UHMWPE for Total Joint Arthroplasty: Past, present and future

Total joint arthroplasty involves the replacement of native articulating joints, such as the hip, knee, shoulder and ankle, with artificial components. In the U.S., these artificial joints are normally composed of plastic and metal. According to THE ORTHOPAEDIC INDUSTRY ANNUAL REPORT®, 2.9 million joint replacements were performed worldwide in 2010, with 1.4 million hip replacements, 1.1 million knee replacements and 100,000 shoulder replacements.1 This report estimates that by 2015, with the aging and more demanding demographics, there will be over 600,000 hip replacements and 1.4 million knee replacements performed in the U.S. alone.

The most commonly-used bearing surface is ultra-high molecular weight polyethylene (UHMWPE), usually articulating against a metal counterface. Since its first introduction in 1962 by Dr. John Charnley, researchers have been striving to improve the properties of UHMWPE to increase its efficacy in total joint replacement surgeries. This paper describes these efforts from the early off-the-shelf engineering material to the highly wear resistant and oxidatively-stable formulations used today.

History of UHMWPE

Starting from the position that friction was the most important parameter for a bearing surface, Dr. Charnley initially experimented with polytetrafluoroethylene, including glass-filled PTFE, which resulted in severe wear in most of his total hip replacement patients.2 He determined that wear properties, rather than friction, were key to a successful long-term implant. An opportune visit from a sales representative for gears fabricated from UHMWPE made him aware of this material and resulted in the first UHMWPE hip component being implanted in 1962.

For almost three decades, little change was made to Charnley’s original concept of UHMWPE articulating against stainless steel, although a few attempts were made to improve the wear resistance during this time. During the 1970s, scientists at Zimmer incorporated pyrolitic carbon fibers into the UHMWPE in an attempt to improve wear and reduce creep. Wear studies and impact testing demonstrated that the carbon fibers caused both roughening of the metal counterface, leading to greater wear and a reduction of the impact strength of the components.3,4 In another attempt in the 1990s, DePuy developed and marketed a hot, isostatically-consolidated UHMWPE that exhibited an extended chain crystal structure, as opposed to the folded-chain crystal structure normally found in UHMWPE. This material, named Hylamer™, exhibited a higher crystallinity and modulus compared to conventional UHMWPE. However, although initial wear tests showed equal wear to conventional UHMWPE and mechanical tests indicated improved properties, Hylamer was discovered to be susceptible to oxidation, which resulted in rapid degradation of these properties after shelf-storage or implantation.5

With increasing clinical implantation time, a better understanding of the modes of failure of the material was gathered. In particular, implant loosening was occasionally reported in patients having previously stable implants. The initial hypothesis placed the blame on the polymethyl methacrylate-based bone cement that was used to fixate a large number of implants, resulting in the phrase “bone cement disease.” When a new generation of cementless implants exhibited the same loosening behavior, scientists re-examined the evidence and concluded that particulate debris generated from the UHMWPE during articulation resulted in a foreign body immune response with subsequent tissue resorption, ultimately resulting in implant loosening in some cases. The industry recognized that improvements needed to be made to the wear resistance of UHMWPE to minimize particle formation and prolong in vivo duration and patient satisfaction.

First Generation Highly Crosslinked UHMWPE (1998-2007)

When polyethylene is exposed to high energy irradiation, hydrogen abstraction leaves behind unpaired electrons on the carbon atoms on the main chain. These unpaired electrons, or free radicals, can form covalent bonds, or crosslinks, if they combine with free radicals on adjacent polymer chains. It was recognized as early as the 1970s that crosslinking UHMWPE had a beneficial effect on wear behavior,6 when researchers used radiation doses up to 1000 kGy to crosslink the UHMWPE. However, due to the small patient population and poor clinical follow-up, this initial study was not widely disseminated, and the orthopaedic industry would wait another 20 to 30 years before rediscovering the benefits of highly crosslinked UHMWPE.


