Aseptic loosening is the commonest cause for revision of total hip replacements. Advances in technique have improved femoral fixation, but acetabular survival is unchanged. Little has been published about the pressures achieved during acetabular insertion.

Using an experimental model, an acetabular cup was inserted into a model acetabulum using standard surgical technique. The pressures achieved under the centre of the cup, and also just beneath the superior flange were recorded over time via a data logger. The experiment was repeated for different insertion times, cup sizes and cement volumes. To reduce experimental error four runs for each constraint were performed. Complex pressure/time curves were produced for each run of the experiment. An average was then plotted. Peak and mean pressures were calculated from these curves.

Central pressures were found to be similar to flange pressures, although pressure under the flange decayed more quickly as the cement cured.

As long as the cup was inserted during the working phase cement then peak and mean pressures were similar.

Use of a double cement mix with a 43mm cup showed a large increase in pressures over the single mix (peak 693 to 957mmHg, mean 297 to 485mmHg). Both were statistically significant (ANOVA: peak p=0.018, mean p=0.008). Peak pressure occurred with cup insertion, rather than with pre-pressurisation.

The 47mm cup showed an increase of peak and mean pressures over the 43mm cup with a single mix (peak=800mmHg, mean=384mmHg). The pressures for the 47mm cup with a double mix were less than those of the 43mm cup with a double mix.

Insertion of the cup without pre-pressurisation resulted in a rapid loss of pressure, and low mean pressures were achieved (double =262mmHg, single=80mmHg).

Our experiment shows the need for pre-pressurisation, and that the timing of cup insertion is not critical as long as it is during the working phase of the cement. We have also shown an increase in pressure with a larger cup with a single mix of cement even though volumes were adequate to fill the space between cup and acetabulum. Interestingly we obtained significantly increased pressures with a smaller cup and a double volume of cement, this area needs further study.