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Fibre-reinforced Composites In Minimal Invasive Prosthodontics Pekka K. Vallittu

Introduction Modern conservative dental treatment prefers minimal invasive options rather than relatively destructive treatments used in the time of amalgam and gold alloys. Minimal invasive treatments became possible in restorative dentistry by the development of enamel and dentine bonding techniques, and by the improvement of restorative composites and ceramics. However, inadequate mechanical properties, namely flexural and fatigue strength of restorative composites and ceramics have limited their use in fixed prosthodontics. A group of materials whose mechanical properties can be tailored to specific needs is fiber-reinforced composites (FRCs). FRCs have been used in removable prosthododontics, fixed partial dentures (FPD), root canal posts, periodontal splints and in orthodontic treatment as a retention splint. By the use of FRCs, fixed partial dentures can today be made with minimal invasive technique, which means that combinations of various kinds of the retentive (adhesive) elements of the bridge could be incorporated to the bridge (see Fig 2). In practice, e.g. a FRC inlay bridge can be combined to Maryland type of surface retained bridges, to full coverage crowns or to onlays. The use of minimal invasive prostho-

dontics allows treatment of the patient by means of Dynamic Treatment Approach. In the Dynamic Treatment Approach odontological and subjective needs of the patient can better be taken into consideration than in the conventional fixed prosthodontics. Also, the use of FRCs in repairs of old fixed partial dentures has become possible. “Repair rather than renewal” could be considered as one of the principles of the Dynamic Treatment Approach. Structure and properties of FRC FRC consists of reinforcing fibers embedded in a polymer matrix (Fig 1). Rigidity and strength of the construction made from FRC are dependent on the polymer matrix of the FRC and the type of fiber reinforcement. Generally, the factors influencing the properties of FRC are: • Properties of fibres vs. properties of matrix polymer • Impregnation of fibres with resin • Adhesion of fibres to matrix • Quantity fibres • Direction (orientation) of fibres • Location of fibres in construction In dental appliances of relatively small size, the quality of FRC is of a great importance. From this perspective, all of the

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FIBRE-REINFORCED COMPOSITES IN MINIMAL INVASIVE PROSTHODONTICS

Finland) (Fig. 2). This method allowed the use of light-curing dimethacrylate resin system in combination with polymers of denture base acrylics. The advantages of the polymermonomer gel preimpregnation of glass fibers are:

Fig 1 Micrograph of the cross-section of continuous unidirectional FRC. Light grey areas are fibers (thickness of 15 micrometers) which are embedded in a polymer matrix.

aforementioned factors which influence the properties of FRC must be carefully taken into consideration for the successful clinical use of FRC. This is especially important because the masticatory system produces cyclic loads to the dental appliances. Therefore, not only adequate static strength of the appliance, but also adequate dynamic strength is needed. At the same time, it should be noticed that dental appliances are multiphasic in nature. The FRC bridge framework is veneered with particulate filler composite and the FRC is luted on place by composite resin luting cements. All of these require adequate interfacial adhesion between the FRC and another resin material. One of the fundamental requirements for the successful clinical use of FRCs is optimal impregnation of fibers with the matrix resin. A promising method to preimpregnate the fibers is based on the use of polymer-monomer gel which was introduced by the Stick Tech Ltd (Turku,

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• Improved handling characteristics of the FRC material due to the reduced “memory effect” of the straight form of the glass fibres, and due to elimination of fraying of fibres in handling. • High quantity of reinforcing fibres in the resin matrix resulting in high strength FRC. • Semi-IPN structure of the polymer matrix improves adhesion behaviour of the material after being polymerized. The static strength (flexural strength) of the FRC is linearly dependent on the fiber quantity to the level of approximately 70 vol%. The use of polymer-monomer gel preimpregnation method produces FRC material with high fiber quantity and high flexural properties (with E-glass ad 1250 MPa). Adhesion of fibers to the polymer matrix Adhesion of veneering composite resin to the FRC framework of the bridge plays an important role for the longevity of the FRC restoration. Internal adhesion of the FRC influencing “the cohesive” strength of the FRC is based on bonding the fibers to the matrix polymer. In this respect, the most suitable fibers are glass and silica fibres which can be silanated for the adequate adhesion to the polymer matrix. Less suit-


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FIBRE-REINFORCED COMPOSITES IN MINIMAL INVASIVE PROSTHODONTICS

a

b

Fig 2 Laboratory-made FRC bridges. a Inlay bridges replacing one missing premolar: one with inlay adhesive elements only and another having FRC bonding wings to increase the bonding surface area to enamel. b A six unit FRC bridge having full coverage crowns, onlays and a glass FRC root canal post as adhesive elements. The the continuous fibers of the FRC root canal post is part of the FRC framework of the bridge.

