In vitro analysis of the stimulation of bone formation by highly bioactiveapatite- and wollastonite-containing glass-ceramic: Released calcium ions promote osteogenic differentiation in osteoblastic ROS17/2.8 cells

We analyzed the mechanisms of the efficient bone formation on the osteocond uctive surface of apatite- and wollastonite-containing glass-ceramic (AW) b y using an in vitro system. AW releases Ca ions and bonds to bone via a sub micron-thick hydroxycarbonate apatite (HCA) layer. AW disks were conditione d with simulated body fluid (SBF) to grow HCA layers, and the amount of rel eased Ca ion was regulated by modulating the conditioning time from 24 to 2 40 h. Surface-transformed AW disks increased alkaline phosphatase (AP) acti vity in osteoblastic ROS17/2.8 cells by 1.5- to threefold over unconditione d disks. AW disks conditioned for 24 h [AW(24)], which had a homogeneous, s ubmicron-thick apatite layer and increased extracellular ionized Ca concent ration ([Ca2+](e)) in the culture medium to the greatest extent, enhanced t he AP activity the most. High [Ca2+](e) promoted osteogenic differentiation in ROS17/2.8 cells: It increased AP activity in a dose-dependent manner by up to 1.6-fold, and up-regulated the expression of AP, osteocalcin (OC), a nd transforming growth factor-beta 1 mRNAs in dose- and time-dependent mann ers. AW(24) enhanced AP activity in ROS17/2.8 cells as much as AW disks con ditioned with SBF containing serum to exhibit in vivo surface-structure cha nges. AW(24) increased AP activity in ROS17/2.8 cells by 1.6-fold and enhan ced the expression of AP and OC mRNAs significantly, compared with sintered hydroxyapatite (HA). After implantation of AW and HA in the distal metaphy ses of rabbit femurs, thin, newly formed bone lined with cuboidal, osteobla st-like cells was characteristically observed adjacent to the AW surface wi thin 8 days. These results provide evidence for the hypothesis that AW stim ulates bone formation on its surface by increasing [Ca2+](e) to promote the HCA layer formation and the differentiation of osteogenic cells. (C) 1999 John Wiley & Sons, Inc.