Dental materials: 1979 literature review Part II

Journal of Dentistry,

9, No. 4, 1981, pp. 271-298

Dental materials: Part II D. Brown,

Prinred in Great Britain

1979 literature

review

MSc, PhD, CEng, MIM (Editor)

Guy’s Hospital Dental School, London M. Miller,

M. Braden, BSc, PhD, FlnstP, FPRI Dental

School,

London

Hospital

Medical

College

Medical

College

School,

London

E. C. Combe, Turner Dental

Hospital

MSc, PhD, DSc, CChem, FRSC School,

University

of Edinburgh

H. J. Prosser, BSC, PhD

8. E. Causton, MC, PhD Dental

8%. CChem, FRSC

School of Dental Surgery,

Manchester

Laboratory

of the Government

Chemist,

London

N. E. Waters, BSC, PhD. FlnstP School of Dental Surgery,

Royal Denral

Hospital

of London

D. W. Cruickshanks-Boyd, Eastman

Dental

Hospital,

8Sc. CEng. MIM

London

Angela M. Fletcher, BDS, LDS RCS, MIM, AMPRI Dental

School,

C. H. Lloyd, Dental

Universily

College, London

BSc, PhD

School and Hospital,

J. F. McCabe, Dental School,

D. C. Watts, ESc, PhD, CChem, FRSC Turner Dental School,

D. F. Williams,

Manchester

&SC, PhD, CEng, FIM

School of Dental Surgery,

University

of Liverpool

A. D. Wilson, DSC, CChem, FRSC Dundee

BSc, PhD, CChem, MRSC Newcastle-upon-Tyne

Laboratory

of the Government

Chemist,

London

H. J. Wilson, PhD, DSc, CChem, FRSC Dental School,

Birmingham

ABSTRACT This paper, which is presented in two parts, reviews the work on dental materials published in 1979. Included in Part II are sections on impression materials, model, die and investment materials, waxes, acrylic resins, denture base polymers, soft lining materials and tissue conditioners, cast and wrought dental alloys, ceramics, implants and, finally, corrosion. Part I included sections on dental biomechanics, fissure sealants, cements, amalgam, composite filling materials, endodontic materials and microleakage.

INTRODUCTION This review is the seventh in an annual series compiled by the members of the Panel for Dental Materials Studies in the United Kingdom, and is intended as a source of reference work on dental materials published in 1979. It covers the introduction of new materials, the evaluation of existing materials and the development and application of techniques to aid such evaluations, together with assessments of the materials-oriented problems of clinical procedures. A basic list of 37 journals was surveyed by all contributors, but each contributor was free to include material from any other source.

