276
J. Dent. 1990;
18: 276-280
Adaptation of two different calcium hydroxide bases under a composite restoration M. Papadakou,*
I. E. Barnes, * R. W. Wassellt
*Department of Operative Dentistry Newcastle Upon Tyne, UK
and J. F. McCabet
and tDepartment
of Dental Materials
Science, Dental School, University
of
A preliminary scanning electron microscope (SEM) study was carried out to investigate how the adaptation of two calcium hydroxide bases (one chemically cured, one light cured) was affected by the polymerization contraction of a supervening light-cured composite resin restoration. Occlusal cavities were prepared in 40 sound extracted human premolars, divided into two equal groups. In the first group a chemically cured calcium hydroxide (Dycal, De Trey Dentsply, Konstanz, FRG) was placed as a base. In the second group a new light-cured calcium hydroxide product (Prisma VLC Dycal, De Trey Dentsply ) was used. The restorations were completed with an acid-etched, incrementally placed composite resin. The specimens were sectioned vertically and debrided. A replica was made of each half-tooth. The interfaces between composite resin/base and base/dentine were viewed and photographed in the SEM. The marginal adaptation at these two interfaces ..I,,0 plor‘..;l;aA ;*tr. ,-otnnr\r;00 LIC-C”IUIIIE o,.,V.r,-l;nn +r\ -“tn..+ fiF+hn “Q..C tl.n+ r l%mnl WLLi) \rlLlIJJIIItiU llll” &FP.. LIIItiCUXl\rrZj”IIU L”tha LUCcl~lrlll”1 LLItiB_iJJ Ll‘al XI_.._ Wti‘ti,.l.“,W.W*rl ““JC, “CU.In,:,,o llJl,lLLVr . Ub UJti’LL’ base was found to be pulled away from the dentine floor of the cavity as a result of an apparent adhesion to the composite resin during polymerization contraction. Dycal was better adapted to the cavity floor than Prisma WC Dycal. Disorganization of the resin-bonded Prisma VLC Dycal was minimal even after acid etching the enamel, sectioning and ultrasonic debridement. Dycal appeared to be more friable, and occasionally exhibited marked disorganization as a result of these procedures. KEY WORDS: Calcium hydroxide, Composite resins, Adaptation J. Dent. 1990;
18: 276-280
(Received 8 May 1990;
reviewed 20 June 1990;
accepted 11 July 1990)
Correspondence should be addressed to: Mr R. W. Wassell, Department of Operative Dentistry, Dental School, Framlington Place, University of Newcastle Upon Tyne, Newcastle Upon Tyne NE1 8TA. UK.
INTRODUCTION Calcium hydroxide is considered to be the cavity base of choice under composite resin restorations in the UK The material was first introduced as a base by Herman (1930), who proposed its beneficial action as a pulp-capping material. Since then, mixtures of calcium hydroxide have been used with success, not only to cap pulps, but also in other reclamatory endodontic procedures such as pulpotomy, pulp amputations and apexification. Other uses include, the dressing of root canals, the treatment of large periapical lesions, as a root canal sealer in canals with wide apical foramina, the treatment of perforations and root fractures, and the treatment of root resorptions. The development of light-activated catalysis for resin products has led to the recent introduction of a visible@ 1990 Butterworth-Heinemann 0300-5712/!90/050276-05
Ltd.
light-cured (VLC) calcium hydroxide base (Prisma VLC Dycal, De Trey Dentsply, Konstanz, FRG). The main clinical advantage is purported to be the control of its working time by the dentist. The material consists of calcium hydroxide and tillers ofbarium sulphate dispersed in a specially formulated urethane dimethacrylate resin containing initiators and accelerators activated by visible light (Stanley and Pameijer, 1985). Despite significant improvements that have been made to composite resins over the years, there remains a measurable polymerization contraction. Values of around 1.5 per cent volumetric contraction are typical for the newer hybrid materials (McCabe, 1985). Although small, this shrinkage is clinically significant and may allow microleakage along tooth/ restoration interfaces. The resulting passage of bacteria, oral fluids, chemical substances, molecules and ions may
Papadakou: Calcium hydroxide
contribute to marginal staining, tooth discolouration, secondary caries, hypersensitivity of the restored tooth, pulpal damage and a hastening of the breakdown of certain filling materials (Kidd, 1976; Going, 1979). This potential problem of impaired marginal seal can be combated by the use of some form of bond between restorative materials and enamel or dentine. This bond may be provided by the use of micromechanical retention to enamel (Buonocore 1955) and the use of chemical bonding agents either to dentine or enamel. Such bonding, whether mechanical or chemical, results in a great reduction in the shrinkage of the restoration away from the cavity walls during setting. However, polymerization contraction will still occur at the unrestrained occlusal part of the tilling and possibly at the base. If a light-cured calcium hydroxide base adhered to the undersurface of the composite resin restoration the base may be drawn away from the cavity floor to which it is less adherent. McConnell et al. (1986) used a sectioning and SEM study to show that when a light-cured, bulk-tilled composite restoration was placed over the top of a lightcured calcium hydroxide base a gap appeared between the base and the cavity floor. Only a limited number of specimens was used and the effect of incrementally placing the composite was not assessed. The present study was undertaken to assess the effect upon two types of calcium hydroxide liner of the basal polymerization contraction of an acid-etch-retained lightcured composite resin restoration.
