The knee is a complex hinge joint with ligaments that support the joint as bone surfaces glide with knee movements including back and forth, sideways, and bent. Knee joint prostheses are intended for replacement of a knee joint or part of a knee joint.

Design considerations for a knee joint prosthesis include: 

  • Smooth movement
  • Strength
  • Flexibility
  • Biocompatibility
  • Minimal wear of the implant over time. 

There are over one hundred knee prosthesis designs on the market.  The selection and placement of the prosthesis may differ based on the patient’s need and the orthopedic surgeon’s preference. The two classes of knee joint prosthesis discussed in this post are:

The total knee prosthesis also known as the knee joint patellofemorotibial prosthesis

  • A total knee replacement has components attaching to the lower end of the femur, the top surface of the tibia, and and the back surface of the patella. 
  • Components include a metal tibial baseplate and a patellar component with a polymer-based bearing fixed to the metal femoral component. 
  • Metal pieces may be porous-coated.

The knee joint femorotibial or unicompartmental prosthesis 

  • This type of prosthesis is composed of a metal femoral component and a tibial component with a tibial bearing fixed to a metal baseplate. 
  • Metal parts may be porous-coated.

In the United States, both the above prosthesis are classified as class II (special control) devices per the FDA and require extensive testing for regulatory approval. For more information, refer to the FDA Guidance Document Knee Joint Patellofemorotibial and Femorotibial Metal/Polymer Porous-Coated Uncemented Prostheses linked here.

Knee Joint Prosthesis Mechanical Testing

As discussed above, prostheses used in knee joint replacements consist of multiple components. Thus, proper knee joint prosthesis testing includes tiered testing that involves testing of the specific components based on the expected loads. 

Component Testing

It is recommended to dynamically test the tibial baseplate component as it must withstand cyclic loading over time. In addition to a failure analysis, FDA recommends following ASTM F1800 Standard Practice for Cyclic Fatigue Testing of Metal Tibial Tray Components of Total Knee Joint Replacements. During this test, various loads must be applied to the specimen with the pass criteria being surviving 10 million cycles. 

Although there is no specific standardized test method for a unicompartmental knee prosthesis, manufacturers should demonstrate the capability of the device to withstand the expected loads over the specified time.

For UHMWPe tibial bearing component testing, FDA recommends fatigue and wear testing to ensure this component of the knee implant can withstand both sliding and rolling motions seen in a functional knee over an extended period of time. Test methods for this type of testing include ASTM F1715 Standard Guide for Wear Assessment of Prosthetic Knee Designs in Simulator Devices, ASTM F2025 Standard Practice for Gravimetric Measurement of Polymeric Components for Wear Assessment, and ISO 14243-1 & -2 Implants for surgery – Wear of total knee-joint prostheses. In addition, orthopedic devices using UHMWPe (Ultrahigh Molecular Weight Polyethylene), should refer to this FDA guidance document to determine the appropriate information and testing protocols. 

Finally, the posterior-stabilized tibial bearing component must be tested following shear fatigue test methods. 

Component Interlock Strength Testing

As a replacement to the complex knee joint, the knee prosthesis is also a complex prosthesis made of multiple components, including articulating components, interlocked together. Femoral, tibial, and patellar components have several different positions of flexion at 0°, 15°, 30°, 60°, 90°. Components that are interlocked together should be tested following shear test methods depending on their position of flexion as well as in tensile and fatigue testing to ensure they are able to withstand loads.

Prosthesis Testing

The knee prosthesis test methods also include a variety of tests. Due to the nature and functionality of the knee joint, the knee implant should show that it can withstand forces from different directions over an extended period. Test methods include fatigue, wear and range of motion testing. 

Featured Equipment

The featured equipment for knee prosthesis testing is the ADMET eXpert 5951 dual column electrodynamic testing machine equipped with the MTESTQuattro controller and software.

Medical Device Fatigue Testing Machine - Knee Implant Demonstration

eXpert 5951 – Fatigue Testing on Knee Implant

eXpert 5900 series fatigue testing systems are fast acting high efficiency table top electrodynamic testers. Specifications include a 6 in (152 mm) stroke, 15 Hz dynamic capability, and 240 in/min (6.096 mm/min) maximum speed. The knee implant testing configuration included a Stainless Steel T-Slot base to easily mount different test fixtures. Some of the grips and fixtures supplied with the system are manual vise grips, rope grips, and wire crimp grips

In addition, the machine was configured with a manual crosshead lift mechanism with dual acme screws mounted to the crosshead and to the base of the fatigue system. The manual crank mechanism allows the users to rotate the crosshead at a right angle drive resulting in vertical motion.

Knee Implant Fatigue testing

eXpert 5951A with MTESTQuattro for Knee Implant Fatigue Testing

MTESTQuattro controller and software for fatigue testing comes with live graphing capabilities that reports time elapsed from start of the test and the number of cycles completed. ADMET also provides access to the live test data real-time so that you can use external software to log data over long periods of time.

Click here for other orthopedic device testing system configurations.

Conclusion

Orthopedic implants, including the complex knee joint implant, are subject to rigorous endurance tests to ensure that they exceed specified life requirements. At ADMET, we have over 30 years of experience working with medical device manufacturers and researchers to build test equipment tailored to the specific need. Our testing system reliability, price vs. performance, ease of use and responsive customer support make ADMET a valuable partner to your testing laboratory.

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