ISO/TS 20721:2020

TECHNICAL ISO/TS
SPECIFICATION 20721
First edition
2020-09
Implants for surgery — General
guidelines and requirements for
assessment of absorbable metallic
implants
Reference number
©
ISO 2020
© ISO 2020
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ii © ISO 2020 – All rights reserved

Contents Page
Foreword .iv
Introduction .v
1 Scope . 1
2 Normative references . 1
3 Terms and definitions . 1
4 Absorbable metal considerations . 2
4.1 General . 2
4.2 Design considerations. 3
4.2.1 Composition . 3
4.2.2 Coatings . 4
4.2.3 Non-absorbable subcomponents . 4
4.2.4 Microstructure . 4
4.2.5 Implant design and functional performance . 5
4.3 The absorption process . 5
4.3.1 General outline . 5
4.3.2 Metallic conversion . 5
4.3.3 Subsequent degradation reactions . 6
4.3.4 Elemental impact on absorption . 6
4.3.5 Biological absorption . 6
4.3.6 Mechanical loss . 6
5 Metallurgical and manufacturing considerations . 8
5.1 General . 8
5.2 Composition . 8
5.3 Production process . . 8
5.3.1 General. 8
5.3.2 Raw material purity . 8
5.3.3 Metal melting practice . 8
5.3.4 Metal casting. 8
5.3.5 Metal thermo-mechanical processing . 8
5.3.6 Surface considerations . 9
5.3.7 Implant cleaning, sterilization, packaging, storage, and handling . 9
6 Evaluation of in vitro degradation characteristics . 9
6.1 General . 9
6.2 Additional considerations . 9
7 Biological evaluation .10
7.1 General .10
7.2 Biocompatibility of degradation products .10
7.3 In vitro biological evaluation .10
7.4 In vivo biological evaluation .10
7.4.1 Biocompatibility end point studies .10
7.4.2 Animal safety and implant performance studies .11
Annex A (informative) Nomenclature of absorb, degrade and related terms .12
Bibliography .13
Foreword
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This document was prepared by Technical Committee ISO/TC 150, Implants for surgery.
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iv © ISO 2020 – All rights reserved

Introduction
This document provides a general introduction to the field of absorbable metals. It outlines design
considerations which differ from non-absorbable metals and provides a detailed description of the
absorption process.
Metallurgical evaluation of absorbable metals is discussed, with reference to ASTM F3160 and
commentary on the impact of composition and production processes on final performance.
In vitro degradation corrosion testing is discussed, with reference to ASTM F3268 and commentary on
the importance of environmental conditions in the tests.
Both in vitro and in vivo biological assessment are discussed, with reference to several parts of the
1) 2)
ISO 10993 series, ISO/TS 37137-1 and the under-development ISO/TR 37137-2 .
NOTE ISO/TS 37137-1 applies to all absorbable materials, including metals and polymers. ISO/TR 37137-2 is
specific to absorbable magnesium-based materials.
The interrelation of the absorbable-specific reference documents can be viewed in Figure 1.
Figure 1 — Interrelation of standards specific to absorbable implants
The guide can be useful to both material suppliers and implant manufacturers.
Absorbable polymers used in conjunction with absorbable metals, either for performance modification
or drug delivery, are not addressed. However, it is expected that a polymer coating, absorbable or non-
absorbable, can influence absorption and performance of the underlying absorbable metal. ASTM F2902
addresses absorbable polymers.
Some existing standards address specific absorbable implants (e.g. ISO/TS 17137 addresses absorbable
cardiovascular implants) made of either polymer or metal.
1) Under preparation. Stage at the time of publication: ISO/TS/CD 37137-1:2020.
2) Under preparation. Stage at the time of publication: ISO/TS/CD 37137-2:2020.
TECHNICAL SPECIFICATION ISO/TS 20721:2020(E)
Implants for surgery — General guidelines and
requirements for assessment of absorbable metallic
implants
1 Scope
This document established the currently recognized approaches and special considerations needed
when evaluating the in vitro and in vivo performance of absorbable metals and implants fabricated, in
whole or in part, from them. This document describes how the evaluation of these metals can differ
from those utilized for permanent non-absorbable implantable implants (or subcomponents), in that
absorbable metal implants (or subcomponents) are — by design — intended to be absorbed in their
entirety by the host.
This document provides guidance regarding the materials considerations, in vitro degradation/
fatigue characterization, and biological evaluation of medical implants made of absorbable metals. The
provided content is intended to deliver added clarity to the evaluation of these materials and implants
to increase awareness of critical factors and reduce potential for generation of erroneous or misleading
test results.
While this document and the herein described referenced standards contain many suggested alterations
or modifications to currently practiced procedures or specifications, the provided content is intended
to complement, and not replace, current conventions regarding the assessment of implantable implants.
This document covers the evaluation of absorbable metal specific attributes in general and is not
intended to cover application or implant specific considerations. Thus, it is important to consult relevan
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