In the late 1990s, several orthopaedic manufacturers introduced highly crosslinked UHMWPE. Using either electron beam radiation (e-beam) or gamma ray sources, manufacturers irradiated UHMWPE with doses from 50 to 100 kGy. (See Table 1.) However, it quickly became apparent that residual free radicals that do not participate in crosslinking can react with oxygen, creating carbonyls that weaken the main polymer chains, leading to scissioning and embrittlement. Recognizing this, manufacturers sought methods for reducing the residual free-radicals post-crosslinking. These methods involved heating the irradiated UHMWPE to temperatures just below the melting point (annealing), or above the melting point (re-melt). The re-melting process is more effective at quenching residual free radicals, but suffers from a decrease in some mechanical properties because the freshly crosslinked material cannot recrystallize as effectively and the resulting drop in crystallinity impacts some mechanical properties. In contrast, the annealing process leaves more residual free radicals, making this material more susceptible to oxidation, but has improved intrinsic mechanical properties.

Table 1: First generation highly crosslinked UHMWPE (hip)†

Material

Marathon™

Prolong™

Crossfire™

Durasul™

Longevity™

XLPE™

Company

DePuy

Zimmer

Stryker

Zimmer

Zimmer

Smith&Nephew

Radiation Dose [kGy]

50

65

75

95

100

100

Post-process

Re-melt

Re-melt

Anneal

Re-melt

Re-melt

Re-melt

 †Since the release of first generation materials, several smaller orthopedic companies have released products with similar formulations.

 

The wear properties of the first generation of crosslinked UHMWPE were a vast improvement over conventional (uncrosslinked) UHMWPE, reducing the wear rate by several orders of magnitude. However, because of the manner aforementioned post-irradiation processing conditions, these materials exhibited reduced mechanical properties (such as toughness) relative to their conventional counterparts. As a result, there was a strong push to improve the mechanical properties of the crosslinked UHMWPE while maintaining the excellent wear and oxidation resistance in order to reduce issues with fatigue crack propagation. This desirable combination of properties would allow thinner liners, and have greater use in the demanding loads found in the knee. The second generation highly crosslinked UHMWPE were developed to address this need.

Second Generation Highly Crosslinked UHMWPE (2005-2012)

In 2005, Biomet introduced Arcom Xl™, a 50 kGy irradiated UHMWPE that is isostatically extruded to mechanically-anneal the free radicals. The same year, Stryker replaced Crossfire™ with X3™, a UHMWPE with repeated crosslinking and annealing steps, achieving a total dose of 90 kGy. Biomet’s E1™ was introduced in 2007, the first marketed medical grade UHMWPE to incorporate an antioxidant, Vitamin E. In Biomet’s E1 process, the UHMWPE is first irradiated to approximately 100 kGy, thus achieving similar crosslink densities (and properties) to the first generation materials. It is then soaked in Vitamin E to diffuse the antioxidant throughout the bulk of the crosslinked UHMWPE, and then terminally sterilized following machining. The Vitamin E stabilizes residual free radicals, eliminating the need for thermal treatment. Irradiated blends of Vitamin E in UHMWPE are also being marketed by some other companies (See for example Stelkast’s EXp™, cleared by FDA in 2011), and DePuy received FDA clearance for knee systems made from UHMWPE containing a hindered phenol antioxidant (AOX™). Zimmer is expected to release a Vitamin E-stabilized UHMWPE in 2012.


Using a different strategy, researchers at Cambridge Polymer Group and the Massachusetts General Hospital have developed two second generation UHMWPE (E-CIMA and CIMA). CIMA is a cold-irradiated (100 kGy), mechanically-annealed UHMWPE that has mechanical properties close to conventional UHMWPE, is oxidatively stable and exhibits the low wear of highly crosslinked UHMWPE. E-CIMA incorporates Vitamin E, which is blended in prior to irradiation for added long-term oxidation stabilization, prior to mechanical annealing. As the presence of Vitamin E will partially inhibit crosslinking, a higher radiation dose (120-150 kGy) is normally used for E-CIMA to impart the same degree of crosslinking as the 100 kGy CIMA. E-CIMA is currently being sold in Europe by Corin  and marketed as ECiMa™. (See Exhibit 1.) A comparison of properties of two of the second generation UHMWPE to the first generation materials, shown in Table 2, demonstrates the improvement in mechanical properties and oxidation resistance, while maintaining the good wear behavior of the first generation.

Exhibit 1: Corin’s ECiMA™
Corin's ECiMA

 

Table 2: Properties of second generation highly crosslinked UHMWPE relative to first generation.

