Dental materials – what goes where? class I and II cavities

A brief history of dental amalgam

The history of amalgam is uncertain: however, there is a report of the use of a silver paste being used as early as 659AD in China,1 with its first use as a dental material being reported in France in 1826.2 The years passed, with many other metals being combined with mercury, until GV Black produced a formula, in 1895, for a dental amalgam which provided reasonable clinical performance. This remained unchanged for circa 70 years3 until Eames4 recognized the benefit of a 1 to 1 ratio of mercury to alloy, thus allowing a substantial reduction from the levels previously recommended (as high as 8 to 5). High copper content alloys followed, with these creating a copper-tin phase which was less susceptible to corrosion than the tin-mercury gamma 2 phase present in low copper content alloys.3

The content of amalgam alloys in current use is as follows:3

Trace elements, including indium (up to 4%), palladium (0.5%) and zinc (up to 1%) may also be present. The inclusion of zinc has been reported to delay the expansion of amalgam. However, it is the opinion of Berry and co-workers3 that selection of a non-zinc containing alloy to avoid expansion is unnecessary. The manufactured alloy can be processed for clinical use as either spherical, lathe cut or admix (a mixture of the two) particles, each of which has differing handling properties, with spherical being considered to be the easiest to condense.3 The alloy is then mixed with mercury (up to 50% by weight) to form the dental amalgam.

Alloys in which the mercury was completely or partially replaced by gallium, a metal which is liquid at room temperature in the same group of the periodic table as mercury, were introduced in the 1960s, becoming popular in the 1980s and 1990s after adverse publicity regarding mercury. One such material (Galloy, SDI, Melbourne, Australia) received the American Dental Association's Seal of Approval, but this was withdrawn when published research indicated that materials of this type expanded under the humid intra-oral conditions,5 causing fractured cusps and pulp pressure symptoms, and with Dunne and colleagues in the UK publishing illustrations of fractured cusps as a result.6 To the authors' knowledge, only one paper produced results which indicated favourable findings after two years’ clinical evaluation, although the authors stated that ‘the material is very sensitive to moisture and restorations must be protected from saliva in the early post-operative hours’. In this paper,7 the gallium-containing restorations were sealed with unfilled resin, with rubber dam isolation continuing to be kept in place for a time following restoration placement. This technique sensitivity, plus the suboptimal clinical results, led to the demise of gallium as a mercury alternative.

Toxicity of amalgam?

A review of the history of dental amalgam would be incomplete if it did not mention the turbulent history of the material. Debates on dental amalgam and its relation to disease go back as far as 1833,8 with so-called ‘amalgam wars’ taking place between those who were protagonists and antagonists of the material in the first half of the 20th century. Mercury has been considered to be an ubiquitous environmental toxin9 and, for a more complete résumé of the safety of dental amalgam, readers are referred to the review articles by Osborne and Albino8 and Osborne and Swift.9

Regarding release of mercury vapour by dental amalgams, a paper by Vimy and Lorscheider, in 1985,10 which reported that 27 micrograms of mercury are released per 12 amalgams per day, caused widespread consternation. However, it was later demonstrated that their calculations were incorrect and that they had overestimated exposure by around 16 times,11 with Berglund, in 1990, estimating that the correct figure was 1.7 micrograms of mercury from 12 amalgams per day.12

This, however, did not prevent the publication of many media articles, implicating dental amalgam in many illnesses, for example, memory loss, Alzheimer‘s Disease and autism, with the results from scientific publications on the relationship (or not) of dental amalgam and some illnesses being summarized in Table 1.9^,13^,14^,15 These indicate no correlation between dental amalgam and a variety of illnesses. Review articles16^,17^,18^,19 also confirm the view that amalgam does not constitute a health risk to patients. This view is substantiated by two well-designed studies, examining potential adverse effects of dental amalgam in patients under the age of 12 years.20^,21 The results of these studies failed to produce any results which could give rise to anxieties. It could therefore be considered that there would be no problems in adult patients, given the much lower toxicity risk in larger/heavier patients.20^,21

Readers also are directed, in particular, to the two well-referenced papers (n = 282 references) by Wahl,17^,18 in Quintessence International, which detail the myths related to amalgam restorations, and a further paper by the same author in Dental Update.22 The authors of the present paper suggest that dentists who are still using amalgam should have these papers available in their practices in order to provide information for patients who express anxieties when amalgam restorations are proposed.

