Why should I be interested?
Orthopaedic surgeons are often barraged with offers to examine and use a variety of modified or new surgical techniques. These techniques often involve new instruments, new implants, or devices that have new materials associated with them.
The selection of a specific surgical device, method, or approach is affected by a number of factors, such as the device's suitability for the procedure and whether it is sufficiently strong, compliant, and biocompatible.
Ideally, to ensure safety, sufficient technique and implant evaluations would be performed and the device would be tested according to approved consensus standards before clinical use. In many cases, implants have undergone extensive testing both in vitro and in vivo. Consensus standards have enabled the evaluation of modified or new implant designs on a comparison basis. The consensus standards are living documents that can be revised if necessary. This article will provide background on how consensus standards for implant devices are developed, maintained, and used.
Who develops and maintains these standards?
The overall evaluation of orthopaedic implants lies within the combined efforts and responsibility of the federal Food and Drug Administration (FDA), standards organizations such as the American Society for Testing and Materials (ASTM) International and the International Standards Organization (ISO), manufacturers, and others. "Others" is a big group that includes researchers of all types, testing laboratories, biomaterial producers, surgeons, and more. The AAOS Biomedical Engineering Committee and the Orthopaedic Device Forum are also involved in this process, as may be the AAOS Biological Implants Committee.
As a surgeon, you should be aware of the process leading to performance standards for the implants you are offering to your patients.
Performance standards define how an implant should be tested. An example is the ASTM F2068, Standard Specification for Femoral Prostheses–Metallic Implants, which requires that representative implant samples must be able to support a specified loading profile for 10 million cycles, representing years of in vivo function (Fig. 1). As noted in the standard, this testing has been based on the correlation of clinical fractures and associated laboratory simulations to characterize fatigue strength performance.
Thus, new or significantly modified implants must pass this cyclic loading test to be considered compliant. However, other factors—including the location and assembly of parts during placement; patient size, weight, and activity level; and the presence of progressive diseases—may affect the clinical outcome of treatment.
These minimum loading profiles were determined only after the forces that cause clinical fractures were recognized. As a result, you, as an orthopaedic surgeon, should be confident that a design that meets this consensus standard will not break down structurally, unless there are highly irregular size, fixation, or other issues.
Frequently, ASTM standards are proposed and developed only after a known and significant clinical outcome or concern has been determined and the need arises for more clinically applicable testing to enhance clinical outcomes. For example, ASTM F1800, Standard Practice for Cyclic Fatigue Testing of Metal Tibial Tray Components of Total Knee Joint Replacements, was developed after a series of tibial trays required revision due to the absence of sufficient medial tibial bone support (Fig. 2).
Similarly, after reports of device fractures, ASTM F2345, Standard Test Methods for Determination of Static and Cyclic Fatigue Strength of Ceramic Modular Femoral Heads, was developed to help evaluate the mechanical strength of these heads (Fig. 3). These standards help ensure that these problems would not occur, while recognizing that it is difficult to anticipate every mode leading to loss of function.
Other ASTM standards clearly recommend that certain types of testing should be performed by referring to that test method. For example, ASTM F1378, Standard Specification for Shoulder Prostheses, states that porous coatings should be tested according to ASTM Test Method F1044 (shear strength) (Fig. 4) and F1147 (tensile strength). However, neither the shoulder specification nor the test methods specify a strength minimum.
Therefore, when an orthopaedic surgeon is presented with a new design with some type of porous coating, such as a 3-D additive metal printed porous surface, he or she might ask if the strength of the coating has been tested. It is very important to also ask how this design compared with an existing design used previously with a similar type of coating.
These examples of performance standards have led to numerous safe implant designs and should allow for many more. The need for surgeon involvement in the development and revision of consensus standards remains. For example, an orthopaedic surgeon can help improve standards for testing by alerting not just the manufacturer of a clinical problem, but also regulatory bodies such as the FDA. Previous articles in AAOS Now and The Journal of the AAOS have discussed the use of the FDA MedWatch program to report adverse events. The sooner details of a clinical failure are reported, the sooner testing procedures and associated studies can be developed to minimize such events.
As a surgeon, when should you be concerned about a new implantable device or method? What questions should you ask when presented with a new implant or surgical technique with revised instruments?
First, consider how this device should perform under the conditions of use. Could it break down mechanically? Corrode? Loosen? Generate excessive or unusual wear? You might ask how has this implant has been tested. How does it compare to existing, successful devices? If the device is considerably different, you can ask whether new testing procedures were developed to evaluate the device and ask for specific data on that testing.
Even with the best possible testing procedures in place, unforeseen complications may result from the use of some implants or methods. More input and reporting from surgeons who use these devices will improve these testing procedures and the associated consensus standards and will enhance future clinical outcomes. Your help is critical! Asking questions about the testing of new implants or methods leads to continuous improvement.
ASTM meets twice a year, and all areas of clinical expertise in orthopaedic surgery can be used to help improve consensus standards. Any interested surgeons can contact the authors to learn more about getting involved, or visit www.astm.org
To obtain a copy of the complete standards cited in this article, visit www.astm.org
Frederick W. Werner, M Eng, is Professor, Department of Orthopaedic Surgery, at SUNY Upstate Medical University, and is a member of the AAOS Biomedical Engineering Committee.
- ASTM F2068-15, Standard Specification for Femoral Prostheses—Metallic Implants, ASTM International, West Conshohocken, PA, 2015, DOI: 10.1520/F2068-15, www.astm.org
- ASTM F1800-12, Standard Practice for Cyclic Fatigue Testing of Metal Tibial Tray Components of Total Knee Joint Replacements, ASTM International, West Conshohocken, PA, 2012, DOI: 10.1520/F1800-12, www.astm.org
- ASTM F2345-03(2013), Standard Test Methods for Determination of Static and Cyclic Fatigue Strength of Ceramic Modular Femoral Heads, ASTM International, West Conshohocken, PA, 2013, DOI: 10.1520/F2345-03R13, www.astm.org
- ASTM F1378-12, Standard Specification for Shoulder Prostheses, ASTM International, West Conshohocken, PA, 2012, DOI: 10.1520/F1378-12, www.astm.org
- ASTM F1044-05(2011)e1, Standard Test Method for Shear Testing of Calcium Phosphate Coatings and Metallic Coatings, ASTM International, West Conshohocken, PA, 2011, DOI: 10.1520/F1044-05R11E01, www.astm.org
- ASTM F1147-05(2011), Standard Test Method for Tension Testing of Calcium Phosphate and Metallic Coatings, ASTM International, West Conshohocken, PA, 2011, DOI: 10.1520/F1147-05R11, www.astm.org
- Mihalko, W.M., GreenWald, A.S., Lemons, J., Kirkpatrick, J. "Reporting and Notification of Adverse Events in Orthopaedics". J Am Acad Orthop Surg:18, 193-198, 2010.
- McKee, J. "Using the FDA's MedWatch Program". AAOS Now, June 2011.