"Hip New World," Feature Article, October 2006

Focus on Bioengineering

hip new world

Once reserved for the aging, this achievement of mechanical engineering is offered to younger adults who still want to jog, run, and play golf.

by Alan S. Brown, Associate Editor

golfing greats Arnold Palmer and Jack Nicklaus each have an artificial hip. So do former tennis star Jimmy Connors, television chef Ainsley Harriott, and Motley Crue guitarist Mick Mars. Bo Jackson returned to baseball after his implant, though without his blazing speed. After years of pain, Floyd Landis planned hip surgery after this year's Tour de France.

Companies that manufacture artificial hips encourage celebrities—often with lucrative contracts—to talk about their implants. They may not be as young or active as they once were, but they often retain the image of vitality for those old enough to remember their prime.

This is no accident. Not that long ago, doctors usually counseled patients under 60 years old to put off hip surgery as long as possible. They warned that hip implants lasted only 10 or 15 years, and each new surgery, or revision, ate away more of the body's natural bone. Not everyone could have a third revision.

A typical hip replacement includes (from left to right) a femoral stem, a bearing, a liner, and an acetabular cap.

Today's medical implants last longer, often much longer. Equally important, implants act more like the natural hips they replace. By publicizing athletes and rock stars who have opted for hip implants, manufacturers hope to encourage younger adults to leave behind the pain caused by arthritis and other hip conditions, and reclaim their active lifestyles.

The pitch seems to be working. Patients aged 40 to 59 now receive as many hip implants as those over 60, and their number is swelling rapidly. According to Philadelphia technology consultant Exponent Inc., 216,000 patients received new hips in 2006, 38 percent more than in 1996. It expects that number to reach 572,000 (plus another 97,000 revisions) by 2030. Many of those patients are not babying their implants. They bicycle, jog, hike, swim, ski, golf, and play tennis.

The modern hip implant, while not a complete triumph, is an outstanding achievement of mechanical engineering, from materials selection and design through machining and surgical procedure.

Like many other engineering achievements, it was built on failure.

The Exile

Surgeons have been rebuilding hips damaged by crippling arthritis for more than 100 years. They tried smoothing body parts roughened by arthritis. They lined the pelvic socket (acetabulum) and thighbone (femoral head) with gold, magnesium, zinc, and even chromatized pig bladder. Some placed molds between the joints. Others replaced the femoral head with a metal bearing, or resurfaced the acetabulum with acrylic.

Most early designs, materials, machining, and surgical procedures failed to withstand the enormous forces placed on the human hip. The most successful operations left patients with a limited range of motion for a limited amount of time.

A major barrier to success was the way orthopedists looked at the problem. Inherently conservative, they sought to fix a femoral head or acetabulum surface. No one really looked at hip replacement as a systems problem until British orthopedist John Charnley did so in 1958.

Charnley's radical notions about totally rebuilding hip joints unnerved many of his colleagues. In 1962, he moved to an isolated British hospital outside Manchester that had previously served as a tuberculosis sanatorium. There, he began performing the first modern hip replacements. Anyone familiar with snap-fit plastics would understand his method.

First, Charnley cut off the upper part of the femur. In its place he put a metal ball on a metal stem sunk into the remaining bone. Then he used dental cement to glue a cup into a hollowed-out area of the acetabulum. The cup consisted of an ultrahigh-molecular-weight polyethylene liner inside a metal shell. When he pushed the metal ball into the liner, the plastic deformed until the ball snapped into place.

The result was a ball-and-socket bearing. Surgeons found it easy to implant. It replaced the hit-and-miss results of past surgeries with very predictable reductions in pain and increases in range of motion. Most hips lasted 10 to 15 years.

Yet Charnley (later knighted Sir John Charnley) had to make a critical compromise. His hip used a bearing much smaller than the natural femoral head it replaced. He did this to minimize the amount of metal surface rubbing against the polyethylene liner with each step. This reduced wear. Patients could still stand, sit, and walk comfortably. Yet the smaller bearings were not as stable as the original femur. They proved prone to dislocations and limited patient activity.

Fixation

When it came time for revisions, surgeons found something else about the Charnley hip. It dissolved the bone around it. This left surgeons with less bone to work with for each subsequent revision. Third revisions were often impossible. As a result, surgeons generally advised patients to put off hip implants as long as possible or risk losing their ability to walk in 20 or 30 years.

They called the problem "cement disease."

