Comparative study on bone regeneration by synthetic octacalcium phosphate with various granule sizes Yoshihisa Murakami a,b , Yoshitomo Honda b , Takahisa Anada b , Hidetoshi Shimauchi a , Osamu Suzuki b, * a Division of Periodontology and Endodontology, Tohoku University Graduate School of Dentistry, Sendai, Japan b Division of Craniofacial Function Engineering, Tohoku University Graduate School of Dentistry, Sendai, Japan article info Article history: Received 24 June 2009 Received in revised form 6 October 2009 Accepted 13 October 2009 Available online 1 November 2009 Keywords: Octacalcium phosphate Granule Bone regeneration Biodegradation Osteoclasts abstract The present study was designed to investigate whether the granule size of synthetic octacalcium phos- phate (OCP) and the resultant intergranular spaces between the granules formed by the filling affect its osteoconductive and biodegradable characteristics in a mouse calvaria critical-sized defect up to 10 weeks after implantation. Mercury intrusion porosimetry showed that OCP granules having distinct diameter sizes ranging from 53 to 300 (S-OCP), 300 to 500 (I-OCP) and 500 to 1000 lm (L-OCP) produced distinct intergranular spaces between OCP granules ranging from 28.8 to 176.6 lm. The dissolution rate of OCP, estimated by the phosphate concentration in the culture medium, was the highest in S-OCP, fol- lowed by I-OCP and L-OCP, while the specific surface area of OCP decreased. Histological and histomor- phometric analyses showed that bone formation around the implanted granules increased significantly with increasing granule size coupled with activating the appearance of TRAP- and cathepsin K-positive osteoclastic cells. The rate of new bone formation formed with L-OCP was two times higher than that formed with S-OCP at 10 weeks after implantation. The results indicated that the osteoconductive and biodegradable properties of OCP can be augmented by increasing the granule size, most probably by thus providing enough spaces between the granules, suggesting that the intergranular spaces formed by the granules may work similarly to pores, as reported in porous ceramic materials. It seems likely that the enhancement of bone formation by OCP is accompanied by simultaneous activation of osteoclastic resorption of OCP. Ó 2009 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. 1. Introduction Biodegradable calcium phosphate compounds, such as b-trical- cium phosphate (b-TCP) [1] and octacalcium phosphate (OCP) [2] biomaterials, have been extensively investigated as bone substitute materials in the repair of various bone defects. This is because these calcium phosphate materials are anticipated to be replaced with higher volume of new bone than non-biodegradable calcium phos- phate, such as sintered hydroxyapatite (HA), which has a stoichi- ometric Ca/P molar ratio 1.67 if implanted in bone defects, even critical-sized ones [3]. OCP has been proposed as a precursor of bio- logical apatite crystals in bone and tooth [4], although the chemical nature of the first mineral formed in vertebrate biomineralization remains controversial [5,6]. Apart from what kind of mineral is formed first at the onset in osteogenesis, recent intensive studies on the experimental application of synthetic OCP have shown that it has the potential to enhance new bone formation [7–12]. Previous studies reported that OCP is more capable of facilitat- ing the differentiation of mouse bone marrow stromal ST-2 cells into osteoblastic cells than non-sintered Ca-deficient HA formed via OCP origin [13] or sintered HA ceramics [14] if incubated on calcium phosphate coatings in vitro even in the absence of osteo- genic supplements, such as dexamethazone and b-glycerophos- phate. The OCP crystal assembly also has the potential to support the differentiation of rat bone marrow stromal cells into osteo- blasts [15,16]. Although the solution-mediated and osteoclastic cell-mediated biodegradable properties of OCP could be one of the factors leading to higher osteoconduction in vivo [7,17–21], it has been suggested that the transitory nature of OCP into HA in a physiological environment also plays a role in stimulating osteo- blastic cells to differentiate both in vitro and in vivo during OCP– HA conversion [13,22–25]. The osteoconductive characteristics of OCP were first demonstrated in granular form in the subperiosteal region of mouse calvaria [22,23], showing the rapid appearance of new bone more clearly than HA or Ca-deficient HA [22]. Further- more, it was confirmed that OCP was unique in that fine filaments and granular materials were formed around the OCP particles, the structure of which was very similar to that of the starting locus of intramembranous bone mineralization or so-called bone nodules 1742-7061/$ - see front matter Ó 2009 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.actbio.2009.10.023 * Corresponding author. Address: Division of Craniofacial Function Engineering, Tohoku University Graduate School of Dentistry, 4-1, Seiryo-machi, Aoba-ku, Sendai 980-8575, Japan. E-mail address: suzuki-o@m.tains.tohoku.ac.jp (O. Suzuki). Acta Biomaterialia 6 (2010) 1542–1548 Contents lists available at ScienceDirect Acta Biomaterialia journal homepage: www.elsevier.com/locate/actabiomat