Preparations for full coverage crowns

Preparations for full coverage crowns

Introduction

Joint prosthesis is the art and manner of reconstructing and replacing damaged or missing teeth with cast metal, metal-ceramic and ceramic-ceramic crowns and bridges.

Among the full coverage crowns we distinguish: 

• the cast crown.

• The crown with vestibular inlay.

• The ceramic-metal crown.

• The jacket crown.

Successful fixed prosthesis treatment requires a careful combination of various aspects of dental therapy. 

1. Clinical preparation:

The preparation (or cutting) of the stump consists of reducing the coronal surfaces of a certain volume of tissue, in order to separate them from the adjacent and antagonistic teeth, in order to interpose the prosthetic element.

Two methods that provide precise quantitative control of the thickness of the preparation can be used:

• The preparation method called controlled penetration, by creating driving grooves.
• The control method using a silicone key cut along a vestibulo-lingual axis, serving as a reduction guide.

1.1. Size of a posterior tooth:

We take the 1st lower molar as an example.

• The instrumentation will be quoted according to the phase of the size and the face considered.
• If the tooth is vital, local-regional anesthesia is administered.
• Throughout the cut, it is necessary to work along the chosen insertion axis and under water cooling to avoid harming the health of the tissues, especially the pulp (risk of necrosis).

1st ^step : Reduction of the FO:

Its objective is to disengage this FO by creating sufficient space (1 mm) in relation to the opposing teeth for the thickness of the metal. 

For this reduction, a diamond wheel bur (or grinding wheel) or a thick cylindro-conical bur can be used. The latter will be directed towards the center of the FO (MD groove) while respecting the inclination of the cusps (cusp slope). The successive passes of the bur mesio-distally on the V side and the L side must result in a uniform reduction that maintains the general appearance of the concave inclination of the occlusal face.

2nd ^step : Reduction of the proximal faces:

• The quality of a stump lies mainly in the preparation of its proximal faces (retention). Trimming consists of removing the points of contact with the neighboring teeth, as well as the anatomical bulges of these faces.
• To do this, a fine truncated cone cutter mounted on a turbine begins the reduction in a VL direction then in an LV direction until the contact point M is eliminated, then D.
• In no case should the neighboring teeth be touched (otherwise protection by a metal matrix).
• A larger conical cutter will then give the general orientation of the M and D faces, increasing the separation with the adjacent ones, and setting the 1st degree of convergence towards the FO (draft) from 3° to 10° depending on the crown height.

3rd ^step : Reductions of the V. and L. faces:

Its purpose is to remove the V. and L. bulges using a truncated cone bur with a rounded tip. The cervical limit will be started at a supragingival limit while giving a clearance to these two faces (convergence towards the FO) (angle of 5° to 10°).

4th ^step : Round the proximo-lateral angles:

The connecting angles V.Iv1. and VD and the angles LM and LD are rounded using the round-end conical milling cutter, thus joining the faces V and L to the proximal faces (Met D).

5th ^step : Chamfering the external edges of the FO:

The peripheral edge of the FO joining it to the VLM and D faces will be chamfered and rounded using the conical cutter with rounded end.

The chamfer will be wider on the external slope of the support cusps (V for the 1st ^lower Mol.), this is to have a sufficient thickness of metal at this level and to have a correct morphology of the cast crown.

6th ^time : Finishing the cervical limit:

The conical bur with a round end or an oblong conical bur with a rounded end will allow the cervical limit to be finished in the form of a fillet (or quarter round) at a juxta-gingival level on the V side (if the coronal height is sufficient), on the L side and at the level of the proximal faces.

Thus, the fillet will be peripheral with a width between 0.4 and 0.6mm.

7th ^step : polishing the stump:

The surfaces cut using diamond burs are rough (pulling during impression taking). Their polishing is essential and will be done with multi-blade tungsten carbide finishing burs, then using silicone cones and grinding wheels used under water spray.

