PhD Thesis

UNIVERSITY OF SIENA School of Dental Medicine

PhD PROGRAM: “DENTAL

MATERIALS

AND

THEIR

CLINICAL

APPLICATIONS”

PhD THESIS OF: Federica Papacchini_________________________________________

TITLE A study into the materials and techniques for improving the composite-repair bond.

Academic Year 2006/07

December 2006 Siena, Italy Committee: Promoter Prof. Marco Ferrari Co-Promoter Prof. Antonella Polimeni

TITLE: A study into the materials and techniques for improving the composite-repair bond.

CANDIDATE Federica Papacchini

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CONTENTS Chapter 1 1.1 General Introduction…………………………………………………..6 1.2 Resin composite repair as a minimally invasive treatment……………8 1.3 Longevity of direct composite restorations…………………………..11 1.4 Reasons of failure of direct resin composite restorations……………12 1.5 Repair versus replacement: making a clinical choice………………..15 1.6 Basic formulation of resin composites……………………………….16 1.7 Factors affecting the composite repair strength……………………...17 References………………………………………………………………..20 Chapter 2. 2.1. The properties of the composite surface as a bonding substrate: physical and chemical factors affecting repair strength………………….29 References………………………………………………………………..31 2.2. Effect of oxygen inhibition on composite repair strength over time.……………………………………………………………………...33 References………………………………………………………………..47 2.3. Composite to composite microtensile bond strength in the repair of a micro-filled hybrid resin: effect of surface treatment and oxygen inhibition…………………………………………………………………50 References………………………………………………………………..67 2.4 A study into the application of hydrogen peroxide in composite repair……………………………………………………………………..71 References………………………………………………………………..87

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Chapter 3. 3.1.Optimising

the

application

mode

of

traditional

silane-based

intermediate agents in composite-to-composite bond……………………92 References………………………………………………………………..94 3.2. Effect of air-drying temperature on the effectiveness of silane primers and

coupling

blends

in

the

repair

of

a

micro-hybrid

resin

composite………………………………………………………………...95 References………………………………………………………………110 Chapter 4. 4.1 Exploring alternative resin-based materials and procedures for composite repair………………………………………………………...116 References………………………………………………………………118 4.2 Flowable composites as intermediate agents without adhesive application in resin composite repair: a bond strength and SEM study…………………………………………………………………….119 References………………………………………………………………137 4.3 Effect of intermediate agents and pre-heating of repairing resin on composite repair bond strength…………………………………………143 References………………………………………………………………161 Chapter 5 5.1. Comparing intermediate agents after challenging conditions……...166 References………………………………………………………………168 5.2. Effect of thermocycling on composite-to-composite bond mediated by different coupling agents………………………………………………..170 References………………………………………………………………187

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Chapter 6 6.1 Summary……………………………………………………………191 6.2 Conclusions and Future directions………………………………….195 6.3 Riassunto, conclusioni e direzioni future……………………………..X 6.4 Resumé, discussion, conclusions and directions futures……………...X 6.5 Resumen, discusion, conclusiones y direcciones futuras……………..X 6.6 Zusammenfassung, allgemeine Abhandlung, Schlussfolgerungen und zukünftige, Ausrichtung……………………………………………….….X References………………………………………………………………...X Complete list of References………………………………………………X Curriculum Vitae………………………………………………………….X Acknowledgments…………………………………………………….…..X

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Chapter 1 1.1 General Introduction The use of direct resin-based composite materials has become an active part of contemporary Operative Dentistry. The esthetic appearance associated with conservative cavity preparations and the constantly improved properties have made these materials the main choice for all classes of restorations (Roulet et al., 2001). However, resin composites in common with the majority of dental materials, undergo deterioration and degradation in the intraoral environment (Söderholm et al., 1984) (Finer and Santerre, 2004). Being technique-sensitive materials, failure at the tooth-restoration interface may also occur (Roulet, 1997). As a result, managing of failed restorations is a common problem encountered in daily practice. For years, the traditional behavior consisted in remaking the entire restoration, also in presence of minor imperfections. In recent times, with more insight into cariology and dental materials science, a minimally invasive operative philosophy has prevailed and the advantages of repairing rather than replacing composite restorations have been increasingly emphasized (Tyas et al., 2000) (Mjör and Gordan, 2002). Repair of fractured, worn and discolored restorations is a simple procedure consisting in the addition of a fresh resin composite over the existing material (Boyer, 1978). Despite the repair of a partially lost resinbased fissure sealant is a well-accepted technique (Simonsen, 1982) (Simonsen, 2002), the same concept with failed resin-based composite restorations is so far not well-recognized (Mjör and Gordan, 2002). Besides the clinical doubt of leaving secondary caries underneath a repaired restoration, the possibility of achieving a reliable composite-to6

composite bond is one of the major concerns related to this conservative procedure (Mjör et al., 2000) (Blum et al., 2003). This thesis contains a study into different aspects related to the repair of aged composite substrates, with the purpose of identifying factors affecting the composite-to-composite adhesion, as well as selecting improved materials and procedures for enhancing coupling potential. Microtensile bond strength test was used to perform mechanical trials, while stereo- and scanning electron microscopy (SEM) provided a mean to assess improvements in failure patterns and interfacial quality. An overview of the literature was provided in order to present the background information existing on composite repair: the longevity of direct resin-based restorations, the reasons of failure as well as the clinical aspects guiding the operative choice between repair and replacement were analyzed and discussed. The potential factors and variables involved in composite-tocomposite bond were investigated (Shahdad et al., 1998). The composite surface as a bonding substrate was evaluated as a first variable. On this topic, three in vitro studies were performed with the purpose of evaluating changes in chemical bonding potential of the aged substrate over time, selecting the best chemical and/or mechanical surface treatment able for providing a reliable micro-mechanical retention and verifying if any new technique may be proposed over the already known procedures to increase repair strength. The choice of the intermediate agent is another factor potentially affecting repair strength (Teixeira et al., 2005). Different techniques are currently used in dental laboratory and industrial applications to maximize the interfacial coupling between biomaterials mediated by silane agents. This is the case of ceramic repairs (Roulet et al., 1995) and resin composite manufacturing (Antonucci et al., 2005). A study was conducted 7

