The challenges of composite restorations of non-carious cervical lesions

Difficulties related to lesion anatomy

Depending on the severity of the wear, a lesion may have a shape that is poorly suited to retention. A shallow, early lesion has few walls available for bonding. Thus, retention will be reduced, especially when the lesion is extensive, but shallow as a result of erosion, for example.

Additionally, the vast majority of lesions have an apical limit beyond the cemento-enamel junction. This requires bonding to a root zone without the enamel that is the most favourable and predictable tissue for bonding.¹³

Isolation difficulties

One of the difficulties in NCCL restorations is isolation. The location of NCCLs in close contact with the marginal gingiva is a real challenge for restorative treatment. The risks of contamination by saliva, blood and sulcus fluid during reconstruction are high.

In addition, the intrinsic anatomical and morphological characteristics of the cervical region create limitations in the placement of the rubber dam and clamp.¹⁴ Proper isolation is very difficult, sometimes impossible, when lesions extend proximally or under the gingiva.¹⁵

Difficulties related to the dental substrate

The enamel in the cervical area has few prisms and is aprismatic in its most apical area. Bonding of the enamel results from the infiltration of the adhesive into the micro-relief created by the acid etching of the enamel prisms, therefore, bonding to the cervical enamel is less effective.¹⁶

Cervical lesions are usually hypermineralized dentine lesions, characterized by the presence of sclerotic dentine casts within the dentine tubules. It has been suggested that the cervical sclerotic dentine, unlike sound dentine, exhibits extensive variations in hybrid layer thickness making the adhesion process more difficult.¹⁷

The presence of the hypermineralized layer is responsible for a 26% decrease in bonding effectiveness.¹⁸ The phosphoric acid used during bonding is unable to completely dissolve this sclerotic layer, which is also present in the tubules, hence the absence of resin tag formation.¹⁷ Intra-tubular casts are more resistant to acid etching than the smear layer and intra-tubular dentine.¹⁹

Nature of constraints

Some authors consider that the poor retention rates of RCs placed in NCCLs can also be explained by continued tooth flexure in the cervical area during mastication, which contrasts with the high elastic modulus of the restorative material. The use of materials with a low elastic modulus can absorb the compression produced by the toothflexure stress, thereby preventing restoration dislodgement.⁵

Preparation of dental substrates

Considering the enamel substrate, the placement of an enamel bevel may be a good option, because this procedure makes the enamel prism more reactive to conditioning and, consequently, increases the bonding performance of adhesive and reduces marginal microleakage yield to improve aesthetics.²⁰

It is advisable to remove the surface dentine layer using rotary instrumentation with light milling (Figure 2), so that the bonding occurs only on the sclerotic dentine. This creates, however, an acidresistant, hypermineralized layer that requires conditioning with a strong acid such as 37% phosphoric acid, as the mild acids of self-etching systems cannot dissolve it.²¹ Air abrasion can also be used to clean the surface of the enamel and dentine for better adhesion.

Isolation

Proper isolation is important for the success of NCCL restorations. A recently published meta-analysis stated that the use of rubber dam positively influences the performance of adhesive NCCL restorations, resulting in less restoration loss and better marginal adaptation. Therefore, if the clinical situation allows, absolute isolation with a rubber dam should be applied.²²

If the location of the lesion limit is supra-gingival, then a single-tooth dam is used. When incisors, canines and premolars are involved, using a rubber dam clamp is recommended.

When lesions extend proximally or under the gingiva, proper isolation is very difficult, sometimes impossible. There are risks of pinching the gum, slippage of the clamp and the latex sheet covering the lesion, as well as access to the lesion being difficult). In such cases, a subgingival clamp can be used to isolate and retract the gingiva at the same time.

