General principles of tooth preparation for restoration
Goals :
The goal of conservative treatment is of course the preservation of a healthy and vital pulp, while allowing the normal function of the tooth. By normal function we mean the biological and biomechanical integration of the tooth.
This means that it must be possible to completely and reliably reconstruct the normal anatomy of the tooth, including surface condition and aesthetics. All techniques used for cavity preparation must meet these criteria.
Only lesions with cavitation require restorative treatment.
The initial lesions of the enamel, and even for some of the dentine, are the responsibility of remineralization treatments, essentially by fluoride vectors: varnishes, gels or mouthwashes.
The concepts of conservative treatments are today structured around three main notions:
1* treatments must be as non-mutilating as possible.
2* the biological integration of materials, in particular pulp protection, is ensured under the composites by adhesives.
3*current bonding techniques allow very good biomechanical integration.
For non-bonded fillings, stability and support are ensured by the residual dental panels which must therefore be resistant,
Whereas tooth-composite bonding, on the other hand, allows for a harmonious distribution of stresses – certainly not equivalent to a healthy tooth – but sufficient to strengthen the residual sections.
Three new concepts have therefore emerged
Collage
Tissue economy
Bio-integration
I- Biological criteria and means of detersion
The removal of pathological tissues that interfere with either pulp physiology or reconstruction techniques is the initial step in any treatment.
From a histopathological point of view, it is possible to differentiate different strata in a dentin caries. The main thing is the eviction of softened infected dentin.
of the infected area characterized by demineralization of the peritubular dentin
For decades, dental caries removal has been carried out mechanically, either using manual instruments (such as excavators) or using rotating instruments mounted on a contra-angle or turbine.
This mechanical eviction is not specific to the tissue to be eliminated, and can extend to healthy tissue.
It is therefore necessary to control speed, thermal effect and pressure
Vibrations: They depend mainly on the rotation speed:
-maximum vibration amplitude and frequency between 3000 and 30,000 rpm.
– above 400,000 rpm, there may be a bending of the cutter and the appearance of parasitic vibrations.
They also depend on the cutter used (diameter and length)
Ex; The existence of play in the head of the rotating instrument due to poor manufacturing or wear can also be the cause of harmful vibrations.
Pressure; It is a function of:
– the rotation speed: the higher it is, the lower the pressure exerted on the cutter to obtain a cut of mineralized hard tissues
Temperature elevation is one of the most harmful elements for pulp tissue.
At the same speed, heating is greater with a large cutter and a worn cutter
→ Consequences on tissues
-On the enamel: no reaction at the pulp level because the enamel is a poor thermal conductor
-Carbonized surface layers
Evaporation of fluid from dentinal tubules
-Inflammatory phenomenon at the level of the pulp from a simple disorder of the odontoblastic layer to the induction of a pulp necrosis process. Above 52˚C,
How to combat overheating: with spray cooling
Water cooling alone is insufficient due to the low pressure of the water jet.
Nature of rotating instruments; steel cutter: steel wears very quickly and loses its cutting edge, which increases the friction surface and therefore the temperature rise.
Diamond burs: even with cooling, they are much more dangerous than tungsten carbide instruments. Their friction surfaces are large, they get clogged more quickly so the temperature rise during milling increases.
Tungsten carbide cutter: better tolerated with a lower temperature rise. They can be monitored which reduces the working surface, the heating produced and promotes the diffusion of the spray (added blades
Desiccation of dentin; The use of the jet to control the level of the preparation to dry the cavity causes dehydration of the tooth, aspiration of the moisture contained in the canaliculi, sometimes even displacement of the nuclei of the odontoblasts in the canaliculi.
Use a jet of warm air under low pressure or small cotton balls.
Pulpo-dentin organ reaction
Depending on the nature and severity of the irritant agent, the reaction of the pulpo-dentin organ will be located within the dentin, at the periphery of the pulp or within the pulp tissue:
Histopathological reactions range from simple alteration of the odontoblast layer to pulp necrosis.
Preparation of dentinal tissues by milling or curettage results in the formation of a coating layer commonly known as the smear layer. It may contain bacteria and may also promote bacterial growth. This coating contributes to the increase in the tooth-material interface.
It is therefore essential to finish the preparation by disinfecting the dentin wound, we classically speak of grooming the preparation. (sodium hypochlorite)
The action of phosphoric acid, as the first step in dentin conditioning for adhesion, is sometimes likened to grooming. Indeed, its use has been shown to reduce the amount of bacteria present under the filling by approximately 50%.
Some cavity preparation procedures have been more recently developed and are of undeniable clinical interest: in particular, the use of diamond inserts mounted on sound or ultrasonic vibration generators allows much easier access to crevice areas than rotary instruments; for example, in the case of cavities, tunnels or by vestibulolingual access. In addition, these inserts are only active on their diamond face, which allows a very selective action.
II- Mechanical principles of preparation
The preparation of the cavities will have to follow the following rules:
*respect for dental structures;
* the stability and support of the material in the cavity;
*the mechanical resistance of the filling material
*retention of material in the cavity;
* prophylaxis and extension of the preparation.
These mechanical principles which once dominated for metallic restorations, are considerably moderated by current biological concepts.
1/ Respect for dental structures
a* Resistance of enamel walls
The prismatic structure of enamel makes it easily cleavable perpendicular to its external surface. When preparing cavities for adhesive materials, however, it is conceivable to leave enamel unsupported by dentine, especially on the vestibular side for aesthetic reasons;
It is then the composite which ensures the support role and avoids shearing, in addition the beveling makes it possible to include the edge of the enamel
Black’s concepts taught us to preserve the structures necessary for the mechanical resistance of the tooth such as the enamel bridge of the upper molars, and to eliminate weakened structures subjected to occlusal constraints such as the marginal ridges.