While data suggest that mercury hygiene practices have improved in UK dental practices,23 proper management of amalgam waste remains dentists' responsibility (as part of their duty of care) within their practices.24 This includes proper ventilation of the operatory and the mandatory use of appropriate amalgam separators in water lines. This topic will be addressed in a future paper in Dental Update.

In summary, therefore, there is no evidence that the placement of amalgam restorations causes medical harm to patients.

Toxicity to dentists?

While a large number of (non peer-reviewed) publications have raised anxieties concerning toxicity to patients, and with the scientific basis for these being unproven (vide supra), it could be expected that it would be dentists, arguably more exposed than any patient, who would be the sufferers if dental amalgam/mercury was toxic. However, there is a paucity of literature on this subject.

Readers' attention is drawn to one paper which, despite being published as long ago as 1976, remains a lesson for all who use mercury in a dental surgery. It describes four cases of non-fatal mercury intoxication which occurred in one practice.25 The problem arose following a mercury spillage, which was pooled beneath and then vapourized by a dry heat sterilizer. When the surgery windows were closed during the winter months, the mercury vapour was concentrated in the surgery. The dentist and his nurse developed headaches, diplopia and loss of fine muscular co-ordination; all signs of mercury intoxication. The dentist and nurse in an adjacent surgery were affected more gradually. All eventually recovered and the surgeries were decontaminated.

More recently, Ritchie and colleagues examined the effect of chronic exposure to mercury on cognitive function and health in 180 dental practices in the West of Scotland.26 They found that dentists had a urinary mercury concentration of four times that of the control subjects and that dentists were significantly more likely to have had kidney disorders and disturbances of memory. The authors stated that, while these differences could not be directly attributed to dentists' exposure to mercury, as similar health effects may be associated with mercury exposure, health surveillance of dental staff should be implemented. A related paper recommended greater emphasis on safer handling of amalgam in the training of dentists.27 It may be of interest to note that the BBC website, on the day of publication of the paper in 2002, stated ‘Fillings make dentists ill’. Perhaps there is a warning here to all who still use amalgam?

Survival of amalgam restorations

In spite of its failings, however, results from a ½ million restoration database in England and Wales have indicated 53% survival of Class I amalgam restorations at 10 years28 and a review of other research has indicated reasonable performance, with annual failure rates of 3% for amalgam restorations in stress-bearing cavities in posterior teeth, with a variety of factors being involved in restoration longevity.29

The future of dental amalgam?

The debate surrounding amalgam versus composite restorations for posterior teeth has been ongoing since the first dedicated posterior composite material was introduced in the mid-1980s. Some see amalgam as the beautiful silver maiden30 (Figure 1), and the banning of amalgam in some countries as a tragedy.31 Others may consider it as unaesthetic (Figure 2) in an era where the public is more focused on beauty than ever before. Nevertheless, it has been suggested that amalgam should be the material of choice if aesthetic results are not an overriding concern.3

Amalgam has helped maintain dental public health for 150 years. It will eventually be replaced by a more aesthetic material, not because of health hazards, but because of environmental concerns and patient demands for a tooth-coloured restorative material for their posterior teeth. In this regard, few things in life from 150 years ago have survived in a similar form. Most have been replaced because something came along that did the job better, or cheaper (look at the horse vs the car!). That has not happened yet with amalgam – the dream composite is not yet with us (although discussed below), despite the fact that composite has many advantages, for example, good aesthetics and, principally, the capability of being bonded to much smaller cavities than are typical for amalgam. Perhaps by the 50th anniversary of Dental Update the dream composite will have arrived?

Amalgam usage is decreasing, worldwide,32 partly as a result of caries incidence decreasing in the developed world, but also because much of the developed world has embraced posterior composites, with increasing demand from patients for a more aesthetic material than amalgam, and much of Scandinavia using amalgam rarely.32

Why continue to use amalgam?

