Synthesis and characterization of porous β-tricalcium phosphate blocks

Acceleration of segmental bone regeneration in a rabbit model by strontium-doped calcium polyphosphate scaffold through stimulating VEGF and bFGF secretion from osteoblasts

The development of suitable bioactive three-dimensional scaffold for the promotion of bone regeneration is critical in bone tissue engineering. The purpose of this study was to investigate in vivo osteogenesis of the porous strontium-doped calcium polyphosphate (SCPP) scaffolds for bone repair, as well as the relationship between osteogenic properties of SCPP scaffolds and the secretion of bFGF and VEGF from osteoblasts stimulated by SCPP. Besides, the advantages of scaffolds seeded with mesenchymal stem cells (MSCs) for bone repair were also studied. Firstly, the bone repair evaluation of scaffolds was performed on a rabbit segmental bony defects model over a period of 16 weeks by histology combined with X-ray microradiography. And then, in order to avoid the influence from the other factors such as hypoxia which emerge in vivo study and affect the secretion of VEGF and bFGF from host cells, human osteoblast-like cells (MG63) were seeded to SCPP, CPP and HA scaffolds in vitro to determine the ability of these scaffolds to stimulate the secretion of angiogenic growth factors (VEGF and bFGF) from MG63 and further explore the reason for the better osteogenic properties of SCPP scaffolds. The histological and X-ray microradiographic results showed that the SCPP scaffolds presented better osteogenic potential than CPP and HA scaffolds, when combined with MSCs, the SCPP scaffolds could further accelerate the bone repair. And the amounts of VEGF measured by ELISA assay in SCPP, CPP and HA groups after cultured for 7 days were about 364.989 pg/mL, 244.035 pg/mL and 232.785 pg/mL, respectively. Accordingly, the amounts of bFGF were about 27.085 pg/mL, 15.727 pg/mL and 8.326 pg/mL. The results revealed that the SCPP scaffolds significantly enhanced the bFGF and VEGF secretion compared with other scaffolds. The results presented in vivo and in vitro study demonstrated that the SCPP could accelerate bone formation through stimulating the secretion of VEGF and bFGF from osteoblasts, making it attractive for bone regeneration.

Processing and characterization of innovative scaffolds for bone tissue engineering

A new protocol, based on a modified replication method, is proposed to obtain bioactive glass scaffolds. The main feature of these samples, named "shell scaffolds", is their external surface that, like a compact and porous shell, provides both high permeability to fluids and mechanical support. In this work, two different scaffolds were prepared using the following slurry components: 59 % water, 29 % 45S5 Bioglass(®) and 12 % polyvinylic binder and 51 % water, 34 % 45S5 Bioglass(®), 10 % polyvinylic binder and 5 % polyethylene. All the proposed samples were characterized by a widespread microporosity and an interconnected macroporosity, with a total porosity of 80 % vol. After immersion in a simulated body fluid (SBF), the scaffolds showed strong ability to develop hydroxyapatite, enhanced by the high specific surface of the porous systems. Moreover preliminary biological evaluations suggested a promising role of the shell scaffolds for applications in bone tissue regeneration. As regards the mechanical behaviour, the shell scaffolds could be easily handled without damages, due to their resistant external surface. More specifically, they possessed suitable mechanical properties for bone regeneration, as proved by compression tests performed before and after immersion in SBF.

Interconnectivity analysis of supercritical CO₂-foamed scaffolds

This paper describes a computer algorithm for the determination of the interconnectivity of the pore space inside scaffolds used for tissue engineering. To validate the algorithm and its computer implementation, the algorithm was applied to a computer-generated scaffold consisting of a set of overlapping spherical pores, for which the interconnectivity was calculated exactly. The algorithm was then applied to micro-computed X-ray tomography images of supercritical CO(2)-foamed scaffolds made from poly(lactic-co-glycolic acid) (PLGA), whereby the effect of using different weight average molecular weight polymer on the interconnectivity was investigated.

Sol-gel method to fabricate CaP scaffolds by robocasting for tissue engineering

Highly porous calcium phosphate (CaP) scaffolds for bone-tissue engineering were fabricated by combining a robocasting process with a sol-gel synthesis that mixed Calcium Nitrate Tetrahydrate and Triethyl Phosphite precursors in an aqueous medium. The resulting gels were used to print scaffolds by robocasting without the use of binder to increase the viscosity of the paste. X-ray diffraction analysis confirmed that the process yielded hydroxyapatite and β-tricalcium phosphate biphasic composite powders. Thus, the scaffold composition after crystallization of the amorphous structure could be easily modified by varying the initial Ca/P ratio during synthesis. The compressive strengths of the scaffolds are ~6 MPa, which is in the range of human cancellous bone (2-12 MPa). These highly porous scaffolds (~73 vol% porosity) are composed of macro-pores of ~260 μm in size; such porosity is expected to enable bone ingrowth into the scaffold for bone repair applications. The chemistry, porosity, and surface topography of such scaffolds can also be modified by the process parameters to favor bone formation. The studied sol-gel process can be used to coat these scaffolds by dip-coating, which induces a significant enhancement of mechanical properties. This can adjust scaffold properties such as composition and surface morphology, which consequently may improve their performances.

