Introduction

In many ways, the title of this presentation would lead the reader to assume that the use of dental composite, instead of dental amalgam, to restore teeth was a regressive step. From the perspective of the 2020s and with the current attributes of dental composite, the author no longer believes this is true. There are still some disadvantages to dental composite but, compared with dental amalgam, which has remained relatively unchanged in the past 30 years, they are few.

Dental amalgam has been the mainstay of directly-placed dental restorations for over a century1 and during times when pulp biology and the response of the caries process to exclusion of substrate were poorly understood. Over this long period of time, countless experiential studies were performed of inadequate power, poor experimental design and with dubious or, more likely, no ethical approval, to work out how amalgam could be used, abused and applied to restore teeth, maintain dentitions and to a considerable extent, fund dentists' lifestyles.

We know that amalgam can be used to restore any shape of cavity possessing adequate retention and resistance form, in any tooth in the mouth and may last for decades, if correctly placed and maintained.2 Most dentists will recognise the exuberance of inexperience comes crashing down when they try to replace extensive amalgams with composite to find moisture control is impossible and matrices do not seem to fit, leading the operator to the conclusion, 'how did they do that with amalgam'? Furthermore, in the deep, dark recesses of the distal aspects of upper molars, it is comforting to be able to see an amalgam restoration margin to assist with placement of a finishing line for fixed prosthetic tooth preparations.

Thus, it seems a sensible approach to follow the process of preparation and restoration of a tooth with a direct restoration comparing and contrasting composite resin and amalgam when used as the final restorative material.

Caries removal

Complete caries removal is no longer a prerequisite for successful restoration of a tooth. Indeed, experiments using amalgam were among the first to demonstrate that separating caries from a nutrient source would slow or arrest the process and thereby inhibit or arrest lesion progression, in some cases permitting lesion remineralisation.3 Current thinking is that only sufficient caries removal with an excavator, back to hard tissue at the cavity periphery and removal of soft tissue on the pulpal aspect, in order to place an adequate bulk of restorative material and thereby create a bacteria-tight seal, is required.4 Clearly, a composite seal can be achieved through the combined bactericidal action of etching, priming and bonding, followed by placement of composite in such a manner that polymerisation shrinkage does not compromise the integrity of the interface between tooth and composite.5 Amalgam restorations require a leap of faith that modern, low corrosion amalgam alloys will ultimately contribute to sealing the restoration/tooth interface. Possibly, a combination of the sealing properties of an underlying resin-based liner and good amalgam packing technique are sufficient to achieve a similarly effective seal to composite resin for the purposes of affecting the caries process.

Cavity design

Having undertaken sufficient caries removal, amalgam must utilise retention and resistance form within the prepared cavity to remain stable when set within the cavity. If such features have not developed during the process of caries removal, then further removal of caries and/or tooth structure is required to achieve these essential cavity features.6 It is feasible to retain amalgam within a cavity using an interface of resin restorative material between the amalgam and the tooth structure and this was the basis of the so-called bonded amalgam.7 However, it is likely that the term 'sealed amalgam' may be more appropriate and there is some evidence in this scarcely-more researched field that this seal is compromised over longer periods of time to a more accelerated degree than that seal achieved between composite and tooth structure.8 Clearly, the ability to bond composite to tooth structure does not require retentive features to be cut into cavities and under conditions of occlusal loading, several authors suggest composite does not require to be placed in sufficient bulk to withstand such forces.9,10

Cavity lining

Composite resin restorative material is a thermal and electrical insulator which bonds directly to tooth structure in a chemically and physically innocuous manner.11 When composite is used as the restorative material, the primacy of the bond between composite and tooth structure is generally recognised as being of utmost importance.12 However, it is less clear with amalgam restorations. Remaining dentine thickness is important but can be difficult to judge.13 Many dentists continue to place a resin-based insulating lining between the pulpal aspect of dentine in a cavity and the amalgam restoration to protect against thermal and electrical insults, but there is a paucity of evidence in this area.14 The bulk of such linings is now minimal and unlikely to materially impact on the depth of cavity preparation.

The nature of the composite and amalgam used

Amalgam as a restorative material has not materially altered since the 1980s, when low corrosion alloys were introduced.15 Over the same period, composite resin has changed substantially. There has been greater understanding of the preparation of both the tooth and other restorative material surfaces before composite bonding. There is continually-evolving stronger bonding of composite resin, to more diverse materials with simplified bonding instructions. There have been many changes to the resin itself: notably, a reduction of the polymerisation contraction and an increase in the increment size suitable for placement. Much of this substantial development has been evidence-based using in vitro studies and we are seeing the practical results in vivo. Comparisons with amalgam are difficult, as reasons for use of either material can be skewed by patient preference or nationalised health service regulations. However, it is clear that many dental schools graduate students with much greater experience of composite resin directly-placed restorations, compared to amalgam.16

Placement techniques

For both amalgam and composite, placement of restorative material within a dry field is important. For composite, it is critical, although evidence shows that rubber dam use to achieve such moisture control is not always beneficial and increases complexity.17 For amalgam, a dry field is desirable but not critical and many dentists will recognise amalgams have been placed when moisture control has been 'challenging', only to provide years of clinically acceptable service. There is no doubt that amalgam is the more forgiving material is this respect.

