Comparative biomechanical and radiological characterization of osseointegration of a biodegradable magnesium alloy pin and a copolymeric control for osteosynthesis.

Magnesium alloys offer great advantages as degradable implant material for pediatric fracture fixation and hold the potential to overcome certain critical shortcomings inherent to currently used degradable (co)polymers. Besides good biocompatibility and appropriate degradation kinetics, sufficient implant anchorage in host bone is critical to prevent implant failure. Bone-implant anchorage of biodegradable magnesium alloys, however, has not yet been related and compared to that of copolymers, their degradable counterparts currently in clinical use. The aim of this study, therefore, was to comparatively assess bone-implant interface strength and the amount of peri-implant bone of a biodegradable magnesium alloy pin (Mg-Y-Nd-HRE) and a self-reinforced copolymeric control (85/15 poly(l-lactic-co-glycolic acid)). To this purpose, push-out testing, microfocus computed tomography (μCT), histological and scanning electron microscopic examination was performed after 4, 12 and 24 weeks of transcortical implantation in 72 rats. Biomechanical testing revealed significantly higher ultimate shear strength for the magnesium alloy pins than for the copolymeric controls at all 3 timepoints (P≤0.001 for all comparisons). As evaluated by μCT, the amount of bone present near the interface and in a wider radius (up to 0.5mm) around it was higher in the magnesium alloy implants at 4 weeks, without significant differences at 12 and 24 weeks. Histological examination confirmed direct bone-to-implant contact for both implant types. In vivo degradation of implants did not induce any noticeable local or systemic inflammation. This data suggests that the investigated degradable magnesium alloy rod exhibits markedly superior bone-implant interface strength and a greater amount of peri-implant bone than a self-reinforced copolymeric control currently in use; thus it fulfills a crucial prerequisite for its successful clinical deployment as an alternative degradable orthopedic implant material. Further studies, however, are warranted to evaluate the long-term degradation behavior and biocompatibility of the investigated degradable magnesium-based alloy.