J. Dent. 1992;

20: 3-10

Review

Microleakage M. J. Taylor and E. Lynch* Department of Prosthetic Dentistry and *Department Hospital, Whitechapel, London, UK

of Conservative

Dentistry,

Dental institute,

Royal London

ABSTRACT A critical review of techniques used in the assessment of microleakage in dental restorations is presented. These techniques include the use of air pressure, bacteria, radioisotopes, electrochemistry, chemical tracers and dye penetration. Wide variations in methodologies are revealed. KEY WORDS: J. Dent. 1992;

Microleakage, Dentistry, Review 20: 3-l 0 (Received 25 March 1991;

accepted 30 June 1991)

Correspondenceshouldbeaddressed to: Mr M. J.Taylor, Department of Prosthetic Dentistry, Dental Institute, Royal London Hospital, Whitechapel, London El 2AD. UK.

INTRODUCTION Given a remit to design the perfect restorative material, most readers would no doubt include in their specifications the ability to bond to tooth substance in such a way that there is a complete and perfect seal between the margin of the restoration and the tissue of the tooth. This elusive ability to prevent leakage continues to be sought as the methods used to assess materials become more exacting. In an effort to update existing reviews of the subject (Going, 1972; Kidd, 1976; Shortall, 1982; Trowbridge, 1987), the following is a critical review of the methods which have been, and those which continue to be, used to determine the ability of a restorative material to develop a marginal seal.

MICROLEAKAGE The effects of bacterial leakage upon the dental pulp are well documented (Bergenholtz et al., 1982; BrannstrGm, 1981, 1987). Prevention of bacterial access along the margins of restorations is therefore a high priority. As early as 1861, in an effort to determine the effectiveness of dental restoratives as sealants, microscopic examination of amalgam marginal contraction was carried out by Tomes (Blackwell, 1955), followed by experiments into the leakage of dye indicators around the margins of amalgam packed into glass tubing. Since these early experiments, countless workers have attempted to demonstrate leakage of materials and to improve the marginal seal. It is perhaps a measure of the success in this field that this is today termed ‘micro@(1992) Butterworth-Heinemann 0300-5712/92/010003-08

Ltd.

leakage’. Microleakage may be defined as the passage of bacteria, fluids, molecules or ions between a cavity wall and the restorative material applied to it (Kidd, 1976). Many different techniques have been used to demonstrate that, despite what clinicians may like to think, the margins of restorations allow the active movement of ions and molecules. These techniques include the use of bacteria, compressed air, chemical and radioactive tracers, electrochemical investigations, scanning electron microscopy and, perhaps most commonly of all, the use of dye penetration studies. These methods have also been used with varying degrees of success in the study of modern endodontic materials. Investigation of leakage has been carried out both in tivo and in titro, but the latter is more common. In vitro experiments fall broadly into two categories; those which use a clinically relevant model which attempts to reproduce the oral situation, and those in which the model does not represent this and is purely a test of the materials’ behaviour.

MICROLEAKAGE

STUDIES

Air pressure In

vitro experiments using compressed air to test the integrity of the marginal seal of restorations were first carried out in 1912 by Harper using Class II amalgam restorations placed in pre-cut steel dies which were then immersed in water.

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It was not until the 1950s that experiments using extracted human teeth began to be reported using acrylic restorations (Fiasconaro and Sherman, 1952) and later using amalgam (Pickard and Gaynford, 1965). Both techniques involved the introduction of compressed air through the root canal and pulp chamber of the tooth and the measurement of the loss of pressure within a static system. Microscopic examination of the release of air bubbles from the margin of the submerged restoration provided a subjective view of the marginal integrity. Pickard and Gaynford successfully designed a means by which leakage could be studied on a longitudinal basis using standardized techniques. This method has the advantage that it is not destructive of tooth tissue (although the investigators reported that amalgam fragments were occasionally dislodged by the pressure), but does not qualify the information obtained other than to define the smallest path ofleakage which gives no detail of the true leakage pattern. Air pressure tests otherwise treat the restoration as leaking equally along the entire margin when this is unlikely to be the case. They do not represent the clinical situation nor is the drying effect of compressed air passing through the restoration taken into account. It is also possible that some leakage may occur through clinically sound tooth tissue.