Heat treatment of Na2O-CaO-P2O5-SiO2 bioactive glasses: densification processes and postsintering bioactivity

Because of their excellent bioactivity, bioactive glasses are increasingly diffused to produce biomedical devices for bone prostheses, to face the dysfunctions that may be caused by traumatic events, diseases, or even natural aging. However, several processing routes, such as the production of scaffolds or the deposition of coatings, include a thermal treatment to apply or sinter the glass. The exposure to high temperature may induce a devetrification phenomenon, altering the properties and, in particular, the bioactivity of the glass. The present contribution offers an overview of the thermal behavior and properties of two glasses belonging to the Na2O-CaO-P2O5-SiO2 system, to be compared to the standard 45S5 Bioglass(®). The basic goal is to understand the effect of both the original composition and the thermal treatment on the performance of the sintered glasses. The new glasses, the one (BG_Na) with a high content of Na2O, the other (BG_Ca) with a high content of CaO, were fully characterized and sintering tests were performed to define the most interesting firing cycles. The sintered samples, treated at 880°C and 800°C respectively, were investigated from a microstructural point of view and their mechanical properties were compared to those of the bulk (not sintered) glass counterparts. The effect of sintering was especially striking on the BG_Ca material, whose Vickers hardness increased from 598.9 ± 46.7 HV to 1053.4 ± 35.0 HV. The in vitro tests confirmed the ability of the glasses, both in bulk and sintered form, of generating a hydroxyapatite surface layer when immersed in a simulated body fluid. More accurate biological tests performed on the sintered glasses proved the high bioactivity of the CaO-rich composition even after a heat treatment.

Effect of subvoxel processes on non-destructive characterization of β-tricalcium phosphate bone graft substitutes

The geometric features of bone graft substitutes, such as the pore and pore interconnection sizes, are of paramount importance for their biological performance. Such features are generally characterized by micro-computed tomography (μCT). Unfortunately, the resolution of μCT is often too limited. The aim of this study was to look at the effect of μCT resolution on the geometric characterization of four different bone graft substitutes. An attempt was also made to improve the characterization of these materials by applying a subvoxelization algorithm. The results revealed that both approaches increased the accuracy of the geometric characterization. They also showed that the interconnection size in particular was affected. Comparing the results obtained from the scanned and numerical subvoxelization datasets revealed a minor difference of less than 2.5% for the porosity values. The difference for the pore sizes was up to 10%. Considerable differences of up to 35-50% were found for the interconnection sizes. The present study demonstrates how complex geometric characterization is and how important it is for biomaterial researchers to be aware of the impact of μCT resolution on the pore and pore interconnection sizes.

Personalized implant for high tibial opening wedge: Combination of solid freeform fabrication with combustion synthesis process

In this work a new generation of bioceramic personalized implants were developed. This technique combines the processes of solid freeform fabrication (SFF) and combustion synthesis (CS) to create personalized bioceramic implants with tricalcium phosphate (TCP) and hydroxyapatite (HA). These porous bioceramics will be used to fill the tibial bone gap created by the opening wedge high tibial osteotomy (OWHTO). A freeform fabrication with three-dimensional printing (3DP) technique was used to fabricate a metallic mold with the same shape required to fill the gap in the opening wedge osteotomy. The mold was subsequently used in a CS process to fabricate the personalized ceramic implants with TCP and HA compositions. The mold geometry was designed on commercial 3D CAD software. The final personalized bioceramic implant was produced using a CS process. This technique was chosen because it exploits the exothermic reaction between P2O5 and CaO. Also, chemical composition and distribution of pores in the implant could be controlled. To determine the chemical composition, the microstructure, and the mechanical properties of the implant, cylindrical shapes were also fabricated using different fabrication parameters. Chemical composition was performed by X-ray diffraction. Pore size and pore interconnectivity was measured and analyzed using an electronic microscope system. Mechanical properties were determined by a mechanical testing system. The porous TCP and HA obtained have an open porous structure with an average 400 µm channel size. The mechanical behavior shows great stiffness and higher load to failure for both ceramics. Finally, this personalized ceramic implant facilitated the regeneration of new bone in the gap created by OWHTO and provides additional strength to allow accelerated rehabilitation.

