Direct posterior composite resin restorations: A review. 1. Factors influencing case selection Roland W. Bryant*
Key words: Case selection, clinical problems, composite resin, posterior composites, review. Abstract The principal factors that influence case selection for direct composite resin restorations in posterior teeth are discussed. These include the perceived preference for tooth-coloured aesthetics, survival rate and replacementof posterior composites, clinical problems and concerns such as occlusal surface defects and the numerous effects of the material's polymerization contraction, and the availability of alternative tooth-coloured techniques for restoring posterior teeth. Specific guidelines in case selection are suggested. A subsequent paper reviews controversial aspects of the restorative technique for posterior composites and, on this basis, defines important principles in the clinical technique. (Received for publication June 1991. Revised December 1991. Accepted January 1992.)
Introduction Although the initial over-adventurous enthusiasm has declined, there can be little doubt, judged from the interest in posterior composites at continuing education courses and trade exhibits, that direct enamel-bonded composite resin will endure as a technique for selected restorations in posterior teeth. In the light of clinical experience, clinical research and a vast amount of literature on the subject, it is appropriate to review current restorative techniques after first considering some of the many factors that influence the clinical decision to place
*Associate Professor and Head, Department of Operative Dentistry, The University of Sydney. Australian Dental Journal 1992;37(2):81-7
a posterior composite. Careful case selection and meticulous attention to the restorative technique are recognized as essential requirements for achieving longer-term clinical success with these restorations. This paper reviews the use, survival rate and replacement of posterior composites, selection of composite resin, clinical problems and concerns, and currently available tooth-coloured restorative alternatives for posterior teeth. In this context, suggestions are made regarding the case for using direct composite resin in the restoration of posterior teeth. Particular emphasis is given to clinical situations such as occlusal and proximal restorations which, in previous years, would routinely have been restored with dental amalgam.
The use of composite resin in posterior teeth Although recent data from Australian dentists are not available it is probable, because of the active marketing of materials in this country, that the tendency to use composite resin in posterior teeth is similar to that in the USA. A detailed 1988 survey' found that, while 25 per cent of dentists did not place any posterior composites and 4 per cent of dentists used composite resin in more than 50 per cent of the posterior restorations they placed, a majority of the dentists were using composite resin in up to 25-30 per cent of posterior restorations. Leinfelder* observed that the increased placement of posterior composite restorations has been associated with an increasing preference within the community for tooth-coloured restorations and with the concern by some about mercury in dental amalgam. Unbalanced media interest has at times been associated with both these i ~ s u e s . ~ , ~ Although most patients accept assurances by medical and dental authorities that the presence of mercury in dental amalgam is not harmful in normal use, there is an increasing feeling by patients
and dentists that if something else could be used it would be preferable. Currently, the convenient 'alternative' is considered by many to be directly placed enamel-bonded composite resin. The clinical techniques and principles of directly placed composite resin restorations in posterior teeth are now being taught in a majority of USA dental school^.^ At the University of Sydney, five classes of dental students have, to-date (1992), received didactic and practical instruction in the conservative use of composite resin in posterior teeth, in an attempt to increase the likelihood of these restorations being placed selectively and with the appropriate technique and care when the students have graduated. Herrid reported that pressure on USA dental schools to teach posterior composites was perceived to come from students (in 60 per cent of schools), patients (51 per cent) and faculty staff (in 43 per cent of dental schools). On average, dentists have reported that fewer than 10 per cent of patients request posterior composites.' Both Leinfelder7 and Bales8 have expressed concern about the role of the manufacturers and distributors of these materials. Bales8 suggested that, if dentists are going to continue to buy unproven products, manufacturers are going to continue to promote and market them. Leinfelder7 indicated that the responsibility is with the manufacturer to ensure adequate pre-clinical and clinical testing prior to marketing. The alternative is that dentists in general practice are, unknowingly, doing the experimental work for the manufacturers. From an ethical point of view, in the absence of proven allergy, there is currently no justification for the routine replacement of sound amalgam restorations with composite re sir^.^^'^ Survival rate and replacement of posterior composites Most of the currently available materials, and the principles of good restorative technique, have been used for sufficient time for the reporting of four - and five - year data from large clinical studies. These have tended to suggest a failure rate in posterior teeth of composite resin that is relatively low (approximately 10 per cent at four or five years),".12 but is approximately twice that of amalgam (approximately 4 per cent at four or five years)." During the four or five years after placement, failure of posterior composites in controlled clinical studies has most commonly been associated with wear andor fracture of the restoration. Sensitivity, recurrent caries and pulpal problems have been the other principal causes of replacement.",'2 82
Because sound posterior composite technique is regarded as technically demanding and timeconsuming, compared with the 'forgiving' nature of amalgam technique, caution should be exercised in applying these findings to general dental practice. Results reported in the dental literature are usually for carefully controlled clinical studies in which the restorations are placed in dental schools by graduate students with an awareness that the restorations will be meticulously evaluated and scrutinized at frequent intervals by their peers and supervisors. With careful case selection, the continuing improvement in materials and the careful application of current restorative techniques, there is justification for Leinfelder's ~tatement'~ that 'a posterior composite placed under the appropriate conditions and monitored routinely can be expected to last ten years or longer.' Selection of a composite resin Many clinical and laboratory studies have examined the increasing number of composite resin products distributed for use in posterior teeth. Although long-term wear rates may be predicted from sixmonth data,14 longer-term studies are required to evaluate margin adaptation (particularly at proximal margins) and fracture resistance. Research has demonstrated that chemicallyactivated (auto-cured) c o m p ~ s i t e s ' and ~ . ~ compo~ sites containing coarse m a ~ r o f i l l e r ~should ~ - * ~ not be used for restoring occlusal surfaces of posterior teeth. Microfilled composites have generally exhibited satisfactory wear resistance'8-21with wear occurring at a linear rate over an extended period of time.22 Mazer, Leinfelder and Russellz3 described the mechanism of failure of microfilled composites under occlusal loading. Failure is initiated at the area of occlusal contact and is directly related to the tensile fatigue of the resin matrix. There has been a trend in the last four or five years towards the use of composites with a higher inorganic filler content ('filler loading') which contain predominantly small ground particles of 1-6 pm average size and a small content of microfiller. Modifications to the type of macrofiller have yielded improved wear resistan~e.'~Unfortunately many composites of this type, particularly those with smaller average particle size, have exhibited more difficult handling characteristics ('stickiness') than the heavily loaded microfilled or coarse macrofilled composites. Australian Dental Journal 1992;372.
