Abstract
Introduction
The role of porosity in the longevity of polymethylmethacrylate (PMMA) bone cement mantles remains unclear, although porosity reduction is probably desirable. It is not known whether pore patterns, pore distribution or pore morphology contribute to failure, since it is difficult to assess these features with traditional techniques. We used a novel microtomographic technique to quantitatively and qualitatively assess porosity in PMMA cements of differing viscosities to establish whether pore distribution can be effectively assessed and to document any differences in porosity (in both quantity, distribution and morphology). Each cement was also examined with and without the addition of vacuum, since this is thought to reduce porosity.
Methods and materials
Four PMMA bone cements of different viscosities (three of the same brand and the fourth chosen due to its popularity) were prepared and moulded according to established protocols (ASTM F451-99a), with and without the addition of vacuum. 25 samples per group (200 total) were prepared and densities for each sample calculated using Archimedes' principle. Four samples per group (total 32) were randomly selected for further analysis. These samples underwent micro-computer tomography (micro-CT) at a magnification of 20× and slice thickness of 13.67μm and reconstructed images were analysed with in-house developed software to measure pore size and volume. Results were analysed and compared with the two-sample T-test assuming significance at P<0.05. Qualitative assessment of pore character and distribution was made using three dimensional (3D) reconstruction.
Results
Densities and mean total pore volume showed an overall reduction in porosity with vacuum. Mean pore volume (MPV) was not significantly different between samples due to the huge range of pore sizes. Qualitative assessment revealed a striking, distinctive pore distribution between cements with an asymmetric distribution in the high viscosity group. Pore size was also qualitatively distinct between cement types and groups with and without vacuum.
Conclusion
Micro-CT allows detailed in vitro quantitative and qualitative assessment of porosity in PMMA cement. Pore volume, architecture and volumetric distribution differ in cements of differing viscosity and with or without the application of vacuum. This effect is marked enough to allow distinction between cements based on their micro-CT appearances. Further study using this technique in combination with other methods (such as quantitative analysis of 3D pore distribution) may shed light on the failure mechanisms in PMMA cement mantles, particularly with regard to the role of pore orientation, distribution and size.
