Originally Published MDDI March 2005

Material



Ultra-high-molecular-weight polyethylene has been the gold standard for 40 years. New technology and new materials can now offer even more options.

Steven M. Kurtz, Harvey L. Stein, and Gunther Redeker

When disease, trauma, or overuse cause natural joints to fail, they can be replaced by artificial ones to regain function and offset debilitating pain. Most often, artificial joints contain one or more metallic components integrated with bone, and a polymer component that promotes easy movement.

The polymer must be biocompatible and tough enough to handle the loads imposed on the joint by normal living. It must also resist wear and mechanical damage, and have excellent lubricity, among other properties. This article discusses properties of ultra-high-molecular-weight polyethylene (UHMWPE). It will also explore options from new materials and new processing techniques for joint replacement.

Roughly 570,000 primary total hip and knee replacements are currently performed in the United States each year.¹ According to the American Academy of Orthopedic Surgeons, the procedures performed in the United States are expected to increase to at least 750,000 per year by 2030.

Many manufacturers of artificial hips, knees, shoulders, and other joints use UHMWPE. The polymer is suited for load bearing and movement, given its strength, low wear, and other properties (see Table I). Although more knees are now replaced than hips, historically, hip replacement was the first use of UHMWPE for orthopedic joints.

UHMWPE is unique among polymerized substances. Each molecule has more than 200,000 ethylene units (a molecular weight of 3–6 million). The molecule is equivalent to a tangled length of spaghetti more than a kilometer long. Its carbon backbone folds neatly into crystalline lamellae (sheetlike structures) embedded in amorphous regions. Processed UHMWPE is typically made up of approximately 50% crystalline content. The size and orientation of the crystalline portions depends on factors such as molecular weight and processing conditions.

Although UHMWPE never truly flows in a liquid state, because it has high viscosity, its smaller crystalline regions start to melt at around 60°C. And most of the crystalline regions have completed melting between 135° and 140°C. When UHMWPE is cooled below the latter range during processing, the lamellae that form are 10–50 nm thick and 10–50 µm long. These have an average separation of about 50 nm. Tie molecules between the lamellae create a network of amorphous and crystalline regions.

UHMWPE components are usually machined from rod or sheet stock made by compression molding or ram extrusion. Both processes use heat and pressure to sinter powdered UHMWPE into a uniform solid. The rod and sheet are annealed before machining to remove the internal stresses that occurred in processing. Components are then sterilized, either with low doses of gamma radiation (in barrier packaging with a low-oxygen environment) or by nonionizing methods such as gas plasma or EtO (in gas-permeable packaging). Some grades are made more resist-ant to wear by cross-linking with exposure to high doses of gamma radiation and then reheating to eliminate free radicals.

UHMWPE in Orthopedics

In the late 1950s, British joint-replacement pioneer John Charnley, MD, found that natural joints have a low coefficient of friction because the cartilage expels synovial fluid between the contacting surfaces as it is compressed. The pressurized fluid film protects cartilage from wear. Arthritis and other joint diseases can cause cartilage to lose its ability to lubricate joints.

Replacement joints mimic this lubricity, but depend on the boundary lubrication of the materials in contact with each other. Charnley's earliest artificial hip joints in 1958 relied on polytetrafluoroethylene (PTFE) for the polymeric bearing material. The earliest design had a PTFE acetabular element articulating against a PTFE femoral component.

Although these implants initially restored pain-free mobility to those suffering from joint disease, they wore out rapidly. Debris from PTFE wear led to inflammation and pain and the need to revise nearly all of the implanted joints just two to three years after implantation.

In 1962, Charnley moved to UHMWPE as an alternative to PTFE to increase joint longevity.² Initially, it was available only to a limited number of surgeons trained personally by Charnley. It became more widely used during the 1970s.