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IMPRESSION MATERIALS Although much of the current work on elastomeric impression materials merely confirms earlier findings, it is desirable that such work should be continuing not only because comparison of the new addition-cured silicones with the older materials is needed, but also because new brands appear, and because modifications of existing brands occur, often without notification by the manufacturers. Eames et al. (1979b) compared the accuracy and stability of impressions made with the four available types of elastomeric impression material by pouring stone casts to the impression of a stainless steel crown preparation after 30-minute and 24-hour intervals. They used a heat-cured tray allowing 2.4-mm spacing and allowed the material to set at 32°C for 3 minutes longer than the minimum laid down in the ADA specification. The range of dimensional error they found on all the materials was 0.1 l-045 per cent after 30 minutes and O*l&OG34 per cent after 24 hours. The authors drew the conclusion that provided the impressions were poured immediately after removal, the stability characteristics of all the materials were similar, so the dentist can then base his choice of material on considerations other than accuracy. But where immediate pouring is precluded, only the most stable materials should be selected. Conventional silicones were the least stable. Addition-reaction silicones were similar to the polyethers, which were the most stable. The putty systems were as good as polyethers up to 30 minutes, but less stable at 24 hours. Polysulphides were as good as polyethers up to 30 minutes, but varied considerably according to brand at 24 hours. In another paper Eames et al. (1979a) examined the effect on accuracy of the bulk of the impression material. Other workers had variously recommended spacings 2 mm maximum, 2-4 mm and l-5 mm. In this work trays were constructed to give 2-, 4- and 6-mm spacings round a tapered stainless steel die with score marks for measurement. The impressions were measured immediately after removal from the water bath at 37 “C and again after 24 hours. The average error for all the materials tested was least for the 2-mm thickness and greatest for the 6-mm thickness. These averages were shrinkages, but three results, including two putty and wash systems, showed expansions. In one case the error at 24 hours was greater than the permitted maximum in ADA specification No. 19. When castings were made from impressions in the most stable, the least stable and an average brand, it was found that the lift of the castings where 4-mm and 6-mm spacings were used would not be clinically acceptable. The results of this work confirmed the recommendation of Phillips that a maximum of 2-mm space be allowed. Another approach to accuracy in elastomers by Luebke et al. (1979) was to find the effect of second pours of stone in the same impression. They used a master of four square posts in brass, a surprising choice of metal in view of the known affinity of polysulphides for brass. Acrylic trays were vacuum formed to give 3-mm relief, and the impression materials were allowed to set for 15 minutes. Compounded measurements of distances between the posts showed no significant difference between first and second pours made after the same time interval. The time intervals extended to 48 hours. Results were not significantly different from the master if poured within 15 minutes, and the results for the polyether varied so little that the authors commented that it was ‘remarkably stable’. Their findings on polysulphide were probably valueless first because they used only one brand, and that was the one found by Eames et al. (1979a) to be the least stable and therefore not typical, and secondly because the adhesion of polysulphide to brass would vitiate the results anyway.

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This reaction between copper in metals and sulphur in polysulphide, which is a reason for the success of the copper-ring technique, was used by Shigeto et al. (1979) to obtain improved tray adhesion. They found that copper-plated acrylic tray resin gave a tensile bond to polysulphide of 2.64 kg, whilst the resin coated with adhesive gave only 1.66 kg. Since the thinner layers of polysulphide required for greater accuracy result in greater stress at the interface between tray and elastomer, this stronger bond could be valuable. Polyether and polysulphide materials were compared by Nayyer et al. (1979) by measuring working time, setting time and compression set by ADA specification and other methods. Their conclusions were confirmation of already-known facts, namely the slower setting of polysulphides, the superior elastic recovery of polyether and the improvement in elastic properties when more time is allowed for setting, but the comparison of clinical, rheometer and manufacturer’s working and setting times was interesting. The working time of the addition-cured silicones was considered by Stannard and Craig (1979) to be too short, which suggests that these materials may have been modified since work mentioned in this review for 1978, which found them to be slower than the conventional silicones. Two methods of modifying the setting rate were tried. The first was the addition of phenylpropiolic acid (PPA), whose triple bonds retard the rate of polymerization, and the second was by altering the catalyst/base ratio. They reported an increase in catalyst/base ratio as increasing the working time, which seems curious, but either this method or the addition of PPA increased the time available to get the material into the mouth from 2 minutes to 6 minutes. The elastic properties were only slightly worsened and remained well within the limits of the ADA specification. This facility was considered to have a potential use for teaching syringe-tray techniques. In these techniques viscosity is an equally important factor, and Rehberg (1979) studied elastomer materials using the parallel plate spread test to show that low viscosity and the application of only small amounts of paste were prerequisites for obtaining the thinnest possible layer for correcting first impressions in the double mix technique. With most of the recent work on elastomers it was a change to find a paper on alginates. Coleman et al. (1979) took impressions of a metal mandibular arch master in four brands of alginate and poured the cast after storage periods from 2 to 24 hours, some wet, some dry. They found no significant difference between the immediate pour and those after 10 minutes wet, 30 minutes dry, and 1 hour wet, but the 24-hour results were significantly different. The 30minute dry result is surprising and suggests a high ambient humidity. Mack (1979) estimated the amount of alginate powder likely to be inhaled during mixing and concluded there was no health hazard from the major ingredients. The potential hazard from minor ingredients was ignored. Initial spatulation should be gentle to minimize nasal contamination. When gypsum casts are made from the hydrophobic polysulphide or silicone impressions the casts may be spoiled by bubble entrapment due to poor wetting. Hydrocolloids, being hydrophilic, do not give this trouble, and polyethers have a low wetting angle which approaches that of hydrocolloid. Norling and Reisbick (1979) sought to improve this situation by adding nonylphenoxypoly(ethyleneoxy)ethanol homologues to the base paste of polysulphide or silicone before mixing. From wetting angle measurements they selected the best homologue and concentration and then used this with the polysulphide and silicone. Die stone casts showed a large reduction in the number of bubbles compared with the unmodified material, the bubble counts approaching that for polyether. Working time was

