This article analyzes changes in sterilization techniques that can substantially increase the life of implants. A result of the work on sterilization methods has led to a related development—crosslinking—that greatly increases the wear resistance of the joints’ polyethylene mating surfaces. The vast majority of the prostheses used in a half-million hip replacement procedures performed worldwide each year are a combination of metal and ultrahigh molecular weight polyethylene. Some experts believe that the overall effects of gamma radiation in air have been misrepresented. Howmedica recently developed a highly crosslinked polyethylene, called Crossfire, which achieves a 90 percent reduction in wear compared to standard polyethylene. The crosslinking is achieved with an elevated dose of gamma radiation, followed by heating the material close to its melting point. The polyethylene is then machined and packaged in a nitrogen atmosphere, and sterilized with gamma radiation.
ARTIFICIAL HIP JOINTS must perform reliably for many years of use and millions of cycles. A typical patient with a hip replacement joint may take as many as a million steps a year. Yet researchers say that changes in sterilization techniques can substantially increase the life of implants.
A result of the work on sterilization methods has led to a related development-crosslinking-that greatly increases the wear resistance of the joints' polyethylene mating surfaces. Crosslinking is the bonding of adjacent molecular chains, creating a molecular structure that better resists sliding forces.
The vast majority of the prostheses used in a half-million hip replacement procedures performed worldwide each year are a combination of metal and ultrahigh molecular weight polyethylene. The prosthesis consists of a metal shaft that is inserted into the femur, and a polyethylene cup, usually housed in a metal shell, that is affixed to the hip. A metal ball at the end of the shaft rides in the cup.
According to Thierry Blanchet, associate professor of mechanical engineering at Rensselaer Polytechnic Institute in Troy, N.Y., ganul1a radiation of the polyethylene cup to sterilize it during manufacturing sets the stage for a chain of events that eventually may lead to a cause qf prosthesis failure.
The high molecular weight polyethylene depends on long polymer chains for its mechanical properties, Blanchet said. Irradiating the polyethylene has the unintended result of stripping hydrogen from the hydrocarbon chains. This leaves "free radicals"-essentially unpaired electronsalong the chain's length. These free radicals are unstable sites that react with oxygen if oxygen is available when the prosthesis is in storage, or even in the human body.
The oxidation causes a scission reaction in the polyethylene, in which the long chains are cut, resulting in a loss of the molecular weight of the polymer. This leads to a loss of wear resistance at the surface of the polyethylene. The polymer surface abrades, releasing submicron polyethylene particles in the body. The presence of this debris triggers an inflauu11atory response in the body, resulting in bone loss around the joint and causing the implant to loosen.
Blanchet said the problem is not that the polyethylene, which is generally about one centimeter thick, wears through, but that the biological reaction weakens the interface between the implant and the bone. Loosening of the metal shaft in the femur and loosening of the polyethylene cup's shell housing that attaches to the hip have been observed.
Researchers have addressed the problem by crosslinking the polymer, essentially tying up the free radicals that are the root of the problem. To avoid oxidation, manufacturers today steer clear of packaging components under atmospheric conditions, which was common practice until about five years ago. An alternative is to package the component in foil, pull a vacuum on it, and irradiate the implant through the foil. This eliminates the oxidation during storage, but doesn't totally solve the problem of free radicals, which can persist for years, Blanchet said.
To speed up crosslinking while the part is still in a vacuum, some companies increase the temperature, in addition to irradiating the part. Increasing the temperature is an attempt to stabilize the material by linking up the free radicals before the package is opened for surgery. The temperature can be increased either during or immediately after radiation.
One approach is to machine the part, irradiate it, and heat it to below the material's melting point of abo ut 137°C, maintaining the dimensional integrity of the machined part. As a semi-crystalline material, polyethylene is composed of both crystalline and amorphous regions.
However, free radicals in the crystalline regions are less likely to link together than their mobile counterparts in the amorphous areas.
Another method is post-irradiation melting, in which the polyethylene is heated above its melt temperature. The higher tempera ture increases the mobility of free radicals trapped in crystalline regions and makes possible more crosslinking overall. R esearchers at Rensselaer have heated material to as high as 200°C.
