Osteoinductive potential of highly purified porous β-TCP in mice

Hypoxia Drives Material-Induced Heterotopic Bone Formation by Enhancing Osteoclastogenesis via M2/Lipid-Loaded Macrophage Axis

Heterotopic ossification (HO) is a double-edged sword. Pathological HO presents as an undesired clinical complication, whereas controlled heterotopic bone formation by synthetic osteoinductive materials shows promising therapeutic potentials for bone regeneration. However, the mechanism of material-induced heterotopic bone formation remains largely unknown. Early acquired HO being usually accompanied by severe tissue hypoxia prompts the hypothesis that hypoxia caused by the implantation coordinates serial cellular events and ultimately induces heterotopic bone formation in osteoinductive materials. The data presented herein shows a link between hypoxia, macrophage polarization to M2, osteoclastogenesis, and material-induced bone formation. Hypoxia inducible factor-1α (HIF-1α), a crucial mediator of cellular responses to hypoxia, is highly expressed in an osteoinductive calcium phosphate ceramic (CaP) during the early phase of implantation, while pharmacological inhibition of HIF-1α significantly inhibits M2 macrophage, subsequent osteoclast, and material-induced bone formation. Similarly, in vitro, hypoxia enhances M2 macrophage and osteoclast formation. Osteoclast-conditioned medium enhances osteogenic differentiation of mesenchymal stem cells, such enhancement disappears with the presence of HIF-1α inhibitor. Furthermore, metabolomics analysis reveals that hypoxia enhances osteoclastogenesis via the axis of M2/lipid-loaded macrophages. The current findings shed new light on the mechanism of HO and favor the design of more potent osteoinductive materials for bone regeneration.

Calcium Orthophosphate (CaPO4)-Based Bioceramics: Preparation, Properties, and Applications

Various types of materials have been traditionally used to restore damaged bones. In the late 1960s, a strong interest was raised in studying ceramics as potential bone grafts due to their biomechanical properties. A short time later, such synthetic biomaterials were called bioceramics. Bioceramics can be prepared from diverse inorganic substances, but this review is limited to calcium orthophosphate (CaPO4)-based formulations only, due to its chemical similarity to mammalian bones and teeth. During the past 50 years, there have been a number of important achievements in this field. Namely, after the initial development of bioceramics that was just tolerated in the physiological environment, an emphasis was shifted towards the formulations able to form direct chemical bonds with the adjacent bones. Afterwards, by the structural and compositional controls, it became possible to choose whether the CaPO4-based implants would remain biologically stable once incorporated into the skeletal structure or whether they would be resorbed over time. At the turn of the millennium, a new concept of regenerative bioceramics was developed, and such formulations became an integrated part of the tissue engineering approach. Now, CaPO4-based scaffolds are designed to induce bone formation and vascularization. These scaffolds are usually porous and harbor various biomolecules and/or cells. Therefore, current biomedical applications of CaPO4-based bioceramics include artificial bone grafts, bone augmentations, maxillofacial reconstruction, spinal fusion, and periodontal disease repairs, as well as bone fillers after tumor surgery. Prospective future applications comprise drug delivery and tissue engineering purposes because CaPO4 appear to be promising carriers of growth factors, bioactive peptides, and various types of cells.

Sustained local ionic homeostatic imbalance caused by calcification modulates inflammation to trigger heterotopic ossification

