General Principles of Preparation of Anchors in Fixed Prosthesis

General Principles of Preparation of Anchors in Fixed Prosthesis

INTRODUCTION  : Respect for dental and periodontal tissues: concerns hard tissues (enamel and dentin), pulp and periodontium. Occlusal integration: integration of the prosthesis in relation to the antagonist teeth.

• Dental tissues differ from other tissues in the rest of the body by their inability to regenerate, hence the importance of any intervention on these tissues for the success of tooth preparation and restoration depends on the simultaneous consideration of three main considerations: 

Biological considerations, which influence the health of oral tissues

Mechanical considerations, which affect the integrity and durability of the restoration.

Occlusal considerations

Aesthetic considerations, which affect the patient’s appearance.

1. BIOLOGICAL CONSIDERATIONS: 
   1. Protection of adjacent teeth: Iatrogenic damage to an adjacent tooth is a common and frequent error in dentistry. Even if a damaged proximal contact area is carefully reshaped and polished, it is more susceptible to dental caries than was the original undamaged tooth surface.

• Proper tooth preparation technique helps avoid and prevent damage to adjacent proximal surfaces:
  • Manual skill: Having good address with better attention and a keen eye.  
  • A metal matrix band placed around the adjacent tooth to protect it may be helpful; however, the thin band can be punctured and the underlying enamel may be damaged.
  • The preferred method is to use the proximal enamel of the tooth being prepared to protect adjacent structures. 
  • The teeth are 1.5 to 2 mm wider at the contact area than at the cemento-enamel junction. Therefore, a fine, tapered diamond bur can be passed through the interproximal contact area while leaving a slight “lip” or “fin” of enamel without resulting in a loss of effectiveness. 

General Principles of Preparation of Anchors in Fixed Prosthesis

2. Saving of hard tissues: Dentine and enamel are tissues that have no healing or regeneration potential after their development. Losses of substances caused by pathological or therapeutic processes are therefore irreversible. This observation should encourage the therapist to save these tissues as much as possible to ensure the solidity of teeth supporting prosthetic restorations in particular.

• Thus, during fixed prosthesis preparations, the reduction thicknesses must be strictly limited to what is necessary to respect the thicknesses of the restoration material (which vary depending on the materials) and to comply with general mechanical principles.

3. Respect for the pulp:

General Principles of Preparation of Anchors in Fixed Prosthesis

1. Protection against immediate pulp attacks: they are linked to the clinical procedures involved in the production of a fixed prosthesis. The dentinal tubules represent entry points to the pulp that can transmit variations in hydrostatic pressure, a rise in temperature, and allow chemical or bacterial agents to pass through. All these aggressive phenomena can lead to inflammation of the pulp, or even necrosis.

• Should we preserve the vitality of a tooth intended to receive a fixed prosthesis? 
  • Clinical and practical elements supporting the preservation of pulp vitality:
    • Support teeth in good position with favorable insertion axis, 
    • Use of water jet and air jet during tissue decortication
  • Elements that encourage the pulp removal of teeth supporting joint prosthesis:
    • Risk of pulp necrosis linked to immediate and mediate attacks
    • Complications occurring during treatment: perforation, unbearable pain,  

• In practice: the risk of complication due to pulp necrosis will be all the greater as the preparation approaches the pulp. The risk therefore increases on teeth that are severely dilapidated, with high and triangular clinical crowns, in the presence of a large pulp volume.
• A corono-peripheral restoration (crown) is most often justified by significant decay which often requires prior endodontic treatment. If the decay is slight or the tooth is undamaged, another type of anchorage or prosthesis may be preferred (partial coronal anchorage, implant, etc.). The indications for corono-peripheral preparation on pulped teeth are therefore not the majority but when they are placed they must lead to an appropriate attitude.
• Solution to implement to prepare a pulped tooth:
• During preparation, heating occurs due to the work of the rotating instruments. This heating can be reduced by using a cooling water spray that equips the turbines or contra-angles. The use of diamond burs also reduces heating compared to tungsten blade burs, particularly if the diamond grains are of large diameter (green or black ring bur). Similarly, the instrument must sweep the surface to be prepared without excessive insistence on a limited area and by exerting low pressure. 
• After preparation, a dentin protection treatment will be implemented. A 30-second etching with orthophosphoric acid will eliminate the dentin smears resulting from the preparation, create micro-roughness on the surface of the dentin and open the dentin tubules. After rinsing and moderate drying, a fluid resin (adhesive) will seal the tubules with a microscopic film. After dentin treatment, the preparation can no longer be retouched without destroying the protection put in place.
• Treatment time: on pulped teeth, the treatment time must be as short as possible to limit bacterial infiltration that temporary restorations only partially prevent. 
• Note: Partial coronal preparations, for veneer, inlay or onlay, less mutilating than coronal-peripheral preparations are generally better tolerated by the pulp.