to verify whether a procedure commonly used in extra-oral conditions may be modified for intra-oral use and be effective in chair-side repair procedures. Resinous intermediate agents may vary in chemical composition as well as in physical properties (Labella et al., 1999). Resin-based materials with different chemical and physical properties were compared in this research. A further laboratory study was performed to evaluate if clinically inducible changes in rheological characteristics of the repairing composite may have an influence on interfacial coupling. In the final part of this thesis, water aging and thermocycling were used to test the hydrolytic stability of composite repairs mediated by resinbased, silane-based and combined intermediate agents. 1.2 Resin composite repair as a minimally invasive treatment. Restoring the tooth to a long-term condition of health, function, and esthetic appearance as well as preventing caries recurrence are the goals pursued by each restorative treatment in dentistry (Lutz et al., 1997). An invasive approach to caries management has prevailed in the past decades, and sound tooth structure has been often sacrificed to make up for the limitations of the available operative techniques and filling materials (Black, 1895) (Black, 1908). The synthesis of the first adhesive resin (Hagger, 1948) (McLean, 1952) and the enamel acid-etching (Buonocore et al., 1955) have marked the start of a major revolution in dentistry. Non-invasive pit and fissure sealing (Cueto and Buonocore, 1965) and preventive resin restorations (Simonsen et al., 1971) (Simonsen et al., 1991) have been the earliest precursors of minimally invasive treatments. The increased understanding of the caries process and the advances in adhesive dentistry have promoted 8

a gradual shift in the operative philosophy from the “extension for prevention” proposed by GV Black (Black, 1895) (Black, 1908) toward “prevention of extension”. As a result, the traditional surgical approach to caries lesions has been steadily superseded by a biological approach, focused on the individual caries risk assessment, the disease control, the healing potential of early carious lesions (Mount and Ngo, 2000-535) (Elderton, 2003) and the selective removal of cavitated lesions (Mount and Ngo, 2000-621). These aspects characterize a refined model of care known in daily practice as “Minimal Intervention Dentistry” (Mount and Hume, 1997) (Mount and Ngo, 2000) (Mount and Ngo, 2000-527) (Tyas et al., 2000) (Peters and McLean, 2001). Minimal intervention also provides for a conservative treatment of failed restorations (Staehle et al., 1999) (Tyas et al., 2000) (Peters and McLean, 2001) (Murdoch-Kinch et al., 2003) (Mjör et al., 2003) (Chalmers, 2006). For many years, traditional taught considered as necessary to completely remake restorations not satisfying strict quality requirements. Replacement of failed restorations accounted for about 60% of the operative activity in general dental practice (Mjör, 1989). Laboratory (Krejci et al., 1995) (Gordan, 2000) (Gordan et al., 2002) and clinical (Gordan, 2001) studies have shown that replacing a resinbased composite restoration inevitably increases the size of the new cavity preparation, which may extends to areas remote from the original site of failure. Due to the esthetic quality of resin composites, with shadematching and light-transmitting properties similar to the surrounding dental tissues, the visual and tactile identification of the bonded resintooth interface may be impaired (Gordan et al., 2002). Either over- or under-treatment are likely to occur and finally result, respectively, in unnecessary loss of tooth structure or in incomplete removal of resin remnants from the substrate (Krejci et al., 1995). As resin residues may 9

prejudice a complete demineralization of the substrate, and presumably affect the bonding potential of the new restoration, the cavity preparation is often extended beyond the resin-impregnated, beveled margins at the time of replacement (Krejci et al., 1995) (Gordan et al., 2000) (Gordan et al., 2001). The use of chemical softening agents selectively dissolving the composite resin matrix has been proposed without success (Cruickshank and Chadwick, 1998). Conversely, the optical contrast to the tooth substance and the purely mechanical retention of amalgam restorations, make their replacement more conservative as compared to the removal of failed resin restorations (Hunter et al., 1995) (Mjör et al., 1998). There is consensus in dental literature that replacement of resin composites is a technically-demanding and time-consuming procedure, likely to result in weakening of the tooth and renewed insult to the pulp tissue (Krejci et al., 1995) (Tyas et al., 2000) (Mjör and Gordan, 2002). The re-restoration cycle may even lead to tooth loss (Simonsen, 1991-25). Considering these concerns, the repair of an existing restoration may be conceived as a viable and minimally invasive alternative to total replacement, providing that the repaired restoration is clinically acceptable.

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1.3 Longevity of direct composite restoration In the Dictionary of Dentistry, Jablonski (1992) described a permanent restoration as “a restoration designed to remain in service for not less than 20 to 30 years…”. Based on this definition, a resin composite restoration cannot be regarded as permanent, on account of significantly lower survival times reported in the literature. The need of placing effective long-lasting restorations has stimulated a continuous research in the field of adhesive resins, with the aim of achieving a reliable adhesion of the composite restoration to enamel and dentin. The long-term survival of direct composite restorations has interested a great number of researchers in the last decades. In spite of the wide variation in longevity reported, median survival time has been calculated to range between 6 and 8 years, and it has been evaluated by retrospective (Redman et al., 2003) (Forss and Widstrom, 2004) (Opdam et al., 2004) (Opdam et al., 2006) and prospective (Van Dijken, 2001) (Van Nieuwenhuysen et al., 2003) (Van Dijken and Sunnegardh-Gronberg, 2005) (Kramer et al., 2005) studies. Relevant reviews on this issue have been published (Downer et al., 1999) (Hickel and Manhart, 2001) (Brunthaler et al., 2003) (Manhart et al., 2004) (Sarrett, 2005). Patient-, dentist-, and material-related factors seem to affect the life span of bonded restorations (Hickel and Manhart, 2001). A direct comparison of the longevity of resin restorations is difficult if not impossible to be performed, owing to the differences in study designs, materials on trial and observation periods (Manhart et al., 2005). Details of technical procedures are not always exhaustive. Many studies on longevity of composite restorations are retrospective in nature. Retrospective cross-sectional studies differ from controlled longitudinal investigations in which selected clinicians treat selected 11