Clinical problems with posterior composites A previous article25reviewed many of the clinical problems and concerns associated with posterior composite resin restorations. The topics discussed included wear and loss of anatomical form, occlusal contact relationships, sensitivity after placement, radiopacity, proximal contact and contour, the clinical detection of failure, and time and cost. Many of these perceived problems are still concerns although more is understood about them. For example, it is widely acknowledged now that a Class I1 composite resin restoration will require at least 50 per cent more time to place than an equivalentsized amalgam.26 Currently, clinical problems associated with posterior composites that are of major concern for clinicians and researchers include occlusal surface defects, clinical symptoms and signs attributable to, or associated with, the polymerization contraction of the composite resin, and the definition of clinical failure with respect to posterior composites. 1. Occlusal surface defects In response to abrasion, fatigue and chemical degradation, composite restorations exhibit defects at the margins and on the surface of the restorations. Until recently, most research interest has centred on wear at the occlusal surface margins. It is now recognized that, with the substantial improvement in the wear resistance of current composite resins, resulting in annual wear rates of 10-30 pn, other occlusal surface defects are of equal or relatively greater concern. Four types of occlusal margin defects have been described for composite restoration~.~’ 1. Wear - the progressive exposure of the axially-directed cavity wall. 2. Crevice formation (ditching, marginal fracture). 3. Surface fracture of excess composite resin material. 4. Voids (porosities), resulting from the inadvertent incorporation of air at the margin region during pla~ement.~~.~’ Other studies28,29have associated a wear defect with macrofilled and coarser hybrid types of composites and a greater frequency of marginal fracture (crevice) with microfilled and small particle hybrid composites.
2. The effects of polymerization contraction Composite resins contract between 1.0 and 3.5 per cent (by volume) as they polymeri~e.~’The setting contraction of the visible light-activated composite resin induces substantial stress at the Australian Dental Journal 1992;372.
interface of the composite resin and the acid-etched enamel. The magnitude of this stress has been estimated at approximately 13 MPa.31 There is evidence that, if this interfacial stress associated with the composite’s polymerization can be controlled adequately, the potential for dimensional changes associated with the thermal coefficient of expansion is of little clinical c o n ~ e q u e n c e . ~ ~ Polymerization contraction stresses are responsible for several significant clinical problems. (a) Marginal leakage The major factor influencing the adaptation of composite resins to the surrounding tooth structure is the polymerization ont traction.^^ The most common finding in all microleakage studies has been the observation that composite resin restorations leak more at the gingival margin than at the occlusal margin. One of the probable causes is thin or absent enamel at the gingival wall of the cavity, resulting in relatively poor bonding. The cavity preparation for posterior composites and the restorative technique should therefore aim to minimize the leakage at this vulnerable site by controlling the effects of this contraction. In situations where this is unlikely to be achieved, an alternative restorative technique is indicated. (b) Sensitivity after placement of the composite resin The Occurrence of several types of sensitivity afier placement of a composite restoration has been associated with the increasingly large size of bonded composite restorations and the use of visible light activation of polymerization. Patients have reported sensitivity to sweet stimuli, sensitivity to cold (and possibly hot) stimuli and tenderness to touch with restorations of this type. Although most, if not all, dentists have been worried by patients with these symptoms, most of the evidence is anecdotal and has not been adequately quantified and explained in the literature. i. Sensitivity to sweet and thermal stimuli It has been largely assumed that increased sensitivity to sweet and thermal stimuli in composite-restored teeth is associated with pulpal irritation, presumably resulting from loss of smear layer and leakage with resultant osmotic, thermal and bacterial sequelae. Possible contributing causes include failure to obtain or maintain a seal at the margin, because of bending or contraction of the composite resin, and etching of the dentine. Unlike amalgam restorations, composite restorations sensitive to thermal stimuli tend to remain sensitive and early intervention is usually indicated. 83
ii. Tenderness to touch Another type of sensitivity associated often with large composite restorations is a tenderness to touch (in the absence of premature contact). This may vary from a mild discomfort to extreme tenderness of the tooth to light touching by the tongue or a finger. Although reports of this type of sensitivity were quite common in early studies on posterior composites, in recent years, clinical studies have rarely reported this phenomenon probably because of adaptations made to clinical technique. This type of tenderness is, however, a surprisingly common symptom reported by many patients. Dentists tend to be reluctant to acknowledge the problem, possibly due to ignorance of its cause, treatment and prevention. The tenderness rarely diminishes until the tooth fractures or marginal adaptation fails. This type of tenderness has been attributed to cuspal deformation and may usually be relieved by dividing the large restoration in a mesio-distal direction with a bur-cut through to the glass-ionomer cement base.
c. Deformation and fracture of weakened cusps It has been found experimentally that weakened cusps (in a large MO, DO or MOD restoration) may be deformed, by the polymerization contraction of light-activated composite resin, 15 pm in the first 15 minutes33 to a total of 18-30 pm34 and up to 2 per cent over one week.3s It appears that a number of factors may contribute to the deformation of weakened cusps by the polymerization contraction stresses. 1. The application of a tight matrix may deform weakened cusps 10-65 pm.36 2. Bulk rather than incremental placement of the compo~ite.~’ 3. The relatively rapid onset of polymerization (and contraction) with visible light-activated composites compared with chemically-activated LeinfeldeP has found evidence of the fracture of adjacent enamel that can occur soon after polymerization of a moderate-large occlusal or MOD composite restoration. Fracture is evident on the buccal or lingual surface of the tooth as a horizontal line two to three millimetres gingival to the cusp tip.
d. Potential long-term effects of the polymerization contraction Having placed a large enamel-bonded, lightactivated composite resin restoration in a tooth, there are several possible long-term results. 04
1. The marginal seal remains intact and the tooth is of adequate strength to resist deformation. The restoration is successful. 2. The marginal seal fails and leakage ensues. The tooth may be sensitive to sweet and (possibly) thermal stimuli, and pulpal irritation may result from the ingress of bacteria and their toxins. Marginal discoloration may occur and may be followed by secondary (recurrent) caries. 3. The marginal seal remains intact but the weakened cusps are deformed. These deformed cusps may be sensitive to touch, ranging from mild to extreme tenderness. 4. The marginal seal remains intact, but a weakened and deformed cusp fractures. There are two clinical options to minimize these problems with large tooth-coloured restorations. 1. The careful placement of a direct, enamelbonded composite restoration. This is achieved by reducing the total bulk of composite so that glassionomer cement not only covers but replaces the lost dentine and by the careful incremental placement of the composite resin, which replaces only the lost enamel. 2. The use of an alternative type of restorative technique, such as a composite inlay or onlay. 3. Definition of clinical failure - the decision to replace It could be argued that no restoration should be placed unless the clinician has clearly defined criteria which will govern a subsequent decision to intervene to replace the restoration. Although most clinicians would subscribe to the general principles of minimum intervention, the cost of delayed intervention with respect to composite resin restorations may be high. Large composite restorations in posterior teeth usually have a long margin with the tooth, much of which may be in sites where the integrity of the margin is difficult to assess. Lacking amalgam’s ‘self-sealing’and glassionomer’s fluoride release, a defective composite restoration has far more serious consequences. For a composite resin restoration, 1. What type or size of marginal defect justifies intervention? 2. Should a restoration with this defect be repaired or replaced? 3. How reliable are radiographs in assisting the diagnosis of secondary caries under proximal or occlusal composite restorations? 4. How much wear of a composite restoration can be tolerated before replacement? That is, how much Australian Dental Journal 1992;37:2.
