PULP-DENTIN HEALING
INTRODUCTION :
Teeth can be subjected to all kinds of attacks, particularly carious attacks.
One of the natural means of defense available to the pulp-dentin complex is the formation of tertiary dentin, which allows the establishment of a sort of protective barrier between the pulp and the attack: this is the dentinogenic process.
REMINDER ON THE PULPO DENTIN COMPLEX
The pulp-dentin complex is composed of two parts:
- one mineralized: dentin
- the other non-mineralized (conjunctiva): the dental pulp.
In this complex, the dentin ensures the protection of the pulp (hard tissue), and the pulp tissue ensures the “vitality” of the dental organ (vascularization).
- -Dentin
Dentin is a partially mineralized tissue (70% hydroxyapatite crystals), which is composed, in its organic phase (20%), of a collagen I framework which contains within its “meshes” a certain number of non-collagenous matrix proteins, initially secreted by the odontoblasts, then which are enclosed and protected by the mineralization process.
Among these numerous matrix proteins, we find a large number of growth factors, notably those of the TGF-β, VEGF, ADM family, which will play a major role in the pulp healing process.
Dentin is a permeable tissue; crossed by tubular structures from one side to the other (the dentin tubules) from the enamel-dentin junction (ADJ) or (CDJ) to the pulp cavity.
These tubules contain on the one hand the dentin fluid as well as the odontoblastic extensions which penetrate the mineralized tissue, their cell bodies being located outside the dentin, on the pulp side.
- – The pulp
The pulp is a loose connective tissue of mesenchymal origin, included inside an anatomical cavity located in the internal and central part of the tooth.
It is surrounded by dentin into which it sends branches and from which it is separated by the dentinogenic zone.
The pulp is composed on average of 25% organic matter (cells and extracellular matrix) and 75% water
Its composition is linked to its properties: dentinogenic, nutritional, sensory, vascular and defensive.
Aging pulp is less rich in cells and contains more fiber.
- DEFENSE CAPACITIES OF THE PULPO-DENTIN COMPLEX:
The pulp-dentin complex is called upon to defend itself against physical, chemical or bacterial irritants. Faced with these irritants, the pulp-dentin complex has defensive capabilities
Dentin and pulp react interdependently to environmental influences; everything that affects dentin is reflected in the pulp and vice versa.
Role of the odontoblast
Odontoblasts are highly specialized pulp cells, located at the periphery of the pulp tissue and found in a single-cell layer (palisade).
The cells are in direct contact with each other, united by gap junctions, which gives this palisade a membrane structure separating the mineralized dentin tissues on one side, and the pulp connective tissue on the other.
Odontoblasts are mainly known for their dentinogenic role, responsible for dentin secretion on the one hand, and its mineralization on the other.
In the event of carious aggression, the odontoblasts which have entered a so-called “quiescent” phase of synthesis can be reactivated and then synthesize a tertiary dentin, called reaction dentin.
Beyond this synthetic role, the odontoblast has two other specific abilities,
-A role of immunocompetence:
In the case of bacterial aggression under a cavity, for example, bacterial toxins are detected by the odontoblast using membrane TLRs.
The coupling of these two entities triggers internal cellular signaling within the odontoblast (immune behavior of this cell) which, in a second step, will be able to transmit another message to the underlying pulp cells in order to trigger, when necessary, the establishment of an inflammatory process.
The odontoblast thus plays a role of barrier and protection of the pulp, by keeping the aggressors outside, and by transforming the signal to make it appropriate and understandable by the resident immune cells of the pulp.
-A mechanosensory role:
The specific structure of the odontoblast, combining a cell body with a long cytoplasmic extension, gives it a significant resemblance to the cells of the nerve line (axon and synapse). The latter is capable of transforming received information into emitted information that can be interpreted by the underlying pulp tissues.
-In the context of dentin hypersensitivity, where the baroreceptors of these cells are capable of analyzing a pressure differential and transforming this information into a nerve impulse, for example transmitted to the underlying pulp.
Role of the pulp
The pulp is a connective tissue comprising several cell populations embedded in a loose extracellular matrix.
These cells include so-called “pulp” fibroblasts, immune cells (more or less differentiated cells) and so-called stem cells (Dental Pulp Stem Cells or DPSCs). A highly developed and well-organized vascular and nervous network, ensuring both the physiology of the tissue and its protection, thanks to its ability to trigger an immune and inflammatory reaction in a very short time. These networks also play a role in the healing and regeneration process of tissues destroyed or eliminated for therapeutic reasons.
Intrapulpal pressure is higher than the external pressure of the tooth. This internal excess pressure tends to push the fluid outwards and thus limits the entry of bacteria and toxins
External stimuli increase both the apposition of peritubular dentin into the tubules and the apposition of additional dentin layers, which reduces permeability and increases the distance to the pulp.
Beneath the odontoblastic layer is a richly vascularized area that enables the elimination function.
Substances entering the pulp via the dentinal tubules can be removed from the interstitial tissues by microcirculation.
- PULP-DENTIN HEALING
The mechanisms that take place there are effective and all work towards reducing the permeability of the attacked dentin.