patients under almost ideal conditions for the materials investigated (Hickel and Manhart, 2001) (Opdam et al., 2006). Most of the existing information on composite restoration quality from prospective studies is based on a method of clinical evaluation commonly known as USPHS/Ryge system (Cvar and Ryge, 1971) (Ryge and Snyder, 1972). The evaluation parameters include colour-matching ability, marginal adaptation, loss of anatomic form, marginal discoloration, recurrent caries, post-operative sensitivity and retention rate. For each of the criteria, the degrees of clinical acceptability are further scored. As this method requires an accurate calibration of the examiners (Ryge et al., 1981), several researchers (Burke et al., 2001) (Hickel and Manhart, 2001) (Mjör, 2000) (Mjör, 2002) (Manhart et al., 2004) pointed out its difficult applicability in daily practice, where clinicians are not calibrated. Therefore, the clinical assessment of restoration failure is likely to differ according to the diagnostic criteria used and to reflect the subjective interpretative variability of different operators (Hickel and Manhart, 2001). Although less strictly defined than longitudinal studies, cross-sectional practice-based studies may be regarded as a source from which clinically relevant problems can come into view (Mjör, 2004). 1.4 Reasons of failure of direct resin composite restorations. Insufficient wear resistance leading to loss of anatomic form and interproximal contacts in addition to general deterioration of the restorative material were the main problems related to direct resin-based restorations up to the early ‘80s (Mjör, 1981). Improvements in filler technology and new resin formulations allowed to minimize wear-related failures and a

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change in the reasons for restoration replacement has progressively occurred. According to Hickel (2001), a basic clinical distinction must be made between early and late failures. The early failures occur within days, weeks or months, owing to severe dentist-related treatment faults. Improper handling of the material, incorrect shade selection in aestheticdemanding areas, insufficient resin polymerization, deficiencies in the operative sequences or postoperative symptoms may require an early reintervention. As regards late failures, clinical data indicated that secondary caries, bulk/marginal fractures, bulk/marginal discoloration and tooth fracture are the most frequent reasons of failure of resin-based composites, usually experienced after some years of clinical service (Mjör et al., 2000) (Burke et al., 2001) (Van Nieuwenhuysen et al., 2003) (Opdam et al., 2004) (Manhart et al., 2004). Failure may involve the entire restoration or may be localized and limited in extent: in any case, a complex decision-making process is necessary (Ettinger, 1990). 1.5. Repair versus replacement: making a clinical choice A recent literature review of practice-based studies (Sarrett, 2005) has indicated that secondary caries is the primary reason given for replacement of composite restorations, accounting for 30-60% of all the operative reinterventions. These findings are similar to those reported in a previous review of prospective clinical studies (Brunthaler et al., 2003), even though significantly lower failure rates related to secondary caries were recorded. Several Authors argued that this difference is likely to reflect a poor sensitivity and specificity for the evaluation of secondary caries in 13

clinical settings (Tyas et al., 2000) (Hickel and Manhart, 2001) (Sarrett, 2005). Because of the importance traditionally attributed to microleakage for the occurrence of secondary caries (Kidd, 1976), stains at the margins of tooth-coloured restorations are prone to be misdiagnosed as recurrent carious lesions (Mjör, 2002), leading to preventive replacement of the restoration. However, a correlation between the width of a marginal discrepancy and the presence of recurrent caries only exists when frankly cavitated lesions are detected at the restoration margins (Frencken et al., 1995) (Mjör and Toffenetti, 2000). As secondary carious lesions are known to be localised and delineated defects, a reconsideration of the conventional treatment approach has been recently recommended. In deciding whether to repair or to replace a defective restoration, a “minimal treatment” should be preferred. Simple re-contouring and re-polishing of small marginal defects should be performed as a first option (Mjör, 2002), mainly in patients with a low caries-risk status (Tyas et al., 2000). Conversely, if any clinical doubt exists in areas prone to plaque accumulation, in presence of larger defects and higher caries risk, an exploratory preparation into the composite material at the tooth/resin composite interface may help in diagnosing the existence and the size of the lesion (Mjör, 2002) (Mount et al., 2006). Being localized in nature, it rarely progress along the tooth/resin composite interface (Mjör and Toffenetti, 2000). When sound tooth tissue is exposed, the exploratory cavity may be repaired using a conventional restorative technique (Gordan et al., 2004). In the same way, non-carious, degraded or ditched margins may be successfully restored by re-finishing and re-polishing methods (Mjör, 2005). Based on the same concepts, no replacement of any restoration with bulk discoloration in aesthetic areas should be planned without first 14

evaluating that the unsatisfactory appearance can be treated and improved by resurfacing/veneering or refurbishing procedures. Similarly, clinical reports showed that bulk fractures limited to the composite material may be repaired by bonding a new resin composite to the old restoration (Mjör, 2002). Despite composite repair is a conservative option ethically and theoretically valid, more and more accepted and practised (Gordan et al., 2003) (Blum et al., 2005), little objective evidence is available on the increased longevity of repaired and refurbished restorations. While the first studies on composite repair procedures dates back to the late ‘70s (Inoue, 1978), only in recent times composite repair has received a greater attention. This delay probably explains the existing lack of information. Promising data have been recently reported in a 2-year prospective clinical study on repaired restorations (Gordan et al., 2006). Scientifically rigorous research protocols on this topic have been promoted by The Cochrane Collaboration and are currently in progress (Catleugh et al., 2006). Therefore, it has been suggested that in the temporary absence of evidence-based guidelines, the clinical choice of repair rather than replacement must be based on the individual caries-risk status assessment, the professional evaluation of benefits versus risks, and the conservative principles of cavity preparation (Tyas et al., 2000) (Murdoch-Kinch and McLean, 2003) (Jacobsen, 2004).

