Application of low-crystalline carbonate apatite granules in 2-stage sinus floor augmentation: a prospective clinical trial and histomorphometric evaluation

In vivo trial of bioresorbable mesh cages contained bone graft granules in rabbit femoral bone defects

Bone graft granules implanted in bone defects come into physical contact with the host bone and form interconnected porous structure. However, there exists an accidental displacement of granules to unintended locations and leakage of granules from bone defects. Although covering the defect with a barrier membrane prevents granule emanation, this procedure is troublesome. To resolve these problems, we fabricated bioresorbable mesh cages (BRMc) in this study. Bone graft granules composed of carbonate apatite alone (Gr) and bioresorbable mesh cages (BRMc/Gr) introduced the bone graft granules and were implanted into the bone defect in the rabbit femur. Micro-computed tomography and histological analysis were conducted at 4 and 12 weeks after implantation. Osteoprogenitors in the bloodstream from the host bone passed through the pores of BRMc, penetrated the porous structure of graft granules, and might interact with individual granules. Then bone remodeling could progress actively and new bone was formed. The new bone formation was similar to the host bone at 12 weeks and there were minimal signs of local tissue inflammation. BRMc/Gr could reduce the risk of unwanted new bone formation occurring due to loss of granules from the bone defects compared with Gr because BRMc enclosed granules and prevent granules leakage from bone defects and BRMc could not induce unfavorable effects to forme new bone. Additionally, BRMc/Gr could keep granules assembled in one place, avoid displacement of granules to unintended locations, and carry easily. These results demonstrated that BRMc/Gr was effective in bone regeneration and improved clinical handling.

Biological Properties and Medical Applications of Carbonate Apatite: A Systematic Review

Bone defects represent an everyday challenge for clinicians who work in the fields of orthopedic surgery, maxillofacial and oral surgery, otorhinolaryngology, and dental implantology. Various bone substitutes have been developed and utilized, according to the needs of bone reconstructive surgery. Carbonate apatite has gained popularity in recent years, due to its excellent tissue behavior and osteoconductive potential. This systematic review aims to evaluate the role of carbonate apatite in bone reconstructive surgery and tissue engineering, analyze its advantages and limitations, and suggest further directions for research and development. The Web of Science, PubMed, and Scopus electronic databases were searched for relevant review articles, published from January 2014 to 21 July 2023. The study was conducted in accordance with the Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) guidelines. Eighteen studies were included in the present review. The biological properties and medical applications of carbonate apatite (CO3Ap) are discussed and evaluated. The majority of articles demonstrated that CO3Ap has excellent biocompatibility, resorbability, and osteoconductivity. Furthermore, it resembles bone tissue and causes minimal immunological reactions. Therefore, it may be successfully utilized in various medical applications, such as bone substitution, scaffolding, implant coating, drug delivery, and tissue engineering.

Volume change after maxillary sinus floor elevation with apatite carbonate and octacalcium phosphate

PURPOSE

Maxillary molars have low alveolar bone height diameter due to the presence of the maxillary sinus; thus, a sinus lift may be required in some cases. Changes in the volume of bone substitutes can affect the success of implant therapy. Therefore, this study aimed to compare the changes in the volume of two different bone substitutes-one based on carbonate apatite and the other on octacalcium phosphate-used in maxillary sinus floor elevation.

METHODS

Nineteen patients and 20 sites requiring maxillary sinus floor elevation were included in the study. Digital Imaging and Communications in Medicine data for each patient obtained preoperatively and immediately and 6 months postoperatively were used to measure the volume of the bone grafting material using a three-dimensional image analysis software. The immediate postoperative volume of octacalcium phosphate was 95.3775 mm3 per piece of grafting material used. It was multiplied by the number of pieces used and converted to mL to determine the immediate postoperative volume.

RESULTS

The mean resorption values of carbonate apatite and octacalcium phosphate were 12.7 ± 3.6% and 17.3 ± 3.9%, respectively. A significant difference in the amount of resorption of the two bone replacement materials was observed (P = 0.04).

