Dental materials: Part I N. E. WaterS,

1974 literature review

MSc, PhD, FlnstP (Editor)

School of Dental Surgery, Royal Dental Hospital M. Braden, D. Brown,

T. C. Tranter,

BSc, PhD, FlnstP, AIRI, API

Dental School, London Hospital Medical

of London

College

BSC, PhD, AIM

Dental Department,

E. C. Combe,

Guy’s Hospital, London

MSc, PhD, CChem, FRIC

Department of Prosthetics, Turner Dental School, Manchester

D. F. Williams,

MSC, PhD, MIM, CChem, FRIC

Eastman Dental Hospital, London BSC, FRIC

School of Dental Surgery, University of Edinburgh

ABSTRACT This paper, which is to be presented in two parts, reviews the work on dental materials published in 1974. Included in Part I are sections on amalgam, casting alloys, denture base polymers, composite filling materials, soft lining materials, fissure sealants, impression materials, dental ceramics and

model and die materials. A review of the literature relating to cements, dental implants and dental biomechanics will be presented in Part II.

INTRODUCTION review is the second in an annual series compiled by the Panel for Dental Materials Studies in the United Kingdom, and is intended as a source of reference for work on dental materials published in 1974. It covers the introduction of new materials, the evaluation of existing materials or their modification and developments in certain relevant techniques. A basic list of 37 journals was surveyed by all the contributors, but each contributor was free to include material from other sources.

THIS

BSC, PhD, AIM

School of Dental Surgery, University of Liverpool, and Visiting Associate Professor, Clemson University, USA

A. WikOn, A. J. von Fraunhofer, M. Miller,

BSC, PhD

Dental School, Welsh National School of Medicine, Cardiff

DSC, FRIC

Laboratory of the Government Chemist, London

H. J. WikOn,

PhD, DSC, FRIC

Department of Prosthetics, Birmingham Dental School

METALS Dental amalgam Dental amalgam remains the most widely used restorative material and the literature reflects in part the continuing interest in the material. Whilst the number of clinically oriented communications has continued to fall over the years, dental amalgam remains a fertile field for scientific investigation. This is shown by the large number of International Association for Dental Research abstracts reviewed here (vi& infra).

Only one major review article appears to have been published on dental amalgam in 1974 (Lewis, 1974), and the important manipulative parameters together with their influence on handling characteristics have been discussed. Various workers have studied the cytotoxicity of dental amalgam in relation to the epithelium (Leirskar, 1974), whilst the mercury content of gingival tissues has been investigated by Fred& et al. (1974), and there is clearly considerable scope for further work in this field.

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The vexatious problem of mercury in dental amalgam has led to the development of a semiquantitative test for mercury in air (Jorgensen, 1974). Other workers (Barone et al., 1974; Mackert et al., 1974) have studied the loss of mercury from capsules during mechanical trituration and found that the type of capsule and the mixing speed were important. Mercury loss was observed during mixing, but lower losses were found at low speeds than at high speeds. Similarly, at low speed, mercury loss was independent of time, but it increased with time with high speed mixing. In general, lowest mercury losses were found with screw-type non-disposable capsules if used at low speeds or for short periods of time at high speeds. Studies of the rate of reaction of mercury with silver-tin amalgam (Taylor and Smith, 1974) have shown that the absorption rate was dependent upon the particle size and the tin content of the alloy, with faster rates being found with decreasing particle size and greater tin content. Furthermore, it was concluded that the normal setting expansion was due, at least in part, to solid-solid reactions rather than to the assumed solid-liquid reactions that occur during setting of dental amalgam. A radiotracer study (Okabe et al., 1974) of mercury diffusion in a silver-tin dental alloy yielded volume diffusion coefficients similar to those found previously by immersion techniques. These reports emphasize the importance of trituration, and in fact the effect on the strength of amalgam of air-borne contaminants introduced during trituration has been studied by Hansen et al. (1974). Undertrituration combined with good condensation has been shown to result in restorations with poor strength and reduced corrosion resistance but greater resistance to marginal failure in the clinical situation (Osborne and Gale, 1974). A study of marginal failure of amalgam restorations (Mathewson et al., 1974) showed that the presence of retention grooves in the restoration had no influence on the incidence of failure, but alloy selection was critical, with a dispersed phase alloy being superior to a conventional alloy. A further study (Mahler and Van Eysden, 1974) investigated the occurrence of occlusal extrusion of

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clinical amalgam restorations. It was found that occlusal extrusion of the order of 100 urn did occur after 4 years’ service and the extent of extrusion was directly related to the marginal fracture characteristics of the alloys investigated. This indicates that marginal failure of restorations is related to the creep characteristics of dental amalgams, and clearly the creep behaviour of an amalgam is an important parameter. Various workers have studied the creep behaviour of dental amalgam. It has been shown that creep increases with increased residual mercury levels, although the composition of alloy, particularly if it contains copper as a dispersed phase, can markedly affect the creep resistance (Civjan et al., 1974). The properties of a modified single composition alloy (rather than a blend as employed for the commercial dispersed phase alloy) confirmed these findings, and increased creep resistance was achieved (Asgar, 1974). Other workers (Vrijhoef and Driessens, 1974a, b) have also demonstrated the increased creep resistance of copper-containing amalgams. The phase composition of various dental amalgams have been studied by X-ray diffraction (Gubbels et al., 1974a, b; Marshaal and Hochman, 1974; Vrijhoef and Driessens, 1974a), whilst the long term phase changes in dental amalgam after setting have been investigated by Vrijhoef and Driessens (1974b). Microstructural studies by means of an electrochemical etching technique have been reported (Staheli and von Fraunhofer, 1974), whilst the surface morphology of dental amalgam has been studied by Johnston et al. (1974). The use of Auger spectroscopy for examining the surface film formed on Ag,Sn has been reported by Grenga et al. (1974). Various investigations into the mechanical properties of dental amalgam were presented in the literature, such as a study of working time by means of ultrasound techniques (Davidson et al., 1974). Another communication showed that the modulus and rigidity of cement bases were critical in providing support for amalgam restorations (Farah et al., 1974). The bonding between pins and amalgam restorations has been considered by Galindo et al. (1974a, b), and a scanning analysis of

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dual gold-amalgam restorations has been reported by Liatukas (1974). Finally, the search for new and improved alloys together with the clinical appraisal of modern dental amalgams continues. Coppercontaining amalgams were mentioned earlier, and there have been reports on gold-containing amalgams (Malhotra and Lawless, 1974) and an interesting cadmium-nickel alloy capable of plastic deformation and good crack resistance (Tobeler et al., 1974). Studies on the clinical performance of zinc-containing and zinc-free amalgams were also reported (Ryge et al., 1974); no significant differences were found between the two types of restoration over a 3-year period. Overall, the dental amalgam restoration continues to hold the interest of materials scientist and clinician alike and the wide literature in the field supports this contention, Casting alloys The work on dental casting alloys reported during 1974 mainly concerned base metal alloys. Many investigations detailed additions to cobalt-chromium alloys as well as heat treatments of these new alloy formulations. There were four reports on the cobaltchromium-nickel-tantalum ‘superalloy’ system which had been introduced in the previous year (Mohammed, Abdullah and Mumford, 1974; Mohammed and Asgar, 1974; Mohammed, Grass0 and Abdulla, 1974; Rowe et al., 1974; Williams, 1975). This alloy system consists of 40 per cent Co, 30 per cent Cr and 30 per cent Ni to which varying amounts of Ta are added. At low concentrations, Ta goes into solution in the face-centred cubic matrix alloy and causes some order hardening. As the concentration of Ta is increased, a hexagonal structure, a-Co,Ta, is formed as an interdendritic precipitate. At Ta levels of less than 13-14 per cent, very little slip interference is effected, but at higher concentrations the interdendritic precipitate interferes with the slip mechanism to a dramatic extent. Thus, at these higher concentrations there is a sudden increase in the tensile strength and in the yield point as well as a sharp drop in the ductility. Various heat treatments, such as solutioning at

