cosmetic dentistry_ beauty & science

I 33 review _ surface-conditioning I cosmeticdentistry 2_2013 Glassfibre-reinforcedcomposites A relatively new group of dental biomaterials, the glass fibre-reinforced composites, is used in fixed partial dentures, removable prosthodontics, periodontal splints and retention splints.25 The ad- hesion between the glass fibre and resin composite is improved by adding a silane coupling agent. The silane forms siloxane linkages with the surface hy- droxyl groups of glass fibre. The organo-functional groups of silane react with the functional group in the resin composite. Thus, the bonding strength is increasedbetweenresincompositeandglassfibre.27 Resincompositefillingmaterials Nowadays, dental resin composites are com- posedofaresinmatrixthatcontainsmonomersand cross-linkingmonomers,aswellasafree-radicalini- tiator, an inhibitor, colouring pigments, filler mate- rials such as barium glass, silica, apatite and a silane couplingagent.27,28 Thelatterenhancesthebonding betweenthefillerparticlesandtheresinmatrix.29 The fillerparticlesaddedtotheresinmatrixalsoimprove the physical and mechanical properties of the resin composite. Moreover, the addition of fillers reduces volume shrinkage after polymerisation, and im- proves the aesthetic appearance and radiopacity.30 Titanium,noblemetalandbasemetalalloys Titanium, noble metals and cobalt–chromium (base metal) alloys are commonly used for remov- able partial and complete dentures with a metal frame incorporated and metal–resin cement resto- rations.31,32 Forthesemetalandmetalalloys,surface conditioning by sand-blasting using silica-coated alumina particles produces a silica-coated layer on the surface. Application of a silane coupling agent to the silica-coated surfaces forms a durable silox- ane linkage. This is followed by cementation. _Limitations of silanes as adhesion promoters Silanes are good at promoting adhesion between resincompositesanddentalrestorativematerialsbut there are some limitations to silane coupling agents. The adhesion of silane coupling agents and non- silica-based restorative materials such as alumina, zirconia or metals is weaker than the silica coating of these materials.33, 34 Therefore, a surface pretreat- ment with silica coating is required so that durable bonds (siloxane bonds) are formed between silane and silica-coated restorative materials. For noble metals or noble metal alloys, thione or thiol-based coupling agents are used to promote adhesion.35 Thesecouplingagentshavedifferentbondingmech- anisms with various dental restorative materials. _Current trends and future development of coupling agents in dentistry Nowadays, other coupling agents (such as phosphate ester) are added to self-adhesive resin cements and adhesive primers, metal and alloy primers, and carboxylic acid primers used in dental restoration.36, 37 Phosphate esters can bond directly to non-silica-based ceramics such as zirconia.38 It has been reported that using this phosphate ester can enhance the hydrolytic stability of bonding more than using silane coupling agents can.39 Themainproblemofresincompositesbondedtosil- ica-coatedrestorativematerialswiththeapplicationof commercial silane coupling agents is the bond degra- dationovertimeunderartificialageing.40,41 Inorderto increase the hydrolytic stability of the bonding at the interfacial layer, novel surface treatments of restora- tivematerialsandthedesignofnovelsilanemonomers can solve this problem. Silane coupling agents with long hydrocarbon chains are more hydrophobic than those with short hydrocarbon chains. The bonding at theinterfaciallayerismoreresistancetowaterageing. Thesetwoapproachescouldresolvetheproblem. It could be said that silane coupling agents can fulfil the clinical requirements for dental restora- tions.Nowadays,astandardlaboratoryprotocolfor dental restorations entails surface conditioning of dental materials, silanisation and cementation. The problem of hydrolytic stability of the siloxane link- age formed from silane coupling agents with resin composites and dental restorative materials is cur- rently being addressed. It is not an exaggeration to claim that silane coupling agents have wide appli- cation in industry, dentistry and medicine and will play an important role in biomaterials science._ This review is based on the article “Aspects of silane coupling agents and surface conditioning in dentistry: An overview”, Dental Materials, 28 (2012): 467–77. Acompletelistofreferencesisavailablefromthepublisher. Dr Christie Lung Ying Kei is a postdoctoral fellow in Dental Materials Science at the Faculty of Dentistry at the University of Hong Kong. He can be contacted at cyklung@hku.hk. _about the author cosmeticdentistry