Aseptic loosening remains a long-term problem in total hip replacement. This phenomenon is prevalent even if modern cementing techniques seem to have reduced its incidence. Osteolysis has been deemed as a disease of access to fixation interfaces (1), either the stem- or bone-cement interface in hip replacement. This can be attributed in part to the quality of the cement in the proximity of the stem. It has been noted that due to thermal effects, polymerisation of bone cement starts at the bone-cement interface and gradually moves inwards towards the stem.

Femoral component heating was first proposed as a method to reduce the curing time of bone cement (2). This practice was later found to reduce the porosity at the stem-cement interface (3) and also to improve the interface shear strength (4). This study aimed to investigate the effect of femoral stem heating on two bone cements (Simplex P (Stryker) and Palacos R (Biomet Merck)) over a range of mantle thicknessess.

The model femora used for this study were maintained at a constant temperature of 37C while the stem temperature varied between 21, 37 and 44C. The femoral moulds were formed from dental plaster with a similar thermal conductivity to bone. Mould sizes were created to generate cement mantles of 2, 5 and 7.5mm thickness.

In the 2mm Simplex P cement mantles there was very little porosity evident. It was concentrated in the proximity of the stem when the component was kept at 21C and disappeared as the stem was heated to higher temperatures. Minimal porosity could be identified in the thicker mantles with no apparent differences between temperatures. There were no temperature trends evident from within this cement group. Palacos R cement has been reported to have a higher porosity than Simplex in a number of studies (5, 6). With the 2mm Pala-cos mantles, the increased stem temperatures reduced the porosity at the stem-cement interface. There was however no obvious difference between the 37 and 44C temperatures, where porosity seemed to be evident in the midsection of the mantle. This trend was also identified in the thicker cement mantles. The porosity did not extend out to the cement-bone interface under any conditions.

This study analyses the changes in porosity across the mantle of the cement as the temperature of the stem component is increased. The initial results confirm that the porosity at the stem cement mantle is decreased but indicate that the porosity within the body of the cement is increased as the temperature of the stem is increased.