Heterotopic ossification (HO) is a condition triggered by an injury leading to the formation of mature lamellar bone in extraskeletal soft tissues. Despite being a frequent complication of orthopedic and trauma surgery, brain and spinal injury, the etiology of HO is poorly understood. The aim of this study is to evaluate the hypothesis that a sustained local ionic homeostatic imbalance (SLIHI) created by mineral formation during tissue calcification modulates inflammation to trigger HO. This evaluation also considers the role SLIHI could play for the design of cell-free, drug-free osteoinductive bone graft substitutes. The evaluation contains five main sections. The first section defines relevant concepts in the context of HO and provides a summary of proposed causes of HO. The second section starts with a detailed analysis of the occurrence and involvement of calcification in HO. It is followed by an explanation of the causes of calcification and its consequences. This allows to speculate on the potential chemical modulators of inflammation and triggers of HO. The end of this second section is devoted to in vitro mineralization tests used to predict the ectopic potential of materials. The third section reviews the biological cascade of events occurring during pathological and material-induced HO, and attempts to propose a quantitative timeline of HO formation. The fourth section looks at potential ways to control HO formation, either acting on SLIHI or on inflammation. Chemical, physical, and drug-based approaches are considered. Finally, the evaluation finishes with a critical assessment of the definition of osteoinduction. STATEMENT OF SIGNIFICANCE: The ability to regenerate bone in a spatially controlled and reproducible manner is an essential prerequisite for the treatment of large bone defects. As such, understanding the mechanism leading to heterotopic ossification (HO), a condition triggered by an injury leading to the formation of mature lamellar bone in extraskeletal soft tissues, would be very useful. Unfortunately, the mechanism(s) behind HO is(are) poorly understood. The present study reviews the literature on HO and based on it, proposes that HO can be caused by a combination of inflammation and calcification. This mechanism helps to better understand current strategies to prevent and treat HO. It also shows new opportunities to improve the treatment of bone defects in orthopedic and dental procedures.

Effects of ultraviolet irradiation on beta-tricalcium phosphate as a bone graft substitute

Surface modification of various materials using ultraviolet (UV) irradiation improves their wettability. The purpose of this study was to investigate the wettability of a β-tricalcium phosphate (TCP) surface and the composition changes and bioactivity of β-TCP after UV irradiation. We applied 172 nm UV treatment to a β-TCP surface and measured the contact angle before and after UV irradiation. Energy-dispersive X-ray and Fourier transform infrared spectroscopy examinations were performed on the β-TCP disk with or without UV treatment. In an adhesion test of bone marrow cells using β-TCP disks with and without UV irradiation, cell attachment was measured 10, 30, 50, and 70 h after β-TCP insertion. UV-irradiated β-TCP osteogenesis and absorption of bone substitutes were evaluated using hematoxylin and eosin and tartrate-resistant acid phosphatase (TRAP) staining in a rabbit sinus model. The contact angle on the TCP surface decreased from 70° to 10° owing to UV irradiation. Conversely, UV irradiation did not change the composition of carbon, oxygen, and phosphorus. In the cell adhesion test, UV-irradiated β-TCP significantly increased cell adhesion compared with UV-unirradiated β-TCP after 10 to 30 h of culture. In the rabbit sinus model, TRAP staining showed that UV-irradiated β-TCP significantly increased the number of TRAP-positive cells compared with unirradiated β-TCP granules in the central part of β-TCP. Our results indicate that the UV irradiation of β-TCP improves its clinical utility for surgical bone augmentation in the oral and maxillofacial region.

Serial cellular events in bone formation initiated by calcium phosphate ceramics

To better understand the biological mechanisms triggered by osteoinductive materials in vivo, we evaluated the timeline of cellular responses to osteoinductive materials subcutaneously implanted in FVB mice. More F4/80-positive macrophages were present in osteoinductive tri-CaP ceramic (TCP) with submicron surface topography (TCPs) than non-osteoinductive TCP with micron surface topography (TCPb) at week 1. Moreover, TCPs (but not TCPb) significantly enhanced osteoclastogenesis, and induced macrophages to polarize from M1 to M2 in the first week. The time sequence and relevance of macrophages and osteoclasts responses involved in bone formation was then evaluated through peri-implant injection of specific chemicals in mice implanted with osteoinductive TCPs. Day-1 injection of clodronate liposomes (LipClod) depleted macrophages, inhibited macrophage polarization to M2, blocked osteoclastogenesis and bone formation, while the day-6 injection was less effective. Anti-RANKL antibody (aRANKL) did not affect macrophage colonization but inhibited osteoclastogenesis. Injection of aRANKL before week 2 aborted bone formation in TCPs, while injection at week 4 partially inhibited bone formation. The overall data show that following ectopic implantation, osteoinductive materials allow macrophage colonization in hours to days, macrophage polarization to M2 in days (within 7 days), osteoclastogenesis in weeks (e.g. in 2 weeks) and bone formation thereafter (after 4 weeks). The serial cellular events verified herein bring a new insight on material-induced bone formation and pave the way to further explore the mechanisms triggered by osteoinductive materials. STATEMENT OF SIGNIFICANCE: A series of key cellular events triggered by osteoinductive calcium phosphate ceramic was revealed: macrophages colonized within hours to days, polarization of M2 macrophages occurred within 7 days, osteoclastogenesis mainly occurred in weeks (e.g. in 2 weeks) and bone formation finally arose thereafter (after 4 weeks). Moreover, such time sequence of cellular events was confirmed with specific chemicals (clodronate liposomes and anti-RANKL antibody). The findings verified herein bring a new insight on material-induced bone formation and pave the way to further explore the mechanisms triggered by osteoinductive materials.

