Failure of a Total Knee Prosthesis Tibial Tray Due to the Fatigue Wear
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Some studies report the failure of a tibial insert of knee prostheses due to the subsurface cracking and delamination wear . The present study starts by analysing retrieved total knee prosthesis revealing the loss of a functionality of the prosthesis due to the fact that a larger part of the polyethylene tibial insert was dislocated. Some qualitatively remarks on the depreciative phenomenon are done based on optical and microscopic analysis of the affected areas of the polyethylene component. The failure mechanism was postulated as a combination of the high level of loading during normal activities and a non-conforming contact mechanism between the metallic femoral condyles and the tibial insert. The question that arises is: Could be this phenomenon evaluated quantitatively a priori, e.g. could be the failure due to delamination wear predicted? In order to do that some finite element simulations were performed to dynamically determinate the area of the contact and the contact pressure for three different activities – normal walking, stair ascending and stairs descending – that could be considered as defining the level of the activity of the patient. The finite element model uses a modern contact mechanism, including friction, between the metallic femoral condyles (considered rigid) and the tibial polyethylene insert. The results reveal the disadvantages induced by the nonconformity of the contact, it means the fact that the loads are transferred by relatively small areas of higher stresses. A special summation technique was adopted in order to consider the cumulative effect of all activities, based on the frequency of each activity and on the magnitude of the determined damage function. The computed surface of cumulative maximum damage traversing the layers of the polyethylene insert could be identified as the cracking surface in the retrieved prosthesis. The conclusion is that a partial answer to the question enunciated before could be achieved by the method presented here, that is able to determinate the path of the subsequent failure of the polyethylene inserts even that for an accurate and complete answer some extensions of the simulations needed to be done.
- 2006 fascicula8