5.1 This test method is intended to help assess the degradation rates (that is, the mass loss rate) and changes in material or structural properties, or both, of HDP materials used in surgical implants. Polymers that are known to degrade primarily by hydrolysis include but are not limited to homopolymers and copolymers of l-lactide, d-lactide, d,l-lactide glycolide, caprolactone, and p-dioxanone.7

5.2 This test method may not be appropriate for all types of implant applications or for all known absorbable polymers. The user is cautioned to consider the appropriateness of the test method in view of the materials being tested and their potential application (see X1.1.1).

5.3 Since it is well known that mechanical loading can increase the degradation rate of absorbable polymers, the presence and extent of such loading needs to be considered when comparing in vitro behavior with that expected or observed in vivo.

5.3.1 Mechanically Unloaded Hydrolytic Evaluation—Conditioning of a hydrolysable device under mechanically unchallenged hydrolytic conditions at 37 °C in buffered physiological soaking solution is a common means to obtain a first approximation of the degradation profile of an absorbable material or device. It does not necessarily represent actual in vivo service conditions, which can include mechanical loading in a variety of forms (for example, static tensile, cyclic tensile, shear, bending, and so forth). If the performance of a device under its indicated use includes loading, hydrolytic aging alone is NOT sufficient to fully characterize the device.

5.3.2 Mechanically Loaded Hydrolytic Evaluation—The objective of loading is to approximate the expected device service conditions so as to better understand potential physicochemical changes that may occur. Such testing can be considered as necessary if loading can be reasonably expected under in vivo service conditions. When feasible, test specimens should be loaded in a manner that simulates in vivo conditions, both in magnitude and type of loading. Clinically relevant cyclic load tests may include testing to failure or for a specified number of cycles followed by testing to evaluate physicochemical properties.

5.3.2.1 Static Loading—It is notable that for some polymeric materials it has been shown that a constant load results in the same failure mechanism (for example, creep) and is the worst case when compared to a cyclic load (where the maximum amplitude of the cyclic load is equal to the constant load). Thus, in specific cases it may be acceptable to simplify the test by using a constant load even when the anticipated in vivo loading is cyclic. It is encumbent upon the user of this test method to demonstrate through experiment or specific reference that this simplification is applicable to the polymer under investigation and does not alter the failure mode of the test specimen. If such evidence is not available, it is necessary to recognize that static loading and cyclic loading are measuring different material properties and are not comparable. Using one to replace the other could lead to misinterpretation of the results.

Note 3: Caution must be taken to ensure that fixturing does not introduce artifactual performance or degradation issues, or both. An example is the use of rigid foam block, which restricts swelling and expansion and can elevate pullout strength test results from sample compression within the block. Additionally, restricted perfusion due to the closed cell nature of the foam can result in concentration of acidic byproducts that result in accelerated degradation when compared to a normally perfused and buffered in vivo condition.

Note 4: When performing degradation testing under load, it may be necessary to consider and monitor polymer creep during testing, which may be significant.

5.4 Absorbable devices subjected to flow conditions (for example, vascular stents, particularly those with a drug eluting component) may degrade more rapidly than the same device maintained under static degradation test conditions. When it is feasible to estimate the flow conditions that an implant will be subjected to in vivo and replicate them in vitro the degradation study should be conducted under flow conditions. However, details regarding appropriate flow modeling are beyond the scope of this test method.

5.5 Sterilization of HDP materials should be expected to cause changes in molar mass or structure, or both, of the polymers. This can affect the initial mechanical and physical properties of a material or device, as well as its subsequent rate of degradation. Therefore, if a test is intended to be representative of actual performance in vivo, specimens shall be packaged and sterilized in a manner consistent with that of the final device. Non-sterilized specimens may be included for comparative purposes.

1.1 This test method covers in vitro degradation of hydrolytically degradable polymers (HDP) intended for use in surgical implants. It provides a detailed methodology for conditioning samples and suggests quantitative techniques for evaluating changes in properties (for example, mass, molar mass, and mechanical strength) over time.

1.1.1 For many types of HDPs, a correlation has been established between in vitro degradation per this practice and in vivo degradation for unloaded specimens (see X1.1.1).

1.2 The requirements of this test method apply to HDPs in various forms:

1.2.1 Virgin polymer resins, or

1.2.2 Any form fabricated from virgin polymer such as a semi-finished component of a finished product, a finished product, which may include packaged and sterilized implants, or a specially fabricated test specimen.

1.2.2.1 Use of this test method for conditioning virgin resins, semi-finished forms, or special test specimens may provide information that is useful for material and/or product development. However, those results are not sufficient for predicting the degradation behavior of the final, sterilized implant. Subsections 5.5, 8.2, and X1.2 provide additional guidance.

1.3 This standard provides guidance for mechanical loading or fluid flow, or both, when relevant to the device being evaluated. The specifics of loading type, magnitude, and frequency for a given application are beyond the scope of this test method.

1.4 This standard is not applicable to conditioning of polymers that degrade primarily through mechanisms other than hydrolysis (for example, enzymatic or oxidative degradation). See X1.1.1 for more information.

1.5 The values stated in SI units are to be regarded as standard. No other units of measurement are included in this standard.

1.6 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.

1.7 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.