Posterior composites: An update

Advantages of posterior composites

Disadvantages and contra-indications

While advances in resin composite materials and adhesive technologies have reduced or eliminated most of their historical problems, posterior composites remain technique sensitive, particularly with regard to cavity design, management of operatively exposed dentine, moisture control, contact point formation, adhesive protocols, material/adhesive selection and placement technique,^7,8 where failure to manage polymerization shrinkage stress increases the risk of post-operative sensitivity, hard tissue flexure/fracture and microleakage.⁵

As posterior composites require specialized skills and are demanding in terms of clinical time,⁴ this paper aims to provide an update of the latest evidence-based protocols that will enable the optimization of every clinical stage of posterior composite restorations.

Case selection

Minimally invasive direct restorative techniques focus on biologically respectful interventions designed to address aetiological factors, maximize preservation of natural tooth tissues and promote smaller restorations of increased longevity that are easy to maintain, renovate and repair and leave future restorative options open, when necessary.¹

While more than 50% of all dental procedures involve the replacement of failed existing restorations,¹⁵ primary interventions are only indicated where carious lesions have advanced to obvious cavitation and where remineralization techniques have reached their limit. The general aims of restorative dentistry are to:

As previously described, there are very few contra-indications for the use of posterior composites, but patients must be made aware that:

Regardless of material, the clinical survival of dental restorations is often poor¹⁵ and becomes significantly worse as they increase in size and complexity.⁶

Pre-operative assessment

Pre-operative assessment should include a detailed risk analysis because, in poorly motivated, high-risk patients, adhesive restorations show reduced long-term survival.¹

Periodontal and pulp status should be assessed using appropriate investigations including radiographs, where justified.¹⁶

Prior to isolation, intercuspal and excursive occlusal contacts should be assessed. Preparation outlines should be designed to avoid heavy loading on restorative margins. Photography of pre-operative articulating paper marks provides a useful reference to help reduce finishing time and enables the accurate reproduction of the occlusal scheme.

Opposing and adjacent teeth should be examined and, if their position or contour is likely to compromise successful restoration, they should be adjusted appropriately.

The shade should be recorded before isolation, as teeth rapidly dehydrate and lighten in colour.⁴ A proprietary shade guide may be used, or a test sample of composite may be applied to the tooth and light cured (as there may be a shade shift on polymerization). Shape is a more important functional and aesthetic consideration than shade and a slight shade mis-match assists finishing and will reduce the risk of cavity enlargement during restoration removal.

Isolation

Good moisture control is critical to the success of all adhesive procedures. While rubber dam is widely accepted as the optimal method of isolation,³ it is rarely or never used by the majority of practitioners, who cite poor experience, time and financial constraints, inadequate training and limited benefit as barriers to its routine use.⁷

It has been demonstrated that good results may be achieved with cotton wool rolls and aspiration.⁷ This technique also allows immediate occlusal adjustments.¹

To improve skills with rubber dam, repetitious practice is recommended, beginning with simple, single hole isolation for Class I cavities. Isolation of Class II cavities requires a minimum of three dam holes and the rubber dam clamp should be placed on a tooth distal to the one being restored.

In deep Class II cavities, it may be necessary to cut the dam interproximally to prevent it impinging on the cervical margin of boxes and hindering matrix placement.¹⁷

Cavity preparation

As small restorations last longer than large ones it is important to maximize the amount of residual tooth structure.¹⁸ The rationale behind minimally invasive cavity preparation may be summarized as follows,^1,19 and is illustrated (along with the restoration) in Figure 2.

Linings?

The management of operatively exposed dentine, particularly in deep cavities, remains a confusing subject and this is not helped by variation in teaching.^7,8

In contemporary adhesive restorative dentistry, there is no clinical indication to place a ‘protective’ lining under resin composite materials.^1,12 Modern bonding agents satisfy the ideal properties of a lining material by sealing the dentine tubules opened by cavity preparation, preventing microleakage and therefore pulpal inflammation.²⁰

Although linings remain popular with practitioners concerned about post-operative sensitivity,⁷ they are unnecessary as resin composites are insulators rather than conductors of heat. In addition, the avoidance of a separate layer of lining material increases the available surface area for bonding and stimulates the repair of affected dentine.⁸

While immediate, transient, post-operative sensitivity may be expected as a result of trauma to the dentine-pulp complex and the polymerization of overlying composite material,²¹ 20% of dentists still frequently report problems with posterior composites, the most common of which is post-operative sensitivity.^7,12

Deep cavities have an increased density and diameter of open dentine tubules, resulting in an increase in intrinsic moisture and in the risk of microbial ingress which may lead to pulpal inflammation. When the estimated residual dentine thickness is less than 0.5 mm, a biocompatible material may be used as an indirect pulp cap.

