Abstract
Introduction: In orthopaedic surgery the autologous bone graft is often applied. However, harvesting of this material is not without complications. Tissue engineering of bone is a promising alternative. Many authors have shown the proof of the concept and currently the first human studies are performed. However, little is actually known
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of the technique. Based on the available literature, we set the following study aims:
1) To investigate the role of cell viability in autologous bone grafts;
2) To investigate the role of cell viability in TE constructs;
3) To develop a labeling technique to trace cells used for TE;
4) To investigate methods to optimize bone tissue engineering;
5) To investigate the applicability of TE in clinically relevant models.
Methods: All studies were performed with goat bone or bone marrow stromal cells (BMSC’s). Tissue engineered constructs were made by seeding the cells on several porous ceramic scaffolds.
Results
Ad. 1:
We investigated the role of viability in autologous bone grafts. We observed that osteogenesis was far more advanced in viable transplanted grafts, especially when implanted in the muscles (ectopically). Orthotopically, environmental bone formation probably shaded the osteogenic activity of the grafted cells. Although it is tempting to conclude that cells do survive and make bone after transplantation, this study only permits the conclusion that cell viability has an obvious effect on bone formation. This provides a rationale for cell-based bone tissue engineering.
Ad. 2:
The rationale for cell-based bone tissue engineering was confirmed in the goat model of ectopic bone formation in TE constructs (in the paraspinal muscles).
Ad. 3:
We investigated the "off the shelf" CM-Dil label and had to conclude this label was only applicable for evaluation at short implantation periods As a next step, we developed a retroviral labeling method based on the surface marker NGFR. We could proof the direct coupling between the goat BMSC's and bone formation.
Ad 4:
We found an advantage for constructs that were cultured for one week after seeding of the cells. When the constructs were not cultured, which makes the procedure considerably less complicated, bone formation appeared to be feasible as well. We further optimized the TE technique both with respect to the yield of bone and the ease of the procedure. By using another type of scaffold (biphasic calcium phosphate BCP), culture of the constructs appeared not to be advantageous. The method of peroperatively combining cryopreserved cells with the ceramic scaffold considerably reduced the logistics to the level of a standard elective procedure.
Ad. 5:
We used a bilateral iliac wing CSD model. A moderate effect of tissue engineering could be observed only at the evaluation time of 9 weeks. It is plausible that the scaffold itself was responsible for the absence of an obvious effect. Not because the scaffold is unsuitable, but on the contrary, is that potent that the majority of bone is by conduction/induction.
Conclusion: Irrespective of the outcome, the studies drastically improved our knowledge on bone tissue engineering. Scientifically, the technique is intriguing and provides the opportunity to improve our knowledge on many aspects of bone formation. The answer to improvement of functionality of the tissue engineered bone may be derived from further fundamental investigations or, as often is the case in medicine, found by serendipity. The final and most important conclusion that can be drawn at the end of this thesis is:
Cell-based bone tissue engineering has the potential to become an ideal autologous graft substitute.
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