able fibers are ultra-high molecular weight polyethylene fibres (UHMWP, e.g. Ribbond, Connect) which have been proved to be difficult to adhere to resins even though the fiber surface has been treated with e.g. various types of plasma treatments. Bonding of veneering and luting composite resins Some studies suggest that problems can occur in adhering the veneering composite resin to the FRC framework of Vectris material (Ivoclar, Liechtenstein). It is likely that the adhesional problems were related to the highly cross-linked polymer matrix of the Vectris FRC material. Attempts were made to use silane coupling agents to improve the adhesion. This, however, could improve adhesion of the new resin to the glass fibre surface only. Similar, and well known adhesional problems occur in repairs of aged restorative filling compos-

ites. The adhesional problems can be overcome by retaining the oxygen inhibition layer intact on the FRC surface before application of the veneering composite. This approach is used in FibreKor (Jeneric Pentron, CT, USA) system and in Stick (Stick Tech, Turku, Finland) system when the FRC framework is veneered with the composite resin. Unfortunately, the use of the oxygen inhibition layer cannot be used in bonding surfaces of indirectly made (laboratory made) FRC bridges because they are polymerized into the final degree of conversion in the dental laboratory. Dental laboratory treatments inactivate the oxygen inhibition layer. For this reason the adhesion of many FRC bridges to the composite luting cements can be inadequate. This might limit the use of some FRC materials in surface retained bridges whose function is based on durable bond of composite resin luting cements to the etched enamel and to the FRC framework of the bridge.

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FIBRE-REINFORCED COMPOSITES IN MINIMAL INVASIVE PROSTHODONTICS

means of IPN bonding. The basis for this is in the composition of the polymer matrix of Stick and everStick FRC. Stick and everStick fibre reinforcements are composed of silanated glass fibres in linear polymethyl methacrylate (PMMA) matrix containing also cross-linked polymer. The linear PMMA phases could be dissolved by monomers of the new resin and be used as basis for the IPN bonding. Fig 3 Polymer-monomer gel preimpregnated glass fibers (everStick) give easy handling properties for the fiber reinforcement.

Another approach to adhering new resinous material to the already polymerised FRC is based on interdiffusion of monomers of the new resin into the polymer structure of substrate. This phenomenon, called an interpenetrating polymer networks (IPN), has successfully been used in prosthodontics for decades. Repair of fractured pieces of removable dentures is based on adhering the repair resin to the pieces of dentures by IPN method. Formation of IPN layer between the “old” polymer and the new resin is possible to obtain if: • polymer substrate (“old” polymer) is totally or partially linear polymer (not cross-linked) • monomers of the new resin can dissolve the polymer of the substrate, i.e. the monomers can diffuse into the spaces between linear polymer chains. The semi-IPN structure of Stick and everStick FRC allowed new composite resin to adhere to the FRC surface by

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Adhesion of oral microbes to FRC In fixed bridges made of FRC, the fibers are usually covered with the matrix polymer or veneering composite. In some areas fibers may become exposed and offer a bonding surface for oral microbes. During last years, an intensive research has been focused on the phenomenon of oral microbes’ adhesion to the dental FRCs by Tanner et al. At the moment, it could be concluded that adhesion of bacteria to glass fiber composite and carbon/graphite fiber composite are quite similar to those of restorative filling composites. Polyethylene fiber composites (Connect, Ribbond) seemed to have much higher surface roughness than glass and carbon/graphite fiber composites. Therefore, polyethylene fiber composites adhere significantly more S. mutans bacteria. Luting technique for laboratory-made Stick bridges A FRC bridge made of Stick of everStick glass fiber reinforcements can be luted on place by all composite resin luting cements. The step-by-step procedure to lute a FRC bridge is the following:


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FIBRE-REINFORCED COMPOSITES IN MINIMAL INVASIVE PROSTHODONTICS

• Cleaning the enamel surface with pumice. • Etching the enamel with wide marginal (37 % phosphoric acid for 30 sec). • Roughening the bonding surface of the FPD with silicon carbide stone or by micro etching sandblaster. • Applying the adhesive resin to the bonding surfaces and protecting the bridge from light for 3–5 minutes. This could be made by covering the bridge e.g. with a metal cup. • Applying adhesives on the enamel surface. • Luting the bridge with dual-curing or light-curing composite luting cement. • Finishing the cementation and adjusting the occlusion. Special care should be taken to eliminate cutting of the fibers during finishing the interdental cleaning space of the bridge. The strength of the bridge is dependent on the continuous intact fibers starting from the abutment and going to the connector, pontic, connector, and to another abutment. Example of a directly made FRC bridge A 35-years-old woman was missing her first premolar and she was treated with a minimal invasive direct FRC bridge (Fig. 4 a - g). The space between upper and lower teeth in centric occlusion allowed positioning everStick fibers without tooth preparation. The enamel surface was cleaned with pumice and etched from the region of reinforcing fibers. Adhesives and small amount of flowable restorative composite were applied on the enamel. A piece of

everStick fiber was positioned on the buccal cusp of the second premolar and palatal surface of the canine and pressed against the teeth with Refix silicone instrument. The everStick was initially light-cured through the instrument. An additional fiber was pressed from the labial surface of canine to distal direction. The everStick framework was veneered with restorative composite and characterized with lightcuring composite paints (e.g. Tetric Color, Vivadent). The occlusion was adjusted and the surface was finished. Concluding remarks The development of the FRC materials have been rapid during the last few years. The high quality dental FRCs fulfills the strength requirement of the masticatory system if the fiber framework design of the FRC bridge is correct. The successful use of FRCs necessarily requires education of dentists and dental technicians. The educated dental professionals can achieve excellent treatment outcomes with the FRC technology. One of the most important factors is the possibility to save healthy tooth substance of the patient and to reduce the treatment costs. The FRC technology seems to the able to fulfil the increased demands of the patients in terms of esthetics and minimal invasiveness.