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increased slightly for polysulphide and markedly for silicone but permanent set was not affected. Rinsing the impression with water did not reduce the effectiveness (the additive is insoluble in water), so the effect appeared to be due to an increase in the surface free energy of the elastomer. Meiners and Dittmer (1979) measured the exothermic heat from polymerization of a number of elastomers and found that polyethers and polysulphides gave around ten times as much heat as did silicones. The continuing doubt as to the suitability of tissue conditioning materials for impression taking has provoked a paper by Razek (1979) who took impressions of a metallic anatomic surface mounted in a hinged articulator. Although no quantitative results were quoted he reported that there was no significant difference between casts from zinc oxide/eugenol or polysulphhide and those from Visco-gel or Coecomfort, whether the impression was removed at the setting time or after 2 hours’ compression, and he concluded that Visco-gel and Coecomfort were valid as impression materials. The conclusion regarding Coe-comfort differs from that of McCarthy and Moser (quoted in this review for 1978) who considered it more suitable as a tissue conditioner than as a functional impression material. Razek’s fmding that a 3*0-mm space gives better dimensions than a 1*5-mm space with these materials is in agreement with McCarthy and Moser. The requirements for tissue conditioners and functional impression materials appears to.be a field meriting considerably more investigation.

MODEL, DIE AND INVESTMENT

MATERIALS

Research has continued on measuring the properties of gypsum materials. Hosoda and Ohsawa (1979) measured the dimensional accuracy of stone models, and demonstrated that a new method which they used-an optical feeler technique-was accurate, gave reproducible results and had little or no harmful effects on stone dies. For measuring the strength of dental gympsum, Boswell and Stevens (1979) suggested an alternative technique to the widely used diametral compression test. This was termed double-punch testing; in this method a cylindrical specimen of gypsum is compressed between two flat punches which are placed along the central axis of the specimen. This technique gave data with lower coefficients of variation and less dependence of the results on specimen size, when compared to diametral compression. Probst and Duncanson (1979) have studied the fractography of die stones. They showed by scanning electron microscopy that with the use of two commercially available water substitutes for mixing with the die stones, there was no correlation between crystal morphology and improved mechanical properties. A problem with gypsum slurries is their comparative inability to wet elastomeric impression materials, particularly polysulphides and silicones. This problem can be allieviated by incorporation of non-ionic surfactants into the elastomer (Norling and Reisbick, 1979); this technique can be applied to addition-cured silicones with no harmful effects on dimensional accuracy and stability (Brukl et al., 1979). Vermilyea et al. (1979) have evaluated three resin-containing die materials. Detailed composition of these were not given, though one of them was an epoxy resin, and their flier contents were 26,48 and 49 per cent by weight. Though such materials can produce hard and strong dies, yet they suffer from a number of disadvantages, e.g. relatively long setting times, incompatibility with some impression materials, and polymerization shrinkage. Electroplated dies also may not be accurate, because of dimensional changes in the electro-

