Hydroxyapatite Specimens Research Articles

Mechanical properties of sintered hydroxyapatite and tricalcium phosphate ceramic.

The ultimate values for compressive strength, Young's modulus, and toughness of cylindrical specimens of unitary aspect ratios and uniform grain-size distributions were extrapolated for hydroxyapatite (HAP) to 70 MPa, 9.2 GPa, and 0.36 J cm(-3), and for tricalcium phosphate (TCP), to 315 MPa, 21 GPa, and 2.34 J cm(-3). For total volume porosities of 50%, the corresponding values were determined: for HAP, 9.3 MPa, 1.2 GPa, 0.042 J cm(-3), for TCP, 13 MPa, 1.6 GP, 0.077 J cm(-3). Porosities of HAP specimens ranged from 3%-50%; TCP from 10%-70%. Two pore-size distributions were employed. Exponential dependencies of the mechanical properties were found upon porosity (p < 0.0001). No differences in measured mechanical properties, as determined in compression, could be attributed to pore size. The superiority of TCP increases with density and suggests that a larger or more selective pore-size distribution could be effectively employed in TCP biological implants. This work also suggests the dominant role of secondary calcium phosphates in increasing compressive strengths.

Biocompatibility analysis of different biomaterials in human bone marrow cell cultures.

A cell culture system for biocompatibility testing of hip implant materials is described. Human bone marrow cells have been chosen because these cells are in direct contact with the biomaterial after implantation in situ. The sensitivity of this method is evaluated for materials which are already being used as implants in humans and animal, e.g., hydroxyapatite (HA) ceramic, pure titanium, and ultra-high-molecular-weight polyethylene (UHMWPE). As indicative parameters of biocompatibility primary cell adherence, cell number, cell proliferation, production of extracellular matrix, cell vitality, and cell differentiation are described. After 2 weeks in culture, obvious differences between the biomaterials with respect to the indicative parameters could be observed. Cell numbers were greatest on the HA specimens. In the case of titanium alloys, we observed a decreased number of cells. The production of extracellular matrix was high for the HA ceramics but reduced for titanium specimens. The polymers allowed only a few adherent cells and showed no signs of extracellular matrix production. The results can be correlated astonishingly well to animal experiments and clinical experiences. Therefore, we suggest that this cell culture system seems to be a useful tool for biocompatibility testing of bone implantation materials. It also helps reduce animal experiments. With the help of flow cytophotometry, we analyzed the influence of biomaterials on large numbers of cells with respect to differentiation. There were similar populations of T cells and monocytes on all specimens tested. Extended B-cell and granulocyte populations, however, were observed with titanium and UHMWPE. Most osteocalcin-containing cells adhered to the HA ceramics.

Hydroxyapatite impregnated bone cement: in vitro and in vivo studies

Hydroxyapatite (HA) particle impregnated polymethylmethacrylate (PMMA) bone cement was fabricated in order to induce bony tissue ingrowth into the pores left behind by resorption of the HA. The amount of HA was 0%, 10%, and 30% by weight. After mixing the HA with bone cement it was cast in a 4 mm inside diameter glass tube (10 mm long). Each specimen was cut into half and used for in vitro and in vivo experiment. Diametral tensile and flexural bend tests were made to evaluate its mechanical properties. SEM (scanning electron microscope) and ESCA (electron spectroscopy for chemical analysis) were used to evaluate the surface characteristics of the specimen. Push-out test and histology studies were made after implanting the specimens for six weeks in the distal portion of rabbit femur. The mechanical test results showed significant decrease of the flexural and diametral tensile strength linear with the increased amount of HA (p < 0.05). The SEM and ESCA analyses showed no definite exposure of HA particles on the surface of the specimens due to the covering by PMMA. The interfacial shear strength of the implanted specimens showed significant increases in the 30% HA specimens compared with the 0% ones but the 10% specimens did not show any difference. Histological observation showed little HA particle apposition to new bone in 10% specimens but more in the 30% ones. In conclusion, this study showed the decreased mechanical properties both by increasing the amount of HA particles and with the bone particles impregnated bone cement. In vivo study showed much lower interfacial shear strength due to implant site compared to other studies but the 30% HA specimens showed statistically significant increase in the push-out strength after implanting into the distal end of rabbit femora.

Biomechanical assessment of bone ingrowth in porous hydroxyapatite.

