J. Dent. 1992;

20: 302-310

A confocal microscopic evaluation of the interface between Syntac adhesive and tooth tissue T. F. Watson and D. M. de J. Wilmot Department

of Conservative


United Medical

and Dental Schools, Guys Hospital,



ABSTRACT The aim of this study was to microscopically evaluate the effect of dentine smear layer thickness, tubular orientation and immediate stress application on a modem dentine bonding agent. Eighty mesial and distal wedge-shaped cavities were cut into dentine/cementum cervically in 40 extracted human lower third molars. The thickness of the dentine smear layer was reduced by polishing the cavity surface in half the samples. Each component of the bonding agent (Syntac: Ivoclar Vivadent) was labelled with a fluorescent dye, the unfilled resin being light cured for 30 s with the composite restoration placed in one increment and light cured for 40 s. The samples were longitudinally sectioned using a slow speed diamond saw under water, either immediately or 24 h post placement. The sectioned surfaces were then viewed using a confocal optical microscope. The thickness of the smeared layer only effected the penetration of adhesive in dentine tubules which were separated from the pulp chamber by the cavity design. These areas were well filled with adhesive: however, areas communicating with the pulp chamber showed no penetration differences due to smear layer reduction by polishing. The interdependence of the adhesive components was illustrated by failure to achieve a wellimpregnated tooth/adhesive hybrid layer when the materials were incorrectly handled. The Syntac/dentine interface was generally able to withstand the stress of sectioning immediately post placement, but showed signs of failure in the cavity line angle when the applied stresses were greatly increased. This study points to the need for clinically realistic in vitro testing regimens for dentine bonding agents, especially with regard to tubule orientation and the application of immediate stress to the tooth-restoration interface. KEY WORDS:

Dentine bonding, Confocal microscopy,

J. Dent. 1992; 1992)

20: 302-3

Restorative dentistry

10 (Received 29 January 1992;

reviewed 19 February 1992;

Correspondence should be addressed to: Dr T. F. Watson, Department of Conservative Medical and Dental Schools, Guys Hospital, London Bridge, London SE1 9RT. UK.

INTRODUCTION Many of the third generation bonding agents are reported to be producingin vitro dentine/restoration bond strengths approaching those of acid-etched enamel (Setcos, 1988). This could suggest that these systems are now able to withstand all the competing stresses that are set up either in function, or as a resin composite restoration is polymerized. Unfortunately, many bond strength studies have used dentine samples which have not been maintained in physiological or even wet conditions, and so results can often be unnaturally favourable to certain types of adhesive (Rueggeberg, 1991). Similarly, the testing will often fail to take account of anatomical variations in @ 1992 Butterworth-Heinemann 0300-5712/92/050302-09


accepted 16 March



the tooth and how these may affect the performance of the adhesive in different parts of a test cavity (Prati et al., 1991). Modern adhesives are generally applied in a number of layers or stages, but very few studies have been able to evaluate the interdependence of these components, either at the interface, or within the structure of the tooth itself (Watson and Boyde, 1987; Watson, 1989). Variables in the preparation method of the tooth (such as smear layer thickness) have of course been studied extensively (Prati et al., 1990). However, when such studies use SEM, only the gross dentine penetration of adhesive and not individual components of the system can be imaged, it also being impossible to examine an intact interface (Watson and Boyde, 1991).


and Wilmot:

Confocal optical microscopy enables high resolution images to be made of intact tooth-restoration interfaces, below the surface of a sample. These microscopes produce an optical tomogram, making thin optical sections within translucent samples. If fluorescent dyes are added to the components of an adhesive system then it is possible to track their movement both within the tooth and the restoration (Watson, 1991; Watson et al., 1991). The aim of this study was to evaluate the mechanisms of action for a new dentine adhesive (Syntac, Ivoclar Vivadent, Schaan, Liechtenstein) using a confocal optical microscope, comparing this with other materials which had previously been evaluated using this technique. Syntac is an adhesive system where the primer consists of a volatile solvent, a polyalkenoate acid and a low molecular weight resin in water; this is followed by an adhesive of increased molecular weight in an aqueous solution containing glutaraldehyde. These are applied to the tooth and evaporated, whilst the bond is completed by the addition of a light-cured unfilled resin, containing resins compatible with the primer and adhesive. Factors in the use of Syntac to be examined included: 1. The effect of the thickness of the smeared layer on the penetration and adaptation of the adhesive. 2. The effect of tooth structure on the penetration of the adhesive. 3. The penetration and interrelatedness of the various adhesive components. 4. The ability of the adhesive to seal the cavity surface, both from the pulp chamber and from external microleakage. 5. The ability of the adhesive to withstand early post placement stresses.



Forty human lower third molars were extracted as close to experimental use as possible: these were then kept in phosphate buffered saline (PBS), in a refrigerator, prior to use at 37°C. Samples were checked before use for any damage caused by their removal. Two wedge-shaped cavities (similar to an abrasion-type cavity) were cut with a diamond bur in an ultra-high-speed handpiece and then finished with a steel bur in a slow-speed handpiece under a profuse water spray. Cavities were finished onto dentine-cementum cervically, with no intentional enamel bevel (Fig. 1). In each tooth the mesial cavity was cleaned with a polishing paste (supplied with the Syntac adhesive for use in abrasion type cavities where limited preparation is required) to reduce the smear layer thickness, whereas the distal cavity was not. This was to give an indication of the importance of smeared layer thickness with respect to the penetration of the dentine adhesive, as well as the adhesives’ adaptation to dentine. The majority (68/80) of the restorations had enamel etched for 30 s and then washed for 30 s; the gel etchant was applied very carefully


of Syntac/dentine



Fig. 7. Diagram of longitudinally sectioned tooth showing cavity shapes (stippled) and schematic orientation of dentine tubules to the cavity surfaces. Notch in root indicates orientation. to avoid

dentine contamination and then washed off, away from the dentine. This enabled the competing stresses between the acid-etch retained resin composite and the dentine bonding agent to be evaluated in terms of the presence of absence of gaps at the cervical margin. The influence of any accidental run-off of phosphoric acid onto the dentine close to the enamel-dentine junction (EDJ) would also be highlighted by its effect on the penetration of bonding agent in this region. Control samples did not use enamel etching in order to compare the appearance of the adhesive interface at the EDJ. The primer and adhesive were both applied for 20 s and then dried, followed by the untilled resin which was brushed on as a thin layer and air thinned prior to light curing for 30 s. The distribution of the primer within the dentine and smeared layer was shown by the incorporation of fluorescent dye (rhodamine B) into the primer. This was attached to the monomer by the manufacturer. In eight restorations the primer was labelled with fluorescein or auramine 0 for the purposes of dual labelling and also to act as a control on the distribution of the dye. Water was used as a control to see if the adhesive alone had any effect on the smeared layer (Watson, 1989). Fluorescent dye (rhodamine B, fluorescein or auramine 0) was incorporated into the adhesive to enable its distribution to be visualized. The dye dissolved easily and homogeneously into the adhesive. The control situation of no adhesive or unlabelled adhesive was used to check that the primer/untilled resin gave no significant interactions. Forty restorations had fluorescein label incorporated in the unfilled resin, the remaining 40 were unlabelled. The labelled resin samples were used to indicate the extent of any mixing between the adhesive layers and the unfilled resin. Thirty-four of the unlabelled restorations (i.e. 17