 

First Generation

Second Generation

Property

100 kGy UHMWPE, annealed(7)

100 kGy, remelted(7)

E-CIMA(7)

CIMA (8)

Ultimate tensile strength [MPa]

56

43

50

49

Yield Strength [MPa]

22

19

23

23

Elongation to break [%]

315

303

256

281

Wear rate [mg/million cycle]

1.4

1.7

1.2

0.1

Oxidation index after accelerated aging

0.2

0.1

<0.05

<0.05


 

The Future of UHMWPE

Researchers are already working on third generation technologies. Gradients in crosslink densities will allow high crosslink densities on the wear surfaces, and high mechanical properties in the bulk of the material, which should allow thinner constructs and more flexibility with locking mechanism design. (See Exhibit 2.) Alternative antioxidants are also being considered, along with creative use of spatial distribution of antioxidants.

A common question reasonably asked by surgeons is, “Why should we believe that the new technologies will not have long-term issues like Hylamer and Poly II?” The simple answer is that the industry’s bench-top and in vitro testing methodologies have improved markedly as we have increased our understanding of the failure mechanisms in total joint components composed of UHMWPE. Wear simulators, accelerated aging protocols, mechanical tests and chemical analyses that were not available at the time of the early UHMWPE development are now in routine use and are able to indicate issues that would impact clinical performance. The clinical success rates of hip and knee replacements are amongst the highest for major surgical interventions, and the changes in the formulations of UHMWPE will only serve to increase these success rates, making this surgical option one of the most reliable and life changing interventions currently available.

 

Exhibit 2: Gradient Vitamin E UHMWPE developed at Cambridge Polymer Group and Massachusetts General Hospital.
Gradient Vitamin E

 

 

 

 

 

 

 

REFERENCES

  1. ORTHOWORLD Inc., THE ORTHOPAEDIC INDUSTRY ANNUAL REPORT, 2010-2011. (2011).
  2. Kurtz, S., 2009, The UHMWPE Handbook: Ultra-High Molecular Weight Polyethylene in Total Joint Replacement (Elsevier, San Diego).
  3. Connelly, G. M., C. M. Rimnac, T. M. Wright, R. W. Herzberg, and J. A. Manson, "Fatigue crack propagation behavior of ultrahigh molecular weight polyethylene" J Orthop. Res. 2, 119-125, (1984).
  4. Peterson, C. D., B. M. Hillberry, and D. A. Heck, "Component wear of total knee prostheses using Ti-6AL-4V, titanium nitride coated Ti-6Al-4V, and cobalt-chromium-molybdenum femoral components" J Biomed Mater Res. 22, 887-903, (1988).
  5. Kurtz, S. M., O. Muratoglu, M. Evans, and A. A. Edidin, "Advances in the processing, sterilization, and crosslinking of ultra-high molecular weight polyethylene for total hip arthroplasty" Biomaterials 20, (1999).
  6. Oonishi, H., M. Kuno, Y. Ikada, A. Fujisawa, and S. Masuda,"Super Low Wear Cross-Linked UHMWPE by Heavy-Dose Gamma Radiation," WPOA 2nd Congress of Hip Section, 1996).
  7. Wannomae, K. K., B. R. Micheli, A. J. Lozynsky, and O. K. Muratoglu,"A new method of stabilizing irradiated UHMWPE using Vitamin E and mechanical annealing," 56th Annual Meeting of the Orthopedic Research Society, 2290, (2010).
  8. Malhi, A. S., K. K. Wannomae, W. H. Harris, and O. K. Muratoglu,"Comparison of resins in a second generation highly crosslinked UHMWPE for high stress applications," 51st Annual Meeting of the Orthopaedic Research Society, 1674, (2005).

Stephen Spiegelberg is President and co-founder of Cambridge Polymer Group, Inc. (CPG). He received his B.S. in Chemical Engineering from the University of Wisconsin-Madison and his Ph.D. in Chemical Engineering from the Massachusetts Institute of Technology. He was a post-doctoral fellow at Harvard University. He holds patents in analytical instrumentation and materials for biomedical application. At CPG, he directs a team of scientists performing contract research and testing on polymeric materials for the biomedical community and other fields. He chairs task groups in ASTM on the cleanliness of biomedical devices, medical device shipping and characterization methods for thermoplastics. Please send inquiries to This email address is being protected from spambots. You need JavaScript enabled to view it.

Cambridge Polymer Group, Inc.
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