Coronal-radicular anchors

Coronal-radicular anchors

Introduction :

Coronoradicular restorations are the last resort in the treatment of major loss of substance from the dental organ caused by caries or trauma.

The success of such a prosthetic restoration depends on the exploitation of the root housing; this is called coronoradicular anchorage (CRA).

Coronal restorations with corono-radicular anchors are intended to compensate for the tissue, mechanical and aesthetic deficit of dilapidated teeth. It is traditional to divide them into two main families:

• Cast metal CR reconstructions (the Richmond, the inlay-core or false stump).
• CR reconstructions made with materials crushed in a plastic state (fiber tenon reconstruction).

1. Definition:

Coronoradicular restorations or replacement crowns replace the dental crown that no longer exists.

It completely replaces the natural crown with another artificial crown. These crowns are supported on the remaining roots by a metal root tenon.

Coronal-radicular anchors

2. Reminders:

2.1. Root anatomy:

Whether in the canal shaping for endodontic treatment, in the mechanical preparation of the post housing or in the choice of the post itself, the root morphologies of the different groups of teeth present varying difficulties.

In the oral cavity there are easy teeth and high-risk teeth.

2.1.1. “Easy” teeth:

Teeth with one or more straight and rounded roots are considered to be such. These are the maxillary incisors and canines as well as the mandibular canines and premolars.

2.1.2. “High risk” tooth:

These are mainly teeth with curved and flattened roots. The most “dangerous” roots are in descending order:

• the roots of the maxillary premolars (mainly the palatine);
• the vestibular roots of the maxillary molars;
• the mesial roots of the mandibular molars.

2.2. Root tenon:

To fulfill its role, the tenon must meet many requirements.

2.2.1. Rigidity:

The stiffness is proportional to the modulus of elasticity of the materials constituting the tenon and to its diameter.

A tenon that is not very rigid will undergo bending under the action of occlusal forces which may exceed the elasticity limits of the root dentine and cause cracks or fractures.

If the post is too rigid, it will transmit stress directly to the dental structures; if these stresses exceed the resistance threshold of the dentine, there is a risk of fracture.

Among all existing types of studs, nickel-chromium appears to be the most rigid material, followed by steel, titanium and semi-precious alloys.

2.2.2. Concept of retention: 

The tenon’s retention is based on three parameters: shape, length and surface condition.

The shape: The prefabricated metal tenons come in 3 shapes: 

• Cylindrical-conical posts: recognized as very retentive, their insertion requires very wide canal shaping in the apical half of the root.
• Tapered tenons: are more economical but their retention is much lower than that of cylindrical tenons.
• Cylindrical-conical tenons: combine a cylindrical part improving the retention and stability of the tenon in its housing and a conical apical part that is more respectful of root morphology. 

Length:

Tensile strength increases with length. In theory, the length of the post should be between one-half and two-thirds of the root length. In all cases, this should be greater than the crown height.

Surface condition:        

The anatomical tenons are systematically sandblasted to increase the adhesion of the sealing cements.

Prefabricated tenons are classified into two groups:

• Dentin tapping posts, the most resistant but to be avoided by many authors, cause screwing constraints on the dentin, leading to cracks.
  • The posts without dentin tapping all have horizontal or helical striations over all or part of their length.

3. The root tenon crown: the Richmond

3.1. Definition:

The Richmond belongs to the family of replacement crowns; It consists of completely replacing the coronal part of a generally very dilapidated tooth by resting mainly on a root tenon. It is a corono-radicular restoration, it consists of:

• A small metal cap that surrounds the cervical plate while being attached to the root tenon;
• The tenon and the coping form the infrastructure of the Richmond;
• On this infrastructure, the crown is developed, which includes a palatal or lingual metal support called a heel and a cosmetic element on the vestibular part, all of which constitutes the supra structure. 

