The material design of octacalcium phosphate bone substitute: increased dissolution and osteogenecity

Octacalcium phosphate (OCP) has been advocated as a precursor of bone apatite crystals. Recent studies have shown that synthetic OCP exhibits highly osteoconductive properties as a bone substitute material that stems from its ability to activate bone tissue-related cells, such as osteoblasts, osteocytes, and osteoclasts. Accumulated experimental evidence supports the proposition that the OCP-apatite phase conversion under physiological conditions increases the stimulatory capacity of OCP. The conversion of OCP progresses by hydrolysis toward Ca-deficient hydroxyapatite with Ca²⁺ ion incorporation and inorganic phosphate ion release with concomitant increases in the solid Ca/P molar ratio, specific surface area, and serum protein adsorption affinity. The ionic dissolution rate during the hydrolysis reaction was controlled by introducing a high-density edge dislocation within the OCP lattice by preparing it through co-precipitation with gelatin. The enhanced dissolution intensifies the material biodegradation rate and degree of osteogenecity of OCP. Controlling the biodegradation rate relative to the dissolution acceleration may be vital for controlling the osteogenecity of OCP materials. This study investigates the effects of the ionic dissolution of OCP, focusing on the structural defects in OCP, as the enhanced metastability of the OCP phase modulates biodegradability followed by new bone formation. STATEMENT OF SIGNIFICANCE: Octacalcium phosphate (OCP) is recognized as a highly osteoconductive material that is biodegradable by osteoclastic resorption, followed by new bone formation by osteoblasts. However, if the degradation rate of OCP is increased by maintaining the original osteoconductivity or acquiring a bioactivity better than its current properties, then early replacement with new bone can be expected. Although cell introduction or growth factor addition by scaffold materials is the standard method for tissue engineering, material activity can be augmented by introducing dislocations into the lattice of the OCP. This review article summarizes the effects of introducing structural defects on activating OCP, which was obtained by co-precipitation with gelatin, as a bone substitute material and the mechanism of improved bone replacement performance.

Effect of Octacalcium Phosphate Crystals on the Osteogenic Differentiation of Tendon Stem/Progenitor Cells In Vitro

Synthetic octacalcium phosphate (OCP) activates bone tissue-related cells, such as osteoblasts, osteoclasts, and vascular endothelial cells. However, the effect of OCP on tendon-related cell activation remains unknown. This study examined the response of rat tendon stem/progenitor cells (TSPCs) to OCP and related calcium phosphate crystals in vitro. TSPCs were cultured with OCP and Ca-deficient hydroxyapatite (CDHA) obtained from the original OCP hydrolysis to assess the activity of alkaline phosphatase (ALP) and the expression of osteogenesis-related genes. Compared with CDHA, the effect of OCP on promoting the osteogenic differentiation of TSPCs was apparent: the ALP activity and mRNA expression of RUNX2, Col1a1, OCN, and OPN were higher in OCP than in CDHA. To estimate the changes in the chemical environment caused by OCP and CDHA, we measured the calcium ion (Ca²⁺) and inorganic phosphate (Pi) ion concentrations and pH values of the TSPCs medium. The results suggest that the difference in the osteogenic differentiation of the TSPCs is related to the ionic environment induced by OCP and CDHA, which could be related to the progress of OCP hydrolysis into CDHA. These results support the previous in vivo observation that OCP has the healing function of rabbit rotator cuff tendon in vivo.

Osteogenic capacity of octacalcium phosphate involving macrophage polarization

Previous research has found that octacalcium phosphate (OCP) increases macrophage accumulation and alters the initial inflammatory response. However, the role of the immune response induced by OCP in osteogenesis remains unknown. This study investigated the behavior of macrophages and bone regeneration capacity during the early inflammatory stage of OCP-mediated osteogenesis. To assess the change in macrophage polarization and osteogenic capacity, we used a standardized rat defect model filled with OCP or calcium-deficient hydroxyapatite (CDHA)-a material obtained through the hydrolysis of the original OCP. OCP or CDHA granules were incubated with RAW264 cells for 5 days to investigate the effect of physicochemical characteristics on macrophage cytokine/chemokine expression in vitro. Our in vivo results show that due to the OCP implantation, macrophages in the rat tibial defect area tend to polarize to the M2 phenotype (anti-inflammatory) and inhibit the formation of the M1 phenotype (pro-inflammatory). In comparison to CDHA, OCP exhibited superior bone regeneration potential due to its rapid promotion of cortical bone healing and stimulation of macrophage-related growth factors. Furthermore, our in vitro results have shown that OCP regulates the expression of macrophage chemokines over time. Compared to incubation with CDHA, incubation with OCP caused changes in the ionic microenvironment. These findings suggest that the OCP-mediated macrophage polarization and secretion profile not only regulate immune function but also positively affect osteogenesis.