Increased Osteogenic Potential of Pre-Osteoblasts on Three-Dimensional Printed Scaffolds Compared to Porous Scaffolds for Bone Regeneration

Background

One of the main challenges with conventional scaffold fabrication methods is the inability to control scaffold architecture. Recently, scaffolds with controlled shape and architecture have been fabricated using three-dimensional printing (3DP). Herein, we aimed to determine whether the much tighter control of microstructure of 3DP poly(lactic-co-glycolic) acid/β-tricalcium phosphate (PLGA/β-TCP) scaffolds is more effective in promoting osteogenesis than porous scaffolds produced by solvent casting/porogen leaching.

Methods

Physical and mechanical properties of porous and 3DP scaffolds were studied. The response of pre-osteoblasts to the scaffolds was analyzed after 14 days.

Results

TThe 3DP scaffolds had a smoother surface (Ra: 22 ± 3 µm) relative to the highly rough surface of porous scaffolds (Ra: 110 ± 15 µm). Water contact angle was 112 ± 4° on porous and 76 ± 6° on 3DP scaffolds. Porous and 3DP scaffolds had the pore size of 408 ± 90 and 315 ± 17 µm and porosity of 85 ± 5% and 39 ± 7%, respectively. Compressive strength of 3DP scaffolds (4.0 ± 0.3 MPa) was higher than porous scaffolds (1.7 ± 0.2 MPa). Collagenous matrix deposition was similar on both scaffolds. Cells proliferated from day 1 to day 14 by fourfold in porous and by 3.8-fold in 3DP scaffolds. Alkaline phosphatase (ALP) activity was 21-fold higher in 3DP scaffolds than porous scaffolds.

Conclusion

The 3DP scaffolds show enhanced mechanical properties and ALP activity compared to porous scaffolds in vitro, suggesting that 3DP PLGA/β-TCP scaffolds are possibly more favorable for bone formation.

Influence of Mesenchymal Stem Cell Sources on Their Regenerative Capacities on Different Surfaces

Current gold-standard strategies for bone regeneration do not achieve the optimal recovery of bone biomechanical properties. To bypass these limitations, tissue engineering techniques based on hybrid materials made up of osteoprogenitor cells-such as mesenchymal stem cells (MSCs)-and bioactive ceramic scaffolds-such as calcium phosphate-based (CaPs) bioceramics-seem promising. The biological properties of MSCs are influenced by the tissue source. This study aims to define the optimal MSC source and construct (i.e., the MSC-CaP combination) for clinical application in bone regeneration. A previous iTRAQ analysis generated the hypothesis that anatomical proximity to bone has a direct effect on MSC phenotype. MSCs were isolated from adipose tissue, bone marrow, and dental pulp, then cultured both on a plastic surface and on CaPs (hydroxyapatite and β-tricalcium phosphate), to compare their biological features. On plastic, MSCs isolated from dental pulp (DPSCs) presented the highest proliferation capacity and the greatest osteogenic potential. On both CaPs, DPSCs demonstrated the greatest capacity to colonise the bioceramics. Furthermore, the results demonstrated a trend that DPSCs had the most robust increase in ALP activity. Regarding CaPs, β-tricalcium phosphate obtained the best viability results, while hydroxyapatite had the highest ALP activity values. Therefore, we propose DPSCs as suitable MSCs for cell-based bone regeneration strategies.