dislocation of opposing and adjacent contacts can be tolerated before intervention? 5. How should a recently placed composite’s sensitivity to cold or sweet or touch be managed? These are serious questions. Not all the answers are known. One thing is certain. Dentists are learning to be increasingly cautious with composite resin restorations. The rate of progression of caries under leaking posterior composites is, at times, surprisingly rapid. Greater caution with these restorations should be directed at thoughtful case selection and meticulous restorative technique (refer subsequent paper).
Case selection for direct posterior composites With the development and improvement of toothcoloured alternatives for restoring posterior teeth, the indications for direct posterior composites may be restricted to situations where they are likely to be more successful. The principal clinical situations for which the use of a direct composite restoration may be indicated are: a need for tooth-coloured aesthetics, and either the replacement of a small-medium-sized restoration, or the restoration of a small-medium-sized lesion. Large MOD and cusp replacement direct composite restorations are technically demanding and therefore time-consuming, and, because of their long margin and large occlusal area, are more likely to exhibit defective adaptation or loss of anatomical form. Large direct composite restorations may occasionally be indicated for short-term diagnostic reasons (for example, when a fractured cusp is suspected) or for temporization. Other requirements that should influence case selection are: (a) Adequate access. This is essential in order to achieve the required quality of cavity preparation and placement. (b) Adequate isolation. Because of the numerous stages in the technique and the need to avoid any contamination to ensure optimum seal and clinical performance, simple isolation with rubber dam is sensible and almost mandatory. (c) Small proximal extensions. Ideally, after removal of caries, the proximal margins are placed just beyond contact with the adjacent tooth, to enable removal of already fractured enamel. Broad extension in a bucco-lingual direction will often compromise proximal contour and contact. Deep extension gingivally often compromises the seal at this vulnerable site. Australian Dental Journal 1992;37:2.
Finally, both patient and dentist should be aware of the time-consuming, technically demanding nature of this restorative procedure, which provides a restoration of somewhat uncertain longevity compared with the relatively predictable and less expensive dental amalgam restoration.
Tooth-colouredalternatives to direct composite resin for posterior teeth After the initial enthusiasm for direct composite resin restorations in posterior teeth and the realization of their limitations, modes of clinical failure, demanding technique and the time required for their placement, a number of tooth-coloured alternatives have been developed for restoring posterior teeth. Many of these have not been widely researched and some have inherent problems and disadvantages. They are briefly reviewed here because they do offer alternatives to the more widely used direct posterior composites. Preventive fissure treatments, such as fissure sealing, are mfl-iwasive, much-researched and widely accepted and require no further comment. The minimally invasive, preventive resin restoration is ideal for the restoration of small carious lesions in the pits and fissures of posterior teeth.39 The small occlusal lesion is restored with composite resin and any adjacent, potentially carious fissures are treated by fissure sealing either with an unfilled opaque resin sealant or with a low viscosity composite mixed with masking agent, or by fissure filling with a composite resin. Regular check-ups are required. The principles of this technique are sound and it has wide application particularly in young patients with fluoride-protected teeth where occlusal lesions are frequently minimal in extent. Several restorative techniques combine the use of two materials to achieve the required level of tooth-coloured aesthetics. (1) A simple, reliable and satisfying option, providing aesthetic improvement, is the placement of a composite resin facing on the buccal or facial portion of a widely extended sound amalgam restoration.40 The mesio-buccal portions of large restorations in premolar teeth may be significantly improved in this way without compromising the anticipated longevity of the underlying amalgam restoration. (2) Many dentists have for some years been combining a cermet cement at the gingival portion of a proximal restoration with a composite resin in the contact and occlusal regions of the restoration. The rationale for this has been that the cermet (a radiopaque modified glass-ionomer cement) provides a more reliable seal than composite resin 85
when the gingival margin is on the root surface or very thin enamel. However, to date, there has been very little documented evidence to support this practice. A small study (20 restorations) found no clinical failures at two years.41 Two laboratory studies found that the cermet did not prevent leakage at the gingival m a ~ g i n . ~ ’ , ~ ~ (3) Tunnel restoration^^^.^^ that restore small proximal lesions with cermet cement without removal of the marginal ridge offer a conservative option but one that may be of great concern to a conscientious operator. Vision and access are poor, the risk of damaging the adjacent tooth is real, and the chances of removing all already fractured enamel at the proximal margins are minimal. To date, there has been an absence of data from carehlly controlled clinical studies, in contrast with the considerable number of references reporting essentially anecdotal data and speculations about the likely success. In virro studies have consistently indicated that the restorations 1eak.46,47 Largely to overcome or minimize the problems caused by the polymerization contraction of the direct posterior composite restoration, techniques for composite and ceramic inlays have been updated and promoted commercially. These inlays and onlays are usually enamel-bonded using a dualcured composite luting agent. Summary 1. Careful case selection is essential in order to minimize the potential difficulties and problems associated with the restoration of a posterior tooth with directly placed composite resin. 2. Clinical problems associated with the polymerization contraction of the composite resin require understanding and demand meticulous attention to the restorative technique. 3. Failing composite restorations require careful assessment and thoughtful consideration before electing not to intervene. 4. Although tooth-coloured alternatives are available, many of these have, to date, not been subjected to the intense evaluation from carefully controlled clinical studies that is necessary to provide confidence for the less ‘adventurous’ clinician. 5. A subsequent paper will review clinical technique for the placement of direct composite resin restorations. References 1. Davis EL, Laura JC, Joynt RB, Wieczkowski Jr G. Determination of demand for posterior resin restorations. J Prosthet Dent 1988;59:242-8. 86