Acceleration of bone formation by octacalcium phosphate composite in a rat tibia critical-sized defect

Background

The osteogenic capabilities and biodegradability of octacalcium phosphate (OCP) composites make them unique. Despite the excellent characteristics of OCP, their use is limited due to handling difficulties. In this study, we aimed to evaluate and compare three types of OCPs (cemented OCP (C-OCP), C-OCP with collagen (OCP/Col), and synthetic OCP (S-OCP) with alginate (OCP/Alg)) versus commercially available β-tricalcium phosphate (β-TCP) regarding their potential to accelerate bone formation in defective rat tibias.

Methods

The specimens with OCP composite were manufactured into 5 ​mm cubes and inserted into the segmental defects of rat tibias fixed with an external fixator. In addition, 3 ​mm-hole defects in rat tibias were evaluated to compare the graft material properties in different clinical situations. Serial X-ray studies were evaluated weekly and the tibias were harvested at postoperative 6 weeks or 8 weeks for radiologic evaluation. Histological and histomorphometric analyses were performed to evaluate the acceleration of bone formation.

Results

In the critical-defect model, OCP/Alg showed bone bridges between segmentally resected bone ends that were comparable to those of β-TCP. However, differences were observed in the residual graft materials. Most β-TCP was maintained until 8 weeks postoperatively; however, OCP/Alg was more biodegradable. In addition calcification in the β-TCP occurred at the directly contacted area between graft particles and bony ingrowth was observed in the region adjacent resected surface of tibia. In contrast, no direct bony ingrowth was observed in OCP-based materials, but osteogenesis induced from resected surface of tibia was more active. In the hole-defect model, OCP/Col accelerated bone formation. β-TCP and OCP/Alg showed similar patterns with relatively higher biodegradability. In histology, among the OCP-based materials, directly contacted new bone was formed only in OCP/Alg group. The new bone formation in the periphery area of graft materials was much more active in the OCP-based materials, and the newly formed bone showed a thicker trabecular and more mature appearance than the β-TCP group.

Conclusions

In this study, OCP/Alg was equivalent to β-TCP in the acceleration of bone formation with better biodegradability appropriate for clinical situations in different circumstances. Our OCP/Col composite showed fast degradation, which makes it unsuitable for use in mechanical stress conditions in clinical orthopedic settings.

The Translational Potential of this Article

In our research, we compared our various manufactured OCP composites to commercially available β-TCP in critical-defect rat tibia model. OCP/Col showed acceleration in hole-defect model as previous studies in dental field but in our critical-sized defect model it resorbed fast without acceleration of bony union. OCP/Alg showed matched results compared to β-TCP and relatively fast resorption so we showed market value in special clinical indication depending on treatment strategy. This is the first OCP composite study in orthopaedics with animal critical-sized tibia bone study and further study should be considered for clinical application based on this study.

Octacalcium Phosphate/Gelatin Composite (OCP/Gel) Enhances Bone Repair in a Critical-sized Transcortical Femoral Defect Rat Model

BACKGROUND

Bone grafting is widely used to treat large bone defects. A porous composite of a bioactive octacalcium phosphate material with gelatin sponge (OCP/Gel) has been shown to biodegrade promptly and be replaced with new bone both in animal models of a membranous bone defect and a long bone defect. However, it is unclear whether OCP/Gel can regenerate bone in more severe bone defects, such as a critical-size transcortical defect.

QUESTIONS/PURPOSES

Using an in vivo rat femur model of a standardized, transcortical, critical-size bone defect, we asked: Compared with a Gel control, does OCP/Gel result in more newly formed bone as determined by (1) micro-CT evaluation, (2) histologic and histomorphometric measures, and (3) osteocalcin staining and tartrate-resistant acid phosphatase staining?

METHODS

Thirty-four 12-week-old male Sprague-Dawley rats (weight 356 ± 25.6 g) were used. Gel and OCP/Gel composites were prepared in our laboratory. Porous cylinders 3 mm in diameter and 4 mm in height were manufactured from both materials. The OCP/Gel and Gel cylinders were implanted into a 3-mm-diameter transcortical critical-size bone defect model in the left rat femur. The OCP/Gel and Gel were randomly assigned, and the cylinders were implanted. The biological responses of the defect regions were evaluated radiologically and histologically. At 4 and 8 weeks after implantation, CT evaluation, histological examination of decalcified samples, and immunostaining were quantitatively performed to evaluate new bone formation and remaining bone graft substitutes and activity of osteoblasts and osteoclast-like cells (n = 24). Qualitative histological evaluation was performed on undecalcified samples at 3 weeks postimplantation (n = 10). CT and decalcified tissue analysis was not performed blinded, but an analysis of undecalcified specimens was performed under blinded conditions.