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1.6. Basic formulation of resin composites By definition a composite is a mixture of two or more materials with properties superior to any one constituent. A dental resin composite presents an organic resin matrix and an inorganic filler component bonded to each other through a silane coupling agent (Phillips, 1982). The resin matrix is the chemically active part of the resin composite, based on organic difunctional monomers, such as Bis-GMA (Bisphenol Adiglycidylmethacrylate) and UDMA (Urethane dimethacrylate). Other monomers, for instance TEGDMA (Triethylene glycol dimethacrylate), may be added in various concentrations as resin diluents to lower viscosity. Depending on the curing mode, inhibitors, as well as activator/initiator systems may be present (van Noort, 2002). The fillers used in resin composite formulations include a variety of materials, namely quartz, silica and glasses made of lithium, barium, strontium etc. Addition of fillers particles to the resin matrix improves the overall physical properties of the resin composite: as a general rule, the higher the loading, the higher the strength of the cured resin-based restoration. (Fortin and Vargas, 2000). Different classifications have been proposed for resin-based composites, mainly based on fillers size and content (macrofilled, microfilled, small-particle, hybrid composites) (van Noort, 2002). At the present, resin composites commonly used in dental practice are hybrid materials, containing both macrofillers and microfillers to improve mechanical and esthetical properties. A mixture of small particles (0.5-3.0 µm) is present. In the last years, composites with even smaller particles size have been introduced, with the purpose of increasing polishability and esthetical result: this is the case of microhybrid (fillers: 0.04-0.7 µm) and nanohybrid resin composites (fillers:20-75 nm).

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The role of silane as interfacial coupling agent will be discussed in Chapter 3. 1.7. Factors affecting the composite repair strength. Repair of fractured and/or worn restorations and resurfacing of discolored restorations are achieved by layering a fresh resin composite over the existing material (Inoue, 1978). The coupling effectiveness between the resin composite substrate and the repairing resin represents one of the major concerns raised in clinical practice when the repair option is preferred (Gordan et al., 2003). Indeed, the success of this procedure relies on interfacial coupling and long-term retention between the composite surfaces involved in the repair. At the present, the minimum bond strength for retention of a composite repair in the intraoral environment is not known. The individual maximum bite force capability and the estimated biting force on specific teeth may reduce the success rate (Özcan, 2003). Flaws within the materials or at the bonded interface, in the form of voids, phase separations or non-uniform film thicknesses, may also represent stress concentration and crack propagation points (Pashley et al., 1995). The bond strength between increments of composite is known to be equal to the cohesive strength of the material (Lloyd et al., 1980). However, if the composite surface has been contaminated (Eiriksson et al., 2004) polished (Boyer et al., 1978) (Shahdad and Kennedy, 1998) or aged (Boyer et al., 1984) (Mitsaki-Matsou et al., 1991), direct bonding of a fresh resin may result significantly impaired.

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Moreover, the cohesive strength of resin-based materials varies as a function of time (Figure 1).

24 hours 1 week 1 month 6 months

100

[MPa]

75

50

25

0 H-XRV

QAS

P4

GRA

DUR

Figure 1. Cohesive strength of different resin composites measured using the trimming technique of the microtensile bond strength test after different storage periods in water (unpublished results). 1 Several chemical and mechanical methods have been studied in an attempt to find an ideal surface treatment for composite repair procedures (Kupiec and Barkmeier, 1996) (Shahdad and Kennedy, 1998) (Trajtenberg and Powers, 2004) (Bonstein et al., 2005). The use of low-viscosity adhesion promoters, such as silanes and/or adhesive resins has been regarded as a further variable affecting composite-to-composite interfacial coupling (Matsumura et al., 1995).

1

H-XRV: Herculite XRV; Kerr, Orange, CA, USA – hybrid resin composite; QAS: Quadrant Anterior Shine, Cavex Holland BV, Haarlem, The Netherlands – microhybrid resin composite; P4: Point 4; Kerr Co., Orange, CA, USA; GRA: Gradia Direct, GC Corp., Tokyo, Japan – micro-filled hybrid resin composite; DUR: Durafill, Heraeus Kulzer, Hanau, Germany – microfilled resin composite.

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Since the brand of pre-existent resin is often unknown when a failed restoration is being repaired, the chemical compatibility between materials differing in organic matrix and polymerization method has also received some attention (Kao et al., 1988) (Gregory et al., 1990) (Mitsaki-Matsou et al., 1991) (Sau et al., 1999). The time elapsed after repair is also considered a factor that may have an effect on the composite-to-composite bond: besides water, the salivary enzymatic activity has shown to accelerate the biodegradation of composite resins and interfacial couplings (Finer and Santerre, 2004). It is evident that multiple variables may play a role in the bond at the repair site: various combinations of these factors may exert a different influence on the repair strength. Conflicting results have often been produced when evaluating these variables: this lack of univocal information is probably one of the possible explanations for the limited taught and the wide diversity in composite repair procedures documented in many European and American dental schools curricula (Blum et al., 2003) (Gordan et al., 2003).

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H,

Karanika-Kouma

A,

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Y,

Theodoridou-Pahine S. An in vitro study of the tensile strength of composite resins repaired with the same or another composite resin. Quintessence Int 1991;22:475-481. Mjör IA. Placement and replacement of restorations. Oper Dent 1981;6:49-54. Mjör IA. Amalgam and composite resin restorations: longevity and reasons for replacement. In: Anusavice K. Quality evaluation of dental restorations. Chicago: Quintessence, 1989;61-80. Mjör IA, Reep RL, Kubilis PS, Mondragon BE. The change in size of replaced amalgam restoration: a methodological study. Oper Dent 1998;23:272-277. Mjör IA, Moorhead JE, Dahl JE. Reasons for replacement of restorations in permanent teeth in general dental practice. Int Dent J 2000;50:361-366. Mjör IA, Toffenetti F. Secondary caries: a literature review with case reports. Quintessence Int 2000;31:165-179. Mjör IA, Gordan VV. Failure, repair, refurbishing and longevity of restorations. Oper Dent 2002;27:528-534. Mjör IA. A recurring problem: research in restorative dentistry. J Dent Res 2004;83:92.