Indirect pulp capping

Resin-modified glass ionomers (RMGI) eg Vitrebond™ (3M, St Paul, MI, USA) may be considered to be the material of choice for indirect pulp capping as they have the following advantages:

Intermediate RMGI layers, if used, should be kept to minimal thickness as they have inferior mechanical properties and will fatigue over time.⁵ The outline form of the restoration should be entirely in composite as open sandwich techniques are no longer recommended.⁵

Direct pulp capping

While the routine use of calcium hydroxide lining materials is no longer relevant in restorative dentistry, they may still be used as a direct pulp cap over obvious pulpal exposures.⁸ Only small, localized amounts of material are required and, once set, they should be covered with a layer of RMGI to protect from subsequent etching.

Therapeutic tricalcium silicate materials are also now available for direct pulp capping, eg Biodentine™ (Septodont, France). These extremely biocompatible materials have antimicrobial properties and induce pulpal cell proliferation/repair at a faster rate than calcium hydroxide²⁰ (Figure 5).

Adhesion

Successful bonding is a fundamental requirement for long-lasting posterior composites. Modern bonding systems create a predictable union between resin composites and residual tooth tissue,¹² and reduce the need for more destructive traditional cavity preparations.¹

As it is such a critical stage and dentists still commonly report problems with post-operative sensitivity, premature reduction/loss of bond strength and marginal degradation, the following detailed guide to adhesion is designed to help clinicians optimize bonding procedures.

Function of adhesives

Enamel bonding is predictable and successful when etched with phosphoric acid and the immediate bonding of most contemporary adhesive systems is excellent.²³

Dentine is a much more challenging substrate for bonding as it is less mineralized than enamel and is intrinsically moist. Deeper cavity surfaces have greater permeability and exhibit higher levels of moisture. The volume of water within the demineralized dentine is difficult to control and excess water contributes to incomplete infiltration of resin into the demineralized collagen matrix.²³

Contemporary adhesive systems rely on the formation of a zone made up of both dentinal collagen and polymerized adhesive resin. This is known as the hybrid layer and its formation aims to fulfil the following functions (Figure 6):

Although it is possible to bond to caries affected dentine successfully, hybrid layers tend to be thicker and more poorly infiltrated (regardless of the type of adhesive used).²³

Classification of adhesives

While the last decade has seen dramatic improvements in dental adhesives,¹ classification of the ever-increasing range of bonding agents has now become impossibly complicated. Fortunately, all adhesive systems are compatible with any composite and meticulous attention to following the manufacturer's protocols may be considered more important than the type of bonding resin selected.¹²

Two-step etch and rinse (aka total etch) systems are the most popular wordwide.^7,12 The adhesive systems that have been shown to deliver the highest bond strengths comprise three steps: etch followed by separate primer and adhesive stages.²³ These ‘gold standard’ materials are recommended where restoration retention relies primarily on the adhesive bond and in clinical situations where the functional forces are expected to be high.

The main problem with three-step adhesive systems is their widely reported ‘technique sensitivity’ and the recognition that failure to optimize every stage is likely to affect bond quality, strength and durability.^24,25

Etch and rinse systems that combine the primer and adhesive in one bottle are the most commonly used,^7,12 and may be recommended for deep cavities because of their reduced technique sensitivity.²³

A range of bonding agents with simplified adhesive protocols are now available but may exhibit lower bond strengths and poorer durability.^1,23

Practical guidelines for adhesives

While the most important factor governing the success of bonding procedures is how closely the operator follows the manufacturer's instructions,¹² widely accepted general guidelines for etch and rinse systems are presented in Table 1.

