44 Australasian Dentist

There are currently three main options for intracoronal posterior aesthetic restora-

tions, namely: direct composite, indirect com-posite and indirect ceramic. Of these, direct composites are, of course, the most widely used but can be technically quite taxing, leading to difficulties with polymerisation shrinkage, open proximal contacts (in large Class II resto-rations), durability and safety. Indirect ceramics undoubtedly produce excellent restorations but are expensive, extremely technique sensitive and, again, there are question marks, especially over the degree of wear shown on opposing teeth if the ceramic is finished incorrectly.

There is, however, a middle ground that offers the best of both the direct composite and indirect ceramic techniques – the indirect hybrid composite restoration, in particular the latest zirconium silicate materials (e.g. Ceram-age by Shofu). (Figure 1).

The technique produces high quality, dura-ble restorations (Barone, 2008) and offers the following advantages:u Less technique sensitiveu Significantly lower fracture rates with flex-

ural and compressive strength >140MPau Less wear of opposing tooth structure and

excellent abrasion resistance of opposing dentition

u Excellent marginal adaptationu Ease of adjustment with excellent

‘polishability’

u Cost-effective.Importantly, indirect composites are supe-

rior to direct composites because the bulk of polymerization shrinkage takes place outside the mouth and consequently there is less stress at the tooth-restoration margin and as a result there is:u Less microleakageu Less marginal breakdownu Less post-operative sensitivityu Less marginal staining.

IndicationsThe indirect composite technique is ideally suited to inlays, onlays and, in selected cases, full crowns.

They are ideal for use in premolars and first molars but care should be exercised if second molars are to be restored – consideration should perhaps be given to a stronger alternative.

As with porcelain restorations, indirect composite restorations must be bonded in place, not cemented, and it is important that the tooth is kept dry during the bonding procedure.

Consequently, deep subgingival margins pose a considerable problem (Gerdolle, 2005).

Wherever possible rubber dam should be used.

PreparationThe main feature of such restorations is the use of butt joint margins. These are recommended throughout the preparation for the following reasons:u Thin bevelled margins can break off easily

during seatingu Bevelled margins are more likely to fracture

under occlusal forcesu Bevelled margins are more difficult to

prepareu Bevelled margins are more difficult to finish

in the laboratoryu Bevelled margins tend to remove more

tooth structure. Recommendations for spe-cific restorations follow.

Indirect composite inlays and onlays

by Dr David Penn

Figure 1. Ceramage – Composite with zirconia silicate

Dr David Penn

clinical ArtIcle AwAItIng cPD PoInt APProvAl from the DentAl boArD. Please refer to our website for updates: www.australasiandentist.com.au

Australasian Dentist 45

InlaysThe basic principles are shown in Figure 2. Essentially, the preparation should be a minimum of 2.5mm deep, with rounded internal line angles preferred. Recommended practice is to place a dual curing glass ionomer lining cement over all deep dentine. Any roughness and deficiencies in enamel are removed so that there is a very smooth margin. Internal, denti-nal, undercuts, however, need not be removed as they will be blocked out in the laboratory and filled with resin luting agent at cementation.

However, the smaller such voids are, the less will be the degree of shrinkage of the luting cement. Ten to 15 degrees of taper towards the occlu-sal is recommended. Since 100% of the restoration will be bonded, nearly parallel walls are not needed and could pose a problem during seating.

onlaysThe same principles apply to onlay preparations. Again, butt joints are essential, plus any cuspal reduction should be at least 2.5 mm; an adequate thickness of material is essential as strength is directly proportional to bulk of material when loaded vertically.

try-inThe inlay should slide into place quite easily if the above guidelines have been followed. If not, do not panic as the likely cause is the various surface irregularities that can appear on the fitting surface following blocking out of undercuts in the laboratory. These can be removed with a carbide finish-ing bur. Do not check the occlusion until after the restoration has been fully bonded, as the inlay is much stronger after bonding and there is consequently less risk of damage or even fracture.

bondingOnce the fit has been assessed and corrected where necessary, the fitting surface should be cleaned, acetone being one of the most widely recom-mended agents for this purpose (Swift, 1995; Della Bona, 1994).

There is some debate as to whether the fitting surface of an indirect com-posite restoration should then be silanated. In theory, this would enable a chemical bond to form between the glass particles on the surface of the res-toration and the luting resin. This may not, however, be so effective in zirco-nium-based ceramics such as Ceram-

age. The balance of evidence currently available is that silanation should be performed (Matinlinna, 2007). Plued-demann (1991) demonstrated the com-plex, yet reliable, nature of this bond between the silane coupling agents and both organic and inorganic sub-strates. Activation of the silane agent starts when it is mixed with water. The hydrolysis results in the forma-tion of silanol, which subsequently reacts with silanol on the surface of the porcelain (Soderholm, 1993).

Once applied, the silane must be left for 60 seconds to dry, thus remov-ing any excess absorbed water. If the mixed silane is not used within several hours it will polymerise to an unreac-tive and ineffective polysiloxane (Suh, 1991). As a rule, two part products are preferred because of their longer shelf life – most single-component silanes are only suitable for use six months from manufacture because of their susceptibility to rapid solvent evapo-ration. A good indicator is the appear-ance of the liquid: a clear solution is useable whereas a milky looking one should be discarded (Blatz, 2003).