Total hip replacement for both femoral and acetabular surfaces experienced explosive popularity during the 1980s and then grew steadily thereafter. These joints initially had a UHMWPE cup cemented into the acetabulum articulating against a metal head. The metal head was then attached to a stem inserted into the femur. Over the past 20 years, cementless designs have been adopted. The cementless design involves a polymer component fixed to the pelvis by a metal shell and articulating against a metal or ceramic femoral head. Figure 1 illustrates a cementless design.

On average, more than 90% of all total joint replacements that use UHMWPE bearing surfaces survive more than 10 years, according to the Swedish hip registry. Such joints tend to be revised at a rate of about 1% per year in the first decade after implantation. Data show that implant survivorship drops after 10 years, especially for patients younger than 55 years old.³
The clinical performance of these joints and their pattern of wear and surface damage depend on surgical and implant factors, and a variety of patient variables. Long-term wear of the UHMWPE component in hip replacement occurs at an average rate of 0.1 mm per year.⁴

Each day of patient activity releases millions of microscopic UHMWPE wear particles into tissues around the hip joint. The accumulation of this debris can initiate a tissue response that eventually causes osteolysis (bone loss) and aseptic loosening of the components tied into the bone.

Other Joint Materials

Other weight-bearing combinations have also been involved in the evolution of artificial joints. Although more than 90% of all total hip replacements worldwide in the past decade have involved UHMWPE, metal-on-metal (MOM) or ceramic-on-ceramic (COC) articulations are also used.⁵

MOM joints are usually made from alloys of cobalt, chromium, and molybdenum (CoCr). These alloys have excellent hardness and strength, but may raise questions about long-term metal-ion exposure.⁶ As they wear, MOM hip implants have been estimated to release about 100 times more wear particles than conventional UHMWPE hip implants because their particles tend to be smaller.⁷ The CoCr particles may be more easily digested by cells, bound into proteins, and dissolved into body fluids. Individuals with MOM hip implants often have elevated levels of chromium and cobalt in their blood and urine.^6,8 So far, however, the long-term health effects associated with elevated metal-ion levels remain unknown.

The advent of new ceramics (primarily alumina and aluminazirconia composites) having excellent hardness, and thus high wear and scratch resistance, allowed for improved COC and ceramicon-UHMWPE bearing designs.

Ceramic particles are seen as biologically inert, so COC joints do not have a biological effect.

COC implants can fracture, however, because ceramics are brittle materials.^9-11 Although rare, fracture is a serious complication that must be revised immediately. This can be a complex task because ceramic femoral heads typically break into multiple fragments that are hard to clean from surrounding tissue.

MOM and COC bearings are considered most viable in young and active patients given their relatively low wear rates.¹² They are not expected to displace UHMWPE bearings for older adults. MOM and COC bearings are somewhat higher in cost compared with traditional UHMWPE.¹³

Making UHMWPE More Durable

Research to improve implant survival led to commercial, highly cross-linked UHMWPE grades. Versions introduced in 1998 are more resistant to wear and generate less wear debris than conventional UHMWPE.¹⁴ These grades are made by exposing standard UHMWPE in an inert atmosphere to gamma radiation doses of 50–105 kilogray (kGy,) or to an E-beam. This dosage is well above the 25–40-kGy dose normally applied in sterilizing UHMWPE.

High levels of radiation foster covalent bonding among adjacent molecules in the amorphous phase of the polymer to create a more wear-resistant UHMWPE. Irradiated grades then undergo thermal stabilization to eliminate free radicals (unpaired electrons) knocked loose in the UHMWPE by the radiation. If not eliminated, these radicals can potentially react with oxygen sources in the body and shorten the long polyethylene molecules, thereby reducing the mechanical properties of the UHMWPE.¹⁵

Thermal treatment involves remelting or annealing. Remelting, which occurs above the melt temperature, makes the molecules more mobile and increases the likelihood that free radicals will react to form cross-links. Total melting reduces crystal size and material strength. However, that loss can be avoided by annealing the material at just below its melt temperature (i.e., at 130° to 135° C) instead of remelting it. Annealing preserves the original crystal structure, but does not completely remove all residual free radicals. The process yields higher crystallinity, and improves ultimate strength and resistance to plastic deformation than remelted formulations (see Table II).