Porous hydroxyapatite (Endobon) specimens were implanted into the femoral condyle of New Zealand White rabbits for up to 6 months. After sacrifice, specimens were sectioned for histology and mechanical testing, where the extent of reinforcement by bony ingrowth was assessed by compression testing and fixation was assessed by push-out testing. From histological observations, it was established that the majority of bone ingrowth occurred between 10 day and 5 weeks after implantation and proceeded predominantly from the deep end of the trephined defect, with some integration from the circumferential sides. At 3 months, the implants were fully integrated, exhibiting bony ingrowth, vascularization and bone marrow stroma within the internal macropores. After 5 weeks, the mean ultimate compressive strength of retrieved implants (6.9 MPa) was found to be greater than that of the original implant (2.2 MPa), and by 3 months the fully integrated implants attained a compressive strength of approximately 20 MPa. Push-out testing demonstrated that after 5 weeks in vivo, the interfacial shear strength reached 3.2 MPa, increasing to 7.3 MPa at 3 and 6 months.

Osteoinduction in porous hydroxyapatite implanted in heterotopic sites of different animal models

Previous studies have demonstrated the induction of bone in coral-derived porous hydroxyapatite when implanted intramuscularly in baboons. This hydroxyapatite-induced bone differentiation model was used to study the effect of different animal species on heterotopic bone formation. Porous hydroxyapatite, obtained after hydrothermal conversion of the calcium carbonate exoskeleton of coral (genus Goniopora), was implanted in the rectus abdominis of adult rabbits, dogs and baboons ( Papio ursinus). Specimens were harvested on day 90 after implantation and subjected to histological and histomorphometrical analysis. Minimal amounts of bone formed in hydroxyapatite specimens harvested from rabbits and dogs. Substantial bone differentiation did occur, however, in hydroxyapatite specimens harvested from the rectus abdominis of the baboons. In primates, the porous hydroxyapatite, as used in this study, may act as a solid matrix for adsorption, storage and controlled release of circulating or locally produced bone morphogenetic proteins, which locally initiate bone formation. The results of this study on heterotopic bone formation in porous hydroxyapatite underscore the importance of primate models in biomaterial research, which should be exploited for the formulation of porous substrata with intrinsic osteoinductive activity.

Effect of compaction pressure and atmosphere on sintering of nanometric iron powders : J. Bigot et al (CRNS, Vitry sur Seine, France)

This study describes the effect of sintering temperature on the microstructural, calcium/phosphorus (Ca/P) ion ratios and mechanical properties of non-separated biowastes processed hydroxyapatite (HAp) prepared through a low cold compaction protocol. The HAp was produced by a sintering temperature of 900 °C. Furthermore, HAp sintered at 900 °C was subjected to sintering temperatures of 1000 and 1100 °C.The structural and morphological evolution of the fabricated biomaterials were characterized by X-ray diffraction (XRD) and scanning electron microscopy (SEM) equipped with electron dispersive X-ray analysis (EDX) respectively. Uniaxial compaction using a pressure of 500 pa was used to produce rectangular shaped pellets to investigate the influence of sintering temperature on the mechanical properties of the produced pellets. From XRD analysis, it was found that hydroxyapatite derived from the biowastes showed good thermal stability and did not exhibit phase instability with traces of other calcium phosphates. The SEM micrographs showed microporous structure of the biomaterials and an increase in temperature reduced the porosity and enhanced the mechanical properties. It was also noticed that the trend of transformation of the average shape of pores was from strongly flattened to round at higher sintering temperatures. Electron dispersive X-ray analysis (EDX) revealed that the atomic Ca/P ratios of the as-sintered HAp specimens ranged from 1.58 to 1.79 for sintering temperatures of 900–1100 °C. The synthesized hydroxyapatite powder showed inclusion of the fluorapatite phase at sintering temperature of 1000 °C with a reduction in the crystallite size. For both scenarios (sintering temperature and compaction pressure), a consistent trend in mechanical properties (microhardness, fracture toughness and Young's modulus) is noticed at every point of measurement except for compressive strength. The reduction in compressive strength when compaction pressure was applied could be as a result of the stress induced in the HAp powders during compaction which may have made it more susceptible to cracks. The hardness value obtained for the synthesized hydroxyapatite pellets is in the range of that of actual human femoral cortical bone.

Dentine hypersensitivity: the measurement in vitro of streaming potentials with fluid flow across dentine and hydroxyapatite

Stimulus transmission across dentine, in conditions such as dentine hypersensitivity, is considered to occur via a hydrodynamic mechanism. This fluid flow in dentine may then induce a mechanoreceptor response in pulpal nerves. However, when fluid flows through a porous structure electrical potentials are also generated. The aim of this study was to develop a reproducible model system to measure streaming potential across dentine and hydroxyapatite and determine the influence of pressure. Using an acrylic cell, with silver electrodes, streaming potentials were recorded across dimensionally standardized dentine and hydroxyapatite specimens, over a pressure range of 1-6 atmospheres. Streaming potentials were found to be directly proportional to pressure and dependent on the electrical conductivity of the saline used in the cell. The results confirm the limited existing data on streaming potentials across dentine and indicate that at these low pressures excitation of pulpal nerves would not occur. However, if, as may be the case, stimuli applied to dentine create very high pressures, the resultant potentials generated could indeed evoke a neural response. The model system is worthy of further use to study this phenomenon and the factors which may influence it.