1.1. Size of an anterior tooth:

Not all teeth are suitable for a composite crown. It is essential to be able to reduce the tooth sufficiently, without damaging the pulp, while maintaining a secure support for the crown. In other words, the tooth must be of a suitable thickness.

Otherwise, the thicknesses of resin and metal being too thin, it is the solidity and the aesthetics of the tooth crown which are compromised.

We take the upper central incisor as an example. 

Free edge size:

The free incisal edge should be flat and inclined in the palatal direction for the upper teeth and in the vestibular direction for the lower teeth.

In the case of a canine, it will be necessary to respect both sides of the cuspid tip.

Vestibular face: 

• Preparation of the vestibular reference grooves on the incisal and cervical part: They are made using a flat-tipped conical diamond bur. The vestibular face of the incisor is reduced in two planes: one cervical, one incisal to avoid any pulpal lesion and to have an equal thickness of the prosthetic material on this face.
• Reduction of the incisal part of the vestibular face By eliminating the remaining enamel sections between the different reference grooves. 
• Reduction of the vestibular cervical part and development of the vestibular shoulder and proximal flats: To do this, the flat-tipped conical diamond bur is held perpendicular to the cervico-vestibular plane and follows the gingival ring without damaging it. The shoulder is cut at a right angle to the vestibular face and extends on the proximal faces beyond the contact points to end in a chamfer on the palatal face. 
• This shoulder is first cut at a juxta-gingival level then lowered to its final sub-gingival level of 5 to 6/10 mm in depth for aesthetic reasons.
• For finishing the shoulder, a smooth-walled shoulder milling cutter with an abrasive flat end is used. 

Palatal face:

• Reduction of the angular part: This is done with a diamond bur with a rounded tip; the chamfer is cut straight away to its final level. The bur is held parallel to the cervical part of this face. The chamfer extends over the proximal faces until it meets the shoulder. The cervical 1/3 of the stump forms a cylindrical ring, which constitutes the main retention zone at the level of the incisors and canines. 
• Reduction of palatal concavity: The thickness to be reduced depends on the occlusal relationships with the antagonist teeth. Sufficient space must be left for the thickness of the metal (2mm). 

Finishing, checking and polishing the preparation:

• All corners and edges must be rounded.
• Check that the vestibular surface of the stump is set back from the neighboring teeth.
• Regularize the shoulder using an enamel chisel and check its vestibular subgingival level.
• Polish the preparation as for the CC.
• Before releasing the patient, the preparation must be protected with a temporary cap to restore aesthetics and function.

Preparations for full coverage crowns

3. The cast crown:

3.1- Definition: 

The cast crown or metal covering crown (CMR) is a simple prosthetic device, entirely cast in  alloy (precious, semi-precious, or non-precious) which consists of completely covering the coronal part of a tooth, previously prepared using specific instrumentation, in order to give it a morphology adapted to its different functions.

3.2. Indications:

• The cast crown is indicated for both pulped and pulpless posterior teeth.
• Mainly indicated to protect reconstituted teeth which have fragile walls (complex reconstructions).
•  Improve the occlusal function on a posterior tooth whose occlusal surface does not physiologically mesh with the antagonists (malposition).
• Indicated as a preventive PPA hook support in a mouth sensitive to caries.
• To restore the FO of egressed teeth whose height must be reduced (restoration of the occlusal plane in general).
• Indicated as a means of anchoring bridges and as an intra or extra coronary attachment support in the case of connections with removable prostheses (composite prostheses).

3.3. Contraindications: 

• Apical or periapical lesions that are difficult to treat and stabilize.
• On the anterior teeth where the aesthetic factor takes precedence.
• When the opposing tooth is a CCM.
• Intra-radicular lesions (furcations) especially maxillary because in the mandible, the hemisection of the tooth allows one of the two roots to be kept.
• Bone lysis decreasing the clinical crown-root ratio.
• Unstabilized periodontal disease.
• Significant subgingival fractures or root fractures, in the shape of a flute, not allowing a hermetic crown (absence of hermeticity).
• In the case of low coronal height within a tight occlusion.
• In the vicinity of an evolving or supernumerary included tooth (temporary contraindication). 

