Key Engineering Materials Vols. 361-363

Abstract: The hydrolysis of brushite in calcium phosphate cements to form hydroxyapatite is known to result in the long term stability of the material in the body. It has previously been established that this hydrolysis reaction can be influenced by implant volume, media refreshment rate and media composition. In this study, the effect of macroporosity on the rate of degradation of the material is investigated. Macroporosity was incorporated into the material using calcium alginate beads mixed into the cement paste. The inclusion of the calcium alginate beads did not influence the degree of conversion of the material and allowed the incorporation of porosity at up to maximum of 57%. The macroporosity weakened the cement matrix (from 46.5 to 3.2 MPa). When aged the brushite in the macroporous cement dissolved completely from the matrix resulting in a weight loss of 60wt% in a period of 28 days. This suggests that the controlled incorporation of calcium alginate beads into brushite cements in vivo can be used to control implant degradation rate.

Abstract: Injectable resorbable bone cements for bone void fillings are gaining in interest. The materials resorb in vivo with loss of void filling capacity and strength as a consequence. The objective with this study is to qualitative determining the dissolution behaviour for a calcium sulphate and a calcium phosphate cement as function of storage time in different storage medium and correlate to their strength development. Experiments were performed on a calcium phosphate based cement, Norian SRS, and a calcium sulphate based cement, MIIG X3. In the resorbtion study, the materials dissolution at different pH (3, 5 and 7) was compared over a period of 11 weeks. The materials compressive and biaxial flexural strength was measured after aging in phosphate buffer saline for up to 12 weeks. The proposed qualitative method to study dissolution behaviour of injectable biomaterials as function of time and medium were evaluated and proved to be useful. Both materials were dissolved after 3 weeks of storage in pH 3. MIIG X3 dissolved faster than Norian SRS at pH 5. At pH 7 both materials were stable over the test period of 11 weeks. For both materials the strength decreases with storage time. Norian had a higher compressive strength than MIIG X3 for the first week, after the first week the compressive strength was similar for the two materials. MIIG X3 showed a higher flexural strength than Norian during the full test period.

Abstract: Apatite foam (AP foam) is an ideal material for bone substitutes and scaffolds in bone tissue regeneration. This is because its highly porous interconnected pores provide the space for cell growth and tissue penetration, and that its composition induces excellent tissue response and good osteoconductivity. In the present study, the feasibility of apatite foam fabrication was evaluated based on so-called dissolution-reprecipitation reaction of α-tricalcium phosphate (α-TCP) foam granules. When α-TCP foam granules were placed in water at 37°C for 24h, no reaction was observed. However, α-TCP foam set to form AP foam when treated hydrothermally at 200°C. The network of fully interconnected pores was retained, and porosity was as high as 82%. Pore size ranged from 50 to 300 0m with average pore size at 160 0m. Compressive strength was 207 kPa. Although no setting reaction was observed at 37°C, setting reaction caused by hydrothermal treatment of α-TCP foam granules at 200°C allows AP foam of any shape to be fabricated. Therefore, this method was suggested to be useful for the fabrication of bone substitutes and the scaffold in bone tissue regeneration.

Abstract: The design and processing of 3D macroporous bioactive scaffolds is one of the milestones for the progress of bone tissue engineering and bone regeneration. Calcium phosphate based ceramics are among the most suitable materials, due to their similarity to the bone mineral. Specifically, beta-tricalcium phosphate (β-TCP) is known to be a resorbable and bioactive material, with well established applications as bone regeneration material. The aim of this work is to explore a new route to obtain β-TCP macroporous scaffolds starting from calcium phosphate cements. To this end foamed calcium phosphate cement, composed of alpha tricalcium phosphate as starting powder was used as initial material. The set foamed structures, made of calcium deficient hydroxyapatite (CDHA) were sintered to obtain the final β-TCP macroporous architecture. The interconnected macroporosity was maintained, whereas the porosity in the nanometric range was strongly reduced by the sintering process. The sintering produced also an increase in the mechanical properties of the scaffold.

Abstract: There is a current need for the localized delivery of antibiotics in order to treat implant based infections. In this study, the efficacy of hydroxyapatite (HA) gels, HA cements, and silica gels in the delivery of vancomycin have been investigated and compared. Vancomycin release was monitored at set time points using a UV/VIS spectrophotometer (288 nm). The activity of the vancomycin released from the cements and gels was assessed using an agar diffusion test with Staphylococcus aureus. Vancomycin was released rapidly from both HA matrices, and the silica gel in the first day of the experiment, but the release rate was slowed considerably after 3 days for the HA gels. Following ten days of aging, 70% of the vancomycin remained in the HA gel matrix and the quantity released from the gel was shown to retain its effectiveness against Staphylococcus aureus.