J. Dent.



No. 5

Fig. 2. Uncleaned smear layer viewed en face (left hand side). The thickness was measured by through focusing until dentine tubules became visible. Typically, the smear layer was 4-5 pm thick. The feint diagonal colour banding is due to grooves on the surface of the sample showing as focus level differences because of chromatic aberration. The surface smear was reduced by polishing and the effect of this can be seen on the right hand side. Dentine tubules can be seen at the top of the picture with a patch of remaining smear at the bottom. Any remaining patches of smeared dentine were typically reduced to 2-3 urn in thickness. (X 100/l .4 NA oil imm. Fieldwidth: 50 pm.) Fig. 3. Low magnification view of cavity line angle (see Fig. 1) showing massive penetration of rhodamine-labelled primer into cleaned occlusal dentine (OD), but no apparent movement into pulpal dentine (PD). There is considerable pooling of the Heliobond unfilled resin in the cavity line angle, where the material has been inadequately air thinned, but the primer and adhesive component occupy a thin layer. (5461600 nm. X 2010.8 NA oil. Fieldwidth: 250 pm.) Fig, 4. High magnification view of composite (C), Syntac and dentinetubules (D) communicating with the pulp chamber. The top portion of the image is a combined reflection and fluorescence image showing some dentine structure, whilst the bottom is a fluorescence image showing the rhodamine-labelled adhesive distribution, alone. The hybrid layer between dentine and composite can be seen with the differential penetration of the materials in this region. The sample was sectioned immediately after placement and shows no evidence of cracking at the interface. The adhesive has penetrated about 20 pm. (546/nm top, 546/600 nm bottom x 100/l .4 NA oil. Fieldwidth: 50 pm.) Fig. 5. Dentine tubules (D) in the occlusal part of a polished cavity, where the primer was omitted. The rhodamine-labelled adhesive has penetrated some of the tubules to a far smaller extent than normal. (546/600 nm. X 60/l .4 NA oil. Fieldwidth: 63 pm.) Fig. 6. Dentine tubules (D) connecting with the pulp chamber where the adhesive has been omitted. The Heliobond has failed to adequately wet the surface of the dentine and a void can be seen in the interface (arrowed) where the primer and resin have mixed to a limited extent. (546/600 nm. X 60/l .4 NA oil. Fieldwidth: 63 pm.) Fig. 7. Polished dentine tubules (D) in the occlusal part of a cavity partially impregnated by rhodamine-labelled primer (red) on the right hand side of the micrograph. Notice the apparent void in the tubules close to the composite (C) interface. Adhesive has been labelled with fluorescein and its distribution can be seen in the left hand side of the micrograph. The adhesive (pale green) has penetrated more of the tubules than the primer and shows as a thin film at the composite interface 6 pm through focus sequence. (546/600 (rhodamine, right) and 4901520 nm (fluorescein, left). X 60/l .4 NA oil.) teeth) were either imbibed with dye from the pulpal aspect (soaked) for at least 24 h or subjected to external microleakage with the same dye (fluorescein was dissolved in PBS and the appropriate parts of the tooth sealed with nail varnish). These soaking samples were designed to see if there was any movement of aqueous fluorescent label along the interface (i.e. the sealing ability of the adhesive). It was possible to discriminate between rhodamine and fluorescein in the same sample by the appropriate use of filters (Watson and Boyde, 1991). The Heliomolar resin composite was placed with maximum layer thickness of 2 mm, and cured with a Heliolux II light source for 40 s. The teeth were sectioned, once, through the centre line of the two restorations (longitudinal sections) using a Labcut diamond saw at slow speed, under water. Each tooth was microscopically sampled twice, by examining the tooth on either side of the 300 urn wide saw cut. Most samples were examined 24 h after placement, but ten of the teeth (i.e. 20 restorations) were sectioned as soon after placement as possible (within 5 min). The ability of the adhesive to withstand the stresses of early cutting could therefore be assessed. A confocal microscope of the tandem scanning type (TSM: Noran Instruments, 2551 W. Beltline Highway, Madison, WI, USA) was used to examine the cut surfaces of the teeth, focusing below the layer disturbed by sectioning. Optical reconstructions, or extended focus images, were made of some of the samples to ensure that the full penetration of adhesive was being recorded (Watson, 1991). Samples were kept at 100% humidity and examined using oil immersion objectives. Randomly selected samples were subjected to hard tissue removal, using 0.2 N hydrochloric acid and sodium hypochlorite,