CONCLUSIONS

The results of this study indicate that both bone substitute materials tend to resorb. The two bone grafting materials that are currently medically approved in Japan have not been in the market for a long time, and their long-term prognosis has not yet been reported. Further clinical data are warranted.

Carbonate apatite increases gene expression of osterix and bone morphogenetic protein 2 in the alveolar ridge after socket grafting

PURPOSE

After tooth extraction, preservation of the alveolar ridge by socket grafting attenuates bone resorption. Runt-related transcription factor 2 (RUNX2) and SP7/Osterix (OSX) are transcription factors playing an important role in osteoblast differentiation. The purpose of this study was to evaluate the effects of carbonate apatite (CO3Ap) on osteoblast-related gene and protein expression after socket grafting.

METHODS

Alveolar bone and new bone after CO3Ap grafting were collected at the time of implant placement. Levels of mRNA for RUNX2, SP7/OSX, bone morphogenetic protein 2 (BMP2), BMP7 and platelet derived growth factor B were determined by real-time PCR. Immunostaining was performed using antibodies against RUNX2, SP7/OSX, vimentin and cytokeratin. To evaluate bone resorption rates, cone-beam CT (CBCT) imaging was performed after socket grafting and before implant placement.

RESULTS

CBCT imaging showed that the average degree of bone resorption at the CO3Ap graft site was 7.15 ± 3.79%. At the graft sites, levels of SP7/OSX and BMP2 mRNA were significantly increased. Replacement of CO3Ap with osteoid was evident histologically, and in the osteoid osteoblast-like cells were stained for SP7/OSX and vimentin.

CONCLUSION

These results show that gene expression of both SP7/OSX and BMP2 can be induced by CO3Ap, suggesting that increased expression of SP7/OSX and vimentin may be involved in the BMP pathway.

Evaluation of Eruption of Permanent Teeth in Beagle Dog Extraction Sites Filled with Carbonate Apatite

Autologous bone grafting is the primary method for treating alveolar clefts. However, bone grafting materials are desired as alternatives to autogenous bone to reduce surgical invasiveness. Here, we present an animal study evaluating the effect of carbonate apatite (CA) on the spontaneous eruption of permanent teeth. The bone grafting materials included CA, natural bovine bone (BB), and hydroxyapatite (HA). In 15 8-week-old male beagle dogs, the left mandibular deciduous premolars (DP) two and three were extracted and subsequently filled with CA, BB, and HA. The animals were euthanized after a predetermined number of days, and samples were collected for microcomputed tomography and histological evaluation. Spontaneous eruption of the succeeding permanent teeth (P3 and P4) was observed in the CA group at 14 weeks. Delayed eruption of the succeeding permanent teeth was observed in the BB and HA groups. CA could serve as a viable alternative to autogenous bone for treating alveolar clefts.

Clinical outcomes of periodontal regenerative therapy with carbonate apatite granules for treatments of intrabony defects, Class II and Class III furcation involvements: A 9-month prospective pilot clinical study

Introduction

Carbonated apatite (CO3Ap) has unique properties as an alloplastic bone substitute and has been reported the safety and efficacy for bone regeneration. However, no previous studies reported the clinical application of CO3Ap for periodontal regeneration therapy. The aim of this study was to evaluate the safety and efficacy of periodontal regeneration with CO3Ap in treating intrabony defects, Class II and Class III furcation involvement (FI).

Methods

A single-arm and single-center prospective pilot clinical study was performed to verify the safety and efficacy of CO3Ap in patients with periodontitis. A total of four patients with seven teeth, including three deep intrabony defects, two Class II FI, and two Class III FI, were treated with CO3Ap. The clinical parameters, including probing pocket depth (PPD), clinical attachment level (CAL), bleeding on probing (BOP), tooth mobility (Mo), Plaque index (PI), and Gingival index (GI) were evaluated at baseline, 6 months, and 9 months after the surgery. Radiographic analysis was conducted on images of dental X-ray and cone beam computed tomography (CBCT) at baseline and 9 months post-surgery.