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Mjör IA. Clinical diagnosis of recurrent caries. J Am Dent Ass 2005;136:1426-1433. Mount GJ, Hume WR. A revised classification of carious lesions by site and size. Quintessence Int 1997;28:301-303. Mount GJ. Classification for minimal intervention. Quintessence Int 2000;31:375-376. Mount GJ, Ngo H. Minimal intervention: a new concept in operative dentistry. Quintessence Int 2000;31:527-533. Mount GJ, Ngo H. Minimal intervention: early lesions. Quintessence Int 2000;31:535-546. Mount GJ, Ngo H. Minimal intervention: advanced lesions. Quintessence Int 2000;31:621-629. Mount GJ, Tyas JM, Duke ES, Hume WR, Lasfargues JJ, Kaleka R. A proposal for a new classification of lesions of exposed tooth surfaces. Int Dent J 2006;56:82-91. Murdoch-Kinch CA, McLean ME. Minimally invasive dentistry. J Am Dent Ass 2003;134:87-95. Opdam NJ, Bronkhorst EM, Roeters JM, Loomans BA. A retrospective clinical study on longevity of posterior composite and amalgam restorations. Dent Mater, in press. Opdam NJ, Loomans BA, Roeters FJ, Bronkhorst EM. Five-year clinical performance of posterior resin composite restorations placed by dental students. J Dent 2004;32:379-383. Özcan M. Adhesion of resin composites to biomaterials in dentistry: an evaluation of surface conditioning methods. PhD Thesis, University of Groningen, 2003. Özcan M, Alander P, Vallittu PK, Huysmans MC, Kalk W. Effect of three surface conditioning methods to improve bond strength of particulate filler resin composites. J Mater Sci Mater Med 2005;16:21-27. 25

Pashley DH, Sano H, Ciucchi B, Yoshiama M, Carvalho R. Adhesion testing of denting bonding agents: A review. Dent Mater 1995;11:117-125. Peters MC, McLean ME. Minimally invasive operative care. I. Minimal intervention and concepts for minimally invasive cavity preparations. J Adhes Dent 2001;3:7-16. Phillips RW. Science of dental materials. 8th ed. Philadelphia: Saunders; 1982:224. Redman CDJ, Hemmings KW, Good JA. The survival and clinical performance of resin-based composite restorations used to treat localized anterior tooth wear. Br Dent J 2003;194:566-572. Roulet JF, Söderholm KJ, Longmate J. Effects of treatment and storage conditions on ceramic/composite bond strength. J Dent Res 1995;74:381-387. Roulet JF. Benefits and disadvantages of tooth-coloured alternatives to amalgam. J Dent 1997;25:459-473. Roulet JF, Wilson NHF, Fuzzi M. Advances in Operative Dentistry. Volume 1: Contemporary clinical practice. Quintessence Publishing Co Inc, 2001. Ryge G, Snyder MA. Evaluating the clinical quality of restorations. J Am Dent Ass 1972;87:369-377. Ryge G, Jendresen MD, Glantz PD, Mjör IA. Standardization of clinical investigators for studies of restorative materials. Swed Dent J 1981;5:235-239. Sarrett DC. Clinical challenges and the relevance of materials testing for posterior composite restorations. Dent Mater 2005;21:9-20. Shahdad SA, Kennedy JG. Bond strength of repaired anterior composite resins: an in vitro study. J Dent 1998;26:685-694. Simonsen RJ, Stallard RE. Sealant restorations utilizing a diluted filled resin: one year results. Quintessence Int 1977;6:77. 26

Simonsen RJ. Potential uses of pit-and-fissure sealants in innovative ways: a review. J Public Health Dent 1982:42:305-311. Simonsen RJ. New materials on the horizon. J Am Dent Ass 1991;122:25-31. Simonsen RJ. Retention and effectiveness of dental sealant after 15 years. J Am Dent Ass 1991;122:34-42. Simonsen RJ. Pit and fissure sealant: review of the literature. Pediatr Dent 2002;24:393-414. Söderholm KJ, Zigan M, Ragan M, Fischlschweiger W, Bergman M. Hydrolytic degradation of dental composites. J Dent Res 1984;63:12481254. Staehle HJ. Minimally invasive restorative treatment. J Adhes Dent 1999;1:267-284. Teixeira EC, Bayne SC, Thompson JY, Ritter AV, Swift EJ. Shear bond strength of self-etching bonding systems in combination with various composites used for repairing aged composites. J Adhes Dent 2005;7:159164. Trajtenberg CP, Powers JM. Bond strengths of repaired laboratory composites using three surface treatments and three primers. Am J Dent 2004;17:123-126. Tyas MJ, Anusavice KJ, Frencken JE, Mount GJ. Minimal intervention dentistry – a review. Int Dent J 2000;50:1-12. Van Dijken JWV. Durability of new restorative materials in Class III cavities. J Adhes Dent 2001;3:65-70. van Noort R. Introduction to dental materials. 2nd Ed, Mosby: St Louis, 2002. Van Dijken JWV, Sunnegardh-Gronberg K. A four-year clinical evaluation of a highly filled hybrid resin composite in posterior cavities. J Adhes Dent 2005;7:343-349. 27

Van Nieuwenhuysen JP, D’Hoore W, Carvalho J, Qvist J. Long-term evaluation of extensive restorations in permanent teeth. J Dent 2003;31:395-405. Wilson NHF, Cheung SW, Mjör IA. Influence of patient age factors on age of restorations at failure and reasons for their placement and replacement. J Dent 2001;29:317-324.

