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UHMWPE for Total Joint Arthroplasty: Past, present and future

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.

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