Results

The postoperative healing in all cases was uneventful, with no abnormal bleeding, pain, or swelling. The mean PPD reduction and CAL gain were 5.0 ± 1.0 mm, 4.5 ± 0.7 mm, 1.5 ± 0.7 mm, and 4.7 ± 1.2 mm, 4.5 ± 0.7 mm, 0.0 mm for intrabony defect, Class II and Class III FI, respectively. According to radiographic analysis, linear bone height in intrabony defects and vertical subclassification of FI in Class II FI were improved.

Conclusions

The clinical application of CO3Ap for the treatment of intrabony defects and Class II FI could be effective for periodontal regeneration, although its efficacy in treating Class III FI might be limited. Despite the limitations of this study, the findings in this study suggested that CO3Ap has the potential to be a promising bone graft substitute for periodontal regeneration.

The Effectiveness and Practicality of a Novel Barrier Membrane for the Open Window in Maxillary Sinus Augmentation with a Lateral Approach, with Risk Indicators for Bone Graft Displacement and Bone Height Decrease: A Prospective Study in Humans

Maxillary sinus augmentation with a lateral approach (MSA) is a well-established treatment. In this prospective study, we evaluated risk factors for postoperative bone graft displacement and reported the clinical application of long-term resorbable L-lactic acid/-caprolactone (PLA/PCL) as a barrier membrane to cover the open window in the lateral wall in MSA. Twenty-four patients underwent MSA according to the relevant criteria; CT data obtained before and 1 week (1 w) and 5-6 months (5 m) post-MSA, bone height changes, bone height reduction rates at 1 w and 5 m post-MSA, bone graft displacement measurements, and risk factors were examined. All patients showed bone height increments (p < 0.005). However, no difference was observed between 1 w and 5 m post-MSA. Bone graft displacement was observed in eight patients; the reduction rate from 1 w to 5 m post-MSA was 8.38% ± 4.88%. Sex, septa, maxillary sinus floor-palatal bone distance, and maxillary sinus floor-maxillary ostium distance were associated with bone graft displacement (p < 0.05). The height from the maxillary sinus floor to the palatal bone and the sinus angle influenced the augmentation degree (p < 0.05). The PLA/PCL membrane is compared favorably with other membranes and may be useful as a barrier membrane for the MSA open window.

A Prospective Study of the Assessment of the Efficacy of a Biodegradable Poly(l-lactic acid/ε-caprolactone) Membrane for Guided Bone Regeneration

Biodegradable guided bone regeneration (GBR) membranes consist primarily of collagen and aliphatic polyesters. This study assessed the comparative efficacy of a poly(l-lactic-caprolactone) [P(LA/CL)] membrane versus that of a collagen membrane in GBR. Patients requiring GBR simultaneously or before dental implant placement in edentulous regions were randomly assigned to one of two membranes. Within each membrane, they were subdivided into 3 groups: dental implants were placed simultaneously with GBR in groups A and B, and 180 days post-GBR in group C. The augmented bone width was measured at 1, 3, and 6 mm from the implant's neck (groups A and B) or the reference line (group C), utilizing cone-beam computed tomography images, immediately and 150 days post-surgery. A histological study was performed to evaluate bone formation in group C. No adverse events were observed. In the collagen group, the absorbed ratios of the augmented bone were 40.9 ± 36.7%, 29.4 ± 30.1%, and 11.1 ± 22.0% at 1, 3, and 6 mm, respectively; the ratio at 6 mm was significantly lower than that at 1 mm (p = 0.0442). In the P(LA/CL) group, those were 26.2 ± 27.3%, 17.1 ± 19.7%, and 13.3 ± 16.4% at 1, 3, and 6 mm, respectively, with no significant difference at each point. No significant inter-membrane differences were observed. The bone augmentation potential of the P(LA/CL) membrane matched that of the collagen membrane. P(LA/CL) could be used as a safe and effective membrane in GBR.