Reasons for continuing to use amalgam include:

However amalgam:

Most importantly, in relation to the potential demise of amalgam, is the recommendation of a recent World Health Organization Report33 on the future use of materials for dental restoration, with the report being written as a result of a two-day meeting of invited authorities. The conclusions of the report33 are fairly limited. They state that:

The report also charges the manufacturers of dental materials with improving the quality and affordability of composite resins. It also states that high quality alternatives are now available in some of the wealthiest countries, although it does not specifically state what these are. The report contains scant mention of a major advantage of resin composite as a material for posterior teeth, namely, its ability to be bonded into minimally retentive cavities which prevent unnecessary tooth destruction, notwithstanding the fact that it looks better.

What are the driving factors for the use of aesthetic material for posterior teeth?

These include:

Because ‘traditional’ methods of maintaining practice ‘busyness’ are falling, it is the authors' view that practitioners' operative treatment profiles are changing for a variety of reasons, including the financial climate.

The infrastructure of RBC materials comprises three phases, namely:

In addition, pigments, initiators and stabilizers are incorporated.

The above principal constituents will now be examined in turn.

A brief history of RBC materials with regard to filler characteristics and loading

Early RBC materials from the 1970s contained large (circa 100 microns) filler particles, embedded in, but not well bonded to, the BIs-GMA resin matrix. As a result of the large filler particles, these materials were difficult, indeed impossible, to polish and therefore readily discoloured as they collected food stains. Moreover, because the filler particles were not well bonded to the resin matrix, these were lost as the antagonist teeth moved across them, ie their wear resistance was poor. In this respect, these old macro-filled RBC materials were damned by a paper published in 1973 whose results demonstrated their poor wear resistance.42 Micro-filled materials were then developed, with submicron particles.35 These possessed good aesthetic properties as they were readily polishable but, because a high level of filler loading could not be achieved because of the frictional characteristics of the filler particles, they exhibited chip fractures when placed on, for example, incisal edges.35 It took circa 20 years for manufacturers to refine the materials and, by the 1990s, so-called hybrid composites had been developed (Figure 4).43 The wear resistance of these materials was satisfactory, this having been achieved by the development of silane materials which effectively bonded the filler particles to the resin and by the filler particles also possessing their own innate wear resistance.35 However, because the majority of today's materials are hybrid RBCs, with particles of two or more size ranges, and which may have filler loads of up to 85% by weight, a revised classification was introduced. This distinguishes between three types of composites classified by the size of their largest fillers, as shown in Table 2.44

However, this classification has now become outmoded, as Figure 4 indicates, following the development of materials containing only nano-sized filler particles. These may be classified as nano-filled materials, with only one material truly being a member of that group, Filtek Supreme (3M ESPE, St Paul, MN, USA). Some of its nano-sized filler particles are agglomerated. The material has been reported to achieve high polishability and polish retention,45 not unexpected since polishability has been considered to improve with smaller sizes of filler particles.

In summary:

Flowable composites

An increasingly used group of materials are the flowable RBCs, with lower levels of filler loading than hybrid materials. These may be presented in a wide variety of shades in syringes with small gauge needle tips, which facilitate their placement into Class V cavities or their use as a base layer under conventional posterior composites, a role in which they have been shown to reduce microleakage at the gingival margin of Class II restorations, probably as a result of their ability to wet the surface better and because they have lower levels of polymerization contraction stress.46

A derivative of flowable RBC technology is SDR (Smart Dentine Replacement/Stress Decreasing Resin) from Dentsply, intended for use as a time-saving bulk-fill material for large cavities in posterior teeth, capable of up to 5 mm depth of cure, but needing to be ‘topped’ by a conventional composite. Results from recent research with this material have indicated shrinkage stress levels lower than a range of ‘conventional’ flowable materials, with which SDR was compared, and a lower rate of stress build-up, although the authors comment that ‘its effect on interfacial stress build-up is difficult to predict’.47 Even more recently, results of a cusp deflection (movement) in vitro experiment carried out in Dublin showed that cusp movement with SDR was circa one-third that of a conventional RBC material with which it was compared.48 A recently-introduced material from this developing group of bulk-fill RBCs, (Xtra-Base, VOCO, Cuxhaven, Germany) produced low cusp movement results similar to SDR.48

Readers are referred to the paper by Mackenzie et al,49 recently published in Dental Update, for a detailed description of the clinical use of SDR^TM. Indeed, readers are also referred to another paper by Mackenzie and colleagues for a complete exposition of posterior composite placement techniques.50

