What is UHMWPE?
Ultra-high molecular weight polyethylene. A polymer consisting of long chains of carbon and hydrogen atoms.
What is annealing?
Heating the polyethylene below the melt point. The temperature for annealing UHWMPE is typically 125-135°C.
What is melt-annealing or re-melting?
Heating the polyethylene above the melt point. The temperature for melt-annealing UHWMPE is typically around 150°C.
What is virgin polyethylene?
Untreated UHMWPE is referred to as ”virgin”. It results from forming polyethylene resin into a solid through compression molding or extrusion. Virgin polyethylene has not received any ionizing radiation such as the gamma and e-beam irradiation used in sterilization and crosslinking processes.
Do any manufacturers sell virgin polyethylene?
Yes. DePuy’s Enduron™ and Smith and Nephew’s standard polyethylene are in virgin form. That is, the virgin material is machined, packaged, and sterilized using EtO or gas plasma. They have NO crosslinking.
What is crosslinking?
UHMWPE consists of long molecular chains of carbon and hydrogen. When it is energized by the application of radiation, some of the molecular bonds are broken, creating open bonding sites. These connect to one another, creating bridges between chains, which are called crosslinks.
What is a free radical?
Open or “free” molecular bonding sites created during irradiation when the material is energized and hydrogen atoms released.
What is conventional polyethylene, and is it crosslinked?
Conventional polyethylene is packaged then sterilized using 2.5-4 Mrad of gamma radiation. During this process, both crosslinks and free radicals are formed.
Does the sterilization dose in conventional polyethylene improve its wear over virgin polyethylene?
Yes, the amount of crosslinking achieved during sterilization improves the wear when compared to virgin polyethylene.
What is the difference between crosslinked, moderately crosslinked, and highly crosslinked polyethylene?
Crosslinked polyethylene is a material that has received some amount of ionizing radiation. Moderately crosslinked materials are irradiated to about 5-7 Mrad. Highly crosslinked materials are irradiated about 8 Mrad.
Is there an issue with free radicals in conventional polyethylene?
Yes. Free radicals remain in the material after gamma irradiation.
How is X-alt™ highly crosslinked polyethylene produced?
- GUR 1020 polyethylene powder resin consolidated into sheets by compression molding.
- Long, circular bars cut from sheets.
- Bars submitted to 7.5 Mrad of gamma radiation at room temperature.
- Bars heated to about 150ºC (above melt).
- Bars machined into final liner geometry.
- Liners packaged and gas-plasma sterilized.
Is there clinical data to support current melt-annealed highly crosslinked polyethylenes?
Yes. A survey of the clinical performance of first generation crosslinked polyethylene in the December 2007 JBJS summarized 12 clinical reports. The authors concluded that “the percent reductions in the wear rate…have been comparable with what was predicted from preclinical hip-simulator testing of the highly cross-linked polyethylenes”.
What are the amorphous regions of polyethylene?
Areas where the chains appear tangled and disorderly. The amorphous regions are where most of the crosslinking occurs, and thus these regions are related closely to wear properties.
What are the crystalline regions of polyethylene?
Areas where polyethylene chains fold into an orderly, laminar structure.
Jacobs CA, Christensen CP, Greenwald AS, McKellop H. Clinical Performance of Highly Cross-Linked Polyethylenes in Total Hip Arthroplasty, JBJS, 89: 2779.
Hopper RH Jr, Young AM, Orishimo KF, Engh CA Jr. Effect of terminal sterilization with gas plasma or gamma radiation on wear of polyethylene liners, JBJS. 2003 Mar;85-A(3):464-8.