3.4. Advantages and disadvantages  : 

• Only one drawback: unsightly (metal).
• On the other hand, many advantages: 

• Tough, solid.
• Well tolerated by the marginal gingiva.
•  Little damage to dental tissue.
•  Low cost.
•  Easy lab construction.

3.5. Preparation principle:

The cervical limit is in the form of a peripheral cone (quarter round) at a supragingival level (if the coronal height is sufficient) and of a width between 0.4 and 0.6 mm.

Preparations for full coverage crowns

4. The vestibular inlay crown:

4.1. Definition:

The vestibular inlay crown is a full-coverage crown made of cast metal, bearing on its vestibular face a cosmetic element made of porcelain or resin, for aesthetic purposes. The cosmetic element is a mask made by firing porcelain powder in the cavity made on the vestibular face of the cast metal structure.

4.2. Advantages:

• It meets mechanical requirements, while preserving dental integrity and the vitality of the prepared tooth (robustness, retention).
• Strength of a metal-covered crown.
• Restoration of aesthetics.
• Can be made in the material of our choice (precious, non-precious).
• Satisfies most patients.
• Well adjusted, it is well tolerated by the gum.
• Inexpensive for cosmetic restoration.
• Easily integrated into bridge reconstructions.
• It is also a good support for removable prosthesis.

4.3. Disadvantages of “resin CIV”:

They reside in the facet and in its union with the underlying metal

• Lack of adhesion of the resin to the underlying metal.
• Risk of loosening of the veneers.
• Aging of the resin.
• Discoloration of the resin by infiltration.
• Lack of hardness.
• Rapid wear under the influence of brushing or clasps of a removable partial prosthesis.

4.4. Indications:

They are a combination of the full metal crown indications and the jacket crown indications.

• For any single or multiple prosthetic reconstruction of anterior teeth (incisors, canines) in the maxilla and mandible.
• On the PM.
• On severely decayed living teeth, after endodontic treatment.
• On fractured teeth, when the fracture is not significant.
• On a devitalized tooth, which has changed color.
• CIV can be used as an anchor for the bridge.
• It can be an attachment support (removable prostheses)

4.5. Contraindications:

• In case of untreated apical lesions.
• Unstabilized periodontal disease.
• Subgingival fracture.
• On devitalized and severely damaged teeth.
• Significant mobility.
• On young teeth with large pulp (significant mutilation for the creation of the vestibular shoulder).

4.6. Preparation principle:

• CIV is characterized by: 

• A significant reduction in FV up to the point of contact (1.2 mm).
• This reduction allows the use of two materials: metal and ceramic or resin. 
• The reduction will be limited (0.6 to 0.8 mm) on the other faces.
• The shape of the cervical boundary of the CIV at the level of the FV:

• We have either a fillet with bevel according to STEIN for a metal-tooth joint. 
• This is a wide fillet for a ceramic-metal-tooth seal according to WEISS-KUWATA. 
• Consider a rounded internal angle shoulder with a width of 1 to 1.2 mm for a ceramic-tooth seal.

• the cervical limit of the CIV at the level of the FV is subgingival 5 to 6/10 mm deep for aesthetic reasons.
• Location of the cervical limit of the CIV at the level of the other faces:

• Juxta-gingival on the proximal surfaces and supra-gingival on the lingual surface.
• Perhaps subgingival in the case of reduced coronal height.

5. The ceramic-metal crown:

5.1. Definition:

The metal-ceramic crown is a full-coverage crown consisting of a metal coping made of a precious or non-precious alloy covered entirely or partially with a cosmetic ceramic layer. The metal coping constitutes the infrastructure and the ceramic layer the superstructure. It is one of the so-called aesthetic crowns.

Properly executed, it combines the hardness and precision of cast metal with the aesthetics of ceramic.