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deposit (Stackhouse, 1979). To avoid use of silver-plated dies, Presswood et al. (1979) developed an elaborate die preparation technique for fabrication of porcelain jacket crowns. Included in this procedure was the painting of the die with a cyanoacrylate cement containing 80 per cent acetone. Cyanoacrylates have also been used for cementation of dowel pins; Smith et al. (1979) reported a technique whereby a dowel pin is oriented and cemented after the working cast has been separated from the impression. Stone dies have been treated with dimethacrylate polymers, to form a film of about 30-m thickness on the die, to act as a spacer (Jergensen and Finger, 1979). This thickness provides adequate space for a zinc phosphate luting cement. The above work is of significance in the development of refractory investment materials. Jorgensen and Finger (1979) have developed a gypsum-bonded investment with little or no setting expansion, but sufficient thermal expansion (e.g. l-8 per cent) to compensate for the contraction on cooling of a cast gold alloy.

WAXES A new method for measuring the percentage expansion of dental waxes over the temperature range 23-48 “C has been described by Grajower (1978).

ACRYLIC

RESINS

The evaluation

of denture base poly(methy1 methacrylate) physical properties has been studied fairly exhaustively over the years, but limited data is available on the nature, magnitude and rate of the strains that actually occur in dentures. Stafford and Griffiths (1979) have made a study, using strain gauges, of such important parameters in maxillary complete dentures. The difficulty in producing satisfactory clear poly(methy1 methacrylate) dentures is well known. Keng et al. (1979) have made a careful study of the factors influencing the production of an unblemished product, and have established the importance of separating medium, porosity consequent on too-rapid curing and model dryness. A brief qualitative study has been reported of the evaluation of heat-cured poly(methy1 methacrylate) in terms of porosity (Gay and King, 1979). Whilst not directly on acrylic resins, a paper on denture cleansers is worth noting for its comprehensive literature review of the subject (Budtz-J$rgensen, 1979). Whilst it has long been suspected that heating poly(methy1 methacrylate) appliances towards the T, of the polymer would release internal strains, little information is available. Lorton and Phillips (1979) have now produced valuable quantitative data on this and the consequent distortion. Evidently temperatures of 90°C or more are sufficient to cause changes of tit on a gypsum cast. This is extremely interesting because the data is for heatcured resins, where one would expect the T, to be - 120 “C. It is a matter of speculation and concern as to what the corresponding Lorton and Phillips’ temperature would be for an autopolymerizing resin with a much lower To such as the pourable denture base resins. Manley et al. (1979) have reported further on their work on denture base poly(methy1 methacrylate) reinforced with carbon tibres in terms of tensile and fatigue properties, and also in terms of biological tests to establish possible toxic and carcinogenic properties. The results of the latter were such as to permit the commencement of clinical trials.