"Today, cementing is almost gone, especially on the
acetabular side," said Paul Berman, product director of hip bearings at DePuy Orthopedics Inc. in Warsaw, Ind. Instead of cementing, manufacturers spray implant surfaces with small beads to form porous coatings.

Hip resurfacing takes advantage of larger metal bearings that mimic a natural femoral head. This product is not available commercially in the U.S.

Surgeons attach pore-coated devices by drilling or cutting a slightly smaller space and hammering the implant into the opening. Press fitting locks the part in place. Patients can even walk on it within a day or two of the operation. During the next six to eight weeks, natural bone grows into the pores and forms an even stronger bond.

"Porous coatings have largely solved the industry's fixation issues," Berman said. "It is a better option because cement will eventually loosen. Here, there's one less component to fail."

This is good news for patients. Yet switching to porous coatings did not stop bone resorption. As it turned out, "cement disease" had nothing to do with cement at all. It had to do with wear.

Every day, the femoral ball rubs against the ultrahigh-molecular-weight polyethylene acetabular cup hundreds or even thousands of times, explained Jon Moseley, technical director of Wright Medical Technology Inc.'s implant research group. "This created wear particles that were just big enough to activate the body's cellular defenses," Moseley said.

"The body's white blood cells ingested the particles, but they couldn't digest them. So they kept eating until they burst. This reintroduced the particles back into the joint space, as well as enzymes from the macrophages that started to break down bone," he added.

Crosslinking

Once researchers understood the cause of bone loss, they could attack it with more wear-resistant materials. Their obvious candidate was crosslinked polyethylene. Irradiating the ultrahigh-molecular-weight polymer injected enough energy to break some of its long molecular chains. These broken segments then linked to adjacent molecules, creating an intertwined network of molecules that stood up better to wear.

Although crosslinking had been around for decades, researchers had never considered using it before. That was because they were unable to measure wear in hips accurately. "Academia didn't have a good way to tell if they were making changes that helped," explained Cheryl Blanchard, senior vice president and chief scientific officer of Warsaw-based orthopedic giant Zimmer Inc.

"We did lots of iterations on wear test machines to understand what made these things wear," she continued. "We were testing ultrahigh-molecular-weight polyethylene with linear reciprocating motion, and it didn't wear. If you did the test that way, you could convince yourself that you didn't have a problem. When we began testing with cross-motion patterning, you could see that it really did wear."

Even then, it took time to optimize materials. While crosslinking improves hardness and wear resistance, it also makes polyethylene less elastic and more brittle. Several research teams developed crosslinked materials with the right balance of wear and toughness.

"Since the stresses are predominantly compressive, you can compromise a little on structural strength," said Edward Ebramzadeh, director of the Implant Performance Labs at Los Angeles Orthopaedic Hospital.

Ebramzadeh is an expert in measuring implant wear with X-rays. Since the polyethylene liner is shielded from X-rays by the metal cup, he measures the height of the femoral ball in the acetabular socket. His software uses algorithms to analyze height changes so slight, they fall below the resolution of the X-rays. "So far, crosslinked liners have shown very low rates of wear," he said.

Big Heads

Blanchard and other device makers believe that crosslinking reduces wear by 90 percent when compared with conventional polyethylene. She rates it among the most important hip implant advances since Charnley. Crosslinking, she explained, not only reduced bone resorption, but also enabled designers to increase femoral ball size to better mimic the biomechanics of a natural hip.

Charnley, remember, went with small femoral balls to manage wear. He did so at the expense of stability. A small ball has to move only the distance of its radius before its metal stem impinges the side of the acetabular cup. A little extra pressure leverages the ball out of the cup, causing a dislocation.

Larger bearings change that. Their greater radius and surface area improve stability and give the leg a longer swing plane. Patients can jog, run, and play other active sports. Crosslinked polyethylene stands up well to larger bearings.

"There's been a dramatic shift," said Berman. "If you go back five or six years, 90 percent of all bearings were 28 millimeters in diameter. Today, 55 percent of what we sell is 36 millimeters and above. As we go to 36 millimeters and beyond, dislocations decline considerably."

A cutaway of a metal-on-metal hip implant shows the shell, liner, ball, and stem.

Large bearings offer so many advantages that designers have rushed to increase sizes. Today, many companies offer bearings that exceed 50 millimeters in diameter. This approaches but does not yet match the size of a natural femoral head.

Larger bearings, however, present new problems. "You can't make bearings that big and use polyethylene," noted Moseley, referring to Wright's line of 50-millimeter-plus femoral heads. "Even crosslinked material will wear at that size."