5.2. Indications:

• Ceramic-ceramic crowns are indicated on teeth that require total coverage (pulped or pulpless decayed teeth) when aesthetics and mechanical resistance are simultaneously sought. 
• It can be used as a single restoration or as a bridge anchor.
• Within certain limits, this restoration can also be used to correct the occlusal plane.

5.3. Contraindications:

• Contraindications for metal-ceramic crowns are almost the same as for all fixed restorations, including patients with active caries or untreated periodontal disease. 
•  Teeth with increased mobility and reduced periodontium.
• Teeth with periapical reaction difficult to stabilize.
• Teeth that are too short or too thin in the vestibulo-palatal direction are a contraindication for this restoration.
• In young patients with large dental pulps, the metal-ceramic crown is also contraindicated due to the high risk of pulp exposure.

5.4 Advantages:

• The ceramic-metal crown is very aesthetic (natural appearance) when it is made according to standards in the laboratory.
• It combines the hardness and precision of cast metal with the aesthetics of ceramic and thus increases the resilience of the latter.
• It has good biological integration due to the tolerance of the ceramic by the soft tissues in contact.
• It has a relatively long lifespan.
• The degree of difficulty of its preparation is comparable to that of the preparation of other full coverage crowns.

5.5. Disadvantages:

• The brittle fracture and the very low tensile and shear strength are often the cause of the major drawback of the metal-ceramic crown which is the fracture of the superstructure. This drawback is due to the vitreous nature of the ceramic. 
• Preparation for a metal-ceramic crown requires extensive tissue mutilation, particularly at the cervical margins, in order to create sufficient space for the two restoration materials (metal and ceramic).
• To achieve a good aesthetic result, the cervical limit is placed at an infragingival level, this can increase the potential for periodontal diseases of the crowned tooth.
• Wear of opposing teeth can be accelerated due to the hardness of the ceramic especially if the occlusion is not balanced.
• Choosing the exact colour of the ceramic and communicating it to the laboratory is a common, often underestimated problem.

5.6. Preparation principle:

The principles of cutting and the chronology of the faces are the same regardless of the anchorage considered. For the metal-ceramic crown, it is the shape and the situation of the cervical limit which will change according to the nature of the tooth concerned (anterior or posterior) and the face concerned by the cutting.

The size of the CCM support teeth is the most mutilating; enough space must be left for the two superimposed materials: metal and ceramic: 

• The thickness of the non-precious metal varies from 0.5 to 0.6 mm.
• The thickness of the ceramic must not be less than 0.7 mm for an aesthetic result.

5.6.1. Preparation of anterior teeth:

• The cervical limit will be a subgingival shoulder 1.2 to 1.5 mm wide on the vestibular surface and crossing the proximo-vestibular angles up to half of the proximal surfaces and will be beveled using a torpedo bur and brought to a subgingival limit in the selcus 0.2 to 0.3 mm deep, then relayed by a juxtagingival fillet 0.4 to 0.6 mm wide up to the palatal surface.
• The free edge will be reduced by 1.5 to 2 mm in the vestibulo-palatal direction.
• For the lateral incisor: 

• If the tooth is large, the preparation is identical to that of the central. 
• On the other hand, if it is thin, and there is a risk of damaging the pulp when cutting, it must be pulped and skewered.
• For the maxillary canine: 

• The FV: the preparation is identical to that of the incisors, but the shoulder or the wide cone will stop at ¼ of the FD to save tissue. 
• The FP: is prepared in the form of inclined planes which serve as support for guidance, in lateral movements. 
• The free edge is prepared in the form of a dihedral.
• Lower incisors:

• Usually they need to be pulped and skewered. 
• The cervical preparation will be a wide fillet up to the middle of the proximal faces, extended by a metal bevel.
• The lower canine: 

• Very rectangular tooth, the cervical preparation is similar to the lower incisors.
• when pruning, reproduce the inclination of the free edge to allow lateral movements.