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1200 “C under vacuum for 2 hours; aging at 700 “C for 16 hours and over-aging at 1000 “C can in turn further accentuate these changes in the mechanical properties. One member of this ‘superalloy’ system containing 4 per cent Ta was compared with a gold alloy and with two commercial base metal alloys (Dentillium CB and Howmedica III) for crown and bridge work (Mohammed, Grass0 and Abdullah, 1974). The new alloy demonstrated mechanical and casting properties intermediate between the two base metal alloys and better than the gold. It is therefore adequate for this application. The first report on a projected series of studies on a new nickel-based alloy (Ticonium Premium 100) for cast partial dentures was presented by Lewis (1974). The elemental composition of the alloy was worked out as a baseline for future studies which are to investigate the effects of melting, casting and remelting on the composition as well as on the mechanical properties of the alloy. -Hollander and Wulff (1974) reported on the effects of a hot isostatic pressing (HIP) treatment on the mechanical properties of a cast cobalt-chromium-molybdenum-carbon surgical implant alloy. This treatment involves maintaining the casting in an argon atmosphere at a pressure over 100 MPa and at 1200 “C. It was found that the HIP treatment increased the tensile strength, the yield point, the ductility and the hardness of the cast alloy. The effects of three different casting methods on the mechanical properties of two cobaltchromium alloys (Vitallium and Wironit G) were investigated by von Marx (1974). Huget and Civjan (1974) provided a state of the art report on the commercially available palladium-silver-based alloys for crown and bridge work. POLYMERIC

MATERIALS

Denture base polymers A very important paper which may miss the dental audience at large has been published by De Wijn (1974). This shows how to reduce drastically the exotherm as manifested by temperature rise involved in the polymerization

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of methyl methacrylate. Surprisingly, this is achieved by the incorporation of as little as 0.2 per cent 1,4 cyclohexadiene, without much reduction in polymerization time. Cyclohexyl methacrylate also works, but not nearly so effectively. (The widespread use of cyclohexadiene may be circumscribed by its priceapproximately &SOper 100 g.) Kusy and Turner (1974) have studied the fractography of acrylic polymers, and deduced a correlation between strength and mode of fracture. Composite filling materials The current year (1974) has seen little innovation in composites, but rather a consolidation of existing materials and techniques. An excellent and comprehensive review has been published by Paffenbarger and Rupp (1974). Miiller (1974) has studied glass ceramic powders with potential use in composite filling materials, with particular reference to X-ray opacity. Transparent glass ceramics of ultralow thermal expansion have been prepared using ZrOz and Ta,O, nucleating agents, and La,O, as an X-ray opacifier. The latter does not adversely affect optical transparency as do BaO and PbO. Brady et al. (1974) have measured the thermal conductivities of a number of composite filling materials. As thermal diffusivities are more relevant to the clinical situation, these findings are of limited value. Plant et al. (1974) have studied the heat evolved during the setting of restorative materials including composites. A clinically realistic method of evaluating exotherm has been developed, and a number of materials studied. Furthermore, the temperature at which fluids are taken in the mouth, and in vitro intra-temperature rises were measured, and the results discussed in terms of possible damage to the dental pulp. Asmussen (1974) has studied the effect of temperature changes on the adaptation of resin fillings. It is postulated that initially water absorption swells the restoration, and that the consequent elastic forces counteract the formation of marginal gaps. It was found that fillings could be cooled through a certain temperature

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range without marginal gaps occurring. Previous heating up to 60 “C before cooling increased the subsequent gap size at 2 “C only in the case of unfilled resins. McCurdy et al. (1974) have compared in vitro and in vivo tests for microleakage and concluded that there is a reasonable correlation, justifying the use of laboratory screening tests. Gjerdet and Hals (1974) have carried out an interesting study of the microstructure of composite filling materials. Somewhat surprisingly, it transpires that the polymer matrix is birefringent. Hals and Kvinnsland (1974) have examined lesion formation, both in vitro and in vivo, around two composite filling materials. The tendency to form ‘wall’ lesions was negligible, indicating a narrow microspace. This would seem to be consistent with the work of Asmussen (1974). Hayashi et al. (1974) have studied the discoloration of composites with particular reference to finishing methods. Discoloration was much less if polishing was carried out after 48 hours than at 15 minutes. Very marked differences in discoloration were noted after polishing at 48 hours between various proprietary materials. A clinical comparison of composites, amalgams and silicates was reported by Eames et al. (1974). The findings substantiate those of other workers in that: 1. Amalgam is superior to composite in its ability to retain anatomical form. 2. Composites are superior to amalgam in marginal adaptation. 3. Composites are superior to silicate both in the retention of anatomical form and marginal adaptation. 4. Pulp studies with primates show that composites have a mild to moderate inflammatory response, indicating the desirability of a calcium hydroxide or polycarboxylate liner. Grajower et al. (1974) have examined the effect of pulp-insulating materials on composite resins by scanning electron microscopy. The roughness of the composite surface adjacent to the pulp-insulating material is assumed to be due to chemical reaction or mutual dissolution. This v _~~_ mav ~~~, reduce the transverse ~~._ _roughness

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strength of the restoration or the lining. The surface roughness observed decreased in the order zinc oxide/eugenol, calcium hydroxide, zinc polycarboxylate. The important subject of abrasion resistance of composites has been studied further by Powers et al. (1974). A two-body abrasion test was used and the following important findings were made : 1. Unfilled resins abraded more quickly than filled. 2. Treatment of the filler with silanecoupling agents increased abrasion resistance. 3. Quartz was better at reducing abrasion than lithium aluminium silicate and barium aluminium silicate. 4. There were major differences between composites. 5. The abrasion rate of amalgam was similar to the composites. A clinical and laboratory study of the restoration of cervical wear lesions was conducted by Harris et al. (1974). This nonoperative acid-etch technique was shown to be of doubtful value at the current stage of investigation. The important acid-etch technique has been the subject of further studies. Mitcham and Turner (1974) studied eight resin systems in vitro. Major differences in retentive strength were found, but these were not related to the strength of the resin itself. Retention was said to depend primarily on the ability of the resin to wet the surface. Nelson et al. (1974) have compared materials and methods in the acidetch technique. The Nuva system was ranked best followed by Restodent. However, other variables were important, such as the strength of the acid used. Harsanyi et al. (1974) have examined subcutaneous tissue response to composite resins in dogs. The composite resins as a group were markedly less irritant than zinc oxide/eugenol. McLundie and Murray (1974) have studied the finishing of composites using scanning electron microscopy. No polishing method is able to produce a surface as smooth as that obtained by a polyester matrix. The use of tungsten carbide burs, either alone or followed by fine discs, or coarse and fine discs or strips