Synthesis, Characterization, In Vitro Cytological Responses, and In Vivo Bone Regeneration Effects of Low-Crystalline Nanocarbonated Hydroxyapatite

Hydroxyapatite (HA) has been commonly used as an alternative bone substitute. But it has drawbacks, such as poor degradation and limited osteogenesis. Low-crystalline carbonated hydroxyapatite (L-CHA), which has greater biodegradability than HA, is suggested as one of the main components of bone minerals, but the exact mechanism behind the roles of carbonate substituted in biological behaviors of low-crystalline HA is still a mystery. In this study, L-CHAs with different carbonate contents were prepared, and the effects of the content on the physicochemical properties, in vitro cytological responses, and in vivo bone defects repair effects of L-CHAs were investigated. The results demonstrated that CO₃^2- had successfully entered the lattice structure of L-CHAs with a maximum content of 9.2 wt %. Both low-crystalline undoped HA (L-HA) and L-CHAs were nanocrystalline (20-30 nm) with significantly higher specific surface areas, protein adsorption capacities, and biodegradability compared to high-crystalline HA (H-HA) with submicron crystalline size (200-400 nm). Besides, the amounts of the adsorbed protein and released Ca²⁺ ions increased in a carbonate-content-dependent manner. Compared to L-HA and H-HA, L-CHAs promoted the adhesion and proliferation of bone marrow mesenchymal stem cells and significantly upregulated the levels of alkaline phosphatase (ALP) activity and the expression of osteogenesis-related genes. In addition, L-CHA-9 not only showed a faster biodegradation rate but also effectively promoted bone regeneration when implanted in the critical-sized bone defects of rabbit femora. This study provided evidence for the development of L-CHA as a promising biodegradable and bioactive material with great osteoconductivity and osteogenic capability with respect to conventional HA.

Preclinical evaluation of the effect of periodontal regeneration by carbonate apatite in a canine one-wall intrabony defect model

Objective

This study aimed to histologically compare periodontal regeneration of one-wall intrabony defects treated with open flap debridement, β-tricalcium phosphate (β-TCP), and carbonate apatite (CO₃Ap) in dogs.

Methods

The mandibular third premolars of four beagle dogs were extracted. Twelve weeks after the extraction, a one-wall bone defect of 4 mm × 5 mm (mesio-distal width × depth) was created on the distal side of the mandibular second premolar and mesial side of the fourth premolar. Each defect was randomly allocated to open flap debridement (control group), periodontal regeneration utilizing β-TCP, or CO₃Ap. Eight weeks after the surgery, histologic and histometric analyses were performed.

Results

No ankylosis, infection, or acute inflammation was observed at any of the experimental sites. Newly formed bone and cementum were observed in all experimental groups. The mineral apposition rate of the alveolar bone crest was higher in the CO₃Ap group than in the control and β-TCP groups. The ratio of the new bone area was significantly higher in the CO₃Ap group than in the control group (P < 0.05). The bone contact percentage of the residual granules was significantly higher in the CO₃Ap group than in the β-TCP group (P < 0.05).

Conclusion

Although this study has limitations, the findings revealed the safety and efficacy of CO₃Ap for periodontal regeneration in one-wall intrabony defects in dogs, and CO₃Ap has a better ability to integrate with bone than β-TCP.

Reconstruction of rabbit mandibular bone defects using carbonate apatite honeycomb blocks with an interconnected porous structure

