Magnesium hydroxide temporarily enhancing osteoblast activity and decreasing the osteoclast number in peri-implant bone remodelling

Repeated observations of enhanced bone growth around various degradable magnesium alloys in vivo raise the question: what is the major mutual origin of this biological stimulus? Several possible origins, e.g. the metal surface properties, electrochemical interactions and biological effects of alloying elements, can be excluded by investigating the sole bone response to the purified major corrosion product of all magnesium alloys, magnesium hydroxide (Mg(OH)2). Isostatically compressed cylinders of pure Mg(OH)2 were implanted into rabbit femur condyles for 2–6 weeks. We observed a temporarily increased bone volume (BV/TV) in the vicinity of Mg(OH)2 at 4 weeks that returned to a level that was equal to the control at 6 weeks. The osteoclast surface (OcS/BS) was significantly reduced during the first four weeks around the Mg(OH)2 cylinder, while an increase in osteoid surface (OS/BS) was observed at the same time. At 6 weeks, the OcS/BS adjacent to the Mg(OH)2 cylinder was back within the same range of the control. The mineral apposition rate (MAR) was extensively enhanced until 4 weeks in the Mg(OH)2 group before matching the control. Thus, the enhanced bone formation and temporarily decreased bone resorption resulted in a higher bone mass around the slowly dissolving Mg(OH)2 cylinder. These data support the hypothesis that the major corrosion product Mg(OH)2 from any magnesium alloy is the major origin of the observed enhanced bone growth in vivo. Further studies have to evaluate if the enhanced bone growth is mainly due to the local magnesium ion concentration or the local alkalosis accompanying the Mg(OH)2 dissolution.