Self-etch adhesives

Since the 1990s, attempts have been made to simplify bonding by the development of more ‘user-friendly’ self-etch systems that do not require a separate phosphoric acid etching stage, as they have acidic molecules contained within the bonding resin. Smear layer components become incorporated with the hybrid layer, which has been demonstrated to be thinner and less well-defined. Self-etching adhesives exhibit reduced microtensile bond strength compared to etch and rinse systems, mainly due to their limited depth of enamel etching.²⁵ For this reason, the latest innovative self-etch materials are more strongly acidic. They are known as universal adhesives and have been defined as:

Bond failure

While enamel bonds are predictable and long-lasting and immediate bond strengths to dentine are high, loss of dentine bond integrity over time is widely reported.²⁵ Bond failure may lead to secondary caries, which is the most commonly cited reason for restoration replacement. Research has identified a complex range of mechanisms that cause breakdown of one or both components of the hybrid layer (dentinal collagen and adhesive).²³ The latest developments in adhesive technology aim to stabilize the dentine-bonded interface by resisting hydrolytic degradation of collagen fibrils and the hydrolysis of resin polymers.²³

Matrix systems

Matrix technique has been demonstrated to be the most important determinant in the restoration of tight, anatomically correct proximal contacts and contours.⁴

Unfortunately, high failure rates are reported for Class II posterior composites^6,26 and a high percentage of dentists complain of regular problems with open contacts.⁷ This is primarily as a result of reliance on out-moded matrices designed for amalgam, for example the notoriously unsuitable Siqveland matrix system.⁸

A range of specialized matrices, wedges and contact forming instruments are available and are designed to:^1,8,17

Sectional matrix systems

Specialized sectional matrix systems have been demonstrated to give the best proximal contacts when placing posterior composites. They comprise a thin, contoured sectional matrix, a wedge and a separation ring (Figure 9) which, along with the wedge, separates the teeth to account for matrix thickness. The separation ring also seals the matrix laterally to reduce the risk of marginal excess. In cases with wide proximal boxes, matrix deformation may be avoided with the use of broad face separation rings (Figure 10).

Matrices must be burnished against adjacent teeth because, unlike amalgam, composite will not readily push matrices out.

Despite being taught at the majority of dental schools, usage of sectional matrix systems has only been adopted by a small fraction (<10%) of practices.^7,8 As the UK amalgam phase-down continues, it is essential that clinicians equip themselves with one of the wide range of sectional matrix systems. They are used with the following general instructions:^4,17

Thin, contoured sectional matrices are placed with their concave edge orientated towards the occlusal surface and the convex side towards the adjacent tooth;

A wedge is applied to seal the matrix cervically;

A separation ring is then applied using the designated forceps. Care must be taken to ensure that the cervical seal has not been compromised by ring placement.

Removal of sectional matrices can be difficult as this technique generates very tight contacts but specialized forceps (+/- specialized matrices) or artery forceps may be used to facilitate removal.

Circumferential matrices

In larger cavities and those with wider proximal boxes, specialized circumferential matrices, eg SuperMat™ (KavoKerr, Bioggio, Switzerland) are recommended as they confer a number of advantages¹³ (Figure 11):

Single use eliminates cross-infection risk;

Simple to use and make ideal contacts easier to form;

On tightening they impart a more natural, rounded proximal contour and are less likely to flex weak cusps;

The tightening spool may be placed buccally or lingually/palatally and absence of a matrix holder permits wedge placement from any direction;

They help to hold rubber dam in place and offer improved access;

They are cost-effective and may also be used for amalgam restorations.

Matrices are applied to the tooth, tightened with the matched instrument and should be burnished against the adjacent tooth with a suitable instrument as composite will not push matrices out. Only metal matrices are recommended for posterior composites as transparent versions are thick, difficult to insert and stabilize and cannot be burnished.⁶ Light-transmitting wedges are also unnecessary.⁴

In extensive cavities with limited residual coronal tissue, freehand build-up of missing cusps may be carried out to facilitate circumferential matrix retention and stabilization.

Wedging

Wedging matrices is essential to create a tight cervical seal. The wedge also separates the teeth slightly to compensate for matrix thickness. When working without rubber dam isolation, wedging assists haemostasis and improves moisture control.

Specialized plastic wedges, eg FlexiWedges™ (Common Sense Dental Products Inc, Springlake, MI, USA) are recommended over traditional wooden wedges as they are easier to place and stay firmly in position. They are concave on the gingival surface so that, while optimizing the cervical seal, they compress the interdental papilla and do not deform the matrix or encroach on the contact area (Figure 12).

Occasionally, more than one wedge may be necessary. They may be placed from opposite embrasures and certain systems are also designed to allow stacking (Figure 13).

Wedges may also be introduced early in the operative procedure (pre-wedging) to initiate tooth separation.¹⁷

Contact forming instruments

Suitably shaped or specially designed hand instruments, eg TriMax™ (Adent Inc Danbury, CT, USA) are very useful in helping to create tight contacts. They are applied to the initial composite increment and push the matrix against the adjacent tooth (Figure 14).