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effectively reduce excess material and give clinically acceptable surfaces. Two techniques for the fabrication of pinretained composite cores for posterior teeth have been described by McPhee (1974). An important clinical evaluation was made by Chandler et al. (1974) of a tooth restoration coupling agent (NPG-GMA) which was found to be effective. A comparative study of the toxicity, both in vitro and in vivo, of methyl methacrylate and a Bowen-resin-based material was made by Tronstad and Spawberg (1974). One methyl-methacrylate-based material behaved surprisingly well. Soft lining materials Buckner (1974) has described a one-step method for preparing an obturator shell and denture with permanent soft acrylic lining. However, no mention of the choice of soft lining material or its properties was made. Lechner (1974) found that papillary hyperplasia in geriatric patients normally responded quite rapidly to the tissue-conditioning materials. He also suggested the use of soft lining materials to give a cushion effect in cases where there had been atrophy of the submucosa. Lechner observed that the availablesoftliningmaterials are not entirely satisfactory. They had the disadvantages of impermanence, poor adhesion to the denture base and a tendency to encourage bacterial growth, and were incapable of adjustment or of taking a high surface gloss. Unfortunately, no mention was made of the individual materials which had been studied. McCabe and Wilson (1974), in a review article entitled ‘Polymers in dentistry’ have described the chemical compositions and basic mechanical properties of the available soft lining and tissue-conditioning materials. Fissure sealants Interest in fissure sealants has continued at a high level. Considerable effort has been directed towards obtaining more detailed information regarding the precise mode of action of the etchant. Silverstone (1974) found more rapid action of phosphoric acid at lower concentrations, the most retentive conditions occurring with a 30 per cent solution. Smooth enamel

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surfaces exhibited a fairly uniform depth of etch, whilst on occlusal surfaces the base of the fissure was either etched to a lesser extent than the cuspal slopes or showed no effects at all. This difference was also observed by Hinding and Sveen (1974) in permanent teeth, but in deciduous teeth they found the dissolution more ordered and consistent with both cusp and pit and fissure walls being affected. This may be a result of the greater shallowness of pits and fissures in deciduous teeth, allowing betteraccessoftheconditioningfluid.Deciduous teeth showed less topographical change than permanent teeth, which may be a consequence of the longer time for which they have been in the oral cavity. It is now becoming increasingly clear that the difference in the mode of attack of dilute mineral acids and chelating agents is not as clear cut as was believed initially. Retief (1974a, b), using an epoxy resin formulation with 50 per cent phosphoric acid as the etchant, concluded that the sites of tensile failure could only be classified reliably by examination at high magnification since otherwise small fractures within the enamel would not be observed. The lowest bond strength at which a fracture occurred in the enamel was 14001bf/in2, which compares favourably with the mean linear tensile bond strength of enamel reported by Bowen and Rodriguez (1962) of 1500+370 lbf/in2 After interfacial failure the enamel surface was relatively featureless. This was attributed to fracture of tags which remained in the enamel surface. Calcium determinations showed that spicules of fractured enamel were also retained within the interface of the adhesive. Rod-like projections from the resin surface were, however, found to be due to penetration of dentinal tubules exposed during the polishing procedure. The ability of the adhesive to wet the adherend surface is a factor of considerable importance in relation to bond strength. Retief (1974b) has reported considerable reductions in the contact angle of an uncured epoxy resin (Epikote 828) after using three different etchants-14%’ for 50 per cent H,PO,, 17.9” for 50 per cent H,P0,+7 per cent ZnO by weight and 14.3” for 50 per cent citric acid. The rate of etch was greatest for

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50 per cent H,POI, followed by the attenuated H,PO,, whilst citric acid had the mildest action. The amount of enamel dissolved in the first 15 seconds was greater than in the second 15-second interval for the H,PO, solutions, but no difference was noted in the case of citric acid. The bond strength showed a marked increase after use of all three conditioning solutions. Rock (1974a), using four fissure sealants, found increases in tensile bond strengths after etching in all cases greater than the 200 per cent reported by Eden et al. (1970). For Nuva-Seal the increase was 513 per cent. The bond strengths were often so high that the tensile strength of the resin was exceeded. Use of a surface primer after etching, however, served no useful purpose, the bond strength actually being reduced. Practitioners have expressed fears that etching might predispose teeth to subsequent carious attack. The results of Arana (1974) support the view expressed by other research workers that these fears are groundless. A photographic study showed that remineralization occurred more rapidly in younger patients, but the return to normal texture was essentially completed in 48 hours regardless of the patient’s age. There was also no evidence of more rapid plaque build-up on the treated areas. Von Loew et al. (1974), using OsborneMendel rats, reported no increase in susceptibility to carious attack in molars treated with Nuva-Seal conditioning fluid, the experiments suggesting that remineralization was complete within 3 days. Williams et al. (1974), using acrylic buttons attached to enamel, measured bond strengths after 7 days and 6 months at 37 “C in tap water. Nuva-Seal had the highest bond strength, with no change occurring during this time interval. The bond strengths of Epoxylite 9075 and of ESPE 717 decreased significantly whilst those of ASPA and Poly-F appeared to increase with time. The increase in the case of ASPA may be due to the leaching out of low molecular weight entities. Silverstone (1974) also found differences in the stability of these polymeric films. Epoxylite 9075, Nuva-Seal and TP 2206 showed no evidence of separation at the enamel interface, but

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Epoxylite 9070 and Elmex sealant showed little evidence of adhesion. It should, however, be noted that no etchant is used in the case of Elmex. Film stability is well demonstrated by some impressive results of demineralizing experiments by Silverstone (1974). When a tooth sealed with TP 2206 was left in lactic acid for periods in excess of 1 week, although the entire enamel cap was removed, the sealant remained in the fissure with about 20 urn of enamel on either side which was not removed by further treatment with acid. It seems likely that tag formation plays an important role in relation to these retention differences and so it is hardly surprising that considerable experimental effort has been directed towards establishing their precise form. Silverstone (1974) has reported somewhat longer tag lengths (40-100 urn), but ultraviolet fluorescent light microscopy confirmed that the entire length consisted of sealant. Scanning electron microscope examination of the fitting surface of an entire coating of Nuva-Seal after demineralization showed the tags projecting at various angles and not parallel to each other as they appear in demineralized sections. Myers et al. (1974) used a modified scanning electron microscope technique which made it possible to reveal much more exactly the boundaries of the tag-like extensions. The underside of the adhesive resin showed an hexagonal array of tags of dimensions corresponding with the prism cores which had presumably been preferentially attacked by the etchant. However, trough or ridge-like tags could also be seen, and these presumably arose from preferential attack of the prism sheaths. Silverstone (1974) noted that the periphery of the fitting surface of the sealant to a depth of about 20 urn exhibited negative birefringence relative to the tag length. Since this layer was also more radio-opaque, the author attributes the observed birefringence to entrapped enamel crystallites rather than to strain effects. This would certainly imply a very intimate relationship between enamel and adhesive. The possibility of caries progressing beneath the sealant has been raised on a number of occasions. It would seem that if complete