2. Leinfelder KF. Composite resins. Dent Clin North Am 1985;29:359-71. 3. Bryant RW. Long-term implications for composite resin restorations. Ann R Aust Coll Dent Surg 1989;10:84-90. 4. Truono EJ. Response to ‘60 Minutes’. J Am Dent Assoc 1991; 122:10-4. 5. Wilson NHF, Setcos JC. The teaching of posterior composites: a worldwide survey. J Dent 1989;17:S29-S33. 6. Herrin HK, Harrison JL, von der Lehr W. The status of posterior composites in the dental curriculum. J Dent Educ 1987;51:252-3. 7. Leinfelder KF. Evaluation ofcriteria used for assessing the clinical performance of composite resins in posterior teeth. Quintessence Int 1987;18:531-42. 8. Bales DJ. Posterior composites: for routine use in today’s practice? Oper Dent 1987;12:41. 9. National Institute of Dental Reseach. Workshop: Biocompatibility of metals in dentistry. J Am Dent Assoc 1984;109:469-71. 10. Gilbert JA. Posterior composites: an ethical issue. Oper Dent 1987;12:79-81. 11. Bayne SC, Taylor DF, Roberson TM, Wilder AD, Sturdevant JR, Heymann HO, Lisk MW. Long term clinical failures i n posterior composites. J Dent Res 1989;68:185:Abstr 32. 12. Letzel H. Survival rates and reasons for failure of posterior composite restorations in multicentre clinical trial. J Dent 1989;17:SlO-SI 7. 13. Leinfelder KF. Posterior composite resins. J Am Dent Assoc 1988; 117~21E-26E. 14. Leinfelder KF, Wilder AD, Teixeira LC. Wear rates of posterior composite resins. J Am Dent Assoc 1986;112: 829-33. 15. Wilder AD, May KN, Leinfelder KF. Three-year clinical study of UV-cured composite resins in posterior teeth. J Prosthet Dent 1983;50:26-30. 16. Fortunato J, Santos F, Leinfelder KF. Microporosities in composite resins. J Dent Res 1983;62:671:Abstr 191. 17. Leinfelder KF, Sluder TB, Santos JFF, Wall JT. Five-year clinical evaluation of anterior and posterior restorations of composite resin. Oper Dent 1980;5:57-65. 18. Skeeters TM, Timmons JH, Richards ND, Laswell HR, Mitchell RJ. A three-year study of Class I composite resin restorations. J Dent Res 1986;85:303:Abstr 1204. 19. Hendriks FHJ, Letzel H, VrijhoefMMA. Composite versus amalgam restorations. J Oral Rehabil 1986;13:401-11. 20. Gilpatrick RO, Goldberg AJ, Simonsen RJ. Clinical assessment of four composite resins for posterior teeth. J Dent Res 1987;66:129:Abstr 180. 2 1. Zinck JH, McInnes-Ledoux P. Clinical evaluation of Distalite and Adaptic I1 in posterior teeth: one year report. J Dent Res 1987;66:129:Abstr 184. 22. Leinfelder KF. Wear patterns and rates of posterior composite resins. Int Dent J 1987;37:152-7. 23. Mazer RB, Leinfelder K, Russell C. Mechanism of failure in a microfilled composite resin. J Dent Res 1989;68:233:Abstr 418. 24. Mazer RB, Leinfelder KF. Clinical evaluation of a posterior composite resin containing a new type of filler particle. J Esthet Dent 1988;1:66-70. 25. Bryant RW. Posterior composite resin restorations - a review of clinical problems. Aust Prosthod J 1987;1:41-50. 26. Anonymous. Class I1 resins - update review. Clinical Research Associates Newsletter 1989;13:2. Australian Dental Journal 1992;37:2.
27. Fukushima M, Setcos JC, Phillips RW. Marginal fracture of posterior composite resins. J Am Dent Assoc 1988; 117~577-83. 28. McComb D, Brown J. Two year clinical evaluation ofthree composite resins as posterior materials. J Dent Res 1985; 64:352:Abstr 1601. 29. Bryant RW, Hodge K-LV. Marginal defects around posterior composite resin restorations. J Dent Res 1989;68:547: Abstr 65. 30. Felzer AJ, de Gee AJ. Davidson CL. Curing contraction of composite and glass-ionomer cements. J Prosthet Dent 1988;59:297-300. 31. Craig RG, ed. Restorative dental materials. St Louis: Mosby, 1989:262. 32. Robinson PB, Moore BK, Swartz ML. Comparison of microleakage in direct and indirect composite resin restorations in vitro. Oper Dent 1987;12:113-116. 33. McCullock AJ, Smith BGN. In oirro studies of cuspal movement produced by adhesive restorative materials. Br Dent J 1986;161:405-9. 34. Pearson GJ, Hegany SM. Cusp movement in molar teeth using dentine adhesives and composite filling materials. Biomaterials 1987;8:473-6. 35. Causton BE, Miller B, Sefton J. The deformation of cusps by bonded posterior composite restorations: an in virro study. Br Dent J 1985;159:397-400. 36. Hood JA. Cusp flexion. Br Dent J 1985;158:268. 37. Goetsch T, Krejci I, Lutz F, Reich T . Deformation ofcavity walls induced by different composite restorative techniques. J Dent Res 1989;68:342:Abstr 1282. 38. Davidson CL. Resisting the curing contraction with adhesive composites. J Prosthet Dent 1986;55:446-7. 39. Simonsen RJ. The preventive resin restoration: a minimally invasive, nonmetallic restoration. Compend Contin Educ Dent 1987;8:428-32.
Australian Dental Journal 1992;37:2
40. Zoldan P. Expanded use of light cured composites. Dent Outlook 1983;9:45-54. 41. Grogono AL, McInnes-Ledoux PM, Zinck JH, Weinberg R. Posterior composite and glass ionomerkomposite laminate restorations - a clinical investigation. J Dent Res 1989;68: 186:Abstr 40. 42. Hinkelman KW, Dederich D, Albert A. Microleakage in restorations placed by the glass ionomer 'sandwich' technique. J Dent Res 1989;68:208:Abstr 209. 43. Guelmann M, Fuks AB, Holan G, Grajower R. Marginal leakage of Class I1 glass-ionomer-silverrestorations, with and without posterior composite coverage: an in virro study. J Dent Child 1989;56:277-82. 44. McLean JW. Limitations of posterior composite resins and extending their use with glass ionomer cements. Quintessence Int 1987;18:517-29. 45. Knight GM. The use of adhesive materials in the conservative restoration of selected posterior teeth. Aust Dent J 1984;29:324-31. 46. Robbins JW, Cooley RL. Microleakage of Ketac-Silver in the tunnel preparation. Oper Dent 1988;13:8-11. 47. Hotz PR, Holzer A. Microleakage and quality of conservative Class I1 and tunnel restorations. J Dent Res 1989;68:208:Abstr 21 1.
