Hip, knee and disc replacements are the most common orthopedic replacement procedures. During hip or knee surgery, the damaged ball and socket joint is replaced with an artificial implant. The materials used in the artificial implants depend on several factors, including age and activity level of the patient. Although a wide range of implant materials are available, metal and ultra high molecular weight polyethylene (UHMWPE) are commonly used.
Ultra high molecular weight polyethylene is the material of choice with its lower wear rate and better fatigue performance when compared to other polymers. However recent developments to minimize wear in joint replacements by cross-linking UHMWPE resins have led to reductions in toughness and fatigue crack resistance. Therefore, the determination of fatigue crack growth resistance is of prime importance to implant manufacturers.
The fatigue resistance of UHMWPE is of great interest due to the cyclic nature of the loads applied to joint replacements. The fatigue properties refer to its ability to resist damage under repetitive load. The damage can result from microscopic cracks that form in the polymer because of stress or due to pre-existing effects. Under repeated cyclic loading, these cracks propagate until final fracture.
To determine the rate at which a crack will grow under an applied cyclic stress, fatigue crack growth testing has to be carried out. A traditional approach to design for cyclic loading has involved S-N curve data. However, this approach does not separate out crack initiation and propagation stages. By characterizing sub-critical crack growth using fracture mechanics parameters, it is possible to predict the number of cycles required for a crack to extend from initial length to a predetermined length of interest to the designer.
Over the years, Vollmer-Gray, in partnership with major orthopedic clients, has successfully developed viable test methods for characterization of UHMWPE. For instance, during fatigue crack propagation testing, the crack growth is monitored and recorded visually with the aid of high speed, high resolution cameras. The test data is then processed using Vollmer-Gray’s custom developed, proprietary software to calculate crack growth parameters.
Vollmer-Gray also provides routine physical and mechanical testing of UHMWPE material per ASTM F648 and D4020 per the Food and Drug Administration (FDA) guidelines.
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