RESULTS

Radiologic analysis revealed that the OCP/Gel group showed radiopaque regions around the OCP granules and at the edge of the defect margin 4 weeks after implantation, suggesting that new bone formation occurred in two ways. In contrast, the rat femurs in the Gel group had a limited radiopaque zone at the edge of the defect region. The amount of new bone volume analyzed by micro-CT was higher in the OCP/Gel group than in the Gel group at 4 and 8 weeks after implantation (​​4 weeks after implantation: OCP/Gel versus Gel: 6.1 ± 1.6 mm 3 versus 3.4 ± 0.7 mm 3 , mean difference 2.7 [95% confidence interval (CI) 0.9 to 4.5]; p = 0.002; intraclass correlation coefficient [ICC] 0.72 [95% CI 0.29 to 0.91]; 8 weeks after implantation: OCP/Gel versus Gel: 3.9 ± 0.7 mm 3 versus 1.4 ± 1.1 mm 3 , mean difference 2.5 [95% CI 0.8 to 4.3]; p = 0.004; ICC 0.81 [95% CI 0.47 to 0.94]). Histologic evaluation also showed there was a higher percentage of new bone formation in the OCP/Gel group at 4 and 8 weeks after implantation (​​4 weeks after implantation: OCP/Gel versus Gel: 31.2% ± 5.3% versus 13.6% ± 4.0%, mean difference 17.6% [95% CI 14.2% to 29.2%]; p < 0.001; ICC 0.83 [95% CI 0.53 to 0.95]; 8 weeks after implantation: OCP/Gel versus Gel: 28.3% ± 6.2% versus 9.5% ± 1.9%, mean difference 18.8% [95% CI 11.3% to 26.3%]; p < 0.001; ICC 0.90 [95% CI 0.69 to 0.97]). Bridging of the defect area started earlier in the OCP/Gel group than in the Gel group at 4 weeks after implantation. Osteocalcin immunostaining showed that the number of mature osteoblasts was higher in the OCP/Gel group than in the Gel group at 4 weeks (OCP/Gel versus Gel: 42.1 ± 6.5/mm 2 versus 17.4 ± 5.4/mm 2 , mean difference 24.7 [95% CI 16.2 to 33.2]; p < 0.001; ICC 0.99 [95% CI 0.97 to 0.99]). At 4 weeks, the number of osteoclast-like cells was higher in the OCP/Gel composite group than in the Gel group (OCP/Gel versus Gel: 3.2 ± 0.6/mm 2 versus 0.9 ± 0.4/mm 2 , mean difference 2.3 [95% CI 1.3 to 3.5]; p < 0.001; ICC 0.79 [95% CI 0.35 to 0.94]).

CONCLUSION

OCP/Gel composites induced early bone remodeling and cortical bone repair in less time than did the Gel control in a rat critical-size, transcortical femoral defect, suggesting that OCP/Gel could be used as a bone replacement material to treat severe bone defects.

CLINICAL RELEVANCE

In a transcortical bone defect model of critical size in the rat femur, the OCP/Gel composite demonstrated successful bone regeneration. Several future studies are needed to evaluate the clinical application of this interesting bone graft substitute, including bone formation capacity in refractory fracture and spinal fusion models and the comparison of bone strength after repair with OCP/Gel composite to that of autologous bone.

A multi-modal exploration of heterogeneous physico-chemical properties of DCIS breast microcalcifications

Ductal carcinoma in situ (DCIS) is frequently associated with breast calcification. This study combines multiple analytical techniques to investigate the heterogeneity of these calcifications at the micrometre scale. X-ray diffraction, scanning electron microscopy and Raman and Fourier-transform infrared spectroscopy were used to determine the physicochemical and crystallographic properties of type II breast calcifications located in formalin fixed paraffin embedded DCIS breast tissue samples. Multiple calcium phosphate phases were identified across the calcifications, distributed in different patterns. Hydroxyapatite was the dominant mineral, with magnesium whitlockite found at the calcification edge. Amorphous calcium phosphate and octacalcium phosphate were also identified close to the calcification edge at the apparent mineral/matrix barrier. Crystallographic features of hydroxyapatite also varied across the calcifications, with higher crystallinity centrally, and highest carbonate substitution at the calcification edge. Protein was also differentially distributed across the calcification and the surrounding soft tissue, with collagen and β-pleated protein features present to differing extents. Combination of analytical techniques in this study was essential to understand the heterogeneity of breast calcifications and how this may link crystallographic and physicochemical properties of calcifications to the surrounding tissue microenvironment.