under composite
resin
277
(Etch’n Bond, ICI Dental, Macclesfield, UK). The etched surfaces were thoroughly washed for 30 s and dried with compressed air. Occlusin composite resin (ICI Dental) was placed in the cavities, using an incremental technique in which the material was laid down in horizontal layers each no more than 2 mm in depth. Each layer was light-cured for 30 s with a light-activating unit (Luxor, ICI Dental). The cavities were filled and contoured without further finishing. The restored teeth, which had been stored in distilled water at 37°C for 24 h, were sectioned mesiodistally (specimens a and b). A fine diamond-impregnated (120 pm grit size) annular saw was used under copious irrigation with distilled water. Grinding debris was removed from the interfaces by immersing the sectioned teeth in an aqueous ultrasonic bath for 20 s, following which they were superficially dried with compressed air. A replica of each section was made for SEM examination. This was done immediately to prevent the sectioned teeth drying out. The replication technique is detailed below. A two-stage putty wash impression was taken of the cut surface, using an addition silicone rubber (President Putty and Light-Body, Coltene AG, Altstatten, Switzerland). An embedding resin (Spur-r, Taab, Reading, UK) was poured into the impressions and cured in an oven at 50°C for 48 h (Beynon, 1987). The set models (replicas) were mounted on stubs, sputter coated with gold to a thickness of approximately 15 nm, and examined in the SEM.
MATERIALS AND METHODS Forty intact, caries-free, untilled human premolar teeth, extracted for orthodontic reasons, were used in this study. Prior to use the teeth were stored for 3 weeks in 10 per cent v/v formal saline. A Class I occlusal cavity was cut in each tooth using a flat-ended tapered diamond fissure bur in an airotor with water coolant. Each cavity was eliptical in shape, being 3.5 mm in its mesiodistal aspect, 2.5 mm buccolingually, and between 3 and 4 mm in depth. The cavity walls diverged at an angle similar to that of the bur. Two proprietary calcium hydroxide bases were studied: one was chemically cured (Dycal, De Trey Dentsply), the other being resin based and light-cured (Prisma VLC Dycal, De Trey Dentsply). These cements were placed on the dentine of the cavity floor, to a depth of 0.5 mm; this being the diameter of the bulb of the Dycal instrument that was used to apply them, and which proved to be an effective measuring gauge. The teeth were divided into two groups of 20. Dycal was placed as a cavity base in one group, and was allowed to set in a humidor (100 per cent relative humidity, 37 “C) for 15 min. Prisma VLC Dycal was used in the second group, being light cured for 30 s according to the manufacturers’ instructions. When the bases had set the enamel of the cavity walls was etched for 60 s with phosphoric acid etching gel
SCANNING ELECTRON MICROSCOPY The cut surface of the replicas of the hemisected teeth were examined in the SEM (Cambridge Stereoscan, Cambridge, UK), in plan view at low magnification (25 X) and were photographed. A further photomicrograph was made at higher magnification (200X) of each replica showing greater detail of a representative site along the two interfaces, namely between composite and base, and base and tooth. The photographs were studied, and the integrity of the two interfaces assessed in random order by two examiners to reach a consensus decision. It was found that where a gap had formed it was either small (less than 5 urn) or comparatively wide (greater than 20 urn). Consequently, each interface was scored as follows: I
II III
No discernible gap, or just discernible extending less than half way along the interface (Figs 1, 2, composite/base interface). Gap discernible over half the length of the interface but less than 5 pm wide (Fig, 3). Gap discernible over half the length of the interface, greater than 20 urn wide and sometimes tilled with debris (Figs I, 2. 4).
The Chi-squared test was used to analyse any difference between the scores for Prisma VLC Dycal and Dycal at
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J. Dent. 1990;
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Fig. 7. Prisma VLC Dycal. Spec 3b. Field width 500 pm. C, composite; B, base; D, dentine. The base/composite interface scored I. The base/dentine interface scored III.
Fig. 2. Prisma VLC Dycal. Spec 2b. Field width 500 pm. C, composite; B, base; D, dentine. The base/composite interface scored I. The base/dentine interface scored III.
Fig. 3. Dycal. Spec 14a. Field width 500 pm. C, composite; B, base; D, dentine. The base/composite interface scored II. The base/dentine interface scored II.