Evaluation of 3-D bioactive glass scaffolds dissolution in a perfusion flow system with X-ray microtomography

Bioactive glass has high potential for bone regeneration due to its ability to bond to bone and stimulate osteogenesis whilst dissolving in the body. Although three-dimensional (3-D) bioactive glass scaffolds with favorable pore networks can be made from the sol-gel process, compositional and structural evolutions in their porous structures during degradation in vivo, or in vitro, have not been quantified. In this study, bioactive glass scaffolds were put in a simulated body fluid flow environment through a perfusion bioreactor. X-ray microtomography (μCT) was used to non-destructively image the scaffolds at different degradation stages. A new 3-D image processing methodology was developed to quantify the scaffold's pore size, interconnect size and connectivity from μCT images. The accurate measurement of individual interconnect size was made possible by a principal component analysis-based algorithm. During 28 days of dissolution, the modal interconnect size in the scaffold was reduced from 254 to 206 μm due to the deposition of mineral phases. However, the pore size remained unchanged, with a mode of 682 μm. The data presented are important for making bioactive glass scaffolds into clinical products. The technique described for imaging and quantifying scaffold pore structures as a function of degradation time is applicable to most scaffold systems.

Structural and material approaches to bone tissue engineering in powder-based three-dimensional printing

This article reviews the current state of knowledge concerning the use of powder-based three-dimensional printing (3DP) for the synthesis of bone tissue engineering scaffolds. 3DP is a solid free-form fabrication (SFF) technique building up complex open porous 3D structures layer by layer (a bottom-up approach). In contrast to traditional fabrication techniques generally subtracting material step by step (a top-down approach), SFF approaches allow nearly unlimited designs and a large variety of materials to be used for scaffold engineering. Today's state of the art materials, as well as the mechanical and structural requirements for bone scaffolds, are summarized and discussed in relation to the technical feasibility of their use in 3DP. Advances in the field of 3DP are presented and compared with other SFF methods. Existing strategies on material and design control of scaffolds are reviewed. Finally, the possibilities and limiting factors are addressed and potential strategies to improve 3DP for scaffold engineering are proposed.

Porous hydroxyapatite and gelatin/hydroxyapatite microspheres obtained by calcium phosphate cement emulsion

Hydroxyapatite and hybrid gelatine/hydroxyapatite microspheres were obtained through a water in oil emulsion of a calcium phosphate cement (CPC). The setting reaction of the CPC, in this case the hydrolysis of α-tricalcium phosphate, was responsible for the consolidation of the microspheres. After the setting reaction, the microspheres consisted of an entangled network of hydroxyapatite crystals, with a high porosity and pore sizes ranging between 0.5 and 5 μm. The size of the microspheres was tailored by controlling the viscosity of the hydrophobic phase, the rotation speed, and the initial powder size of the CPC. The incorporation of gelatin increased the sphericity of the microspheres, as well as their size and size dispersion. To assess the feasibility of using the microspheres as cell microcarriers, Saos-2 cells were cultured on the microspheres. Fluorescent staining, SEM studies, and LDH quantification showed that the microspheres were able to sustain cell growth. Cell adhesion and proliferation was significantly improved in the hybrid gelatin/hydroxyapatite microspheres as compared to the hydroxyapatite ones.

Platelet-rich plasma on calcium phosphate granules promotes metaphyseal bone healing in mini-pigs

The role of platelet-rich plasma (PRP) as a promoter of bone healing remains controversial. The aim of this study was to investigate the effect of PRP in combination with calcium phosphate granules (CPG) on bone defect healing in a metaphyseal long bone defect. A metaphyseal bone defect at the proximal tibia of 16 mini-pigs was filled with CPG combined with autologous PRP or CPG solely (control group). The PRP showed 4.4-fold more platelets compared to peripheral blood. Six weeks after surgery the radiological and histomorphometrical evaluations showed significantly more bone formation in the PRP group in the central area of the defect zone (p < 0.01) as well as the cortical defect zone (p < 0.04). Furthermore, the resorption rate of CPG was increased in animals who received PRP. Nevertheless there were only isolated instances of complete osseous bridging of the bone defects even in the PRP group. This study demonstrates that a PRP-CPG composit promotes bone regeneration but does not lead to a solid fusion of a tibial defect in mini-pigs.

Biofilm formation on bone grafts and bone graft substitutes: comparison of different materials by a standard in vitro test and microcalorimetry