Differentiation of committed osteoblast progenitors by octacalcium phosphate compared to calcium-deficient hydroxyapatite in Lepr-cre/Tomato mouse tibia

This study aimed to investigate the accumulation and differentiation of mesenchymal stem cells (MSCs) around octacalcium phosphate (OCP) compared with those around calcium-deficient hydroxyapatite (CDHA), a material obtained through hydrolysis of the original OCP. Leptin receptor (Lepr)-expressing bone marrow-derived MSCs around the OCP and CDHA were pursued utilizing genetically modified Lepr-cre/Tomato mice. OCP and CDHA granules were implanted into the tibia defect of the mice for 10 weeks and subjected to histomorphometric and immunohistochemical analyses. The structural properties of OCP and CDHA after inoculation into mouse subcutaneous tissue (until 4 weeks) or culture mediums (14 days) were analyzed using physicochemical techniques. In vitro osteoblastic differentiation of primary MSCs was examined with the materials for 14 days. While Lepr-cre/Tomato positive cells (red) accumulated around both OCP and CDHA, Lepr and osteocalcin double-positive osteoblastic cells (yellow) were significantly more abundant around OCP than around CDHA in the early implantation period. OCP enhanced the osteoblastic differentiation of MSCs more than CDHA in vitro. Physicochemical and structual analyses provided evidence that OCP tended to convert to the apatitic phase in the tested physiological environments. The higher osteoconductivity of OCP originated from a capacity-enhancing osteoblastic differentiation of committed osteoblast progenitors in bone marrow accompanied by OCP hydrolysis. STATEMENT OF SIGNIFICANCE: MSCs play a key role in bone regeneration through osteoblastic differentiation. Calcium phosphates have been widely applied as bone substitute materials, and OCP has a better ability to promote osteoblast differentiation of MSCs than that of HA in vitro. However, it is not clear how MSCs accumulate in the bone marrow and differentiate into osteoblasts during bone regeneration in vivo. In this study, we focused on the leptin receptor, a marker of bone marrow-derived MSCs. Using genetically modified mice labeled with the red fluorescent protein Tomato, we observed the accumulation of MSCs around calcium phosphates implanted in tibia bone defects and their differentiation into osteoblasts.

Macrophage Polarization Related to Crystal Phases of Calcium Phosphate Biomaterials

Calcium phosphate (CaP) materials influence macrophage polarization during bone healing. However, the effect of the crystal phase of CaP materials on the immune response of bone remains unclear. In this study, the effect of the crystal phases of CaP materials on the regulation of macrophage polarization was investigated. Human THP-1 cells and mouse RAW 264 cells were cultured with octacalcium phosphate (OCP) and its hydrolyzed form Ca-deficient hydroxyapatite to assess the expression of pro-inflammatory M1 and anti-inflammatory M2 macrophage-related genes. OCP inhibited the excessive inflammatory response and switched macrophages to the anti-inflammatory M2 phenotype, which promoted the expression of the interleukin 10 (IL10) gene. In contrast, HL stimulated an excessive inflammatory response by promoting the expression of pro-inflammatory M1 macrophage-related genes. To observe changes in the microenvironment induced by OCP and HL, inorganic phosphate (Pi) and calcium ion (Ca²⁺) concentrations and pH value in the medium were measured. The expression of the pro-inflammatory M1 macrophage-related genes (tumor necrosis factor alpha (TNFα) and interlukin 1beta (IL1β)) was closely related to the increase in ion concentration caused by the increase in the CaP dose. Together, these results suggest that the microenvironment caused by the crystal phase of CaP materials may be involved in the immune-regulation capacity of CaP materials.

