The influence of a liner on deep bulk-fill restorations: Randomized clinical trial

Abstract

Objective

The aim of this study was to evaluate the influence of a liner on the clinical performance of deep posterior restorations performed with bulk-fill composite.

Methods

30 subjects received two restorations on deep preparations in posterior teeth, without pulpal exposure, after selective carious-tissue removal in one-stage. The internal walls reached the inner quarter of dentin, but with a radiographically detectable zone of firm dentin remaining. The pulpal protection using a layer of glass ionomer composite liner (Ionoseal, Voco) was applied in just one preparation. The adhesive system Futurabond U (Voco) was applied in all preparations, which were restored with the bulk-fill pure Ormocer nanohybrid composite (Admira Fusion Xtra - Voco), in up to 4 mm thick increments. All restorations were evaluated using the FDI criteria after 7 days, 6, 12 and 24 months postoperatively.

Results

After 24-months, 25 patients attended the recall and 50 restorations were evaluated. The Fisher’s statistical analysis (5%) showed no difference between the techniques for the esthetic, functional and biological properties. No postoperative sensitivity was reported for both groups.

Conclusion

The application of a liner did not influence the clinical performance of deep restorations with bulk-fill Ormocer composite.

Clinical relevance

The application of a liner with a GIC-based material did not have a significant effect on the clinical performance of bulk-fill restorations and seems to be unnecessary for the material tested.

Introduction

The dentin and pulp are histologically, embryologically and functionally interconnected tissues, and can be considered a form of complex. The effort to maintain the pulp vitality is paramount to have a functional viable tooth in occlusion [1], deeply influencing the restorative procedures. The concepts about the removal of caries-affected dentin tissue during the preparation step have been changed through the years. Instead of the “non-selective carious-tissue removal”, with complete removal of demineralized softened dentin, the current recommendation is the “selective carious-tissue removal” until reach the soft or the firm (but still demineralized) dentin; since radiographic evaluation indicates that caries has progressed no deeper than the pulpal quarter, with a remaining dentin layer [2]. The scientific evidences show that is it more important to seal the cavity than to remove the softened dentin [3,4], as the pulp tissue has an innate ability to repair if the bacterial challenge is removed and the tooth is properly restored [5].

Different sources of pulp irritation can play a role in a restored tooth. When the caries-softened dentin is partially removed, remaining bacteria or their by-products may produce irritative effect on the pulp cells, which can be proportional to the residual amount. However, even when the carious tissue is completely removed, restorative materials can also produce insults to the pulp tissue on deep preparations, which can result in mild to severe inflammation and pain [6]. Those insults can be consequences of the contact of material’s components with the pulp cells, due to the long-term release of leachable non-polymerized monomers or other substances, which penetrate the dentin tubules and negatively affect the cell function and metabolism [6]. Some studies showed that the degree of conversion of monomers into polymers is correlated to the existence of leachable monomers, inducing inflammatory responses of the dental pulp cells [7,8].

Traditionally, different protective materials have been indicated for application on the dentin on deeper areas of the preparation, as a liner between it and the restorative material, which could protect the pulp from the negative effects of the direct contact with the restoration [9]. The application of some liners could also stimulate the remineralization of caries-affected dentin, as well as contribute on prevention of secondary caries by fluoride release. For that, the calcium hydroxide, glass ionomer cements and hydraulic calcium silicate have been indicated [[10], [11], [12], [13]].

In relation to the irritative effect of the restorative materials, the importance of the application of a pulp protection material seems to be related with the thickness of remaining dentin, since this tissue can by itself reduce the irritative effects on the pulp [10]. Previous studies showed that a 0.5 mm thick hard dentin remaining was able to reduce the toxicity level of restorative materials up to 75 %, while 1 mm thick reduced up to 90 % [14]. With 2 mm thick hard dentin remaining, no pulpal response was observed in most cases [15,16]. Therefore, the preservation of the remaining tooth structure is more important to the pulp than the application of a lining material. However, as the remaining dentin thickness cannot be precisely assessed clinically, on deep caries lesions, the use of a highly biocompatible material is still recommended [2,9,11,13].

The development of the bulk-fill composites led to some concern about pulp response, due to higher potential of incomplete cure of the monomers [17]. Created in an attempt to simplify the restorative procedure, thicker layers of materials are applied and light cured at once. Although those materials are more translucent, which could increase the depth of cure, the degree of conversion in the bottom layers, which contact the pulpal and gingival walls of the preparation, has shown to be critically low [18]. The lower the polymerization of the material in contact with the dentin, the higher the chances of irritative effects on the pulpal tissue [19]. Therefore, the use of a highly biocompatible lining material on this area could be beneficial. In addition, a low curing of the composite in the gingival wall predispose the polymer to degradation in tooth-restoration interface, increasing interfacial defects and microleakage, eventually contributing to pulpal irritation and secondary caries lesion [20].

Although traditional composites are formulated mainly based on methacrylate monomers, some other alternatives were created and are available on the market. The Ormocer molecule (organically modified ceramic) is an oligomer having an inorganic silica backbone. Due to its higher molecular weight, the volumetric shrinkage and the stress are reduced; and due to its many polymerizable groups, the presence of leachable monomers is reduced producing a more biocompatible material [21]. Nevertheless, there is not enough clinical evidence in relation to its behavior as a bulk-fill material.

The importance of liner application in deep preparations on posterior teeth, after selective carious-tissue removal until reach the firm dentin, when associated with the new bulk-fill composites was little investigated. It can be an effective way of reducing postoperative sensitivity and secondary caries, in cases of impaired light-curing in the hard to reach areas. However, if not proven to be relevant, its use can be abolished, simplifying the restorative procedure. Therefore, the aim of this study was to evaluate the influence of a liner application on the clinical performance of pure Ormocer bulk-fill material on deep cavities after selective caries tissue removal. The null hypothesis was that the liner does not influence the clinical performance of the restorations.