Osteogenin, a bone morphogenetic protein, adsorbed on porous hydroxyapatite substrata, induces rapid bone differentiation in calvarial defects of adult primates.

Osteogenin, a bone morphogenetic protein, in conjunction with insoluble collagenous bone matrix initiates local endochondral bone differentiation by induction in vivo. This study, by exploiting the affinity of native osteogenin for hydroxyapatite, was designed to construct a delivery system for the expression of the biologic activity of osteogenin in nonhealing calvarial defects of adult primates. After exposure of the calvaria, 64 cranial defects, 25 mm in diameter, were prepared in 16 adult male baboons (Papio ursinus). Defects were implanted with disks of porous nonresorbable and resorbable hydroxyapatite substrata obtained after hydrothermal conversion of calcium carbonate exoskeletons of corals. In each animal, one disk of each hydroxyapatite preparation was treated with osteogenin isolated and purified from baboon bone matrix after sequential chromatography on heparin-Sepharose, hydroxyapatite, and Sephacryl S-200 gel filtration columns. The remaining two defects were implanted with one disk of each hydroxyapatite preparation without osteogenin as control. Histomorphometry on decalcified sections prepared on days 30 and 90 showed superior osteogenesis in osteogenin-treated nonresorbable hydroxyapatite specimens as compared with controls. On day 90, substantial bone formation also had occurred in control nonresorbable hydroxyapatite specimens. On day 90, but not on day 30, significantly greater amounts of bone had formed in osteogenin-treated resorbable specimens as compared with resorbable controls. Overall, resorbable substrata performed poorly when compared with nonresorbable substrata, perhaps due to a premature dissolution of the implants. These results provide evidence that the biologic activity of osteogenin can be restored and delivered by a substratum other than the organic collagenous matrix, inducing rapid bone differentiation in calvarial defects of adult nonhuman primates. The adsorption strategy of osteogenin on porous inorganic nonimmunogenic substrata may help to design appropriate osteogenic delivery systems for craniofacial and orthopedic applications in humans.

Effect of Pore Topology on the Remineralization Rate of Surface-Softened Enamel and Synthetic Hydroxyapatite: An Experimental Comparison and Mathematical Model

Experiments with surface-softened synthetic hydroxyapatite show similar remineralization rates as surface-softened human enamel. The mineral gain with respect to time relates to parameters governing the precipitation of mineral in the specimens. A mathematical model regarding the mineral precipitation on the pore surface is fitted to the experimental values. We obtained comparable precipitation rates for apatite and tooth enamel. The geometry of the pores, however, proved to be different: a mostly tube-like pore structure must be attributed to demineralized enamel, while the synthetic hydroxyapatite specimens remineralize like a ramified, crack-like microstructure of pores.

Use of a transmission ultrasonic technique for the in vitro evaluation of bone ingrowth

The objective of this work was to apply the technique to a group of porous coralline hydroxyapatite (CH) specimens retrieved after implantation in dogs. The ultrasonic results were compared to traditional histomorphometric measures of ingrowth. Though the in vitro technique is not without limitations, it can be used as a non-destructive, qualitative tool to evaluate bone ingrowth into porous media

The adsorption of human submandibular-sublingual salivary proteins by synthesized and commercial hydroxyapatite (author's transl)

A hydroxyapatite (HA) with the crystallinity similar to enamel powder of teeth, determined by analyses of X-ray diffraction, was chemically synthesized. Commercial HA (Lot. 30307, Seikagaku-kogyo Co., Tokyo) was lower in the crystallinity (determined by X-ray diffraction) than that of enamel powder and higher than that of dentin powder. It was studied on the adsorption of submandibular-sublingual (SM-SL) salivary proteins by these two types of HA, synthesized HA (S-HA) and commercial HA (C-HA). Fifteen ml of SM-SL saliva was added to 3.4g of S-HA and C-HA, respectively. Eighty-seven percent of salivary proteins added was adsorbed by C-HA, whereas only 58% of those added was adsorbed by S-HA. There were three salivary proteins which adsorbed on to C-HA but not on to S-HA. These subunit molecular weight were estimated by poly a crylamide gel electrophoresis in sodium dodecyl sulfate (SDS) and β-mercaptoethanol to be approximately 70, 000, 60, 000, and 56, 000. These proteins could be eluted from C-HA by a low concentration of phosphate buffer. Two proteins with estimated subunit molecular weight of approximately 120, 000 and 30, 000 were tightly adsorbed by both HA specimens. A salivary glycoprotein with high molecular weight also adsorbed on to both HA specimens. SDS-polyacrylamide gel electrophoresis in the absence and in the presence of β-mercaptoethanol indicated that the glycoprotein is composed of subunits linked by disulfide bridges.