Highly cross-linked UHMWPE is the most widely used alternative to conventional UHMWPE in total hip and knee replacements.5,13 Thus far, these highly cross-linked materials have demonstrated better short-term in vivo wear performance compared with conventional UHMWPE.^16-18

One clinical trial found that the linear wear rate for a highly cross-linked UHMWPE (irradiated at 100 kGy and annealed) in implants was 65% less after two years than those with conventional UHMWPE.¹⁹ Another clinical trial with the same material found an 85% reduction in wear rate.²⁰ Other short-term outcomes have also been reported with remelted highly cross-linked UHMWPE materials.

Highly cross-linked UHMWPE was only introduced in 1998, so its long-term performance is still being closely studied. Evaluations over the next decade will address long-term clinical wear rate associated with this material, the occurrence of osteolysis, and the need for revision surgery.

Ranges of Use

As with hips, the polymer has been used in knee replacement since the late 1960s, when individual condyles of the femur and tibia were resurfaced. At this point, UHMWPE is the only widely used polymeric material for articulation with metallic components in fixed- and mobile-bearing total knee designs. These designs range from replacing individual condyles to total knee arthroplasty, i.e., the articulation between the femur and tibia and between the femur and patella. More than 90% of the knee joints that use UHMWPE survive for 10 years in the body.^21,22

Shoulder arthroplasty, the third most prevalent joint replacement operation, relies on UHMWPE for motion and load bearing. This involves either hemiarthroplasty (just the humeral surface of the glenohumeral joint) or total shoulder arthroplasty (both the humeral and glenoid articular surfaces). The success of total shoulder replacement also exceeds 90% after 10 years.^23,24

In the spine, surgical replacement of pathological intervertebral disks offers a relatively new alternative to spinal fusion and spinal diskectomy. Originating from the Charité Hospital in Berlin, this novel technology may eliminate the recurrent pain and disk pathology associated with the standard procedures.²⁵ Unlike fusion, artificial disks can restore the spine's natural motion and kinematics. Disk replacement at the Charité hospital involves use of a UHMWPE bearing surface as an inlay or insert between CoCr alloy end plates. The Charité total disk replacement received FDA approval for use in the United States in October 2004.²⁶ Many other designs for artificial disks are still under investigation in the United States and continue to be reviewed by FDA.

Conclusion

For more than 40 years, UHMWPE has been a popular bearing material because it has a low coefficient of friction, wears relatively little over the long term, and is relatively inert (biocompatible) in the body. While it has been placed in knee and hip joints since the 1960s, it has also been used in shoulder, elbow, wrist, ankle, and great toe replacements. And current techniques are also finding use for UHMWPE in the spine.

The standard bearing couple in hips and knees has involved one or more CoCr elements anchored in bone that articulate against a UHMWPE component. Variations on this have emerged based on alternative materials, such as MOM, COC, and highly cross-linked UHMWPE. Up to now, MOM and COC have had relatively limited adoption so far in the United States, but that may change as the technology matures.

Highly cross-linked UHMWPE has recently become the alternative bearing material of choice in hip replacements, because it is relatively inexpensive and has biocompatible properties. It is an economical material alternative and offers design flexibility, including smaller sizes. Highly cross-linked–grade formulations are also good candidates for the creation of longer-wearing and safer joints than conventional UHMWPE for joint replacement.

Acknowledgments

The authors thank Clare Rimnac, PhD, from Case Western Reserve University for her contributions to this article.

References

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Steven M. Kurtz is a principal engineer at Exponent Inc. (Menlo Park, CA). Harvey L. Stein and Gunther Redeker are the global products specialist and the global business segment manager, respectively, for Ticona (Summit, NJ).

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