ANSI - American National Standards Institute
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Standards Protect Quality and Safety of Joint Replacements

New York, Dec 09, 2003

Since the first hip replacement surgery was performed in 1960, advances in joint replacement surgical technology have greatly increased the effectiveness of this procedure. Today, more than 168,000 total hip replacements are performed each year in the United States. Standards from the public and private sector play a key role in keeping these revolutionary surgical practices as safe, effective and resilient as possible.

From supporting the manufacturers of implants to guiding tests of materials and performance, standards affect joint replacement at many levels. The International Organization for Standardization (ISO) is one source of guidelines concerning joint replacement and related technology and equipment. ISO 21535:2002, Non-active surgical implants -- Joint replacement implants -- Specific requirements for hip-joint replacement implants, gives safety requirements for intended performance, design attributes, materials, design evaluation, manufacture, sterilization, packaging and information supplied by the manufacturer, and methods of test. ISO 21534:2002, Non-active surgical implants -- Joint replacement implants -- Particular requirements, specifies particular requirements for total and partial joint replacement implants, artificial ligaments and bone cement.

The F04 committee of ASTM International, an ANSI member and accredited standards developer, currently has jurisdiction of over 165 standards related to materials, orthopedic devices, testing, tissue engineering, and medical/surgical instruments. One of these documents, ASTM F2033-00a, Standard Specification for Total Hip Joint Prosthesis and Hip Endoprosthesis Bearing Surfaces Made of Metallic, Ceramic, and Polymeric Materials, gives a standard practice regarding the design of total hip joint bearing surfaces. The specification covers the sphericity, surface finish requirements, and dimensional tolerances for the spherical articulating metallic or ceramic femoral heads of total hip joint prostheses.

With an increase in younger patients undergoing joint replacement surgeries, it has become more important than ever to ensure the longevity of the materials used in these procedures. A new reference material from ANSI member the National Institute of Standards and Technology (NIST) utilizes an ASTM standard test method to determine what methods of irradiating the plastic parts in joint replacements during manufacturing will best increase their wear resistance.

Ionizing radiation can create new chemical bonds between adjacent molecular chains in a special form of polyethylene used to make the socket for the metal ball and shaft in an artificial hip. This method of “crosslinking” creates a structure that resists sliding forces and wear. According to NIST, manufacturers and researchers need to control radiation conditions to achieve the intended wear resistance in this material; too much radiation causes brittleness, and too little can result in poor wear resistance.

NIST Reference Material 8457 consists of 10 small, identical cubes of polyethylene, intended for use as control samples in the ASTM standard test method F2214-02 Standard Test Method for In Situ Determination of Network Parameters of Crosslinked Ultra High Molecular Weight Polyethylene (UHMWPE). The test method involves immersing cubes in an organic liquid and measuring how much the material swells. The reference material will help researchers and implant manufacturers control or optimize a variety of processing parameters, such as the type (gamma radiation or electron beams), timing, and doses of radiation used for crosslinking.

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