Carbonate apatite (CO₃Ap) granules are useful as a bone substitute because they can be remodeled to new natural bone in a manner that conforms to the bone remodeling process. However, reconstructing large bone defects using CO₃Ap granules is difficult because of their granular shape. Therefore, we fabricated CO₃Ap honeycomb blocks (HCBs) with continuous unidirectional pores. We aimed to elucidate the tissue response and availability of CO₃Ap HCBs in the reconstruction of rabbit mandibular bone defects after marginal mandibulectomy. The percentages of the remaining CO₃Ap area and calcified bone area (newly formed bone) were estimated from the histological images. CO₃Ap area was 49.1 ± 4.9%, 30.3 ± 3.5%, and 25.5 ± 8.8%, whereas newly formed bone area was 3.0 ± 0.6%, 24.3 ± 3.3%, and 34.7 ± 4.8% at 4, 8, and 12 weeks, respectively, after implantation. Thus, CO₃Ap HCBs were gradually resorbed and replaced by new bone. The newly formed bone penetrated most of the pores in the CO₃Ap HCBs at 12 weeks after implantation. By contrast, the granulation tissue scarcely invaded the CO₃Ap HCBs. Some osteoclasts invaded the wall of CO₃Ap HCBs, making resorption pits. Furthermore, many osteoblasts were found on the newly formed bone, indicating ongoing bone remodeling. Blood vessels were also formed inside most of the pores in the CO₃Ap HCBs. These findings suggest that CO₃Ap HCBs have good osteoconductivity and can be used for the reconstruction of large mandibular bone defects. The CO₃Ap HCB were gradually resorbed and replaced by newly formed bone.

An exploratory clinical trial to evaluate the safety and efficacy of combination therapy of REGROTH® and Cytrans® granules for severe periodontitis with intrabony defects

Introduction

Currently, flap operation (FOP) using REGROTH® (0.3% basic fibroblast growth factor [FGF-2]) is the standard treatment for periodontal regenerative therapy in Japan. However, the periodontal tissue regenerative effect with REGROTH® monotherapy is inadequate for severe alveolar bone defects. Therefore, in this study, we evaluated the safety and effectiveness of periodontal regenerative therapy for patients with severe periodontitis using REGROTH® (test medicine) combined with Cytrans® Granules (test device: carbonated apatite granules), which is a new artificial bone.

Methods

The study participants included 10 patients with severe periodontitis (mean age: 47.4 years). All participants provided written informed consents. In each patient, the intrabony defect site (mean bone defect depth: 5.7 mm) was defined as the test site. FOP was performed for the test site after the baseline investigation; moreover, the test medicine and test device were administered simultaneously. Furthermore, the observation of subjects’ general condition and test sites was conducted and the blood, urine, and periodontal tissue tests were performed up to 36 weeks after FOP. The rate of bone increase (%), clinical attachment level (CAL), probing pocket depth (PPD), bleeding on probing (BOP), tooth mobility (Mo), width of keratinized gingiva (KG), gingival recession (REC), gingival index (GI), and plaque index (PlI) were evaluated during the periodontal tissue investigation.

Results

As the primary endpoint, no adverse events related to the test medicine and test device occurred during the entire observation period of this study. Regarding the secondary endpoints, there was a significant increase in new alveolar bone (p = 0.003) and CAL acquisition (p = 0.001) as well as decrease in PPD (p = 0.002) and BOP (p = 0.016) at 36 weeks after administration of the test medicine and test device compared with the preoperative values. Furthermore, at 36 weeks after surgery, the Mo, GI, and PlI decreased to preoperative levels at 40%, 60%, and 30% of sites, respectively. However, at 36 weeks after surgery, there was no difference in KG and REC compared with their preoperative values.

Conclusions

The safety of periodontal regenerative therapy using the test medicine in combination with the abovementioned test device was confirmed. In addition, it was suggested that this periodontal regenerative therapy is effective for tissue regeneration in severe alveolar bone defects.

This clinical trial was conducted after registering and publicizing as a specified clinical trial in the Japan registry of clinical trials (jRCTs051190045).

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The safety of flap operation using 0.3% FGF-2 and carbonated apatite was confirmed.

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The administration of 0.3% FGF-2 and carbonated apatite improved periodontitis.

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Combining 0.3% FGF-2 and carbonated apatite for severe alveolar bone defects.

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Periodontal regenerative therapy combining both could be effective.