to ascertain that the fluorescence image corresponded with the physical presence of adhesive tags (Watson and Boyde, 1987; Watson, 1989). Presence or absence of gaps was noted during examination and fields of view recorded using a 35 mm camera, or video recording methods. In all, 160 restoration interfaces were examined and recorded. Photographs were taken in two positions along the cavity margin: the occlusal portion up to the EDJ, and the pulpal wall (although relevant information was noted for the entire interface). The samples were examined using X 60 objectives for details of the or X 100 oil immersion interface, or alternatively, areas such as the cavity line angle were examined at lower magnification (X 20) to give an overall picture of the adhesive distribution. The transparencies were assessed by the two authors, working single blind with one describing to the other the parameters visible in each section. Discrimination of the following variables was undertaken: 1. The part of the tooth under examination. 2. Whether or not the fluorescence image was attributable to the presence of primer or adhesive (it became apparent from an early stage that there was a very characteristic penetration of the primer and adhesive). 3. Whether or not the cavity surface had been cleaned.

RESULTS The effect of the thickness of the smeared layer on the penetration and adaptation of the adhesive was only apparent in areas of the cavity where the dentine tubules were excommunicated from the pulp, by virtue of the cavity shape (Fig. I). The use of the polishing paste patchily reduced the smeared layer thickness, or removed

Watson and Wilmot:

Evaluation of Syntac/dentine




J. Dent.



No. 5

Watson and Wilmot:

Evaluation of Syntac/dentine



Fig, 8. Occlusal dentine (D) interface with Syntac and composite (C). The Heliobond component of the adhesive system has been labelled with auramine. Short tags of resin can be seen within the openings of the dentine tubules, corresponding to the ‘voids’in Fig. 7. The air-inhibited layer of the resin can be seen to have mixed with the overlying resin composite. (490/520 nm. X 60/l .4 NA oil. Fieldwidth: 63 pm.) Fig. 9. Dentine tubules (D) communicating with the pulpchamber.TheSyntac has been labelled with auramine in the primer and rhodamine in the adhesive. The distribution of the primer (P) can be seen at the top and the wider band of adhesive (A) at the bottom. Mixing of both of these components and the Heliobond resin has occurred at the interface, whilst the distribution of the primer and adhesive within the dentine appears to be comparable (cf. dentine with no pulpal connection: Figs 3, 7). Sample sectioned immediately post placement. (450/520 nm top, 546/600 nm bottom, X 60/l .4 NA oil. Fieldwidth: 63 pm.) Fig. 10. Cavity line angle (LA) with Heliobond labelled with auramine. Sample was sectioned under a heavy load, immediately post placement, and shows cracking along the pulpal interface (arrowed). This was not evident on the occlusal dentine and stopped well shot-t of the cervical margin. Such an appearance was not seen in equivalent samples sectioned after 24 h. (450/ 520 nm. X 60/l .4 NA oil. Fieldwidth: 63 ,um.) Fig. 7 7. Cervical margin of restoration subjected to external microleakage using rhodamine dye solution. The dentine (D) can be seen on the left, with the composite (C) on the right. No fluorescent label has been incorporated into the adhesive complex: this was just discernible as a dark band when examining the sample. The deep yellow dye has not penetrated along the adhesive interface. (546/600 nm. X 60/l .4 NA oil. Fieldwidth: 63 pm.) Fig. 72. Reflection image of a cavity line angle (LA) showing how the orientation of dentine tubules can allow tubules to be cut off from the pulp by cutting a cavity. The fluorescent dye from the pulp chamber is not distinguishable in this image. (546/nm. x 20/0.8 NA oil. Fieldwidth: 250 pm.) Fig. 13. If there was a failure to seal in the occlusal part of the cavity, then dye from the pulpal aspects of the tooth would penetrate along this aspect. This is the same field as Fig. 72, showing rhodamine dye penetration up to, but not past, the cavity line angle (arrowed). (546/600 nm. X 20/0.8 NA oil. Fieldwidth: 250 pm.)

