Consisting of a stem, femoral head, and an acetabular cup that is frequently outfitted with a liner, the replacement ball-and-socket joint should ideally couple high mechanical strength with range of motion and durability. Wear resistance, however, is the critical property by which bearing surface success is measured.
Surface friction between the femoral head and the liner of a man-made hip prosthesis generates tiny particles of debris, which, in turn, triggers an autoimmune response by the body. Because the debris often settles close to the implant, the body ends up attacking the surrounding bone, resulting in bone loss referred to as osteolysis. Osteolysis is considered to be the primary factor contributing to inflammation, implant loosening, and the eventual need for revision surgery for hip-replacement systems.
The rate at which an implant experiences wear and produces debris is dependent on the bearing surfaces selected for the ball-and-socket portions. Throughout their evolution, hip prostheses have incorporated metal-on-metal (all-metal), metal-on-polyethylene, ceramic-on-polyethylene, and ceramic-on-ceramic (all-ceramic) implant designs, each with its own unique advantages and distinct drawbacks. And new combinations of--and improvements in--materials are beginning to emerge as well. Despite progress, there is still no clear frontrunner in terms of the perfect bearing surface material combination--but that doesn't mean that suppliers and OEMs aren't trying to figure it out.
Metal-on-metal implants have had a history fraught with dramatic highs and lows in the industry. Because the hip is a load-bearing joint, various metals have found favor over the years among implant manufacturers and surgeons alike owing to their exceptional mechanical strength. A trailblazer in the industry, the all-metal implant also set the standard in the early days of hip arthoplasty procedures. These initial designs, however, were sometimes marred by poor process control for casting, inferior designs, and weak structures. As a result, the rise of the better-performing metal-on-polyethylene prostheses supplanted all-metal constructions as the preferred design for several decades.
Improved process control and the incorporation of stronger, more wear-resistant materials such as cobalt-chromium and titanium alloys during the past decade have spurred a renewed interest in metal-on-metal hip prostheses; they presently account for roughly one-third of hip implants in the United States. "I think it actually goes beyond the actual material selection as all the current offerings available have their benefits and have clinical evidence to back them," says Bob Bruce, European sales and marketing director for Sandvik (Sheffield, UK; www.smt.sandvik.com), which specializes in forging and casting of cobalt chrome, titanium, and stainless steel for use in hip implants. "Material handling and processing are key factors in the quality of the product sold in the market be it cast, forged, or even machined from solid."
Use of metal-on-metal designs has been further bolstered by their more-recent success in hip-resurfacing applications. An alternative to total hip replacement, resurfacing entails reshaping the patient's natural femoral head and then capping it with a metal implant rather than replacing the entire joint. "This [trend] is really surgeon and OEM led," Bruce observes. "In the case of resurfacing, the logic is very sound: Why sacrifice good bone when only the surface is damaged?"
Controversy is brewing, however, as orthopedic surgeons once again cast a skeptical eye on all-metal total hip replacements. An article in the New York Times just last month thrust artificial hip joints into the spotlight by discussing mounting concern among orthopedic professionals regarding all-metal implant debris, for example. Concern for the metallic ions produced by surface friction and the potential need for revision procedures is further compounded by speculation that metallic ions in the bloodstream could be toxic or could even be linked to cancer.
In response to this backlash, some surgeons are swapping metal-on-metal implants for metal-on-polyethylene or other material combinations. As evidence of this trend, the article cited a recent editorial in The Journal of Arthroplasty advising surgeons to use metal-on-metal implants with "great caution, if at all" and the fact that the renowned Mayo Clinic had reduced its use of all-metal artificial hips by 80% in the past year.
Amid the negative press, some OEMs of all-metal hips are standing by their designs and working hard to restore faith in the mechanically strong implants by investigating ways to reduce wear. To accommodate these customers, some metal suppliers such as Sandvik are researching entirely new material compositions. "Sandvik is currently developing a process where the [implant] actually goes through a phased transformation at the surface, which we believe will greatly enhance the wear properties and will also be homogeneous to the parent material," Bruce says.
Material advancements in recent years have yielded ceramic heads, such as this one by C5, that couple wear resistance with high mechanical strength.
Hardness, ultra-smooth surfaces, and very low wear rates have established ceramic as a popular choice for younger patients so as to delay or prevent the need for revision procedures. But these materials, too, have risks. Ceramics are more fragile than metals or polymers employed in artificial hips. As such, they are more vulnerable to fracture and, consequently, an implant failure of catastrophic proportions.
"Back in the 1970s in Europe when they started using alumina ceramic, there were issues because the material quality was maybe not as good as it is today; strengths were lower," Hughes notes. "What companies like ours have done over the years, is improved our powders and improved our processes. There's a lot of processing done now that wasn't done back then that gives inherently higher strength." A manufacturer of ceramic powders such as alumina, aluminum oxide, ZTA, and zirconia, C5 fabricates ceramic femoral heads and liners for clients in-house.
To mitigate the risk of fracture, companies such as C5 have labored over the development of advanced composite ceramics. The strength of some ceramic compositions has been enhanced through increased chemical purity and reduced grain sizes. Moreover, various composites developed within the past five years double or even triple the strength of ceramic ball heads compared with previous materials, Hughes states.