Effects of Pore Interconnectivity on Bone Regeneration in Carbonate Apatite Blocks

Porous architecture in bone substitutes, notably the interconnectivity of pores, is a critical factor for bone ingrowth. However, controlling the pore interconnectivity while maintaining the microarchitecture has not yet been achieved using conventional methods, such as sintering. Herein, we fabricated a porous block using the crystal growth of calcium sulfate dihydrate, and controlled the pore interconnectivity by limiting the region of crystal growth. The calcium sulfate dihydrate blocks were transformed to bone apatite, carbonate apatite (CO₃Ap) through dissolution–precipitation reactions. Thus, CO₃Ap blocks with 15% and 30% interconnected pore volumes were obtained while maintaining the microarchitecture: they were designated as CO₃Ap-15 and CO₃Ap-30, respectively. At 4 weeks after implantation in a rabbit femur defect, new bone formed throughout CO₃Ap-30, whereas little bone was formed in the center region of CO₃Ap-15. At 12 weeks after implantation, a large portion of CO₃Ap-30 was replaced with new bone and the boundary with the host bone became blurred. In contrast, CO₃Ap-15 remained in the defect and the boundary with the host bone was still clear. Thus, the interconnected pores promote bone ingrowth, followed by replacement of the material with new bone. These findings provide a useful guide for designing bone substitutes for rapid bone regeneration.

Effects of the carbonate content in carbonate apatite on bone replacement

Carbonate apatite (CO₃ Ap), an inorganic component of human bone, has been clinically applied as an artificial bone substitute. In this study, the effects of the CO₃ content in CO₃ Ap on the replacement by new bone were studied by fabricating CO₃ Ap granules containing 0.9-8.3 wt% of CO₃ . The dissolution rate of CO₃ Ap in a weak acidic solution, mimicking the Howship's lacunae, was rapid for the CO₃ Ap granules containing a larger amount of CO₃ . Histological analyses demonstrated the rapid resorption in CO₃ Ap and replacement by natural bone tissue when the CO₃ content was increased. Therefore, the CO₃ content in CO₃ Ap is a key factor that influences the replacement of the bone tissue.

Improved In Vivo Delivery of Small RNA Based on the Calcium Phosphate Method

In the past few years, we have demonstrated the efficacy of a nanoparticle system, super carbonate apatite (sCA), for the in vivo delivery of siRNA/miRNA. Intravenous injection of sCA loaded with small RNAs results in safe, high tumor delivery in mouse models. To further improve the efficiency of tumor delivery and avoid liver toxicity, we successfully developed an inorganic nanoparticle device (iNaD) via high-frequency ultrasonic pulverization combined with PEG blending during the production of sCA. Compared to sCA loaded with 24 μg of miRNA, systemic administration of iNaD loaded with 0.75 μg of miRNA demonstrated similar delivery efficiency to mouse tumors with little accumulation in the liver. In the mouse therapeutic model, iNaD loaded with 3 μg of the tumor suppressor small RNA MIRTX resulted in an improved anti-tumor effect compared to sCA loaded with 24 μg. Our findings on the bio-distribution and therapeutic effect of iNaD provide new perspectives for future nanomedicine engineering.

Staged Sinus Floor Elevation Using Novel Low-Crystalline Carbonate Apatite Granules: Prospective Results after 3-Year Functional Loading

The aim of this study was to evaluate clinical outcomes of staged sinus floor elevation (SFE) using novel low-crystalline carbonate apatite (CO₃Ap) granules. Patients who needed SFE for implant placement were recruited into this clinical trial. A staged procedure (lateral window technique using CO₃Ap granules, followed by implant placement after 7 ± 2 months) was employed in 13 patients. Bone-height increase and insertion torque values (ITVs) were assessed along with histological evaluation. The survival and success rates of 3-year functioning implants were also evaluated. Mean of bone-height increase after SFE using CO₃Ap granules was 7.2 ± 2.5 mm and this increase allowed implant placement in all cases (17 implants). Mean of ITV was 25.1 ± 13.2 Ncm and primary stability was achieved successfully in all cases. Histological analyses revealed mature new bone formation (36.8 ± 17.3%) and residual CO₃Ap granules (16.2 ± 10.1%) in the compartment after SFE. The survival and success rates after 3-year functional loading were 100% and no complications were found. These results clearly indicate the clinical usefulness of CO₃Ap granules for SFE.