it (Fig. 2). The penetration of both the primer and the adhesive was approximately doubled in the occlusal dentine of samples which had the smear layer reduced by polishing. The appearance of the cavities which had been phosphoric acid etched on the occlusal enamel was indistinguishable from those which were unetched. On the pulpal wall of the cavities it was impossible for the examiners to discriminate between areas which had or had not been cleaned. The hybrid dentine-bonding agent layer was clearly labelled as a thin line by the primer and the adhesive, indicating a thin film thickness (Fig. 3). It was rare to see any pooling of the Syntac adhesive, although the Heliobond resin could be subject to pooling if it was inadequately air thinned. The relationship of the cavity surface to the pulp chamber had a profound effect on the penetration of the adhesive. The examiners could always determine whether a particular field of view in a transparency showed dentine tubules which communicated with the pulp (‘pulpal dentine’), or were removed from pulpal influence in the occlusal part of the cavity (‘occlusal dentine’). At low magnifications there was a striking difference between the adhesive penetration up to and past the cavity line angle (Fig, 3) with apparently no penetration into the pulpal dentine. However, at higher magnifications it was evident that the adhesive complex was able to penetrate the smeared layer (< 30 pm), when the tubules communicated with the pulp chamber (Fig. 4). When individual components of the Syntac adhesive system were omitted, the restoration showed poor adaptation (Fig. 5) and a linear breakdown at the adhesive-tooth interface, with the composite pulling away from the partially infiltrated dentine (Fig. 6). It was noticeable that in the areas of occlusal dentine, the primer advanced ahead of the adhesive in many tubules and showed greater

penetration (< 300 pm), although it was always present in the smeared layer. This characteristic component separation could be distinguished in the relevant samples by the examiners working single blind. When samples were examined with double fluorescent labels and extended focus images, it became evident that the primer penetrated fewer tubules in the occlusal dentine than the adhesive (Fig 7). This was also apparent when the image was recorded with an extremely sensitive SIT video camera, with full automatic gain control compensating for any differences in signal intensity or dye concentration between the two fluorescent dyes. These samples also showed an apparent gap between the adhesive and primer in the tubules and the overlying resin composite. By labelling the Heliobond resin it was ascertained that this tubular gap was filled by the resin, completing the transition from dentine to composite (Fig. 8). Dentine tubules which communicated with the pulp chamber (Fig. I) showed a more complete mixing of materials at the interface. The primer and adhesive were constrained in a much narrower band than previously, with little evidence for separation of the components (Fig. 9). The representative samples which were subjected to removal of tooth tissue by acid erosion showed a good correlation between the fluorescence adhesive image and the structural image of the resin tags. The majority of the samples were sectioned after 24 h and showed no evidence of bond failure, where the adhesive had been correctly applied. In the teeth which were sectioned immediately post placement, under the same cutting conditions, no evidence of delamination or fracture in the interface was found (Figs 4, 8, 9). If. however, the weight applied to the sample during cutting was greatly increased, then bond failure was noted to spread from the cavity line angle but seldom extended to either the occlusal or the cervical margin (Fig. 10).


J. Dent.



No. 5

In the samples which were assessed for external microleakage, complete sealing was achieved against the fluorescent dye solution (Fig. 11). The internal sealing ability of the Syntac was sufficient to prevent any movement of dye along the occlusal portion of the cavity, the cut-off being at the line angle (Figs 12, 13). In the situation of an incorrectly applied adhesive, there was profound leakage in all cases.