Fig. 4. Dycal. Spec 12b. Field width 600 pm. C, composite; B, base; D, dentine. The base/composite intetfacescored III. The base/dentine interface scored II.
each interface. The lowest score for the two sections each tooth was used in the analysis.
and close adaptation in 17. Substantial gaps were observed between the base and the cavity floor in 18 teeth; a small gap in no teeth and close adaptation was confirmed in only two.
of
RESULTS The results are shown in Table I. The scores for the two sections of each tooth never varied by more than one. The number of teeth in each group of 20 with the scores in agreement was as follows: Prisma Prisma
VLC Dycal-composite/base interface-l 1 teeth VLC Dycal-base/tooth interface -18 teeth Dycal-composite/base interface-13 teeth Dycal-base/tooth interface - 16 teeth
A different pattern of gap formation two bases. as detailed below.
Dycal In the Dycal specimens substantial gaps between base and composite filling were seen in seven of the 20 teeth, a small gap in live and close adaptation in eight. Substantial gaps between the base and the dentine of the cavity floor were seen in seven teeth, a small gap in 10 and close adaptation in three.
was seen for the
Prisma VLC Dycal There were substantial gaps (score III) between base and restoration in only one of the 20 teeth; a small gap in two
Statistical
analysis
Prisma VLC Dycal tended to be more closely adapted to the composite than Dycal, however the statistical significance was weak (0.05 < P < 0.01). Dycal was more closely adapted to the tooth than Prisma VLC Dycal (P < 0.01).
Papadakou: Calcium hydroxide
under composite
resin
279
Table 1. Gap formation above and below two calcium hvdroxide bases Composite/base NO. Score
Prisma VLC Dycal
Dycal
Base/tooth Score
NO.
1
I II III
6 18
I II III
8 5 7
I II III
3 10 7
I II III
17 2
The number of teeth obtaining scores I, II or Ill at the composite/ base and base/tooth interfaces for the base materials Prisma VLC Dycal and Dycal. The section with the lowest score was taken to represent each tooth. I II
Ill
No discernible gap, or just discernible extending less than half way along the interface. Gap discernibleover half the length of the interface but less than 5 pm wide.
Gapdiscernibleoverhalfthe lengthof the interface,greaterthan 20 pm wide and sometimes filled with debris.
DISCUSSION Four aspects of the experimental protocol merit consideration. First, Class I cavities only were investigated, as it was felt that in a preliminary study the intricacies of the more complex Class II cavity form would serve simply to confuse. Second, the use of replicas eliminates the risk of gross distortion of tooth structure in the vacuum chamber of the scanning electron microscope, a factor that has led to the non-validity of several studies on the adaptation of restorative materials (as reported by Going, 1972). However. there is one disadvantage to the replica technique inherent wherever small marginal gaps are being assessed, namely the retention of small fragments of tom impression material in the deficiency, with the resultant artefactual ‘tilling of the gap’ in the replica (Fig. 5). Third, sectioning can obscure the interface between two materials by two mechanisms: either (a) smearing or (b) filling of the gaps with debris. Light grinding followed by ultrasonification can eliminate these problems but care must be taken on interpretation of the SEM micrographs. Further it must be accepted that a certain number of gaps may be opened up by the sectioning procedure itself. The time of 20 s ultrasonic cleansing was determined by many experiments, and successfully fell between disruption of the base material as a result of too long an ultrasonic cycle, and insufficient removal of the smeared superficial debris as a result of too short a cycle. The results show clearly that substantial gaps formed between the resin-based Prisma VLC Dycal and the underlying dentine. Two mechanisms may have caused this effect. First, the base may have bonded to the overlying composite resin which shrinks towards the light source on polymerization. The polymerization shrinkage of the composite resins that are used for posterior fillings
Fig. 5. Dycal. Spec la. Field width 4 mm. The base/ composite interface scored III. The base/dentin@ interface scored I. The interface between the composite and the base is probably artefactual, representing fractured impression material caught and lying between a substantial gap.
is relatively low, but is nevertheless clinically significant. When placed in etched cavities it has been found that the material keys to the roughened enamel margin with the result that polymerization shrinkage causes flexion of the cusps. Conversely, where part of the material is unrestrained, as may occur occlusally, on the pulpal wall of the cavity or along the dentinal surface of a gingival margin, shrinkage forces may be unrestrained, so that significant marginal deficiencies arise, with the recognized consequences of sensitivity, microleakage and recurrent caries. Secondly, the Prisma VLC Dycal base may have shrunk towards the light source before the composite was placed occlusally. It would be worth testing for this effect in future studies. The interfacial gaps observed with the Dycal specimens were similarly distributed above and below the base. The marginal disruption seen in many of these specimens suggests that the sectioning technique may have displaced some of the friable base material resulting in gap formation. It is important that this effect be taken into consideration in subsequent investigations. The interfacial gaps seen in this study have a number of clinical implications. First, from a mechanical standpoint any disruption to the integrity between tooth and restoration is liable to weaken the restoration under occlusal load. Significantly, no teeth in either the Prisma VLC Dycal group or the Dycal group had perfect adaptation at both interfaces. Secondly, from a biological point of view if the base is pulled away from the dentine the therapeutic effect ofthe alkaline calcium hydroxide will be diminished. Agap between tooth and base was seen most frequently in the Prisma VLC Dycal group. This is particularly disturbing as the antibacterial properties of the resinbased Prisma VLC Dycal are limited by its inherent insolubility. Bacteria remaining after tooth preparation, or as a result of microleakage, may proliferate in the gap.