Bioactivity Evaluation of Biphasic Hydroxyapatite Bone Substitutes Immersed and Grown with Supersaturated Calcium Phosphate Solution

Recently, the frequency of use of bone substitute materials for the purpose of bone augmentation has increased in implant treatment, but bone formation with bone substitute materials alone is limited. Calcification of bone in the body progresses as Ca²⁺, H₂PO^4-, and HPO₄^2- in the body form hydroxyapatite (HA) crystals. In this study, therefore, we prepared a biphasic bone substitute with biological activity to promote bone formation by inducing precipitation and growth of HA crystals on the surface of a bone substitute and evaluated it. Biphasic bone substitute granules were prepared by immersing HA granules in a supersaturated calcium phosphate solution prepared by mixing five medical infusion solutions, the precipitate was analyzed, and the biological activities of biphasic HA granules were evaluated in vitro and in vivo. As a result, the precipitated calcium phosphate crystals were identified as low crystalline HA. On the surface of the HA granules, low-crystalline HA grew markedly as needle-shaped crystals and significantly promoted cell proliferation and bone differentiation. In animal experiments, biphasic HA granules had a significantly higher bone mineral density, new bone volume ratio, and new bone area ratio. Therefore, it suggests that biphasic hydroxyapatite is a useful bone substitute for bone augmentation in dental implant treatment.

Carbonate apatite artificial bone

Bone apatite is not hydroxyapatite (HAp), it is carbonate apatite (CO3Ap), which contains 6-9 mass% carbonate in an apatitic structure. The CO3Ap block cannot be fabricated by sintering because of its thermal decomposition at the sintering temperature. Chemically pure (100%) CO3Ap artificial bone was recently fabricated through a dissolution-precipitation reaction in an aqueous solution using a precursor, such as a calcium carbonate block. In this paper, methods of fabricating CO3Ap artificial bone are reviewed along with their clinical and animal results. CO3Ap artificial bone is resorbed by osteoclasts and upregulates the differentiation of osteoblasts. As a result, CO3Ap demonstrates much higher osteoconductivity than HAp and is replaced by new bone via bone remodeling. Granular-type CO3Ap artificial bone was approved for clinical use in Japan in 2017. Honeycomb-type CO3Ap artificial bone is fabricated using an extruder and a CaCO3 honeycomb block as a precursor. Honeycomb CO3Ap artificial bone allows vertical bone augmentation. A CO3Ap-coated titanium plate has also been fabricated using a CaCO3-coated titanium plate as a precursor. The adhesive strength is as high as 76.8 MPa, with excellent tissue response and high osteoconductivity.

Fabrication and Histological Evaluation of a Fully Interconnected Porous CO3Ap Block Formed by Hydrate Expansion of CaO Granules

Carbonate apatite (CO₃Ap) fabricated by a dissolution-precipitation reaction from a precursor exhibits excellent osteoconductivity and is readily replaced by bone. In the present study, a fully interconnected porous CO₃Ap block was fabricated by hydrate expansion and carbonation of CaO granules, and the resulting CaCO₃ was then converted to CO₃Ap. When CaO granules were exposed to 100% humidity CO₂ in a closed vessel, the CaO granules were hydrated and expanded to form a porous Ca(OH)₂ block. The block was then carbonated to form a porous CaCO₃ block, which was then immersed in a Na₂HPO₄ solution to convert it to a porous CO₃Ap block. The resulting CO₃Ap block possessed a fully interconnected porous structure. Histological analyses 4 and 8 weeks after implantation in rabbits revealed that the porous CO₃Ap block resulted in more significant material resorption and bone formation than the dense CO₃Ap block. Therefore, it was concluded that a fully interconnected porous CO₃Ap block fabricated by the hydrate expansion of CaO granules has potential value as a bone substitute.