Altered pore characteristics and geometry in diaphyseal cortical bone affected by experimental inflammatory arthritis

In experimental inflammatory arthritis (EIA), a loss of fracture toughness has been demonstrated in femoral diaphyseal bone. This may be attributed to porosity within the cortex. The purpose of this study was to contrast pore characteristics and femoral geometry of normal and arthritic bone. EIA was induced by repeated carrageenan injection into the right ribio-femoral joint of ten rabbits. Ten normal rabbits served as a control group. Cross-sections of the 20 right femora were analysed using the LECO 2001 image analysis system which allowed for characterization of the porosity. Femora from EIA animals were shown to have greater percentage porosity than normal femora (8.1% ± 0.39 SEM compared with 5.0% ± 0.13 SEM, respectively, p < 0.0001). In addition, the increase in porosity was restricted to the anterior and lateral portions of the cortex. The diameters and areas of the largest pores in the arthritic group were larger than those of the normal group. The cortical geometry of the arthritic bones was found to be different from normal. Moment of inertia (p < 0.025), cross-sectional area (p < 0.028) and cortical thickness (p < 0.0001) were all significantly reduced in the femora of the EIA animals. By acting as stress concentrators, pores can significantly reduce the mechanical integrity of a material. The increased femoral diaphyseal porosity could be responsible for the previously observed loss of fracture toughness in the model.