Before deciding on the construction of a full veneer crown the following must be considered:
- The health of the pulp.
- The extent of any caries
- Existing restorations to be covered by the crown
- The danger of exposing the pulp.
- The potential of the remaining natural crown for retention. Short tapered clinical crowns with less than 4mm approximal height will probably need some additional retension.
- The gingival and periodontal health.
- The bone support of the tooth and its occlusal stresses.
Preparation for full veneer crown:
Stage 1: Buccal and lingual surface reduction. As with the partial veneer crowns, the preparation is made easier if the buccal and ligual surfaces are reduced before the approximal surfaces. A tapered diamond point (course grade) with a rounded end is used for this purpose. Sufficient tooth is removed to establish the axial walls and to define a cavo-syrface angle immediadtely above the gingival margin. An untouched section of tooth may then stillremain between the prepared axial wall and the occlusal surface, especially on an upper molar. This is then reduced sufficiently to allow for the correct contour of the finished crown. Failure to remove enough tooth at this stage will result in an a bulbous crown, but there is no need to removed all the enalmel if all these conditions have been satisfied. If the crown is to have bonded porcelain facing, then sufficient tooth must be removed in the region of the facing to allow a thickness of 1.5mm to contain gold and porcelain.
Stage 2: Approximal Reduction. Approximal surfaces may be reduced using a fine-tapered instrument in mthe air-turbine handpiece.
Stage 3: Occlusal Surface Reduction. The occlusal surface is reduced sufficiently to allow space for gold to cover it. A cylindrical diamond may be used in the air-turbine for this reduction. The general shape of the prepared surface should be similar to that of the tooth itself, but it mmay be necessary to remove a greater amount of tissue in the fissures than elsewhere to ensures that all caries has been removed. Deepening the fissures also increases resistance of the crownto displacement. This should be done with a tapered fissure tungsten carbide bar or diamond instrument.
After the occlusalsurface is reduced, it is important that the occlusion should be checked in all positions of the mandible, to ensure that sufficient tissue has been removed. It is helpful to get the patient to bite and chew on wax to reveal points of inadequate clearance.
A thicker occlusal section must be provided where teeth show heavyocclusal wear facets as these are evidence of damaging parafunctional movementscausing extra stress and wear. A simple additional precaution is to persuade such patients to wear an acrylic splint, covering incisaland occlusal surfaces, at night if the nocturnal bruxism has notbeen eliminated.
Stage 4: Finishing the Preparation. The junction of pcclusal and axial surfaces should be rounded slightly, but excessive reduced will lead to some loss of rention. The axial walls should be re-checked this stage and the gingival finishing line checked for definition. The clarity of this finishing line is essential to enable the technician to form a wax pattern with a clearly defined margin and thus ensurea perfect marginal seal and a non-bulging contour of the completed crown. Excpet when conditions indicate otherwise the finishing line should be kept just supregingival in a position where the opatient can clear the junction between tooth and crown.
Very fine-grade diamond points are now available that will produce an adequately finished cavo-surface angle at 135degrees. Sharp angles between the occlusal and axial walls should be rounded lightly with a tapered multibladed tungsten-carbide finishing bur.
Most posterior teeth requiring a full veneer crown have been extensively by dental caries. When the caries is not removed in the course of the standard preperation it must be dealth with in with in one of several ways:
Minor areas of residual caries are removed and replaced with a dental cement. The preparation is then completed in the usual way, treating the cement as if it were sound dentine.
While cement is suitable for elimination of small undercuts it is not strong enough to replace replace large amounts of lost tooth substance. Where caries is extensive, amalgam or a composite should be used as a core to replace the lost tooth substance because their crushing strength are approximately three times that dental cements. Amalgam is probably the the better material.
All periferal caries and unsupported enamel is removed. Any central staining is only removed if there is softening of the dentine. Isolated areas of stained dentine overlying the pulp are best deal with by indirect pulp capping.
If it is anticipate that after crown preparation retention for the corematerial will be inadequate, it may be increased with pins inserted into the dentine.
