Metal Mania

The success of total joint replacements is based largely on the design and processing of the materials used in the implant. To replicate the action of a ball-and-socket hip joint, implants used in THA are composed of three parts: the stem, which fits into the leg bone; the ball (or head), which replaces the sphere-shaped head of the leg bone; and the shell (or liner), which replaces the worn-out hip socket.

Before Charnley found the source of the squeaking noise in the prostheses of the 1950s and early 1960s, all three components of hip implants were made of metal. In 1960, Charnley developed a low-friction device using Teflon shells, but massive amounts of debris led to its failure. After experimenting extensively with the Teflon without success, Charnley replaced it with high-density polyethylene that was not as friction-free but was 1,000 times more wear-resistant. This prototype of THA, developed in 1962, was the basis for future designs.

“In the earliest development of orthopedics, 316LVM stainless steel was the workhorse of the market. It was one of the first metals to be used, and it was a very nice combination of corrosion-resistance for long-term implants, hardness and strength for wear resistance and to take the shock-loading of a hip,” noted Mark Michael, president and chief operating officer of Fort Wayne Metals, a company based in Fort Wayne, Ind., that provides centerless ground bar and wire products for the orthopedic industry. “[Stainless steel] 316 was a very good material, and even to this day it’s a decent material to use for orthopedic implants. Its shortcomings and the reason the other two families [of metals] entered the picture is that, generally, cobalt-based super alloys have a higher yield strength—almost twice as high as stainless steels. They have a much higher impact strength and are much harder materials in terms of wear perspective.”

The amount of wear particles, or debris, produced by artificial joints is one of the main factors that can affect the type of material used for the implant. These debris particles are not absorbed by the body and can cause an adverse reaction in surrounding tissue and bone. High concentrations of wear particles ultimately can lead to osteolysis, or bone cell death.

(Editor’s note: For more information on wear debris and its effect on implants, see the feature story by contributing writers Dr. Janet Cotton and Simon Demnitz).

While the proper combination of materials can minimize the amount of debris emitted from the implant, wear particles can never totally be eliminated, experts said.

Concern about debris has led to the development of plastic and ceramic implant components over the last several decades, with the ball being made of ceramic materials such as aluminum oxide or zirconium oxide, and the acetabular socket composed of ultra-high-molecular-weight polyethylene (or a combination of polyethylene backed by metal).

The metal parts of artificial joints are usually made of either cobalt-chrome or Titanium, two metals that are extremely durable and resistant to corrosion, industry experts told Orthopedic Design & Technology. Titanium, though, has other properties that have made it a more popular choice for implants in recent years: It is non-magnetic; it weighs less than most other alloys; it transfers heat well; it has a higher melting point than steel; and it is biocompatible.

“Titanium alloys are much more inert than either of the other two [Cobalt-chrome or stainless steel], so for very long-term implants in younger patients, the Titaniums are a good choice,” Michael said. “One of the advantages of the Titanium is the modulus of the metal is closer to bone. There’s still a significant difference, but it’s certainly closer to bone than either the Cobalt-chrome or stainless steel. It’s as close as metal can get to bone.”

There’s a chance it could get a lot closer, though. Scientists have discovered a way to produce new metal surfaces that improve healing and help the body better accept metal prostheses. According to research published in the technical journal Nano Letters, scientists capitalized on the latest advances in nanotechnology to change the way metals influence cell growth and development in the body.

Scientists in Canada and Brazil collaborated on the research that may one day lead to a new generation of orthopedic implants. The multidisciplinary team applied chemical compounds to modify the surface of metals such as Titanium, and then exposed the material to selected etching mixtures of acids and oxidants to create surfaces with a sponge-like pattern of tiny pits. “We demonstrated that some cells stick better to these surfaces than they do to the traditional smooth ones,” said Antonio Nanci, a professor at the Université de Montréal’s Faculty of Dentistry and a senior author of the study published in Nano Letters.

The scientists tested the effects of the nanoporous Titanium surfaces on cell growth and development. They found that the treated surfaces increased the growth of bone cells, decreased the growth of unwanted cells and stimulated stem cells. The treated surfaces also produced more genes for cell adhesion and growth.

Metals Made Better

Since the introduction of polyethylene in THA more than four decades ago, the material has become a standard choice for the liner of the implant. Studies that have shown metal-on-plastic implants produce less wear debris than their metal-on-metal counterparts have only added to polyethylene’s popularity and widespread usage.