Bone Tissue Response to Different Grown Crystal Batches of Octacalcium Phosphate in Rat Long Bone Intramedullary Canal Area

The microstructure of biomaterials influences the cellular and biological responses in the bone. Octacalcium phosphate (OCP) exhibits higher biodegradability and osteoconductivity than hydroxyapatite (HA) during the conversion process from OCP to HA. However, the effect of the microstructure of OCP crystals on long tubular bones has not been clarified. In this study, two types of OCPs with different microstructures, fine-OCP (F-OCP) and coarse-OCP (C-OCP), were implanted in rat tibia for 4 weeks. F-OCP promoted cortical bone regeneration compared with C-OCP. The osteoclasts appearance was significantly higher in the C-OCP group than in the control group (defect only) at 1-week post-implantation. To investigate whether the solubility equilibrium depends on the different particle sizes of OCPs, Nano-OCP, which consisted of nanometer-sized OCPs, was prepared. The degree of supersaturation (DS) tended to decrease modestly in the order of C-OCP, F-OCP, and Nano-OCP with respect to HA and OCP in Tris-HCl buffer. F-OCP showed a higher phosphate ion concentration and lower calcium ion concentration after immersion in the buffer than C-OCP. The crystal structures of both OCPs tended to be converted to HA by rat abdominal implantation. These results suggest that differences in the microstructure of OCPs may affect osteoclastogenesis and result in osteoconductivity of this material in long tubular bone by altering dissolution behavior.

Involvement of distant octacalcium phosphate scaffolds in enhancing early differentiation of osteocytes during bone regeneration

This study hypothesized that distant octacalcium phosphate (OCP) scaffolds may enhance osteocyte differentiation in newly formed bone matrices. The results obtained were compared with those of Ca-deficient hydroxyapatite (OCP hydrolyzate, referred to as HL hereafter). Granular OCP and HL, 300-500 µm in diameter, were implanted in critical-sized rat calvarial defects for eight weeks and subjected to histology, immunohistochemistry, histomorphometry, and transmission electron microscopy (TEM). Early osteocyte differentiation from an osteoblastic cell line (IDG-SW3) was examined using materials without contacting the surfaces for 10 days. The material properties and the medium composition were analyzed through selected area electron diffraction (SAED) using TEM observation and curve fitting of Fourier transform infrared (FT-IR) spectroscopy. The number of positive cells of an osteocyte earlier differentiation marker podoplanin (PDPN) in bone matrices, along the direction of bone formation, was significantly higher in OCP than that in HL. The ultrastructure around the OCP surfaces observed by TEM showed the infiltration of some cells, including osteocytes adjacent to the OCP surface layers. The OCP structure remained unchanged by SAED analysis. Nanoparticle deposition and hydrolysis on OCP surfaces were detected by TEM and FT-IR, respectively, during early osteocyte differentiation in vitro. The medium saturation degree varied in accord with ionic dissolution, resulting in possible hydroxyapatite formation on OCP but not on HL. These results suggested that OCP stimulates early osteocyte differentiation in the bone matrix from a distance through its metastable chemical properties. STATEMENT OF SIGNIFICANCE: This study demonstrated that octacalcium phosphate (OCP) implanted in critical-sized rat calvaria bone defects is capable of enhancing the early differentiation of osteocytes embedded in newly formed bone matrices, even when the surface OCP is separated from the osteocytes. This prominent bioactive property of OCP was demonstrated by comparing the in vivo and in vitro performances with a control material, Ca-deficient hydroxyapatite (OCP hydrolyzate). The findings were elucidated by histomorphometry, which analyzed the differentiation of osteocytes along the parallel direction of new bone growth by osteoblasts. Therefore, OCP should stimulate osteocyte differentiation through ionic dissolution even in vivo owing to its metastable chemical properties, as previously reported in an in vitro study (Acta Biomater 69:362, 2018).

Mutual chemical effect of autograft and octacalcium phosphate implantation on enhancing intramembranous bone regeneration

This study examined the effect of a mixture of octacalcium phosphate (OCP) and autologous bone on bone regeneration in rat calvaria critical-sized defect (CSD). Mechanically mixed OCP and autologous bone granules (OCP+Auto), approximately 500 to 1000 μm in diameter, and each individual material were implanted in rat CSD for 8 weeks, and subjected to X-ray micro-computed tomography (micro-CT), histology, tartrate-resistant acid phosphatase (TRAP) staining, and histomorphometry for bone regeneration. Osteoblastic differentiation from mesenchymal stem cells (D1 cells) was examined in the presence of non-contacting materials by alkaline phosphatase (ALP) activity for 21 days. The material properties and medium composition before and after the incubation were determined by selected area electron diffraction (SAED) under transmission electron microscopy (TEM), Fourier transform infrared (FT-IR) spectroscopy, scanning electron microscopy (SEM), and chemical analysis. The results showed that while bone formation coupled with TRAP-positive osteoclastic resorption and cellular ALP activity were the highest in the Auto group, a positive effect per OCP weight or per autologous bone weight on ALP activity was found. Although the OCP structure was maintained even after the incubation (SAED), micro-deposits were grown on OCP surfaces (TEM). Fibrous tissue was also exposed on the autologous bone surfaces (SEM). Through FT-IR absorption, it was determined that bone mineral-like characteristics of the phosphate group increased in the OCP + Auto group. These findings were interpreted as a structural change from OCP to the apatitic phase, a conclusion supported by the medium degree of saturation changes. The results demonstrate the mutual chemical effect of mixing OCP with autologous bone as an active bone substitute material.