Once light cured, the proximal composite holds the matrix in position while further increments are placed. Additionally, by dividing the proximal composite into two halves, the tendency for the forces of polymerization shrinkage to pull on both proximal walls simultaneously is reduced.

Material selection

Most modern composites have a similar chemical composition/formulation and are suitable for use in all areas of the mouth.^5,14

Highly-filled hybrid materials with a combination of large and small filler particles are recommended for load-bearing direct posterior restorations. Another factor determining the successful long-term performance of modern posterior composite restorations is that they now have similar mechanical properties to amalgam and feldspathic porcelain.¹⁴

Unfortunately, when choosing a posterior composite there is little correlation between laboratory studies and clinical performance.¹³ After long periods of evaluation, the type of composite used has been shown not to play a significant role in restoration survival, compared to the number of surfaces restored, the amount and quality of residual tooth structure, caries risk and operator factors.⁵

For these reasons it is recommended that clinicians select composite materials based on personal preference with regard to the following:⁴

It is also recommended that clinicians choose vigorously tested composites from well-established manufacturers, rather than cheaper alternatives, which the authors consider to be a false economy and may deliver suboptimal clinical outcomes.

Polymerization shrinkage stress

When composite resin is light cured there is volumetric shrinkage (average 2–3%) as approximately 60% of the resin constituents convert from monomer to polymer molecules.

As composites shrink it is inevitable that internal stresses are generated within the material and are transferred to bonded interfaces. The amount of polymerization shrinkage stress is dependent on the volume of shrinkage, the modulus of elasticity of the material and the degree of conversion to polymer. The stress is more important than the actual dimensional change and may result in a range of complications, the most common of which is post-operative sensitivity. This results from gap formation at debonded interfaces, which fill up with dentinal tubular fluid, which in turn reacts to thermal stimuli and pressure from occlusal loading.

The risk of post-operative sensitivity should be minimized as treatment of persistent problems usually necessitates removal of the entire restoration.⁴ Other complications of polymerization shrinkage stress include cuspal flexure, enamel cracking/fracture, reduction in the mechanical properties of the material and marginal gap formation, which may subsequently allow the ingress of cariogenic bacteria or stain.

Stress also results as the restoration cools from the elevated temperature caused by the reaction exotherm and heat from the light curing unit.

Development of low shrinkage stress composites is an area of ongoing research and aims to produce composites with a targeted shrinkage of 0.5–1.0% by volume, which will counterbalance post-operative water absorption and result in a negligible net volumetric change.²²

Composite placement

Placement technique is one of the most important determinants in the long-term success of adhesively bonded restorations. Operator skill and experience are important factors governing the quality of marginal adaptation, the elimination of non-homogeneous layers and the shaping of an anatomically correct final layer.²²

For a long time, incremental placement technique has been recognized as the standard of care when placing composite restorations.²² The successive placement of small (≤2.0 mm) increments has been shown to:

Incremental placement technique has some disadvantages as it is inherently time-consuming, increases the risk of incorporation of voids, and of contamination when rubber dam is not used.²¹

A wide range of specialized instruments is available, that are designed to simplify placement and shaping (Figure 15).

It is important that they are kept spotless and remain unscratched to prevent composite from sticking to them. While specialized instrument wetting resins are available, the use of bonding resins to lubricate instruments is contra-indicated, as they contain solvents that will degrade the properties of the restorative material.

The aesthetics of posterior composite restorations may be enhanced by the use of polychromatic layering techniques and/or application of tinted flowable composite materials (Figure 16).

Based on current evidence there is currently no laboratory or clinical data to prove decisively the most appropriate placement technique for posterior composites.^14,22

Bulk-fill placement technique

Over the last ten years, the successful long-term clinical performance of posterior composites has been attributed to an improvement in the physical properties of composite materials and the effectiveness of bonding agents.

The most recent innovations are the introduction of bulk-fill materials, which have further reduced technique sensitivity by partially or completely eliminating the need for traditional incremental placement techniques. Development of these ‘low shrinkage stress’ materials is one of the fastest developing areas in materials research. Low shrinkage stress is achieved by designing bulk-fill materials that:^{27, 28, 29}

Bulk-fill materials may be divided into two groups, depending on their one or two stage placement protocol.²⁸

Bulk-fill materials are proving popular with clinicians worldwide and an increasing number of independent studies have been published reporting their significant advantages, which include:^6,20,28,29

Good matrix technique is essential when using low viscosity materials to reduce the risk of ledges and overhangs,²⁹ and translucent bulk-fill materials will not mask darkly stained cavity floors.