However, enhancements to metals in recent years have improved their wear quality and biocompatibility. Some studies have even suggested that metal-on-metal implants have fewer wear particles and last longer than those made of metal and polyethylene.

Wright Medical Technology, an Arlington, Tenn.-based manufacturer of orthopedic devices and

Enhancements to metals have improved ther wear quality and biocompatibility, leading to a resurgence of interest in metal-on-metal implants. Photo courtesy of Heraus.

biologics, sells a metal hip replacement system that purportedly wears less (between three and 10 times less) and lasts longer than implants made of metal and plastic. According to Wright, its A-CLASS Metal-on-Metal Total Hip features a large diameter femoral head that more closely matches a patient’s natural anatomy. The company also claims that the surface finish for the implant’s metal components is 0.008 micrometers, which is much smoother than traditional metal-on-plastic devices.

Zimmer has developed a material called Trabecular Metal, which resembles bone and assumes its physical and mechanical properties more closely than other prosthetic substances. According to data from the Warsaw, Ind.-based orthopedic device and instrument manufacturer, Trabecular Metal is highly porous and conducive to bone formation.

“Trabecular Metal consists of interconnecting pores resulting in a structural biomaterial that is 80 percent porous, allowing approximately two to three times greater bone ingrowth compared to conventional porous coatings and double the interface shear strength,” according to information on the company’s Web site. “Trabecular Metal implants are [created] using elemental tantalum metal and vapor deposition techniques that create a metallic strut configuration similar to trabecular bone. The crystalline microtexture of a Trabecular Metal strut is conductive to direct bone apposition.”

Zimmer makes various products with this material, including an acetabular cup, a metal revision shell, a modular acetabular system, a humeral stem, and a reverse shoulder system.

ATI Wah Chang, a manufacturer of specialty metals for the medical market, has developed ATI 425 Titanium, a metal that is as strong as regular Titanium (Ti-AL-4V) but can be fabricated more easily. It is relatively easy to produce and form, according to the Albany, Ore.-based firm, and exhibits good fracture toughness.

Enhancements to implant metals have benefitted other materials as well. Smith said he has noticed an increase in the use of Cobalt-chrome for spinal implants. “We are seeing a bigger demand for small diameter Cobalt-chrome rods, as small as .1875,” he said. “We’re definitely seeing an increase in demand for that as well as 3/16, 5.5 and six millimeter. We’ve seen this over the last year as spine/disc replacement surgeries have started to take place.”

Complicating the demand last year for metals such as Cobalt-chrome, Titanium and stainless steel was the worldwide recession and price volatility. The price of Cobalt-chrome, for example, seesawed throughout 2008 and much of 2009, going from $24/ pound in January 2008 to $52/pound in June that year, to $11/pound at year’s end. The metal is currently trading at about $18/pound.

The price of hot-rolled 316LVM stainless steel followed a similar pattern, increasing from $6,660/ton in March 2008 to $7,022/ton in May of that year, according to MEPS International LTD., a consultancy firm serving the worldwide steel industry. By January of 2009, the price had dropped by more than half; the metal was down to $2,771/ton in May.

“Over the last two years, material prices have been flattening out,” said Jodie Gilmore, vice president of strategic business development for Onyx Medical Corporation, a Memphis, Tenn.-based contract manufacturer for the orthopedic industry. “We are not seeing a trend of decreases, but we also are not seeing the same kind of increases across all purchases as we experienced in 2004 through 2007. Small diameter fine wire and cannulated material prices still seem to be increasing.”

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The use of metals in orthopedic implants dates back more than seven decades. The earliest joint replacements used stainless steel, but wear debris and the ultimate failure of these devices led to the development of Cobalt-chrome and Titanium components. Ultimately, polyethylene was paired with the various types of metals to reduce wear particles and minimize the risk of osteolysis. Polyethylene is now a standard material in most hip implants.

Innovation by companies, however, has helped improve the wear quality and biocompatibility of metals in recent years. The enhancements have led to a resurgence of interest in metal-on-metal implants, as questions arise about polyethylene’s durability and wear resistance. The development of metals that help promote bone growth can only serve to increase interest in these new metal-on-metal configurations. The capabilities of these improved metals, however, are not as important as the kinds of characteristics they must possess: strength, flexibility, smooth movement, and the same mechanical properties as the joint it is replacing. “These four elements are really essential to the success of any implant,” Smith concluded.