Complementary and synergistic effects on osteogenic and angiogenic properties of copper-incorporated silicocarnotite bioceramic: In vitro and in vivo studies

Promoting bone regeneration to treat bone defects is a challenging problem in orthopedics, and developing novel biomaterials with both osteogenic and angiogenic activities is sought as a feasible solution. Here, copper-silicocarnotite [Cu-Ca₅(PO₄)₂SiO₄, Cu-CPS] was designed and fabricated. In this study, the Cu-CPS ceramics demonstrated better mechanical, osteogenic, and angiogenic properties in vitro and in vivo than pure CPS one. Particularly, CPS with 1.0 wt% CuO (1.0Cu-CPS) exhibited the best performance. Additionally, hydroxyapatite with 1.0 wt% CuO (1.0Cu-HA) was used to explore the respective effects of copper and silicon (Si). According to the in vitro results, it indicated that Cu enhanced the osteogenic activity of CPS ceramics although Si played a dominate role in the osteogenic process. Moreover, Cu could promote an early stage of angiogenesis, and the complementary effect of Si and Cu was found in the late phase. Furthermore, the in vivo results illustrated that the synergistic effect of Cu and Si improved bone and vessel regeneration during the degradation of Cu-CPS scaffolds (P < 0.05). Therefore, Cu-CPS ceramics could improve osteogenesis and angiogenesis through the simultaneous effects of Cu and Si, thus, offering a promising treatment option in orthopedic application for bone tissue regeneration.

Osteoinductivity of nanostructured hydroxyapatite-functionalized gelatin modulated by human and endogenous mesenchymal stromal cells

The demand of new strategies for the induction of bone regeneration is continuously increasing. Biomimetic porous gelatin-nanocrystalline hydroxyapatite scaffolds with tailored properties were previously developed, showing a positive response in terms of cell adhesion, proliferation, and differentiation. In the present paper, we focused on their osteoinductive properties. The effect of scaffolds on osteogenic differentiation of human mesenchymal stromal cells (hMSCs) was investigated in vitro. hMSCs were seeded on GEL (type A gelatin) and GEL containing 10 wt% hydroxyapatite (GEL-HA) and cultured in osteogenic medium. Results showed that GEL and GEL-HA10 sustained hMSC differentiation, with an increased ALP activity and a higher expression of bone specific genes. The osteoinductive ability of these scaffolds was then studied in vivo in a heterotopic bone formation model in nude mice. The influence of hMSCs within the implants was examined as well. Both GEL and GEL-HA10 scaffolds mineralized when implanted without hMSCs. On the contrary, the presence of hMSC abolished or reduced mineralization of GEL and GEL-HA10 scaffolds. However, we could observe a species-specific response to the presence of HA, which stimulated osteogenic differentiation of human cells only. In conclusion, the scaffolds showed promising osteoinductive properties and may be suitable for use in confined critical defects. © 2017 Wiley Periodicals, Inc. J Biomed Mater Res Part A: 106A: 914-923, 2018.

Fabrication of dicalcium phosphate dihydrate-coated β-TCP granules and evaluation of their osteoconductivity using experimental rats

β-Tricalcium phosphate (β-TCP) has attracted much attention as an artificial bone substitute owing to its biocompatibility and osteoconductivity. In this study, osteoconductivity of β-TCP bone substitute was enhanced without using growth factors or cells. Dicalcium phosphate dihydrate (DCPD), which is known to possess the highest solubility among calcium phosphates, was coated on β-TCP granules by exposing their surface with acidic calcium phosphate solution. The amount of coated DCPD was regulated by changing the reaction time between β-TCP granules and acidic calcium phosphate solution. Histomorphometry analysis obtained from histological results revealed that the approximately 10mol% DCPD-coated β-TCP granules showed the largest new bone formation compared to DCPD-free β-TCP granules, approximately 2.5mol% DCPD-coated β-TCP granules, or approximately 27mol% DCPD-coated β-TCP granules after 2 and 4weeks of implantation. Based on this finding, we demonstrate that the osteoconductivity of β-TCP granules could be improved by coating their surface with an appropriate amount of DCPD.