Gas Formation and Its Effect After Implantation of Bioabsorbable Metal Magnesium

Category: Basic Sciences/Biologics Introduction/Purpose: Metallic biomaterials (stainless steel and titanium) are most commonly used for osteotomy and fusion surgery in foot and ankle. But it results in the physical and psychological burden to the patients owing to the second surgery (hardware removal). Bioabsorbable polymer has the disadvantages such as mechanically weak strength and osteolysis. To overcome these, bioabsorbable metal such as magnesium (Mg) implant have been developed recently and applied clinically, but it has also drawback such as gas formation by the biodegradation of magnesium alloys. However, there is no study on the characteristics of the gas formation and its effect on the surrounding tissue. The purpose of this study is to identify the characteristics of gas formation and histological reaction during the absorption of Mg on surrounding tissue using in vitro and in vivo rodent model. Methods: Pure Mg plate (15 × 15 × 2 mm) was immersed in EBSS (Earle’s balanced salt solution) for in vitro immersion test for 30 days. Intramuscular implantation of Mg plate (25×5×0.25 mm) was performed on twenty-four rat’s back muscle and divided as two groups: implantation and sham surgery. Gas volume and compositions, corrosion rate and surface morphology of Mg plate, and histological exam were evaluated at every 5 days for 30 days. Results: Gas is mainly composed of H2 (2.483 ×104± 0.028 μV), but a small amount of CO (93.428±0.14 μV) and CO2 (57.18±0.53 μV) was detected in in vitro immersion test. Gas volume shows significantly higher at 5 days (0.084 ml) than other times (0.005 mL for 30 days) after implantation, and is gradually decreased over time. Corrosion rate is also faster in at 5 days and decreased over time in both in vitro and in vivo study. Surface morphology shows a lot of cracks and the formation of apatite paticles on Mg plate. Histological examination shows that nuclei gathered densely around the small and multiple air bubble pocket at 5 days and the number of gas bubbles and their volumes were reduced over time in tissue surrounded implant, and there is no difference in kidney and liver tissue between implant and sham surgery. Conclusion: The immersion of Mg in EBSS generated H2 as the major forming gas, and CO and CO2. The in vivo implantation of Mg into rats’ back muscle generated gas pockets inside the tissues because the initial gas formation rate was faster than the gas- absorbing rate; however, gas volume decreased after 15 days as the gas formation and absorbing rates became similar. The amount of Mg ions inside the liver and kidneys changed as well, but it was within a normal range, such that inflammation and tissue deformation were not found.