The tooth which has been prepared and finished carefully requires an equally careful impression technique. Reliable materials must be used with some understanding of their limitations. Without this the impression stage can nullify earlier achievements in the preparation.
An accurate wax pattern is essential to achieve a good casting. Precise adaption to the details of preperation is necessary to take full advantage of the properties of resistance and retention. It should also reproduce and contour the tooth surface. A wax pattern may be prepared ‘directly’ in the tooth. Alternatively, it is prepared ‘indirectly’ on a model made from impression of the tooth.
The direct technique is useful for simple inlay cavities where access is good. It can be used for most cavities in anterior teeth, cervical cavities and other simple cavities in posterior teeth.
Some finishing of the casting has to be done in the mouth because there is no die, and therefore more time is spent at the chairside than for the indirect technique. A direct wax pattern is not easy to manipulate in thin section, which makes it unsuitable for cuspal coverage or where other surfaces need a thin layer of wax.
The indirect technique requires less charside time than the direct techique. It is suitable for both simple and complex inlay cavities, all crowns and any preparation with margins inaccessible to wax-carving instruments. It is easier to form a wax pattern accurately on a die in the laboratory than in the mouth and if the casting fails the patient need not be recalled. The technique demands a precise impression using materials which will reproduce accurately the form of the preperation and the adjacent teeth.
When a non-elastic impression material is used, it may be necessary to rduce natural undercuts of the tooth surface and also to remove considerable amounts of sound tooth substance to do this.
The accuracy of the subsequent casting will depend on the properties of the wax that is used as well as the care that goes into shaping it and the reliability of the casting technique. The wax needs a high melting point in order to produce a direct pattern. Otherwise the pattern will remain soft at mouth temperature and be difficult to carve to a fine margin. It will also distort readily.
Little information is available on the exact composition of these waxes, but most of them contain various combinations of paraffin wax,beeswax, and carnauba wax, together with resins. The wax for a pattern must be heated sufficiently to flowunder compression and reproduce the details of the cavity. The wax must be heated uniformly and carefully because too hot a flame may volatilize some ingredients and alter its properties.
Particular care should be taken to ensure that the crown is fully seated onto the prepared tooth. This is not always easy to determine if a restoration has margins entirely within the gingival crevice. The curvature of the buccal and lingual walls should also be checked, since in the interest of the gingival health the crown must reproduce th optimum contour of the tooth.
When cementing a full crown it is sometimes difficulties to seat it completely owing to trapped air or cement. It has been shown that the relief of the internal occlusal surface of the crown is a satifactory way to overcome this problem. This relief may be provided by tin-foilon the occlusal surface of the die in the laboratory stages.
Alternatively, a channel or groove may be cut with a round bur down the fitting surface of the crown from the occlusal surface to 1mm from the margin.
Pin Retained Amalgam
Additional retention can be provided for amalgam by using a pin to construct a pin-retained amalgum core. The two main types of pin used are (1) Friction retained pins; and (2) Threaded pins. The former require a slightly undersized pinhole in the dentine and the latter cut their own threads into walls of a pre-drilled hole. Threaded pins display significantly more retention of the amalgam than the friction-retained pins. The threaded pins are manufactured in two sizes: A standard pin size of 0.76mm diameter is inserted into an 0.68mm diameter pinhole. Thinner pins are 0.6mm in diameter, and are inserted into an 0.53mm diameter pinhole. Although they increase amalgam retebntion in large cavities considerably, pins also weaken the amalgam.
The way in which pins are fitted is summarised below:
- A starting pit of approximately 1mm from the enamel-dentine junction is made in the dentine, using a threaded pin (size half round bur).
- A twist drill is used to prepare a pinhole in the dentine, which is parallel to the external adjacent tooth surface.
- The pinhole is washed with a warm air-water spray and dried with paper points to prevent dehydration. The holes are sometimes lined with a layer of protective cavity varnish.
- The pin is then positioned, and in the case of the threaded pin, the shank is first latched into a contra-angle handpiece. The pin is screwed into the predrilled hole at a low speed, and shears off at the neck when it has threaded into the 2mm pinhole.