BACTERIAL STUDIES Bacterial studies are fraught with difficulties, particularly in controlling the bacterial population. Early studies using amalgam packed into glass tubing (Fraser, 1929) were purely qualitative based upon the presence or absence of clouding in the medium below the restoration. Further studies (Kraus and Kraus, 1951; Seltzer, 1955; Rose et al., 1955) involved the immersion of restored teeth in cultured broths and the later investigation of the underlying dentine, again providing purely qualitative results depending upon the presence or absence of bacteria in the piece of dentine examined. Bacterial techniques continue to be used (Mortensen et al.. 1965; Mejare et al., 1979; Fayyad and Ball, 1987) as these techniques have some clinical relevance but continue to be qualitative rather than quantitative. Marginal gaps allowing the leakage of bacteria would be expected to be in the region of 0.5-1.0 pm or larger. These techniques do not therefore take into account gaps which are smaller than this and while not allowing bacterial penetration allow the passage of toxins and other bacterial products which could be detrimental to the tooth. Perhaps the most relevant of bacterial leakage studies are those in which secondary caries-like lesions are produced in the in vitro situation thus demonstrating the ability of the bacteria to gain access along the restoration/ tooth interface. Experiments using cultured bacteria (Ellis and Brown, 1967) are used less today in favour of acidified artificial caries media such as acidified gelatin (Kiddet al.,

1978; Greive and Glyn Jones, 1980) which produces more consistent results while still being relevant to the clinical situation. Artificial media however no longer demonstrate bacterial penetration of the restoration margin and give at best a simulation of the effect caused by leakage of bacterial products with far smaller dimensions.

RADIOISOTOPE STUDIES The development of radioactive isotopes for medical investigation has resulted in their increased availability in dental research. Isotopes used have been as diverse as 45Ca, 1311, 35S, **Na, 32P, 86Rb and 14C. Going (1964) suggested that the use of radioisotopes provides finer detail in leakage studies as the smaller isotope molecules measure only 40 nm compared with the smaller dye particles (120 nm). In his study comparing 45Ca and a gentian violet dye solution he showed that both indicators would pass easily through dentinal tubules to the pulp, but that at the restoration margin the isotope was more likely to penetrate than the dye. Going,alongwithotherworkers(HembreeandAndrews, 1978; Crim et al., 1985; Gottlieb et al., 1985; Hembree, 1989; Fitchie et al., 1990; Saunders et al., 1990), has used autoradiography to demonstrate isotope presence. This depends upon standardized exposure and development of the autoradiographs. However the results are compromised by the subjective assessment of the degree of leakage. Most workers continue to assess leakage on a scoring system which increases with severity, although Gottlieb and coworkers (1985) used a more objective system of scoring using the position of the amelodentinal junction as a guide to isotope penetration. However, as large (halftooth) sections were exposed using the autoradiograph method, interpretation relied heavily upon the relationship between cavosurface angle and the main X-ray beam. The fact that a two-dimensional autoradiograph showed maximal leakage which may have been at only one small point in the beam’s path was not taken into account. Other factors which may affect the resolution of an autoradiograph include: 1. The choice of isotope. A high energy isotope produces more scatter on the film artificially increasing the apparent leakage. Isotopes with low energies improve resolution. 2. Distance between source and emulsion. Increasing this distance magnifies the image but reduces the resolution. 3. Length of exposure. The longer the film is exposed the greater the chance that due to the random behaviour of beta particle emission the area of emulsion exposed will increase. 4. Rinsing. Rinsing of sections prior to exposure where water-soluble tracers have been used creates the likely risk of spreading isotope into previously uncontaminated areas (Delivanis and Chapman, 1982).

