New perspectives

Are stem cells the implants of the future?

Therapy using mesenchymal stem cells (MSCs) is an up-and-coming treatment avenue in implant dentistry. It is still in its pre-clinical stage, but may soon present a promising alternative for autogenous bone grafts. The benefit of using MSC therapy is that it would overcome the drawbacks traditionally associated with bone grafts (insufficient amount of bone, resorption and donor morbidity). At the pre-clinical level, the results of MSC therapy have been widely demonstrated (Shanbhag et al., 2017). The challenge now is to bridge the gap between lab and clinic.

MSCs have perivascular origins, and are found in almost all tissues in the human body. Due to their limited numbers, however, they should be isolated, cultured, expanded and even reprogrammed ex vivo. This is an alternative approach for using bone marrow as a clinical source of MSC, where stem cells make up a small (and unevenly distributed) fraction of the tissue.

As well as bone marrow, MSCs can be obtained from dental pulp or adipose tissues. Although they share many surface biomarkers, stem cells harvested from different areas have been reported to behave in different ways. Outcomes are highly dependent on the conditions of the culture and how it is manipulated in the lab, where good manufacturing practice (GMP) protocols should be followed to safeguard the cultures. 

At first, MSCs from adipose tissue (ADCs) show a lower osteogenic capacity than those from bone marrow. However, they can be selectively induced to osteogenic pre-differentiation. ADCs are easily accessible from subcutaneous fat, and in vivo experiments have shown that they can be cultured on 3D scaffolds, implanted and then applied in ongoing clinical trials. Bone marrow MSCs have been shown to promote ectopic bone formation when subcutaneously transplanted in mice. However, they were reluctant to form bone in intra-oral osseous defects in humans (Meijer et al., 2008).

The speaker had been working in a line of research dedicated to bone marrow derived MSCs as part of the EU project, ‘REBORNE’[1], where various phase-II clinical trials were conducted.

The first step was protocol validation, which had recently been accomplished. A study concluded that the clinical use of MSCs, manufactured according to a specific standardised protocol (which was GMP compliant), was capable of bone regeneration (Rojewsky et al., 2019). Thereby in three weeks, a hundred million cells can be successfully delivered via syringe, even though the culturing lab is far from the clinical site. Cells are attached to granules of microporous biphasic calcium phosphate mixed with HA/TCP (Biomatlante™), in an optimal density of 10 x 106 cells in 5 cm3 of granules (Brennan 2014) (figs 1–2). In a pre-clinical study, this protocol was applied in a split-mouth mini-pig model and the regenerative potential of the MSCs was clearly demonstrated (Gjerde et al., 2017).

The next step has been a human clinical study on 13 patients with bone defects ≦4 mm wide, where implant placement without augmentation is not possible (Gjerde et al., 2018).  Cells were harvested from the marrow of the iliac crest by aspiration; they were then tested for quality, cultured and expanded in the lab at University of ULM, Germany and finally sent back to the clinical site. In Bergen, surgery was performed (with no special measures required), like a GBR membrane procedure with just any type of particulate graft. Biopsies at six months showed significant formation of new bone in all 11 patients. In two cases, the expansion of MSCs was stopped due to insufficient levels of stem cells found in the aspirate. The authors concluded that this novel augmentation procedure could even challenge the current gold standard protocol of autogenous bone grafts.

After the promising data from this study, a comparative multicentre RCT has been undertaken as part of the European project MAXIBONE[2], including 150 patients in eight clinical centres across six countries. The intention is to use personalised grafts (instead of granules) with scaffolds made with 3D printing to mimic the morphology of the defects.

Bioprinting is an interesting and relatively new technology capable of inserting osteoblast-like cells directly into scaffolds to optimise distribution. Since cell viability remains the same, this approach may prove effective in the near future. It is also possible to print the bone marrow-derived MSCs with the scaffold and the culture medium simultaneously in 3D structures with up to sixteen layers (Ojansivu et al., 2019) (figs 3–4).

The most recent evidence comes from a systematic review and meta-analysis (Shanbhag et al., 2019) and a consensus report from the 15th EWP (Sanz et al., 2019). Both concluded that cell therapies show superior results in pre-clinical in vivo studies, although these effects did not seem to be repeatable in clinical trials. There is a gap between pre-clinical models and clinical application that still needs to be overcome. Currently, thorough research suggests that MSC therapy will become a new approach in the very near future of bone regeneration.