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sealing is achieved then no mechanism exists by which this could occur. However, should sealant loss occur, even over a small part of the fissure system, then this assumption would no longer be justifiable. The extent of any microleakage is, therefore, a matter of some importance. Rudolph et al. (1974) using the 45Ca technique, found that Nuva-Seal and Epoxyiite 9075 showed little or no leakage following 3 months’ storage in water, even when the teeth were subjected to thermal cycling. It is pointed out, however, that achieving an adequate seal clinically is unquestionably more difficult. Dennison et al. (1974) using an improved neutron activation method, reached a similar conclusion. Whilst it was not possible to conclude that the sealant completely eliminated leakage, any leakage that did occur was small and well within the confidence limits for the means of the appropriate control specimens. Rather surprisingly, however, etching had little effect in improving the seal for all three materials. This potential to seal fissures effectively is also supported by the work of Mednick et al. (1974) who tested the ability of Nuva-Seal to prevent leakage into sterile cavity preparations in human primary molars in viva. More than half the samples did permit bacterial leakage, but quantification of this leakage indicated that it was certainly minimal. It also appeared that the ability of bacteria to survive under a sealant was considerably impaired, presumably as a result of an inadequate supply of nutrients. Some bacteria did, however, remain viable and so their cariogenic potential remains to be established. The heat evolved during the curing of fissure sealants has received little attention to date. Von Fraunhofer and Williams (1974) have reported that a glass-ionomer cement (ASPA) had an endothermic setting reaction, Poly-F showed no heat exchange during setting whilst, with a volume of material comparable to that used in clinical practice, Epoxylite 9075, Nuva-Seal and ESPE 717 showed mean temperature rises of between 5 and 35 “C, with Epoxylite 9075 showing a much greater rise than the other two. Excess Epoxylite 9075 if allowed to spill over on to the mucous

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membrane might elicit a painful response in the patient, but no other problems seem likely to arise. Rock (1974c) has reviewed the health hazards associated with the use of ultraviolet light sources. The main output of all three lamps tested was between 364 and 367 pm. Until the safety of long wavelength ultraviolet radiation has been conclusively proved, dental personnel using the lamps should be advised to exercise care. The end of the light guide should not be viewed directly and should be shuttered when not in use. Provided that these precautions are observed, any risk is likely to be very small. Rock et al. (1974) have examined the tissue reactions to Nuva-Seal and Alpha-Seal using subcutaneous implants in laboratory white rats. The results suggested that these two sealants are relatively non-irritant, but care should, however, be exercised in extrapolating the lack of mucosal reaction to the clinical situation because of structural differences between the rat and human buccal mucosa. The final test of fissure sealant efficacy must be clinical trials. A fair amount of data now exists covering trials extending up to 2 years. Rock (1974b) has reported 2-year results for four fissure sealants. Nuva-Seal was intact on 80 per cent of treated teeth and had produced a 99.3 per cent caries reduction against the controls. Epoxylite was intact on 51.5 per cent of teeth and had produced a 64.6 per cent caries reduction. For Epoxylite 9070 the caries reduction was only 15.2 per cent, and Elmex Protector had no effect at all. Using a ultraviolet-light-sensitive resin, TP 2206, incorporating a small quantity of Tinoval dye which fluoresced blue in ultraviolet light, the resin was intact on 82 per cent of treated teeth after 1 year. The numbers remaining fully sealed after 2 years were very similar to the numbers retaining sealant 6 months after application, which is probably a clear indication of the importance of the application technique. The effective life of the sealant appeared to be considerably in excess of 2 years. For Epoxylite 9075 the caries incidence was lower than would have been expected from the retention figures, suggesting that more sealant had remained on

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the teeth than was clinically detectable. Hinding (1974) has also presented some clinical evidence of extended cariostasis following sealant loss. Hinding and Buonocore (1974) also found that teeth showing high sealant loss could apparently be totally protected from caries. However, Risager and Poulsen (1974), in a study involving Danish schoolchildren, obtained comparable retention figures to other workers for Nuva-Seal but could find no cariespreventive effect. It should, however, be noted that contralateral controls were not used in this study and an extremely low caries increment in the entire population made it difficult to evaluate the caries-preventive effect with any certainty. In a trial in Kalispell, Montana, involving 350 schoolchildren, Horowitz et al. (1974) found that the sealant was fully retained on 73 per cent of the treated teeth after 2 years and was 67 per cent effective in inhibiting occlusal caries. McLean and Wilson (1974), using an adhesive glass-ionomer cement (ASPA) as a fissure sealant, obtained excellent retention figures. The cement was completely lost in only 10 per cent of the treated teeth in the first year and from a further 4 per cent during the second year. Absence of controls and the size of the sample precluded a firm statement on the effectiveness of the treatment when the sealant was lost. However, it could be affirmed with certainty that intact ASPA provided a complete protection against caries, not one case being observed when the sealant was retained. This cement, however, is only suitable for sealing fissures exceeding 100 pm in width at the entrance. Hinding and Buonocore (1974), in a 2-year trial with a bis-GMA resin, examined the effect of sealant viscosity, type of conditioning agent and length of conditioning time. High caries reductions were found in all cases. The biggest effect was noted with viscosity, better retentions resulting with lower viscosities. Premolars showed better retentions than molars. This may result from the different type of occlusion, different morphology or the fact that premolars are usually easier to treat. Going et al. (1974) using Nuva-Seal found that complete retention at 12 months was at least 70 per cent for all types of teeth except

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maxillary second molars (27 per cent), whilst 11 per cent of the treated teeth had become carious compared with 29 per cent of the control teeth (caries reduction 15 per cent). The most spectacular caries reduction was achieved with mandibular second molars, where 15 per cent of the treated teeth became carious compared with 77 per cent of the controls. No difference in caries rate was found between control teeth and treated teeth showing partial loss of sealant, which is contrary to the extended cariostasis found by some other workers. Cons et al. (1974), in a large scale trial involving first permanent molars sealed with Nuva-Seal, found that after 12 months 76.8 per cent of the original 4462 sites sealed were still completely covered, 7.3 per cent were partially covered and sealant was lost from 15.9 per cent of the sites. The caries attack rate for the control teeth was 9.5 per cent and 3.3 per cent for the sealed teeth, an overall caries reduction increment of 65.3 per cent. Richardson et al. (1974) used Nuva-Seal in the treatment of mentally retarded children. After 1 year partial or complete loss was observed in 15 per cent of the treated teeth, and the caries rates in the control teeth and those where the sealant had failed were much higher but detailed results have not yet been presented. Newbrun et al. (1974), using three commercial sealants on rat molors after the induction of initial lesions, found significantly more lesions in the treated teeth than in the controls after periods of up to 43 days. El-Kafrawy and Mitchell (1975) however, claimed that two experimental fissure sealants were effective in caries prevention when applied to sound rat molars. Neither sealant adhered to preexisting car ious surfaces so that the subsequent progress of the caries was not affected. This may explain the negative results of Newbrun et al.( 1974).Von Reithmann and Ketter1(1974), using Osborne-Mendel rats, also found a significant caries reduction in molars sealed with Nuva-Seal, but this was smaller than that normally observed in human dentitions. Overall, therefore, real doubt must exist regarding the suitability of rats for assessment