Thus, in response to carious processes, attrition, abrasion, erosion, trauma or even dental restoration procedures, there is a secretion of tertiary dentin by the odontoblasts.
- IN THE FACE OF ATTACKS:
A- Facing caries :
Faced with an attack from the oral cavity, the pulp seeks to preserve its vitality by synthesizing scar tissue called tertiary dentin.
The latter includes two types of dentin:
reaction dentin and reparative dentin
A-1. Reactional dentin:
It is produced by odontoblasts.
It is generally encountered under initial, shallow caries that develop chronically.
Topographically, we distinguish:
- sclerotic reaction dentin DRS
- peripulpal reaction dentin DRP
*Sclerotic reaction dentin DRS:
Sclerotic reaction dentin (SRD), formed at the expense of the lumen of the dentinal tubules. It is highly mineralized and has a translucent appearance.
*Peripulpal reaction dentin DRP
It is formed at the dentin-pulp interface following the increase in the deposition of predentin. This is the result of the acceleration of the formation of peripulpal dentin by odontoblasts.
Molecules from the styling material diffuse through the tubules and stimulate the formation of this reaction dentin.
Among the molecules present in the dentin matrix and stimulating the formation of DRP Transforming growth factor beta-1 (TGFβ-1), bone morphogenesis protein 7 (BMP-7), an adhesive glycoprotein of the extracellular matrix, fibronectin. Enzymatic activity is also increased, notably that of alkaline phosphatase and ATPases. These enzymes allow the release of phosphates and the production of the energy necessary for rapid mineralization of the newly deposited dentin framework.
A-2 . Reparative dentin:
Following necrosis of the odontoblasts, the cells from the subodontoblastic region will divide, then migrate into contact with the necrotic area and differentiate into replacement odontoblasts or second-generation odontoblasts.
They synthesize an extracellular matrix with a composition similar to that of fibrodentin. This extracellular matrix will mineralize to form orthodentin, which contains tubules and the cells are aligned at the periphery.
B/ Faced with trauma:
In trauma resulting in pulp exposure, the first manifestation observed is hemorrhage accompanied by local inflammation.
The pulp exposure is then rapidly covered with a layer of fibrin. There may be capillary budding at the superficial part, followed by proliferation of leukocytes and histiocytes.
This pulp response helps prevent contamination of the wound by saliva. Forty-eight hours after the trauma, pulp inflammation is limited to a depth of 2 mm.
As the inflammation progresses, the risk of bacterial contamination increases. Therefore, the chances of maintaining pulp vitality are lower.
-There can be no spontaneous healing of the pulp.
- FACING OUR THERAPIES: PULP STYLING
The term “pulp repair” is used to characterize a set of therapies whose goal is to promote the healing of a pulp that is still living.
These therapies consist of the treatment of pulp-dentin wounds by indirect or direct pulp capping or by pulpotomy.
- Indirect pulp capping
In indirect pulp capping, the capping material is placed on top of a layer of demineralized dentin, which is left in place to act as a protective barrier to reduce further damage to the pulp and allow it to heal.
Its purpose is to protect primary odontoblasts and promote the formation of reaction dentin.
The biomaterial also ensures the destruction of the last germs present near the pulp
Indirect capping is generally performed when the thickness of residual juxta-pulp dentin after eviction is less than 0.5 mm.
- Direct styling
This therapy consists of covering the pulp exposure (less than 0.5 mm) with a biomaterial placed directly in contact with the pulp.
It is indicated in cases of minimal pulp exposure during dentin curettage or recent coronary trauma (less than 48 hours).
In the case of pulp exposure, there is destruction of the primary odontoblastic layer and septic inoculation.
The objective of therapy is therefore to clean the pulp and allow the recruitment and differentiation of progenitor cells in order to produce reparative dentin.
- Biomaterials used :
Materials with bioactive properties are used, the purpose of which is:
– Partially demineralize the dentin and allow the release of matrix proteins.
-To induce the formation of a dentin bridge
-Biological closure of the wound;
These materials are represented by calcium hydroxide, MTA, and Biodentine.
a‐ Calcium Hydroxide
Also called hydrated lime or slaked lime, is a fine, white, odorless powder resulting from the following chemical reaction; Cao+H2O Ca(OH)2
The hardening form (self-curing or light-curing) is the only form of calcium hydroxide used as a pulp capping material.
It is characterized by:
Its high pH which varies between 12.5 and 12.8 (the OH- ions will alkalize the environment and thus have a strong antibacterial action).
a‐1 . Pulp repair mechanism
This material has the ability to dissolve dentin (prolonged and persistent dissolution), and thus gradually release growth factors (TGFβ).
These released factors could travel through the tubules towards the pulp parenchyma, and thus trigger a pulp reaction, notably odontoblastic.
Thus stimulated, these cells would come out of their quiescent phase and secrete tertiary dentin:
- either by the formation of reaction dentin if the odontoblasts and cells of the Höhl layer are still alive and biologically active,
- either in response to a pulp breach, by the formation of reparative dentin secreted by neoodontoblasts
During its ionic dissociation, there will be a release of Ca2+ ions which will promote the production and mineralization of hard tissues, and will have a hemostatic and anti-inflammatory action.
a‐2 .Disadvantages of the material :
The formation of a mineral barrier (minoralized dentin bridge) is not uniform, nor welded to the dentin walls (weak adhesion). Sealing is therefore not achieved in the long term.