- If necessary, each pin is bent into the cavity so as to be within the confines of the proposed amalgam core
A copper band makes a convenient matrix for the amalgum, as it is thick, and when removed leaves the amalgum ot of contact with the adjacent teeth. This allows easier prepapration of approximal surfaces. The tooth is next prepared for the veneer crown, treating the amalgam as sound tooth substance. The outline of the veneer crown preparation must extend onto the sound tooth substance beyond the amalgum core.
Impression for Full Veneer Crowns
Eleastic Impression Materials. Elestomeric materials are probably the most convenient and accurate for impressions for the type of preparation in question, and in particular for multiple restorations in the same jaw.
Impression Compound. A copper band with impression compound is often suitable for impressions of single teeth. The impression is taken in a similar way to that described for inlays. It is necessary to fit the band tightly at the gingival margin, and it must be loaded with compound before being seated on the prepared tooth. It is essentil to chill the ring thoroughly and to rem,ove all excess compound from the periphery of the ring before removing the impression from the tooth. This will reduce the vulnerability of important parts of the impression, like those found at the cavity margin.
Sources of Inaccuracy and Shortcomings of Veneer Crowns
ADD YOUR OWN COMMENTS BASED ON WHAT YOU DID IN THE PRACTICAL AND IDENTIFY SOURCES OF ERROR AT ANY STAGE OF THE TECHNIQUES YOU USED WHICH MAY BE SIGNIFICANT IN PREVENTING GOOD FIT OF CROWN/HOW CAN SITUATION BE IMPROVED, ETC.
Amalgam.
The failure of an amalgum restoration may result in secondary caries, gross fracture of the amalgum or tooth, or marginal breakdown. (though the fracture of either the amalgam marigins or enamel margins). The most common reason for replacing amalgams is secondary caries. The failures can be grouped into three main categories: Faults in cavity design, poor clinical technique and limitations of materials. These are discussed below.
Faults in Cavity Design: The tooth is progreesively weakened as more tooth tissue is removed. The amalgum, although acting as a ‘space filler’, has no adhesive qualities, and so does not aid in the strengthening the underlying tooth. Therefore special care should be taken when cutting into the tooth. By cutting enamel along the plane parallel to the prism direction, it is possible to keep outline form to a minimum. This method also ensures that cavo-surface angles will be close to 90°, which is optimal for the amalgum. Te employment of an adhesive to allow bonding of the amalgum to the tooth tissues provides additional support to the restoration and the cusps, and should help to strengthen the restored tooth crown.
Sharp Internal Line Angles: The presence of sharp internal line angles concentrates stress at these sites. This increases the likelihood of fracture of both the tooth or the filling. It is desirable to avoid using sharp internal angles, and instead aim for a rounded internal surface.
Incorrect Cavo-Surface Angles: The main cause of marginal breakdown of a restored tooth is the presence of an incorrect cavo-surface angle, leading to marginal fracture of the enamel or the amalgum. Creation of flat walls and floors in an enamel can give rise to undermined enamel. The unsupported enamel will break free, leaving a gap that could lead to recurrent caries. Marginal breakdown is enhanced when the amalgam has an acute marginal angle. Since amalgam is brittle, any thin wedges will fracture very easily as they bend when an external load (such as the veneer crown?check) is applied. Gold alloy inlays do not show symptoms of marginal breakdown.
Shallow Preparations: When dental amalgams are placed in thin sections, they are subject to bending forces which lead to breakage. For large restorations, there must be sufficient depth to the cavity on the occlusal floor, so that the resulting bulky amalgam may resist bending forces. This may require removal of large amounts of sound tooth tissue, resulting in the problems discussed earlier.
Poor Clinical Technique- by Dr M. A. Duberia, specialist in grotchy fillings
Residual Caries: All residual caries must be removed prior to the filling of the cavity, otherwise the caries will spread and undermine the cusps, eventually causing it to fracture. The leakage of bacterial toxins will also lead to pulpal inflammation.