Taylor and Lynch:

The use of isotopes such as 45Ca which may have an affinity for tooth structure or restorative materials could also lead to a misleading distribution on the autoradiograph. Autoradiography therefore is still a qualitative, technique-sensitive means of determining leakage. Other workers (Powis et al., 1988) have developed a technique using radioactive sucrose (14C) by which longterm monitoring of microleakage can be achieved, however this is both destructive of tooth tissue and has questionable clinical relevance. Reverse radioactive absorption is a technique where the radioactive tracer is placed as a lining below the restorative material. The tooth is then immersed in a non-radioactive solution and the level of radioactivity in this solution is measured against time. Vasudev et al. (1981) and Herrin and Shen (1985) have both demonstrated that the leakage of a radioactive tracer placed in this way can be shown to increase with time. However it is not clear whether this leakage is due entirely to any gap which may exist at the restoration margin or if diffusion either through tooth substance or the material itself may be involved.

NEUTRON

ACTIVATION

ANALYSIS

Going et al. (1968) and, more recently, Douglas et al. (1980) have used a technique in which a chemical marker (manganese) is allowed to leak around the margin of a restoration in vivo, the tooth can then be extracted and placed in the core of a nuclear reactor where bombardment with neutrons energizes the tracer (56Mn). The radiation subsequently emitted by the tooth can then quantify the volume of tracer present. Again the technique does not identify at which point the restoration has leaked nor does it take account of manganese absorption at sites other than the restoration margin.

ELECTROCHEMICAL

STUDIES

The use of electrochemistry in the detection of marginal leakage in coronal restorations has been adapted from similar techniques used in endodontic research (Jacobsen and von Fraunhofer, 1975; Delivanis and Chapman, 1982; Mattison and von Fraunhofer, 1983; Momoi et al., 1990). The principle involves the insertion of an electrode into the root of an extracted tooth in such a way that it makes contact with the base of the restoration. Once restored the tooth is suitably sealed to prevent electrical leakage through normal tooth structure and immersed in an electrolyte bath. A potential is then applied between the tooth and the bath and leakage is assessed by measuring the current flowing across a serial resistor. The technique is obviously unsuitable for metallic restorations. Delivanis and Chapman (1982) used this method to determine leakage in endodontically treated maxillary incisors, comparing the efficiency of this method with autoradiography and dye penetration using 45Ca and 2 per

Microleakage

5

cent methylene blue respectively. They concluded that neither dye penetration or autoradiography correlated well with mid-range values of current, while at the two extremes of current flow there was a close relationship. However as both dye sections and autoradiographs were scored only as exhibiting ‘high leakage’ and ‘low leakage’, this mid-range discrepancy seems not altogether surprising. Lim (1987) further adapted this technique to compare the leakage of two treatments of glass (ionomer) polyalkenoate cements placed in coronal restorations using a longitudinal study over 30 days. The results suggested a gradual decrease in current flow (leakage) over time, although there were wide variations in the values. The electrochemical method of assessing leakage is again destructive of tooth substance and cannot be used in the in vivo situation. It does not take account of the dielectric properties of the restorative material or that these (especially in the case of glass-ionomer polyalkenoate cements) may change with time as setting reactions continue. As with air pressure studies there is a tendency to assume that leakage occurs evenly along the restoration margin. Lim attempted to investigate this using a silverstaining technique, but as this used separate tooth preparations few conclusions can be drawn from the results.

SCANNING

ELECTRON

MICROSCOPY

Many workers have used microscopic analysis of cavity margins to help correlate findings from other methods of determining leakage and provide an overall picture of the behaviour of restorative materials. The availability of the scanning electron microscope in dental research has important implications for the investigation of marginal adaptation, particularly when compared with results from leakage studies. A review of marginal adaptation and analysis although intimately involved with the study of microleakage is outside the scope of this article.

THERMAL

AND MECHANICAL

CYCLING

The use of repeated thermal and/or mechanical stressing of restorations now has a widespread role in the demonstration of marginal adaptation and microleakage. This is due in a great part to work by Nelsen et al. (1952), who showed that restored teeth when warmed from placement in iced water exuded small droplets of water from the restoration margin. Temperature cycles used in thermal stressing vary between workers, with few studies on the normal temperature variation in the mouth (Peterson et pal., 1966; Harper et al., 1980) while work by Harper et al. (1980) suggests that the temperature variation in the mouth may be quite small due to diffusion through the materials involved.

