1

Introduction

Perfect anterior restorations act as an advertisement for the skills of the dental professional. Most operative interventions in anterior teeth are accomplished with the direct placement of composite resins. The skill of the dentist in achieving a natural anatomical shape and color match with the adjacent teeth are prerequisites to achieving a pleasing aesthetic result, which can also be assessed easily by the patients themselves. Type of composite resin, methods and materials to condition the tooth structure (enamel etching, self-etching, no etching) as well as the operative procedure (bevelling of enamel margin, rubber dam application) may also influence both the aesthetic results and the longevity of the restoration.

Before the development of composite resins and the acid-etch technique of the enamel, carious lesions in anterior teeth were mainly restored with silicate cement, which required a retentive preparation pattern . Restorations that involve the proximal part of an anterior tooth but not the incisal edge are defined as Class III restorations.

In the early days, the building up of fractured teeth was only possible with indirect restorations, such as full-coverage crowns, because bonding to the remaining tooth substance had not yet been established as an operative procedure. Already in the nineteen-fifties, the enamel etch technique with phosphoric acid was developed by Buonocore . However, it took about 20 years until this technique has been introduced into clinical dentistry. This technique made it possible to directly restore fractured anterior teeth with composite resin, to close diastemata or to build up worn teeth. Restorations that involve a part of the incisal edge are defined as Class IV restorations.

At that time, there was a dispute as to whether it is necessary to place an unfilled resin bonding material on the etched enamel or whether high-viscosity resin composites could be placed directly on the etched enamel. The operative procedures have been gradually simplified and the materials improved since then. First, the application times of both enamel etching and rinsing were reduced from 60 to 30 s , dentin bonding agents made liners superfluous and increased the bonding strength to the tooth structure . Then, self-etching adhesive systems were introduced . Capable of establishing a bond to both the enamel and dentin, these materials streamlined the operative procedure because they eliminated the need for a separate rinsing step.

Most contemporary dental composite resins still contain a monomer which was already developed in the late 1950s of the last century by M. Bowen : it is called Bisphenol-A glycidylmethacrylate or simply Bis-GMA or Bowen’s resin Microfilled composites and later hybrid and nano-hybrid composites replaced the macrofilled composites, which were the first dental resins on the market . Polymerization curing lights were first introduced at the end of 1970s of last century . They allowed these materials to be cured on demand, which facilitated the customization of anterior restorations, because they could be built up step-by-step with several layers that have different optical properties.

With the reduction of caries prevalence in most countries, the prevalence of Class III restorations due to proximal caries has also dropped. However the prevalence of traumatic injuries to anterior teeth has significantly increased over the last 20 years due to an increase in sports activities undertaken during leisure time. In some countries, particularly in Scandinavia, children and adolescents have nowadays more teeth damaged by traumatic injuries than by caries . The restoration of fractured teeth (Class IV) with composite is usually the first treatment option.

The question arises as to how effective such a treatment is in terms of aesthetics, function and longevity. In the databases SCOPUS and PubMed no meta-analysis or systematic review on the efficacy of Class III or Class IV restorations has been found. There are several systematic reviews on posterior composite restorations and cervical restorations . However, it is inadequate to extrapolate from the longevity of posterior composite restoration to that of anterior restorations.

The aim of this review was to systematically evaluate prospective clinical trials on anterior resin composite restorations without restricting the search to the publication year or the type of resin or adhesive system used.

The following factors affecting the clinical outcome were to be specifically evaluated:

• –

  type of cavity (Class III, Class IV)

• –

  type of enamel/dentin conditioning

• –

  type of resin composite

• –

  operative techniques:

  • •

    bevelling of enamel

  • •

    absolute versus relative isolation

These factors were to be assessed by the following outcome criteria:

• –

  time elapsed until replacement and reason for replacement (marginal caries, fracture of filling, retention loss, etc.)

• –

  color match and surface texture

• –

  marginal integrity and marginal staining

• –

  anatomical form (shape)

• –

  chipping and fracture

The following hypotheses were examined:

• 1.

  Class IV restorations mostly suffer from chippings and fractures and have a reduced longevity compared to Class III restorations.

• 2.

  The type of composite resin does not influence the overall longevity of Class III restorations.

• 3.

  Class IV restorations with hybrid composites show a better longevity than Class IV restorations with microfilled composites.

• 4.

  Restorations based on glass ionomer derivates have a reduced longevity compared to composite resin restorations and compomers.