To avoid the possibility of altering the dimensions of the finished part by heating the polyethylene above its melting point, some manufacturers heat the part before machining it. Essentially, an unmachined blank is irradiated, brought above its melting point to promote crosslinking, and then machined into final shape. However, the machining itself exposes the material to contamination, requiring sterilization after machining with one of several non-irradiation techniques, such as gas fumigation or gas plasma.
Recent research has fo cused on manipulating various factors to promote better crosslinking. Manufacturers and researchers have been improving the wear resistance techluques by heating to higher temperatures, inunersing the part in various gases during hea ting, or irradiating to higher dose levels. Research at R ensselaer, for example, has attempted to improve wear characteristics by inunersing the part in an atmosphere of acetylene, ethylene, and hydrogen during the heating stage. So far, Blanchet and graduate student Brian Burroughs have found that parts exposed to hydrogen or ethyl ene have shown better wear resistance than parts heated in a vacuum.
Other research has tried to improve resistance to wear by increasing the radiation dosage range above the typical sterilization level of2.5 megarads. Rensselaer has been able to triple the wear resistance of polyetheylene solely through manipulating post-irradiation environment and temperature. Other research has attempted to further improve resistance to wear by increasing the radiation dosage range above the typical level. "People are getting orders of magnitude improvements in wear resistance," Blanchet said.
"The way they are aclueving it is by going to higher doses."
He added that manufac turers have su cceeded in increasing the wear resistance of polyethylene to the point that the wear rate has become immeasurable.
Crosslinking is the key to improving wear, in the view of Orhun Muratoglu, a polymer scientist at Massachusetts General Hospital in Boston, which is developing protocols for crosslinking polyethylene used in hip implants. The hospital has developed three techniques, all of which use electron-beam radiation-a much higher dose rate method than gamma-to irradiate the mate rial. All three methods irradiate the material in its bulk form, which is machined afterward. T he finished part must be sterilized afterward with ethylene-oxide gas.
The first method is IMS, which stands for irradiation in the molten state. During IMS, the piece is irradiated to 8 to 10 mega rads in its molten state, giving the molecular chains enough mobility to recombine all the free radicals that are generated. This results in a highly crosslinked material with no detectable free radicals.
The second method, known as CISM, for cold irradiation with subsequent melting, irradiates the material to 12 to 15 megarads at room temperature and heats the material above its melting point, freeing up chains to increase mobility and allow free radicals to combine.
Known as warm irradiation with adiabatic melting, or WIAM, the third technique has produced the best results, said Muratoglu. In this method, the polyethylene is heated to 125°C, just below its melting point, after which it is irradiated at 8 to 10 megarads. The radiation induces adiabatic heating, which melts the material, causing crosslinking and recombining of the residual free radicals. The result, he said, is a material that shows two melting peaks, suggesting that it has two phases. This process results in material with b e tter mechanical properties-yield strength, elongation at break, and toughness-than the other two techniques, as well as very good wear properties, he said. In tests on a mechanical hip sin1ulator with a peak load of750 Ibs., the material has shown no measurable wear after 20 million cycles, Muratoglu said.
The hospital has licensed its WIAM technology to Sulzer Orthopedics, h eadquartered in Baar, Switzerland, and Zinm1er in Warsaw, Ind . Sulzer has incorporated the technology into its Durasul hip liners, which has recently received FDA approval. Zinuner is using the technology in its Longevity hip liners.
Some experts believe that the overall effects of ganuna radiation in air have been misrepresented. This is the opinion of Aiguo Wang, head of the tribology department at Howmedica Osteonics Corp. in Rutherford, N.J., who said that the overall effect of ganu11a radiation is to increase the crosslinking rather than to cause chain scission. He said the company has developed techniques to maxinuze the benefits of ganm1a radiation while mininuzing the effects of oxidation. The company packages its prostheses in an inert lutrogen atmosphere prior to ganmu radiation, and heats the part to 50°C to promote stabilization.
Howmedica also recently developed a highly crosslinked polyethylene, called Crossfire, which achieves a 90 percent reduction in wear compared to standard polyethylene, said Wang. The crosslinking is achieved with an elevated dose of gamma radiation, followed by heating the material close to its melting point. The polyethylene is then machined and packaged in a nitrogen atmosphere, and sterilized with galmna radiation.