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In addition, the remineralizing effect over a pulpal exposure would be compromised. A small gap produced with the chemically cured Dycal would be less significant because of its better antibacterial effect. It must be remembered that the study was undertaken in vitro. It is unlikely that the post-mortem changes that occur in tooth tissues immediately following extraction (Causton and Johnson, 1979) would affect the results that have been observed, or the conclusions drawn. Nevertheless, further work remains to be done, and caution must be exercised in interpreting the significance of this initial study, certainly until such time as the experiment has been repeated in viva. It is concluded that in vitro, significant gaps were formed between Prisma VLC Dycal and the cavity floor while chemically cured Dycal showed better adaptation.
Acknowledgements The authors would like to thank Mr I. Bell of the Depart-
ment of Oral Biology for his help with the sectioning of the teeth and Mr A. Parker of the Biomedical EM Unit, Newcastle upon Tyne for assistance with the SEM. References Beynon A. D. (1987) Replication technique for studying micro-structure in fossil enamel. Scanning Microsc. 1, 663-669.
Buonocore M. G. (1955) A simple method of increasing the adhesion of acrylic filling materials to enamel surface. J. Dent. Res. 34, 849-853. Burke F. J. T. and Watts D. C. (1986) Weight loss of four calcium hydroxide-based materials following a phosphoric acid etching and washing cycle. J. Dent 14,226-221. Causton B. E. and Johnson N. W. (1979) Changes in the dentine of human teeth following extraction and their implications for in-vitro studies of adhesion to tooth substances. Arch. Oral Biol. 24, 220. Going R. E. (1972) Microleakage around dental restorations: a summarizing review. .I Am. Dent. Assoc. 84, 1349-1357. Going R. E. (1979) Reducing marginal leakage: a review of materials and techniques. J. Am Dent. Assoc. 99, 646-650. Hermann B. W. (1930) Dentinobliteration der Werzelkanal nach der Behandlung mit Kalzium. Zahnarfzti Rand. 39, 888. Kidd E. A. M. (1976) Microleakage: a review. J. Dent 4, 199-205. McCabe J. F. (1985) Anderson S Applied Dental Materials, 6th edn. Oxford, Blackwell Scientific, pp. 165-166. McComb D. (1983) Comparison of physical properties of commercial calcium hydroxide lining cements. J. Am. Dent. Assoc. 46, 610-613. McConnell R. J., Boksman L., Hunter J. K. et al. (1986) The effect of restorative materials on the adaptation of two bases and dentine bonding agent to the internal cavity walls. Quintessence Int. 17, 703-710. Stanley H. E. and Pameijer C. H. (1985) Pulp capping with a new visible-light-curing calcium hydroxide composition (Prisma VLC Dycal). Oper. Dent. 10, 156-163.
Book Review Physiology for Dental Students. D. B. Ferguson. Pp. 362. 1988. Guildford, Wright Imprint of Butterworth-Heinemann Ltd. Softback, f 19.50 This is a concise, readable and attractive textbook for undergraduate dental students, which may serve also as a background resource for FDS candidates. The text avoids the conventional tour of the systems of the body, which tends to obscure the common principles of physiology necessary for a proper understanding of how the body works, and instead, in the first part sets out general guidelines for physiological processes. Systems are dealt with in the second part, bringing together the general concepts already introduced. Finally, control mechanisms are discussed in a third section, including growth, haemostasis, exercise, digestion (including salivary function) and endocrinology. The material draws extensively on dentally relevant examples, while not ignoring the needs of dental students
for a broad coverage to permit an understanding of general medicine, surgery, pharmacology and pathology. Special aspects of physiology, such as mastication, speech and taste, are dealt with in detail appropriate for a first year dental course, but which may require more development later. The text is admirably clear and unfussy, as are the line illustrations. The more inquisitive student will require fuller explanation of some of the experimental background which is not emphasized, but to which he is guided in recommendations for further reading at the end of each chapter. The current extension of the basic science content of dental courses will stimulate the need for a modern, basic textbook of physiology which this extremely practical book is likely to meet. It should prove a firm favourite with students. W. M. Edgar.
