International Standards of testing orthopaedic implants

A consensus standard of testing serves as a guide, outlining requirements and methodologies to ensure consistency in test results across different laboratories adhering to the standard. This consistency is crucial for obtaining meaningful and practically achievable results. (Source: http://www.astm.org/ABOUT/full_overview.html)   

Before we delve into the specifics, it is worth noting that, to date (April 1, 2025), there are no established standards for testing locking implants. Cortical screws are tested for their torsional behaviour during insertion and extraction, and for their pull-out strength. Plates are tested in single-cycle bending.

Key Organizations:

Three major independent organizations that develop consensus standards are:

1.      ASTM International (formerly the American Society for Testing and Materials)   

2.      ISO (International Organization for Standardization)   

3.      AAMI (Association for the Advancement of Medical Instrumentation)   

These organizations develop and manage voluntary consensus technical standards, along with other documents, across diverse fields encompassing materials, products, systems, and services.   

ASTM Standards for Orthopaedic Implants:

While ASTM, ISO, and AAMI all have standards relevant to orthopaedic implants, this discussion will primarily focus on ASTM standards to illustrate the development and utilization of consensus standards.   

Types of ASTM Documents:

ASTM issues at least six distinct types of documents:

1.      Standard Test Methods: These documents provide specific procedures for generating test results. They are the most commonly applied standards in the mechanical testing of orthopaedic implants.   

2.      Standard Practice: This type of document offers detailed instructions for performing specific operations on a device, such as cleaning or inspection, without generating test results.   

3.      Terminology Standards: These standards provide definitions of terms and explanations of acronyms, symbols, and abbreviations.   

4.      Standard Classifications: These documents describe systematic arrangements of materials, products, or services based on shared characteristics.   

5.      Standard Specifications: These standards define the requirements that a material, product, or service must meet to comply with the standard.   

6.      Standard Guides: These guides offer information on current knowledge and approaches within a subject area. Their application requires professional judgment, as they do not recommend specific courses of action. (Source: http://www.astm.org/ABOUT/full_overview.html, http://www.astm.org/SNEWS/OCTOBER_2000/oct_howto.html)   

Among these document types, Standard Test Methods, Specifications (particularly for materials), Guides, and Practices are most pertinent to the evaluation of medical devices.   

Development of Test Method Consensus Standards:

Standards organizations create test method consensus standards by convening experts in a specific field to form a committee focused on a particular testing standardization need.  Each expert is responsible for contributing in good faith, with a collaborative and collegial approach, to develop the most scientifically sound and relevant method.   

For medical device standards, committees ideally include representation from industry, academia, contract testing facilities, healthcare, and government regulatory agencies. This diverse representation ensures a balanced consideration of stakeholders’ primary needs and concerns.  Industry members primarily advocate for meaningful yet practical testing requirements to avoid undue burden on companies.  Academia emphasizes the importance of scientifically accurate and sound test methods.  Healthcare members help ensure the appropriateness of clinical testing requirements.  Regulatory agency members contribute to verifying that the proposed test methods and requirements are suitable for evaluating the safety and effectiveness of a given device.  The collaboration of participants with these sometimes-competing interests enables the development of test methods that are both practical and meaningful.  This collaborative effort supports the overarching goal shared by all stakeholders: to facilitate the timely provision of safe and effective devices to patients.   

Expert member subcommittee groups are formed to address specific needs.  Within ASTM, Committee F04 on Medical and Surgical Devices is dedicated to developing standards and other documents exclusively for medical devices. (Source: https://www.astm.org/COMMITTEE/F04.htm)  Experts in a particular medical device area collaborate to determine the critical mechanical characteristics of a given implant, the most suitable testing modes for each characteristic, and how to clearly define the testing requirements within the standard.  The precise wording used in the standards is carefully selected to minimize the potential for misinterpretation.  Certain terms are assigned very specific meanings that are essential for the correct implementation of the standard.  For example, the term "shall" in a test instruction indicates an absolute requirement, whereas "should" signifies a recommendation. (Source: https://www.astm.org/Bluebook_Oct2016.pdf)   

Standard Vetting and Publication:

Once a subcommittee develops a draft testing standard, it undergoes a vetting process that may include round-robin testing and always involves multiple stages of review. (ASTM Standards, 2016)  Round-robin testing involves laboratories from various sectors, such as academia, industry, contract testing facilities, and regulatory agencies, conducting the test using specified specimens.  The results of these tests are compared to ensure quality control and reproducibility of the standard method in practice. These results are incorporated into the method as a precision and bias statement, providing information on expected variability within and between laboratories.  Following evaluation, review, and necessary revisions, the standard may be adopted by the organization and published for use.   

ASTM standards are published annually in a multi-volume book set, available in hard copy or on CD.  Individual standards are also available for purchase as PDFs online.  In some instances, "working documents" are utilized in the field before the final version of a standard is fully vetted and ready for publication.  While working documents are available from ASTM for use as needed, test reports based on these documents must not claim adherence to a standard, as working documents do not constitute official standards. (Source: https://www.astm.org/COMMIT/PrecisionBias.pdf)   

Harmonization of Standards:

ISO and AAMI operate similarly to ASTM, and there are ongoing efforts to harmonize the activities of these organizations.  Harmonization aims to minimize duplication and instead focus on developing standards that complement one another.  Historically, each group independently developed standards that sometimes overlapped.  The existence of multiple and potentially conflicting standards for a given device type could lead to confusion within the community and inefficient use of resources.  Harmonization of standards development involves the process of minimizing redundant or conflicting standards. (Source: https://www.astm.org/PRESIDENT/03_05_harmonization.html)   

Interpretation and User Expertise:

It is crucial to recognize that standards for mechanical testing of orthopaedic implants provide only methods and practices.  These standard test methods generally do not guide the user on how to interpret the obtained data or define criteria for successful or unsuccessful results.  Particularly in the context of mechanical testing standards, it is essential that the individual implementing a standard test method possesses a thorough understanding of fundamental mechanical testing procedures. The standards rely heavily on the user's experience and education for proper application and interpretation of the results.

The article is a brief description of Chapter 4 of the book, Mechanical Testing of Orthopaedic Implants, ed Elizabeth Friis, 2017.