Engineered with this improved strength, advanced composites are allowing for new designs, including the potential replacement of metals in hip-resurfacing applications and the fabrication of larger femoral heads, according to Hughes. Driven by surgical interest, the demand for larger-diameter bearings from ceramics, metals, and polymers is on the rise. Larger heads more-closely resemble the body's natural structure and allow for a greater range of motion of the artificial joint. Stability and a reduced opportunity for dislocation are additional benefits.
The enhanced mechanical strength exhibited by recent ceramic composites also has opened the door to the potential next generation of bearing surface combinations: ceramic on metal. Although available outside of the United States, ceramic-on-metal implants have not yet infiltrated the domestic market. This is likely to change soon, however, with the introduction of the Pinnacle CoMplete acetabular hip system manufactured by DePuy Orthopaedics (Warsaw, IN; www.depuy.com). At the time this issue of MPMN went to print, the hip-replacement system had gained recommended approval by an FDA advisory panel and was awaiting FDA approval, which it is expected to obtain soon. Combining the low wear resistance of ceramic-on-ceramic configurations with the strength and design flexibility of metal-on-metal designs, the ceramic-on-metal system demonstrated 90% less wear than metal-on-metal prostheses, according to DePuy. This new combination of bearing materials, Hughes points out, is generating substantial buzz in the industry.
A new cross-linking platform for UHMWPE developed by DSM Biomedical provides increased wear resistance while minimizing the loss of strength and stability.
Boasting a successful track record, polyethylene acetabular cup liners have served as a pivotal component in hip arthroplasty for more than 45 years. Characterized by biocompatibility, stability, fatigue resistance, and strength, ultrahigh-molecular-weight polyethylene (UHMWPE), in particular, features relatively low wear rates that fall in between metals and ceramics. "It is one of the highest wear-resistant polymer materials in the industry, but there is still a bit of wear in the body and the particles are attacked by the body, [which] leads to osteolysis," notes Leo Smit, business director of DSM Biomedical (Geleen, Netherlands; www.dsm.com). The company specializes in polyethylenes for implants, such as its UHMWPE homopolymer MG003.
Even though wear rates of polyethylene are relatively low, the generation of any debris poses a threat to patient health. Motivated to improve wear resistance after 45 years without change to the material's molecular structure, the plastics industry has been increasingly experimenting with cross-linking of polyethylene in recent years. DSM Biomedical is among the companies exploring highly cross-linked polyethylene. The company recently announced its new platform of easily cross-linkable diene-copolymers, and claims to be the first one to apply the technique to UHMWPE.
"Cross-linking basically is when you radiate the material with a very high dose of gamma radiation, which leads to radical formation," Smit explains. "The molecules all tie together into one very big strong network with the intent to reduce wear."
Improved wear resistance is indeed achieved through the process--albeit not without consequence. Cross-linking has been shown to decrease the material's mechanical strength and oxidative stability, thereby making it susceptible to fracture and failure. The DSM cross-linking platform, on the other hand, minimizes this loss of critical properties, according to Smit. He states that it enables the development of highly cross-linked UHMWPE using significantly lower doses of radiation than those employed in conventional cross-linking processes.
"The aim of our platform is if you start with our material, you can radiate the material with three to four times less radiation, which means that you induce [fewer oxygen] radicals," Smit says. "Therefore, the attack of the radicals on the stability of the polymer is much lower, so we expect it to be more stable." Still in the R&D phase, the platform has yielded cross-linked UHMWPE that is 20 to 25% stronger and features two to three times fewer radicals than current cross-linked polyethylenes, according to Smit. "We have proven these numbers in the laboratory and we are now working with a number of major medical device companies to see if we can then translate that into more-stable and stronger implants."
Cross-linking polyethylene is one way to address the need for wear-resistant polymers for hip implant applications; utilizing polycarbonate-urethane (PCU) is another. Replacing UHMWPE with PCU in hip implants represents a completely different approach to solving wear issues of polymeric bearing surfaces--one that has only relatively recently emerged as a viable option.
"If you take a look at the different bearing material used in hip implants, in principle what we've done is go to harder and harder materials," Smit states. "But if you look inside your own body, the cartilage is much softer than all of these materials. There are people who want to go in a different direction and say: What if we use a material that is much more like the cartilage in the human body?"
As the pioneer of this technology, Active Implants Corp. (Memphis, TN; www.activeimplants.com) unveiled its TriboFit hip system last year as the first orthopedic implant to have a PCU bearing surface. Also going against convention, DSM PTG (Berkeley, CA; www.dsm.com), part of DSM Biomedical, supplies Bionate thermoplastic PCU, which is designed for use in total hip arthroplasty, among other applications.
Studies have shown that PCU exhibits less wear than UHMWPE, which many researchers attribute to PCU's resemblance to natural cartilage. While UHMWPE has a modulus of elasticity that is 70 times stiffer than cartilage, PCU features a comparable elasticity. Furthermore, it simulates the role of cartilage in the natural joint and therefore allows for a layer of synovial fluid to form between the artificial bearing surfaces so as to prevent friction. Lubrication is enabled by PCU's hydrophilic properties; UHMWPE, in contrast, is naturally hydrophobic.
"Bionate has the same elasticity and hardness as cartilage, so it absorbs shock much better. Its affinity to water is also more than polyethylene, which means that it helps fluid fill between the bearing surfaces, and that facilitates lower friction and lower wear," Smit says. "This is a radical new type of material for future hip implants."