Long-term biocompatibility, bioabsorption, and bone-fixation capability of magnesium alloy screw

Abstract

Background and Purpose of study Magnesium (Mg) is a potential material for designing absorbable metallic bone-fixation devices because of its biocompatibility and degradability. However, Mg has low mechanical strength and produces hydrogen gas which prevents its clinical acceptance. Various physical, heat, and/or chemical hardening methods have been used to improve mechanical strength including the alloying technique. Surface-coated Mg has been used for controlling hydrogen gas formation. This study developed an absorbable metallic screw with improved mechanical quality using Mg alloy, investigated the long-term biocompatibility and bioabsorption capability of this alloy in vitro and in vivo, and evaluated the possibility of using this Mg alloy screw for mandibular fixation after osteotomy.

Materials and Methods The mechanical properties of the magnesium alloy (WE43) were improved by heating, extrusion, and multi-axial rolling treatment. Screws and cylinders were designed and fabricated using treated Mg alloy. The maximum torque of the screw and rate of hydrogen gas formation in vitro in a corrosion model were measured. Screws and cylinders made of treated Mg alloy were placed in the tibiae and femurs of NZ white rabbits (n=12). To investigate the effect of coating on corrosion, a thin coat of hydroxyapatite (HA) (2-µm thick) was applied on half of the samples. Simple radiographs and microcomputed tomography images were taken for evaluating absorption. Histological sections were prepared and evaluated for biocompatibility at postoperative 3, 12, and 24 months For the mandibular osteotomy model, sagittal split ramus osteotomy (SSRO) was performed on the synthetic mandible (n=60). Biomechanical tests were performed according to the amount of distal segment movement and screw location. Results were compared with those for screws made of titanium (diameter, 1.6 mm), polymer (diameter, 2.5 mm), and pure Mg (diameter, 2.0 mm). Finite element analysis was performed by applying an occlusal force of either 132 or 500 N at the mandibular first molar. The failure risk of the screws was calculated using material yield strength. The yield load and distance of the load cell at the time of fracture of the screw or separation of the segments were measured. Ten dogs received bilateral SSRO via an extraoral approach, and the osteotomies were fixed with treated magnesium alloy screws in half of the animals and titanium screws in the remaining half. After weekly clinical observation and computed tomography imaging 1, 3, and 6 months postoperatively, animals were sacrificed and evaluated using microcomputed tomography and histological sections.

Results After extrusion and multi-axial rolling processing, the ultimate tensile strength and yield strength of the Mg alloy were 303 and 195 MPa, respectively. The maximum torque of the processed screw was 51.1 N, which was almost equal to that of titanium. The volume of gas produced by the Mg alloy with and without HA coating was 9.5 and 15.2 ml, respectively, on the 6th day after immersion in a simulated body fluid. Overt infection and clinical problems related to gas formation were not observed with the coated and uncoated Mg alloys after 24 months. Radiographically, the screw and cylinder shapes were mostly maintained. However, absorption at the head of the screw had progressed. Histologically, treated Mg alloy screws showed excellent biocompatibility with bone formation around screws without any inflammatory reaction. There were no screw fractures or deformations in any synthetic mandibles. The yield load and yield displacement tended to be higher in the Mg alloy screws than the titanium screws. In an FEM study, the maximum equivalent stress of the treated Mg alloy showed values in middle range compared to other materials, and similar pattern was observed regardless of the amount of the distal segment movement. Mg alloy screws were stable under an occlusal force of 132 N when the yield strength of the material was considered. Screw fracture, occlusal displacement, infection, and fistula formation were not observed in the dogs. Mg alloy screw displacement was observed in one animal, but bone fixations were well maintained in all other animals. Radiographically and histologically, bone healing with no inflammatory reaction was observed.

Conclusion The ultimate tensile strength of Mg alloy screws was increased to 303 MPa through physical processing. Partial absorption of these screws occurred during the 2-year observation period. The treated Mg alloy screws were biocompatible and possessed stable bone fixation capability in vitro and in vivo. Screws made of treated magnesium alloy can be a substitute for titanium and polymer screws for bone fixation after mandibular osteotomy.