Standard Guide for Corrosion Fatigue Evaluation of Absorbable Metals

Importancia y uso:

5.1 Evaluation Strategy: 

5.1.1 Corrosion fatigue evaluation of absorbable metals may be useful for screening and validation of alloys, processes, and coatings, and also for verification and validation of fully designed implants. Corrosion fatigue testing may also be useful for informing an in vitro / in vivo correlation (IVIVC). Corrosion fatigue is just one aspect of absorbable metal implant performance. A broad discussion of design aspects specific to absorbable metals can be found in ISO/TS 20721. ISO/TS 17137 has guidance specific to absorbable cardiovascular implants.

5.1.2 Materials—The chosen material’s metallurgical properties should be characterized in accordance with Guide F3160. The material should be assessed regarding its general corrosion resistance and its influence on its predicted service life etc., as well as all potential effects that can limit service life under static or dynamic mechanical loading, such as localized corrosion (e.g. pitting corrosion, cracking) or susceptibility to hydrogen embrittlement. Literature data can be used to support the assessment. The effect of degradation products (including, e.g., hydrogen evolution or pH changes) on the corrosion fatigue behavior of the implant should be considered.

Note 1: Guide F3268 offers guidance on various aspects of in vitro corrosion testing of absorbable metals.

5.1.3 Mechanical Properties—Assessment of the specimen’s mechanical properties may assist in guiding possible corrosion fatigue test settings. Individual specimens used in such mechanical tests (e.g., tensile, torsion, compression) should not subsequently be included in the corrosion fatigue study.

5.1.4 Corrosion Properties—The test specimen may be benchmarked regarding degradation rate, service life prediction, durability, fatigue life, and assessment of susceptibility to special forms of severe damaging mechanisms (e.g., hydrogen embrittlement, stress corrosion cracking). Such benchmark data may help design the corrosion fatigue study and interpret the resulting data.

5.1.5 Designs—Implant designs may be assessed regarding their contribution to a preliminary failure of the implant, when subject to dynamic loading. Factors such as the occurrence of localized accelerated corrosion in regions of the implant with high mechanical stresses or strains may be considered.

5.1.6 Manufacturing Processes—Manufacturing processes should be adequately controlled to provide consistent implant performance. Manufacturing deviations can impact the service life of an implant by altering the elemental homogeneity and microstructure, causing surface defects, or contaminating the implant with substances in or on the surface that can accelerate localized corrosion through galvanic coupling.

5.1.7 Coatings and Surface Modifications—Coatings may be assessed regarding their influence on the service life of the implant. A coating can alter the degradation kinetics of the implant. In case of special forms of corrosion occurring underneath the coating, particles of the coating may be released, especially in areas with high mechanical loading. The potential combination of coating defects and localized in-depth corrosion may be considered.

5.1.8 Clinical Implantation Conditions—It may be considered that, depending on the clinical indication, an absorbable metallic implant may come into contact with other metallic components (e.g., a pre-existing implant, devices used in the acute phase of implantation etc.), potentially influencing the corrosion fatigue behavior.

5.2 Technical Challenges: 

5.2.1 In vitro corrosion fatigue evaluation methods for absorbable metals may have certain limitations imposed on the rigor of a simulation that can be achieved regarding the in vivo application of an implant.

5.2.1.1 Tissue interactions (e.g., ingrowth, shielding from media, multiple fractures without losing overall structural integrity, stationary degradation products/particles, biological interactions, etc.).

5.2.1.2 Conditions at the implantation site (e.g., calcifications, wall apposition, hard/soft tissue, stress shielding, etc.).

5.2.1.3 Uncertain/unknown boundary conditions (e.g., thermodynamics, flow conditions, peak loading, loading sequence, ion concentrations, etc.).

5.2.1.4 Synchronization of degradation rate and test frequency is desirable but may not be achievable for all implants (e.g., a degradable metallic implant with a polymer coating).

Note 2: See 9.2 for additional discussion of this issue.

5.2.1.5 Continuous monitoring of specimen conditions during corrosion fatigue testing is desirable but may not always be possible (e.g., crack initiation on the surface of an absorbable implant underneath a coating).

5.2.2 Therefore, in vitro fatigue test duration or service life is not necessarily similar to in vivo expectations. However, the results of this testing conducted in simulated physiological solutions can provide useful data to determine corrosion fatigue behavior as part of a risk assessment or to compare implants with different materials, designs, or manufacturing processes.

Subcomité:

F04.15

Volúmen:

13.02

Palabras clave:

absorbable; absorbable metallic implant materials; absorption; bioabsorbable; biodegradable; corrosion; corrosion fatigue; degradation;