The problem is not just wear. Bigger bearings need bigger sockets. Polyethylene leaves no place for them. A typical metal-on-polymer acetabular cup consists of a crosslinked polyethylene liner inside a metal shell thick enough to maintain its shape without deforming. "The outer diameter is fixed by the size of the hip," Moseley noted. "The only way to accommodate a larger bearing is to use a thinner liner inside the shell."

Unfortunately, designers cannot slim down polyethylene. "If you make the polymer too thin, it weakens structurally and there's not much room for wear," said Ebramzadeh. "That limits the size of the bearing."

Hard-to-Hard

To use larger femoral balls, designers have turned to metal-on-metal and ceramic-on-ceramic systems. Both are stronger and more wear-resistant than polyethylene. Engineers can form them into thinner liners. Both promise to last for decades rather than years. Some first-generation metal joints are still functioning after 30 years, Moseley said. Yet hard-on-hard systems have issues of their own.

Despite a handful of early successes, surgeons abandoned metal hips. The problem was wear, and it took them years to learn what industrial engineers already knew about bearing design. "Orthopedics is just now catching up," according to Berman, who worked for Carrier Corp. before coming to DePuy. "It's starting to talk about things like fluid film lubrication and proper clearance."

For example, research showed that if the clearance between the femoral bearing and metal liner is 80 to 120 micrometers, larger metal bearings will actually wear less than smaller bearings.

These implantable stems use composites (left) and porous tantalum to emulate the stiffness of natural bone.

"If you optimize the clearance, the faster the swing plane of the bearing in the socket, the more fluid you introduce into the joint from the body's natural lubrication system, and the less you wear," said Berman.

"Metal joints wore down in the past because people didn't realize these tolerances were important," he added.

Yet before the industry could make reliable metal-on-metal systems, it had to re-engineer its metal machining and finishing operations to consistently achieve such close tolerances.

These improvements have revived interest in hip resurfacing, a technique developed and abandoned 40 years ago. Designed to treat people with dying femur heads, it caps the femur head with a metal bearing but preserves the healthy bone below.

"The femurs of these patients are in good condition except for the head," noted John Chopack Jr., who follows orthopedic markets for New York investment firm HealthpointCapital LLC. "Since you always lose more bone with each hip revision, why not conserve as much bone as possible for future operations?"

Ions and Diamonds

Modern metal-on-metal hips seem to have it all. Their large bearings mimic natural biomechanics and let patients remain active. They show so little wear that they may last for decades. Yet their long lifespan poses a problem. Metal rubbing on metal creates nanoscale wear debris. The particles appear small enough for some cobalt and chrome to end up as ions. Both metals have the potential to cause cancer.

"Researchers have found elevated levels of metal in urine and blood, but we don't know whether this causes problems or not," Ebramzadeh said. "If someone gets cancer, how do we know it's from the implant? We have to study a lot of people to know, and there are not yet enough implants to study."

No one has linked ionic metals with cancer. Yet the possibility makes people nervous. The U.S. Food and Drug Administration, for example, does not classify all metal hips as safe and effective, but instead requires each new metal hip to undergo full clinical trials.

"FDA is worried about the elevated heath risk of ions from hip joints that will last for 30 or 40 years," said Jeffery Taylor, chief technology officer of Diamicron Inc. of Orem, Utah. Taylor thinks his company's diamond coatings could solve the problem.

Diamond has the lowest coefficient of friction and greatest hardness among natural substances. The coating is so wear-resistant and its bonding so strong, Diamicron-coated oil drills run three times faster and 160 times farther—240,000 feet—than conventional tungsten carbide.

Taylor believes diamond coatings can keep metal implants from wearing. Two implant manufacturers, Biomet Inc. and Exactech Inc., have partnered with his company to develop the technology. Not everyone is a believer. Zimmer's Blanchard says the coatings are expensive and have unresolved technical issues. "I didn't see it as a commercially viable improvement," she said.

Ceramics

Meanwhile, device manufacturers are scrambling after alloys that leave behind less debris. Some have also introduced ceramic hips. Ceramics are highly biocompatible and so hard and wear-resistant that they are likely to outlast metal.

The ceramics used in hip implants are a triumph of materials science. Ceramics are inherently brittle and crack easily. Materials scientists spent decades developing ways to manufacture and machine ceramics that are able to withstand the pounding taken by a human hip.