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a

b

c

d

e

f Fig 4 A clinical example to replace one missing premolar with directly made FRC bridge. For explanation of a to g, see text.

g

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Selected literature of FRCs Behr M, Rosentritt M, Lang R, Handel G. Flexural properties of fiber reinforced composite using a vacuum/pressure or a manual adapatation manufacturing process. J Dent 2000;28:509-514. Ekstrand K, Ruyter IE, Wellendorf H. Carbon/graphite fiber reinforced polymethylmethacrylate: properties uder dry and wet conditions. J Biomed Mater Res 1987;21:1065-1080. Freilich MA, Karmarker AC, Burstone CJ, Goldberg AL. Development and clinical applications of lightpolymerized fiber reinforced composite. J Prosthet Dent 1998;80:311-318. Kallio TT, Lastumäki TM, Vallittu PK. Bonding of restorative and veneering composite resin to some polymeric composites. Dent Mater 2001;17:80-86. Körber HK, Körber S. Experimentelle Untersuchungen zur Passgenauigkeit con GFK-Bruckengerusten “Vectris”. Quintessenz Zahntech 1998;24:43-53. Lassila LVJ, Nohrström T, Vallittu PK. The influence of short-term water storage on the flexural properties of unidirectional glass fiber-reinforced composite. Biomaterials 2002;23:2221-2229. Miettinen VM, Vallittu PK, Forss H. Release of fluoride from glass fibre reinforced composite with multiphase polymer matrix. J Mater Mater Med 2001;12:503-505. Narva K, Vallittu PK, Yli-Urpo A. Clinical survey of acrylic resin removable denture repairs with glass fiber reinforcement. Int J Prosthodont 2001;14:219-224. Rosentritt M, Behr M, Kolbeck C, Handel G. In vitro repair of three-unit fiber-reinforced composite FPDs. Int J Prosthodont 2001;14:344-349. Ruyter IE, Ekstrand K, Björk N. Development of carbon/graphite fiber reinforced polymethyl methacrylate suitable for implant fixed dental bridges. Dent Mater 1986;2:6-9. Takagi K, Fujimatsu H, Usami H, Ogasawara S. Adhesion between high strength and high modulus polyethylene fibers by use of polyethylene gel as an adhesive. J Adhesion Sci Technol 1996;9:869-882. Tanner J, Vallittu PK, Söderling E. Adherence of streptococcus mutans to an E-glass fiber-reinforced composite and conventional restorative materials used in prosthetic dentistry. J Biomed Mater Res 2000;29:250-256. Tanner J, Vallittu PK, Söderling E. Effect of water storage of E-glass fiber reinforced composite on adhesion of Streptococcus mutans. Biomaterials 2001;22:1613-1618. Vallittu PK. The effect of glass fiber reinforcement on the fracture resistance of a provisional fixed partial denture. J Prosthet Dent 1998;79:125-130. Vallittu PK. Glass fiber reinforcement in repaired acrylic resin removable dentures: preliminary results of a clinical study. Quintessence Int 1997;28:39-44. Vallittu PK. Flexural properties of acrylic polymers rein-

forced with unidirectional and woven glass fibers. J Prosthet Dent 1999;81:318-326. Vallittu PK, Ruyter IE, Ekstrand K. Effect of water storage on the flexural properties of E- glass and silica fiber acrylic resin composite. Int J Prosthodont 1998;11:340-350. Vallittu PK. Effect of 180 weeks water storage on the flexural properties of E-glass and silica fiber acrylic resin composite. Int J Prosthodont 2000;13:334339. Vallittu PK. Ultra-high-modulus polyethylene ribbon as reinforcement for denture polymethyl methcarylate. A short communication. Dent Mater 1997:13:381-382. Vallittu PK, Sevelius C. Resin-bonded, glass fiber-reinforced composite fixed partial dentures: A clinical study. J Prosthet Dent 2000;84:413-418. Waltimo T, Tanner J, Vallittu PK, Haapasalo M. Adherence of Candida albicans to the surface of polymethyl methacrylate – E-glass fiber composite used in dentures.. Int J Prosthodont 1999;12:8386.

Dr. Pekka Vallittu He has obtained certified dental technician degree in 1988 and studied dentistry at the University of Kuopio where he graduated in 1994 and completed his PhD degree in 1994. Docentur in prosthodontic materials science received from the University of Turku 1995 and became a specialist in prosthodontics in 2000. Post doctoral education was made at NIOM (Scandinavian Institute of Dental Materials) in Norway during 1995 – 1997. Current position is at the Department of Prosthtetic Dentistry and Biomaterials Research, Institute of Dentistry, University of Turku, Turku, Finland.

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