Early Osteoconductivity and Degradability of Sintered Porous Bodies of A- and B-Type Carbonated Apatites and Stoichiometric Hydroxyapatite

Abstract

An artificial bone that mimics the chemical composition of biological apatite is one of the promising implants for the treatment of bone defects and is useful as a scaffold for tissue-engineered bone. There are three types of carbonate substitution in hydroxyapatite (HAp) at hydroxyl groups (A-type), phosphate groups (B-type), and both groups (AB-type). Less is known about the role of A-type carbonate apatite (CAp) in influencing the bone regeneration process and there are no comparative studies of these apatites. The present study aims to elucidate osteoconductivity and degradability of porous sintered bodies of stoichiometric HAp, A-type CAp (A-CAp), and B-type CAp (B-CAp) with a constant carbonate content at 5.0 wt%. Both CAp with 75% porosity showed similar ionic dissolution rates which were faster than HAp with 67% porosity. A-CAp had the largest affinity for albumin from time-dependent adsorption tests. The lowest attachment of MC3T3-E1 cells was observed for A-CAp; however, the proliferation rate was similar of both CAp, which was lower than HAp. Early osteoblastic marker (alkaline phosphatase activity) was significantly upregulated for CAp at day 3 relative to HAp. After two weeks of implantation, A-CAp and HAp showed scarce bone formation encapsulated by fibrous tissue. In contrast, B-CAp showed significant bone ingrowth and closely adjoined to the surrounding bone tissue. The rapid degradation of CAp was attributed to osteoclast activities, in which A-CAp showed better capacity in promoting osteoclast maturation. It is concluded that a specific carbonate substitution type significantly impacts osteoconductivity or degradability of artificial bones.