Charlie Campion

The effects of microporosity on osteoinduction of calcium phosphate bone graft substitute biomaterials.

The effect of increasing strut porosity on the osteoinductive ability of silicate substituted calcium phosphate (SiCaP) biomaterials was investigated in an ectopic ovine model. Implants with strut porosities of 22.5%, 32.0% and 46.0% were inserted into the parapsinalis muscle. At 8, 12 and 24 weeks histological sections were prepared. Sections were examined using backscattered scanning electron microscopy and un-decalcified histology. Bone area, implant area and bone-implant contact were quantified. At 8 weeks there was no significant difference between the groups in terms of bone area and implant area. However at 12 weeks, the amount of bone formation observed was significantly greater in SiCaP-46 (6.17 ± 1.51%) when compared with SiCaP-22.5 (1.33 ± 0.84%) p=0.035. Results also showed significantly increased amounts of bone-implant contact to the SiCaP-46 scaffold (3.30 ± 1.17%) compared with SiCaP-22.5 (0.67 ± 0.52%, p=0.043) at 8 weeks and 12 weeks; (SiCaP-46 (21.82 ± 5.59%) vs SiCaP-22.5 (3.06 ± 1.89%), p=0.012). At 24 weeks, bone formation and graft resorption had significantly increased in all groups so that the level of bone formation in the SiCaP-46 group had increased 75-fold to 30.05 ± 8.38%. Bone formation was observed in pores <10 μm. Results suggest that bone graft substitute materials with greater strut porosity are more osteoinductive.

Effect of increased strut porosity of calcium phosphate bone graft substitute biomaterials on osteoinduction

The effect of increasing strut porosity on the osteoinductivity of porous calcium phosphate (CaP) and silicate-substituted calcium phosphate (SiCaP) bone substitute materials was investigated in an ovine ectopic model. One to two millimeter-sized granules or block implants with strut porosities of 10, 20, or 30% were inserted into the left and right paraspinalis muscle. At 12 weeks, histological sections were prepared through the center of each implant and bone contact, bone area and implant area quantified. Backscattered scanning electron microscopy (bSEM) was used to visualize bone within small pores in the struts of the scaffolds. Increased bone formation was measured in the SiCaP with 30% strut porosity (5.482% ± 1.546%) when compared with the nonsilicate CaP with the same morphology (1.160% ± 0.502%, p = 0.02), indicating that silicate substitution may increase osteoinduction. Greater bone formation was seen in scaffolds with increased strut porosity. No bone growth was found in any of the SiCaP scaffold with 10% porosity. There was no significant difference between block and granule specimens. Scanning electron microscopy and EDX in combination with histology demonstrated bone formation within pores <5 μm in size. The use of silicate-substituted CaP material with increased strut porosity may further augment repair and regeneration in bony sites.

Increasing strut porosity in silicate-substituted calcium-phosphate bone graft substitutes enhances osteogenesis

Synthetic, porous silicate-substituted calcium phosphate bone graft matrices (SiCaP; 0.8 wt % Si) with varying strut porosity were applied to ovine critical-sized defect sites as either 1-2 mm microgranules (SiCaP-23G, SiCaP-32G, and SiCaP-46G) or 1-2 mm microgranules in an aqueous poloxamer carrier (SiCaP-23P, SiCaP-32P, and SiCaP-46P). Defect sites treated with SiCaP-23G or SiCaP-23P showed evidence of bone formation at 8 and 12 weeks in central zones. More advanced neovascularization and increased bone contact was observed for graft materials with higher strut porosities. At 12 weeks, graft materials with higher strut porosities (32% and 46%) had statistically significantly higher absolute bone volumes (p < 0.05) versus those with a strut porosity of 23%. Absolute bone volume in defects treated with grafts of matched strut porosities as microgranules, or microgranules with poloxamer carrier, were similar at 12 weeks. Absolute graft volume for SiCaP-46 reduced over 12 weeks (not statistically significant). In conclusion, bone formation patterns in critically-sized defects confirm strut porosity to be a clinically relevant property of porous silicate-substituted calcium phosphate bone grafts in promoting osteogenesis. Increasing graft matrix strut porosity encouraged earlier neovascularization and increased the absolute equilibrium volume of bone growth within the graft without compromising graft stability.

Directed osteogenic differentiation of human mesenchymal stem/precursor cells on silicate substituted calcium phosphate.