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of the effectiveness of fissure sealants in humans. A considerable amount of clinical evidence is now available to show that fissure sealants can produce large reductions in the incidence of new occlusal caries, particularly in molars. However, the scatter in results between different users of these materials must remain a matter of concern until the factors responsible have been more clearly defined. Even at this stage, however, the statement of Stamm (1974) that ‘because only fissure caries is affected and because the materials today have only temporary effects, it is difficult for the dentist to decide if the required time and effort is worthwhile for the patient or the dentist’ seems hardly in accord with the present research position. The Council on Dental Materials and Devices (1974) has now recognized the short term benefits but believes that questions of long term use and effectiveness have not been completely answered. Decisions to use th,% materials should, therefore, continue to rest with the profession. Epoxylite 9075 (Lee Pharmaceuticals) and Nuva-Seal (L.B. Caulk Co.) are now classified as provisionally acceptable. The work of Lee and Ocumpaugh (1974) suggests that these adhesive resins can also be used with advantage as a vehicle for therapeutic agents such as fluoride. With a polyurethane containing disodium monofluorophophate (Epoxylite 9070) the initial fluoride uptake by enamel was similar to that obtained with an acidulated fluoride gel (Luride), but the subsequent retention of fluoride and resistance to acid etching was significantly greater. It is suggested that this is a result of the need to isolate the fluoride ion from the oral environment for a period of at least 6 hours if it is to be transformed into an insoluble form (fluorapatite). The polyurethane coating remains on the teeth for a minimum of 48 hours. The research results during the period under review do nothing to detract from the previously expressed view that these adhesive resins have an extremely promising future in the field of clinical dentistry.

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IMPRESSION

Journal of Dentistry, Vol. ~/NO. 2

MATERIALS

In a general survey of findings on elastomer impression materials reported in recent years, Rehberg and Schwickerath (1974) noted particularly the results of specification tests and their significance with regard to clinical behaviour. They divided the properties into three groups: A, viscosity and working and setting times of the pastes; B, elastic strain and detail reproduction of the set material; C, permanent deformation and dimensional stability. A and B they regarded as characterizing the material while C indicates its quality. The importance of the effect of temperature on working time was stressed. A rise of only 4 above the normal 23 “C can mean a working time of only 1 minute in some cases. Schwickerath (1974) pointed out that specification tests give only an instantaneous photograph of a material and this is not a sufficient indication of properties for the dentist. Some idea of change in properties with time is needed for a fuller picture. Examples from impression materials and cements were given. The properties of two polyether impression materials have been surveyed by Barolet and Desautels (1974). A new technique for silicone rubber impression-taking has been described by Fusayama et al. (1974) and called the ‘laminated single technique’. The light-bodied impression material is spread as a thin layer on the heavybodied material which is mixed at the same time. They claimed that this technique gave stone casts with good surfaces and less dimensional difference than the usual two-mix technique. Uses for alginate impression material other than for the normal impressiontaking have been described by Fairchild (1974). These include indication of occlusal prematurities, indexing, locating pressure spots under dentures and moulds for fluid resins. Stone casts from dentures with undercuts could be obtained by a technique which leaves a IO-mm layer of alginate on the denture. Mixing the alginate in a plastic bag was recommended. Properties of rubber impression materials have been reported from two sources. Marxkors et al. (1974) measured viscosity change with time for some rubber impression materials

using proportions which varied by * 15 per cent as well as normal proportions, and showed how long the different mixes took to reach a viscosity indicating network formation at different temperatures. This should be an important guide to working times. Kaloyannides (1974a) reported breaking strength and Young’s modulus for all elastomer impression materials used in Greece. Tensile loading of moulded rods was used and polysulphides were found to have a lower strength than silicones, but their strength increased with time up to 48 hours whilst that of the silicones and the polyether decreased. Kaloyannides and Kapari (1974a) measured consistency of the same materials by a parallel plate method and setting times by a penetrometer at 20, 25 and 37 “C. This showed silicones and the polyether to be less affected by temperature than the polysulphides. In further papers the same workers reported setting time and consistency for mixtures of different brands of the same impression material type of elastomer (Kaloyannides and Kapari, 1974b, c). The data would enable a dentist to obtain properties intermediate between those of individual brands. Permanent deformation of similar mixtures was also reported (Kaloyannides, 1974b). Dimensional accuracy continues to be the main theme of investigations on rubber impression materials. Cooney (1974) found that stone dies were more accurate than silverplated ones from the silicone Xantopren, for which he describes a satisfactory plating method. On the polysulphide Permlastic he found no difference in accuracy. Henry and Harnist (1974) compared the dimensional accuracy of three polysulphides, four silicones and a polyether by pouring stone casts in impressions of a silver-plated model and measuring the ‘horizontal’ dimensional change between four posts on two ridges. Casts were poured at 30 minutes and 24 hours. In general, polysulphides and the polyether gave positive changes and silicones negative changes, though all double mixes gave a positive change. One silicone showed the same change at 30 minutes and 24 hours, while the polyether showed less change at 24 hours than at 30 minutes, and

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these two materials had the lowest changes. Otherwise, results showed it was best to cast early. Hembree and Nunez (1974) noted the discrepancies of gold castings using two polyether materials to take impressions of wet and dry models. The wet models gave greater discrepancies than the dry, whether the stone was poured immediately or after 1 hour. Sawyer et al. (1974) compared the accuracy of stone casts from three classes of elastomer impression material. Silicones showed smaller deviations than polysulphides, which is the reverse of the known polymerizationshrinkages. Polyethers were the most accurate.

CERAMICS Dental ceramics The reports on dental ceramics published during 1974 can be broadly divided into those concerning restorations in porcelain itself and those where the porcelain has been bonded to metal substrates.

Porcelain D&and (1974a) reported that stresses were generated in porcelain crowns which were fired on platinum foil. These stresses were compressive on the inner surface and tensile on the outer surface and they both contributed to the overall strength of the crown. To determine how the shape of the post preparation affects the stresses generated in porcelain jacket crowns, he used plastic models six times larger than life (D&and, 1974b). He then extended his study to normal-sized porcelain crowns and observed how the point of application and direction of the load affected their ultimate strength (D&and, 1974~). D&and observed that the greatest stresses were generated on the inner sides of the proximal surface and that the compressive and tensile stresses were at a maximum when the loading angle was 60” and the loading points were at the incisal corners. At a loading angle of 60”, the load needed to fracture the crown was smaller than at any other angle. When the lingual surface of the core was concave and provided with high lingual surfaces, the strength of the core was considerably higher

61

than when the surface of the core was plane. Greater stresses were generated in short or asymmetric cores. It was also observed that whilst longitudinal proximal grooves reduced the stresses in the plastic crown models and enhanced their retention, they had little effect on the ultimate strength of the porcelain jacket crowns. The transverse strength of porcelains as a function of composition was reported by Sherrill and O’Brien (1974). They observed although aluminous porcelain was that, superior in strength to feldspathic porcelain and that a rough surface affected the strength of the aluminous porcelain least, the strength of both materials was adversely affected by environmental moisture. Some indication as to why this occurs is given by Southan and Jorgensen (1974), who observed in their study of the endurance limit of dental porcelain that static fatigue was occurring in porcelain jacket crowns (which were nothing more than pigmented glasscrystal composites), owing to the chemical action between the glass and water. The OHions attack the glass, causing cleavage of the siloxane bond. They found that if the porcelain was chemically treated by heating for 44 hours at 600 “C in molten potassium nitrate, then the strength was increased by 50 per cent. The endurance limit was also raised.

