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Fatigue Testing of Femoral Stem Prostheses per FDA Guidelines

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Fatigue Testing of Femoral Stem Prostheses per FDA Guidelines

If you are bringing new femoral stem prostheses to market, you face many challenges. To begin, if your firm is a medical startup, your time to market is fixed by your financing. Within that time, you need to prove your concept and develop your product. As you know, the regulatory aspects are challenging and consume time and resources. Typically mechanical static and fatigue tests are not only required of your device but also of predicate devices as well. Mechanical tests may be helpful during the concept development phase and for sure, are needed to meet regulatory submission requirements of your 510K. The FDA does not require a 510K planning meeting or even dialogue.

The earlier you contact the FDA in your process, the more you will understand their view and expectations. It is a wise choice to pursue a meeting of minds to maximize the chance of succeeding through the FDA on the first try. Contact us as soon as topic of testing arises – we offer experienced application engineering support at no charge.

In the case of femoral stem prostheses, an FDA guidance document is available to address information requirements for your device(s). The recommendations in this document may be used for class II and class III femoral stem prostheses intended as components for cemented or uncemented (i.e., press-fit or for biological fixation) hemi or total hip replacement systems. Femoral stems have a neck and shaft, which extends into the intramedullary canal, and may have either an integral femoral head or cone designed to accept modular heads. Fatigue testing of the worst case stress levels induce on the femoral stems is recommended to demonstrate that your femoral stem will continue to function without failure. In particular, the FDA recommends using the test methods listed below to characterize the fatigue properties of the femoral stem. For each test method used, there may be a different worst-case stress levels of the stem. In addition, there may be femoral stem fatigue test methods (e.g., proximal stem fatigue testing), that mimic demonstrated clinical failure modes that your protocol should address.

Stem Fatigue Test Methods

Recommended Standards: The FDA has recommended the use of standards listed below or equivalent methods: ASTM F1612-95 ASTM F1440-92 ISO 7206-4. Worst-Case Design: The FDA recommends you test the stem dimensions and tolerances that produce the highest stressed components and greatest damage (i.e., worst-case). Potting Medium: The FDA recommends you use either a bone cement or a material that is mechanically similar to bone cement. Potting Level: If you use the test method described in ISO 7206-4:2002, The FDA recommends using a potting level at a minimum of 80 mm ± 2mm below the center of the head as measured vertically along the load application line from the center of the head to the potting level. Alternatively, if you follow ASTM F2068-03 and the test methods described in ASTM F1612-95 or F1440-92, the FDA recommends using a potting level with a minimum “unsupported implant length” of 50mm ± 2mm. Depending on the stem design, additional details are spelled out in the guidance and test standards. For very short stems that lack a sufficient potting medium to maintain fixation for the duration of the fatigue test, the FDA recommends that the length of stem embedded in the potting medium should be, at most, one third of the total femoral stem length measured vertically from the center of the head to the distal tip. If you rely on ISO 7206-8:1995 as a performance criterion and as a basis for comparing the fatigue testing results of the subject device to the majority of legally marketed devices that have been tested using ISO 7206-4:1989, ASTM F1612-95 or F1440-92, the FDA recommends you use a potting level as described above. For stems less than 200mm, therefore, the potting level FDA recommends differs from the protocol outlined in ISO 7206-4:2002. You may need to adjust the potting level such that stress risers (e.g., stems with design features such as slots, ribs, changes in material, surface characteristics, or modular connections) that are near the potting level are above the potting level. Lateral Head Deflection: FDA recommends you measure the lateral head deflection under maximum load at the initiation of each test and include this information in your maximum bending moment calculation. FDA recommends you include exceeding a pre-defined deflection limit as a stem failure. Therefore, you should monitor lateral head deflection during the test and record deflections that exceed the deflection limit defined in your protocol. Fatigue Test Results: When conducting the ISO/ASTM standard test methods, the FDA recommends you do the following: Test six femoral stems and compare your results to the acceptance criteria given in either ISO 7206-8:1995 (cyclic loading with a minimum load of 300N and a maximum load of 2.3kN for five million cycles) or ASTM F2068-03 (clause 6.1.1). Alternatively, the demonstrated fatigue strength of the stem should equal or exceed the demonstrated fatigue strength of a comparable legally marketed predicate femoral stem. The FDA recommends you test six devices for five million cycles.

Femoral Neck Fatigue Test Methods

The FDA recommends you perform femoral neck fatigue testing as described below. The performance standards associated with this method are outlined in Section VI.E.2. Recommended Standard: The FDA recommends using the standard listed below or an equivalent method. ISO 7206-6:1992 Fatigue Testing Considerations. Worst-Case Design: The FDA recommends you test the femoral neck with dimensions and tolerances that produce the highest stressed components and greatest damage (i.e., worst-case). Potting Medium Use a bone cement or a material that is mechanically similar as a potting medium. Fatigue Test Results: When conducting the ISO standard test method, we recommend that you do the following: Test six femoral stems and compare your results to the acceptance criteria in ASTM F2068-03 (clause 6.1.3). Alternatively, demonstrated fatigue strength of the neck should equal or exceed the demonstrated fatigue strength of a comparable legally marketed predicate femoral neck. We also recommend that you test six devices for ten million cycles. In addition, we recommend you provide a rationale for the components and methods you selected are representative of a worst-case scenario.

Notice - TestResources is not associated in any way with the FDA and presents the above information to support our customers as they navigate through the FDA regulatory testing processes. The contents of this web page have been modified and reduced from the more complete information available from the FDA website. It is strongly recommended that you visit their site to get further necessary information.