Dindo Q. Mijares

Biphasic calcium phosphate bioceramics: preparation, properties and applications

Biphasic calcium phosphate (BCP) bioceramics belong to a group of bone substitute biomaterials that consist of an intimate mixture of hydroxyapatite (HA), Ca10(PO4)6(OH)2, and beta-tricalcium phosphate (β-TCP), Ca3(PO4)2, of varying HA/β-TCP ratios. BCP is obtained when a synthetic or biologic calcium-deficient apatite is sintered at temperatures at and above 700 °C. Calcium deficiency depends on the method of preparation (precipitation, hydrolysis or mechanical mixture) including reaction pH and temperature. The HA/β-TCP ratio is determined by the calcium deficiency of the unsintered apatite (the higher the deficiency, the lower the ratio) and the sintering temperature. Properties of BCP bioceramics relating to their medical applications include: macroporosity, microporosity, compressive strength, bioreactivity (associated with formation of carbonate hydroxyapatite on ceramic surfaces in vitro and in vivo), dissolution, and osteoconductivity. Due to the preferential dissolution of the β-TCP component, the bioreactivity is inversely proportional to the HA/β-TCP ratio. Hence, the bioreactivity of BCP bioceramics can be controled by manipulating the composition (HA/β-TCP ratio) and/or the crystallinity of the BCP. Currently, BCP bioceramics is recommended for use as an alternative or additive to autogeneous bone for orthopedic and dental applications. It is available in the form of particulates, blocks, customized designs for specific applications and as an injectible biomaterial in a polymer carrier. BCP ceramic can be used also as grit-blasting abrasive for grit-blasting to modify implant substrate surfaces. Exploratory studies demonstrate the potential uses of BCP ceramic as scaffold for tissue engineering, drug delivery system and carrier of growth factors.

Mg-Substituted Tricalcium Phosphates: Formation and Properties

This study aimed to investigate the formation and properties of mag nesium (Mg)substituted tricalcium phosphate, β-TCMP, its properties and potential as biomaterial for bone repair. β-TCMPs were prepared and characterized using x-ray diffraction, FT-IR and SEM. Dissolution properties were determined in acidic buffer. β-TCMP discs were implanted in surgically created holes in femoral and tibial diaphyses of rabbits. Results demonstrated that the formation of β-TCMP and Mg incorporation in β-TCMP were dependent on reaction pH, temperature and solution Mg/Ca ratios. Sintere d β-TCMP was significantly less soluble than β-TCP. Implanted unsintered β-TCMP showed osteoconductive properties associated with new bone formation. This study suggests that β-TCMP (sintered or unsintered), alone or in combination with other calcium phosphates, may be useful as biomaterials for bone repair and maybe useful in cases where s low r biodegradation than that of β-TCP is desired.

Effect of Calcium Phosphate Coating and rhBMP-2 on Bone Regeneration in Rabbit Calvaria Using Poly(propylene fumarate) Scaffolds

Various calcium phosphate based coatings have been evaluated for better bony integration of metallic implants and are currently being investigated to improve the surface bioactivity of polymeric scaffolds. The aim of this study was to evaluate the role of calcium phosphate coating and simultaneous delivery of recombinant human bone morphogenetic protein-2 (rhBMP-2) on the in vivo bone regeneration capacity of biodegradable, porous poly(propylene fumarate) (PPF) scaffolds. PPF scaffolds were coated with three different calcium phosphate formulations: magnesium-substituted β-tricalcium phosphate (β-TCMP), carbonated hydroxyapatite (synthetic bone mineral, SBM) and biphasic calcium phosphate (BCP). In vivo bone regeneration was evaluated by implantation of scaffolds in a critical-sized rabbit calvarial defect loaded with different doses of rhBMP-2. Our data demonstrated that scaffolds with each of the calcium phosphate coatings were capable of sustaining rhBMP-2 release and retained an open porous structure. After 6weeks of implantation, micro-computed tomography revealed that the rhBMP-2 dose had a significant effect on bone formation within the scaffolds and that the SBM-coated scaffolds regenerated significantly greater bone than BCP-coated scaffolds. Mechanical testing of the defects also indicated restoration of strength in the SBM and β-TCMP with rhBMP-2 delivery. Histology results demonstrated bone growth immediately adjacent to the scaffold surface, indicating good osteointegration and osteoconductivity for coated scaffolds. The results obtained in this study suggest that the coated scaffold platform demonstrated a synergistic effect between calcium phosphate coatings and rhBMP-2 delivery and may provide a promising platform for the functional restoration of large bone defects.

