Indirect coronal-radicular restorations “Endocrowns”

Indirect coronal-radicular restorations “Endocrowns”

Introduction :

It is accepted that tooth loss and the amount of residual tissue are probably the most important predictors of the long-term clinical success of a restoration.

1. General information
   1. Coronary substance loss

If faced with a pathological tooth with limited tissue loss, the treatment to be adopted by the practitioner may be a simple direct reconstruction; for a tooth with much more extensive decay, the decision on a treatment plan is made through a wider choice of possibilities. Generally speaking, the reconstruction must be as watertight and durable as possible and for this, the dental surgeon may use indirect reconstruction techniques involving prosthetic medical devices often made in a prosthetic laboratory.

1. Etiologies:

In order to define the best treatment plan and therefore the best restoration possibility, it is important to know and be able to identify the main causes of loss of dental substance.

• Caries: Whether it is an initial phenomenon or a recovery under filling, caries represents the main cause of loss of dental substance.
• Dental trauma: Dental trauma such as fractures is the second leading cause of significant loss of dental tissue.

There are two categories of invoices: those with pulp breakage (called

“complicated”) that which respects the endodontium (called “uncomplicated”)

• Dental erosions:

Erosions are another cause of tooth tissue loss. They are defined as non-carious wear that does not involve any microorganisms:

The etiology of dental erosion is also multifactorial. It can be of extrinsic origin, involving acidic foods, certain medications such as vitamin C, certain professional environments, etc.), or intrinsic, the cause of which can be gastroesophageal reflux, changes in salivary flow and buffering capacity, etc.

2.  Endodontic treatment:
   1. Objective

Endodontic treatment, also called “biopulpectomy”, aims to treat diseases of the pulp and the periapex and to transform a pathological tooth into a healthy, asymptomatic and functional entity on the arch capable of receiving a waterproof and functionally durable reconstruction.

Many studies agree that the quality of endodontic treatment is the most important factor in the success and health of the periapex, but that the quality of the coronal restoration is also an important criterion that should not be neglected due to its role in ensuring the long-term hermeticity of the root canals and the underlying endodontic treatment.

3.  The loss of the resistance beams

A study on the strength of restored tooth after endodontic treatment was conducted by Reeh, Messer and Douglas in 1989.

The results show a minimal effect of root canal treatment reducing resistance by only 5% compared to 20% attributable to the preparation of an occlusal cavity itself, and that the greatest loss of resistance results from the loss of the marginal ridges: the real resistance beams of the tooth.

Indeed, the preparation of a mesio-occluso-distal cavity destroying the marginal ridges would reduce the relative resistance of the cusps by 63%.

2. The Endocrowns:

“ Cameral anchoring without root weakening”

1.  Historical:

The first study carried out on the concept of the “Monobloc” technique was published by Pissis in 1995. He was able to develop his concept and meet the desired specifications based on tissue economy, economy of means and simplification of protocols, but also the absence of metal and aesthetic nuisance as well as the absence of root anchoring.

It was Bindl and Mörmann who were the first to call this restoration

“endocrown” in 1999, the French translation of which is “endocouronne”

2.  Definition :

The endocrown is a single-unit, monobloc prosthetic restoration for endodontically treated teeth with significant tissue loss.

It is characterized by a coronal portion replacing the defect whose apical projection is made into the cavity of the pulp chamber; thus creating a mechanical macro retention enabled by the exploitation of the pulp walls but also a micro retention with the use of bonding protocols and adhesives.

The cervical preparation itself has a straight shoulder with a rounded internal angle or is completely flat (we speak of “butt margin”) or cervical pavement ; Its most well-known indication remains the posterior teeth whose pulp chamber is deep and large enough to ensure retention, as is mainly the case for molars.

Recently, some authors have sought to extend their application to premolars with still uncertain but encouraging results.

2.3. Advantages:

Mainly made of ceramic, they have the same advantages as bonded restorations:

• Simplicity of implementation: in fact, the implementation is relatively simple as well as the taking of impressions without the need for a technique to access the limits

allow the number of surgical steps to be limited compared to treatments with conventional crowns and the use of intra or juxta-gingival limits.

