Coronal-root restorations
Definition
- A coronal-radicular reconstruction is a restoration that involves both the coronal and radicular parts of the tooth.
- It always involves a complex reconstruction which, to ensure its retention, uses root anchors.
Goals
CPR allows:
- to restore loss of substance due to trauma or carious lesion and their therapies;
- to preserve residual dental tissues;
- to ensure the sealing of the canal obturation;
- to ensure retention of the overlying prosthetic part;
- a balanced distribution of the stresses undergone by the prosthetic restoration and transmitted to the remaining coronal and radicular tissues;
- to contribute to the restoration of the aesthetic aspect of the restoration
TYPES OF CPR
- direct RCRs : using materials inserted in the plastic phase, supported or not by a tutor;
Benefits
The mode of reconstitution by RCR inserted in the plastic phase allows
● a single session production (no relationship with the laboratory),
● tissue savings thanks to gluing because we can keep the undercuts,
●to have a modulus of elasticity close to that of dentin,
● an aesthetic improvement in the case of previous restoration.
Disadvantages
The clinical protocol has the disadvantage of being more demanding and taking longer to implement. In addition, the technical platform and the cost of the latter and the materials are higher.
Indirect RCRs: performed in the laboratory. They are metallic (precious or non-precious alloys) or ceramic.
Benefits
Cast RCRs can be used to respond to a variety of clinical situations and extremely significant carious or traumatic deterioration where RCRs inserted in the plastic phase are no longer sufficient.
Disadvantages
- Reconstruction by inlay-core or cast false stump is a long technique because it requires a second clinical session.
- It is also iatrogenic with respect to residual dental structures because it requires the removal of undercuts.
- It presents risks of corrosion due to the conductivity of the metal.
- The modulus of elasticity of the materials used in cast RCR is significantly higher than that of dentine (20 GPa), which implies a large difference in rigidity and therefore localized and intense stresses, which could be the cause of root fractures.
Therapeutic gradient of choice of CPR
- As early as 1979, an in vitro study showed that the placement of a root post did not increase the resistance to root fracture.
- A certain study even showed that the resistance of a tooth devitalized without a root post was higher than that of teeth with a sealed or glued root post, metallic or carbon fiber.
- Two others have, in this sense, highlighted the dangers linked to the installation of a root tenon
The behavior of the devitalized tooth
For many years, the idea of a “fragility” of the devitalized tooth has been widely accepted. Many studies have sought to highlight the structural differences of the devitalized tooth and their implications in restorative dentistry.
Water concentration of dentin
The notion of dehydration of the devitalized tooth has long been synonymous with vulnerability
The water concentration of the dentin of a pulped tooth is around 13%.
Devitalized teeth had 9% less water than pulped teeth.
For some, this water loss is only free water and not water bound to collagen, and it is less than 9% of total water, which is not a significant deficit.
Hardness and modulus of elasticity
- Many authors have evaluated the changes in the mechanical properties of dentin caused by pulp removal.
- A study showed that no significant decrease in compressive and tensile strength could be demonstrated after pulping.
Comparison of the biomechanical behavior of healthy and pulpless teeth, according to
SEDGLEY and MESSER
Loss of tooth substance
Main cause of weakening. In 2002, a study emphasized the principle of tissue economy of internal and external structures, showing the importance of this element in the prevention of root fractures and the long-term maintenance of the tooth on the arch.
- Related to endodontic treatment
The instrumental stages of endodontics, as seen previously, have only a slight impact on the biomechanical behavior of the tooth.
Particular vigilance is nevertheless recommended when creating access cavities for the upper molars (mesio-vestibular canal) and lower molars (mesial canals).
Indeed, sometimes excessive thinning of the dentinal walls can lead to a weakening of the cervical region.
In addition, a conservative attitude during the access cavity will allow us to keep the maximum amount of residual dentin for the possible bonding of our composite stump.
- Related to caries
The fragility of the devitalized tooth is proportional to the disappearance of the tissues which led to the need for endodontic treatment .
Whether the tooth is vital or not, the greater the amount of tooth substance lost, the more the residual structure is weakened and therefore prone to fractures.