We analyzed the initial adhesion and biofilm formation of Staphylococcus aureus (ATCC 29213) and S. epidermidis RP62A (ATCC 35984) on various bone grafts and bone graft substitutes under standardized in vitro conditions. In parallel, microcalorimetry was evaluated as a real-time microbiological assay in the investigation of biofilm formation and material science research. The materials beta-tricalcium phosphate (beta-TCP), processed human spongiosa (Tutoplast) and poly(methyl methacrylate) (PMMA) were investigated and compared with polyethylene (PE). Bacterial counts (log(10) cfu per sample) were highest on beta-TCP (S. aureus 7.67 +/- 0.17; S. epidermidis 8.14 +/- 0.05) while bacterial density (log(10) cfu per surface) was highest on PMMA (S. aureus 6.12 +/- 0.2, S. epidermidis 7.65 +/- 0.13). Detection time for S. aureus biofilms was shorter for the porous materials (beta-TCP and processed human spongiosa, p < 0.001) compared to the smooth materials (PMMA and PE), with no differences between beta-TCP and processed human spongiosa (p > 0.05) or PMMA and PE (p > 0.05). In contrast, for S. epidermidis biofilms the detection time was different (p < 0.001) between all materials except between processed human spongiosa and PE (p > 0.05). The quantitative analysis by quantitative culture after washing and sonication of the material demonstrated the importance of monitoring factors like specific surface or porosity of the test materials. Isothermal microcalorimetry proved to be a suitable tool for an accurate, non-invasive and real-time microbiological assay, allowing the detection of bacterial biomass without removing the biofilm from the surface.

The progress of early phase bone healing using porous granules produced from calcium phosphate cement

Objective

Bone grafting is a vital component in many surgical procedures to facilitate the repair of bone defects or fusions. Autologous bone has been the gold standard to date in spite of associated donor-site morbidity and the limited amount of available donor bone. The aim of this study was to investigate the progress of bone regeneration and material degradation of calcium phosphate granules (CPG) produced from a calcium phosphate self-setting cement powder compared to the use of autologous bone grafting in the treatment of "critical size defects" on load-bearing long bones of minipigs.

Methods

A critical size defect in the tibial metaphysis of 16 mini-pigs was filled either with autologous cancellous graft or with micro- and macroporous carbonated, apatic calcium phosphate granules (CPG) produced from a calcium phosphate self-setting cement powder. After 6 weeks, the specimens were assessed by X-ray and histological evaluation. The amount of new bone formation was analysed histomorphometrically.

Results

The semi-quantitative analysis of the radiological results showed a complete osseous bridging of the defect in three cases for the autograft group. In the same group five animals showed a beginning, but still incomplete bridging of the defect, whereas in the CPG group just two animals developed this. All other animals of the CPG group showed only a still discontinuous new bone formation. Altogether, radiologically a better osseous bridging was observed in the autograft group compared to the CPG group.

Histomorphometrical analysis after six weeks of healing revealed that the area of new bone was significantly greater in the autograft group concerning the central area of the defect zone (p < 0.001) as well as the cortical defect zone (p < 0.002). All defects showed new bone formation, but only in the autograft group defects regenerated entirely

Conclusions

Within the limits of the present study it could be demonstrated that autologous cancellous grafts lead to a significantly better bone regeneration compared to the application of calcium phosphate granules (CPG) produced from a calcium phosphate self-setting cement powder after 6 weeks. In the early phase of bone-healing, the sole application of CPG appears to be inferior to the autologous cancellous grafts in an in vivo critical size defect on load-bearing long bones of mini-pigs.

Chondrogenic pre-induction of human mesenchymal stem cells on beta-TCP: enhanced bone quality by endochondral heterotopic bone formation

New techniques to heal bone defects include the combination of bone substitute materials with mesenchymal stem cells (MSC). To find solutions not hampered by low material resorbability or high donor variability of human MSC, the potency of such composites is usually evaluated by heterotopic bone formation assays in immunocompromised animals. The aim of this study was to investigate whether resorbable phase-pure beta-tricalcium-phosphate (beta-TCP) could support heterotopic bone formation by MSC comparable to partially resorbable hydroxyapatite/tricalcium-phosphate (HA/TCP). Furthermore, in light of disappointing results with osteogenic in vitro priming of MSC, we tested whether chondrogenic pre-induction of constructs may allow for enhanced bone formation by triggering the endochondral pathway. beta-TCP granules of three different sizes and HA/TCP were seeded with MSC and transplanted subcutaneously into immunocompromised mice either immediately or after a chondrogenic pre-induction for 6 weeks. After 8 weeks, explants were analysed by histology. beta-TCP seeded with unprimed MSC revealed intramembranous bone formation without haematopoietic marrow with 3.8-fold more bone formed with granules smaller than 0.7 mm than with 0.7-1.4mm particles (p< or =0.018). Chondrogenic pre-induction of beta-TCP/MSC composites resulted in collagen type II and proteoglycan-rich cartilage-like tissue which, after transplantation, underwent endochondral ossification, yielding ectopic bone produced by human cells while haematopoietic marrow was derived from the mouse. Transdifferentiation of MSC-derived chondrocytes to osteoblasts or direct osteogenesis of cartilage-resident MSC is postulated to explain the human origin of new bone. In conclusion, beta-TCP was significantly more osteo-permissive (p=0.004) than HA/TCP for human MSC, and chondrogenic priming of beta-TCP/MSC represented a superior approach capable of supporting full bone formation, including marrow organization.

