|Crosslinked Polyethylene Standard Aids Effort to Improve Surgical Implants |
An ASTM standard is now playing an integral role in increasing the long-term durability of orthopedic surgical implants. The standard, F 2214, Test Method for In Situ Determination of Network Parameters of Crosslinked Ultra High Molecular Weight Polyethylene (UHMWPE), was developed by Committee F04 on Medical and Surgical Materials and Devices. NIST Reference Material 8457, a set of 10 small, identical cubes of polyethylene, is used to help researchers and manufacturers control a variety of parameters while they are using the test method, including type of ionizing radiation used for crosslinking (gamma radiation or electron beam), timing and doses of radiation.
Test Method F 2214, which falls uner the jurisdiction of Subcommittee F04.11 on Polymeric Materials, describes how crosslink density can be determined by measuring the swelling ratio of samples immersed in hot ortho-xylene. This is an indirect method of measuring the degree of crosslinking. Samples that expand the most have the least amount of crosslinking, conversely samples with high degree of crosslinking will swell very little. This is important for manufacturers of the polyethylene used in making surgical implants because the amount of radiation used in crosslinking needs to be precise to achieve the intended wear resistance. Too much radiation causes brittleness, while too little can result in poor wear resistance.
According to Hany Demian, Center for Devices and Radiological Health ASTM Liaison for polymeric materials to F04.11 and senior reviewer in the restorative devices branch, Food and Drug Administration, the development of Test Method F 2214 and NIST Reference Material 8457 can be traced back to developments in orthopedic surgery over the last 20 years. While this type of surgery has proven to be very successful, in the late 1980s and early 1990s cases of osteolysis (a condition that destroys the bone) were increasingly being documented in patients who had total hip replacement surgery. At first, doctors believed that the culprit was bone cement particles, however, it was soon discovered that submicron particles of the polyethylene were being produced when the polyethylene cup was used as an articulating surface. The polyethylene was wearing away and migrating to distant points in the bone. These tiny particles would then collect and the body would react by the bone resorbing away, potentially causing loosening of the implant over time.
While it was well-known that polyethylene used for surgical implants could be strengthened by crosslinking, there was not a standard for measuring crosslinked density. People were using different protocols to measure crosslinked density and we could not compare one value with another, Demian says, Finally, an activity was initiated by industry and FDA in a collaborative effort to develop a method so that everybody was measuring crosslink density and swell ratios in the same way. NIST and most manufacturers of polyethylene have been involved in the creation of F 2214, as well as companies that buy crosslinked polyethylene and machine it into implant parts.
For further technical information, contact Hany Demian, FDA, Rockville, Md. (phone: 301/827-8858). Committee F04 meets April 20-23 during the April Committee Week in Salt Lake City, Utah. For membership or meeting details, contact Dan Schultz, ASTM International (phone: 610/832-9716). //
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