- Related to the preparation of a canal accommodation
Drilling the root canal leads to thinning of the root walls and at the same time to the formation of weak points.
Summary:
- Despite the relative dehydration of the devitalized tooth compared to the vital tooth, there appears to be no modification of the mechanical and biological characteristics of the dentin attributing a more fragile character to the non-vital dentin.
- Similarly, well-conducted endodontic treatment does not significantly modify the resistance and hardness of the tooth.
- The loss of marginal ridges induced by carious or traumatic damage or by eviction and restoration therapies dramatically compromises its resistance, thus increasing the risk of fracture.
- The amount of residual dentin must be as large as possible and requires maximum tissue economy in order to reduce the weakening of the devitalized tooth.
The dangers of the root post on the biomechanical behavior of the devitalized tooth
Root cracks and fractures are common consequences because occlusal stresses are applied to root walls thinned by the drilling preparation, often unable to resist the forces transmitted by the post.
Drilling is also dangerous for the integrity of the tooth because it presents:
- A risk of perforation of the pulp floor, the apical third of the root, or its invaginated lateral regions
In the majority of cases, these result in tooth extraction.
- A risk of fracture if the drilling axis is deflected:
The forces will be exerted on a weakened area, creating a wedge effect that dramatically compromises the integrity of the root.
There are also special clinical situations where root posts are dangerous:
- This is the case for curved roots: The end of the tenon must not exceed the curvature zone, which makes some roots difficult to use effectively.
- Another clinical situation is that of oval or flattened section canals :
- The placement of prefabricated posts in such canals leads either to weakening of the root (if precise adaptation of the post is sought) or to a considerable decrease in post retention.
- There is also a danger when the tooth requires re-intervention. Indeed, the removal of an anchor increases the risk of perforations and fractures.
Summary:
Drilling and placing a root post can cause many risks to the integrity of the tooth. It is therefore necessary to avoid anchoring as much as possible, hence the interest in promoting in our therapeutic gradient the realization of an RCR inserted in the plastic phase without a post.
Coronal-radicular anchoring
- The essential role of the root post and the retention of the reconstruction and the strengthening of the root/reconstruction bond
- It does not strengthen the root itself but can on the contrary weaken it by the loss of substance that would result from the preparation of a housing for a tenon of excessive diameter.
- The tenons, in addition to their role of retaining the reconstruction, would distribute, at the root level, the constraints applied at the level of the coronal reconstruction.
- They must be studied in number, shape, length, diameter and materials.
Principles
- The number
To distribute the forces as well as possible, one tenon per root should be used as much as possible.
- The shape
Very little retentive conical post even if it adapts to the canal morphology, it is not recommended for single-rooted teeth.
Very retentive cylindrical tenon , it weakens the apex if it is long, because it does not adapt to the root end, it will be used to increase retention on a short root of a single root
Cylindrical conical tenon the cylindrical part is retentive and the terminal part is closer to the root anatomy at the apex, therefore less fragile. It presents a good compromise and can be frequently used.
Anatomical tenons reproduce the shape of the canal after widening, this shape is the most economical of tissues, it is very often used as long as the retention obtained is sufficient
- The diameter
The larger the diameter, the stronger the tenon, but the weaker the root (risk of fracture)
The diameter depends on the material and its mechanical qualities (precious or not) the most commonly used diameters are those between 1.2_1.6 mm.
It is prudent to leave at least 1mm of root wall thickness around the post. This is easily achieved at the cervical level, but towards the apical end the walls are likely to be too thin for a long, cylindrical post.
- Length
The longer the post, the better the distribution of stresses at the root level, but it is imperative to leave a sufficient length of the root canal obturation so as not to hinder its hermeticity. The accepted standard is to leave an intact plug 4-5 m, reduced to the extreme limit to 3 mm for short roots and where retention is problematic.
Tenon Length Rules
For a single-rooted tooth and therefore a single post:
- 2/3 to 3/4 root height
- At least equal to the height of the crown
- End of the post more apical than the bony edge (loss of bone height in the event of periodontal disease)
For multi-rooted plants, the root length can be reduced so as not to weaken the curved roots.
- Materials
Rigidity appears to be favorable to good stress distribution
We therefore prefer to choose a material with a high modulus of elasticity.