New processing approaches in calcium phosphate cements and their applications in regenerative medicine

The key feature of calcium phosphate cements (CPCs) lies in the setting reaction triggered by mixing one or more solid calcium phosphate salts with an aqueous solution. Upon mixture, the reaction takes place through a dissolution-precipitation process which is macroscopically observed by a gradual hardening of the cement paste. The precipitation of hydroxyapatite nanocrystals at body or room temperature, and the fact that those materials can be used as self-setting pastes, have for many years been the most attractive features of CPCs. However, the need to develop materials able to sustain bone tissue ingrowth and be capable of delivering drugs and bioactive molecules, together with the continuous requirement from surgeons to develop more easily handling cements, has pushed the development of new processing routes that can accommodate all these requirements, taking advantage of the possibility of manipulating the self-setting CPC paste. It is the goal of this paper to provide a brief overview of the new processing developments in the area of CPCs and to identify the most significant achievements.

Aqueous impregnation of porous beta-tricalcium phosphate scaffolds

The ability of a porous bone graft substitute to be impregnated with an aqueous solution is of great importance for tissue engineering and in vivo applications. This study presents an impregnation test setup and assesses the effect of various synthesis parameters such as sintering temperature, composition, macroporosity and macropore size on the impregnation properties of porous beta-tricalcium phosphate scaffolds dipped in water. Among those parameters, the macropore size had by far the largest effect; generally, the bigger the macropore size, the lower the saturation level. The results also showed that impregnation was less complete when the samples were fully dipped in water than when they were only partially dipped, owing to the requirement for the system to create air bubbles under water.

Generating Porous Ceramic Scaffolds: Processing and Properties

For tissue regeneration in medicine three-dimensional scaffolds with specific characteristics are required. A very important property is a high, interconnecting porosity to enable tissue ingrowth into the scaffold. Pore size distribution and pore geometry should be adapted to the respective tissue. Additionally, the scaffolds should have a basic stability for handling during implantation, which is provided by ceramic scaffolds. Various methods to produce such ceramic 3D scaffolds exist. In this paper conventional and new fabrication techniques are reviewed. Conventional methods cover the replica of synthetic and natural templates, the use of sacrificial templates and direct foaming. Rapid prototyping techniques are the new methods listed in this work. They include fused deposition modelling, robocasting and dispense-plotting, ink jet printing, stereolithography, 3D-printing, selective laser sintering/melting and a negative mould technique also involving rapid prototyping. The various fabrication methods are described and the characteristics of the resulting scaffolds are pointed out. Finally, the techniques are compared to find out their disadvantages and advantages.

Geometric analysis of porous bone substitutes using micro-computed tomography and fuzzy distance transform

There is increased interest in resorbable bone substitutes for skeletal reconstruction. Important geometric design measures of bone substitute include pore size, interconnection size, porosity, permeability and surface area of the substitute. In this study, four substitute groups with variable geometric features but constant porosity were scanned using micro-computed tomography (microCT) and their geometric measures were determined using an advanced image-processing algorithm based on fuzzy distance transform and new pore size definition. The substitutes were produced using the calcium phosphate emulsion method. The geometric analysis revealed that the reproducibility of the emulsion method was high, within 5%. The average porosity of the four groups was 52.3 + or - 1.5. The pore diameter of the four bone substitute groups was measured to be 170 + or - 1.7, 217 + or - 5.2, 416 + or - 19, and 972 + or - 11 microm. Despite this significant change in pore size, the interconnection size only increased slightly with an increase of pore size. The specific surface decreased with increasing pore size. The permeability increased with the pore size and was inversely proportional to the specific surface. The combination of microCT and the fuzzy image-processing tool enables accurate geometric analysis, even if pore size and image resolution are in the same range, such as in the case of the smallest pore size. Moreover, it is an exciting tool to understand the structure of the substitute with the hope of designing better bone substitutes.

Direct write assembly of calcium phosphate scaffolds using a water-based hydrogel

The development of materials to support bone regeneration requires flexible fabrication technologies able to tailor chemistry and architecture for specific applications. In this work we describe the preparation of ceramic-based inks for robotic-assisted deposition (robocasting) using Pluronic F-127 solutions. This approach allows the preparation of pseudoplastic inks with solid contents ranging between 30 and 50 vol.%, enabling them to flow through a narrow printing nozzle while supporting the weight of the printed structure. Ink formulation does not require manipulation of the pH or the use of highly volatile organic components. Therefore, the approach can be used to prepare materials with a wide range of compositions, and here we use it to build hydroxyapatite (HA), beta-tricalcium phosphate (beta-TCP) and biphasic (HA/beta-TCP) structures. The flow of the inks is controlled by the Pluronic content and the particle size distribution of the ceramic powders. The use of wide size distributions favors flow through the narrow printing nozzles and we have been able to use printing nozzles as narrow as 100 microm in diameter, applying relatively low printing pressures. The microporosity of the printed lines increases with increasing Pluronic content and lower sintering temperatures. Microporosity can play a key role in determining the biological response to the materials, but it also affects the strength of the structure.