2. In prosthetic position:

• After the production of the prosthesis: they are linked to the presence of the prosthesis which, even if well produced, will remain an artifice reproducing nature only imperfectly.
• Problem data: The possibility of inserting a joint prosthesis on a tooth implies the presence of a space between the prosthesis and the prepared tooth surface. 
• This space is filled at the time of assembly with a material (cement or glue) to give the dento-prosthetic joint. The hermeticity of this joint, essential to avoid bacterial infiltration, depends on its thickness and the properties of the assembly material used.
• Solution
• a thin seal must be obtained. This is directly linked to the precision of the preparations, which must particularly respect the requirements related to insertion.
• Resin-based cements and adhesives, when used under good conditions, provide better sealing than mineral cements. 

4. Respect for the periodontium: ( situations of cervical limits and indications) The closer the limit is placed to the gum, the more it attacks the surrounding living tissues. (see course on cervical limits types and access) )
   1. Respect for the periodontium during the prosthetic phase  : clinical procedures are aggressive for the periodontium: a certain number of measures must be implemented during the phases of prosthesis production to limit this aggression.

• during preparation: a cord placed in the sulcus indicates the maximum depth of exploitable sulcus while protecting the epithelial-connective tissue attachment from injury caused by rotating instruments. ( or even access to the cervical limits) )
• with the temporary prosthesis: they must respect a certain number of criteria so as not to harm the good health of the periodontium: 

• Vertical and transverse adaptation
• Quality contact points with adjacent teeth
• Open cervical embrasures and emergence profiles respected
• Perfectly polished surface condition  
• Deflecting architecture of the vestibular and lingual bulges to improve the oral kinematics of the food bolus.

• during impressions: like preparation, the working impression requires the opening of the sulcus by placing cords.
• during sealing or bonding: here again, placing a cord in the sulcus prevents fluid from rising onto the preparation at the time of assembly while avoiding flares of assembly material, which are difficult to eliminate, in the bottom of the sulcus.

2. Periodontal integration of the prosthesis after delivery: the presence of the dento-prosthetic joint in contact with the gum causes permanent aggression. This aggression must be reduced by obtaining a good quality dento-prosthetic joint so that it is bearable for the periodontium. Furthermore, the shape and surface condition of the crowns, bridge intermediaries and connection areas must not be direct or indirect irritating factors (by preventing suitable cleaning).

• Quality criteria enabling long-term periodontal integration of the prosthesis:
  • Vertical and horizontal cervical adaptation: joint thin and continuous with the residual part of the tooth.
  • Hermeticity and polished surface condition of the seal.
  • Choice of biologically compatible restorative materials.
  • Accessibility to hygiene permitted or even improved by the prosthesis.
  • Restoration of contact points with adjacent teeth, preventing food from being packed between the teeth.
  • Deflecting architecture of the vestibular and lingual bulges to push back the food bolus.

2. MECHANICAL CONSIDERATIONS IN FIXED PROSTHESIS  : The design of tooth preparation for fixed prosthesis must respect certain mechanical principles, otherwise the restoration may become dislodged, deformed or fractured during various functions. These principles have evolved from theoretical and clinical observations and are supported by experimental studies. Mechanical considerations can be divided into several categories:
3. Geometric: The insertion axis corresponds to an imaginary line along which the prosthesis can be put in place and removed.

• For a single crown the insertion axis generally coincides with the major longitudinal axis of the tooth. It must also take into account the inclination of the proximal faces of the adjacent teeth.
• The axis of insertion having been determined, the rotating instruments (burs) used to carry out the preparation will be held parallel to this axis and it is the shape of the instrument (cylindrical-conical) which will allow the clearance of the axial walls. Thus the degree of conicity of the instrument will determine the degree of convergence of the walls.
• Theoretically, the convergence should not exceed 10°.