dentine bonding agents has been extensively studied by Pashley and his co-workers (Pashley and Pashley, 1991). It has been noted in a previous study using a confocal microscope that the presence of the pulp reduced the penetration of GLUMA, although it still showed a significant impregnation of the pulpal dentine (Watson and Boyde, 1987). We have found that the Syntac adhesive was capable of massive impregnation of the dentine tubules in the occlusal part of the cavity, where these no longer had a communication with the pulp chamber. It would seem that the partially dried dentine tubules exerted a considerable capillary effect on the adhesive system. Where the tubules had a remaining connection to the pulp chamber, then the penetration of the adhesive system was altogether more restrained, embedding < 30 urn of the surface dentine. This is probably sufficient for a strong bond at 24 h where there was no evidence of delamination on sectioning. A reduction in pulpal penetration of an adhesive will reduce the likelihood of any pulpal reaction to the constituents of the material. The teeth in this study were not, of course, restored in vivo but were freshly extracted and stored in a physiological storage medium. The system used in these experiments probably represents the situation of an anaesthetized tooth, where the pulpal fluid pressure is reduced due to the vasoconstrictor effect of the local anaesthetic (Pashley and Pashley, 1991). The pulp chamber was only acting as a fluid reservoir in these experiments, but was still able to exert an influence on the gross penetration ofthe Syntac and also perhaps its ability to wet the dentine surface. This finding would have important implications for bond testing studies where blocks of dentine (especially bovine dentine) could easily

DISCUSSION Many dentine bonding agents use quite acidic conditioners or primers to remove or reduce the thickness of the smeared layer on cut dentine. This may have the effect of increasing fluid flow from the pulp, if the tubules are opened sufficiently, which could be a problem for some adhesive systems (Prati et al., 1991). With the Syntac system the primer contains a weak acid, but this study has shown that the penetration of the primer can be enhanced when the thickness of the smear layer is reduced by polishing. The manufacturers recommend this for abrasion type cavities to remove surface debris, where limited cavity preparation is required. In this study the increased penetration was only noted in dentine tubules which did not communicate with the pulp, with its effect elsewhere being equivocal. An implication of this is that polishing will only improve the adhesive adaptation in an area which is already going to be well impregnated by adhesive. Therefore, routine polishing is not indicated for cavities which have involved dentine preparation. One of the most striking findings was the relationship between the adhesive penetration and its position along the cavity margin. The effect of pulpal fluid pressure on

Table 1. Overall numbers variables

of restorations

Restorations Primer


+ + Rh

6 2 5 1

+ Rh + FI



: 5 13 82 2 14 2 80


in this study,


the operative

and microscopic


+ + + + + _

FVAu Rh Rh Rh

+ + FI


+ + + Rh + Rh + + + Rh FI/Au _ + + Rh + Rh + Rh

Unfilled resin

+ + + +



3 7 (6) 3 (3) 0 1

2 (7 7) 1

4 (7-Y 1 4

Immediate sectioning

1 4

2 (4)

; 4

4 (8) &T,

+ FI + FI + FI FVAu + FI

Internal leakage

1 (5)

+ + Rh + FVAu +

External leakage

1 1 7


44 (70, 3 (9) 1




1 4 1 4


1 1 6



Data from a number of controlled subgroups have been combined, giving an apparent random distribution of samples. Corresponding figure numbers are given in italics. Numbers in columns indicate numbers of restorations for each category. +. Material present; -, material absent; Rh, rhodamine B; FI, fluorescein; Au, auramine 0.

Watson and Wilmot:

be dissociated from their fluid supply, so giving an unnaturally high bond strength. Such variations in sample preparation technique would also partly account for the virtual incompatibility between bond strength measurements from different institutions. This study has only used one particulartype of standardized cavity, mimicking a severe abrasion lesion. The occlusal disconnection of tubules from the pulp chamber could, however, easily occur in other cavity designs, such as in the lateral walls of proximal cavities. The implication of this is that the adhesive bond strength will differ significantly for various parts of the same tooth, as shown by the reduced bond strength implicit in the failure at the pulpal aspect of the cavity line angle, when the samples were immediately loaded. This would complicate attempts to model the strengths or weaknesses of the adhesive interface in a simple form of finite element analysis. As one would expect, the adhesive system was not fully effective if important stages were left out. The correct use of the primer, adhesive and untilled resin was particularly important for the pulpal aspects of these restorations. If the adhesive was left out, then gaps formed between the acidic primer and the Heliobond, but the superficial dentine had clearly been altered by the primer. The acidic and volatile primer was capable of wetting the dentine surface extremely well, clearing a pathway for adhesive to follow. The development of the technique for making threedimensional reconstructions of the Syntac-tooth interface, in conjunction with double fluorescent labelling, has enabled a more complete understanding of the interrelatedness ofthe primer, adhesive and untilled resin to be reached. It was possible to analyse the differential penetration of the primer and adhesive in the same volume slice of dentine at high resolution, which would not be possible using any alternative microscopic techniques. The primer penetrated ahead of the adhesive, into tubules which were no longer connected to the pulp, but the adhesive actually impregnated a larger number of the tubules. In the dentine which was connected to the pulp chamber, the primer, adhesive and untilled resin tended to mix more thoroughly at the interface indicating that pulpal fluid decreased the separation of the components. The untilled resin played an important part in the bond to make a trilayer in the hybrid layer between dentine and restoration, with a true transition from the wet dentine to the hydrophobic resin composite (Nakabayashi et al., 1982). The differential distribution of the primer, adhesive and unfilled resin within the hybrid layer was therefore made visible by using the fluorescent labelling techniques in conjunction with confocal microscopy. The Syntac adhesive system consists of a number of layers to be applied, but only the final layer of resin requires light curing. Although not directly examined in this study, it would seem likely (based on the microscopic flow and wetting properties of the primer and adhesive (Griffiths and Watson, 1992)) that the system could be applied more rapidly than recommended and still be

Evaluation of Syntac/dentine



effective. Therefore, this rapidly applied multilayered adhesive system is more user friendly than many apparently simpler systems. These systems may, in fact, be more technique sensitive because they often require application and possible curing of components for a lengthy time period. The primer and adhesive showed a minimal film thickness, which would be important for indirect restorations, where the luting composite would replace the Heliobond. It would be important that compatibility be maintained between the luting agents and the adhesive system. Other commercially available adhesives are apparently sensitive to adhesive film thickness, because of the need for a thick resin layer in order to reduce the proportion of the overall film occupied by the airinhibited layer; these would be less suitable in luting applications (Watson, 1989). Microleakage studies are normally undertaken following thermocycling in order to stress the bond and so allow gross dye penetration of the samples, subsequently using a macroscopic examination of the interface. The use of fluorescent dyes and confocal microscopy produces an equally sensitive system, without needing to resort to thermal stressing (Chong er al., 1991). The Syntac adhesive was highly effective at sealing the cervical margin against leakage in this study, and this was confirmed by examination of the samples with ‘internal’ leakage, where the adhesive stopped the penetration of any dye past the cavity line angle. The most important requirement of a dentine adhesive is its ability to withstand early post placement stresses in a restoration and achieve a high early bond strength. All of the samples where the system had been used correctly showed no de-lamination when sectioned 24 h after placement. When sectioned immediately, failure was only seen when the stresses induced by the saw were increased to a maximum and the saw blade was tending to ‘tear out’ the restoration. Failure was only seen in the cavity line angle, extending towards the cervical margin but not producing an open margin, perhaps indicating a reduced polymerization of the composite/adhesive in this area. Microscopic examination of the tooth-restoration interface under load would produce valuable comparative insights into the weaker areas of new adhesive materials. In conclusion, the adhesive under study performed well in a number ofin vitro tests relating to the likely success of a dentine bonding agent, with fluorescence confocal microscopy being used to indicate the distribution of various adhesive components. Adhesive penetration was at its greatest in areas where the dentine tubules were excommunicated from the pulp chamber. Adhesive failure was only seen when the samples were sectioned immediately (with considerable force) rather than at the more normal 24 h. This study points to the need for clinically realistic in vitro testing regimes for dentine bonding agents, especially with regard to tubule orientation and the application of immediate stress to the toothrestoration interface.


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No. 5

Acknowledgements We would like to thank our colleagues in Dental Photography for all their skilled help with this project and Paul Sheppard for a steady supply of teeth for this and other projects.