• the concept of therapeutic gradient: they make it possible to delay the loss of the tooth on the arch as much as possible by slowing down the cycle of invasive restorations and by avoiding the need for root anchoring which could weaken the tooth.
• the advantage of being able to choose the base shade but also of being able to be made up for optimal mimicry with the peripheral tissues.
• Biocompatibility : Ceramics used in prosthetic dentistry are bioinert materials (chemical, electrical and thermal inertia). Their chemical structure gives them great stability and therefore good general biocompatibility.
• They are much more stable than metals and resins and do not present degradation by corrosion, finally they are smooth and without porosity and therefore reduce the adhesion of dental plaque and the periodontal inflammation which could result from it.
• the absence of a post and minimal preparation: the preparation can be carried out on low coronal heights because it exploits the pulp chamber for its macro and micromechanical retention and this without exploitation or weakening of the root integrity. In addition, the placement and removal of a post remain risky for the practitioner (perforation, false route, fracture, weakening by loss of root substance).
• The possibility of re-intervention on the endodontic treatment: in fact, it is possible to transfix the prosthetic piece in order to have access to the canal entrances. This seems simpler and less risky in terms of fracture than having to remove a sealed or glued intraradicular anchor.
• the absence of interference on X-rays: thanks to the use of ceramics, there is a reduction in optical nuisances during examination and therefore the possibility of more easily identifying any possible carious recurrence
• the reduction in the number of bonding interfaces compared to therapy with a classic post.

• Strengthening residual tooth structures: Bonded partial restorations strengthen residual tissues by reducing flexibility. According to some studies, restored teeth have a rigidity equal to or even greater than that of undamaged natural teeth (watertightness: the bonding joint of the ceramics as well as the good marginal adaptation of the ceramics allow a good long-term watertightness and reduce the penetration of microorganisms.
• hygiene and brushing for the patient made easier since the prosthetic joint is supragingival the possibility of being entirely produced by CAD/CAM technique in the office by the dental surgeon (CAD/CAM), a better peripheral joint than that found for direct bonded reconstructions the lower manufacturing cost and therefore cost for the patient compared to conventional therapy with anchoring and ceramic crown
• Preservation of periodontal health through supragingival limits: all authors recommend the supragingival limit as the only guarantor of periodontal integration.

4. Disadvantages and imperatives:

The concepts that should not be neglected, however, are:

• The need to place an operating field (the dam) in order to have a good watertight seal.
• the need for supragingival limits for bonding.
• the assessment, by the practitioner, of the mechanical, architectural and occlusal imperatives in order to best distribute the forces exerted by the rigorous bonding protocol;
• the need to have a sufficiently wide and deep pulp chamber.
• the risk of detachment and root fracture due to the difference between the moduli of ceramics and that of dentin.

5. The different materials used:
   1. The specifications:

The ideal restorative material, by extension from that for bonded partial reconstructions, must be able to meet the following specifications

:

• allow the most conservative approach to healthy tissues possible when arranging the necessary prosthetic space
• restore a natural and functional morphology of the dental complex
• restore mechanical resistance to the tooth, compatible with its function
• ensure optimal adaptation at the edges and interfaces be biocompatible be radiopaque be aesthetic
• ensure the greatest possible longevity and the best adhesion and bonding properties to the tooth

The two materials that are most likely to meet this specification are ceramics and composite resins.

• Ceramics

Definition : are a type of glass obtained by melting metal oxides at high temperature which solidify at room temperature.

Dental ceramics, more precisely, are composite structural materials comprising a vitreous structure called a glass matrix, reinforced by different crystalline phases which allow the thermal expansion coefficient of the material to be adapted and their properties to be modified.

They are manufactured by heating the mixture above the melting temperature of the glassy matrix and below that of the crystals by a process of

“sintering”.