Oral bone loss induced by mineral deficiency in a rat model: Effect of a synthetic bone mineral (SBM) preparation

UNLABELLED Osteoporosis affects the craniofacial and oral structures and has been associated with periodontal bone loss, tooth loss and reduced jaw bone mass. OBJECTIVE This study aimed to test the therapeutic efficacy of synthetic bone mineral (SBM) in minimizing alveolar bone loss induced by mineral deficiency in a rat model. SBM consists of a calcium carbonate apatite (similar to bone apatite) matrix incorporating magnesium, zinc, and fluoride ions. DESIGN Thirty female Sprague Dawley rats (2 months old) were randomly distributed into 3 groups (10 rats per group): GA (control), on basic diet; GB, on mineral deficient (MD) diet; and GC, on MD+SBM. The rats were sacrificed after 3 months, the jawbones were isolated and the soft tissues removed. Bone density was determined using X-ray radiography (Faxitron); mandibular cortical width, panoramic mandibular index, and alveolar resorption degree (M/M ratio) using BioquantOsteo; and bone micro-architecture micro-computed tomography and scanning electron microscopy. RESULTS Compared to control (GA), the rats on MD diet (GB) experienced significant mandibular bone loss while the rats on MD+SBM diet (GC) experienced significantly less bone loss compared to the GB group. CONCLUSION SBM, administered orally, may have the potential as an osteoporosis therapeutic agent in minimizing or preventing alveolar bone loss induced by mineral deficiency.

Amorphous Calcium Phosphates (ACP): Formation and Stability

Our earlier studies showed that several ions inhibit the crystal growth of apatite and promote the formation of amorphous calcium phosphates (ACP). These ions include: magnesium (Mg), zinc (Zn), stannous (Sn), ferrous (Fe), carbonate (CO3), pyrophosphate (P2O7). The purpose of this study was to investigate the effect of combination of these ions (e.g., Mg & CO3, Mg & P2O7, Mg & Zn, etc) on the formation and stability of ACP. ACP compounds containing the different ions were prepared at 25 and 37oC according to the method we previously described. Chemical stability was investigated by suspending the different ACP preparations in solutions with or without inhibitory ions. Thermal stability was determined by sintering the ACP at different temperatures. Dissolution properties were determined in acidic buffer. The ACP before and after chemical or thermal treatment were analyzed using X-ray diffraction, infrared spectroscopy, and thermogravimetry. Results showed synergistic effects of inhibitory ions on the formation of ACP. ACP materials, regardless of their composition, remained amorphous even after heat treatment at 400oC. Transformation of ACP to other calcium phosphate phases depended on the pH and on the solution composition.

Experimental antibacterial and mineralizing calcium phosphate-based treatment for dentin surfaces

Objectives: This study aimed to determine the efficacy of experimental calcium phosphate-based solutions (sCaP) containing fluoride (F), with and without zinc (Zn) ions on reducing susceptibility to acid dissolution and Streptococcus mutans (S. mutans) colonization of dentin surfaces. Methods: Dentin sections were treated with double distilled water (control) and with sCaP solutions differing in pH and in F− and/or Zn2+ ion concentrations. Solutions A (pH 7); B, C, and D (pH 5.5); solution C, twice Zn2+ and F− ion concentration compared to B; solution D is similar to C but without Zn2+. The dentin surfaces were characterized using scanning electron microscopy (SEM), x-ray diffraction, and Fourier Transform Infrared spectroscopy. Dissolution was determined in acidic buffer. Bacterial (S. mutans) attachment and growth were evaluated using SEM and Bioquant. Statistical analyses applied analysis of variance (ANOVA) and Duncan’s multiple Range test. Results: Compared to control, dentin surfaces treated with sCaP solutions showed: (a) occluded dentin tubules; (b)reduced susceptibility to acid dissolution; and (c) Zn2+ ions were more effective than F− ions in inhibiting bacterial colonization. Significance: Acidic sCaP containing both F and Zn ions have mineralizing, acid resistance, and antibacterial effects and may be potentially useful as a strategy against dentin caries formation and progression.

