INTRODUCTION:

A composite filling is a tooth-coloured plastic and glass mixture used to restore decayed teeth. Composites are also used for cosmetic improvements of the smile by changing the colour of the teeth or reshaping disfigured teeth. Secondary caries is the lesion at the margin of a restoration which has been widely considered as the most important and common reason for restoration replacement, regardless of the restorative material type [1]. Secondary caries are mainly caused by bacterial microflora[2,3] and the formation of a biofilm (plaque) at and within the restoration-tooth margin [4]^.. It is most likely facilitated by a gap forming between the restoration-tooth margin that allows bacterial colonization[5]. While cyclic loading is also considered as a known potential cause of failure for both tooth tissue and restorative materials [6-11],margin failure and gap propagation may also occur due to the cyclic loading experienced by restorations during mastication [12-15].

Secondary caries lesions are the main late complication of dental restorations, limiting their life span and generating costs by repeated reinterventions. [16]. There are two kinds of lesions that can occur adjacent to the restoration: they are secondary caries and residual caries( remaining caries). The residual caries are the residual demineralised tissue left, due to the failure of eliminating all infected dentine or/and enamel during the cavity preparation. Therefore, it becomes very difficult for clinicians to make an accurate diagnosis of secondary caries and provide a clear terminology. Nowadays, it is generally acknowledged that secondary caries or recurrent caries is a primary carious lesion of tooth at the margin of an existing restoration, which occurs after the restoration has been used for some time [17].

SITE OF OCCURRENCE OF SECONDARY CARIES

Secondary caries is also initially caused by the activities of microorganisms in dental plaque, so it is possible for any site on the restored teeth which is prone to the bacterial stagnation to develop secondary caries. Secondary caries was detected predominately on the gingival margins of Class II and Class I restorations, while sometimes on the Class I restorations and the occlusal part of Class II restorations [18,19]. A number of factors contribute to the more frequent occurrence of secondary caries on the gingival surface. First of all, the gingival aspect of any restorations is more difficult for patients to keep plaque free than any other parts, especially if it is located inter-proximally, while the occlusal surface is not a plaque stagnation area and toothbrushing can easily reach this area to clean the plaque [20,21]. Secondly, during the restorative operation, the gingival surface is prone to contamination by gingival fluid and saliva, which cause the impossible visual inspection of the gingival floor and the deficiencies of insertion of restorative materials. And these deficiencies may lead to secondary caries more easily ²¹. The clinically diagnosed secondary caries has been shown to be principal cause for the replacement of all types of restorations both in permanent and primary teeth, 50%-60% of restorations are replaced as a result of the diagnosis of secondary caries [22].

RELATION BETWEEN COMPOSITE AND SECONDARY CARIES

Composite shrinkage occurs during the polymerization process as a result of short-range covalent bonds forming between the monomer units, which result in shortening of the final network polymer [23]. Walls, McCabe and Murray (1988) related four variables that influence polymerization shrinkage of the composites: a) size of the monomer molecules, where the larger molecules present lower shrinkage; b) the volume fraction of filler that within limited parameters provides reduced shrinkage; c) the degree of polymerization, which is directly proportional to increased shrinkage and d) the nature of the resin and consequently the mechanism of polymerization [24]. Polymerization shrinkage may result in gap formation around the margins of the restoration due to the stress produced in these areas, which disrupts the bond of the composite to dental tissues. This shrinkage may cause marginal leakage and lead to further irritation or pulp necrosis [25]. For these reasons, polymerization shrinkage is one of the main factors that determines the longevity of composite restorations [23]. A review of the literature suggests that the average lifetime of posterior dental composites is only six years [26]. Furthermore, a majority of restorations are replacements of failed restorations [27]. The most common reason for the replacements is secondary caries occurring at the margins [28-31].

PREVENTION OF SECONDARY CARIES IN COMPOSITE RESTORATIONS

Many materials have antimicrobial properties: copper, zinc, silver, various silica-based glasses, etc [32,33]. Bioactive glass (BAG) has been shown to have both an antimicrobial effect on oral bacteria and the ability to remineralize adjacent mineralised tissues [34-39]. The antimicrobial effect of BAG is attributed in part to the release of ions (e.g., calcium and phosphate) that have a toxic effect on the bacteria and cause neutralisation of the local acidic environment [40], the latter leads to a local increase in pH that is not well tolerated by many oral bacteria [41]. The first Bioactive glass was developed more than 40 years ago but its potential use in resin based dental restorative composites has begun very recently [42-44]. It has been already discussed that composites containing up to 15% by weight non-silanated Bioactive glass filler can have mechanical properties comparable to, or superior to, commercial composites [43] and that the addition of Bioactive glass fillers does not compromise the degree of monomer conversion[44]. Furthermore, despite Bioactive glass ions leaching out of the composite the degradation of mechanical properties with ageing is no worse, or better than, many commercial composites [43]. It has been described in a study that bacterial penetration for 0 Bioactive glass composite samples was always to the full depth of the restoration, sometimes propagating along the floor of the cavity under the restoration. In contrast, the average depth of penetration for the 15 Bioactive glass samples was only 61% of the depth of the gap [45]. Indirect composites for fabrication of onlays are polymerised outside the oral environment and luted to the tooth with a compatible resin cement indirect composite inlays or onlays reduce wear and leakage and overcome some of the limitations of resin composites such as secondary caries etc.

CONCLUSION:

Secondary caries with composites is to some extent associated to the restorative material, as significantly more caries occurred with composites than with amalgam. The mechanisms behind the development of secondary caries are much less clear and are most probably multifactorial. Composites also seem to favour the growth of cariogenic bacteria on their surface, which has been associated with their specific surface properties, release of components and lack of antibacterial properties. So, in order to reduce the secondary caries due to composite restorations, Bioactive glass (BAG) filler particles are added to the composites because Bioactive glass (BAG) has an antimicrobial effect on the oral bacteria and they can help in preventing the secondaries caries.

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Received on 10.06.2016 Modified on 03.08.2016

Research J. Pharm. and Tech. 2017; 10(1): 362-364.

DOI: 10.5958/0974-360X.2017.00073.7