1. Feldspathic ceramics They are known to be beautiful but fragile.
2. Aluminous ceramics

These ceramics contain a significant proportion of alumina in order to strengthen them for clinical use.

3. Glass ceramics

These are materials formed into a glass state that undergo a voluntary, controlled, and partial crystallization heat treatment. They have very good aesthetic properties.

• Composites

A composite material is a material formed from several materials of different nature or origins whose mechanical characteristics are superior to those of the materials included in its composition.

For this definition to be valid, it is necessary that the cohesion of the whole be ensured by mechanical, physical or chemical bonds.

In dentistry, we speak of “composite resin” when a material is made up of an organic resin matrix and a reinforcement made up of fillers.

The cohesion between these two materials is ensured by a coupling agent: a silane.

• Hybrids (ceramic-composite)

These are new materials composed of a three-dimensional structure of feldspar ceramic reinforced by methacrylate polymers derived from acrylic resin.

Indeed, ceramics, due to their high chemical stability, have good optical and mechanical properties as well as excellent biocompatibility, but repairs remain problematic once placed in the mouth.

Conversely, composites are easier to handle and repair but have inferior mechanical and biocompatible properties to ceramics.

• Liaison Officer:
  • The bonding material forms the critical interface between the restoration and the prepared tooth.
    • The material must not only have good adhesive properties, but its modulus of elasticity is also important, as the material must be able to accommodate stresses such as the enamel-dentin junction does.
    • The interface includes all prepared surfaces. If the product is to be light-cured, a high-power lamp must be used that can reach the photoinitiators at the pulp floor, under layers of ceramic that sometimes exceed 7 mm.

3. Indications and contraindications

• Endocrown is indicated for all molars, and more particularly those with a low clinical crown, calcified root canals or very thin roots.
• Endocrown is contraindicated if adhesion cannot be guaranteed, if the pulp chamber depth is less than 3 mm, or if the cervical contour is less than 2 mm wide over most of its circumference.

4. Lifespan and efficiency:

Several authors have concluded that endocrowns are more resistant to compressive forces than conventional crowns. More recently, a finite element analysis has highlighted the advantage of endocrowns in stress distribution.

.

5. Morphology of the preparation

• The butt joint, or cervical pavement, is the basis of the restoration with a peripheral enamel band that optimizes adhesion.
• The prepared surface should be parallel to the occlusal plane to provide resistance to stresses along the major axis of the tooth.
• The pulp chamber cavity provides retention and stability. Its shape – trapezoidal in lower molars and triangular in upper molars – improves the stability of the restoration.
• No further preparation is necessary.
• The saddle-shaped pulp floor provides increased stability.
• This anatomy, combined with the adhesive qualities of the sealing material, makes any anchoring in the root canals unnecessary.
• In fact, root canals do not require any shaping; they are therefore not weakened by milling and they will not be exposed to the stresses resulting from their use as anchors.
• Finally, the compression load is reduced, being distributed over the cervical contour and the walls of the pulp chamber.

(a). Pressed endocrown with sprues, placed on a model (b).

6.  Preparation methodology:
7. Occlusal preparation

• The preparation aims to reduce the overall height of the occlusal surface by at least 2 mm in the axial direction.
• This reduction can be achieved by making 2 mm deep grooves as guides and then using a diamond wheel bur (green ring) to reduce the occlusal surface.
• The bur is oriented along the main axis of the tooth and held parallel to the occlusal plane.
• Its shape allows the orientation of the reduction to be controlled and a flat surface to be obtained, which will determine the cervical limit (or cervical sidewalk).
• This contour should be in a supragingival position, but it can also follow the gingival contour if clinical or aesthetic factors require it.
• The level differences between the various segments of the cervical contour must have a slope of at most 60° to avoid any staircase effect.

Enamel walls less than 2 mm thick should be removed.

Creation of guide grooves on an isolated tooth and in situ.

Preparation of the cervical sidewalk using a wheel burr held parallel to the occlusal plane.