Porcelain-metal

composites

Whilst the gold alloys are still popular, alternative systems of metal restorations faced with porcelain are being used. Lugassy and Kumamoto (1974) studied the creep properties of three non-precious alloys and compared them with one gold alloy, with a view to explaining why these systems frequently fail to fit. They observed that, despite their greater degree of oxidation, the base-metal systems showed significantly lower creep values than the gold alloy. Two status reports have emphasized the need for further research into the various treatment variables which can affect the porcelain-metal bond (Huget and Civjan, 1974; Moffa and Jenkins, 1974). The latter concentrated on the nickel-chromium alloys whilst the former

62

surveyed the palladium-silver systems. The high palladium alloys tend to give a yellowishgreen discoloration to the porcelain, and consequently a coating agent is recommended. Measurements of the porcelain-metal bond strength have again exercised the ingenuity of several workers. Carter (1974) fused porcelain to both sides of a thin rectangular metal substrate and then twisted it about its long axis. Well-bonded porcelains cracked in tension at 45” to the torsion axis and then failed catastrophically in compression. Poorly bonded porcelains failed by shear at low torques. This method requires a sophisticated mathematical interpretation, unlike the system devised by Civjan et al. (1974). In this, cast metal discs were fused to porcelain cylinders. The porcelain was held in acrylic and the cast disc removed in a loading machine in the diametral direction. Of 108 specimens tested, 74 showed shear failures at the alloy-porcelain interface and 34 failed within the porcelain. The gold alloys showed values in the range 7000-81001bf/in2, whereas the base-metal systems had values in the range 4300-5800 lbf/in2 when tested in this manner. Model, die and mould materials Several workers have reported results of research into the development of gypsum products with improved properties. For example, Bajada and Makinson (1974) studied the effects of three surface treatment agents on one model and one die stone. The agents were (a) a concrete hardener, (b) a 10 per cent solution of polystyrene in amyl acetate and (c) a fast-curing epoxy resin. The first two of these materials were brushed on to the surface, and the epoxy resin was rubbed into the material. It was found that the polystyrene had no effect on the hardness, but the other two additives softened the material. In all cases the abrasion resistance was improved, except for the epoxy resin/die stone combination. Dimensional change due to the epoxy resin was not assessed; the other agents had no effect on this property. In contrast to this work, Sanad et al. (1974) used epoxy resin in acetone solution, and found considerable improvement in the strength, hardness and abrasion resistance of a

Journal of Dentistry, Vol. ~/NO. 2

die stone, without any loss of dimensional accuracy. An alternative approach has been put forward by Earnshaw (1974), who impregnated gypsum with methyl methacrylate containing 2 per cent benzoyl peroxide. The material was wrapped in tinfoil and heated at 70°C for 24 hours. This improved the tensile strength (as evaluated by diametral compression) and abrasion resistance of both plaster and stone. Reports have also been given of techniques of using gypsum products. Winkler and Puengphob (1974) have studied gypsum as an alternative to hydrocolloids as an investing medium in the fabrication of dentures from pour resins. Lord et al. (1974) have reported a rapid technique for boxing impressions prior to obtaining casts. Comparative studies of dental materials have been made by Cooney (1974) and Ewers (1974). The former author compared silverplated and stone dies prepared from rubberbase impressions. Ewers (1974) has used a scanning electron microscope to study the compatibility of three dental stones with two alginates and two rubber-base impression materials. Two reports on investment materials have been presented. Mabie (1974) has demonstrated that a coating of zircon (ZrSiO& in a solution of methylene chloride and zirconium octoate, applied to a refractory cast or model, could reduce considerably the roughness of the casting. Tidy (1974) has measured both porosity and permeability of a phosphate- and gypsum-bonded investment material. This author also discusses the theory of permeation in detail, and the effects of such experimental variables as powder : liquid ratio and burn-out temperature. REFERENCES

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BARONEJ. J., CIVJANS., SWARTZ R. J. and STONE R. C. (1974) Mercury leakage of disposable and nondisposable amalgam capsules. J. Dent. Res.

53, 62.

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Materials

&JAN S., BARONEJ. J. and SWARTZ R. J. (1974) Static creep and residual mercury of eleven dental amalgams. J. Dent. Res. 53, 60. DAVIDSONC. L., ARENDS J. and MARKEL E. T. (1974) An ultrasonic investigation into the working time of amalgam. J. Oral Rehabil. 1, 99-106. FARAH J. W., HOOD J. A. A. and CRAIG R. C. (1974) Effect of cement bases on the stresses in amalgam restorations. J. Dent. Res. 53,278. FRED~N H., HELLD~NL. and MILLEDINGP. (1974) Mercury content in gingival tissues adjacent to amalgam fillings. Odont. Revy 25, 207-210. GALINDO Y., MCLACHLAN K. and KASLOFF Z. (1974a) Mechanical tests of bonds between retention pins and amalgam. J. Dent. Res. 53, Ill. GALINDO Y., MCLACHLAN K. and KASLOFF Z. (1974b) Microscopic studies of pin amalgam specimens forming a metallurgical bond. J. Dent. Res. 53, 67. GRENGA H. E., CARDEN J. L., OKABE T. and HOCHMANR. F. (1974) Auger analysis of surface films on Ag,Sn. J. Dent. Res. 53,6. GUBBELS G. H. M., VRIJHOEF M. M. A. and DRIESSENS F. C. M. (1974a) A short communication on the concentration of copper in silver-tin phases of dental amalgam alloy. J. Oral Rehabil. 1, 371-372.

GUBBELS G. H. M., VRIJHOEF M. M. A. and DRIESSENSF. C. M. (1974b) Distribution of zinc in dental amalgam alloy. J. Dent. Res. 53, 1294129.5. HANSEND. A., HOLLANDJ. W. and LEWIS S. A. (1974) A study of the influence during trituration of air-borne impurities on the strength of amalgam. J. Biomed. Mater. Res. 8, 61-68. JOHNSTONW., O’BRIEN W. J. and HEINKELD. E. (1974) Surface morphology of dental amalgam during setting. J. Dent. Res. 53, 61. JORGENSENK. D. (1974) A semi-quantitative test for mercury in air. Acta Odontol. Stand. 32,305308. LEIRSKP.I:J. (1974) On the mechanism of cytotoxicity of silver and copper amalgams in a cellculture medium. Odont. Tidskr. 82,74-81. LEWIS J. (1974) The silver amalgam restoration. Northwest

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LIATUKAS E. L. (1974) Scanning analysis of a dual-alloy restoration. J. Prosthet. Dent. 31, 435438.

MACKERTJ. R., MOOREB. K., GREENERE. H. and GOLDBERG A. F. (1974) Variables affecting mercury loss during trituration of amalgam. J. Dent. Res. 53, 62.