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Chapter 2 2.1. The properties of the composite surface as a bonding substrate: chemical and physical factors affecting repair strength Surface characteristics of solid materials have been one of the main research fields in mechanical and biomedical engineering over years (Erli et al., 2003) (Caliskan and Karihaloo, 2004). Likewise adhesion of biomaterials to dental substrates, bonding between biomaterials may be achieved through chemical and/or physical surface interactions. Chemical bonding between layers of resin composite relies on copolymerization between new resin monomers and residual unreacted methacrylate groups (Vankerckhoven et al., 1982). In a composite repair scenario, in which the composite surface is presumably aged, the possibility to obtain a dependable co-polymerization with the repairing resin monomers may be compromised, because of reduced availability of unreacted methacrylate groups in the cross-linked matrix of the composite substrate. It has been suggested that the greatest reactivity of the composite surface to the formation of covalent bonds with fresh resin composite can be found during the first 24 h after polymerization (Saunders, 1990), but it tends to decrease with time (Burtscher, 1993). Modification of the composite surface texture by chemical and mechanical methods has been performed in an attempt to promote a composite-to-composite physical interlocking (Caliskan and Karihaloo, 2003). Abrasion with carborundum stones (Brosh et al., 1997), diamond rotary instruments (Bonstein et al., 2005) and airborne particle abrasion with aluminum oxide (Lucena-Martín et al., 2001) have been proposed as reliable methods for preparing the composite surface to be repaired. Other 29

mechanical features, such as small superficial undercuts proved to be not suitable in composite repair, as difficult to be filled completely and resulting in areas of high stress concentration (Shen et al., 2004). Hydrofluoric acid, a well-known etching agent for ceramic substrates (Blatz et al., 2003), has led to contrasting results when applied as a conditioning agent of damaged resin composites (Brosh et al., 1997) (Trajtenberg and Powers, 2004) (Özcan et al., 2005). Unfilled bonding resin as an intermediate agent has been considered a critical component of the repaired joint (Swift et al., 1994) (Brosh et al., 1997) (Kallio et al., 2001) (Lucena-Martín et al., 2001). Three are the possible mechanisms of adhesion attributed to resin-based bonding agents: a) chemical bonding to the composite resin matrix, b) physical/chemical bonding to the exposed filler particles, c) micro-mechanical retention promoted by penetration of the unfilled resin into surface irregulaties (Brosh et al., 1997) (Lucena-Martín et al., 2001). Conversely, in spite of clear benefits showed in composite-ceramic adhesion, the true benefit provided by silane coupling agents in composite-to-composite bond is still equivocal (Swift et al., 1994) (Brosh et al., 1997) (Matsumura et al., 1995) (Tezvergil et al., 2003). In this chapter, it was of interest to verify whether and to what extent a resin restoration may be repaired without modifying the composite surface texture. As a further step, when chemical bonding of the substrate is inadequate and a physical interaction is needed, the best surface treatment for maximizing repair strength was investigated. The possibility to increase micro-mechanical bonding through a recently proposed etching step and adhesive or silane/adhesive coating was also considered.

30

References Blatz MB, Sadan A, Kern M. Resin-ceramic bonding: a review of the literature. J Prosthet Dent 2003;89:268-274. Bonstein T, Garlapo D, Donarummo J Jr, Bush PJ. Evaluation of varied repair protocols applied to aged composite resin. J Adhes Dent 2005;7:41-49. Burtscher P. Stability of radicals in cured composite materials. Dent Mater 1993;9:218-222. Brosh T, Pilo R, Bichacho N, Blutstein R. Effect of combinations of surface treatments and bonding agents on the bond strength of repaired composites. J Prosthet Dent, 1997;77:122-126. Caliskan S, Karihaloo BL. Effect of surface roughness, type and size of model aggregates on the bond strength of aggregate/mortar interface. Interf Sci 2004;12:361-374. Drummond CJ, Chan DYC. Van der Waals interaction, surface free energies and contact angles: dispersive polymers and liquids. Langmuir 1997;13:3890-3895. Erli HJ, Marx R, Paar O, Niethard FU, Weber M, Wirtz DC. Surface pretreatments for medical application of adhesion. Biomed Eng OnLine 2003;2:15. Kallio TT, Lastumaki TM, Vallittu PK. Bonding of restorative and veneering composite resin to some polymeric composites. Dent Mater 2001;17:80-86. Lucena-Martín C, González-López S, Navajas-Rodríguez de Mondelo JM. The effect of various surface treatments and bonding agents on the repaired strength of heat-treated composites. J Prosthet Dent 2001;86:481-488.

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Matsumura H, Hisamatsu N, Atsuta M. Effect of unfilled resins and a silane primer on bonding between layers of a light-activated composite resin veneering material. J Prosthet Dent 1995;73:386-391. Newman S. Kinetics of wetting of surfaces by polymers: capillary flow. J Colloid Interf Sci 1968; 26: 209-213. Özcan M, Alander P, Vallittu PK, Huysmans MC, Kalk W. Effect of three surface conditioning methods to improve bond strength of particulate filler resin composites. J Mater Sci Mater Med 2005;16:21-27. Saunders WP. Effect of fatigue upon the interfacial bond strength of repaired composite resins. J Dent 1990;18:158-162. Swift EJ, Cloe BC, Boyer DB. Effect of a silane coupling agent on composite repair bond strength. Am J Dent 1994;7:200-202. Tezvergil A, Lassila LVJ, Vallittu PK. Composite-composite repair bond strength: effect of different adhesion primers. J Dent 2003;31:521525. Trajtenberg CP, Powers JM. Bond strengths of repaired laboratory composites using three surface treatments and three primers. Am J Dent 2004;17:123-126. Vankerckhoven H, Lambrechts P, Van Beylen M, Davidson CL, Vanherle G. Unreacted methacrylate groups on the surfaces of composite resins. J Dent Res 1982;61:791-795.