Address for correspondenceheprints: Department of Operative Dentistry, The University of Sydney, C/o Westmead Hospital Dental Clinical School, Westmead, New South Wales, 2145
87
Key words: Case selection, clinical problems, composite resin, posterior composites, review. Abstract The principal factors that influence case selection for direct composite resin restorations in posterior teeth are discussed. These include the perceived preference for tooth-coloured aesthetics, survival rate and replacementof posterior composites, clinical problems and concerns such as occlusal surface defects and the numerous effects of the material's polymerization contraction, and the availability of alternative tooth-coloured techniques for restoring posterior teeth. Specific guidelines in case selection are suggested. A subsequent paper reviews controversial aspects of the restorative technique for posterior composites and, on this basis, defines important principles in the clinical technique. (Received for publication June 1991. Revised December 1991. Accepted January 1992.)
Introduction Although the initial over-adventurous enthusiasm has declined, there can be little doubt, judged from the interest in posterior composites at continuing education courses and trade exhibits, that direct enamel-bonded composite resin will endure as a technique for selected restorations in posterior teeth. In the light of clinical experience, clinical research and a vast amount of literature on the subject, it is appropriate to review current restorative techniques after first considering some of the many factors that influence the clinical decision to place
*Associate Professor and Head, Department of Operative Dentistry, The University of Sydney. Australian Dental Journal 1992;37(2):81-7
a posterior composite. Careful case selection and meticulous attention to the restorative technique are recognized as essential requirements for achieving longer-term clinical success with these restorations. This paper reviews the use, survival rate and replacement of posterior composites, selection of composite resin, clinical problems and concerns, and currently available tooth-coloured restorative alternatives for posterior teeth. In this context, suggestions are made regarding the case for using direct composite resin in the restoration of posterior teeth. Particular emphasis is given to clinical situations such as occlusal and proximal restorations which, in previous years, would routinely have been restored with dental amalgam.
The use of composite resin in posterior teeth Although recent data from Australian dentists are not available it is probable, because of the active marketing of materials in this country, that the tendency to use composite resin in posterior teeth is similar to that in the USA. A detailed 1988 survey' found that, while 25 per cent of dentists did not place any posterior composites and 4 per cent of dentists used composite resin in more than 50 per cent of the posterior restorations they placed, a majority of the dentists were using composite resin in up to 25-30 per cent of posterior restorations. Leinfelder* observed that the increased placement of posterior composite restorations has been associated with an increasing preference within the community for tooth-coloured restorations and with the concern by some about mercury in dental amalgam. Unbalanced media interest has at times been associated with both these i ~ s u e s . ~ , ~ Although most patients accept assurances by medical and dental authorities that the presence of mercury in dental amalgam is not harmful in normal use, there is an increasing feeling by patients
and dentists that if something else could be used it would be preferable. Currently, the convenient 'alternative' is considered by many to be directly placed enamel-bonded composite resin. The clinical techniques and principles of directly placed composite resin restorations in posterior teeth are now being taught in a majority of USA dental school^.^ At the University of Sydney, five classes of dental students have, to-date (1992), received didactic and practical instruction in the conservative use of composite resin in posterior teeth, in an attempt to increase the likelihood of these restorations being placed selectively and with the appropriate technique and care when the students have graduated. Herrid reported that pressure on USA dental schools to teach posterior composites was perceived to come from students (in 60 per cent of schools), patients (51 per cent) and faculty staff (in 43 per cent of dental schools). On average, dentists have reported that fewer than 10 per cent of patients request posterior composites.' Both Leinfelder7 and Bales8 have expressed concern about the role of the manufacturers and distributors of these materials. Bales8 suggested that, if dentists are going to continue to buy unproven products, manufacturers are going to continue to promote and market them. Leinfelder7 indicated that the responsibility is with the manufacturer to ensure adequate pre-clinical and clinical testing prior to marketing. The alternative is that dentists in general practice are, unknowingly, doing the experimental work for the manufacturers. From an ethical point of view, in the absence of proven allergy, there is currently no justification for the routine replacement of sound amalgam restorations with composite re sir^.^^'^ Survival rate and replacement of posterior composites Most of the currently available materials, and the principles of good restorative technique, have been used for sufficient time for the reporting of four - and five - year data from large clinical studies. These have tended to suggest a failure rate in posterior teeth of composite resin that is relatively low (approximately 10 per cent at four or five years),".12 but is approximately twice that of amalgam (approximately 4 per cent at four or five years)." During the four or five years after placement, failure of posterior composites in controlled clinical studies has most commonly been associated with wear andor fracture of the restoration. Sensitivity, recurrent caries and pulpal problems have been the other principal causes of replacement.",'2 82
Because sound posterior composite technique is regarded as technically demanding and timeconsuming, compared with the 'forgiving' nature of amalgam technique, caution should be exercised in applying these findings to general dental practice. Results reported in the dental literature are usually for carefully controlled clinical studies in which the restorations are placed in dental schools by graduate students with an awareness that the restorations will be meticulously evaluated and scrutinized at frequent intervals by their peers and supervisors. With careful case selection, the continuing improvement in materials and the careful application of current restorative techniques, there is justification for Leinfelder's ~tatement'~ that 'a posterior composite placed under the appropriate conditions and monitored routinely can be expected to last ten years or longer.' Selection of a composite resin Many clinical and laboratory studies have examined the increasing number of composite resin products distributed for use in posterior teeth. Although long-term wear rates may be predicted from sixmonth data,14 longer-term studies are required to evaluate margin adaptation (particularly at proximal margins) and fracture resistance. Research has demonstrated that chemicallyactivated (auto-cured) c o m p ~ s i t e s ' and ~ . ~ compo~ sites containing coarse m a ~ r o f i l l e r ~should ~ - * ~ not be used for restoring occlusal surfaces of posterior teeth. Microfilled composites have generally exhibited satisfactory wear resistance'8-21with wear occurring at a linear rate over an extended period of time.22 Mazer, Leinfelder and Russellz3 described the mechanism of failure of microfilled composites under occlusal loading. Failure is initiated at the area of occlusal contact and is directly related to the tensile fatigue of the resin matrix. There has been a trend in the last four or five years towards the use of composites with a higher inorganic filler content ('filler loading') which contain predominantly small ground particles of 1-6 pm average size and a small content of microfiller. Modifications to the type of macrofiller have yielded improved wear resistan~e.'~Unfortunately many composites of this type, particularly those with smaller average particle size, have exhibited more difficult handling characteristics ('stickiness') than the heavily loaded microfilled or coarse macrofilled composites. Australian Dental Journal 1992;372.