1. Stabilization: The application of an oblique force to a prosthetic crown causes it to destabilize by a rotational movement around an axis.

• It is clear that the higher the preparation, the greater the resistance surface, promoting the development of compressive forces at the cement level opposing loosening.
• The taper plays an important role because the resistance surface decreases as the clearance increases. It is therefore advisable to make high and weakly convergent preparations.
• The shorter the preparation, the lower the stability.
• Two preparations of the same perimeter: the narrower one is more stable “lever arm”
• The greater the taper, the less stability there will be .
• Auxiliary retention means, wells, grooves and boxes, correct the harmful effects of a preparation of low height or presenting too marked a conicity.

2. Retention : 

• The fixity of sealed prosthetic elements therefore depends essentially on the geometric configuration of the preparation. Sealing cements with variable mechanical properties cannot alone ensure the retention of fixed prostheses.
• It is linked to:
• The developed surface of the preparation depends on its perimeter and its height: 

• with equal perimeter, the highest preparation ensures the best retention,
• at equal height the preparation of larger perimeter ensures the best retention.
• It is therefore necessary to maintain the maximum height and the maximum perimeter. The occlusal preparation should remove only the amount necessary for the requirements of the prosthetic material of the future restoration. The height of the prepared tooth should never be less than 4 mm.
• Likewise, axial preparation should not be excessive in order to maintain the maximum perimeter.
• It is possible to increase the developed surface area of ​​the preparation by using additional retention means such as grooves, grooves, wells and boxes. 
• The clearance or convergence of the walls of the preparation is necessary for insertion.
• If theoretically the optimal clearance to obtain the best retention is 6 to 10°, clinically clearance values ​​of 16 to 20° remain acceptable and allow a more complete installation and therefore a reduced dento-prosthetic joint. This last objective represents one of the essential elements of the specifications of the fixed prosthesis in terms of biological integration.
• The nature of the surface conditions: of the prepared tooth and the prosthetic intrados: The presence of macroscopic roughness on the teeth causes micro-keying of the cement. They are obtained on the prepared dental surface by the work of diamond burs, and in the prosthetic intrados by sandblasting (surface treatment by projection of particles similar to sand). The prosthetic intrados must never be polished.

3. Resistance : 

-The dento-prosthetic assembly must withstand the energies applied to it in order to last over time. Tissue economy during preparation, corono-radicular reconstructions and the choice of materials used are essential. 

-Restorative materials must be used in thicknesses compatible with their mechanical resistance. This is one of the criteria that determines the desired reduction thicknesses.

3. OCCLUSAL CONSIDERATIONS  : Occlusal integration consists of creating a prosthesis that is in accordance with the occlusal relief of the opposing teeth. The prosthesis must not, by its presence, modify the relationships between the maxillary and mandibular teeth.
4. AESTHETIC CONSIDERATIONS: 

• The restorative dentist must develop skills in determining the patient’s esthetic expectations. Most patients prefer their dental restorations to look as natural as possible. However, esthetic considerations should not be at the expense of the patient’s long-term prognosis for oral health or function. expectations. Most patients prefer their dental restorations to look as natural as possible. However, esthetic considerations should not be at the expense of the patient’s long-term oral health or function prognosis. of the patient’s long-term
• During the initial examination, the dentist fully assesses each patient’s appearance and their aesthetic expectations should be discussed in relation to their oral hygiene needs. 
• proposed to him the different possible fixed therapeutic solutions.

General Principles of Preparation of Anchors in Fixed Prosthesis

5. CONCLUSION: Achieving the objectives of these considerations during the preparations represents the motto of the success of fixed therapy in partial prosthesis.

Deep cavities may require root canal treatment.
Interdental brushes effectively clean between teeth.
Misaligned teeth can cause chewing problems.
Untreated dental infections can spread to other parts of the body.
Whitening trays are used for gradual results.
Cracked teeth can be repaired with composite resins.
Proper hydration helps maintain a healthy mouth.
 

General Principles of Preparation of Anchors in Fixed Prosthesis