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Mechanical cycling of restored teeth may result in the production of marginal gaps either permanently or only while the tooth is under stress (Jorgensen et al., 1976). The effect of mechanical cycling on marginal leakage is apparently quite small when compared with the effects of thermal changes (Munksgaard et al., 1985).

CHEMICAL

TRACERS

The use of non-radioactive chemical tracers is distinct from the use of dyes in that these tracers rely upon the reaction between one or more of the chemicals used. The usual method involves the use of two colourless compounds to produce an opaque precipitate, usually a silver salt using established photographic techniques. In all cases these methods rely upon the penetration of both chemicals, hence precipitation will not occur when only the smaller of the two molecules can penetrate. In practice silver halide particles have been shown penetrating dentinal tubules with ease, often making interpretation of results difficult. Early chemical tracing of leakage is reported by Korntield (1953) who described a method where an acrylic resin had lead glass incorporated into it so that when immersed in a solution of barium sulphide marginal leakage could be demonstrated by the black precipitate of lead sulphide. Leinfelder et al. (1986) describe a technique where the pH change caused by the leakage of soluble calcium hydroxide liners is used to demonstrate leakage as the alkaline products escape at the margins of the restoration, causing colour changes in sensitive indicator papers. More commonly a 50 per cent silver nitrate solution is used to immerse the in vitro specimen which is subsequently reacted with a photographic developer such as benzene 1,4-diol (hydroquinone). Variations of this technique continue to be reported (Wu et al., 1983; Hammesfahr et al., 1987; Darbyshire et al., 1988; Pintado and Douglas, 1988; Barkmeier and Cooley, 1989; Barkmeier et al., 1989; Douglas et al., 1989; Eakle and Ito, 1989; Kanca, 1989; Sheth et al., 1989; Shortall et al., 1989; Swift and Hansen, 1989; Holtan et al., 1990). A similar technique using 1 per cent silver chloride solution has already been described (Lim, 1987). The silver salt/ photographic developer technique seems second only in popularity to the use of dye penetration studies. Douglas et al. (1989) point out that the silver nitrate test is very severe as the ion has an extremely small diameter (0.059 nm) when compared to a typical bacterium (0.51.0 urn). However, as the organic developer molecule has the larger diameter it is the penetration of this chemical which is being observed. It is therefore unlikely that observations are of the full extent of silver ion penetration. Developing techniques vary widely between operators, with developing times varying between 3 and 16 h. Assessment of leakage also varies. Kanca (1989) used a subjective assessment over a five-point scale of increasing

magnitude of leakage. A blind technique was incorporated to produce some standardization of results. Shortall et al. (1989) measured leakage around ceramic crowns expressing results as a percentage of the total length of the visible margin using single sections from each specimen. Holtan et al. (1990) Douglas et al. (1989) and Pintado and Douglas (1988) all measured leakage along the tooth/ restoration interface using single transverse sections cut at the centre of each restoration and observed under a stereomicroscope incorporating a calibrated graticule. The main problems associated with interpretation of these results is that single sections of tooth provide at best a two-dimensional representation of the threedimensional restoration. Position and angle of section can both significantly affect the length of the restoration margin exposed and hence the proportion of leakage. A single section may show the only place where the marginal seal has failed, or, conversely, the only place where it has not. In an attempt to avoid this problem, Swift and Hansen (1989) used similar techniques but took three 150 urn sections through each restoration and calculated the mean leakage. However, measurements of the length of the cavity/restoration margin were not recorded so it was not possible to determine whether the sections were perpendicular to the restoration and so an accurate representation of the extent of leakage. Chemical tracer studies share many of the characteristics of dye leakage studies. Unfortunately this includes many of the same problems, especially those of interpretation.