A recently developed, novel material/placement system is the Sonicfil system from Kerr (Orange, CA, USA). This utilizes an ultrasonic handpiece designed by Kavo (Bibero, Germany), and a material which incorporates a highly filled proprietary resin with special modifiers that react to sonic energy. As the sonic energy is applied through the handpiece, the modifier increases the flowability of the composite, resulting in a less viscous material which may be placed in the cavity in one increment. Another objective is to provide better wetting of the cavity. When the sonic energy is stopped, the material becomes more viscous, enabling contouring and shaping. At the time of writing, there is little published research on this technology (as may be considered to be the case with the majority of recently-introduced materials and techniques).

Packable composites

The ‘opposite’ to flowable materials were developed over 20 years ago. These are the so-called packable or condensable RBCs, which display stiffer viscosity and less stickiness.51 This is said to enable packing of the material in a manner similar to dental amalgam, with the stiffness of the material potentially pushing out the matrix band and facilitating the achievement of a tight contact. However, these materials have been shown to be an inhomogeneous group with differences in physical properties and, while good results have been demonstrated at five years,52 it is the authors' view that these materials are not as popular as previously, with dentists possibly electing to use a material which is suitable for use in anterior and posterior situations (ie universal).

RG filler technology

Giomers are a derivative of resin-composite and glass ionomer technology but, instead of using a conventional filler, as previously described, a pre-reacted glass (PRG) filler is utilized. This filler is synthesized by reacting an acid-reactive glass containing fluoride with polyalkenoic acid in water before incorporating it into the resin material. A material which uses this technology is Beautifil (Shofu, Kyoto, Japan). Clinical results from an 8-year evaluation of Beautifil (Shofu) restorations indicated that, of the 41 restorations examined (from 61 placed originally), no restorations had failed.53 An example of a Beautifil restoration is presented in Figure 5.

A brief history of RBC materials with regard to resin characteristics

The resin developed by Bowen, Bis-GMA (2,2bis[4-(2-hydroxy-3-methacryloxypropoxy)phenyl] propane), still forms the resin matrix for a majority of RBC materials. This is a resin of high viscosity so, in order to make suitable for use in dentistry, a diluent resin of lower viscosity (such as Triethylene Glycol Dimethacrylate [TEGDMA]) is added to make an RBC which is of satisfactory viscosity for clinical use. However, TEGDMA has a higher polymerization contraction then Bis-GMA, so the addition of this leads to a higher overall polymerization contraction of the RBC. Urethane Dimethacrylate (UDMA) resin was developed in 1974 – this has slightly lower viscosity and does not generally require the addition of a diluent. For a more complete exposition of polymerization contraction and its sequelae, readers are directed to the paper by Burke et al in Dental Update54 and the two-year evaluation of a low shrinkage stress RBC which contains a review of the subject.55

However, polymerization contraction stress (not polymerization contraction per se) remains a problem which may be manifested clinically by patient complaints of post-operative pain or sensitivity, in particular, the cusps of the restored tooth being tender to bite upon, and longer term sequelae such as marginal staining and leakage, enamel microcracks (which may show as white lines). The majority of resin composite materials shrink up to 3% on polymerization, resulting in stresses at the (bonded) restoration margin, or within the restorative material itself. The result of these stresses may be:56

The magnitude of the stresses also depends on the modulus of elasticity of the material, its coefficient of thermal expansion, the bonding of the filler particles to the resin and their nature and the configuration of the cavity into which the restoration is placed. In brief, polymerization contraction stress is a function of:

Methods designed to reduce polymerization contraction stress include:

The obvious answer would appear to be for manufacturers to develop and produce a material which has lower polymerization shrinkage than the majority of conventional RBC materials which exhibit shrinkage values in the region of 3%, and some manufacturers have devoted time and finance to the research and development of materials that achieve this. Materials which have lower polymerization shrinkage include the following:

The interfacial phase

Since there is no innate bond between the filler and resin matrix, a coupling agent is required as the ‘adhesive’ between the resin matrix. Most commonly, these are vinyl epoxy silane or methyl silane. These achieve a reliable bond through simultaneous ionic bonding to the inorganic filler and the organic matrix.62

Survival of RBC restorations

Ferracane,43 when considering whether RBCs were ‘state of the art’, concluded that their physical properties were adequate for use in all areas of the mouth, with flexure strength, fracture toughness and tensile strength similar to what could be considered the ‘gold standard’ dental amalgam. Flexural modulus is, however, several times lower, which may result in deformation under high occlusal loading.43

Following a further review of the literature, Ferracane concluded that, although certain correlations existed, the clinical success of composite restorations is unlikely to be predicted accurately by a battery of laboratory tests.63 Therefore, although well-controlled clinical trials are expensive and time consuming, there is no substitute for them.

