Abstract
Background
Bio-Active Glass (BAG) is a promising bone graft substitute for large bone defect reconstruction because of its favourable osteoconductive, antibacterial and angiogenic properties. Potentially, it could also mechanically reinforce the defect, thus making it suitable for load-bearing defects. However, the mechanical properties of the reconstructive layer consisting of BAG/bone allograft mixtures are unknown. The goals of this study therefore were, first, to measure the mechanical properties of different BAG/bone graft mixtures and, second, to investigate to what extent such mixtures could reinforce distal tibial defects using micro-FE analysis and high-resolution CT scans.
Materials and Methods
Four different BAG/bone graft mixtures were impacted in a cylindrical holder, mechanically tested in confined compression and scanned with micro-CT. From these images, bone graft material and glass were segmented using two different threshold values. The interface between bone and BAG was modelled separately by dilating the glass phase. Micro-Finite-Element (FE) models of the composites were made using a Young's modulus of 2.5 GPa for bone and 35 GPa for BAG. The Young's modulus for the interface region was determined by fitting experimental and micro-FE results for the same specimens. (82 μm resolution) CT scans of a 9 mm region of the distal tibia of 3 subjects were used. Micro-FE models of this region were made to determine its stiffness in the original state, with a simulated cortical defect and after a mixture of BAG/bone was modelled in the defect.
Results
The confined compression tests showed a strong dependence of the modulus of the BAG/bone composite on the amount of BAG, ranging from 116.7 ± 18.2 to 654.2 ± 35.2 MPa. The micro-FE results could well reproduce these measured moduli, when using a stiffness of 25 MPa for the interface layer. The micro-FE analyses of the cortical defect demonstrated that the stiffness of the tibial segment would be reduced by 13 ± 3 % with the defect. Treatment with the BAG/bone composite could restore the stiffness to 101 ± 6 % of its original value.
Discussion
The experiments demonstrate that BAG/bone mixtures have a composition-dependent stiffness, in the range of that of trabecular bone, which can be well estimated from micro-FE analyses. Furthermore, the tibial micro-FE analyses demonstrate that these mixtures potentially can restore the stiffness of large bone defects at this site. Future development of the model may predict mechanical behaviour of BAG/bone mixtures patient specifically.