2. Axial preparation

• This preparation mainly involves removing undercuts in the access cavity. A green diamond bur of cylindrical-conical shape, with a total convergence of 7°, is used to make the pulp chamber and the endodontic access cavity continuous.
• By orienting the bur along the long axis of the tooth, the preparation can be carried out without exerting excessive pressure and without touching the pulp floor. Removing too much tissue from the walls of the pulp chamber will reduce the thickness of the walls and reduce the width of the enamel band.
• The cavity must be at least 3 mm deep.

Axial preparation carried out with a cylindrical-conical burr, to make the pulp chamber and the access cavity continuous.

3. Polishing the cervical band

• The bur used during this step has the same taper as that used for axial preparation, but its diameter is larger and the particles are finer.
• The burr should be directed over the entire surface of the cervical band, to eliminate micro-irregularities and produce a flat, polished surface.
• The contour line should be smooth and determine a sharp-edged boundary.

Polishing the cervical band.

Cervical contour before (a) and after (b) polishing.

4. Preparing the cavity floor

• The pulp canal entrance is cleared. Gutta-percha is removed to a maximum depth of 2 mm to take advantage of the saddle-shaped anatomy of the pulp chamber floor. This step should be performed with a non-abrasive instrument to preserve the integrity of the canal entrance.
• No milling of the dentin is performed.

5. Cleaning the pulp chamber

It is recommended to use ultrasound to thoroughly clean the pulp chamber and floor. Abrasion is not indicated.

6. Sealing

Adhesives such as self-adhesive cement, or bonding composites, are used to bond the endocrown to the prepared tooth.

Prepared tooth (a), endocrown (b) and final result after sealing (c)

7. The footprints

Impression taking does not represent a major challenge here since all the limits are supragingival in order to achieve bonding of the ceramic.

It can be performed on the day of preparation if conditions permit, but can also be delayed by a week or more depending on whether a surgical technique to access the margins was necessary to allow bonding.

• Traditional prints

Hydrocolloids and hydroalginates are unsuitable for cavity preparations due to their tearing properties, which limits them to taking impressions of the opposing arch in fixed prosthesis.

Elastomers (addition silicones and polyethers) represent a more common and suitable alternative due to their registration accuracy, good elasticity, tear resistance and ubiquity in dental practices.

The “double mix” impression is the technique indicated for recording cavity preparations with silicones. It is carried out in one step with two materials of different viscosities.

The operator may use a tissue deflection technique to improve the recording of the emergence profile but this is not necessary a priori due to the distance between the prosthetic limit and the gingival contour.

Similarly, an inter-arch report can also be recorded following the patient’s occlusion to transmit information to the prosthetist.

• Optical fingerprints

Developments offer many perspectives in terms of recording and it is now possible to make an optical impression.

The intra-oral optical impression is also based on three separate recordings: the arch concerned; the antagonist arch; a vestibular recording in the maximum intercuspation position allowing the occlusion of the two arches replacing the interarch relationship for traditional impressions.

The advantages of optical impressions are based on precision, ergonomic and unalterable nature, time saving, as well as the possibility of completing an impression already taken without material, without printing and without disinfection protocol or even of detecting an inaccuracy in a preparation form and correcting it immediately.

Patients’ visualization of their arches also helps in understanding the treatment.

The main disadvantage of this method is financing: the material investment remains significant for the dental surgeon.

Some authors such as Carlos et al., advise, in the case of endocrowns, to apply a layer of glass-ionomer cement in order to obtain a flat pulp floor and to seal the canal entrances, which would make the optical impression simpler.

8. The temporization

Ideally, the delay should be as short as possible to avoid any inconvenience to the patient and to allow the prosthetist to perform the endocrown (generally one week).

This step also allows the preparation thicknesses to be validated and a temporary filling can be created using a conventional self-molding technique using acrylic resin (e.g., Unifast®, GC Corp, VOCO) made during a prior session on the intact tooth or during the session from a mold (3M ion) positioned on the prepared tooth.

The temporary is then sealed using temporary cement, preferably without eugenol so as not to affect bonding and polymerization.