Synthesis Bone Mineral (SBM) for Osteoporosis Therapy: Part 1 - Prevention of Bone Loss from Mineral Deficiency

Osteoporosis is a ‘silent’ disease characterized by thinning cortical bone and disorganized trabecular architecture causing bone fragility leading to fracture. Osteoporosis results when the rate of bone resorption far exceeds the rate of bone formation. Current pharmaceutical interventions (estrogen therapy, bisphosphonate-based drugs) focus on inhibiting bone resorption. However, some of these therapies have serious side effects (e.g., cancer risk from estrogen therapy; osteonecrosis of the jaw and delayed fracture healing from bisphosphonate-based drugs). The long term objective of the study was to develop a novel material for potential osteoporosis therapy, prevention and fracture repair. This novel material MZF-CaP or synthetic bone mineral, SBM) incorporates Mg, Zn and F ions in a calcium phosphate matrix. Separately, magnesium (Mg), zinc (Zn) and fluoride (F) ions have been associated with biomineralization and osteoporosis therapy in human and in animals. MZF-CaP or SBM was prepared by a modified hydrolysis method previously described and characterized using x-ray diffraction, FT-IR spectroscopy, inductive coupled plasma and dissolution in acidic buffer. Separately, male and female Sprague-Dawley rats were randomly assigned to the following groups depending on the diet: GA: normal on basic diets; GB: on mineral deficient diets (md); GC: on md + Mg-CaP; GD: on md + Zn-CaP; GE: md+F-CaP; and GF: md+MZF-CaP. The rats were sacrificed after 3 months and the femur bones separated, cleaned of extraneous soft tissues and stored until needed for analyses. Femur bones were analyzed using microradiography (Faxitron), scanning electron microscopy (SEM) and microCT. Results: SEM, Faxitron and microCT analyses showed thinning of cortical bone and disorganized trabecular bone architecture for osteoporotic rats on mineral deficient diet (GB) and prevention of bone loss in rats receiving the supplemented diets (GC,GD,GE,GF). Conclusion: These results indicate that the novel material, MZF-CaP or SBM had a potential for osteoporosis therapy and prevention. Studies to demonstrate the use of SBM in reversing (recovering) bone loss are in progress.

Consequences of Fluoride Incorporation on Properties of Apatites

Fluoride, when incorporated in the apatite, stabilizes the structure. The purpose of this study was to determine the consequences of fluoride (F) substitution on the physico-chemical properties of apatites. F-containing apatites were prepared by precipitation or by hydrolysis of CaHPO4 in solutions containing different F concentrations and characterized using x-ray diffraction, FT-IR spectroscopy, scanning electron microscopy, thermogravimetry and chemical analyses. Results showed that F incorporation have the following effects: (a) decrease in a-axis dimension, (b) increase in crystal size and thickness, (c) decrease in calcium deficiency, and (d) lower solubility.

Destructive periodontal diseases in minority populations.

Disparities in the prevalence and severity of destructive periodontal diseases have been reported for American minority populations and have raised the following questions. Are differences in destructive periodontal disease prevalence and severity due to genetic or other confounding variables associated with ethnicity race? Do risk factors for destructive periodontal diseases differ among American minority populations or differ from the population at large? Answers to these questions will have profound impact on the direction of future research and the allocation of resources to address disparities in destructive periodontal diseases in American minority populations. Risk assessment studies that examined a set of clinical, demographic, immunologic, and microbiologic parameters of Asian Americans, African Americans, and Hispanic Americans resident in the greater New York City region suggest that occupational status, monitored as a surrogate variable for socioeconomic status, may be a more robust risk factor than ethnicity/race for destructive periodontal diseases in these populations.

Electrochemically Deposited Calcium Phosphate Coating on Titanium Alloy Substrates

Plasma-sprayed HA coating combines the strength of the metal and the bioactivity of the HA. However, this method has several disadvantages. Alternatives to the plasma-spray method such as electrochemical deposition (ECD) and biomimetic or precipitation methods are being explored. The purpose of this study was to develop an ECD method for coating Ti alloy substrate with different calcium phosphates (octacalcium phosphate, calcium deficient apatite, carbonatesubstituted apatite, fluoride-substituted apatite). Pairs of Ti6Al4V plates that have been mechanically polished, ultrasonically cleaned, acid etched, rinsed and dried were used as anodes and cathodes. ECD was carried out using programmed pulse time electric fields. Results showed that uniform coating with only the desired calcium phosphate can be obtained using metastable calcium phosphate solutions at different pH and temperature conditions and different electrolyte concentrations. Coating thickness varied with the duration of coating deposition. Crystal size varied with other ECD conditions (e.g., pulse time, current density). This method can be used to obtain uniform coating of the desired calcium phosphate composition at low temperatures (25 to 80oC) on substrates of any type of geometry.