This report documents the results of a finite element method (FEM) "worst-case" failure analysis of a stemmed, porous-coated tibial revision component that was designed with "slots" to aid in removal. A three-dimensional FEM model of a metal-backed tibia! component was subjected to expected peak loads while being restrained at the stem and unsupported under the plateaus. Stresses in the prosthesis plateau adjacent to the stem were found to exceed yield stress in many areas, and to exceed the expected endurance limit in large areas. We conclude that a stemmed prosthesis with slots is flawed and may be expected to fail if implanted without adequate support for the prosthesis plateau. The long-term effects of bone remodeling may further jeopardize the integrity of this prosthesis. We recommend that this prosthesis not be used in its present design.
Recent reports of total knee prosthesis fractures have raised concerns over the long-term structural integrity of metalbacked tibial components. Both the development of a fibrous tissue membrane under the tibial plateau of a total knee prosthesis and loading conditions may seriously alter the fatigue life of the metal tibial tray. The effects of the cement and fibrous tissue at the bone-prosthesis interface were studied. Using the method of threedimensional finite element analysis, peak loads of normal gait were simulated at several locations on the plateau of a generic, single-stemmed, porous-coated, CoCrMo tibial component model, providing information on the effect of abnormal loading patterns. According to the analysis, stresses below the material endurance limit are predicted throughout the prosthesis prior to the development of the fibrous membrane. However, stresses exceeding the yield strength of the material are predicted in a prosthesis that is supported by a fully developed 1 mm membrane, meaning that it has a markedly increased risk of low-cycle fatigue failure. Lateral displacement of the loading is detrimental to prosthesis life because maximum stress increases 100% while posterior displacement of the loading increases maximum stress by only 30%. Anterior loading creates stresses similar to those created by central loading. Because of their susceptibility to low-cycle fatigue failure, simple, single-stemmed prostheses are not recommended in cases of questionable bone stock unless modified. Several design alternatives are proposed.
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