Impact of simultaneous hydrolysis of OCP and PLGA on bone induction of a PLGA-OCP composite scaffold in a rat femoral defect

Effect of the simultaneous hydrolysis of octacalcium phosphate (OCP) and poly (lactic-co-glycolic acid) (PLGA) was investigated on its osteoconductivity. PLGA soaked in phosphate buffered saline with 0%, 20%, and 40% OCP at 37°C for eight weeks indicated that when the OCP dose was increased, 1) the weight loss of PLGA increased, 2) the glass transition temperature of the PLGAs decreased, 3) the saturation degree in the saline moved to nearly saturated condition with respect to hydroxyapatite (HA) but was undersaturated with respect to OCP, and 4) OCP tended to convert to HA by X-ray diffraction and Fourier transform infrared spectroscopy. OCP/PLGA composites of 20% and 40% with more than 92% porosity were produced by combining OCP granules with 1,4-dioxane-solubilizing PLGA followed by lyophilization and then subjected to four- and eight-week in vivo implantation tests in 3 mm diameter rat femora defects. Microfocus X-ray computed tomography, histochemical and histomorphometric analyses showed that while bone formation was very limited with PLGA implantation, the extent of repair tended to increase with increasing OCP content in the PLGA, coupled with PLGA degradation, and bridge the defects with trabecular bone. Tartrate-resistant acid phosphatase-positive osteoclast-like cells were accumulated four weeks after implantation, while osteocalcin-positive osteoblastic cells appeared later at eight weeks, especially in 40% OCP/PLGA. These results suggest that OCP hydrolysis, with phosphate ion release, enhances PLGA hydrolysis, probably through the acid catalysis function of the protons supplied during the hydrolysis of OCP, thereby inducing PLGA biodegradation and new bone formation in the femoral defects. STATEMENT OF SIGNIFICANCE: Octacalcium phosphate (OCP) enhances osteoblasts and osteocytes differentiations during its hydrolysis accompanying inorganic ions exchange in this material. The present study found that the advancement of OCP hydrolysis under physiological conditions had an effect on poly (lactic-co-glycolic acid) (PLGA) degradation through its chemical environmental change around OCP, which was ascertained by the decreases in weight loss and glass transition temperature of PLGA with increasing the dose of OCP co-present. Rat femur-penetrated standardized severe defects were found to repair through bridging the cortical region defect margin. PLGA degradation could be enhanced through an acid catalyst function by protons derived from inorganic phosphate (Pi) ions through OCP hydrolysis under bone forming condition, resulting in showing a prominent bone regenerative capacity in OCP/PLGA composite materials.

Resorption analysis of deproteinized cancellous bovine bone

Commercially available deproteinized cancellous bovine bone (DPBB) has been indicated as non-absorbable bone filling materials. Stoichiometric hydroxyapatite (HA) was not resorbed by osteoclasts while calcium-deficient and carbonate-rich apatite converted from octacalcium (OCP hydrolysate) was resorbed by osteoclasts. We analyzed the chemical composition of DPBB and compared the tissue reactions around two materials after implanting into mouse bone marrow. X-ray diffraction analysis and Fourier transform infrared spectroscopy showed that DPBB was a carbonate-rich apatite. Micro-CT analysis indicated the massive bone formation on both materials at 2 weeks, then gradually resorbed. At 12 weeks, osteoclasts were directly attached to both materials. The ultrastructure of ruffled borders on DPBB was identical to osteoclasts resorbing normal bone while ruffled border on OCP hydrolysate showed irregular shape. These results indicated that DPBB was the absorbed material and that the structure of ruffled border of osteoclasts might be regulated by the size or orientation of HA.

Octacalcium phosphate bone substitute materials: Comparison between properties of biomaterials and other calcium phosphate materials

Octacalcium phosphate (OCP) is a material that can be converted to hydroxyapatite (HA) under physiological environments and is considered a mineral precursor to bone apatite crystals. The structure of OCP consists of apatite layers stacked alternately with hydrated layers, and closely resembles the structure of HA. The performance of OCP as a bone substitute differs from that of HA materials in terms of their osteoconductivity and biodegradability. OCP manifests a cellular phagocytic response through osteoclast-like cells similar to that exhibited by the biodegradable material β-tricalcium phosphate (β-TCP). The use of OCP for human cranial bone defects involves using its granule or composite form with one of the natural polymers, viz., the reconstituted collagen. This review article discusses the differences and similarities in these calcium phosphate (Ca-P)-based materials from the viewpoint of the structure and their material chemistry, and attempts to elucidate why Ca-P materials, particularly OCP, display unique osteoconductive property.