Clinical problems with posterior composites A previous article25reviewed many of the clinical problems and concerns associated with posterior composite resin restorations. The topics discussed included wear and loss of anatomical form, occlusal contact relationships, sensitivity after placement, radiopacity, proximal contact and contour, the clinical detection of failure, and time and cost. Many of these perceived problems are still concerns although more is understood about them. For example, it is widely acknowledged now that a Class I1 composite resin restoration will require at least 50 per cent more time to place than an equivalentsized amalgam.26 Currently, clinical problems associated with posterior composites that are of major concern for clinicians and researchers include occlusal surface defects, clinical symptoms and signs attributable to, or associated with, the polymerization contraction of the composite resin, and the definition of clinical failure with respect to posterior composites. 1. Occlusal surface defects In response to abrasion, fatigue and chemical degradation, composite restorations exhibit defects at the margins and on the surface of the restorations. Until recently, most research interest has centred on wear at the occlusal surface margins. It is now recognized that, with the substantial improvement in the wear resistance of current composite resins, resulting in annual wear rates of 10-30 pn, other occlusal surface defects are of equal or relatively greater concern. Four types of occlusal margin defects have been described for composite restoration~.~’ 1. Wear - the progressive exposure of the axially-directed cavity wall. 2. Crevice formation (ditching, marginal fracture). 3. Surface fracture of excess composite resin material. 4. Voids (porosities), resulting from the inadvertent incorporation of air at the margin region during pla~ement.~~.~’ Other studies28,29have associated a wear defect with macrofilled and coarser hybrid types of composites and a greater frequency of marginal fracture (crevice) with microfilled and small particle hybrid composites.
2. The effects of polymerization contraction Composite resins contract between 1.0 and 3.5 per cent (by volume) as they polymeri~e.~’The setting contraction of the visible light-activated composite resin induces substantial stress at the Australian Dental Journal 1992;372.
interface of the composite resin and the acid-etched enamel. The magnitude of this stress has been estimated at approximately 13 MPa.31 There is evidence that, if this interfacial stress associated with the composite’s polymerization can be controlled adequately, the potential for dimensional changes associated with the thermal coefficient of expansion is of little clinical c o n ~ e q u e n c e . ~ ~ Polymerization contraction stresses are responsible for several significant clinical problems. (a) Marginal leakage The major factor influencing the adaptation of composite resins to the surrounding tooth structure is the polymerization ont traction.^^ The most common finding in all microleakage studies has been the observation that composite resin restorations leak more at the gingival margin than at the occlusal margin. One of the probable causes is thin or absent enamel at the gingival wall of the cavity, resulting in relatively poor bonding. The cavity preparation for posterior composites and the restorative technique should therefore aim to minimize the leakage at this vulnerable site by controlling the effects of this contraction. In situations where this is unlikely to be achieved, an alternative restorative technique is indicated. (b) Sensitivity after placement of the composite resin The Occurrence of several types of sensitivity afier placement of a composite restoration has been associated with the increasingly large size of bonded composite restorations and the use of visible light activation of polymerization. Patients have reported sensitivity to sweet stimuli, sensitivity to cold (and possibly hot) stimuli and tenderness to touch with restorations of this type. Although most, if not all, dentists have been worried by patients with these symptoms, most of the evidence is anecdotal and has not been adequately quantified and explained in the literature. i. Sensitivity to sweet and thermal stimuli It has been largely assumed that increased sensitivity to sweet and thermal stimuli in composite-restored teeth is associated with pulpal irritation, presumably resulting from loss of smear layer and leakage with resultant osmotic, thermal and bacterial sequelae. Possible contributing causes include failure to obtain or maintain a seal at the margin, because of bending or contraction of the composite resin, and etching of the dentine. Unlike amalgam restorations, composite restorations sensitive to thermal stimuli tend to remain sensitive and early intervention is usually indicated. 83
ii. Tenderness to touch Another type of sensitivity associated often with large composite restorations is a tenderness to touch (in the absence of premature contact). This may vary from a mild discomfort to extreme tenderness of the tooth to light touching by the tongue or a finger. Although reports of this type of sensitivity were quite common in early studies on posterior composites, in recent years, clinical studies have rarely reported this phenomenon probably because of adaptations made to clinical technique. This type of tenderness is, however, a surprisingly common symptom reported by many patients. Dentists tend to be reluctant to acknowledge the problem, possibly due to ignorance of its cause, treatment and prevention. The tenderness rarely diminishes until the tooth fractures or marginal adaptation fails. This type of tenderness has been attributed to cuspal deformation and may usually be relieved by dividing the large restoration in a mesio-distal direction with a bur-cut through to the glass-ionomer cement base.
c. Deformation and fracture of weakened cusps It has been found experimentally that weakened cusps (in a large MO, DO or MOD restoration) may be deformed, by the polymerization contraction of light-activated composite resin, 15 pm in the first 15 minutes33 to a total of 18-30 pm34 and up to 2 per cent over one week.3s It appears that a number of factors may contribute to the deformation of weakened cusps by the polymerization contraction stresses. 1. The application of a tight matrix may deform weakened cusps 10-65 pm.36 2. Bulk rather than incremental placement of the compo~ite.~’ 3. The relatively rapid onset of polymerization (and contraction) with visible light-activated composites compared with chemically-activated LeinfeldeP has found evidence of the fracture of adjacent enamel that can occur soon after polymerization of a moderate-large occlusal or MOD composite restoration. Fracture is evident on the buccal or lingual surface of the tooth as a horizontal line two to three millimetres gingival to the cusp tip.
d. Potential long-term effects of the polymerization contraction Having placed a large enamel-bonded, lightactivated composite resin restoration in a tooth, there are several possible long-term results. 04
1. The marginal seal remains intact and the tooth is of adequate strength to resist deformation. The restoration is successful. 2. The marginal seal fails and leakage ensues. The tooth may be sensitive to sweet and (possibly) thermal stimuli, and pulpal irritation may result from the ingress of bacteria and their toxins. Marginal discoloration may occur and may be followed by secondary (recurrent) caries. 3. The marginal seal remains intact but the weakened cusps are deformed. These deformed cusps may be sensitive to touch, ranging from mild to extreme tenderness. 4. The marginal seal remains intact, but a weakened and deformed cusp fractures. There are two clinical options to minimize these problems with large tooth-coloured restorations. 1. The careful placement of a direct, enamelbonded composite restoration. This is achieved by reducing the total bulk of composite so that glassionomer cement not only covers but replaces the lost dentine and by the careful incremental placement of the composite resin, which replaces only the lost enamel. 2. The use of an alternative type of restorative technique, such as a composite inlay or onlay. 3. Definition of clinical failure - the decision to replace It could be argued that no restoration should be placed unless the clinician has clearly defined criteria which will govern a subsequent decision to intervene to replace the restoration. Although most clinicians would subscribe to the general principles of minimum intervention, the cost of delayed intervention with respect to composite resin restorations may be high. Large composite restorations in posterior teeth usually have a long margin with the tooth, much of which may be in sites where the integrity of the margin is difficult to assess. Lacking amalgam’s ‘self-sealing’and glassionomer’s fluoride release, a defective composite restoration has far more serious consequences. For a composite resin restoration, 1. What type or size of marginal defect justifies intervention? 2. Should a restoration with this defect be repaired or replaced? 3. How reliable are radiographs in assisting the diagnosis of secondary caries under proximal or occlusal composite restorations? 4. How much wear of a composite restoration can be tolerated before replacement? That is, how much Australian Dental Journal 1992;37:2.