Poor Matrix Technique: A poorly adapted matrix band can cause proximal overhangs, or poor contact points with adjacent teeth. Proximal overhangs, in particular, are prone to to plaque accumulation, and may initiate secondary caries. Overtighting of the matrix band can cause the fracture of tooth cusps which have ben weakened by the removal of large amounts of tooth tissue.
Contamination: Contamination of the cavity wit blood or saliva will result in poor adaptation of the restoration to the cavity margins. The entrapment of saliva in zinc-containing alloys can result in delayed expansion, as the material is being placed. Water reacts with zinc, resulting in the producion of zinc oxide and bubbles of hydrogen gas within the amalgum. The pressure rises over time, eventually causing an expansion of the restoration. This can give rise tp downward pressure on the pulp (leading to pulpal pain), or cause the restoration to sit clear of the surface (thus leading to marginal breakdown). This shortcoming can be minimised by using proper isolation and the selection of a zinc-free amalgam with a rapid set.
Poor Condensation: This results in porosity of the amalgam and the presence of excess mercury, both of which cause it to be reduced in strength. Marginal adaptation will also be poor, increasing the potential for marginal leakage, secondary caries and corrosion. For good condensation, the amalgam should be well mixed and the appropriate trituration time selected.
Over and Under-Filling and Over-Carving: Over-filling of a cavity followed by insufficient carving results in a ledge, so that the tooth surface-restortion surface trasnsition is not smooth and even. The ledge will eventually fracture, and signs of marginal breakdown of the restoration will appear. This problem can be avoided by ensuring the surface of the restoration is carved and flushed against the tooth surface. Unerfilling or over-carving may result in an acute amalgam margin angle, resulting in marginal breakdown.
Limitation of Materials.
Most short-term failures of amalgams are avoidable if the points discussed above are addressed appropriately. In the long term, amalgams will fail, usually with the onset of creep or corrosion that has caused marginal breakdown. The amalgam alloy should show little or no creep and display high corrosion resistance. A high copper content amalgam is preferred.
Glass ionomer restoration- (for anterior teeth restorations) for class three and five cavities.
The incisor was used for this retoration. The icisor was reduced using a rose head bur to obtain a curved shape. varnish applied to the enamel surface and Acats as aprotective coating for the freshly cut tooth sturcture of the prepared cavity and thus prevents penetration of acid from the acid etch technique. Varnish also acts as a barrier to other substances e.g . if this was an amalgum restoration the varnish could prevent penetration of corrosion products into the dentinal tubules preventind tooth decolouration. Sevral thin layers were applied. The traditional glass ionomer cement is somewhat viscous, which prevent penetrations to the full depth of the surface. So the acid etch technique is used to promote better bonding.
Phosphiric acid primer was placed onto the reducsed tooth surface. Rhe main effect this gas is that it increases the surface roughness of the enamel at a microsopic level. The acid ethch modification of the enamel surfaece allow the micromechanical attachment of the glass ionomer and the enamel surface. Good adherence to mineralized tooth tissue keeps the removal of sound tooth structure to a minimum and the ability of the cement to leak and absorb fluoride decreasing the rate of secondary caries. The acid was washed off . The glass ionomer cement was prepared my mixing equal quatities of a liqiud (polyacrylic acid) and a powder (silicate glass). Using a mixing spatula and a section of smooth shiny paper to mix it onto. When both liquid and pwder are thoughroughly mixed, since they are both different coulours, a uniform mix should mean that no streaks of one coulour should be seen. Both colours should be mergerd. When this occcurs the mixture was taken and built up onto the tooth using a spoon excavator. A matrix strip was then taken and adapted to the tooth. The shaped material, while still soft was light-cured under u.v. light with a u.v. filter for eye protection for ten minutes (until it set). The u.v. light is the inititor in the free radical reaction which takes place which hatdens the ionomer cement. Varnish is then apllied all over the cement to mimic the natural shine of teeth so that the tooth coloured restoration does not stand out from the rest of the natural tooth but complements it.