Adsorption of Serum Albumin onto Octacalcium Phosphate in Supersaturated Solutions Regarding Calcium Phosphate Phases

Octacalcium phosphate (OCP) has been shown to enhance new bone formation, coupled with its own biodegradation, through osteoblasts and osteoclast-like cell activities concomitant with de novo hydroxyapatite (HA) formation and serum protein accumulation on its surface. However, the nature of the chemical environment surrounding OCP and how it affects its metabolism and regulates protein accumulation is unknown. The present study examined how the degree of supersaturation (DS) affects the bovine serum albumin (BSA) adsorption onto OCP in 150 mM Tris-HCl buffer at 37 °C and pH 7.4, by changing the Ca²⁺ ion concentration. The amount of BSA adsorbed onto OCP increased as the DS increased. In addition, the amount of newly formed calcium phosphate, which could be OCP, was increased, not only by increases in DS, but also at lower equilibrium concentrations of BSA. The increased adsorption capacity of BSA was likely related to the formation of calcium phosphate on the adsorbed OCP. Together the results suggested that the formation of new calcium phosphate crystals is dependent on both the DS value and the adsorbate protein concentration, which may control serum protein accumulation on the OCP surface in vivo.

Angiogenesis involvement by octacalcium phosphate-gelatin composite-driven bone regeneration in rat calvaria critical-sized defect

Effect of octacalcium phosphate/gelatin composite (OCP/Gel) on angiogenesis was studied by its implantation in rat calvaria critical-sized defect in relation to bone regeneration for 2 and 4 weeks. The implantation of OCP/Gel disks was analyzed by radiomorphometry using a radiopaque material perfusion (Microfil®) method and histomorphometry by hematoxylin and eosin-staining before and after the decalcification. Effect of the OCP dose in the range up to 4 mg per well on the capillary-like tube formation by human umbilical vein endothelial cells (HUVECs) was also examined in a transwell cell culture. The results showed that the blood vessels formation by OCP/Gel group was significantly higher at 2 weeks than other groups but decreased at 4 weeks during the advancement of new bone formation. The capillary-like tube formation was highest in an OCP dose of 1 mg per well while other OCP doses above or below 1 mg did not show such a stimulatory effect. The results established both in vivo and in vitro confirmed that OCP has a positive effect on angiogenesis during bone regeneration in a suitable dose ranges, suggesting that the angiogenesis stimulated by OCP could be involved in the OCP/Gel-enhanced bone regeneration. STATEMENT OF SIGNIFICANCE: We have reported that octacalcium phosphate (OCP) materials display stimulatory capacities on the bone tissue-related cells. However, the effect of OCP on the angiogenesis and its relation to the OCP-enhanced bone regeneration is unknown. This study confirmed the capacity of OCP on angiogenesis before increasing the new bone mass after the implantation of a composite of OCP and gelatin (OCP/Gel). The blood vessels formation took place associated with the area beginning of the new bone formation, which finally decreased together with development of bone formation. Because OCP was ascertained stimulating the capillary-like tube formation in HUVEC culture with a certain OCP dose, the present study is the first report showing the capacity of OCP on angiogenesis during the OCP/Gel-enhanced bone regeneration.

Effect of calcium phosphate phases affecting the crosstalk between osteoblasts and osteoclasts in vitro

Previous studies have reported that octacalcium phosphate (OCP) enhances osteoblast differentiation and osteoclast formation during the hydrolysis process to hydroxyapatite (HA). However, the crystal phases that affect the crosstalk between osteoclasts and osteoblasts are unknown, which should determine the bone substitute material's property of OCP. The present study was designed to investigate whether the chemical composition and crystal structure of calcium phosphates affect osteoclast formation and the osteoclast-osteoblast crosstalk. Biodegradable β-tricalcium phosphate (β-TCP) was used as the control material. Osteoclasts were cultured on HA/OCP or HA/TCP disks and their cellular responses were assessed. Both OCP and β-TCP had a similar ability to create multinucleated osteoclasts. However, OCP promoted the expression of complement component 3a (C3a), a positive coupling factor, in osteoclasts, whereas β-TCP enhanced that of EphrinB2 (EfnB2) and collagen triple helix repeat containing 1 (Cthrc1). During osteoclast culture, phosphate ions were released from the crystals, and OCP-HA conversion was advanced in HA/OCP mixtures and OCP. X-ray diffraction analysis revealed no remarkable changes in the crystal structures of HA/TCP mixtures and β-TCP before and after osteoclast culture. These results indicate that the distinct chemical environment induced by the calcium phosphate phases affects the crosstalk between osteoclasts and osteoblasts. © 2019 Wiley Periodicals, Inc. J Biomed Mater Res Part A: 107A: 1001-1013, 2019.

