Several publications have shown the phenomenon of osteoinduction by biomaterials to be real. However, whether the ability of a biomaterial to initiate bone formation in ectopic implantation sites improves the performance of such osteoinductive biomaterial in clinically relevant orthotopic sites remains unclear. No studies have been published in which osteoinductive potential of a biomaterial is directly related to its performance orthotopically. In this study, we compared osteoinductive and nonosteoinductive biphasic calcium-phosphate (BCP) ceramics ectopically and in a clinically relevant critical-sized orthotopic defect in goats. The two materials, BCP1150 and BCP1300, had similar chemical compositions, crystallinities, and macrostructures, but their microstructures differed significantly. BCP1150, sintered at a lower temperature, had a large amount of micropores, small average crystal size, and hence a high specific surface area. In contrast, BCP1300, with few micropores, had a significantly lower specific surface area as compared to BCP1150. Twelve-week intramuscular implantation in goats (n ¼ 10) showed that bone was induced in all BCP1150 implants, while no signs of bone formation were found in any of the BCP1300 implants. After 12 weeks of implantation in a bilateral critical-sized iliac wing defect in the same goats, BCP1150 showed significantly more bone than BCP1300. In addition, the analysis of fluorochrome markers, which were administered to the animals 4, 6, and 8 weeks after implantation to follow the bone growth dynamics, showed an earlier start of bone formation in BCP1150 as compared to BCP1300. Significantly better performance of an osteoinductive ceramic in a criticalsized orthotopic defect in a large animal model in comparison to a nonosteoinductive ceramic suggests osteoinduction to be clinically relevant. Further improvement of material osteoinductive properties is thus a significant step forward in the search for alternatives for autologous bone graft.

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Animal experiments help to progress and ensure safety of an increasing number of novel therapies, drug development and chemicals. Unfortunately, these also lead to major ethical concerns, costs and limited experimental capacity. We foresee a coercion of all these issues by implantation of well systems directly into vertebrate animals. Here, we used rapid prototyping to create wells with biomaterials to create a three-dimensional (3D) well-system that can be used in vitro and in vivo. First, the well sizes and numbers were adjusted for 3D cell culture and in vitro screening of molecules. Then, the functionality of the wells was evaluated in vivo under 36 conditions for tissue regeneration involving human mesenchymal stem cells (hMSCs) and bovine primary chondrocytes (bPCs) screened in one animal. Each biocompatible well was controlled to contain μl-size volumes of tissue, which led to tissue penetration from the host and tissue formation under implanted conditions. We quantified both physically and biologically the amounts of extracellular matrix (ECM) components found in each well. Using this new concept the co-culture of hMSCs and bPCs was identified as a positive hit for cartilage tissue repair, which was a comparable result using conventional methods. The in vivo screening of candidate conditions opens an entirely new range of experimental possibilities, which significantly abates experimental animal use and increases the pace of discovery of medical treatments.

The osteoinductive potential, or bone induction potency, of two calcium phosphate ceramics was evaluated after intramuscular implantation in goats. The ceramics were comprised of hydroxyapatite (HA) and biphasic calcium phosphate (BCP), the later of which contained a 85/15 mixture of hydroxyapatite and tricalcium phosphate (TCP). Both ceramics had a similar macroporosity of around 55% and a pore distribution between 100 and 800 microm. Besides the difference in chemistry, BCP was also microporous and hence had a different surface microstructure. After implantation in the back muscles of four goats for 12 weeks, all 8 BCP samples (7x7x7 mm(3)) showed the presence of bone formation in the macropores (1+/-1%), while no bone was found in any of the HA samples. The used BCP can therefore be characterized as an osteoinductive material. Having the ability to induce bone formation in soft tissues, the BCP presented herein may be a useful biomaterial for bone repair when combined with cultured osteogenic cells, growth factors or both.

Multiblock poly(ether-ester)s based on poly(ethylene glycol), butylene terephthalate, and butylene succinate segments were evaluated for their in vivo degradation and biocompatibility in order to establish a correlation with previously reported in vitro results. Porous polymer sheets were implanted subcutaneously for 32 weeks in rats. The degradation was monitored visually (histology), by molecular weight (GPC), and by copolymer composition (NMR). Substitution of the aromatic terephthalate units by aliphatic succinate units was shown to accelerate the degradation rate of the copolymers. Direct correlation of the in vivo and in vitro degradation of the porous implants showed a slightly faster initial molecular weight decrease in vivo. Besides hydrolysis, oxidation occurs in vivo due to the presence of radicals produced by inflammatory cells. In addition, the higher molecular weight plateau of the residue found in vivo indicated a higher solubility of the oligomers in the extracellular fluid compared to a phosphate buffer. Minor changes in the poly(ether-ester) compositions were noted due to degradation. Microscopically, fragmentation of the porous implants was observed in time. At later stages of degradation, macrophages were observed phagocytozing small polymer particles. Both in vitro cytotoxicity studies and histology on in vivo samples proved the biocompatibility of the poly(ether-ester)s.

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Osteoinductive biomaterials are able of inducing bone formation at ectopic, i.e. extraskeletal implantation sites. It is, however, important to investigate whether osteoinductive biomaterials perform better when implanted orthotopically as well, in particular in clinically relevant criticalsized defects. In this study, an osteoinductive and a non-osteoinductive biphasic calcium-phosphate (BCP) ceramic were compared in a critical-sized iliac wing defect that allows for paired comparison. After 12 weeks of implantation in the critical-sized defect, the osteoinductive BCP1150 ceramic showed significantly more bone than the non-osteoinductive BCP1300 ceramic. In addition, the analysis of fluorochrome markers, which were administered to the animals 4, 6 and 8 weeks after implantation in order to visualize the bone growth dynamics, showed an earlier start of bone formation in BCP1150 as compared to BCP1300. Significantly better performance of osteoinductive ceramic in a critical-sized orthotopic defect in a large animal model in comparison to the non-osteoinductive ceramic suggests osteoinduction to be clinically relevant.