Preparations for full coverage crowns

5.6.2. Preparation of posterior teeth:

• a juxtagingival cervical limit in the form of a shoulder (1 to 1.2 mm wide) which crosses the mesio-vestibular angle and stops at the disto-vestibular angle.
• cervical limit in the form of a juxtagingival fillet of 0.4 to 0.6 mm in width on the other faces (FP and FP). 
• Occlusal face: it will be reduced by 1.5 to 2mm compared to the opposing occlusal faces, depending on whether or not it will be covered with ceramic. 
• In the case of teeth that are too short, avoid reducing the occlusal surface too much, which will be half metallic; the ceramic will cover the surface and the vestibular cusp. 

6. The jacket crown:

6.1. Definition:

The jacket crown is a crown that completely covers a tooth, made of ceramic or resin and gives the defective tooth a natural appearance. It is a single restoration that often involves the anterior teeth and is performed on both pulped and pulpless teeth.

6.2. Advantages:

• The ceramic jacket crown: 

• Aesthetics: A well-executed jacket crown cannot be distinguished from other natural teeth.
• Longevity: the jacket crown is durable over time.
• Pulp vitality: The pulp vitality of the prepared tooth can be preserved.
• Invisibility of the seal: the seal at the collar is invisible.
• Ease of execution: no metal casting, directly sculpted.
• The resin jacket crown:

• The gum is preserved at the neck.
• The invisible seal.
• The size is less mutilating, the resin is elastic, so it can be sculpted under a low thickness.

6.3. Disadvantages:

• The ceramic jacket crown: 

• The fully ceramic jacket crown is not very resistant, currently the ceramic is reinforced with alumina or zirconia which makes it more resistant and we speak of ceramic-ceramic. Longevity: the jacket crown is durable over time.
• Difficulty of execution.
• High cost.
• The resin jacket crown:

• Rapid abrasion of the material.
• Infiltration and change of color over time.

6.4. Indications:

• Unsightly anterior teeth, pulpy or depulpated, particularly the upper incisors when affected by cervical caries or apparent proximal caries.
• Teeth affected by dystrophy, mylolysis, dysplasia, angle fracture, coronal fracture without exposure of the pulp.
• Teeth slightly misaligned.
• Teeth affected by color abnormality after pulp mortification.
• Teeth affected by abnormal shape or volume.

6.5. Contraindication:

• Short clinical crown.
• Teeth too thin in the vestibulolingual direction. The peripheral shoulder weakens the tooth.
• Reverse articulated.
• Articulated end to end.
• Tight occlusion.
• The jacket crown cannot, under any circumstances, be used as a bridge pillar.
• Overdeveloped muscles. 
• For function.

6.6. Preparation principle:

• A careful clinical examination is essential before deciding on the use of a ceramic jacket crown. 
• At the end of the preparation, the incisal edge is reduced by 1.5 to 2 mm. It must be beveled obliquely, from bottom to top in the palatal direction for the upper teeth and from top to bottom for the lower teeth.
• The cervical limit of a ceramic jacket crown is a peripheral shoulder with a rounded internal angle of a width between 1 and 1.2 mm. It is subgingival on the FV.
• Check the occlusal relationships: sufficient clearance between the occlusal edge of the preparation and the incisal edge of the opposing teeth in all occlusal positions.

Conclusion :

If for the patient the choice of restoration mainly evokes the aesthetic aspect.

For us practitioners, the choice comes from an in-depth study of the clinical case, respecting the principles of making each crown adapted to different clinical situations.

Preparations for full coverage crowns

Wisdom teeth may need to be extracted if they are too small.
Sealing the grooves protects children’s molars from cavities.
Bad breath can be linked to dental or gum problems.
Bad breath can be linked to dental or gum problems.
Dental veneers improve the appearance of stained or damaged teeth.
Regular scaling prevents the build-up of plaque.
Sensitive teeth can be treated with specific toothpastes.
Early consultation helps detect dental problems in time.
 

Preparations for full coverage crowns