The prepared tooth should also be cleaned prior to etching. Prophylaxis paste containing fluoride is contrain-dicated as the presence of the fluoride has been shown to compromise bond strength. Pumice and water is widely used, although some concern has been expressed about the possibility of pum-ice remaining within dentinal tubules, leading to reduced bond strength.

Figure 2a: Inlay preparation. The preparation depth should be a minimum of 2.5mm. The axial walls of the proximal box should be flared slightly so that the enamel margin does not form an acute angle

Figure 2b: Onlay preparation. The overlaid portion should be at least 2mm deep. Forget conventional gold-only preparations – this is a totally different material that needs bulk at the margins wherever possible

Figure 3. Leaky composite restorations

Figure 4. Composites removed exposing recurrent caries

Figure 5. GIC based placed after caries removal

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46 Australasian Dentist

Dr David Penn is in private practice in Sydney, Australia and is managing director of Southern Cross Dental Laboratories (Australia).

Again, acetone is increasingly being used to clean the tooth surface. The tooth enamel can then be etched (preferably under rubber dam), at the same time avoiding etching the adja-cent proximal surface since excess luting resin can bond to that surface, closing the contact as a result. For many years, the use of etching tooth enamel and dentine with phosphoric acid – the so-called ‘total etch’ – to create micromechanical retention of resin tags has been almost universally accepted by the profession.

More recently, though, the use of weaker acids, which do not require washing of the tooth surface after acid application and which leave the smear layer on the dentine and lead to a less aggressive etching of the enamel, has gained popularity. Such ‘self-etch’ systems are thought to produce much less postoperative tooth sensitivity as compared with the total-etch approach (Perdigao, 2003).

This may be linked to the finding that self-etch dental adhesive systems could contribute to the elimination of residual bacteria and hence reduce the risk of secondary caries (Feuerstein, 2007).

This all begs the question – which etching technique is most appropri-ate? Christensen (2005) advocated the following approach for those prepa-rations that involve both exposed enamel and dentine:1. Selectively etch the enamel with

a well-controlled, viscous phos-phoric acid gel

2. Wash the phosphoric acid gel from the tooth with a significant amount of water spray. If it is washed slowly, the gel acid is spread all over the tooth prepara-tion, thus etching it and reducing the sensitivity-prevention advan-tages obtained with self-etching primer application

3. Place the self-etching primer

and bonding agent on the entire preparation, including the enamel that has been etched with the phosphoric acid gel. Do not cure the primer/bonding agent before seating the restoration, otherwise it will not seat fully owing to the film thickness of the self etching bonding agent. The exception to this is with some of the newer generation self-etching primers, which exhibit extremely thin film thicknesses.A dual-cure bonding agent is

then placed on the preparation, but not light cured at this stage. Equal lengths of a dual-curing luting agent are dispensed just prior to mixing. An excess of the mixture is applied to the silanated surface of the restoration, as well as a thin amount to cover most of the cavity preparation, especially in undercut areas.

Prior to seating the inlay, floss should be placed into the interproxi-mal areas. The inlay is then seated and, with an assistant holding it in place firmly with an instrument, the floss moved in an occlusal/gingival direction, thus removing any excess

luting agent at the gingival margin. The floss is then removed towards the buccal or lingual.

With the restoration still firmly seated, a fine brush can be used to remove 95% of the excess luting agent from the margins. Some operators pre-fer to cure the resin for a very short time, three to four seconds, and then carry out removal of excess resin as they find that the material peels away from the tooth more cleanly.

Despite the earlier warning con-tra-indicating subgingival prepara-tions, should these occur, the latest research suggests that use of a resin modfied glass ionomer cement leads to significantly less microleakage at the enamel margins compared to self-cure or dual-cure resin cements (St Germain, 2002).

finishingWhile the goal is to reduce the amount of finishing required to a minimum, there will always be situations where some adjustment is necessary. The use of a series of finishing grit dia-monds followed by a 30-fluted carbide bur and polishing pastes will produce highly satisfactory results.

Polishing under water spray has also been shown to produce a smoother surface than dry polishing (Haywood, 1989). Despite these apparently reas-suring findings, the general consensus is that the less finishing that needs to be done the better.

If these steps are followed carefully then the result will be well-fitting, durable aesthetic restorations (Figures 3 to 7) u

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Figure 6. Working model

Figure 7. Ceramage Inlays cemented and polished

1. Indirect composite inlays are more likely to produce superior contact points than direct composite True/False

2. Rounded internal line angles are preferred in indirect composite preparations True/False

3. Indirect composites are recommended for second molars True/False

4. Feather edges and bevels are ideal finishing margins True/False

5. Resin modified Glass Ionomer Cement is preferred as a luting agent when the margins in the proximal boxes finish subgingivally True/False

6. The balance of evidence suggests that Zirconium Silicate Hybrid Composite inlays should be silanated prior to cementation True/False

7. The occlusion should be modified prior to cementation True/False

CPD Questionnaire

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