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Barsby and Braden (1979) have investigated the properties of a hydrophilic denture base resin. A degree of hydrophilicity is conferred by the incorporation of some 2-hydroxyethyl methacrylate in the monomer, supposedly to enhance retention. The material is inferior to conventional materials in strength, rigidity, and has a high mechanical loss tangent. Hargreaves (1979) has also reported on the same hydrophilic resin with respect to equilibrium water uptake, and consequent unacceptable dimensional instability. Two review papers on pourable (fluid) denture base resins have been compiled by Hedergird (1978 a, b). Hikage et al. (1979) have published an extremely interesting paper on the interaction between an aromatic dimethacrylate and poly(methy1 methacrylate). For some years facing materials have been available based on poly(methy1 methacrylate) powder with aromatic dimethacrylate monomers, to give the requisite hardening and low water absorption. This paper describes in detail a study of the actual interaction between monomer and polymer. It is evident that the authors have done extensive work in this largely ignored area, judging by about eighteen references largely in the Japanese literature. Anyone interested in this area is strongly advised to pursue these. Bowen (1979) has published the thirteenth of a series of papers on adhesive bonding to hard dental tissues. The current one concerns details of the synthesis of complex chemicals containing both acrylate groups, and moieties capable of chelating. Dulik (1979) has evaluated a number of commercial and newly synthesized amine accelerators ostensibly for composites, but of obvious interest for any room temperature polymerizing acrylic system. Brauer et al. (1979a) have examined further activating amines, again for composites, but of general interest in room temperature polymerizing acrylics. Alexieva (1979) has studied the nature of the bond between hard tissue and polymeric materials. The basis of the model study was the interaction of N-phenyl glycine, and its reaction product with glycidyl methacrylate, and Ca3(P04), . This was followed by a study of NPG with enamel and dentine. Antonucci et al. (1979) have made a fascinating study of new initiating systems for the room temperature polymerization of acrylic resins. These include a range of peroxide activators and asorbic acid and its derivatives. Brauer et al. (1979b) have revived the fortunes of cyanoacrylates as bonding agents for acrylic resins to dentine. Paddon and Wilson (1978) have studied the stress relaxation of both an acrylic and an epimine resin, and analysed the results in terms of linear viscoelastic theory. Duran et al. (1979) have measured some viscoelastic properties of tissue conditioners and soft liners, some of which are of course acrylic polymers. Harrison et al. (1979) have reported a careful study which involves the important feature of correlating abrasion resistance with other physical properties. Studies of mechanical properties of acrylic resins have featured in work by Mom (1979).

DENTURE BASE POLYMERS Investigations into the effect of processing and finishing procedures on the structure and properties of rigid denture base polymers have formed the basis of the majority of the

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information published in 1979. Soni et al. (1979) devised a non-parametric method which employed ranking data to compare the fit of a total of 25 dentures-5 made from each of the following materials: a conventional acrylic resin, 2 rubber-modified acrylics, an acrylic/ vinyl co-polymer and an acrylic/hydroxyethyl methacrylate co-polymer. The acrylic/vinyl co-polymer dentures appeared to fit best of all stages and the rubber-modified acrylic dentures were the worst. The conventional acrylic gave only mediocre results. It should be noted that the assessment was only by visual methods. The effects of the temperature rise during processing, grinding and storage were evaluated by Lorton et al. (1979). After determining the temperature rise in test specimens with thermocouples, they constructed 9 test dentures which incorporated metal markers. These dentures were immersed in water at 70”, 80”, 90” and 100 “C. Their results showed that temperatures above 90°C caused by either grinding or water immersion caused warpage. A stereometric method for determining the dimensions and the dimensional changes of wet and dry dentures was evolved by de Gee et al. (1979) who processed 5 pairs of maxillary dentures using 5 acrylic polymers. Their measurements over a 2-month period indicated that the processing technique was both reliable and reproducible. However, whilst the dentures which they constructed out of self-curing acrylic resins shrank the least on curing, they showed considerable retraction from the palate after being stored in water. The effect of processing temperature on the exotherm, porosity and properties of both heat and selfcured acrylic resins was reported by Faraj and Ellis (1979) who used discs 50 and 80 mm thick to show that porosity only occurred in heat-cured acrylic if it was processed at 100 “C. They also observed that the polymerization shrinkage for self-cured acrylic was 0.26 per cent, whereas that for heat-cured acrylic was 053 per cent, and that the self-cured material had a lower Young’s Modulus. Carbon fibres are now routinely being used by some to reinforce acrylic denture bases and Manley et al. (1979) in animal studies found no evidence of long-term toxicity or carcinogenicity from either implanted carbon fibres or black silk suture material. The tensile strength of a carbon fibre reinforced poly(methy1 methacrylate) was compared with conventional and rubber-modified acrylic and found to be three to four times as great. Fatigue measurements also indicated that carbon fibre reinforcement gave a greater resistance to fatigue, the reinforced material showing an order of magnitude improvement over the unreinforced materials.