Sodium inhibits the formation of ammonium-substituted solid solutions of octacalcium phosphate by filling its substitution site

Octacalcium phosphate (OCP) is a layered type of calcium phosphate that shows promise for pharmaceutical and biomedical applications because it offers both excellent biocompatibility and a unique, robust crystal structure that readily accepts substitution by various molecules. Although several cations can be incorporated into the OCP crystal lattice by ionic substitution, little is known about the relative probability of different ions to substitute into the OCP crystal lattice. In this study, we focus on Na and NH4, which are known to enter the OCP crystal lattice by ionic substitution. We investigate which of these two is most likely to substitute into OCP in a system containing both ions. In this work, OCP is fabricated from monocalcium phosphate monohydrate via hydrolysis in a solution containing 1 mol L-1 (NH4)2HPO4 with NaCl concentrations ranging from 0 to 100 mmol L-1. X-ray diffraction and thermal analyses indicate that the structure characteristic of NH4 ionic substitution in OCP is attenuated as the Na concentration increases. Furthermore, when the Na concentration exceeds 50 mmol L-1 (Na/NH4 ≥ 1/20), the structure characteristic of NH4 ionic substitution into OCP almost completely disappears. These results indicate that Na is more likely to be ionically substituted into the OCP crystal structure than NH4 and thereby inhibits the structural change of OCP caused by ionic substitution of NH4.

Current bone substitutes for implant dentistry

PURPOSE

Alveolar ridge augmentation is essential for success in implant therapy and depends on the biological performance of bone graft materials. This literature review aims to comprehensively explain the clinically relevant capabilities and limitations of currently available bone substitutes for bone augmentation in light of biomaterial science.

STUDY SELECTION

The biological performance of calcium phosphate-based bone substitutes was categorized according to space-making capability, biocompatibility, bioabsorption, and volume maintenance over time. Each category was reviewed based on clinical studies, preclinical animal studies, and in vitro studies.

RESULTS

Currently available bone substitutes provide only osteoconduction as a scaffold but not osteoinduction. Particle size, sensitivity to enzymatic or chemical dissolution, and mechanical properties affect the space-making capability of bone substitutes. The nature of collagen fibers, particulate size, and release of calcium ions influence the biocompatibility of bone substitutes. Bioabsorption of bone substitutes is determined by water solubility (chemical composition) and acid resistance (integrity of apatite structure). Bioabsorption of remnant bone substitute material and volume maintenance of the augmented bone are inversely related.

CONCLUSION

It is necessary to improve the biocompatibility of currently available bone substitutes and to strike an appropriate balance between bioabsorption and volume maintenance to achieve ideal bone remodeling.

Application of an indicator-immobilized-gel-sheet for measuring the pH surrounding a calcium phosphate-based biomaterial

The present study was designed to investigate the local microenvironment of octacalcium phosphate in a granule form upon biomolecules adsorption utilizing an indicator-immobilized-gel-sheet for measuring pH. We previously showed that octacalcium phosphate enhances bone regeneration during its progressive hydrolysis into hydroxyapatite if implanted in bone defects. The gel-sheet was made from a photocrosslinkable prepolymer solution, which can easily immobilize a pH indicator (bromothymol blue; BTB) in the hydrogel. The indicator-immobilized-gel-sheet was mounted on a biochip which was made of polydimethylsiloxane (PDMS) with a flow channel. The pH value was calculated by detecting the color changes in the gel-sheet and displayed as the pH distribution. After pre-adsorption of bovine albumin, β-lactoglobuline or cytochrome C onto octacalcium phosphate granules, the granules with the gel-sheet were further incubated in Tris-HCl buffer solution in the absence or presence of fluoride, known as an accelerator of octacalcium phosphate hydrolysis. pH values of the gel-sheet surrounding octacalcium phosphate granules showed a decrease from pH 7.4 to 6.6 in relation to the proteins adsorbed. Overall, the proposed pH-sensitive gel can be used to detect the pH around octacalcium phosphate granules with a high spatial resolution.