Microporous pure beta-tricalcium phosphate implants for press-fit fixation of anterior cruciate ligament grafts: strength and healing in a sheep model

PURPOSE

A sheep study was conducted to test a press-fit technique using microporous pure beta-tricalcium phosphate (beta-TCP) dowels for fixation of the anterior cruciate ligament (ACL) graft.

METHODS

Microporous (5 mum) cylindrical plugs of beta-TCP (diameter, 7 mm; length, 25 mm) with interconnecting pores were used. The material featured a novel configuration of structure and surface geometry. Implants were tested by use of press-fit fixation of ACL grafts with and without bone blocks in 42 sheep over a period of 24 weeks. Biomechanical, radiologic, histologic, and immunohistochemical evaluations were performed.

RESULTS

In load-to-failure tests at 6, 12, and 24 weeks after surgery, the intra-articular graft always failed, not the fixation. Grafts showed bony fixation in the tunnel at 6 weeks and primary healing at the junction of the tunnel and joint after 24 weeks. Tricalcium phosphate was resorbed and simultaneously replaced by bone. Remodeling was still incomplete at 24 weeks.

CONCLUSIONS

In the sheep model microporous beta-TCP implants used with press-fit fixation of ACL grafts permit early functional rehabilitation. After 6 weeks, the graft is fixed by woven bone or bony integration. Implanted microporous tricalcium phosphate is resorbed and replaced by bone.

CLINICAL RELEVANCE

In a sheep model we showed that primary healing of ACL grafts with resorption and bony replacement of the fixating implant can be achieved by means of press-fit fixation with pure beta-TCP.

Targeted mechanical properties for optimal fluid motion inside artificial bone substitutes

Our goal was to develop a method to identify the optimal elastic modulus, Poisson's ratio, porosity, and permeability values for a mechanically stressed bone substitute. We hypothesized that a porous bone substitute that favors the transport of nutriments, wastes, biochemical signals, and cells, while keeping the fluid-induced shear stress within a range that stimulates osteoblasts, would likely promote osteointegration. Two optimization criteria were used: (i) the fluid volume exchange between the artificial bone substitute and its environment must be maximal and (ii) the fluid-induced shear stress must be between 0.03 and 3 Pa. Biot's poroelastic theory was used to compute the fluid motion due to mechanical stresses. The impact of the elastic modulus, Poisson's ratio, porosity, and permeability on the fluid motion were determined in general and for three different bone substitute sizes used in high tibial osteotomy. We found that fluid motion was optimized in two independent steps. First, fluid transport was maximized by minimizing the elastic modulus, Poisson's ratio, and porosity. Second, the fluid-induced shear stress could be adjusted by tuning the bone substitute permeability so that it stayed within the favorable range of 0.03 to 3 Pa. Such method provides clear guidelines to bone substitute developers and to orthopedic surgeons for using bone substitute materials according to their mechanical environment.

Porosity and pore size of beta-tricalcium phosphate scaffold can influence protein production and osteogenic differentiation of human mesenchymal stem cells: an in vitro and in vivo study

The interaction of stem cells and ceramics in bone regeneration is still poorly understood. The aim of this study was to examine the influence of the porosity (25%, 65% and 75%) of beta-tricalcium phosphate (TCP) ceramics on osteogenic differentiation of mesenchymal stem cells (MSC) in vitro and in vivo. For the in vitro portion of the study, TCP scaffolds loaded with MSC were kept in osteogenic induction medium for 21 days. For the in vivo portion of the study, scaffolds loaded with undifferentiated MSC were implanted subcutaneously into SCID mice for 8 weeks and compared with similarly implanted controls that were not loaded with MSC. Measurements of total protein as well as specific alkaline phosphatase (ALP) activity were taken as indicators of growth/matrix production and osteogenic differentiation. An increase in the total protein concentration was noted from day 1 to day 21 on the in vitro TCP 65% and TCP 75% scaffolds (p<0.05) with no such increase noted in the TCP 25% specimens. However, the specific alkaline phosphatase activity increased from day 1 to day 21 in all three in vitro specimens (p<0.02) and reached similar levels in each specimen by day 21. In vivo, ALP activity of cell-loaded TCP 65% ceramics was higher when compared with both the TCP 25% and TCP 75% specimens (p<0.046), and higher in the TCP 75% than TCP 25% specimens (p=0.008). Histology revealed mineralization by human cells in the pores of the TCP ceramic scaffolds with a trend toward greater calcification in TCP 65% and 75%. In summary, a higher porosity of TCP scaffolds does not necessarily mean a higher ALP activity in vivo. The distribution and size of the pores, as well as the surface structure, might play an important role for osteogenic differentiation in vivo.

