Knee prostheses as well as individual knee replacement components, including femurs, patellas, tibial trays, and tibial inserts, are typically subjected to endurance testing to characterize useful life and durability. In addition to fatigue and wear testing, properties such as range of motion, constraint, contact pressure, and component interlocking strength are characterized through static testing.
Tibial tray fatigue testing is performed by fixating one half of the tibial baseplate and applying a constant amplitude load to the unsupported half of the baseplate. ASTM F1800 Standard Practice for Cyclic Fatigue Testing of Metal Tibial Tray Components of Total Knee Joint Replacements and ISO 14879-1 Implants for surgery – Total knee-joint prostheses Part 1: Determination of endurance properties of knee tibial trays suggest testing to 10M cycles. For submission testing, five samples must survive 10M cycles under at least 900 N of load. Unicondylar trays are also subject to fatigue testing, but in a 3-point bend test setup.
Femoral implants are dynamically-tested during opening and closing conditions to simulate stresses due to poor bone condition. Tibial posts of posterior-stabilized tibial inserts are fatigue-tested to evaluate resistance to failure after loading against the femoral component that has undergone artificial aging per ASTM F2003. Contact area and stress distribution are characterized at multiple flexion angles and loads in reference to ASTM F2083. Similarly, constraint testing per ASTM F1223 Standard Test Method for Determination of Total Knee Replacement Constraint is used to determine the dislocation and motion resistance of the implant system. ASTM F1672 provides guidance on contact area and constraint characterization for patellar components.
Knee implant wear properties are assessed per ISO 14243-1 Implants for surgery – Wear of total knee-joint prostheses Part 1: Loading and displacement parameters for wear-testing machines with load control and corresponding environmental conditions for test or ISO 14243-3 Implants for surgery – Wear of total knee-joint prostheses Part 3: Loading and displacement parameters for wear-testing machines with displacement control and corresponding environmental conditions for test. Tests are typically performed at 1 Hz frequency to 5M cycles with samples removed at various intervals and precisely measured for mass loss per ISO 14243-2. Wear debris is retrieved and subject to particle analysis utilizing ISO 17853 or ASTM F1877. Particle retrieval can be performed referencing methods in ASTM F561.
ASTM F1814 Standard Guide for Evaluating Modular Hip and Knee Joint Components provides an overview of relevant tests depending on the type of knee implant design and addresses the modular connections of a total knee system. In general, modular connections are assessed for both static and fatigue strength. Fatigue testing of connections is often performed in saline or protein solution environments.
Comprehensive testing of knee prostheses requires a variety of equipment. Axial static and fatigue machines are used for uniaxial tests such as ASTM F1800 and tensile disassembly of modular connections. Biaxial testers recreate more advanced loading situations to evaluate knee constraint and post posterior stabilization fatigue. Multiaxial joint simulators are required to replicate complex conditions that include simultaneous motions along four or more axes.
Additional equipment is needed to support mechanical testing. A scanning electron microscope (SEM) with energy dispersive spectroscopy (EDS) capabilities is useful for particle characterization. Precision scales are required to evaluate components for mass loss with sub-milligram accuracy.
MDT tests are conducted in accordance with ANSI/ISO/IEC 17025-2005. Testing schedules and resource needs are determined during project kickoff meetings with MDT engineers and the customer. Services are performed in accordance with standard operating procedures and test protocols specific to the project. Standard tests do not begin without a protocol signed by both the customer and an MDT representative. For feasibility or non-standard tests, MDT will develop a method and protocol with customer input and will review the final process with the customer prior to initiation of the test.
The test equipment software monitors the cycle counts, loads, and displacements throughout the duration of the test. MDT engineers monitor tests daily and send weekly updates to ensure customers are apprised of the status of their test.
Inspection Services Offered:
MDT offers several inspection and monitoring methods including:
• High speed video inspection of devices or fixture validation
• Visual Inspections
• Particle analysis using SEM/EDS
Tests are conducted in accordance with ANSI/ISO/IEC 17025-2005. In compliance with this standard, MDT test reports include comprehensive documentation and summary of the test protocol and results. Any test method deviations, additions, or exclusions are documented and reviewed with the customer prior to beginning the test. Information within the report includes the specific test conditions, statements of compliance with test protocol, and the estimated measurement uncertainty. The reported results only relate to the items tested. Any opinions and interpretations provided by MDT are clearly marked as such. Any additional information required by customer-specific methods will also be noted. At the conclusion of the test, MDT provides the customer with a Microsoft Windows formatted CD containing copies of all raw data, inspection results, and other information collected during the test (see example test reports).
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