J. Dent. 1990;
18: 276-280
Adaptation of two different calcium hydroxide bases under a composite restoration M. Papadakou,*
I. E. Barnes, * R. W. Wassellt
*Department of Operative Dentistry Newcastle Upon Tyne, UK
and J. F. McCabet
and tDepartment
of Dental Materials
Science, Dental School, University
of
A preliminary scanning electron microscope (SEM) study was carried out to investigate how the adaptation of two calcium hydroxide bases (one chemically cured, one light cured) was affected by the polymerization contraction of a supervening light-cured composite resin restoration. Occlusal cavities were prepared in 40 sound extracted human premolars, divided into two equal groups. In the first group a chemically cured calcium hydroxide (Dycal, De Trey Dentsply, Konstanz, FRG) was placed as a base. In the second group a new light-cured calcium hydroxide product (Prisma VLC Dycal, De Trey Dentsply ) was used. The restorations were completed with an acid-etched, incrementally placed composite resin. The specimens were sectioned vertically and debrided. A replica was made of each half-tooth. The interfaces between composite resin/base and base/dentine were viewed and photographed in the SEM. The marginal adaptation at these two interfaces ..I,,0 plor‘..;l;aA ;*tr. ,-otnnr\r;00 LIC-C”IUIIIE o,.,V.r,-l;nn +r\ -“tn..+ fiF+hn “Q..C tl.n+ r l%mnl WLLi) \rlLlIJJIIItiU llll” &FP.. LIIItiCUXl\rrZj”IIU L”tha LUCcl~lrlll”1 LLItiB_iJJ Ll‘al XI_.._ Wti‘ti,.l.“,W.W*rl ““JC, “CU.In,:,,o llJl,lLLVr . Ub UJti’LL’ base was found to be pulled away from the dentine floor of the cavity as a result of an apparent adhesion to the composite resin during polymerization contraction. Dycal was better adapted to the cavity floor than Prisma WC Dycal. Disorganization of the resin-bonded Prisma VLC Dycal was minimal even after acid etching the enamel, sectioning and ultrasonic debridement. Dycal appeared to be more friable, and occasionally exhibited marked disorganization as a result of these procedures. KEY WORDS: Calcium hydroxide, Composite resins, Adaptation J. Dent. 1990;
18: 276-280
(Received 8 May 1990;
reviewed 20 June 1990;
accepted 11 July 1990)
Correspondence should be addressed to: Mr R. W. Wassell, Department of Operative Dentistry, Dental School, Framlington Place, University of Newcastle Upon Tyne, Newcastle Upon Tyne NE1 8TA. UK.
INTRODUCTION Calcium hydroxide is considered to be the cavity base of choice under composite resin restorations in the UK The material was first introduced as a base by Herman (1930), who proposed its beneficial action as a pulp-capping material. Since then, mixtures of calcium hydroxide have been used with success, not only to cap pulps, but also in other reclamatory endodontic procedures such as pulpotomy, pulp amputations and apexification. Other uses include, the dressing of root canals, the treatment of large periapical lesions, as a root canal sealer in canals with wide apical foramina, the treatment of perforations and root fractures, and the treatment of root resorptions. The development of light-activated catalysis for resin products has led to the recent introduction of a visible@ 1990 Butterworth-Heinemann 0300-5712/!90/050276-05
Ltd.
light-cured (VLC) calcium hydroxide base (Prisma VLC Dycal, De Trey Dentsply, Konstanz, FRG). The main clinical advantage is purported to be the control of its working time by the dentist. The material consists of calcium hydroxide and tillers ofbarium sulphate dispersed in a specially formulated urethane dimethacrylate resin containing initiators and accelerators activated by visible light (Stanley and Pameijer, 1985). Despite significant improvements that have been made to composite resins over the years, there remains a measurable polymerization contraction. Values of around 1.5 per cent volumetric contraction are typical for the newer hybrid materials (McCabe, 1985). Although small, this shrinkage is clinically significant and may allow microleakage along tooth/ restoration interfaces. The resulting passage of bacteria, oral fluids, chemical substances, molecules and ions may
Papadakou: Calcium hydroxide
contribute to marginal staining, tooth discolouration, secondary caries, hypersensitivity of the restored tooth, pulpal damage and a hastening of the breakdown of certain filling materials (Kidd, 1976; Going, 1979). This potential problem of impaired marginal seal can be combated by the use of some form of bond between restorative materials and enamel or dentine. This bond may be provided by the use of micromechanical retention to enamel (Buonocore 1955) and the use of chemical bonding agents either to dentine or enamel. Such bonding, whether mechanical or chemical, results in a great reduction in the shrinkage of the restoration away from the cavity walls during setting. However, polymerization contraction will still occur at the unrestrained occlusal part of the tilling and possibly at the base. If a light-cured calcium hydroxide base adhered to the undersurface of the composite resin restoration the base may be drawn away from the cavity floor to which it is less adherent. McConnell et al. (1986) used a sectioning and SEM study to show that when a light-cured, bulk-tilled composite restoration was placed over the top of a lightcured calcium hydroxide base a gap appeared between the base and the cavity floor. Only a limited number of specimens was used and the effect of incrementally placing the composite was not assessed. The present study was undertaken to assess the effect upon two types of calcium hydroxide liner of the basal polymerization contraction of an acid-etch-retained lightcured composite resin restoration.
