Quantifying the degradation of degradable implants and bone formation in the femoral condyle using micro-CT 3D reconstruction

Degradation limits the application of magnesium alloys, and evaluation methods for non-traumatic in vivo quantification of implant degradation and bone formation are imperfect. In the present study, a micro-arc-oxidized AZ31 magnesium alloy was used to evaluate the degradation of implants and new bone formation in 60 male New Zealand white rabbits. Degradation was monitored by weighing the implants prior to and following implantation, and by performing micro-computed tomography (CT) scans and histological analysis after 1, 4, 12, 24, 36, and 48 weeks of implantation. The results indicated that the implants underwent slow degradation in the first 4 weeks, with negligible degradation in the first week, followed by significantly increased degradation during weeks 12-24 (P<0.05), and continued degradation until the end of the 48-week experimental period. The magnesium content decreased as the implant degraded (P<0.05); however, the density of the material exhibited almost no change. Micro-CT results also demonstrated that pin volume, pin mineral density, mean 'pin thickness', bone surface/bone volume and trabecular separation decreased over time (P<0.05), and that the pin surface area/pin volume, bone volume fraction, trabecular thickness, trabecular number and tissue mineral density increased over time (P<0.05), indicating that the number of bones and density of new bone increased as magnesium degraded. These results support the positive effect of magnesium on osteogenesis. However, from the maximum inner diameter of the new bone loop and diameter of the pin in the same position, the magnesium alloy was not capable of creating sufficient bridges between the bones and biomaterials when there were preexisting gaps. Histological analyses indicated that there were no inflammatory responses around the implants. The results of the present study indicate that a micro-arc-oxidized AZ31 magnesium alloy is safe in vivo and efficiently degraded. Furthermore, the novel bone formation increased as the implant degraded. The findings concluded that micro-CT, which is useful for providing non-traumatic, in vivo, quantitative and precise data, has great value for exploring the degradation of implants and novel bone formation.