References Chong B. S., Pitt Ford T. R. and Watson T. F. (1991) The adaptation and sealing ability of light cured glass ionomer retrograde root fillings. Int Endod. J. 24, 223-232. Griffiths B. M. and Waltson T. F. (1992) A real-time confocal microscopic study of dentinal fluid movement. J. Dent. Res. 71, 760 (abstr. 1956). Nakabayashi N., Kojima K. and Masuhara E. (1982) The promotion of adhesion by the infiltration of monomers into tooth substrates. J. Biomed. Mater. Res. 16, 256-273. Pashley D. H. and Pashley E. L. (1991) Dentin permeability and restorative dentistry: a status report for the American Journal of Dentistry Am. J. Dent. 4, 5-9. Prati C., Biagini G., Nucci C. et al. (1990) Effects of chemical pretreatment on dentin bonding. Am. J. Dent. 3, 199-206.

Prati C., Pashley D. H. and Montanari G. (1991) Hydrostatic intrapulpal pressure and bond strength of bonding systems. Dent. Mater. 7, 54-58. Rueggeberg F. A. (1991) Substrate for adhesion testing to tooth structure-review of the literature. Dent. Mater. 7, 2-10. Setcos J. C. (1988) Dentin bonding in perspective. Am. J. Dent. 1, (Special Issue), 173-175. Watson T. F., Billington R. W. and Williams J. A (1991) The interfacial region of the tooth/glass-ionomer restoration: a confocal optical microscope study. Am. J. Dent. 4, 303-310. Watson T. F. and Boyde A. (1987) Tandem scanning reflected light microscopy: applications in clinical dental research. Scanning Microsc. 1, 1971-1981. Watson T. F. and Boyde A. (1991) Confocal light microscopic techniques for examining dental operative procedures and dental materials. Am. J. Dent. 4, 193-200. Watson T. F. (1989) A confocal optical microscope study of the morphology of the tooth/restoration interface using Scotchbond 2 dentin adhesive. J. Dent. Res. 68, 1124-1131. Watson T. F. (1991) Applications of scanning optical microscopy to dentistry. Br. Dent. J. 171,287-291.

Book Review Current Controversies in Orthodontics. B. Melsen. Pp. 3 14. 199 1. New Malden, Quintessence. Hardback, f 70.00. This book, comprising ten chapters, is edited by Birte Melsen. There are 14 contributors, ten working in North America, three in Denmark, and one in France. Most of the contributors are orthodontists, but the disciplines of behavioural science, communicative disorder, growth and development, oral physiology, restorative dentistry and oral and maxillofacial surgery are also represented. The book identifies ten specific areas of controversy in current orthodontics, arranged as follows: 1. (Albino and Tedesco): The aesthetic need for orthodontic treatment. 2. (Baumrind): Prediction in the planning and conduct of treatment. 3. (Warren and Spalding): Dentofacial morphology and breathing. 4. (Bakke and Moller): Occlusion, malocclusion and craniomandibular function. 5. (Norton and Melsen): Functional appliances. 6. (Burstone): The biomechanical rationale of orthodontic treatment.

7. (Melsen): Limitations in adult orthodontics. 8. (Tucker and Thomas): Rigid fixation for orthognathic surgery. 9. (Fontenelle): Lingual orthodontics in adults. 10. (P Vig): The origins, consequences and resolution of orthodontic controversies. The subjects have been well chosen for inclusion as controversies, and the contributors address them squarely and honestly. They are to be commended for their clarity of presentation and quality of argument-indeed in this connection the book is outstanding, and reflects what must have been a considerable editorial effort. Each chapter draws upon research work in that particular area, and attempts as far as possible to identify scientifically supportable points of view. All chapters are augmented with valuable references. The book does not set out simplistically to resolve controversies but rather to air them, to identify the absence of a firm scientific basis for much ‘conventional wisdom’, and to suggest an intellectual and practical approach whereby orthodontics can be put on a more realistic foundation. It is both readable and stimulating, and can be recommended to all orthodontists. J. K. Williams.

A confocal microscopic evaluation of the interface between Syntac adhesive and tooth tissue.

The aim of this study was to microscopically evaluate the effect of dentine smear layer thickness, tubular orientation and immediate stress applicatio...
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