• 5.

  Enamel etching with phosphoric acid reduces the number of restorations that show marginal discoloration and defective marginal integrity compared to self-etching systems and compared to those restorations that were placed with enamel etching but without bonding agent.

• 6.

  The type of isolation or bevelling of the enamel does not influence the clinical outcome.

• 7.

  Hybrid and microfilled composites show a better color match than macrofilled composites.

• 8.

  Hybrid composites maintain their anatomical form more effectively than microfilled composites, compomers and glass ionomer derivates.

2

Materials and methods

2

Materials and methods

3

Statistical analysis

All the clinical outcomes could be expressed as percentages of restorations retaining a given property across the defined period of time, for example the percentage of restorations without a visible marginal discoloration, the percentage of restorations with a good or a very good anatomical form, or the percentage of restorations which did not need replacement, as defined above. To permit a comparison of the rate of deterioration among the various experiments, the percentages observed at the various points of time were divided by the percentage observed at baseline for those experiments where the latter was below 100%.

Let Y ( t ) be a percentage measured at time t (expressed in years). To model the rate of deterioration, we were looking for a model where Y ( t ) is a decreasing function of t ranging from Y (0) = 100% down to 0% for large values of t. A linear model of the form Y ( t ) = 100 − beta × t would for example not be convenient since it would have become negative for large values of t, which was not sensible in our context. We considered instead a deterioration model of the form Y = 100 × exp(−lambda × t ˆalpha) with positive values of alpha and lambda, which is equivalent to stating that Log(−Log Y /100) = beta + alpha × Log( t ), with beta = Log(lambda).

To study how the deterioration process depends on a given factor of interest, we then considered the following statistical model for our empirical percentages Y ( t ):

Log(−Log( Y ( t )/100)) = beta_ j + alpha × Log( t ) + study_effect + experiment_effect + random error.

In this model, beta_ j is a fixed parameter characterizing the rate of deterioration for the level j of the factor of interest, such that the higher the parameter, the faster the deterioration (a value of beta_ j = −2 indicates for example a faster deterioration than a value of beta_ j = −3). The parameter alpha characterizes the shape of the deterioration which does not depend on the factor of interest. A random experiment effect was included to account for the obvious dependencies among the repeated percentages observed in the same experiment along time, while a random study effect was included to account for the fact that the subjects involved in different experiments from the same study were partly the same (split-mouth design).

In our model, the deterioration curve is thus assumed to be different from study to study and from experiment to experiment. Figs. 1–9 below show some of our fitted models as Y = 100 × exp(−lambda_ j × t ˆalpha), with lambda_ j = exp(beta_ j ), which can be interpreted as a median deterioration curve for the level j of the factor of interest (estimated over all studies and experiments).

Estimated median percentage of restorations across the studies and experiments with good or very good color match in relation to the type of restorative material and to the observation time. (Left) Class III, (right) Class IV.

Estimated median percentage of Class III restorations across the studies and experiments with good or very good surface texture in relation to the type of restorative material and to the observation time.

Estimated median percentage of Class III restorations across the studies and experiments with adequate anatomical form in relation to the type of restorative material and to the observation time.

Estimated median percentage of restorations across the studies and experiments without material chipping/fracture to the restoration in relation to the type of restorative material. (Left) Class III, (right) Class IV.

Estimated percentage of restorations across the studies and experiments based on all data without caries adjacent to the restoration in relation to the type of cavity category (Class III/IV).

Estimated median percentage of Class III restorations across the studies and experiments without caries adjacent to the restoration in relation to the type of isolation.

Estimated median percentage of restorations across the studies and experiments without marginal staining in relation to the adhesive technique and adhesive system and to the observation time. (Left) Class III, (right) Class IV.

Estimated median percentage of Class III restorations across the studies and experiments without detectable margins in relation to the composite material (left) and the adhesive technique (right).

Estimated median percentage of restorations across the studies and experiments that were not replaced in relation to the type of restorative material. (Left) Class III, (right) Class IV.

Such a linear mixed model could be fitted using the restricted maximum likelihood method implemented in the routine lme , which can be found in the package nlme from the statistical software R. In this routine, it was also possible to weight each empirical percentage by the corresponding number of restorations (the denominator of the percentage). To test for the statistical significance of the factor of interest, a maximum likelihood ratio test was used, with the number of levels of the factor of interest minus one as number of degrees of freedom. p -Values smaller than 0.05 were considered to be significant.

4

Results