Bone ingrowth in zirconia and hydroxyapatite scaffolds with identical macroporosity

The role of the material composition, porosity and surface topography of scaffolds for promotion of osteogenesis and osseointegration is not fully understood. The aim of the present study was to evaluate the effects of material composition and surface topography on bone ingrowth and bone contact. Designed macroporous ceramic scaffolds of zirconia and hydroxyapatite were used. Using free form fabrication (FFF) techniques an identical macroporosity in both materials was achieved. The scaffolds were implanted in rabbit tibia (cortical bone) and femur (trabecular bone). After 6 weeks of implantation the tissue response was assessed with histology and histomorphometry. The results showed significantly more bone ingrowth and bone contact in the hydroxyapatite scaffolds compared to the zirconia scaffold. Surface topography had no significant effect on bone contact inside the macropores regardless of material. This was observed in both cortical and trabecular bone sites. The study suggests that the difference between hydroxyapatite and zirconia was due to a difference in material chemistry.

Effect of platelet-rich plasma on the in vitro proliferation and osteogenic differentiation of human mesenchymal stem cells on distinct calcium phosphate scaffolds: the specific surface area makes a difference

The in vitro effect of platelet-rich plasma (PRP) on cell loading, proliferation, and osteogenic differentiation of human mesenchymal stem cells (MSC) is assessed on distinct resorbable and synthetic calcium phosphate scaffolds. A high specific surface area scaffold composed of calcium-deficient hydroxyapatite (CDHA; 48m2/g) is compared with one made out of beta-tricalcium phosphate (beta-TCP; surface area <0.5 m2/g). Fivefold concentrated fresh PRP is applied to scaffolds loaded with 2 x 10(5) MSC (n = 5). These constructs are kept in a medium with osteogenic supplements for 3 weeks. The addition of PRP leads to a higher cell loading efficiency of MSC on CDHA (p = 0.0001), that reaches the values of beta-TCP. Proliferation over 21 days is improved by PRP both on CDHA (p = 0.0001) and beta-TCP (p = 0.014) compared to MSC/calcium phosphate composites. Without the addition of PRP, CDHA has a lower cell loading efficiency (p= 0.0001) and proliferation (p= 0.001) than beta-TCP. The ALP activity is higher in the MSC/ceramics groups than in the monolayer controls (p<0.05). The addition of PRP does not significantly affect ALP activity. However, ALP activity varies considerably within the cell donors and different PRP-pools (p = 0.001), while the cell numbers do not vary within these two parameters. PRP generates a positive effect on the loading efficiency of MSC on the high specific surface scaffold CDHA that thereby reaches the loading efficiency of beta-TCP. PRP improved proliferation, but its osteogenic properties on both calcium phosphate scaffolds are weak.

Osteoblast cell response to beta-tricalcium phosphate scaffolds with controlled architecture in flow perfusion culture system

Porous beta-tricalcium phosphate (beta-TCP) scaffolds with controlled architecture and improved mechanical properties were fabricated by combining the gel-casting and rapid prototyping techniques. The pore morphology, size, and distribution of the beta-TCP scaffolds were characterized using a scanning electron microscope. The porosity of the resulting scaffolds with pore size range from 300 to 500 microm was 46%. The average compressive strength was 16.1 MPa. X-ray diffraction was used to determine the crystal structure and chemical composition of scaffolds. The result indicated that the sintering process has not changed the composition of beta-TCP. Flow perfusion culture system was developed in our lab to improve mass transfer for seeded cells. For scaffold/cell constructs cultured under flow perfusion for 4, 8, and 16 days, there was greater scaffold cellularity and alkaline phosphatase activity compared with static culture condition. These results indicated that flow perfusion culture system had evident effects on osteoblast viability and functions in vitro.

Fabrication of porous substrates: a review of processes using pore forming agents in the biomaterial field

This paper is a review of solid and casting manufacturing processes able to create porous materials, mainly in the biomaterial field. The considered methods are based on pore forming agents that are removed either by heating or by dissolution. All techniques lead to products presenting pores with amount, size, and shape are close to those of the initial pore formers. Porosities up to 90% with pores ranging from 1 to 2000 microm are reported. Major differences concern macrointerconnections that are more frequently obtained using foams, or porogens which undergo a melting stage during firing. Casting methods combined with solid free form fabrication are promising for the design of porous network through the manufacturing of 3D scaffolds corresponding to the desired porosity.

Skeletal deformations in medaka (Oryzias latipes) visualized by synchrotron radiation micro-computer tomography (SRmicroCT)

Synchrotron radiation micro-computer tomography (SRmicroCT) offers the possibility to investigate biomineralized structures in high detail. Two animals of adult medaka fish (Oryzias latipes) were analyzed by SRmicroCT with a resolution of 6.55 microm: the wild-type animal was normally developed whereas the second animal showed an idiopathic deformation of the cranial and axial skeleton. These deformations could be followed on the macro- and on the microscale (i.e., on the level of the individual ribs and fin bones). Our study clearly demonstrates that SRmicroCT is an excellent technique to study alterations in the skeletal structure of fish in detail.

