Intramembranous bone tissue response to biodegradable octacalcium phosphate implant
Introduction
The chemical nature of the first mineral formed in vertebrate biomineralization remains controversial. Some studies suggested that octacalcium phosphate (Ca8H2(PO4)6·5H2O; OCP) is a precursor of biological apatite crystals in bone [1], [2]. Other studies suggested that very small poorly crystalline apatite can be formed directly in the initial bone mineralization [3], [4]. Apart from the chemical nature of the first mineral formed in bone, recent intensive studies on the experimental application of synthetic OCP have shown that it has the potential to enhance new bone formation [5], [6], [7], [8], [9], [10], [11].
The osteoconductive nature of synthetic OCP was found first by its subperiosteal implantation in mouse calvaria in comparison with synthetic hydroxyapatite (Ca10(PO4)6(OH)2); HA) [5], [12]. Several studies have been conducted to investigate the possible use of synthetic OCP as a bone regenerative scaffold in various forms, such as coatings on metallic implants [8], [9], [10], [13], microscaffold self-assembled [14], [15] and granules [5], [7], [11], [12], [16]. Recent in vitro studies disclosed that OCP facilitates osteoblastic cell differentiation [11], [14], [15], and that marked increase in osteoblast-related gene expression, such as osterix and alkaline phosphatase (ALP), was observed depending on the dose of OCP [17]. It has been shown that synthetic OCP is converted into HA both in vivo [5], [11], [12], [18] and in vitro [11], [19], [20], [21], [22], [23]. Previous studies showed that a process of OCP–HA conversion involves exchanges of calcium and phosphate ions with surrounding tissue milieu [11], [24], [25] and is involved in promoting osteoblastic cell differentiation [11], [17] and bone regeneration [5], [11], [12]. OCP can be converted topotaxially without changing its original morphology [21], [26], [27] even in vivo, where bone formation was accelerated by its implantation [28] thereby providing a scaffold for osteoblast attachment, proliferation and subsequent differentiation.
It is known that solubility at physiological pH decreases in the order of OCP, β-tricalcium phosphate (β-Ca3(PO4)2; β-TCP) and HA [24]. β-TCP is widely accepted as a biodegradable bioceramic and used clinically [29], [30], [31]. Thus, OCP is the most soluble salt among them, so a lot of attention has been paid to the use of synthetic OCP with the expectation of it acting as potential loci for the nucleation of bone induction in orthotopic sites, which could be replaced with a significantly higher volume of newly formed bone compared with the other calcium phosphate phases such as HA [29], [30] or amorphous carbonated apatite [13]. It has been explained that the biodegradable characteristics of OCP are acquired via its resorption by osteoclast-like multinucleated giant cells (MNGCs) in bone marrow spaces [6], [16], [32] after a larger amount of new bone deposition compared with the amount by HA [16], in addition to its soluble nature in physiological condition. However, it is still uncertain whether the enhanced bone formation is induced coupled with osteoclastic resorption of OCP in not only bone marrow spaces but also in an environment near to intramembranous bone, such as the calvaria. The subperiosteal region of intramembranous bone is considered to be a less reactive site compared with the bone marrow site regarding bone formation [33].
The present study was designed to investigate whether synthetic OCP stimulates bone deposition on its surfaces coupled with osteoclastic cell resorption and/or biodegradation compared with synthetic HA when implanted in the subperiosteal region of mouse calvaria. The characteristics of OCP as a bone substitute material were compared with non-sintered HA histochemically and histomorphometrically. It was reported that synthetic OCP enhances bone formation more than commercially available sintered porous HA ceramics in rabbit bone marrow [16] and in rat critical-sized calvaria bone defects [34]. In the present study, synthetic non-sintered HA was used as a control for OCP because it is known that non-sintered HA composed of nano-particles is relatively soluble compared with dense sintered HA ceramic [30]. Therefore, this type of HA could be a control material for OCP regarding their capability to induce bone deposition and be replaced with new bone over time.
Section snippets
Materials and their characterization
OCP and HA were prepared according to methods described previously [5]. Briefly, OCP was synthesized by mixing Ca solution with phosphate solution at 70 °C and pH 5–6 [5]. HA was synthesized by mixing Ca solution with phosphate solution at 80 °C by adjusting pH 8.5–9.5 by ammonium water [5], [35]. The synthetics were well washed with water, filtered and then dried at 120 °C. The dried cakes were ground using a pestle and mortar, and the ground granules between 16 and 32 mesh sizes (granule sizes
Characterization of synthetic OCP and HA used for implantation
The characterization of synthetic OCP and HA, including chemical composition and microstructure, is summarized in Table 1. Ca and phosphorus contents in these calcium phosphates were determined using inductively coupled plasma analysis [5]. Ca/P molar ratios were close to those of their stoichiometric compositions. Both OCP and HA were porous materials, with >75% porosity (78.4% in OCP granules and 76.5% in HA granules). The average pore diameter of OCP (0. 275 μm) was higher than that of HA
Discussion
The present study provided evidence that synthetic OCP significantly enhances bone deposition on its surface, more than synthetic non-sintered HA, when implanted in the subperiosteal region of mouse calvaria for up to 21 days. The enhanced bone formation was coupled with its own biodegradation facilitated by resorption by TRAP-positive osteoclast-like cells. TRAP-positive osteoclast-like cells appeared prior to the attachment of ALP-positive osteoblastic cells to OCP. Previous study confirmed
Acknowledgments
This study was supported in part by Grants-in-Aid (17076001, 18659567, 19390490, 20659304) from the Ministry of Education, Science, Sports and Culture of Japan. The authors thank Masatoshi Yamada, JGC Corporation, for assistance in SEM-EPMA study.
