5.1 The biological response to materials in the form of small particles, as produced from implant wear, abrasion, or erosion, often is significantly different from that to the same materials in bulk form (that is, an implant component). Additionally, the morphology (for example, size and shape, surface characteristics), volume distribution, and species of these particles are major determinants of device-related biological responses; therefore, this practice provides standardized nomenclature for describing particles. Such a unified nomenclature will be of value in interpretation of biological tests of responses to particles, in that it will facilitate separation of biological responses per different particle characteristics such as size, shape, and volume.

5.2 Particles released due to wear from implants in vivo may result in an adverse biological response which will affect the long-term survival of the device. Characterization of such particles will provide valuable information regarding the safety and effectiveness of device designs or methods of processing components and the mechanisms of wear.

5.3 The morphology of particles produced in laboratory tests of wear and abrasion often is affected by the test conditions, such as the magnitude and rate of load application, device configuration, and test environment. Comparison of the morphology, size, and quantity of particles produced in vitro with those produced in vivo will provide valuable information regarding the degree to which the method simulates the in vivo condition being modeled.

5.4 Particles harvested from particle-release studies (for example, cell culture experiments, third body wear simulation) that are to be used for testing should be representative of the entire spectrum of possible particles produced from clinical use of the device/material under review (for example, due to wear, abrasion, or erosion). Therefore, efforts should be made to ensure that the particles for testing were produced from in vivo / in vitro studies that mimicked the clinical use conditions as much as possible. When there is uncertainty regarding the characteristics of particles produced from in vitro or bench testing, particles from clinical studies (for example, retrievals) can be used to enhance the clinical representativeness of testing and its predictive power for characterizing potential biological responses.

1.1 This practice covers a series of recommendations, generally applicable to all medical devices, for characterization of the morphology, shape, size, and size distribution of particles. The methods utilized include sieves, optical, scanning electron microscopy (SEM), transmission electron microscopy (TEM), and electrooptical.

1.2 While characterizing the quantity or number of particles shed from medical devices is important, this is not covered within the scope of the current document. AAMI TIR 42 and USP <788> provide guidelines for determination of particle quantities in various size ranges.

1.3 These methods are appropriate for particles produced by a number of different methods. These methods can include simulated use approaches such as in vitro wear test machines (Test Method F732), total joint simulation systems (Guides F1714 and F1715), abrasion testing, and vascular durability testing (Guide F2942). Other methods for producing particles such as shatter boxes or pulverizers, as well as commercially available particles, and particles harvested from tissues in animal or clinical studies can be used.

1.4 Except for chemical composition, this standard does not address sample preparation procedures and/or test systems that can be affected by chemical properties (for example, solubility, miscibility). While this standard does not provide detailed recommendations regarding assessment of chemical properties of particles, these should be considered.

1.5 The particles may be metallic, polymeric, or ceramic and are released from medical device materials either acutely or chronically (for example, due to wear).

1.6 The digestion procedures to be used and issues of sterilization of retrieved particles are not the subject of this practice.

1.7 A classification scheme for description of particle morphology is included in Appendix X3.

1.8 When nanoparticles (that is, having at least one dimension less than 100 nm) are known to be present or are expected, other characterization methods may be needed. For information regarding nanoparticle characterization, refer to standards that address nanoparticles (for example, ISO 21363, ISO/TR 10993-22, ISO/TR 16196).

1.9 This standard does not address ions released from medical devices.

1.10 The values stated in SI units, including units officially accepted for use with SI, are to be regarded as standard. No other systems of measurement are included in this standard.

1.11 As a precautionary safety measure for handling test samples during particle characterization analyses, removed particles from implant tissues should be sterilized or minimally disinfected by an appropriate means that does not adversely affect these particles.

1.12 This standard does not purport to address all of the safety concerns, if any, associated with its use. It is the responsibility of the user of this standard to establish appropriate safety, health, and environmental practices and determine the applicability of regulatory limitations prior to use.

1.13 This international standard was developed in accordance with internationally recognized principles on standardization established in the Decision on Principles for the Development of International Standards, Guides and Recommendations issued by the World Trade Organization Technical Barriers to Trade (TBT) Committee.