A NOVEL MAGNESIUM-BASED BONE SUBSTITUTE TO STIMULATE IN VIVO BONE FORMATION FOR ORTHOPAEDIC IMPLANTATION

Summary A promising approach to stimulate in vivo bone formation by using our newly developed magnesium-based bone substitutes, which can be an alternative to treat the patients with bone loss in addition to the anticatabolic drugs and growth factors. Introduction Bone impairment arising from osteoporosis as well as other pathological diseases is a major health problem. Anti-catabolic drugs such as bisphosphonates and other biological agents such as bone morphogenetic proteins and insulin-like growth factor can theoretically apply to stimulate bone formation. However, the formation of more brittle bone and uncontrolled release rate are still a challenge nowadays. Hence, we propose to stimulate bone formation by using a newly developed magnesium-based bone substitute. Indeed, the presence of magnesium ions can stimulate bone growth and healing by enhancing osteoblastic activity. This study aims to investigate the mechanical, in vitro and in vivo properties of this novel bone substitute. Methods The bone substitutes were prepared by incorporating 9% TMSPM-treated Mg granules (i.e. 45μm & 150μm) into biodegradable polymer, polycaprolactone (PCL). The TMSPM silane-coupling agent treatment was used to protect the Mg particles from rapid degradation. Compression test was performed to study the mechanical properties of the bone substitute by using the MTS machine. A 7-day stimulated body fluid (SBF) immersion test was conducted to test their bioactivity. The surface composition was checked by energy dispersive x-ray spectroscopy (EDX) after immersion. The cytocompatibility and osteogenic differentiation properties of the bone substitutes were studied by MTT, ALP assays and qRT-PCR with the use of MC3T3-E1 pre-osteoblasts. Finally, the in vivo response of the bone substitutes was evaluated by using rat model of 2 months. Micro-CT was used to monitor the volume change of bone formation. Pure PCL was used as the control. Results At least 36% higher compressive modulus was found on the new bone substitutes as compared to pure PCL. Calcium and phosphate deposition were detected on the Mg bone substitutes but not on pure PCL after 7-day SBF immersion. Significantly higher cell viabilities and specific ALP activities were found on the new bone substitutes as compared to pure PCL. Additionally, significantly higher ALP, Type I collagen, osteopontin and Runx2 expressions were found on the Mg-based substitutes at different time points. Finally, more than 15% new bone was found on the Mg bone substitutes after 1 week of post-operation and 40% higher after 3 weeks. Discussion/Conclusion The increased compressive moduli of the Mg-based bone substitutes suggested that the mechanical property of PCL could be enhanced by incorporating Mg granules and the values fall within the range of cancellous bone (50 – 800 MPa). Moreover, the detection of the calcium and phosphate on the bone substitutes showed that they might possess osteoinductivity. The in vitro study showed the enhanced cytocompatibility and osteogenic differentiation properties of the new bone substitutes, which was possibly due to the effect of Mg ions release. Our previous study showed that only a low level of Mg ions (i.e. 50ppm) is able to stimulate the growth and differentiation of osteoblasts. Hence, this suggested the importance of controlling the release of Mg ions. This also explained why more new bone formation was found on the new bone substitutes than pure PCL during animal implantation. Hence, all the data presented here suggested our new bone substitutes maybe a potential candidate to stimulate new bone formation.