IN VITRO DEGRADATION TEST OF MAO COATED Mg IN SIMULATED BODY FLUID

INTRODUCTION Anterior cruciate ligament (ACL) is one of the major knee stabilizers. The ACL is also the most commonly inured ligament of the knee, with over 200,000 injuries reported annually in the United States [1]. Currently, ACL reconstruction surgery is the primary method of repair. ACL reconstruction uses a tissue graft, usually derived from the patellar tendon, as a scaffold for the ACL to regrow on [2]. Although ACL reconstruction surgery allows for the patients to return to their daily activities in the short term, about 25% of the patients have reported complications in the long term [1]. To overcome these complications, our lab has developed a magnesium ring device that allows for the natural healing of the ACL. The Mg ring holds together the two transected ends of the ACL to re-stabilize the knee joint, load the injured site, and promote natural healing when coupled with an extracellular matrix (ECM) scaffold [1]. Aside from facilitating natural healing, the Mg ring allows for a less invasive procedure than ACL reconstruction surgery and is completely biodegradable and bioresorbable, thereby not requiring a second surgery to remove the device. Due to Mg’s biodegradability, there has been an increasing interest in developing Mg based materials for clinical practices [3]. Mg has many advantages over other materials for clinical practices: its density is 1.6 to 4.5 times less than aluminum and steel, its elastic modulus and compressive yield strength are comparable to bone, and it is required for human metabolism of calcium [3]. However, Mg degrades relatively rapidly in the body, which causes a problem in that the device could potentially degrade before the ACL has a chance to heal enough to bear loads. Micro-arc oxidation (MAO) coatings have been investigated as a potential solution to the biodegradability problem of Mg [4]. Gu et al. have reported that the corrosion resistance of Mg can be controlled using MAO coating time [4]. When a high applied voltage is coupled with an MAO coating solution, consisting of Na3PO4, Na2SiO3, and KF, an oxide layer is formed on the surface of the Mg disk [5]. This oxide layer can slow down the corrosion of Mg in the body and thereby allow the ring device to last longer at the site of repair. Due to the relative novelty of Mg based biomaterials, not enough research has been done on the effect of MAO coated Mg in the body.