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2.2. Effect of oxygen inhibition on composite repair strength over time. Susanna Dall’Oca, Federica Papacchini, Cecilia Goracci, Álvaro H. Cury, Byoung I. Suh, Franklin R. Tay, Antonella Polimeni, Marco Ferrari. Journal of Biomedical Materials Research. Part B: Applied Biomaterials 2006, in press. Introduction Dental resin composites are mostly based on rapidly curing mixtures of

dimethacrylate

monomers

such

as

2,2-bis

[4-(2-hydroxy-3-

methacryloylopropoxy)phenyl]propane (Bis-GMA) and triethylenglycol dimethacrylate (TEGDMA). These monomers form, after appropriate initiation, a three-dimensional tetra-functional network by radical polymerization of methacrylate C=C double bonds (Lapcik and Jancar, 1998) (Andrzejewska, 2001). It has been suggested that the formation of a radically polymerised network can proceed in two steps: microgelation occurs initially which is followed by connection of microgels, resulting in a heterogeneous network. Depending on the molecular weight and flexibility, microgelation may be followed by cyclisation at an oligomer level. It has been shown that during the copolymerization reaction, about 30% of the total amount of C=C bonds remain unreacted in the form of pendant groups. The amount of unreacted C=C bonds increases substantially in the case of copolymerization of two dimethacrylates such as Bis-GMA and TEGDMA. The quantity of unreacted C=C bonds in the Bis-GMA/TEGDMA system is correlated with the amount of Bis-GMA in the system, as neat TEGDMA exhibits about 10% higher degree of conversion that neat Bis-GMA. It has been suggested that with its increasing concentration, Bis-GMA is probably incorporated in the 33

network by only one reactive C=C site forming large bulky pendant groups. The degree of conversion achieved commonly in thermally cured dental polymers is moderate (typically 5-70%) and is somewhat higher in ultraviolet and visible light-cured unfilled dental resins such as sealants and adhesives (Lapcik and Jancar, 1998). Oxygen reacts with carbon-based polymerising free radicals in a diffusion-controlled manner to form peroxy radicals. As they are much less reactive towards double bonds, the efficiency of initiation is reduced leading

to

significant

polymerization.

These

retardation

(or

undesirable

even

reactions

inhibition) with

of

oxygen

the occur

prevalently at the air interface where the concentration of O2 is around 102

-10-3 M, in contrast with the 10-3-10-4 M present in organic or aqueous

media. Thus, photoinitiated polymerization at the surface is selectively retarded.

Moreover,

oxygen

exerts

a

detrimental

effect

on

photopolymerization as it quenches the excited triplet states of photoinitiators. This limits the initiation stage of a polymerization reaction (Andrzejewska,

2001).

The

degree

of

oxygen-inhibition

during

polymerization may potentially be affected by factors such as monomer functionality and structure (Finger et al., 1996) (Lee et al., 2004), BisGMA/HEMA ratio (Finger et al., 1996), TEDMA content (Lapcik and Jancar,

1998),

the

type

and

concentration

of

photoinitiators

(Andrzejewska, 2001), the type of fillers (Burtscher, 1993), temperature (Lapcik and Jancar, 1998) (Andrzejewska, 2001) (Burtscher, 1993), polymerization conditions (Lovestead et al., 2003) (Guillot et al., 2004), the type of solvent (Nunes et al., 2005) and the surface-to-volume ratio of the resin coating (Andrzejewska, 2001). A controversial issue in the study of resin composite repair is whether an oxygen-inhibited layer is required for the coupling of the repair composite to an existing composite. Whereas a positive correlation 34

between the presence of an oxygen inhibition layer and composite repair strength was suggested in one study (Truffier-Boutry et al., 2003), others reported no significant differences (Kupiec and Barkmeier, 1996) (Suh et al., 2003) (Suh, 2004) or even a negative correlation (Rueggeberg and Margeson, 1990) (Eliades and Caputo, 1989). As previous studies have shown that composites bond even in the absence of an oxygen-inhibited layer, it is speculated that the amount of remaining active free radicals that is available for reacting with resin composite monomers is a crucial factor in direct composite repair. Free radicals have half-lives that slowly decay over time. Consequently, the potential for bonding to either a light-cured composite surface or a lightcured

adhesive

layer

should

decrease

with

time

after

photo-

polymerization, in the absence of a resin activator to rejuvenate these surfaces. A higher resin conversion rate may even be a disadvantage in a composite repair procedure that relies on covalent bonding with unreacted free radicals (Lucena-Martín et al., 2001). Composite-to-composite bond strength has so far been assessed mainly with shear (Truffier-Boutry et al., 2003) (Kupiec and Barkmeier, 1996) (Suh, 2004) (Eliades and Caputo, 1989) (Lucena-Martín et al., 2001) (Brosh et al., 1997) and traditional tensile tests (Mitsaki-Matsou et al., 1991). However, in recent years, an increasing number of researcher has turned to the microtensile technique. This technique is currently considered to be a reliable adhesion test, for it allows the loading stress to be more uniformly distributed by the testing of small-sized specimens. In fact, the likelihood that structural faults may affect interfacial strength measurements is limited by the small bonding surface area being tested. In addition, with the non-trimming variant of the method, multiple specimens can be obtained from a single sample, and the variance associated with testing is usually lowered to 10 to 25%, providing a more accurate method 35

for the assessment of the interfacial bond strength (Pashley et al., 1999). The aims of this study were to examine whether an oxygen inhibition layer is required for the bonding of composites, and to determine the time required for free radicals within a polymerised composite to decay to the extent that there is a significant drop in composite repair strength. Clinically, this will provide information on the optimal time period for refurbishing pre-existing aesthetic composite restorations without compromising the bond strength of the final restoration. Thus, the null hypotheses tested were that: (a) there is no difference in composite repair strength irrespective of whether an oxygen inhibition layer is present, and (b) there is no change in the composite repair strength with time when composite repair is performed in the absence of an oxygen inhibition layer. Materials and Methods