Since the material was likely to dissolve over time, the clinical situation was therefore the same after a few months as for treatments without capping material.
b. Tricalcium silicate materials b-1. Mineral Trioxide Aggregate (MTA )
MTA is a material that is part of the hydraulic cements based on calcium silicates or also called first generation bioceramic cements;
It is a mixture of Portland cement and bismuth oxide (making the material radiopaque) with traces of silica (SiO2), calcium oxide (CaO), magnesium oxide (MgO), potassium sulfate (K2SO4) and sodium sulfate (NaSO4)
MTA has antibacterial and antifungal properties and a lack of cytotoxicity.
Hydration of MTA powder results in the formation of a colloidal gel that hardens. The pH of MTA immediately after mixing is 10.2. Three hours later, it is 12.5.
* Pulp repair mechanism
MTA has advantages which are the small size of its particles, the ability to form a watertight cavity bottom, the alkaline pH following its setting, and the progressive release of calcium ions (mineralizing and hemostatic activity).
MTA is a product that releases calcium hydroxide as a by-product of its hydration.
Its mode of action is therefore considered similar to that of calcium hydroxide in inducing the proliferation of pulp cells, the release of cytokines and the formation of a hard tissue barrier with the synthesis of a homogeneous mineralized dentin interface similar to that of biological hydroxyapatite.
* Main disadvantages of the materials
- its fairly long setup time.
- its incompatibility with other materials.
- its setting time (3 hours).
b-2. Biodentine
Biodentine is a material that is part of the hydrolic cements based on calcium silicates or also called second generation bioceramic cements;
It comes in the form:
* a powder, composed mainly of tricalcium silicates (C3S), calcium carbonate (CaCO3) and zirconium oxide (ZrO2, for radiopacity). Dicalcium silicates (C2S) and calcium oxide (CaO) are also found.
*a liquid containing water, calcium chloride (CaCl2, which accelerates setting, allowing it to set in 12 minutes) and a water-reducing agent: Fluid Premia 150.
It is a modified polycarboxylate-based superplasticizer that helps prevent cracking during material setting.
* Pulp repair mechanism
Several studies have confirmed its biocompatibility and its ability to induce odontoblastic differentiation and mineralization.
After its placement, as with MTA, calcium hydroxide is released as a by-product of hydration. In a recent clinical study, Biodentine showed the formation of a dentin bridge and the absence of pulp inflammatory reaction.
- PROGNOSIS
It is now known that an affected pulp can be healed and kept alive. However, direct pulp capping is a delicate procedure with uncertain success.
- Precautions to promote pulp healing during our treatments:
It is essential to limit attacks on the dentino-pulp complex during restorative treatments so as not to alter the restorative potential;
- Restorative procedures aimed at minimizing the formation of iatrogenic pulp inflammation should be adopted.
- Excessive desiccation of dentin should be avoided during all stages of tooth restoration to prevent:
- fluid changes within the pulp tissue,
- aspiration of odontoblastic nuclei and formation of pulp edema
- Bacterial contamination must be prevented throughout tooth restoration procedures, as it is the main cause of pulp inflammation.
- Care must be taken, as soon as the preparation by milling is finished, to decontaminate the dentin smear, which is a source of germs capable of surviving the restoration procedures, either using 0.12% chlorhexidine solutions or 2.5% sodium hypochlorite solutions or both consecutively.
- Immediate dentin hybridization using an adhesive system can be performed to seal open dentin tubules during milling
Please note that:
-Beyond 1.5 mm of assumed thickness of residual dentin, the use of an enamel-dentin adhesive proves sufficient to protect the pulp tissue
-The juxta-pulpal dentin zones supposed to be less than 0.5 mm (pink in appearance), can be covered punctually by Ca(OH)2. This material presents an antimicrobial activity due to its alkaline pH.
-When the expected thickness is between 0.5 mm and 1.5 mm, it is preferable to interpose a dentin substitute between the restoration and the base of the cavity. In these cases, the use of glass ionomer cements is possible.
CONCLUSION
Advances in research into pulp physiology will increasingly lead practitioners to maintain pulp vitality during styling treatments. The pulp is capable of ensuring its own defense against bacterial attack.
BIBLIOGRAPH
- STÉPHANE SIMON Endodontics/pulp biology 2012.
- PIETTE – GOLDBERG The normal tooth and the pathological tooth
- M. Goldberg Histology of the pulpo-dentin complex. EMC stomatology/odontology;22-007-B- 10;2008;Dentistry;28-115-B10;2008.
- MORGANE GUYOMARD; direct pulp capping: histophysiological aspects, success factors and current biomaterials 2013
- WOHLGEMUTH PIERRE; pulp capping: current techniques 2014
- S. SIMON; F. BERES; M. ZANINI. Pulp-dental healing from biology to the clinic; clinical realities 2016 Vol 27 n’1 pp. 13-26
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