchers have been unable to explain the variability of the results. The company has decided not to develop metal-on-metal hip joints.

Another concern about metal-on-metal joints is that metal debris can become ionized and can travel anywhere in the body. While there is no strong link between metal ions and health problems, patients who have metal-on-metal implants have shown high metal levels in their blood and organs, and there is concern about systemic health effects, Wright said.

The third major combination for artificial hips is ceramic-on-ceramic. Ceramic-on-ceramic joints are very hard and highly polished. The downside is that they are brittle. Over the last 20 years, though, ceramic fabrication has improved, with higher-purity materials and finer grain. The idea has sparked the interest of surgeons in the United States, who have noted very low wear rates, Wright said. However, he added that if ceramic-on-ceramic joints are malpositioned, they will also create wear debris.

Stryker Orthopaedics has Crossfire hip implants, using highly crosslinked polyethylene cups against a metal ball. Wang said the device produces 90 percent fewer wear particles than standard polyethylene.

The company developed a ceramic-on-ceramic joint replacement, which it commercialized in 2003. The new Trident joint uses bearing surfaces of alumina ceramic. The company claims it has scratch resistance, low wear rates, good wettability for lubrication, and no ion release.

Stryker Orthopaedics is also looking at other types of polymer surfaces. In Europe, it has been testing a carbon-reinforced bearing surface of polyetheretherketone, or PEEK, with good results, Wang said. The material is stronger than polyethylene and is flexible. It has greater design potential than polyethylene, because it provides the strength in thinner surfaces, Wang said.

Wear is not really a function of time; it is one of use, Wang observed. More active patients who receive hip implants want to recover their quality of life and activity. That is going to put a lot of cycles on implants, which will require materials that can take some punishment.