dislocation of opposing and adjacent contacts can be tolerated before intervention? 5. How should a recently placed composite’s sensitivity to cold or sweet or touch be managed? These are serious questions. Not all the answers are known. One thing is certain. Dentists are learning to be increasingly cautious with composite resin restorations. The rate of progression of caries under leaking posterior composites is, at times, surprisingly rapid. Greater caution with these restorations should be directed at thoughtful case selection and meticulous restorative technique (refer subsequent paper).
Case selection for direct posterior composites With the development and improvement of toothcoloured alternatives for restoring posterior teeth, the indications for direct posterior composites may be restricted to situations where they are likely to be more successful. The principal clinical situations for which the use of a direct composite restoration may be indicated are: a need for tooth-coloured aesthetics, and either the replacement of a small-medium-sized restoration, or the restoration of a small-medium-sized lesion. Large MOD and cusp replacement direct composite restorations are technically demanding and therefore time-consuming, and, because of their long margin and large occlusal area, are more likely to exhibit defective adaptation or loss of anatomical form. Large direct composite restorations may occasionally be indicated for short-term diagnostic reasons (for example, when a fractured cusp is suspected) or for temporization. Other requirements that should influence case selection are: (a) Adequate access. This is essential in order to achieve the required quality of cavity preparation and placement. (b) Adequate isolation. Because of the numerous stages in the technique and the need to avoid any contamination to ensure optimum seal and clinical performance, simple isolation with rubber dam is sensible and almost mandatory. (c) Small proximal extensions. Ideally, after removal of caries, the proximal margins are placed just beyond contact with the adjacent tooth, to enable removal of already fractured enamel. Broad extension in a bucco-lingual direction will often compromise proximal contour and contact. Deep extension gingivally often compromises the seal at this vulnerable site. Australian Dental Journal 1992;37:2.
Finally, both patient and dentist should be aware of the time-consuming, technically demanding nature of this restorative procedure, which provides a restoration of somewhat uncertain longevity compared with the relatively predictable and less expensive dental amalgam restoration.
Tooth-colouredalternatives to direct composite resin for posterior teeth After the initial enthusiasm for direct composite resin restorations in posterior teeth and the realization of their limitations, modes of clinical failure, demanding technique and the time required for their placement, a number of tooth-coloured alternatives have been developed for restoring posterior teeth. Many of these have not been widely researched and some have inherent problems and disadvantages. They are briefly reviewed here because they do offer alternatives to the more widely used direct posterior composites. Preventive fissure treatments, such as fissure sealing, are mfl-iwasive, much-researched and widely accepted and require no further comment. The minimally invasive, preventive resin restoration is ideal for the restoration of small carious lesions in the pits and fissures of posterior teeth.39 The small occlusal lesion is restored with composite resin and any adjacent, potentially carious fissures are treated by fissure sealing either with an unfilled opaque resin sealant or with a low viscosity composite mixed with masking agent, or by fissure filling with a composite resin. Regular check-ups are required. The principles of this technique are sound and it has wide application particularly in young patients with fluoride-protected teeth where occlusal lesions are frequently minimal in extent. Several restorative techniques combine the use of two materials to achieve the required level of tooth-coloured aesthetics. (1) A simple, reliable and satisfying option, providing aesthetic improvement, is the placement of a composite resin facing on the buccal or facial portion of a widely extended sound amalgam restoration.40 The mesio-buccal portions of large restorations in premolar teeth may be significantly improved in this way without compromising the anticipated longevity of the underlying amalgam restoration. (2) Many dentists have for some years been combining a cermet cement at the gingival portion of a proximal restoration with a composite resin in the contact and occlusal regions of the restoration. The rationale for this has been that the cermet (a radiopaque modified glass-ionomer cement) provides a more reliable seal than composite resin 85
when the gingival margin is on the root surface or very thin enamel. However, to date, there has been very little documented evidence to support this practice. A small study (20 restorations) found no clinical failures at two years.41 Two laboratory studies found that the cermet did not prevent leakage at the gingival m a ~ g i n . ~ ’ , ~ ~ (3) Tunnel restoration^^^.^^ that restore small proximal lesions with cermet cement without removal of the marginal ridge offer a conservative option but one that may be of great concern to a conscientious operator. Vision and access are poor, the risk of damaging the adjacent tooth is real, and the chances of removing all already fractured enamel at the proximal margins are minimal. To date, there has been an absence of data from carehlly controlled clinical studies, in contrast with the considerable number of references reporting essentially anecdotal data and speculations about the likely success. In virro studies have consistently indicated that the restorations 1eak.46,47 Largely to overcome or minimize the problems caused by the polymerization contraction of the direct posterior composite restoration, techniques for composite and ceramic inlays have been updated and promoted commercially. These inlays and onlays are usually enamel-bonded using a dualcured composite luting agent. Summary 1. Careful case selection is essential in order to minimize the potential difficulties and problems associated with the restoration of a posterior tooth with directly placed composite resin. 2. Clinical problems associated with the polymerization contraction of the composite resin require understanding and demand meticulous attention to the restorative technique. 3. Failing composite restorations require careful assessment and thoughtful consideration before electing not to intervene. 4. Although tooth-coloured alternatives are available, many of these have, to date, not been subjected to the intense evaluation from carefully controlled clinical studies that is necessary to provide confidence for the less ‘adventurous’ clinician. 5. A subsequent paper will review clinical technique for the placement of direct composite resin restorations. References 1. Davis EL, Laura JC, Joynt RB, Wieczkowski Jr G. Determination of demand for posterior resin restorations. J Prosthet Dent 1988;59:242-8. 86