9. The design of the prosthetic part

The impressions can be sent to the prosthetic laboratory to be used by the prosthetist or can be used directly in the office if the practitioner has the equipment for the CAD/CAM technique.

Traditional impressions are cast in plaster to obtain a model that can be split into a single positive model (SPM).

This method allows the tooth to be reconstructed to be separated from the rest of the model, while allowing its exact repositioning, in relation to the adjacent teeth.

The goal is to be able to easily access the proximal preparation limits.

• The CAD/CAM Technique*

The CAD/CAM technique allows automated machining of a part based on its computer design.

After digital acquisition of the models in STL (Standard Tessellation Language) files, the operator can design the future prosthetic part using 3D modeling software.

Then, the machining unit produces the prosthetic part using an automated process from a preformed ceramic block which can then be coated.

10. The fitting

It is important to perform a verification of the endocrown on the model before the patient arrives: the marginal adaptation and the proximal contact points must be validated.

The prosthetic part is then disinfected and decontaminated in 90° alcohol then in 2.5% sodium hypochlorite.

In the mouth, the practitioner will take care to eliminate all traces of cement and material

temporary filling that may prevent proper insertion of the endocrown.

This is then tried on the tooth. It must be inserted passively into the preparation. Any forced insertion creates a risk of fracture of the endocrown.

If the part does not enter easily, first check the contact points again using a piece of dental floss.

If the contacts are considered too marked, 40 micron marking paper is interposed between the restoration and the adjacent tooth in order to visualize where, and by how much, the proximal reduction should be made.

These corrections are carried out using a red ring diamond cutter, mounted on a turbine, under air+water spray.

Once passively inserted, marginal adaptation is checked visually and with a probe.

11. The assembly

Hardening by polymerization (covalent bonds) of adhesives gives them a mechanical resistance much superior to cements whose mechanical performance is considered low to average.

Furthermore, in the case of endocrowns, we have seen previously that etchable reinforced ceramics were the materials of choice for manufacturing and that their prosthetic limits were supragingival.

Effective bonding is therefore indicated and possible.

Finally, the possibility of choosing the color and their quality optical properties

allow the glues to match the aesthetic appearance of the ceramics.

The bonding is carried out under protection and isolation by an operating field placed around the tooth in order to guarantee the best results.

12. Treatment of the prosthetic intrados:

The goal is to create a rough surface in which the glue will diffuse. To attack the surface and create these roughnesses, two treatments are possible:

Sandblasting with 50 μm alumina particles etching with 9.5% hydrofluoric acid Etching with hydrofluoric acid for 20 seconds provides the most effective and reliable bonding.

13. Finishes

Once the bonding is complete and the occlusion is adjusted, the last excess is removed using a cold blade of a No. 12 scalpel, dental floss, and metal matrices.

It is important to carry out a final photopolymerization of the prosthetic limits through a layer of glycerin because oxygen alters the hardening of the surface bonding resins and creates an inhibition layer which tends to degrade more quickly than the rest.

Finally, polishing is carried out later so as not to cause any tearing of the assembly material.

Conclusion

Preparation for endocrown placement is streamlined and simple, and can be done quickly.

The root canals are not affected, and the procedure is less traumatic than other treatments .

The supragingival position of the cervical contour preserves the marginal periodontium, facilitates impression taking and preserves the solid substance of the residual tooth.

The all-ceramic monolithic construction, made by pressing or machining, provides mechanical strength to the endocrown .

From a biomechanical point of view, the restoration promotes adaptation to the constraints exerted at the level of the bonding joint.

The forces are distributed across the cervical pavement or cervical joint end-to-end (compressive forces) and the axial walls (shear forces), which alleviates the loads on the pulp floor.

The endocrown fits perfectly into the concept of biointegration and is one of the restorative options available for endodontically treated posterior teeth and severely damaged molars.

Indirect coronal-radicular restorations “Endocrowns”

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Indirect coronal-radicular restorations “Endocrowns”