Bone Response Inside Free-Form Fabricated Macroporous Hydroxyapatite Scaffolds with and without an Open Microporosity

BACKGROUND

The technique of free-form fabrication enables the production of controlled macroporous geometry inside ceramic scaffolds. Using scaffolds with identical macropore design makes it possible to study a relevant biological response linked to other specific changes of the material.

PURPOSE

This study investigates the role of open micropores in hydroxyapatite (HA) scaffold during early bone healing to quantitatively ascertain whether microporosity in otherwise identical macroporous HA scaffolds can influence the bone response in rabbit tibia and femur at 6 weeks.

MATERIALS AND METHODS

HA scaffolds (Ø: 3.8 mm) with and without microporosity were randomly installed in both cortical and trabecular bone sites of New Zealand White rabbits. The animals were sacrificed 6 weeks after surgery. Ground sections obtained from en bloc tissues containing scaffold and recipient bone were subjected to histological evaluation and histomorphometric analysis.

RESULTS

Microscopy showed elevated amounts of bone ingrowth and bone contact inside the microporous HA (mHA) group as compared with non-mHA.

CONCLUSION

The current study indicates that the presence of open scaffold microporosity in HA, as determined by the fabrication process, enhances the ability of ceramic scaffolds to promote bone ingrowth and bone contact.

In vivo behavior of calcium phosphate scaffolds with four different pore sizes

The goal of the present study was to assess the effect of macropore size on the in vivo behavior of ceramic scaffolds. For that purpose, beta-tricalcium phosphate (beta-TCP) cylinders with four different macropore sizes (150, 260, 510, and 1220 microm) were implanted into drill hole defects in cancellous bone of sheep and their resorption behavior was followed for 6, 12 and 24 weeks. The scaffolds were evaluated for biocompatibility, and new bone formation was observed macroscopically, histologically and histomorphometrically. Histomorphometrical measurements were performed for the whole defect area and for the area subdivided into three concentric rings (outer, medial, and inner ring). All implants were tolerated very well as evidenced by the low amount of inflammatory cells and the absence of macroscopic signs of inflammation. Resorption proceeded fast since less than 5% ceramic remained at 24-week implantation. Hardly any effect of macropore size was observed on the in vivo response. Samples with an intermediate macropore size (510 microm) were resorbed significantly faster than samples with smaller macropore sizes (150 and 260 microm). However, this fast resorption was associated with a lower bone content and a higher soft tissue content. At 12 and 24 weeks, the latter differences had disappeared. Bone was more abundant in the outer ring than in the rest of the blocks at 6 weeks, and in the outer and medial ring compared to the inner ring at 12 weeks.

Mechanism of bone incorporation of β-TCP bone substitute in open wedge tibial osteotomy in patients

A histological study was performed of bone biopsies from 16 patients (17 biopsies) treated with open wedge high tibial osteotomies for medial knee osteoarthritis. The open wedge osteotomies were filled with a wedge of osteoconductive beta tricalcium phosphate (beta-TCP) ceramic bone replacement. At the time of removal of the fixation material, core biopsies of the area where the beta-TCP was located were taken at different follow-up periods (6-25 months). beta-TCP resorption, bone ingrowth and bone remodelling were studied. We hypothesized that the incorporation and remodelling process occurs similarly as in animals. Histology showed a good resorption of the beta-TCP with complete incorporation and remodelling into new bone. The different phases as described in animal studies were found. A correlation was found between histological findings and radiological assessment. In conclusion, beta-TCP appeared to be a bone replacement material with optimal biocompatibility, resorption characteristics and bone conduction properties for the clinical use.

Application of synchrotron-radiation-based computer microtomography (SRμCT) to selected biominerals: embryonic snails, statoliths of medusae, and human teeth

Synchrotron-radiation-based computer microtomography (SRmicroCT) was applied to three biomineralised objects First, embryonic snails of the freshwater snail Biomphalaria glabrata, second, rhopalia (complex sense organs) of the medusa Aurelia aurita, and third, human teeth. The high absorption contrast between the soft tissue and mineralised tissues, i.e. the shell in the first case (consisting of calcium carbonate) and the statoliths in the second case (consisting of calcium sulphate hemihydrate), makes this method ideal for the study of biomineralised tissues. The objects can be non-destructively studied on a micrometre scale, and quantitative parameters like the thickness of a forming a snail shell or statolith crystal sizes can be obtained on a length scale of 1-2 mum. Using SRmicroCT, the dentin-enamel border can be clearly identified in X-ray dense teeth.