Some authors recommend the use of celluloid matrix strip. The matrix is inserted into one side of the cavity and fixed into place with a wooden wedge. It is then carefully inserted into the gingival sulcus, involving the entire cervical wall of the cavity. The unattached side of the matrix is positioned by inserting another wedge into the opposite side of the cavity (Figure 3). A light-cured gingival barrier is injected around the matrix in order to stabilize it (Figure 4). This procedure will also allow the necessary volume of restorative material to be inserted with no excess, and adequate penetration between the gingiva and tooth, forming an angle that provides a large enough aperture for the flowable resin insertion.²³

After the isolation, another important and commonly neglected step should be performed: prophylaxis of the cavity. By their nature, NCCLs are lined with a contaminated layer that resists adhesion. The gingival proximity (sometimes partially or totally covering the cavity) makes this procedure a more complex step. In some cases, rotary prophylactic brushes cannot be used to avoid mechanical aggression and bleeding.¹⁵

Material selection

The dislodgement of composite resin restorations from NCCLs that could be attributed to the flexure that occurs in the tooth cervix can be reduced by the use of composite resin with a low elastic modulus.⁵ Thus, flowable restorative resins are used for the restoration of small lesions because of their low viscosity and increased elasticity (Figures 5 and 6).

The filler loading of flowables is lower than for micro-filled resins, resulting in an enhanced flow and a reduced elastic modulus. An improved flow is likely to facilitate adaptation, and the reduced elastic modulus may provide the material with stress-absorbing ability.^23,24

For large cavities, the major disadvantage of the use of flowable composite resins is that the polymerization shrinkage stress is greater than with conventional composite since it is less loaded with a greater amount of resin. Flowable resin-based composites are conventional composites with the filler loading reduced to 37–53% (volume) compared with 50–70% (volume) for conventional minifilled hybrids. This altered filler loading modifies the viscosity of these materials. Indeed, when resin content increases, flowability and shrinkage stress is higher, while elastic modulus is reduced.²⁵ The alternative is to use a lamination technique with the flowable composite as a cavity liner covered by a conventional composite (Figure 7).

Furthermore, it has been shown that the application of a thick layer of composite resin increases polymerization shrinkage stress compared to a thinner layer.

Adhesive selection

The histological characteristics of the NCCL create specific challenges regarding dental adhesion. The sclerotic dentine resists demineralization and, consequently, does not respond well to etching and bonding techniques, which require the formation of a hybrid layer enhanced by the penetration of the bonding agents in microporosities of demineralized dentine.¹⁹

Previous studies have reported better retention rates of NCCL restorations with a three-step etch-and-rinse adhesive and poorer retention rates with the two-step etch-and-rinse adhesive system.^{8,21,22,23,24,25,26} Other systematic reviews have reported significant differences between self-etch adhesive and etch-and-rinse adhesive.^12,27,28

At the level of the enamel, self-etch materials do not produce the same retentive etching pattern as phosphoric acid. This results in a higher incidence of marginal discolouration at the enamel margins that may affect the aesthetics when compared with etch-and-rinse adhesives, and may be responsible for deterioration of the bonding.²⁹

Other authors recommend the use of universal adhesives in total etch or selective enamel etch modes.³⁰ Perdigão et al showed that phosphoric acid etching prior to universal adhesive application increases retention of composite restorations in NCCLs.³¹ Oz et al showed better clinical results for retention, marginal adaptation and marginal staining, when universal adhesive was used and the enamel was etched.³²

Application of the composite

It has been shown that application of a thick layer of composite material (bulk placement) results in greater polymerization shrinkage. This increases the risk of cohesive fracture and, consequently, the appearance of a hiatus. Furthermore, because bond strength to enamel is usually greater than bond strength to dentine, it has been found that this often results in open dentine margins, thus increasing microleakage that could lead to clinical failure.³³

To deal with these problems, it has been suggested that cavities could be restored by the incremental placement of a composite material in an oblique way, avoiding linking the occlusal and gingival margin of the cavity during the application of each increment (Figure 8). It has also been suggested to start incremental placement at the dentine portion of the preparation. This would minimize leakage into the dentine margin.²³

Applying these recommendations could be facilitated by the use of a small mouth spatula and optical aids.

Finishing and polishing

The proper finishing and polishing of cervical restorations are important procedures that enhance aesthetics, gingival health, periodontal integrity and the longevity of restored teeth.

The smoothest surfaces are achieved when resin material is cured against matrix strips. However, properly contoured restorations are seldom achieved without excess material having to be removed.²³

Maintenance

Marginal deterioration that presents as marginal discolouration and small marginal defects are commonly seen in these restorations with ageing; their lifespan can be lengthened by repolishing the restoration margins. If larger defects are present, local repair is indicated.²²