under composite
resin
277
(Etch’n Bond, ICI Dental, Macclesfield, UK). The etched surfaces were thoroughly washed for 30 s and dried with compressed air. Occlusin composite resin (ICI Dental) was placed in the cavities, using an incremental technique in which the material was laid down in horizontal layers each no more than 2 mm in depth. Each layer was light-cured for 30 s with a light-activating unit (Luxor, ICI Dental). The cavities were filled and contoured without further finishing. The restored teeth, which had been stored in distilled water at 37°C for 24 h, were sectioned mesiodistally (specimens a and b). A fine diamond-impregnated (120 pm grit size) annular saw was used under copious irrigation with distilled water. Grinding debris was removed from the interfaces by immersing the sectioned teeth in an aqueous ultrasonic bath for 20 s, following which they were superficially dried with compressed air. A replica of each section was made for SEM examination. This was done immediately to prevent the sectioned teeth drying out. The replication technique is detailed below. A two-stage putty wash impression was taken of the cut surface, using an addition silicone rubber (President Putty and Light-Body, Coltene AG, Altstatten, Switzerland). An embedding resin (Spur-r, Taab, Reading, UK) was poured into the impressions and cured in an oven at 50°C for 48 h (Beynon, 1987). The set models (replicas) were mounted on stubs, sputter coated with gold to a thickness of approximately 15 nm, and examined in the SEM.
MATERIALS AND METHODS Forty intact, caries-free, untilled human premolar teeth, extracted for orthodontic reasons, were used in this study. Prior to use the teeth were stored for 3 weeks in 10 per cent v/v formal saline. A Class I occlusal cavity was cut in each tooth using a flat-ended tapered diamond fissure bur in an airotor with water coolant. Each cavity was eliptical in shape, being 3.5 mm in its mesiodistal aspect, 2.5 mm buccolingually, and between 3 and 4 mm in depth. The cavity walls diverged at an angle similar to that of the bur. Two proprietary calcium hydroxide bases were studied: one was chemically cured (Dycal, De Trey Dentsply), the other being resin based and light-cured (Prisma VLC Dycal, De Trey Dentsply). These cements were placed on the dentine of the cavity floor, to a depth of 0.5 mm; this being the diameter of the bulb of the Dycal instrument that was used to apply them, and which proved to be an effective measuring gauge. The teeth were divided into two groups of 20. Dycal was placed as a cavity base in one group, and was allowed to set in a humidor (100 per cent relative humidity, 37 “C) for 15 min. Prisma VLC Dycal was used in the second group, being light cured for 30 s according to the manufacturers’ instructions. When the bases had set the enamel of the cavity walls was etched for 60 s with phosphoric acid etching gel
SCANNING ELECTRON MICROSCOPY The cut surface of the replicas of the hemisected teeth were examined in the SEM (Cambridge Stereoscan, Cambridge, UK), in plan view at low magnification (25 X) and were photographed. A further photomicrograph was made at higher magnification (200X) of each replica showing greater detail of a representative site along the two interfaces, namely between composite and base, and base and tooth. The photographs were studied, and the integrity of the two interfaces assessed in random order by two examiners to reach a consensus decision. It was found that where a gap had formed it was either small (less than 5 urn) or comparatively wide (greater than 20 urn). Consequently, each interface was scored as follows: I
II III
No discernible gap, or just discernible extending less than half way along the interface (Figs 1, 2, composite/base interface). Gap discernible over half the length of the interface but less than 5 pm wide (Fig, 3). Gap discernible over half the length of the interface, greater than 20 urn wide and sometimes tilled with debris (Figs I, 2. 4).
The Chi-squared test was used to analyse any difference between the scores for Prisma VLC Dycal and Dycal at
278
J. Dent. 1990;
18: No. 5
Fig. 7. Prisma VLC Dycal. Spec 3b. Field width 500 pm. C, composite; B, base; D, dentine. The base/composite interface scored I. The base/dentine interface scored III.
Fig. 2. Prisma VLC Dycal. Spec 2b. Field width 500 pm. C, composite; B, base; D, dentine. The base/composite interface scored I. The base/dentine interface scored III.
Fig. 3. Dycal. Spec 14a. Field width 500 pm. C, composite; B, base; D, dentine. The base/composite interface scored II. The base/dentine interface scored II.