Abstract: Commercially available calcium phosphate cements set by precipitation of nanoapatite or brushite. The goal of this study was to elucidate the most suitable conditions for forming cements from calcium potassium sodium phosphate. Furthermore, the behaviour of these cements after immersion in SBF and/or TRIS solution was investigated. Using varying additives resulted in differences in solubility kinetics. The XRD spectra of all investigated cement compositions displayed Ca2KNa(PO4)2 after setting. However, the various cement compositions differed with respect to apatite formation when immersed in TRIS buffer in and/or SBF solution. Therefore, when investigating calcium phosphate cements we consider it necessary to clearly differentiate between the phases which form after completion of the final setting time when these materials set in air, and the phases which form in a time dependant manner after immersion in different biological fluids.

Abstract: We have successfully developed novel “chelate-setting apatite cement” using hydroxyapatite (HAp) particles surface-modified with inositol phosphate (IP6) . The HAp particles surface-modified with IP6 were mixed with water (HAp/water ratio = 1.00/0.50[w/w]) to fabricate apatite cements. We have examined the biocompatibility of the apatite cement using the culture system of MC3T3-E1 cells and the rabbit model. The cell-culture test using MC3T3-E1 cells has shown that the apatite cement has noncytotoxicity. This cement has been implanted into tibiae of rabbits. When tissue response was examined histologically up to 24 weeks, new bone formation was observed around the surface of the cement. The present work demonstrates that this apatite cement is useful as a material for artificial bone grafting.

Abstract: Effect of added α-tricalcium phosphate (α-TCP) and β-TCP was investigated to understand the setting reaction of apatite cement consisting of tetracalcium phosphate (TTCP) and dicalcium phosphate anhydrous (DCPA). Addition of TCP delayed the initial setting time because TCP was not involved in the initial setting reaction and resulted in the decreased initial mechanical strength. After the initial setting of the cement due to the conversion of TTCP and DCPA into apatite, α-TCP dissolved to supply calcium and phosphate ions and they were consumed for crystal growth of apatite. Therefore, mechanical strength of the apatite cement containing α-TCP was increased. In contrast, added β-TCP showed no reactivity in the cement and thus result in the decreased mechanical strength.

Abstract: Mineral trioxide aggregate (MTA) is a Portland cement (PC) based material used for sealing root canals however it has a long setting time which is undesirable for dental applications. This study investigated the effect of three different calcium sulphate additions for accelerating the initial setting of a PC based dental material, whilst attempting to maintain its high compressive strength and low relative porosity. Anhydrous calcium sulphate (CaS), Plaster of Paris, calcium sulphate hemihydrate (PoP) and Gypsum, calcium sulphate dihydrate (Gyp) were each added to PC at 5wt%, 10wt% and 20wt%. Initial setting times, compressive strengths and relative porosity were measured using the Gilmore Needles Test, a universal testing machine and a helium pycnometer respectively. Scanning electron microscopy (SEM) was used to observe any microstructural changes in cements. PoP and CaS had the most profound influence on the setting of PC. 20wt% CaS had the greatest effect on the setting time of PC (10min) although decreased the compressive strength by up to 40%, which may have arisen from the formation of microcracks, observed by SEM analysis. Additions of 10wt% PoP and CaS may have the potential to reduce the long setting time of PC based dental materials.

Abstract: In the present study, ZrO2 was added into the injectable calcium phosphate cements (CPCs) to improve their mechanical strength. Different mass fractions of ZrO2 (5 %, 10 %, 15 %, 20%) were mixed with the powder components consisted of tricalcium phosphate (α-TCP) and hydroxyapatite (HA). Then formed the paste via adding the liquid component consisted of citric acid. The compressive strength, the injectability, the initial setting time and finial time of CPC were measured, respectively. X-ray diffraction (XRD) was employed to analyse the phase of as-prepared CPC. Scanning Electron Microscope (SEM) and Energy dispersive spertrum (EDS) were used to observe the morphology and indicate the element components of CPC. The compressive strength of ZrO2-CPC was higher than that of CPC without added ZrO2. The compressive strength got the maximal when the mass fraction of ZrO2 was 15%. It had no effect on the injectability with adding ZrO2, which were 89 % to 92 %. It had a slight down-regulation of the initial and final setting time with adding ZrO2. SEM showed that there was amounts needle-like substance in CPC, which might be related to the improvement of compressive strength of CPC. XRD showed that there were HA, a few of α-TCP and ZrO2 diffraction peaks in CPCs. The present results indicate that it is feasible to improve the compressive strength of injectable CPC via adding ZrO2.