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 10(6) 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.

The effect of test environment on the creep behavior of a poly(ethylmethacrylate) bone cement was investigated. The aim of the study was to assess the influence of environment on the inherent material behavior, and so it was convenient to perform tests in tension on well-prepared samples. In addition to control tests in air, the liquid environments studied were water, Ringer's solution and Intralipid. Creep tests were performed in each of these environments with a range of aging times, test temperatures and applied stresses. In order to compare the effects of the environments, the creep curves were fitted to a generalized form, from which a creep rate was determined. The ratio of these creep rates between different environments at each testing condition was then used as a basis for a comparison of the detailed effects of environment. It was found that in all cases the water-based environments (water and Ringer's) had similar effects and gave the largest creep rates. Intralipid was then intermediate and air gave the lowest creep rates. These effects are mainly due to plasticization by water, although with Intralipid, some increased monomer leaching occurred, which served to reduce the creep rates. The influence of environment on the effects of aging time, temperature and stress were complex, although in general any conditions which increased water plasticization (longer aging, higher temperature and to a lesser extent, higher stress) gave an increase in creep rate. The major exception to this was at temperatures of 40 degrees C and above, where the effects of water plasticization were diminished, due to the inherent increase in molecular mobility of the material.