Standard Guide for in vitro Degradation Testing of Absorbable Metals
5.1 This standard provides an itemization of potential in vitro test methods to evaluate the degradation of absorbable metals. The provided approach defers to the user of this standard to pick most appropriate method(s) based on the specific requirements of the intended application. However, a minimum of at least two different corrosion evaluation methods is considered necessary for basic profiling of the material or device, with additional methods potentially needed for an adequate characterization. However, in some instances there may be only one method that correlates to in vivo degradation results.
5.2 It is recognized that not all test methods will be meaningful for every situation. In addition, some methods carry different potential than others regarding their relative approximation to the in vivo conditions within which actual use is to occur. As a result, some discussion and ranking of the relevance of the described methods is provided by this guidance.
5.3 It should be noted that degradation of absorbable metals is not linear. Thus, precautions should be taken that evaluations of the degradation profile of a metal or metal device are appropriately adapted to reflect the varying stages and rates of degradation. Relevant factors can include the amount or percentage (%) of tissue coverage of the implanted device and the metabolic rate of surrounding tissue, which is not necessarily accompanied by a high perfusion rate.
5.4 It is recognized that in vivo environments will impart specialized considerations that can directly affect the corrosion rate, even when compared with other in vivo locations. Thus, a basic understanding of the biochemistry and physiology of the specific targeted implant location (e.g. hard tissue; soft tissue; high, low or zero perfusion areas/tissue; high, low or zero loading environments) is needed to optimize in vitro and in vivo evaluations.
5.5 Within the evaluation of absorbable metals, rate uniformity is considered to be the principle concern and design goal. The recognized primary value for the herein described in vitro testing under static (i.e. not dynamic) conditions is to monitor and screen materials and/or devices for their corrosion consistency. Such an evaluation may provide a practical understanding of the uniformity of the device prior to any subsequent in vivo testing - where device consistency is considered to be critical for optimizing the quality of the obtained observations.
5.6 Once a suitable level of device corrosion consistency has been established (either directly or historically), static and/or dynamic fatigue testing can then be undertaken, if needed, to further enhance the understanding of the corrosion process within the context of the device’s overall design and its intended application/use.
5.7 Depending on the intended application, appropriate levels of implant loading may range from minimal to severe. Thus, this standard does NOT directly address the appropriate level of loading of absorbable metallic devices, guidance for which may be found in documents specific to the intended implant application and the design requirements for the product.
5.8 This standard does NOT directly address dynamic fatigue testing of absorbable metallic devices.
1.1 The purpose of this standard is to outline appropriate experimental approaches for conducting an initial evaluation of the in vitro degradation properties of a device or test sample fabricated from an absorbable metal or alloy.
1.2 The described experimental approaches are intended to control the corrosion test environment through standardization of conditions and utilization of physiologically relevant electrolyte fluids. Evaluation of a standardized degradation control material is also incorporated to facilitate comparison and normalization of results across laboratories.
1.3 The obtained test results may be used to screen materials and/or constructs prior to evaluation of a more refined fabricated device. The described tests may also be utilized to define a device’s performance threshold prior to more extensive in vitro performance evaluations (e.g. fatigue testing) or in vivo evaluations.
1.4 This standard is considered to be applicable to all absorbable metals, including magnesium, iron, and zinc-based metals and alloys.
1.5 The described tests are not considered to be representative of in vivo conditions and could potentially provide a more rapid or slower degradation rate than an absorbable metal’s actual in vivo corrosion rate. The herein described test methods are to be used for material comparison purposes only and are not to act as either a predictor or substitute for evaluation of the in vivo degradation properties of a device.
1.6 This standard only provides guidance regarding the in vitro degradation of absorbable metals and does not address any aspect regarding either in vivo or biocompatibility evaluations.
1.7 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.
1.8 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.
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