Regardless of material, it is recommended to shape the final increment as anatomically as possible to minimize finishing time and marginal excess, which may be difficult to visualize and remove with accuracy after light curing.

Light curing

Optimal light curing is a vitally important stage of direct composite restorations. Numerous studies have reported that suboptimal light curing may result in a range of significant disadvantages:³⁰

Various regimens have been proposed for light curing (eg soft start, ramp and pulse), but the clinical significance of these protocols is questionable and may not be practical, or may have a limited effect on polymerization shrinkage and therefore stress.²²

When light curing, the following generally accepted guidelines should be followed:³¹

Light curing of final composite increments may be carried out through a layer of translucent material, eg glycerine, to eliminate the approximately 10 μm surface oxygen inhibition layer of unreacted resin.

Shaping, finishing and polishing

Accurate shaping of posterior composites demands good understanding of tooth anatomy,³² and can be challenging and time-consuming when restorations are large or complex.¹¹

Despite best efforts, functional and/or aesthetic adjustments are usually necessary and a range of specialized burs and finishing and polishing discs and tips are available to facilitate this (Figure 19).

Finishing burs should be applied intermittently, with light pressure and copious water spray to prevent overheating and potential pulpal irritation. Heavy immediate finishing increases the risk of ‘white line’ enamel fractures, which occur 10–50 μm from restorative margins.

Finishing discs may be used to round line angles and shape marginal ridges where they will reduce the risk of iatrogenic damage to adjacent teeth. As with all clinical stages, the use of magnification facilitates finishing procedures and reduces the risk of iatrogenic damage when removing marginal excess.¹

Specialized low viscosity liquid polishes, eg Biscover™ (Bisco Inc, Schaumburg, IL, USA) may be applied to seal surface defects, impart a smooth finish, improve aesthetics and eliminate the oxygen-inhibited surface layer (Figure 20). NB: Bonding resins should not be used for this purpose as their oxygen inhibition layer will collect stains.

Maintenance

As well as restoring function and aesthetics, restorations should be designed so that they are easily maintained by patients. Oral hygiene instructions given at the planning stage should be reinforced and monitored at regular intervals.¹

The surfaces of all composites will roughen over time.¹⁴ While most modern materials are designed to remain below the threshold for plaque accumulation (0.2 μm),¹⁴ the majority of composite restorations will demonstrate age-related degradation of their aesthetic properties (Figure 21).

Marginal staining is the most commonly occurring complication, but it is very important to remember that:

Renovation and repair

Over 50% of all dental restorations are replacements of existing restorations, with secondary caries cited as the most common reason for replacement.¹⁵ It is widely reported that many failing composite restorations are amenable to minimally invasive renovation/repair techniques that will increase their functional longevity at minimal biological cost.^1,13,15

Repair/renovation of composite resin with defective margins may now be regarded as state-of-the-art care,¹⁴ and repair rather than total replacement should always be considered as it delivers a range of significant advantages:^13,15,33,34

The generally accepted technique for repairing composites is as follows:

Practising posterior composites

Carefully recording which materials, equipment and clinical techniques have been used will enable a long-term clinical audit of restorations.⁴

A vast array of educational resources is available to help clinicians learn and practise techniques with posterior composites. Repetitious independent practice on plastic and extracted natural teeth (Figure 22) is recommended to allow clinicians to refine practical skills, improve confidence and expand their range of predictable applications to include:

Direct composite restorations are easier to monitor than metallic/indirect restorations and may generally be expected to have more retrievable modes of failure (Figure 23).

Routine reviews of posterior composite restorations, ideally aided by magnification and clinical photography, will enable detailed reflection on restorative outcomes and assist refinement of clinical techniques to world class levels.

Summary

Direct posterior composites have revolutionized restorative dentistry and form an integral component of contemporary oral healthcare.

Improvements in materials, equipment, clinical techniques, teaching and operator experience allow direct composite to be used to solve a comprehensive range of restorative problems and, in many clinical situations, may obviate the need for more destructive, irreversible indirect restorative techniques.

By optimizing the evidence-based clinical stages described in this paper, dental teams will be able to enhance patient care by using minimally invasive treatments designed to conserve biological tissues, ensure the life-long health of the teeth that they restore and leave future options open for minimally invasive renovation and repair when signs of failure occur. The time taken to equip practices and refine and master posterior composite techniques will be rewarding for patients and clinicians alike.