References (60)
- et al.
Raman spectroscopic evidence for octacalcium phosphate and other transient mineral species deposited during intramembranous mineralization
Bone
(2006) Transient precursor strategy in mineral formation of bone
Bone
(2006)Transient precursor strategy or very small biological apatite crystals?
Bone
(2007)Human osteoblast response to pulsed laser deposited calcium phosphate coatings
Biomaterials
(2005)- et al.
Bone formation enhanced by implanted octacalcium phosphate involving conversion into Ca-deficient hydroxyapatite
Biomaterials
(2006) - et al.
Maclura pomifera agglutinin-binding glycoconjugates on converted apatite from synthetic octacalcium phosphate implanted into subperiosteal region of mouse calvaria
Bone Miner
(1993) - et al.
Bone tissue engineering on amorphous carbonated apatite and crystalline octacalcium phosphate-coated titanium discs
Biomaterials
(2005) - et al.
Influence of calcium phosphate crystal assemblies on the proliferation and osteogenic gene expression of rat bone marrow stromal cells
Biomaterials
(2007) - et al.
Bone marrow cell gene expression and tissue construct assembly using octacalcium phosphate microscaffolds
Biomaterials
(2006) - et al.
The effects of magnesium and fluoride on the hydrolysis of octacalcium phosphate
Arch Oral Biol
(1992)
Crystal chemistry of octacalcium phosphate
Prog Crystal Growth Charact
The slow resorption with replacement by bone of a hydrothermally synthesized pure calcium-deficient hydroxyapatite
Biomaterials
Elevated extracellular calcium stimulates secretion of bone morphogenetic protein 2 by a macrophage cell line
Biochem Biophys Res Commun
NK Cell lines and primary cell cultures in the study of bone cell biology
Mol Cell Endocrinol
Bioactive glass ceramics: properties and applications
Biomaterials
Self-organization mechanism in a bone-like hydroxyapatite/collagen nanocomposite synthesized in vitro and its biological reaction in vivo
Biomaterials
Effect of partial hydrolysis of octacalcium phosphate on its osteoconductive characteristics
Biomaterials
Inorganic phosphate added exogenously or released from beta-glycerophosphate initiates mineralization of osteoid nodules in vitro
Bone Miner
Inorganic phosphate induces apoptosis of osteoblast-like cells in culture
Bone
Infra-red spectra of hydroxyapatite, octacalcium phosphate and pyrolysed octacalcium phosphate
Arch Oral Biol
A solid-state NMR investigation of the structure of nanocrystalline hydroxyapatite
Magn Reson Chem
Bone formation on synthetic precursors of hydroxyapatite
Tohoku J Exp Med
Bone tissue reaction of octacalcium phosphate
Implantation of octacalcium phosphate (OCP) in rat skull defects enhances bone repair
J Dent Res
In vitro and in vivo degradation of biomimetic octacalcium phosphate and carbonate apatite coatings on titanium implants
J Biomed Mater Res A
Influence of octacalcium phosphate coating on osteoinductive properties of biomaterials
J Mater Sci Mater Med
Comparative study on osteoconductivity by synthetic octacalcium phosphate and sintered hydroxyapatite in rabbit bone marrow
Calcif Tissue Int
Dose-dependent osteogenic effect of octacalcium phosphate on mouse bone marrow stromal cells
Tissue Eng Part A
Phase transformation of octacalcium phosphate in vivo and in vitro
Dent Mater J
Solution-mediated transformation of octacalcium phosphate (OCP) to apatite
Scanning Electron Microsc
Cited by (70)
The material design of octacalcium phosphate bone substitute: increased dissolution and osteogenecity
2023, Acta BiomaterialiaCitation Excerpt :It can be concluded that higher solubility in the calcium phosphate material is an essential factor leading to the induction of new bone tissue, but the higher osteogenic capacity of the material is more essential [32]. When OCP is placed on the surface of cortical bones, osteoblasts initiate bone formation from the existing bone toward the OCP or from the OCP surface [17,72]. Transmission electron microscopy (TEM) of the implanted OCP area using decalcified specimens revealed that fine filaments and granular materials in the newly formed bone matrix were localized at the site of osteogenesis adjacent to the OCP surface [17], the structure of which was very similar to the components of the initial locus of the intramembranous osteogenesis (bone nodules) [75,76].
Acceleration of bone formation by octacalcium phosphate composite in a rat tibia critical-sized defect
2022, Journal of Orthopaedic TranslationCitation Excerpt :Previous studies have reported that OCP promotes osteoblast differentiation in vitro [13]. Studies have shown that the use of OCP as a bone substitute material is of interest from the point of view of osteogenesis in intramembranous ossification [36,37]; however, its clinical utility is still unclear and there is a lack of evidence compared to β-TCP or calcium sulfate, which are widely used in orthopedics. Regarding the phase composition, OCP has a two-layered structure of hydrate and apatite layers, and the apatite layers are involved in ion–exchange reactions at the phase boundary between the material and given environment [38].
Electrospun poly(3-hydroxybutyrate-co-4-hydroxybutyrate) /Octacalcium phosphate Nanofibrous membranes for effective guided bone regeneration
2020, Materials Science and Engineering CEnhanced biocompatibility and improved osteogenesis of coralline hydroxyapatite modified by bone morphogenetic protein 2 incorporated into a biomimetic coating
2019, Materials Science and Engineering CNovel scaffold composites containing octacalcium phosphate and their role in bone repair
2019, Octacalcium Phosphate Biomaterials: Understanding of Bioactive Properties and Application