High Cross-Linked Polyethylene Liner, how much do you know?

In younger and more active patients, the long-term survival of the metal-to-polyethylene (UHMWPE) friction interface is limited by the biological response to the wear of the UHMWPE material.

Highly cross-linked polyethylene liners (XPE liners) are a potential solution to reduce wear. Studies have shown that the wear resistance of XPE liners is superior to traditional UHMWPE liners, which can greatly extend the life of the prosthesis, reduce the risk of osteolysis caused by wear particles around the prosthesis, and thus reduce the incidence of aseptic loosening.

According to the data from the American Joint Registry (AJRR), XPE liners have become the mainstream prosthesis with low friction interface in international clinical practice.

In order to better understand the superior performance of XPE liners, we must understand the intrinsic relationship between XPE and UHMWPE materials first.

1. What is UHMWPE material?

UHMWPE (ultra-high molecular weight polyethylene) is an acronym for ultra-high molecular weight polyethylene. It is a long-chain type with a relative molecular weight of 2~9×106g/mol formed by the polymerization of ethylene monomer (C2H4). Polymer materials, finally formed by molding or plunger extrusion, or hot isostatic pressing, the products are mainly used in the field of joint replacement.

2. What is the performance of UHMWPE material?

Due to the existence of extremely long macromolecular chains, the crystallization ability of UHMWPE is limited. Therefore, the proportion of crystalline phase in UHMWPE material is only about 50%, which is significantly lower than that of standard high-density polyethylene (HDPE) material. The crystalline phase of the UHMWPE material is dispersed in a continuous amorphous phase. This special structure endows the UHMWPE material with the properties required for long-term connection of metal or ceramic joint prosthetic components: high wear resistance, low friction coefficient, high strength, toughness and fatigue resistance, and good biocompatibility.

3. Why consider cross-linking on the basis of UHMWPE material?

The traditional UHMWPE liner is the first-generation UHMWPE liner used in clinical orthopedics. The process that affects the performance of this liner has only two steps: compression molding (including forming nitrogen annealing treatment) and prosthesis sterilization process.

Initially, chemical sterilization was used for the UHMWPE liners, such as formaldehyde steam sterilization, but due to insufficient treatment of spore-forming microorganisms, gamma radiation of 25-40KGy was selected to improve the sterilization quality. At the same time, it was found that the use of this small dose of irradiation caused slight cross-linking inside the material, which improved the wear resistance of UHMWPE to a certain extent, which also inspired the subsequent development of highly cross-linked UHMWPE prostheses to a certain extent.

It can be said that the first generation of UHMWPE has been successfully used in joint replacement and achieved satisfactory clinical results. However, as patients become younger and their mobility needs to increase, long-term survival of first-generation UHMWPE-based prostheses has gradually encountered problems, especially failure due to wear and oxidative degradation. Therefore, follow-up research focuses on the cross-linking modification of UHMWPE materials to further increase the wear resistance and oxidation resistance of the material while maintaining the existing mechanical properties (although the wear resistance of the first-generation UHMWPE materials is already very high).

4. More attention should be paid to the optimal balance of wear resistance, creep resistance, oxidation resistance, and biomechanics

Based on the experience of the first-generation UHMWPE prosthesis, the research and development of the highly cross-linked (second-generation) UHMWPE prosthesis are more comprehensive, and the high wear resistance, creep resistance, oxidation resistance, and other properties of the prosthesis are achieved through a three-step process: radiation Cross-linking, heat treatment, sterilization.

The first step is gamma radiation or electron beam irradiation. This leads to cross-linking of the polymer chains (so-called cross-linking refers to the formation of lateral covalent bonds between the individual macromolecules). To form a three-dimensional network structure between molecular chains. The amount of crosslinking depends on the radiation dose during irradiation. With the increase of the irradiation dose, the cross-linking amount of the molecular chain of the material increases, thereby improving the molecular weight, wear resistance, and creep resistance of the material. When the irradiation dose increases to a certain value, the effect of irradiation on wear resistance is not significant, and the negative impact on the basic mechanical properties of the material (strength, ductility, and toughness deterioration) will increase, and the amount of residual free radicals in the material will increase, so the irradiation dose is not as high as possible.

JUST MEDICAL's cross-linking liner material is made of UHMWPE GUR 1020 imported from Germany. The weight average molecular weight is Mw≈4×106g/mol, and it has a high surface finish and excellent mechanical properties. At the same time, the irradiation dose was selected to be 75KGy, which perfectly balanced the relationship between cross-linking and wear resistance, creep resistance, and other mechanical properties.

The second step is heat treatment. After irradiation of the UHMWPE material, some of the cleaved molecular chains remain in the material in the form of highly reactive residual radicals. These reactive groups can react with air oxygen, causing oxidative degradation, which can accelerate the deterioration of almost all important properties of UHMWPE materials. Therefore, it is absolutely necessary to remove free residual radicals from the material. There are currently 2 heat treatment processes to remove residual radicals from irradiation: Remelting, and heating just above the melting temperature of the polymer (usually about 150°C). And annealing, heating just below the melting point of the polymer (usually 110- 130°C). Compared with pristine UHMWPE, remelting completely removed free radicals in the material, but the mechanical properties of the material were slightly lost compared to conventional UHMWPE. Although the annealing process reduces the loss in mechanical properties, it cannot completely eliminate the free radicals inside the cross-linked UHMWPE material, so the risk of long-term oxidative degradation of the material is not completely eliminated. Therefore, most second-generation UHMWPE producers prefer the remelting process.

JUST MEDICAL's high cross-linked polyethylene adopts a remelting heat treatment process to completely remove residual free radicals without the risk of oxidative degradation.

The third step is sterilization, this is necessary for implants. Recommended sterilization method for highly cross-linked UHMWPE: sterilization with ethylene oxide, or gas plasma sterilization. These two methods are characterized by the fact that they do not affect the mechanical properties of the material and that there are no residual free radicals.

JUST MEDICAL's high cross-linked polyethylene is sterilized by ethylene oxide, which does not affect the mechanical properties of the material, and does not lead to residual free radicals, ensuring the long-term survival rate of the prosthesis.