DYE PENETRATION

STUDIES

The use of coloured agents to demonstrate microleakage continues to be the most popular of techniques which are currently available. This method allows the production of sections showing leakage in contrasting colours to both tooth and restoration without the need for further chemical reaction or exposure to potentially hazardous radiation. However, it is highly technique sensitive and the assessment of results requires careful standardization. In general terms the technique involves the placement of an extracted, restored tooth in a dye solution for a predetermined period. This is followed by the washing and sectioning of the specimen and its examination (usually under magnification) to determine the extent of leakage around the tooth/restoration interface. The main disadvantages with this form of assessment are that it is usually associated with the assigning of a numerical scoring system of increasing degrees of leakage and that this assessment, although often carried out by more than one examiner, is somewhat subjective; also, as previously discussed, the assessment of the restoration as a whole is difficult when viewing only individual small sections of tooth. Most workers have taken single sections from the midpoint of a Class V restoration when there is

Taylor and Lynch: Microleakage

Table I. Dye penetration studies--summary References

Year

Going et al.

1960 1966 1966 1970 1973 1974 1975 1975

Christen and Mitchell Christen and Mitchell Messing Greive and Parkholm Sanders and Dooley Barry and Fried Al-Hamadani and Crabb Fogel Ford Zmener Crim and Mattingly Kwan and Harrington Douglas and Zakariasen Camp and Todd Alperstein et al. O’Neill et al. Tagger et al. Michanowicz and Czonstkowsky Munksgaard et al. Jacobsen et al. Crim et al.

1977 1979 1980 1981 1981 1981 1983 1983 1983 1983

of dye choice Dye used

2.0% 0.1%

0.5% 2.0% 2.0% 0.25% 1 .O% 0.5% 2.0%

20.0%

1984

5.0%

1985 1985 1985

9.0% 0.5%

7

Crystal Violet Fluorescein Rhodamine B FIuorescein* Eosin* Methylene Blue* Methylene Blue* Alcian Blue Methylene Blue* Eosin* Methylene Blue* Basic Fuchsin India Ink* Methylene Blue* Rhodamine B* Fluorescein India Ink* Procion Brilliant Green * Methylene Blue* Erythrosin Methylene Blue* Basic Fuchsin

References Phair and Fuller Eldeeb Pearson and Longman Fayyad and Shortall Zidan et al. Crim and Shay Zidan et al. Yu et al. Glyn Jones et al. Davila et a/. Callis and Paterson Fisbein et al. Fusayama and Kohno Spangberg et al. Guelmann et al. Crim Scherer et al. Goldman et al. Scherer et al. Youngson et al. Krejci and Lutz &reb6 and Raadal Crim Ciucchi et a/. Arcoria et al.

Year

Dye used

1985 1985 1987

2.0% 2.0% 1 .O%

Erythrosin B Methylene Blue* Methylene Blue*

1987 1987 1987 1987 1987 1988 1988 1988 1988 1989 1989 1989 1989 1989 1989 1990 1990 1990 1990 1990 1990 1991

1 .O% 0.5%

Methylene Blue* Basic Fuchsin Basic Fuchsin Methylene Blue* Methylene Blue* Eosin Basic Fuchsin Procion Blue* Basic Fuchsin Chinese Ink Methylene Blue* Basic Fuchsin Basic Fuchsin Basic Fuchsin Crystal Violet* Basic Fuchsin Eosin Basic Fuchsin Methylene Blue Basic Fuchsin Blue Cresyl Methylene Blue

2.0% 4.0% 0.5% 1 .O% 2.0%

2.0% 2.0% 0.5% 1 .O%

0.5% 5.0% 0.5% 0.5% 0.5% 0.5%

*Endodontic studies.