3.2. Indications:

• The Richmond is indicated mainly in the anterior sector when the occlusion is tight and does not allow the use of an inlay core.
• Crown damaged by decay.
• Relatively large coronal fracture (trauma).
• Significant misalignment of a tooth to be used as a bridge abutment.

3.3. Contraindications:

• Root fracture.
• Root perforation.
• Significant mobility.
• Unstable apical or periapical lesions.

3.4. Disadvantages:

• The fact that the tenon is integral with the crown constitutes a major disadvantage because the stresses exerted on the prosthesis will be directly transmitted to the tenon, hence the risk of root fracture or loosening.
• When the Richmond has to be removed, there is a risk of dental decay.

3.5. Clinical preparation:

3.5.1. Stump size:

• Instrumentation: cylindrical or cylindro-conical diamond tip mounted on turbine, diamond grinding wheels and discs. Cutting can be done in 4 phases:
•  Phase 1: Reduction of the coronal height and size of the cervical plateau. Using the turbine, the reduction begins with the free edge using a diamond tip or diamond grinding wheel in the form of a wheel. Oriented perpendicular to the axis of the tooth by making back and forth movements from the mesial edge to the distal edge, the coronal part is reduced to 1 to 2 mm below the gingival margin. And the cervical plateau must be perpendicular to the axis of the tooth.
• Phase 2: Trimming the palatal and proximal faces. The relief faces must be made using a diamond bur held parallel to the root canal axis; at these faces the cervical limit will be a juxta-gingival chamfer.
• Phase 3: Trimming the vestibular slope starting from a mesio-distal line passing through the center of the root canal. The vestibular part is trimmed in a gentle bevel toward the gingival margin up to an intra-sulcular level. This is called flute beak trimming.
• Phase 4: Polishing the preparation; Done using rubber grinding wheels.

3.5.2. Channel boring:

The preparation of the root housing is done in two stages:

• Widening of the canal: this is done using manual instruments (pins, rasps) or using rotary instruments such as drills; the two methods can be combined and this is the most frequent case. 

• The unclogging of the canal entrance can be done with a fine round bur to clear the canal entrance.
• Place the drill in the canal housing and continue widening with drills of increasing sizes until reaching the one chosen for the tooth in question, while using the X-ray as a guide.
• Unblock what remains of the canal preparation using H files. The reaming is carried out to a desired length (generally 2/3 of the root height is unblocked). 
• At the end of the preparation, the root walls must be bare, converging towards the apex which must always remain hermetic (1/3 apical sealed). 
• Pilot hole size:

When the canal reaming is completed, a secondary means of retention is practiced by widening the entrance of the root canal into a funnel using a fissure bur. This pre-hole also constitutes a reinforcement of the most fragile part of the tenon-metal coping connection. 

3.6 . Taking fingerprints:

We start with the impression of the prepared root housing, then, the location impression of the entire arch which includes the stump and the impression of the antagonist arch.

The canal should be cleaned with alcohol and then dried, lubricated with Vaseline.

The canal impression is made using two methods:

• Direct method: performed directly in the mouth using blue inlay wax (type 1).
• Indirect method: (most recent method) uses viscosity-based silicones and a tutor to support the impression material.

Coronal-radicular anchors

4. Inlays-cores or false stumps:

4.1. Definition:

These are fully cast corono-radicular metal cores intended to replace partial or total coronal loss of a devitalized tooth.

When the coronal destruction is partial, they are called inlay-cores; when it is total, they are called false stumps. 

It consists of a metallic infrastructure with corono-radicular support and a supra coronal structure in the form of a peripherally covering crown (CC, CIV or CCM) which restores aesthetics.

4.2. Indications: 

• Crown decay due to caries in the anterior or posterior sector.
• Teeth with wide, juxta- or subgingivally beveled coronal fractures.
• To correct the parallelism of the abutment teeth in the case of large prosthetic reconstructions.

 4.3. Contraindications:

• Root fracture or Rhizalysis.
• Root perforation or unstable apical or periapical lesions.
• Significant mobility.
• Classic restoration proves to be insufficient.