There are no equivalent figures for RBC to those quoted for amalgam from the ½ million restoration database in England and Wales, because RBC restorations were not permitted under General Dental Services regulations. However, a review of other research has indicated good performance, with annual failure rates of 2.2% for RBC materials in stress-bearing cavities in posterior teeth being reported by Manhart et al29 (compared with 3% for amalgam).

A number of papers have been published, recently, worthy of special mention, either because the restorations in the studies were followed for long periods of time, and/or because there were large numbers of restorations in the study:

The final word on longevity should go the work of Demarco and co-workers,69 published in 2012, who carried out a study of survival rates for posterior composite restorations from 34 papers, each with evaluation periods of five years or more. They concluded that ‘composite restorations have been found to perform favourably in posterior teeth, with annual failure rates of 1–3%’. Their work also demonstrated poorer survival rates in molar teeth than in premolars and that larger, multiple surface restorations were more likely to fail than those in Class I cavities. Fracture of the restoration and secondary caries were the most common causes of failure and it was considered that the aesthetic quality of the restoration was unlikely to have a bearing on restoration survival, although the appearance of the restoration is likely to have an impact on patient (and possibly, operator) satisfaction (Figures 8 and 9). The authors also stressed that factors related to the patient and operator are of importance, alongside a preventive and conservative approach.69

In summary, therefore, RBC restorations in posterior teeth have been shown to provide good longevity and, increasingly, survival rates as good as for amalgam. Not only are they suitable for restoration of large cavities (Figures 5 and 7), but also appropriate to minimally-invasive, adhesive, cavity designs (Figure 10).

The future of resin-based composite?

It has been considered, by Demarco and colleagues,69 that ‘due to their aesthetic properties and good clinical service, composites have become the preferred standard for direct posterior restorations’. Patients appear to be increasingly desirous of aesthetic restorations for their posterior teeth (Figure 11), although an exact shade match in these teeth may not be as important as in anterior teeth. It is the view of the authors that, indeed, a slight shade mismatch is a clinical advantage, as this will facilitate finishing of the restoration without the risk of removing enamel adjacent to the tooth, controlled removal, should this become necessary and, furthermore, that, in the authors' experience, patients will accept a slight mismatch, provided that the restoration is lighter in colour than the tooth. In this regard, Burke has demonstrated, in a paper from the early days of the posterior composite era that, once patients have received a tooth-coloured restoration in their posterior teeth, they will be unlikely to request an amalgam, if and when a further restoration is required.70

RBC materials now demonstrate many desirable characteristics (especially the ability to bond them to tooth substance) with excellent physical properties and good wear resistance, but are more demanding and time consuming to place compared with amalgam restorations. If the desirable characteristics of low polymerization contraction stress, as with Filtek Silorane (3M ESPE), could be combined with the self-adhesive characteristic (to enamel) of the luting materials such as RelyXUnicem (3M ESPE) or self-etch dentine bonding agents and a self-adhesive flowable composite (Vertise Flow: Kerr, Orange, CA, USA) plus the 5 mm depth of cure achievable with SDR (Dentsply, Weybridge, UK) (Figure 12), then what would we have? – an easily placed material with no need to etch with phosphoric acid and capable of being placed in bulk for speed of placement. It would also take less time to place, resulting in reduced patient charges for treatment.

Is that a tooth-coloured amalgam replacement? While there can be little doubt that all major manufacturers (both those specializing in resin-based and glass-ionomer based materials) are working on such a material because they know the enormous market there would be, not one can yet be drawn on how long this development will take. Hopefully, before the 50th Anniversary of Dental Update!

Disclosures

Trevor Burke is a member of the Scientific Advisory Board of 3M ESPE, but has no financial interest in the company, nor in any of the products mentioned in this review.