The French began making ceramic hips from alumina about 25 years ago, Ebramzadeh said. It is hard, tough, and biocompatible, and patients have continued to use it with good long-term results. Unfortunately, alumina was not strong enough to form into large femoral balls and thin acetabular liners.

The industry moved to toughened zirconia, which had even better mechanical properties. "For a long time, everyone thought they would never fracture," Ebramzadeh said. That belief was upended after a supplier, France's St. Gobain Desmarquest, changed its manufacturing process and produced balls that failed prematurely.

That blackened zirconia's name, but the ceramic had another problem as well. Although researchers had tested it extensively, they found that implants changed phases, or chemical structures, inside the body. "Any time you see a change and didn't predict it, it makes people nervous," Moseley said.

Ceramic-on-ceramic hips may last for decades, but their thick liners cannot yet accomodate the largest bearings.

The industry is now moving to zirconia-toughened alumina. Because it is stronger than conventional alumina, designers can slim down cup liners and use larger ceramic femoral bearings. Even so, ceramic bearings will not be as large as metal ones. Several companies are now looking at silicon nitride, a stronger and tougher ceramic that might enable still larger ceramic balls.

Ceramics have other downsides. Because they are inherently brittle, implants demand careful surgical technique. "If there's a misalignment, then you're stuck," Blanchard said. "There's also a real risk that they will fracture in an accident, and when they do, they're messy to clean up."

Ceramics also command premium prices, roughly $7,500 compared with $5,500 for metal-on-metal components, according to Chopack. Even so, he expects that their long lifespan and good performance will eventually power them to 20 to 30 percent of the market.

Flexibility

Meanwhile, it turns out that unpalatable particles of polyethylene are only one cause of bone resorption. There are many other causes, including stress shielding.

Ordinarily, natural bone adapts to loads placed on it. Karate students who learn to break bricks, for example, develop denser bones. Conversely, the body resorbs bone when a load is removed. That is what happens when surgeons bury the stem of a femoral bearing 8 to 12 inches down the femur. Since the stem is stiffer than the surrounding bone, it carries most of the load. As a result, the body begins to resorb femur bone.

Zimmer has attacked the problem with two different types of hip stems. Both replicate the stiffness of natural bone.

The first consists of a cobalt-chrome alloy core surrounded by a thick layer of polyetheretherketone resin whose outside layer contains a molded-in weave of titanium fiber. The composite structure has the same stiffness as bone and provides a woven metal surface into which bone can grow.

Ball-and-socket systems. Plastic liners (top) limit the bearing size, while metal liners (middle) allow for larger bearings. Eliminating the liner entirely supports the largest bearings.

Several companies attempted to create composite stems but failed, Blanchard said. "Some went with carbon-reinforced PEEK, but it degraded in the body," she explained. "Others were too flexible for bone to grow into. We didn't want to go with a completely new composite material, so we stepped back and designed a composite structure of metal and resin. It's been extremely successfully clinically, and we've shown that the stem does not lead to any stress shielding."

It took a long time for FDA regulators to okay the new technology. "By the time we launched, surgical preferences in stem geometries had changed. We're now launching more modern geometries and expect it to take off," Blanchard said.

The second technology, acquired by Zimmer in 2004 when it bought New Jersey-based Implex Corp., is trabecular metal. Based on biocompatible tantalum, it is a 70 to 80 percent porous metal with the same compressive strength as bone. While the metal itself can support the body, its pores provide an ideal structural support for natural bone in-growth. The longer it stays in the body, the denser it gets.

The Perfect Hip

Composites and trabecular metals are just two of the new technologies reaching commercialization. Implant manufacturers continue to work on tougher ceramics, more wear-resistant metals, and better coatings. They are developing better ways to make surgery more precise and less prone to human error. Farther afield, they have begun to test proteins that can form new bone and that may one day replace metal, plastics, and ceramics.

"MANY PATIENTS STILL PUT OFF TREATMENT. WHAT CAN WE PROVIDE SO THEY CAN SEEK TREATMENT EARLIER AND NOT HAVE TO LIVE IN PAIN?"

What is the perfect hip of the future?

To Blanchard, it's a hip that combines good biomechanical properties and minimal wear while eliminating negative biological responses and bone resorption. "Today," she said, "hip implants are one of the most successful surgeries you can have. They are 99 percent successful, and they stay that successful for years out.

"Now, we're asking a different question: How can we treat clinical issues orthopedically earlier in the continuum in care? Many people still put off treatment. They live with pain for many years, until they can't stand it any more. We need to ask what we can provide so they can seek treatment earlier and not have to live with pain."