little, if any, evidence to suggest that this will reflect the true state of the other restoration margins. A dye technique which is non-destructive and so allows longitudinal study of restoration margins has been reported (DeTrey, 1976; Tsuchiya et al., 1986). This involves examination of the restoration margin under magnification following exposure to a dye substance and measuring the proportion of the margin which exhibits leakage. This technique however does not give an indication of the behaviour of the material below the restoration margin where large, unrecordable gaps may exist. A similar chemical system (Leinfelder, 1986). measuring the pH at the restoration margin has already been described. Douglas and Zakariasen (1981) have described a technique using an inert dye which is collected from the specimen following leakage to give a quantitative value for the amount of dye taken into the marginal gap. It is not clear however if all the dye taken up by the tooth can be recovered, particularly that taken into dentinal tubules. As in other forms of microleakage study, dye studies have also been used in the investigation of endodontic sealers. The diverse range of dyes, and concentrations used both in endodontic research and in studying leakage around restorations, is illustrated by the examples shown in Table I. Roulet (1976-as reported in Roulet et al., 1989), Spangberg et al. (1989) and Goldman et al. (1989) have shown that placing a specimen in vacuum before immersing in a dye solution significantly increases dye penetration along marginal defects. This is due to the

removal of entrapped air from within the system. However, despite this means of obtaining greater penetration and perhaps a more realistic picture of the pattern of leakage, the choice of dyes used continues to be selected on an apparent ad hoc basis with little mention given to the differing size of dye molecules/particles and their behaviour when used in these situations. Examination of the literature reveals that whilst there are wide variations in choice of dye, individual workers tend to use the same dye. Dyes most frequently used in dental research are provided as either solutions or particle suspensions of differing particle size dependent upon manufacture and the individual behaviour of the dye (M. Braden, 1990, personal communication). It is therefore impractical to continue to use a variety of dyes with the expectation that, even when used with standardized techniques, they will provide consistent results. Christen and Mitchell (1966), for example have shown that different concentrations of two dyes can vary in penetration time between 5 min and over 1 h. It is apparent from this that pilot studies to determine the time necessary for maximal leakage should always be carried out for any differing dye concentration. A potential source of error is also present with any dye which will bind to tooth substance or the restorative material. It is impractical to use a dye particle which has a diameter greater than that of the internal diameter of dentinal tubues (l-4 pm) as it is the likelihood of bacterial penetration of these tissues which is being investigated. Therefore penetration studies in dentine will exhibit some dentine staining which should be distinguished from the actual gap between the cavity wall and restorative.

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Glyn Jones et al. (1988) suggest however that dentine involvement may be used as a relative indicator of marginal leakage and have used image analysis techniques to measure this. Nevertheless image analysers are also prone to some subjective problems in that the threshold for distinguishing between the presence and absence of stain usually needs to be set prior to analysis by the operator. Of the dyes most frequently used in penetration studies, basic fuchsin solutions, particularly those in propyl glycol co-solvent have been shown to bind preferentially with carious dentine (Fusayama and Terachima, 1972; Heinrich and Kunzel, 1986; Kidd et al.. 1989 (1 per cent acid red)). This propensity has been made use of in the manufacture of caries-disclosing agents. However, this must bring into question the effectiveness of using such a substance when attempting to demonstrate an inert space between restoration and tooth substance, as, in crosssection, an area of stained dentine can potentially be mistaken for a larger gap than actually exists. Dye materials which show a propensity for binding to tooth tissue or the restorative material under investigation should therefore be avoided. Dyes must also be colour stable under any conditions expected to arise during investigation. The risks of misinterpretation of chemical indicators, for example aniline blue which becomes colourless in alkaline conditions, such as the presence of a calcium hydroxide liner, are obvious (Roulet et al., 1989).

SUMMARY With the rapid increase in the development of new restorative materials and techniques, researchers will become more involved in determining the ability of each new material to adapt to the tooth without leakage and its consequent risk to the vitality of the tooth, postoperative sensitivity, marginal staining and secondary caries. There are many diverse methods of demonstrating the leakage of restorative materials with advantages and disadvantages for each technique. However, the consequence of such a large choice of methods leads to a lack of standardization between workers to such an extent that it becomes difficult to compare between similar techniques using slightly different apparatus, including the use of differing dyes. Attempts are being made at developing standard scoring systems for the assessment of microleakage, but these continue to be quite subjective. Surely to enable researchers to compare the sealing ability of materials with other workers, a standard technique using established materials and methods of assessment should be available.