Fig. 4. Dycal. Spec 12b. Field width 600 pm. C, composite; B, base; D, dentine. The base/composite intetfacescored III. The base/dentine interface scored II.
each interface. The lowest score for the two sections each tooth was used in the analysis.
and close adaptation in 17. Substantial gaps were observed between the base and the cavity floor in 18 teeth; a small gap in no teeth and close adaptation was confirmed in only two.
of
RESULTS The results are shown in Table I. The scores for the two sections of each tooth never varied by more than one. The number of teeth in each group of 20 with the scores in agreement was as follows: Prisma Prisma
VLC Dycal-composite/base interface-l 1 teeth VLC Dycal-base/tooth interface -18 teeth Dycal-composite/base interface-13 teeth Dycal-base/tooth interface - 16 teeth
A different pattern of gap formation two bases. as detailed below.
Dycal In the Dycal specimens substantial gaps between base and composite filling were seen in seven of the 20 teeth, a small gap in live and close adaptation in eight. Substantial gaps between the base and the dentine of the cavity floor were seen in seven teeth, a small gap in 10 and close adaptation in three.
was seen for the
Prisma VLC Dycal There were substantial gaps (score III) between base and restoration in only one of the 20 teeth; a small gap in two
Statistical
analysis
Prisma VLC Dycal tended to be more closely adapted to the composite than Dycal, however the statistical significance was weak (0.05 < P < 0.01). Dycal was more closely adapted to the tooth than Prisma VLC Dycal (P < 0.01).
Papadakou: Calcium hydroxide
under composite
resin
279
Table 1. Gap formation above and below two calcium hvdroxide bases Composite/base NO. Score
Prisma VLC Dycal
Dycal
Base/tooth Score
NO.
1
I II III
6 18
I II III
8 5 7
I II III
3 10 7
I II III
17 2
The number of teeth obtaining scores I, II or Ill at the composite/ base and base/tooth interfaces for the base materials Prisma VLC Dycal and Dycal. The section with the lowest score was taken to represent each tooth. I II
Ill
No discernible gap, or just discernible extending less than half way along the interface. Gap discernibleover half the length of the interface but less than 5 pm wide.
Gapdiscernibleoverhalfthe lengthof the interface,greaterthan 20 pm wide and sometimes filled with debris.
DISCUSSION Four aspects of the experimental protocol merit consideration. First, Class I cavities only were investigated, as it was felt that in a preliminary study the intricacies of the more complex Class II cavity form would serve simply to confuse. Second, the use of replicas eliminates the risk of gross distortion of tooth structure in the vacuum chamber of the scanning electron microscope, a factor that has led to the non-validity of several studies on the adaptation of restorative materials (as reported by Going, 1972). However. there is one disadvantage to the replica technique inherent wherever small marginal gaps are being assessed, namely the retention of small fragments of tom impression material in the deficiency, with the resultant artefactual ‘tilling of the gap’ in the replica (Fig. 5). Third, sectioning can obscure the interface between two materials by two mechanisms: either (a) smearing or (b) filling of the gaps with debris. Light grinding followed by ultrasonification can eliminate these problems but care must be taken on interpretation of the SEM micrographs. Further it must be accepted that a certain number of gaps may be opened up by the sectioning procedure itself. The time of 20 s ultrasonic cleansing was determined by many experiments, and successfully fell between disruption of the base material as a result of too long an ultrasonic cycle, and insufficient removal of the smeared superficial debris as a result of too short a cycle. The results show clearly that substantial gaps formed between the resin-based Prisma VLC Dycal and the underlying dentine. Two mechanisms may have caused this effect. First, the base may have bonded to the overlying composite resin which shrinks towards the light source on polymerization. The polymerization shrinkage of the composite resins that are used for posterior fillings
Fig. 5. Dycal. Spec la. Field width 4 mm. The base/ composite interface scored III. The base/dentin@ interface scored I. The interface between the composite and the base is probably artefactual, representing fractured impression material caught and lying between a substantial gap.
is relatively low, but is nevertheless clinically significant. When placed in etched cavities it has been found that the material keys to the roughened enamel margin with the result that polymerization shrinkage causes flexion of the cusps. Conversely, where part of the material is unrestrained, as may occur occlusally, on the pulpal wall of the cavity or along the dentinal surface of a gingival margin, shrinkage forces may be unrestrained, so that significant marginal deficiencies arise, with the recognized consequences of sensitivity, microleakage and recurrent caries. Secondly, the Prisma VLC Dycal base may have shrunk towards the light source before the composite was placed occlusally. It would be worth testing for this effect in future studies. The interfacial gaps observed with the Dycal specimens were similarly distributed above and below the base. The marginal disruption seen in many of these specimens suggests that the sectioning technique may have displaced some of the friable base material resulting in gap formation. It is important that this effect be taken into consideration in subsequent investigations. The interfacial gaps seen in this study have a number of clinical implications. First, from a mechanical standpoint any disruption to the integrity between tooth and restoration is liable to weaken the restoration under occlusal load. Significantly, no teeth in either the Prisma VLC Dycal group or the Dycal group had perfect adaptation at both interfaces. Secondly, from a biological point of view if the base is pulled away from the dentine the therapeutic effect ofthe alkaline calcium hydroxide will be diminished. Agap between tooth and base was seen most frequently in the Prisma VLC Dycal group. This is particularly disturbing as the antibacterial properties of the resinbased Prisma VLC Dycal are limited by its inherent insolubility. Bacteria remaining after tooth preparation, or as a result of microleakage, may proliferate in the gap.