2. Leinfelder KF. Composite resins. Dent Clin North Am 1985;29:359-71. 3. Bryant RW. Long-term implications for composite resin restorations. Ann R Aust Coll Dent Surg 1989;10:84-90. 4. Truono EJ. Response to ‘60 Minutes’. J Am Dent Assoc 1991; 122:10-4. 5. Wilson NHF, Setcos JC. The teaching of posterior composites: a worldwide survey. J Dent 1989;17:S29-S33. 6. Herrin HK, Harrison JL, von der Lehr W. The status of posterior composites in the dental curriculum. J Dent Educ 1987;51:252-3. 7. Leinfelder KF. Evaluation ofcriteria used for assessing the clinical performance of composite resins in posterior teeth. Quintessence Int 1987;18:531-42. 8. Bales DJ. Posterior composites: for routine use in today’s practice? Oper Dent 1987;12:41. 9. National Institute of Dental Reseach. Workshop: Biocompatibility of metals in dentistry. J Am Dent Assoc 1984;109:469-71. 10. Gilbert JA. Posterior composites: an ethical issue. Oper Dent 1987;12:79-81. 11. Bayne SC, Taylor DF, Roberson TM, Wilder AD, Sturdevant JR, Heymann HO, Lisk MW. Long term clinical failures i n posterior composites. J Dent Res 1989;68:185:Abstr 32. 12. Letzel H. Survival rates and reasons for failure of posterior composite restorations in multicentre clinical trial. J Dent 1989;17:SlO-SI 7. 13. Leinfelder KF. Posterior composite resins. J Am Dent Assoc 1988; 117~21E-26E. 14. Leinfelder KF, Wilder AD, Teixeira LC. Wear rates of posterior composite resins. J Am Dent Assoc 1986;112: 829-33. 15. Wilder AD, May KN, Leinfelder KF. Three-year clinical study of UV-cured composite resins in posterior teeth. J Prosthet Dent 1983;50:26-30. 16. Fortunato J, Santos F, Leinfelder KF. Microporosities in composite resins. J Dent Res 1983;62:671:Abstr 191. 17. Leinfelder KF, Sluder TB, Santos JFF, Wall JT. Five-year clinical evaluation of anterior and posterior restorations of composite resin. Oper Dent 1980;5:57-65. 18. Skeeters TM, Timmons JH, Richards ND, Laswell HR, Mitchell RJ. A three-year study of Class I composite resin restorations. J Dent Res 1986;85:303:Abstr 1204. 19. Hendriks FHJ, Letzel H, VrijhoefMMA. Composite versus amalgam restorations. J Oral Rehabil 1986;13:401-11. 20. Gilpatrick RO, Goldberg AJ, Simonsen RJ. Clinical assessment of four composite resins for posterior teeth. J Dent Res 1987;66:129:Abstr 180. 2 1. Zinck JH, McInnes-Ledoux P. Clinical evaluation of Distalite and Adaptic I1 in posterior teeth: one year report. J Dent Res 1987;66:129:Abstr 184. 22. Leinfelder KF. Wear patterns and rates of posterior composite resins. Int Dent J 1987;37:152-7. 23. Mazer RB, Leinfelder K, Russell C. Mechanism of failure in a microfilled composite resin. J Dent Res 1989;68:233:Abstr 418. 24. Mazer RB, Leinfelder KF. Clinical evaluation of a posterior composite resin containing a new type of filler particle. J Esthet Dent 1988;1:66-70. 25. Bryant RW. Posterior composite resin restorations - a review of clinical problems. Aust Prosthod J 1987;1:41-50. 26. Anonymous. Class I1 resins - update review. Clinical Research Associates Newsletter 1989;13:2. Australian Dental Journal 1992;37:2.
27. Fukushima M, Setcos JC, Phillips RW. Marginal fracture of posterior composite resins. J Am Dent Assoc 1988; 117~577-83. 28. McComb D, Brown J. Two year clinical evaluation ofthree composite resins as posterior materials. J Dent Res 1985; 64:352:Abstr 1601. 29. Bryant RW, Hodge K-LV. Marginal defects around posterior composite resin restorations. J Dent Res 1989;68:547: Abstr 65. 30. Felzer AJ, de Gee AJ. Davidson CL. Curing contraction of composite and glass-ionomer cements. J Prosthet Dent 1988;59:297-300. 31. Craig RG, ed. Restorative dental materials. St Louis: Mosby, 1989:262. 32. Robinson PB, Moore BK, Swartz ML. Comparison of microleakage in direct and indirect composite resin restorations in vitro. Oper Dent 1987;12:113-116. 33. McCullock AJ, Smith BGN. In oirro studies of cuspal movement produced by adhesive restorative materials. Br Dent J 1986;161:405-9. 34. Pearson GJ, Hegany SM. Cusp movement in molar teeth using dentine adhesives and composite filling materials. Biomaterials 1987;8:473-6. 35. Causton BE, Miller B, Sefton J. The deformation of cusps by bonded posterior composite restorations: an in virro study. Br Dent J 1985;159:397-400. 36. Hood JA. Cusp flexion. Br Dent J 1985;158:268. 37. Goetsch T, Krejci I, Lutz F, Reich T . Deformation ofcavity walls induced by different composite restorative techniques. J Dent Res 1989;68:342:Abstr 1282. 38. Davidson CL. Resisting the curing contraction with adhesive composites. J Prosthet Dent 1986;55:446-7. 39. Simonsen RJ. The preventive resin restoration: a minimally invasive, nonmetallic restoration. Compend Contin Educ Dent 1987;8:428-32.
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40. Zoldan P. Expanded use of light cured composites. Dent Outlook 1983;9:45-54. 41. Grogono AL, McInnes-Ledoux PM, Zinck JH, Weinberg R. Posterior composite and glass ionomerkomposite laminate restorations - a clinical investigation. J Dent Res 1989;68: 186:Abstr 40. 42. Hinkelman KW, Dederich D, Albert A. Microleakage in restorations placed by the glass ionomer 'sandwich' technique. J Dent Res 1989;68:208:Abstr 209. 43. Guelmann M, Fuks AB, Holan G, Grajower R. Marginal leakage of Class I1 glass-ionomer-silverrestorations, with and without posterior composite coverage: an in virro study. J Dent Child 1989;56:277-82. 44. McLean JW. Limitations of posterior composite resins and extending their use with glass ionomer cements. Quintessence Int 1987;18:517-29. 45. Knight GM. The use of adhesive materials in the conservative restoration of selected posterior teeth. Aust Dent J 1984;29:324-31. 46. Robbins JW, Cooley RL. Microleakage of Ketac-Silver in the tunnel preparation. Oper Dent 1988;13:8-11. 47. Hotz PR, Holzer A. Microleakage and quality of conservative Class I1 and tunnel restorations. J Dent Res 1989;68:208:Abstr 21 1.
Address for correspondenceheprints: Department of Operative Dentistry, The University of Sydney, C/o Westmead Hospital Dental Clinical School, Westmead, New South Wales, 2145
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