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MAHLERD. B. and VAN EYSDENJ. (1974) Occlusal extrusion of clinical amalgam restorations. J. Dent. Res. 53, 61.

MALHOTRA M. L. and LAWLESS K. R. (1974) Microstructure and phase identificationof Ag,Sn alloys containing gold using transmission and scanning electron microscopy. J. Dent. Res. 53, 61. MARSHAAL M. E. and HOCHMAN R. F. (1974) X-ray determination of volume fraction of dental amalgam phases. J. Dent. RES. 53, 63. MATHEWSONR. J., RE~ZLAFF A. E. and PORTER D. R. (1974) Marginal failure of amalgam in deciduous teeth-a two-year report. J. Am. Dent. Assoc. 88, 134-136. OKABET., HOCHMAN,R. F. and HINESA. L. (1974) Tracer diffusion of mercury (2U3Hg)in a dental alloy (Ag,Sn). J. Dent. Res. 53, 62. OSBORNEJ. W. and GALE E. N. (1974) A two-, three- and four-year follow-up of a clinical study of the effect of trituration on amalgam restorations. J. Am. Dent. Assoc. 88, 795-797. RYGE G., CVARJ. F., MICIK R. E. and LANGERM. (1974) Clinical studies of restorative materials I; Zinc vs. non-zinc alloys. .I. Dent. Res. 53, 61. STAHELIP. J. and VON FRAUNHOFERJ. A. (1974) Electroetching of dental amalgam. J. Dent. Res. 53,468473.

TAYLOR D. F. and SMITH J. H. (1974) Rate of reaction of rn:rcury with silver-tin amalgam. J. Dent. Res. 53, 63.

TOBELERR. L., ROSTOKERW. and MASSLERM. (1974) Development of a ductile amalgam. J. Dent. Rrs. 53, 937-91 I. VRIJHOEF M. M. A. and DRIESSENSF. C. M. (1974a) Investigation of the phase composition of six dental amalgams by X-ray diffraction. J. Biomed. Mater. Res. 8,443450.

VRIJHOEFM. M. A. and DRIESSENS F. C. M. (1974b) Long-term phase changes in dental amalgam after setting. J. Biomed. Mater. Res. 8, 435-142.

Casting all0 ys

HOLLANDERR. and WULFF J. (1974) New technology for improvement of cast mechanical propwty Co-Cr-MO-C surgical implants. Eng. Med. 3, 8-9. HUGERE. F. and CIVJANS. (1974) Status report on palladium-silver-based crown and bridge alloys. J. Am. Dent. Assoc. 89, 383-385.

LEWISA. J. (1974) The analysis and compositional evaluation of a nickel base partial denture casting alloy. Aust. Dzzt. J. 19, 389-394.

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MARX H. (1974) Zur Wiederverwendbarkeit Dtsch. dentaler Kobalt-Chrom-Legierungen. Zuhnaerztl. Z. 29, 1008-1013. MOHAMMED H., ABDULLAHS. I. and MUMFORDG. (1974) A new dental superalloy system: VI. Heat treatment. J. Dent. Res. 53, 379-384. MOHAMMED H. and ASGARK. (1974) A new dental superalloy system: V. Embrittling phase transformations. J. Dent. Rex 53, 7-14. MOHAMMEDH., GRASSOJ. E. and ABDULLAHS. I. (1974) Application of non precious alloys to crown and bridge procedures. J. Can. Dent. Assoc. 40, 733-740. ROWE A. P., BIGELOWW. C. and ASGAR K. (1974) Effect of tantalum additions to a cobaltchromium-nickel base alloy. J. Dent. Res. 53, 325-332. WILLIAMS D. F. (1975) Dental materials: 1973 literature review. J. Dent. 3, 51-67.

Journal of Dentistry, Vol. ~/NO. 2

GRAJOWERR., HIRSCHFELDZ. and ZALKIND M. (1974) Compatibility of a composite resin with pulp insulating materials. A scanning electron microscope study. J. Prosthet. Dent. 32,70-77. HALS E. and KVINNSLANDI. (1974) Structure of experimental in vitro and in vivo lesions around composite (Addent XV) fillings. &and. J. Dent. Res. 82, 517-526. HARRISR. K., PHILLIPSR. W. and SWARTZ M. L. (1974) An evaluation of two resin systems for restoration of abraded areas. J. Prosthet. Dent. 31, 537-546. HARSANYI B. B., ANGELOPOULOSA. P. and GOURLEY J. M. (1974) Subcutaneous tissue response to composite resins in dogs. Oral Surg. 37,308-319. HAYASHIH., MAEJIMAK., KEZUKA K., OGUSHI K., KONO A. and FUSAYAMAT. (1974) In vitro study of discoloration of composite resins. J. Prosthet. Dent. 32, 66-69. MCCURDY C. R., SWARTZ M. L., PHILLIPS R. W. and RHODES B. F. (1974) A comparison of in vivo and in vitro microleakage of dental restoraDenture base polymers tions. J. Am. Dent. Assoc. 88, 592-602. KUSY R. P. and TURNERD. T. (1974) Fractography MCLUNDIE A. C. and MURRAYF. D. (1974) Comof two-phase acrylic polymers. J. Dent. Res. 53, parison of methods used in finishing composite 520-529. resin-a scanning electron microscope study. DE WIJN J. R. (1974) Reduction of maximum J. Prosthet. Dent. 31, 163-171. temperature in the polymerization of cold- and MCPHEE E. R. (1974) Pin-retained composite heat-curing acrylic resins. J. Biomed. Mater. Rrs. resin cases for posterior teeth. J. Prosthet. Dent. 8.421-434. 31,566-569. MITCHAM J. C. and TURNER L. R. (1974) The retentive strength of acid etched retained resins. J. Am. Dent. Assoc. 89, 1107-1110. MILLER G. (1974) Glass ceramics as composite Composite filling materials fillers. J. Dent. Res. 53, 1342-1345. A~MUSSENE. (1974) The effect of temperature NELSONS. R., TILL M. J. and HINDING J. H. (1974) changes on adaptation of resin fillings I. Acta Comparison of materials and methods used in Odontol. &and. 32, 161-171. acid etch restorative procedures. J. Am. Dent. BRADY A. P., LEE H. and ORLOWSKIJ. A. (1974) Assoc. 89, 1123-1127. Thermal conductivity studies of composite PAFFENBARGER G. C. and RUPP N. W. (1974) dental restorative materials. J. Biomed. Mater. Composite restorative materials in dental pracRes. 8,471-485. tice: a review. Znt. Dent. J. 24, l-l 1. CHANDLERH. H., BOWEN R. L., PAFFENBARGE~ PLANT C. G., JONES D. W. and DARVEL B. W. G. C. and MULLINEAUXA. L. (1974) Clinical (1974) The heat evolved and temperatures evaluation of a tooth-restoration coupling agent. attained during setting of restorative materials. J. Am. Dent. Assoc. 88, 114-l 18. Br. Dent. J. 137, 233-238. EAMESW. B., STRAIN J. D., WEITMANR. T. and WILLIAMSA. K. (1974) Clinical comparison of POWERSJ. M., ALLENL. J. and CRAIG R. G. (1974) Two-body abrasion of commercial and expericomposite, amalgam and silicate restorations. mental restorative and coating resins and an J. Am. Dent. Assoc. 89, 1111-1117. amalgam. J. Am. Dent. Assoc. 89, 1118-1122. GJERDETN. R. and HALS E. (1974) Observations on the microstructure of the composite filling TRONSTADL. and SP~WBERG L. (1974) Biologic materials Concise and Addent XV. Stand. J. tests of a methylmethacrylate composite material. Dent. Res. 82, 608-612. Stand. J. Dent. Res. 82, 93-98. VON