- The metal will be chosen in accordance with the reconstitution material to avoid the problem of corrosion.
- Carbon fiber posts are proposed, on the argument of the modulus of elasticity close to that of dentine.
- Ceramic posts (alumina or zirconia) are also proposed, clinical experience appears insufficient to verify their long-term resistance, their removal also appears problematic if a reintervention proves necessary
- Assembly mode
Sealed post: conventional sealing (zinc oxide, polycarboxylate cement) despite some adhesion to dental tissues, the mechanical quality is low for a durable seal
Adhesive sealing
Conventional or modified CVIs with the addition of resin
Bonded: with resins or bonding composites
Preparing the accommodation
Involves partial removal of the root canal filling, the aim is to leave a watertight apical portion.
- Unclogging
It is performed with rotating instruments with a blunt tip so as not to risk deviating from the canal, and at low speed so as to warm the gutta and manipulate at low pressure in this way the instruments can be guided by the walls of the canal and there is no risk of perforation.
The choice of their diameter must be guided by the knowledge of the root anatomy
of the instrument of choice: largo drill, which prepares the canal shaping at the same time.
When a cylindrical conical tenon is chosen, the drill diameter will be chosen in accordance with the diameter of the apical end
Immediately after the drill has passed, the remaining gutta plug , warmed by unobturation maneuvers, will be condensed with a flat plugger of a suitable diameter to perfect its sealing. This unobturation process involves a canal that is obturated according to the rules
- Root canal shaping
Canal shaping can be limited to the passage of drills. If we wish to protect the canal walls and create an anatomical post, it can be completed by the passage of specialized drills, of increasing diameters to obtain the shape corresponding to a manufactured post (cylindrical or cylindro-conical)
CPR on the move
This reconstruction includes one or more tenons sealed or glued in the root(s) and a material inserted secondarily in the plastic phase.
Materials
- Silver amalgam was once used, with corrosion problems arising with the post on the one hand and the metal crown on the other.
- Composites or CVI are more mechanically resistant, but their marginal sealing is questionable in the long term.
- they can undergo swelling by water inhibition which modifies their volume
- finally the adhesion of certain sealing cements is poor on these composites
- CVIs are less mechanically resistant but their adhesion to dentin does not weaken and the seal is therefore better over time.
Let’s hold on
- They must be electrochemically resistant and compatible with the metals of the superstructure to avoid corrosion. The material of choice currently is titanium and its alloys.
- Some tenons intended for reconstructions using crushed material are threaded. This thread improves their retention and, above all, facilitates possible removal by screwing.
- Carbon fiber tenons are proposed which have inherent strength but insufficient bonding with the reconstruction material,
Advantages and disadvantages
- Fabric Saver
- Fast, simple and inexpensive
However
- Low mechanical resistance due to the intrinsic properties of the materials and their poor bond to the tenons
- Its uncertain watertightness implies a significant covering by the superstructure
Indications
- Teeth with little decay, destruction limited to 1-2 walls and residual walls with at least 1.5mm thickness
- The limit of the reconstruction must be located at least 2mm from JAC
- Height should be >1/3 of the total crown height
- Use on anterior teeth is limited due to the low vest-ling thickness and therefore insufficient resistance of the material.
Implementation protocol
RCR casting
- A cast reconstruction consists of a metal part obtained comprising one or more tenons, of the same alloy as the reconstruction to avoid corrosion.
- Generally the supra structure completely encompasses the reconstruction and ensures the dento-prosthetic joint.
- In some exceptions, the reconstruction ensures the dento-prosthetic joint by a metal strip of around 10 mm.
Advantages and disadvantages
The advantages of cast RCRs are:
- mechanical resistance of the inlay-core linked to its metallic nature
- quality of the cervical joint, linked to the precision of adaptation of the casting
- versatile and easy to implement technique
- sustainability of this type of pre-prosthetic coronal-radicular reconstruction.
For the Cons:
- high modulus of elasticity of the metal tenon.
- difficult re-intervention.
- Not very economical because inserting the part involves stripping the pulp chamber
Method of implementation
1. Direct technique
2. Indirect technique .