Introduction: Bioactive glass (BAG) has been studied widely and seems to be a very promising biomaterial in regeneration of large bone defects and osteomyelitis treatment, because of its bone bonding and antibacterial properties[1]-[5]. Potentially, it could also mechanically reinforce large defects, thus making it suitable for load-bearing applications. However, the mechanical properties of the reconstructive layer and its dependence on BAG:bone allograft mixture composition are unknown.
In this study, we measured the mechanical properties of different impacted BAG/bone graft mixtures. Then these properties were used in micro-Finite Element (FE) patient-specific models to investigate whether these mixtures could restore mechanical properties of large bone defects.
Materials and Methods: Five different S53P4 BAG/bone graft mixtures were impacted in a cylindrical holder, mechanically tested in confined compression and scanned with micro-CT. From these images, the mixture was identified by its three phases: bone, glass or interface region. 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.
High-Resolution peripheral quantitative CT scans of a 9 mm region of the distal tibia of seven subjects were used for studying the reinforcement potential. Micro-FE models of this region were made to determine its stiffness in the original state, with a simulated cortical defect, and after that a mixture of BAG/bone or BAG alone was simulated in the defect.
Results: The confined compression tests showed a strong dependence of the Young’s 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 fairly reproduce these measured moduli, when using a stiffness of 25 MPa for the interface layer (R2=0.678, see Figure 1).
Figure 1. The Young's Modulus measured in the experiment versus predicted by the model after fitting the interface stiffness, for the different BAG contents.
Micro-FE analyses of the cortical defect demonstrated that the stiffness of the tibial segment would be significantly reduced by 16±4 % with the defect. Simulating treatment with the BAG or a BAG/bone mixture could restore the stiffness to 105±9 % and 102±8% of its original value, respectively (Figure 2).
Figure 2. A significant decrease in stiffness was observed with the simulation of a cortical window. The stiffness was significantly restored with the virtual implantation of BAG and a mixture of BAG and bone morsels. The stiffness is converted to percentages with the intact condition as 100%.
Discussion and Conclusion: The experiments demonstrate that well confined 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 to normal values. More validation studies, however, will be needed to investigate its performance in less confined situations as they can be expected in clinical cases.
References:
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