1) In case of tooth decay and a healthy pulp, a resin-based composite is used. 2) In case of tooth decay with an inflamed pulp, a calcium-silicate cement is applied on the pulp before the resin-based composite. 3) When the pulp is partially necrotic, this part is removed and replaced by a combination of calcium-silicate cement and resin-based composite. 4) When the complete pulp is necrotic, it is completely removed and replaced by inert filling material. Future behaviours for the treatment of tooth decay. 3’) When the pulp will be partially removed, it will be replaced by a hydrogel loaded with growth factors in order to attract stem cells from the remaining pulp. 4’) When the pulp will be completely removed, it will be replaced by a hydrogel loaded with stem cells in order to re-create a new dental pulp tissue.
In the treatment of tooth decay, restorative dental materials are required to exhibit excellent mechanical, biological properties and most uniquely, display good aesthetics. The research carried out focuses 1) on the characterization of currently available commercial materials, in relation with clinical requirements and 2) on developing new biomaterials for tooth restoration, from a conventional conservative but also from a more advanced regenerative standpoint.
The use of restorative materials allows for relatively fast treatments as they may be implemented directly in the oral cavity in a matter of minutes. They are also highly versatile. However several concerns exist with regards to the suitability of some materials in terms of mechanical or biological properties. Additionally the very mechanisms responsible for the setting of materials or interactions with the biological are little understood.
a) In vitro-methods
We are continuously invested in determining the most suited set of characterization methods to properly analyze both mechanical and biological properties of commercial materials, leading to innovative experimental research. Our most recent results describe the setting kinetics and mechanical properties of ultra-fast polymerizing resin composites, based on a monoacylphosphate photoinitiator and bioactive calcium silicate cements (Figure 6, item 2 or 3). In collaboration with Pr. Möginger (University of Bonn-Rhein, Germany), an innovative combination of characterization techniques was set up, allowing for a precise analysis of polymerization kinetics in heavily filled composites.
b) In vitro-material development
The formulation of resin composites is fine-tuned ([photoinitiator], resin composition, etc) to quicken kinetic, increase longevity and bring mechanical properties close to that of hard tissues. The use of micro hydroxyapatite particles and amorphous CaP nano particles is currently being investigated for the release of Ca²⁺ and PO₄²ˉ with antibacterial and re-mineralizing potential. The impact of their introduction in model formulations on kinetics and mechanical properties is investigated. Additionally, ceramics are investigated for their use as alternatives of resin composites following root canal treatment (Figure 6, item 4).
c) In vitro-material/cell interactions
The interactions with pulp tissues and oral commensal bacteria are also researched. The potential of apatite-loaded resin composites is being evaluated, aiming both at S.mutans/S.gordonii biofilm reduction and increased dental pulp stem cells (DPSC) viability (induced osteo-differentiation is also analyzed). Further, as resin composites do not polymerize completely, the toxicity of monomers and un-reacted compounds on DPSC is investigated. Even in the absence of toxicity, some monomers may still induce oxidative stress and genotoxic effects. Methods are being developed to quantify ROS production and osteo-differentiation inhibition on a large number of samples.
d) Clinical work
As a result of strong collaborations with the dental clinics, several studies are currently under way, focusing on the analysis of the suitability of resin composites for the treatment of large cavities, in a retrospective manner. Another study underway was designed to investigate the suitability of calcium silicate cement for pulp capping (Figure 6, item 3).
In modern dentistry, there is currently a paradigm shift from restorative procedures to strategies based on regenerative medicine. In this context, alternatives to current clinical restorative strategies where pulp tissue is partially or completely lost (irreversibly inflamed and necrotic dental pulps) must we designed by combining bioactive matrices and dental stem cells in a clinically relevant way.
a) Cells
Dental stem cells are mesenchymal stem cells that may be collected in large amounts from dental tissues. Such cells display a higher proliferation rate than bone marrow stem cells and have better neural and epithelial properties as they originate from the neural crests. Additionally dental stem cells can differentiate in multiple cells types, like osteo- odonto-, adipo-, neuro-, chondroblast-like cells… Among dental stem cells, we selected dental pulp stem cells (DPSCs) and stem cells from the apical papilla (SCAP) for their potential. While we have worked with SCAP (RP89 cell line), originating from one patient and obtained from Dr. Diogenes (University of Texas, USA), we recently created a pool of DPSC and SCAP from 10 different patients. These cell pools will be fully characterized by cell-surface markers analysis, by differentiation potential and by stem cell gene expression and used as an internal standard for all of our work. Such efforts will allow us to have a much genetically diverse and relevant cell source.
b) Scaffold
For the regenerative approach, cells must be properly delivered. The design of an “ideal” bioactive matrix is thus necessary. This one would be biocompatible, injectable and would ideally resemble the native pulp tissues in terms of mechanical properties and allow cell invasion, survival and proliferation. Therefore, we will test in vitro different hydrogels, which will be provided through different collaborations (Prof. Anne des Rieux, UCL; Prof. Berit Strand, NTNU, Norway; Prof. Patrick Henriet, UCL; Prof. Christine Dupont, UCL).
Fibrine/Alginate hydrogels are currently being investigated, testing for DPSC attachment and viability on the medium-term. Once an « ideal » bioactive matrix is designed, it will be implemented in two different regenerative strategies and tested in vitro/in vivo:
-Dental pulp stem cell homing from residual dental pulp tissue in case of partial pulp tissue removal, through the injection of a bioactive scaffold loaded with factors like SDF1, bFGF and TGF-B (Figure 6, item 3’),
-Exogenous dental pulp stem cell delivery in case of complete pulp tissue loss, to regenerate the lost tissue volume into a vascularized, innervated and functional de-novo dentin-pulp complex (Figure 6, item 4’).