Table 1.

Summary of the three kinds of absorbable materials.

Absorbable material		Degradation time	Degradation products	Merits	Demerits
Polymers	PGA	6∼12 months	Glycolic acid → Water, carbon dioxide	High elastic modulus, high mechanical strength	Rapid degradation rate, can result in inflammatory reaction at the implant site
	PLA	Few years	Lactic acid→Water, carbon dioxide	High mechanical stability, slow degradation rate	Poor biocompatibility，acid degradation，hard to control degradation rate
	PLGA	More than 1–2 months	Lactic acid, Glycolic acid→Water, carbon dioxide	Good biocompatibility, controllable degradation rate, almost no inflammatory reaction	Poor mechanical strength, poor osteointegration, unable to bear too much weight
	PCL	2∼4 years	Caproic acid → Water, carbon dioxide	Slow degradation rate, suitable for manufacturing scaffolds	Poor mechanical strength, unable to bear weight
Bioceramics	HA and β-TCP	Few months to few years (depend on the molecular structure of materials)	Ca2+, PO34-	High mechanical stability, suitable degradation rate, good biocompatibility, good osteointegration and osteoconductive, improved environmental pH	Too brittle to be manufactured into implants independently
Metal	Mg- based alloy	About 1∼1.5 years (MAGNEZIX)	Mg2+, OH−, hydrogen and other metal ions	Superior mechanical properties compared with stainless steel or Ti-based alloy, good biocompatibility, good osteointegration and osteoconductive, no cytotoxicity,	Need to improve manufacturing process to control degradation rate, complications related to degradation product hydrogen
	Zn- based alloy	More than 20 months	Zn2+, OH− and other metal ions	Similar mechanical properties to those of human bones, suitable degradation time, good biocompatibility, good osteointegration and osteoconductive, strong antibacterial activity in vivo	Uncertainty about toxicity of local high concentration zinc ions in human body
	Fe- based alloy	Longer than 2 years	Fe2+, Fe3+, OH−, iron oxides and hydroxides, hydrogen	Superior mechanical properties with high elasticity, strength, and plasticity	Slow degradation rate, poor biological activity, high concentration iron ions have biological toxicity