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In addition, the remineralizing effect over a pulpal exposure would be compromised. A small gap produced with the chemically cured Dycal would be less significant because of its better antibacterial effect. It must be remembered that the study was undertaken in vitro. It is unlikely that the post-mortem changes that occur in tooth tissues immediately following extraction (Causton and Johnson, 1979) would affect the results that have been observed, or the conclusions drawn. Nevertheless, further work remains to be done, and caution must be exercised in interpreting the significance of this initial study, certainly until such time as the experiment has been repeated in viva. It is concluded that in vitro, significant gaps were formed between Prisma VLC Dycal and the cavity floor while chemically cured Dycal showed better adaptation.
Acknowledgements The authors would like to thank Mr I. Bell of the Depart-
ment of Oral Biology for his help with the sectioning of the teeth and Mr A. Parker of the Biomedical EM Unit, Newcastle upon Tyne for assistance with the SEM. References Beynon A. D. (1987) Replication technique for studying micro-structure in fossil enamel. Scanning Microsc. 1, 663-669.
Buonocore M. G. (1955) A simple method of increasing the adhesion of acrylic filling materials to enamel surface. J. Dent. Res. 34, 849-853. Burke F. J. T. and Watts D. C. (1986) Weight loss of four calcium hydroxide-based materials following a phosphoric acid etching and washing cycle. J. Dent 14,226-221. Causton B. E. and Johnson N. W. (1979) Changes in the dentine of human teeth following extraction and their implications for in-vitro studies of adhesion to tooth substances. Arch. Oral Biol. 24, 220. Going R. E. (1972) Microleakage around dental restorations: a summarizing review. .I Am. Dent. Assoc. 84, 1349-1357. Going R. E. (1979) Reducing marginal leakage: a review of materials and techniques. J. Am Dent. Assoc. 99, 646-650. Hermann B. W. (1930) Dentinobliteration der Werzelkanal nach der Behandlung mit Kalzium. Zahnarfzti Rand. 39, 888. Kidd E. A. M. (1976) Microleakage: a review. J. Dent 4, 199-205. McCabe J. F. (1985) Anderson S Applied Dental Materials, 6th edn. Oxford, Blackwell Scientific, pp. 165-166. McComb D. (1983) Comparison of physical properties of commercial calcium hydroxide lining cements. J. Am. Dent. Assoc. 46, 610-613. McConnell R. J., Boksman L., Hunter J. K. et al. (1986) The effect of restorative materials on the adaptation of two bases and dentine bonding agent to the internal cavity walls. Quintessence Int. 17, 703-710. Stanley H. E. and Pameijer C. H. (1985) Pulp capping with a new visible-light-curing calcium hydroxide composition (Prisma VLC Dycal). Oper. Dent. 10, 156-163.
Book Review Physiology for Dental Students. D. B. Ferguson. Pp. 362. 1988. Guildford, Wright Imprint of Butterworth-Heinemann Ltd. Softback, f 19.50 This is a concise, readable and attractive textbook for undergraduate dental students, which may serve also as a background resource for FDS candidates. The text avoids the conventional tour of the systems of the body, which tends to obscure the common principles of physiology necessary for a proper understanding of how the body works, and instead, in the first part sets out general guidelines for physiological processes. Systems are dealt with in the second part, bringing together the general concepts already introduced. Finally, control mechanisms are discussed in a third section, including growth, haemostasis, exercise, digestion (including salivary function) and endocrinology. The material draws extensively on dentally relevant examples, while not ignoring the needs of dental students
for a broad coverage to permit an understanding of general medicine, surgery, pharmacology and pathology. Special aspects of physiology, such as mastication, speech and taste, are dealt with in detail appropriate for a first year dental course, but which may require more development later. The text is admirably clear and unfussy, as are the line illustrations. The more inquisitive student will require fuller explanation of some of the experimental background which is not emphasized, but to which he is guided in recommendations for further reading at the end of each chapter. The current extension of the basic science content of dental courses will stimulate the need for a modern, basic textbook of physiology which this extremely practical book is likely to meet. It should prove a firm favourite with students. W. M. Edgar.