4.4. Advantages  :

• It is an excellent bridge abutment because of its mechanical resistance.
• Provides protection and strength to severely damaged teeth.
• Inlay core, can correct the parallelism between the axes of the very divergent abutment teeth.
• The independence between the inlay-core or the false stump and the covering anchorage.

4.5. Clinical preparation:

4.5.1. Size of the residual crown: 

• The remaining portion of the decayed crown will undergo peripheral preparation or trimming of the chosen anchor. The unsupported enamel and dentin portions will be removed, and chamfered with a diamond flame bur.
• The outer peripheral portion of the occlusal plateau of the tooth will be beveled with a flame bur to increase the seal and ensure protection of the dentinal walls. 

4.5.2. Channel boring:

• As with the Richmond, the general rule is:
• The length and diameter of the tenon will depend on the root length.
• The length of the tenon should be 2/3 to 3/4 of the length of the root.
• The end of the root must remain sealed for at least 3mm.
• The length of the tenon must be equal to or greater than that of the crown to be reconstructed.
• The width of the reamed canal should be equal to 1/3 of the mesio-distal diameter of the root. 

4.6. Different types of inlay-cores: 

4.6.1. Inlay-core on mono-roots:

It is a single-piece inlay-core, a partial coronal reconstruction inlay attached to the root post (on the lower canine incisors and premolars).

4.6.2. The bi-radicular inlay-core:  

• In the case of parallel canals, we are in the same case as the monoradiculates.
• In the case of non-parallel canals: one of the tenons will be attached to the inlay-core, the other will be placed secondarily, acting as a key.
• If they are maxillary premolars, the post is placed in the vestibular canal, with another post serving as a lock that guides the placement and prevents rotation of the inlay-rib.

4.6.3. The inlay core on multi-rooted:

The metal reconstruction with tenons on molars is more difficult to achieve.

One of the tenons will be attached to the inlay-core and depending on the coronal deterioration and the desired quality of retention, there will be one or two keys.

• In the maxillary arch: the tenon is intended for the palatine canal.
• In the mandibular arch, the tenon will be in the distal canal because it is straighter.

Coronal-radicular anchors

5. Fibered tenon reconstructions:

Represent an excellent alternative to corono-radicular restorations designed with metal posts.

5.1. Advantages:

• They allow tissue savings, thus preserving the solidity of the depulped tooth, unlike cast ACRs.
• Bonding allows for reinforcement of the remaining dental structures and distribution of functional constraints across the entire dental organ.
• Gluing also provides a watertight seal.
• Fiber post reconstructions also help to improve aesthetics in cases where ceramic-ceramic crowns are made.
• Fiber post reconstructions can be performed in the same session as the endodontic obturation.

5.2. Disadvantages: 

• The protocol for implementing a fibered tenon reconstruction is delicate and, as with any bonding protocol, requires a great deal of rigor.
• The cost of the technical platform and materials is high. 

5.3. Indications:

This type of reconstruction is indicated when:

• 2 or 3 residual walls persist.
• The wall thickness is greater than or equal to 1 mm.
• The height of the residual walls is greater than or equal to half the prosthetic coronal height.
• The cervical margin is in a supragingival position.
• Clinical accessibility to the preparation is sufficient to ensure effective photopolymerization.

Coronal-radicular anchors

Conclusion :

Crown-root reconstructions must be adapted to the clinical situation; their preparation is therefore extremely varied and depends on the clinical situation, the type of tooth to be restored, the coronal and root anatomies and finally the type of damage.

Whatever the type of restoration, one principle will be necessary: ​​that of dentin conservation (both at the coronal and radicular level).

The corono-radicular anchorage must be designed and carried out as foundations, on which the clinician, in the absence of biological problems, can construct and possibly reconstruct a prosthetic supra-structure.

Coronal-radicular anchors

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Coronal-radicular anchors