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Materials

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Soft lining materials

BUCKNER H. (1974) Construction of a denture with hollow obturator, lid and soft acrylic lining. J. Prosthet. Dent. 31, 95-99. LECHNER S. K. (1974) Prosthetic management the geriatric patient. Aust. Dent. J. 19, 17-24.

in

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HINDING J. H. and SVEEN 0. B. (1974) A scanning electron microscope study of the effects of acid conditioning on occlusal enamel of human permanent and deciduous teeth. Arch. Oral Biol. 19, 573-576. HOROWITZ H. S., HEIFETZ S. B. and MCCUNE R. J. (1974) The effectiveness of an adhesive sealant

in preventing occlusal caries: findings after two years in Kalispell, Montana. J. Am. Dent. Assoc. 89, 885-890. LEE H. and OCUMPAUGH D. (1973) Sealing of developmental pits and fissures. V. Comparison

Fissure sealants ARANA E. M. (1974) Clinical observations of enamel after acid-etch procedure. J. Am. Dent. Assoc. 89, 1102-l 106.

BOWENR. L. and RODRIGUEZM. S. (1962) Tensile strength and modulus of elasticity of tooth structure and several restorative materials. J. Am. Dent. Assoc. 64, 378-387.

CONSN. C., POLLARDS. Y. and LESKEG. S. (1974) Adhesive sealant field trial in a fluoridated areafirst years’ experience. Int. Assoc. Dent. Res. Abstr. No. 792, p. 252. COSGROVED. J. (1974) A treatment for damaged anterior teeth associated with bruxism. Aust. Dent. J. 19, 320-321. COUNCIL ON DENTAL MATERIALS AND DEVICES (1974) Pit and fissure sealants. J. Am. Dent. Assoc. 88, 390. DENNISON J. B., MEYER J. M., BIRNHOLTZ S. B. and CRAIG R. G. (1974) Initial leakage under pit and fissure sealants assessed by neutron activation. J. Dent. Res. 53, 1439-1444. EDEN G. T., CRAIG R. G. and PEYTON F. A. (1970) Evaluation of a tensile test for direct filling resins. J. Dent. Rts. 49, 428434. EL-KAFRAWY A. H. and MITCHELL D. F. (1975) Effect of fissure sealants on the prevention and progress of caries in rats. J. Dent. Res. 54, 421. VON FRAUNHOFER J. A. and WILLIAMS B. (1974) Heat liberation during the setting of four fissure sealants. Br. dent. J. 136, 498499. GOING R., HAUGH L., CONTI A. and GRAINGER D. (1974) Two year clinical evaluation of a pit and fissure sealant. Int. Assoc. Dent. Res. Abstr.

No. 795, p. 253. HINDING J. (1974) Extended cariostasis following loss of pit and fissure sealant from human teeth. J. Dent. Child. 41, 41-43. HINDING J. H. and BUONOCORE M. G. (1974) The effects of varying the application protocol on the retention of pit and fissure sealant: a twoyear clinical study. J. Am. Dent. Assoc. 89, 127-131.

of adhesive topical fluoride coating vs. fluoride gels. Biomater. Med. Devices Artif. Organs 1, 163-170. VON LOEW R. K., LAMPERT F., KETTERL W. and EHRENBERG-KIECKEBUSCHW. (1974) Der Einflus des Conditioners bei Versiegelung mittels

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p. 252. RISAGER J. and POULSEN S. (1974) Fissure sealing with Nuva-Seal in a public health program for Danish schoolchildren after 12 months’ observation. Stand. J. Dent. Res. 82,570-573.

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ROCK W. P. (1974a) The effect of etching

of human enamel upon bond strengths with fissure sealant resins. Arch. Oral Biol. 19, 873-877. ROCK W. P. (1974b) Fissure sealants. Br. Dent. J. 136,317-321. ROCK W. P. (1974c) The use of ultra-violet radiation in dentistry. Br. Dent. J. 136,455458. ROCK W. P., BAILEY A. R. and BROWNE R. M. (1974) Tissue reactions to two fissure sealant resins. J. Oral Pathol. 3, 224-231. RUDOLPHJ. J., PHILLIPSR. W. and SWARTZ M. L. (1974) In vitro assessment of micro-leakage of pit and fissure sealing. J. Prosthet. Dent. 32, 62-65.

SILVERSTONEL. M. (1974) Fissure sealantslaboratory studies. Caries Res. 8,2-26. STAMMJ. (1974) A brief look at fissure sealants. J. Dent. Quebec 8, 28. WILLIAMS B. F., VON FRAUNHOFERJ. A. and WINTER G. B. (1974) Tensile bond strength between fissure sealants and enamel. J. Dent. Res. 53,23-27.

KALOYANNIDES T. M. and KAPARI D. J. (1974a) Setting time and consistency of elastomer impression materials. J. Dent. Res. 53, 653-656. KALOYANNIDES T. M. and KAPARI D. J. (1974b) Mixtures of elastomer impression materials of the same group: setting time and consistency. J. Dent. Res. 53, 657-660.

KALOYANNIDES T. M. and KAPARI D. J. (1974~) Mixtures of elastomer impression materials : I. Setting time and consistency. J. Dent. Res. 53, 809-813. MARXKORSR., MEINERSH. and MENDE T. (1974) Einfluss von Temperatur und Dosierung auf die Vernetzungsgeschwindigkeit elastomerer Abformmaterialien. Dtsch. Zuhnaertzl. 2. 29, 810814.

REHBERGH. J. and SCHWICKERATHH. (1974) Der Stand unserer Kentnisse uber die elastomeren Abformmaterialien. Dtsch. Zahnaertzl. Z. 29, 382-389. SAWYERH. F., DILTS W. E., AUBREYM. E. and NEIMANR. (1974) Accuracy of casts from three classes of elastomer impression materials. J. Am. Dent. Assoc. 89, 644-648.

Impression materials BAROLETR. Y. and DESAUTELSP. C. (1974) New elastomeric impression materials. J. Can. Dent. Assoc. 40,488-489. COONEYJ. P. (1974) A comparison

and stone dies from rubber-base J. Prosthet.

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FAIRCHILDJ. M. (1974) Versatile uses for alginate impression material. J. Prosthet. Dent. 31, 266269.

FUSAYAMAT., IWAKU M., DAITO K., KUROSAKIN. and TAKATSUT. (1974) Accuracy of the laminated single impression technique with silicone materials. J. Prosthet. Dent. 32, 270-276. HEMBREEJ. H. and NUNEZ L. J. (1974) Effect of moisture on polyether impression materials. J. Am. Dent. Assoc. 89, 1134-1136. HENRY P. J. and HARNIST D. J. R. (1974) Dimensional stability and accuracy of rubber impression materials. Aust. Dent. J. 19, 162166.

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