References Al-Hamadani adaptation

K. K. and Crabb H. S. (1975) Marginal of composite resins. J. Oral Rehabil. 2, 21-33.

Alperstein K. S., Graver H. T. and Herold R. C. (1983) Marginal leakage of glass ionomer cement restorations. J. Prosthet. Dent. 50, 803-807. Arcoria C. J.. Fisher M. A. and Wagner M. J. (1991) Microleakage in alloy-glass ionomer lined amalgam restorations after thermocycling. J. Oral Rehabil. l&9-14. Barkmeier W. W. and Cooley R. L. (1989) Resin adhesive systems: in vitro evaluation of dentin bond strength and marginal microleakage. J. Esthetic Dent. 1, 67-72. Barkmeier W. W.. Huang C.. Hammesfahr P. D. et al. (1989) J. In vitro evaluation of two new dentin adhesive systems. Esthetic Dent. 1, 164-167. Barry G. N. and Fried I. L. (1975) Sealing quality of two polycarboxylate cements used as root canal sealer. J. Endodont. 1, 107-111. Bergenholtz G., Cox C. F., Loesche W. J. et al. (1982) Bacterial leakage around dental restorations: its effect on the dental pulp. 1 Oral Pathol. 11,439-450. Blackwell R. E. (1955) In: Black’s Operative Dentistry Vol. II -Technical Procedures-Materials, 5th edn. Oxford, Blackwell, pp. 387-388. Brannstriim M. (1981) In: Dentin and Pulp in Restorative Dentistry, 1st edn. Sweden, Dental Therapeutics AB. Brannstriim M. (1987) Infection beneath composite resin restorations: can it be avoided? Oper. Dent. 12, 158-163. Callis P. D. and Paterson A. J. (1988) Microleakage of root fillings: thermoplastic injection compared with lateral condensation. J. Dent. 16, 194-197. Camp L. R. and Todd M. J. (1983) The effect of dowel preparation on the seal of three common obturation techniques. J. Prosthet. Dent. 50, 664-666. Christen A. G. and Mitchell D. F. (1966) A fluorescent dye method for demonstrating leakage around dental restorations. J. Dent. Res. 45, 1485-1492. Ciucchi B., Bouillaguet S. and Holz J. (1990) Proximal adaptation and marginal seal of posterior composite resin restorations placed with direct and indirect techniques. Quintessence ht. 21, 663-669. Crim G. A (1989) Influence of bonding agents and composites on microleakage. J. Prosthet. Dent. 61, 571-574. Crim G. A. (1990) Assessment of microleakage of three dentinal bonding systems. Quintessence ht. 21, 295-297. Crim G. A. and Mattingly S. L. (1981) Evaluation of two methods for assessing marginal leakage. J Prosthet. Dent. 45, 160-163. Crim G. A. and Shay J. S. (1987) Microleakage pattern of a resin-veneered glass-ionomer cavity liner. J. Prosthet. Dent 58, 273-276. Crim G. A., Swartz M. L. and Phillips R. W. (1985) Comparison of four thermocycling techniques. _J.Prosthet. Dent. 53, 50-53. Darbyshire P. A., Messer L. B. and Douglas W. H. (1988) Microleakage in Class II composite restorations bonded to dentin using thermal and load cycling. J. Dent. Res. 67, 585-587. Davila J. M., Gwinnett A. J. and Robles J. C. (1988) Marginal adaptation of composite resins and dentinal bonding agents. ASDC J. Dent. Child. 55,25-28. Delivanis P. D. and Chapman K. A. (1982) Comparison and reliability of techniques for measuring leakage and marginal penetration. Oral Surg. Oral Med. Oral Pathol. 53, 410-416. DeTrey E. R. (1976) Der Einfluss der Kavitatenform und des Versieglersystems auf die Adaptation und den Randschluss von approximalen Frontzahnftillungen, in vivo. Thesis, Zurich. Douglas W. H. and Zakariasen K. L. (1981) Volumetric assessment of apical leakage utilizing a spectrophotometric. dye-recovery method. J. Dent. Res. 60A, (abstr. 512) 627.

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