Prediction of the long-term creep behaviour of hydroxyapatite-filled polyethylmethacrylate bone cements |
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Authors: | J C Arnold Nicholas P Venditti |
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Affiliation: | (1) Materials Research Centre, School of Engineering, University of Wales Swansea, Singleton Park, Swansea, SA2 8PP, UK |
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Abstract: | The creep behaviour of bone cements based on polyethylmethacrylate, with and without addition of hydroxyapatite filler has
been investigated, in order to determine the effect of hydroxyapatite filling and to investigate methods of predicting the
long-term creep behaviour from short-term tests. The materials were produced under laboratory conditions and tested in tension
in Ringer’s solution, as the study was intended to investigate the inherent materials behaviour rather than to simulate realistic
conditions. The effects of adding hydroxyapatite were to increase the short-term stiffness and more significantly to decrease
the creep rate. Short-term creep tests of up to 106 s were conducted at various temperatures, stresses and ageing states. These were then used to investigate various methods
of extrapolation to long-term behaviour. The use of time-temperature superposition was found to be useful, though it takes
no account of ongoing physical ageing and so gives a significant overestimate of long-term creep strains. Stress-time superposition
was less useful and also excludes ageing effects. The use of ‘effective time’ theory was more successful, but requires a large
number of short-term tests. The most effective method was that of the ‘integrated time’ approach, which required fewer tests
yet still gave good correlations with longer-term data. |
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