We must now review these concepts and consider that the important elements are not necessarily those that provide mechanical resistance, but rather those that maintain a functional morphology. More precisely, the interdental contact zone is a fragile anatomical element that is difficult to reconstitute, especially with a non-condensable material such as composite.
b* Importance of occlusal forces
The physiological occlusal forces that will be exerted on the tooth and the restoration influence the choice of whether or not to preserve certain residual walls.
The practitioner must take care to record and visualize the occlusal contacts before preparing the cavity.
2/ Stability and support
The shape of the cavity must allow for harmonious transmission of stresses, so that in the long term these repeated stresses do not cause fracture of the tooth or displacement of the filling in the cavity.
The most traditional concept remains Black’s flat bottom. A flat bottom parallel to the occlusal plane allows for regular transmission of forces to the residual walls.
However, the systematic creation of a flat base is mutilating, it reduces the thickness of residual dentin and reduces
the resistance of the walls while a “stepped” cavity meets the same conditions as the flat bottom, provided that the floors of these floors remain parallel to the occlusion plane
3/ Resistance of the filling material
The material used molds, directly or indirectly, the surfaces of the preparation. It therefore presents in negative all the irregularities of the preparation surfaces, and has the volume given by the cavity. The obturation transmits the chewing forces to the tooth, and the tooth opposes a reaction which can lead to a fracture of the materials
Amalgams are metallic materials that are relatively fragile under shear;
However, excessively thin amalgam thicknesses should be avoided, particularly in all areas subject to mechanical stress.
Composites are more resistant to tension and compression in low thicknesses due to their adhesion to dental structures. However, their resistance to abrasion makes low thicknesses impossible in areas supporting occlusal contacts.
Ability of the different walls of the cavity to withstand masticatory shocks without fracture.
The fracture of one of the walls of the cavity can jeopardize the vitality of the pulp, or even the existence of the dental organ.
In order to avoid this risk, it will be necessary to neutralize as much as possible the effect of transverse forces.
Preparation angles: a rounded angle allows a better adaptation of the filling material
. Undercut walls: Systematic at the level of the proximal walls to avoid the weakening of the marginal ridges.
4/ Retention of the filling material; Care should be taken not to create harmful extensions for the pulp directly or indirectly by increasing dentin permeability or weakening the residual sections by grooves.
Untimely.
For adhesive materials, the retention forces and “classic” devices such as undercuts and grooves apply in the same way, but here as for the
Stabilization and support, it is the adhesion achieved on the enamel and the dentin which will ensure retention
5/ Extension of the preparation
The extension of the preparation is conceived, in the conventional manner, under two aspects: extension necessary for the implementation of the technique – called convenience – and prophylactic extension to avoid carious recurrence.
Extension of convenience:
The shape of the cavity both at the contour line and its walls must allow easy access for preparation instruments, dentin-enamel bonding materials, filling materials and finishing instruments.
This requires sufficient preparation dimensions which depend on the practitioner and his instrumentation.
Rotary instrumentation, on the other hand, is delicate to use for the creation of tunnel cavities or by vestibular approach; in these cases the use of diamond ultrasonic instrumentation can be useful. As already mentioned, the extension of convenience also concerns the access route and it must preserve the interdental area as much as possible.
Prophylactic extension
The finishing of the interface must be as careful as possible both in terms of the preparation of the cavity and the obturation:
The interface must have as few irregularities as possible in order to limit the retention of bacterial plaque, which, at this level, would aggravate the degradation phenomena and lead to recurrence of caries.
III- Applications to cavity preparations. Choice of material
1/ Preparations for bonded materials
In all cases where bonding is possible, retention, stability and support are ensured by this bonding. Bonding obturation techniques are therefore the techniques of choice which best meet the criteria for tissue conservation.
Of course, Black’s classification can no longer accommodate cavity shapes that are so different from amalgam preparations. Mount and Hume in 1997 proposed a classification that is – in a very slightly modified form – used today for cavity preparations for composites.
If the implementation of the adhesive is delicate, the use of a glass ionomer in sandwich technique may prove judicious. The sandwich technique consists of sealing the cavity in two parts;
A deep dentin-reconstituting obturation is performed with a conventional glass ionomer cement (GIC) or hybrid which allows a relatively weak but reliable spontaneous adhesion to the dentin.
2/ Preparations for amalgams
The choice of amalgam filling is generally a second-line choice when bonding is not possible or the technique is too expensive. Amalgam filling will require the cavity to be arranged in such a way as to mechanically ensure retention, stability and support;
However, the preparation of cavities for amalgam has evolved significantly since their systematization by Black.
This development is linked to the improvement of their mechanical and physical properties.
Rodda, in 1972, described the modern cavity type of the “large class 2” as it is currently conceived: the width of the occlusal cavity is less than a quarter of the width of the tooth, the angles are rounded, the axial walls are undercut; the proximal cavity has an occlusal line in an inverted S but the contour line remains at a distance from the brushing limits and the periodontium (within the limit of caries of course). He does not describe axial grooves
Whenever possible, more conservative preparations
will be carried out: Sturdevant in 1987 described the “small class 2” without occlusal prophylactic extension. This type of cavity is very economical in terms of tissue. It represents a quarter of a conventional preparation in volume. Thermal and mechanical stresses are less, the interface line is much shorter and the risk of recurrence is reduced accordingly, especially if hygiene is correct.
Conclusion
The best way to treat a carious lesion is to prevent it, however this goal cannot always be achieved.
The dentist is obliged to apply and respect this new approach to restorative dentistry at a minimum by integrating the si/sta concept into his daily practice, thus helping to delay tooth deterioration as much as possible.
General principles of tooth preparation for restoration
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