The placement technique for amalgam has not changed substantially in many decades. Firm packing of small increments at speed, with over-packing to allow excess mercury to rise to the surface for removal before carving to an anatomically acceptable contour, is what is required for amalgam.18 Matrix placement, assisted by a tight-fitting wedge for proximal box restoration is mandatory to withstand the considerable packing forces and reduces the possibility of ledge formation at the gingival aspect. The packable nature of amalgam, the use of a tightly-fitted wedge and the considerable packing forces used in placement all contribute to the formation of tight contact points between amalgam and adjacent tooth structure, minimising interproximal food packing and subsequent increased risk of periodontal and caries problems.

For the practitioner with the experience of placement of many amalgams, there are important lessons which can be usefully applied in the placement of composite resin, particularly in relation to restoration of the proximal box. However, there are also important differences.

Currently, many commercially available composites suggest a number of increments be placed in a specific manner within a cavity to ensure optimal polymerisation and reduce the overall effect of polymerisation contraction for posterior restorations.19,20 Such contraction stresses the tooth-composite bond, resulting in increased micro-leakage and its consequences.21,22 However, the rapid current development of composite resin seeks to find new formulations to facilitate much larger increment placement, with much reduced contraction thus offering the realistic opportunity of reliably placing composite in a large cavity in one increment and achieving full polymerisation within the furthest recesses of the cavity. While there are a number of reviews for bulk-fill composites,23 they have not been widely used by undergraduates taught by this author and so are not discussed further here.

However, the lack of a truly packable composite resin still means the effective restoration of a proximal contact point is a challenge using composite resin. A variety of techniques, matrices and other instruments are available to help the practitioner achieve this contact point and such variety is probably testament to the genuine difficulties practitioners face with this aspect of composite placement. Methods seek to separate teeth, enlarging the interproximal space before, during or post proximal box preparation; and to use contoured matrices, facilitating the reconstruction of anatomically-correct proximal contours of the tooth and wedges to ensure close adaptation of any matrix, reducing ingress of oral fluids and egress of composite resin.24

Restoration finishing

The replication of a polished and anatomically correct amalgam restoration was undoubtedly a skilled and time-consuming procedure that, for some dentists, was regarded as a defining attribute of excellent clinical practice. There are no data to indicate correctly-replicated anatomical form, other than maintenance of occlusal contacts and a mirror-polish, directly contribute to the longevity of an amalgam restoration.25 Some dentists may believe that a practitioner capable of such work would also likely have superior skills in other areas of amalgam restoration placement and this may, in turn, result in more successful restorations. However, evidence to support these statements is more to do with expert opinion than published clinical studies.6 The author's current teaching of amalgam placement and contouring supports restoration of proximal contacts with no positive or negative restoration margins and replication of pre-operative occlusal contacts. There is no requirement for anatomically correct reproduction of occlusal form or polishing of the amalgam surface although it should be burnished to reduce roughness and subsequent plaque accumulation.

Overall, finishing of composite resin restorations is more straightforward. Any composite cured against a smooth matrix will have a very smooth surface, with no oxygen-inhibited layer. The aim, as with all restorations, is to reduce the ability for plaque to accumulate on the finished restoration surface.26 Thus, proximal finishing is mostly confined to ensuring a smooth tooth-to-composite interface to ensure minimal plaque-retention potential. Occlusal finishing aims to also produce a smooth tooth-to-composite interface and maintain pre-operative occlusal contacts. Increasing demands of aesthetic dentistry may additionally seek to ensure fissure morphology and that appearance is replicated but there is no evidence this is connected to the longevity and functionality of the composite resin restoration.

Restoration monitoring

All restorations placed in teeth must be checked regularly for signs of failure. It sounds obvious, but the key element in this process is to identify the restoration. With amalgam this is easy, due to the colour difference from natural tooth structure. Such identification can be significantly more difficult for composite resin, which may also be confused with many other tooth-coloured materials used for placement of indirect restorations. This problem is compounded when there is a change in dentist and even with the age of the dentist who may have trouble actually seeing a composite resin restoration which has successfully copied the visual appearance of an unrestored tooth. Older dentists have the benefit of having lived through the development of composite resin and know that early composite resins with large particle sizes were actually quite abrasive and base and catalyst systems could never be mixed with a metal spatula without turning the resultant mixture a subtle shade of grey. For some composites, that abrasive property can still be useful for identification, by scratching the surface texture with a dental probe. The feel of a composite is often different to that of a natural tooth surface; a factor that can sometimes be useful to detect what is composite and what is tooth!

Having identified the restoration, the tooth-restoration interface should be evaluated. For amalgam, gap formation of more than 0.5 mm, demineralisation of adjacent enamel or frank cavity formation are all signs that attention is required.27 The same is true for composite resin although some practitioners would argue that a white line or brown staining at the composite-tooth interface is a clinical sign of restoration microleakage. Guidance is not clear whether or not this affects the long-term viability of the restoration.28

Restoration repair or replacement

Should the practitioner decide a restoration has failed, then a decision has to be made about repair or replacement. If the restoration is largely intact and a small accessible aspect of the tooth-restoration interface is all that is needed, then many restorations can be repaired by removal of the small area of affected tooth tissue and restoration with composite resin, or even glass ionomer if indicated. Repair is often less invasive than replacement.29 However, when replacement is indicated, amalgam can be easily distinguished from tooth structure in removal with a high-speed bur; cutting from the centre of the restoration to the periphery of the cavity will often allow the practitioner to chip out the amalgam with minimal increase in overall cavity size. This is more difficult with replacement composite resin restorations as dentists find it difficult to assess where the edge of the cavity is. Evidence would suggest there is often an increase in overall cavity size after removal of a composite resin.30,31

Conclusions

In keeping with the title of this paper, it is possible to sum up much of the previous text in Table 1.

Table 1 Problems and solutions
Full size table