Insufficient, underactive, or inappropriate osteoblast function results in serious clinical conditions such as osteoporosis, osteogenesis imperfecta and fracture nonunion and therefore the control of osteogenesis is a medical priority. In vitro mesenchymal stem cells (MSCs) can be directed to form osteoblasts through the addition of soluble factors such as β-glycerophosphate, ascorbic acid, and dexamethasone; however this is unlikely to be practical in the clinical setting. An alternative approach would be to use a scaffold or matrix engineered to provide cues for differentiation without the need for soluble factors. Here we describe studies using Silicate-substituted calcium phosphate (Si-CaP) and unmodified hydroxyapatite (HA) to test whether these materials are capable of promoting osteogenic differentiation of MSCs in the absence of soluble factors. Si-CaP supported attachment and proliferation of MSCs and induced osteogenesis to a greater extent than HA, as evidenced through upregulation of the osteoblast-related genes: Runx2 (1.2 fold), Col1a1 (2 fold), Pth1r (1.5 fold), and Bglap (1.7 fold) Dmp1 (1.1 fold), respectively. Osteogenic-associated proteins, alkaline phosphatase (1.4 fold), RUNX2, COL1A1, and BGLAP, were also upregulated and there was an increased production of mineralized bone matrix (1.75 fold), as detected by the Von Kossa Assay. These data indicate that inorganic substrates are capable of directing the differentiation programme of stem cells in the absence of known chemical drivers and therefore may provide the basis for bone repair in the clinical setting.

The effect of particle size on the osteointegration of injectable silicate-substituted calcium phosphate bone substitute materials†‡

Calcium phosphate (CaP) particles as a carrier in an injectable bone filler allows less invasive treatment of bony defects. The effect of changing granule size within a poloxamer filler on the osteointegration of silicate-substituted calcium phosphate (SiCaP) bone substitute materials was investigated in an ovine critical-sized femoral condyle defect model. Treatment group (TG) 1 consisted of SiCaP granules sized 1000–2000 μm in diameter (100 vol %). TG2 investigated a granule size of 250–500 μm (75 vol %), TG3 a granule size of 90–125 μm (75 vol %) and TG4 a granule size of 90–125 μm (50 vol %). Following a 4 and 8 week in vivo period, bone area, bone-implant contact, and remaining implant area were quantified within each defect. At 4 weeks, significantly increased bone formation was measured in TG2 (13.32% ± 1.38%) when compared with all other groups (p = 0.021 in all cases). Bone in contact with the bone substitute surface was also significantly higher in TG2. At 8 weeks most new bone was associated within defects containing the smallest granule size investigated (at the lower volume) (TG4) (42.78 ± 3.36%) however this group was also associated with higher amounts of fragmented SiCaP. These smaller particles were phagocytosed by macrophages and did not appear to have a negative influence on healing. In conclusion, SiCaP granules of 250–500 μm in size may be a more suitable scaffold when used as an injectable bone filler and may be a convenient method for treating bony defects.

The effect of increased microporosity on bone formation within silicate-substituted scaffolds in an ovine posterolateral spinal fusion model

This study compared the bone forming capacity of the same formulation of silicate-substituted bone graft substitute materials with different microporosity in an instrumented posterolateral spinal fusion ovine model. Materials with a strut porosity of (i) 22.5% (SiCaP) or (ii) 36.0% (SiCaP(+)) were packed along either side of the spine. Bone apposition rates, % new bone formation, % bone-implant contact, and % graft resorption were quantified at 8, 12, and 24 weeks post surgery. Computed Tomography (CT) was used to grade the formation of fusion bridges between vertebrae. Results showed no significant difference in bone apposition rates, % new bone formation, and % bone-implant contact when the two materials were compared. However, at 8 weeks, a significantly higher CT score was obtained in the SiCaP(+) group (0.83 ± 0.17) when compared with the SiCaP group (0.17 ± 0.17; p = 0.027). Significantly less scaffold remained in the SiCaP(+) group at 12 weeks (p = 0.018). Both SiCaP and SiCaP(+) formulations augmented bone formation. Increasing the strut porosity did not significantly increase bone formation however, at 8 weeks it promoted the formation of more highly mineralized bone resulting in a significantly higher CT score, suggesting the bone tissue formed was more mature.