The materials used in the study and their chemical compositions are listed in Table 1. Ten resin composite slabs (20 x 15 x 8mm) were prepared from Gradia Direct Anterior (GC Corp., Tokyo, Japan; shade A3) using the incremental polymerization technique. Each increment was condensed with a clean plastic filling instrument and light-cured for 20 s using a quartz-tungstenhalogen light curing unit (VIP, Bisco Inc., Schaumburg, IL, USA) with an output intensity of 500 mW/cm2. Light activation was performed by placing the curing tip at 1 mm away from the composite surface. The polymerised composite slabs were polished with 600-grit silicon carbide paper to create slabs with parallel surfaces. These parallel composite slabs were divided into an experimental group in which polymerization was 36

effectuated in absence of oxygen, and a control group in which polymerization occurred in presence of oxygen. Table 1. Composition, batch number and manufacturer of the materials used in this study. Batch Manufacturer number Gradia Direct UDMA, silica, pre- A3: 0305231 GC Corp., Anterior polymerised fillers, B3: 0305221 Tokyo, Japan pigments, catalysts D/E Resin Bis-GMA, UDMA, 0300002890 Bisco Inc., HEMA Schaumburg, IL, USA Materials

Composition

Abbreviations - Bis-GMA: Bisphenol A-diglycidylmethacrylate; HEMA: Hydroxyethil methacrylate; UDMA: Urethane dimethacrylate.

Two layers of bonding resin (D/E Resin, Bisco) were applied to the flat surface of the composite slabs. For both the control and experimental groups, the first bonding resin layer was light-cured for 20 s in atmospheric air. For the control group, the second bonding resin layer was also light-cured for 20 s in atmospheric air, while for the experimental group, this layer was light-cured in a nitrogen atmosphere to completely eliminate the oxygen inhibition layer. The nitrogen atmosphere was created inside a nitrogen chamber (Figure 1) in which the adhesive-coated composite slab could be inserted by unscrewing the plastic screw cap. This cap was connected on one side to a plastic hose for the delivery of nitrogen. The pressure within the chamber was monitored by a manometer that was attached to the other side of the screw cap. To obtain an oxygenfree environment inside the chamber, nitrogen was first introduced until a chamber pressure of 25 psi was initially achieved. The nitrogen gas was purged by pressing the exhaust valve at the base of the chamber. Purging

37

of the nitrogen was repeated five times to completely eliminate the oxygen. Then the pressure was released to permit free flow of nitrogen gas through the chamber during the light-curing process. The time employed for light-curing of the bonding resin within the chamber was increased from 20 s to 40 s to compensate for the increased distance from the light source.

Figure 1. Nitrogen chamber used for specimens of the study group, in order to obtain an oxygen-deprived atmosphere of curing. Also, in order to standardize the light intensity that reached the resin, a device was placed within the chamber that allowed aligning the specimen to a pre-determined distance from the external light source. Finally, to ensure optimal polymerization of the bonding resin layer, curing of the latter within the nitrogen chamber was followed by conventional lightcuring in atmospheric air for an additional 20 s after the initially lightactivated specimen was removed from the nitrogen chamber. After polymerization of the bonding resin layers, specimens from the control and experimental groups were stored in atmospheric air in a dark 38

room at 37°C. At specific time intervals (viz. after 1 h, 2 h, 1 day, 14 days and 30 days), one specimen was randomly chosen from each group. Shade B3 of Gradia Direct Anterior was incrementally light-cured in the manner previously described in order to obtain a 20 x 15 x 8mm composite repair on top of the bonding resin layer. These specimens simulated the coupling of a fresh repair composite to an existing composite substrate fabricated in the presence or absence of an oxygen-inhibited interface. Different shades were intentionally chosen for the “substrate” and for the “repairing material” to facilitate orientation of the “repairing interface” during microtensile bond strength evaluation.

Microtensile bond strength testing Each repaired composite slab was mounted on a slow-speed cutting machine (Isomet, Buehler, Lake Bluff, IL, USA) that was equipped with a water-cooled diamond blade. The slab was serially sectioned to produce beams with the shade A3 substrate composite on one end and the shade B3 repair composite on the other for microtensile bond strength evaluation using the non-trimming technique (Pashley et al., 1999). Each beam had a cross-sectional area of about 0.8 mm2 and was secured at the ends by cyanoacrylate glue (Zapit, Dental Ventures of America, CA, USA) to a Gerardeli jig (Perdigão et al., 2002). The latter was designed to transmit purely tensile forces to the specimen when mounted on a universal testing machine (Controls, Milano, Italy). The test was conducted at a cross head speed of 0.5 mm/min until failure. The load at failure was recorded in Newtons, and the fractured fragments were removed from the fixtures with a scalpel blade. The cross-sectional area at the site of fracture was measured to the nearest 0.01 mm with a digital caliper. Bond strength was expressed in MegaPascals (MPa). The fractured specimens were examined with an optical microscope (Bausch&Lomb, Rochester, USA) at 60X 39

magnification to determine the failure modes, which were classified as adhesive (within the bonding resin layer), or cohesive in either the substrate or the repair composite.

Statistical Analysis As the data were normally distributed (Kolmogorov-Smirnov test), and groups exhibited homogeneous variance (Levene test), a two-way ANOVA was employed for examining the effect of “curing atmosphere” and “time of composite repair” on the microtensile bond strengths of the repaired composite beams. Multiple comparisons were performed using the Tukey test. All analyses were processed by the SPSS 11.0 software (SPSS, Chicago, IL, USA), with significance set at the 95% probability level. Results The means and standard deviations of the microtensile bond strengths for all the tested subgroups are presented in Table 2. Two-way ANOVA showed that the atmosphere under which the adhesive layer was cured did not significantly affect the composite repair strength (p=0.82). Conversely, the timing of composite repair and the interaction between these two factors significantly affected the composite repair strength (p