SOFT LINING MATERIALS

AND TISSUE CONDITIONERS

In a review article Douglas (1979) outlined the requirements of soft polymers used to line dentures and concluded that there is a need for better materials. One of the major problems associated with soft polymers is that they are readily colonized by fungi and there is a need for a material which is anticandidal. Ellis et al. (1979) have characterized a commonly used soft lining material (Coe-soft) with respect to composition and diffusion characteristics. The material consists of a poly(ethy1 methacrylate) powder and a liquid mixture of dibutylphthalate, benzyl salicylate and ethanol. Rates of ethanol desorption and water absorption were measured. Murray (1979) utilized a resilient lining material for the retention of maxilIo-facial prostheses. The choice of material was a heat-cured silicone rubber (Molloplast B) which was chosen after consulting the in vitro test results of other groups of workers. Razek (1979) found that two tissue conditioners (Visco-gel and Coe-comfort) could be

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used successfully as functional impression materials. The use of 15 or 3-mm thickness of material was satisfactory and best results were achieved by pouring casts within 2 hours. Duran et al. (1979) studied the viscoelastic properties of soft liners and tissue conditioners. Seven materials were included in the study; two tissue conditioners, four acrylic soft liners and one silicone rubber soft liner. All the materials showed deviation from linear viscoelastic behaviour. Under static conditions the tissue conditioners functioned as viscous liquids, whilst the liners were more elastic. Under dynamic conditions all the materials appeared to be stiffer. The materials containing ethanol as a component of the liquid were most affected by water storage. CAST AND WROUGHT DENTAL ALLOYS Casting alloys The replacement of high gold content casting alloys by less costly substitutes has continued to dominate the literature with research directed towards both casting technology and the effect of alloy composition and structure upon properties. That the casting accuracy of base metal casting alloys is inferior to type IV gold has been reconfirmed by Pulskamp (1979). A simulated partial denture cast with a nickel base ahoy into the recommended gypsumbonded investment was on average O-42 per cent undersize due to inadequate expansion of the investment, whereas a cobalt-base alloy cast into silica-bonded investment was 0.61 per cent undersize. These compared with 0.16 per cent oversize for a type IV gold alloy. To minimize the unwanted forces exerted by a misfitting base alloy denture Pulskamp has suggested that wrought-wire clasps should be used. Lorey et al. (1979) have reported that the marginal fit of seven base alloys was less acceptable than gold when cast for full coverage restorations, yet cast as inlays possessed very accurate clinical margins. The fit of a number of base alloy crowns has also been shown to be generally poor (only 12 per cent were adequate) with the choice of the alloy/investment combination a significant factor for determining accuracy (Huget et al., 1979). The use of a ringless, hygroscopically expanded, mould was stated to offer the greatest potential for increased expansion to produce more accurate castings, although moulds produced by this technique were overexpanded (and therefore base alloy casts were oversize). The accuracy of Ni-base castings has also been examined by Cole and Vincent (1979) who again found that hygroscopically treated investment contained in waxed paper rings overexpanded to produce a loose-fitting crown. They discovered that the optimum expansion occurred when the investment in the paper ring was not expanded hygroscopically! When discussing the castibility of an alloy (or completeness of castings) the choice of investment is a variable too-often ignored. Barreto et al. (1979) found that both the choice of investment and how it was stored after burnout affected the castibility of base and noble alloys alike. The flow of six dental alloys into the mould cavity has been shown to obey pressure distribution equations but the marginal filling was also dependent upon the surface tension of the melt (i.e. on the degree of superheat and alloy composition), as well as the metal pressure (Pines et al., 1979). In another study (DeWald, 1979) the flow pattern developed during certrifugal casting was obtained to determine completeness in the casting. This is most probable when the wax pattern is angled away from the sprue towards the outer, lower part of the ring. In an extension to an earlier investigation Ogura et al. (1979) have shown

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that the surface roughness of a crown interior is affected by the variables of alloy composition, alloy temperature, mould temperature and position in the casting ring, but not by choice of casting machine. The need to control or detect porosity in partial denture castings has been demonstrated by Wictorin et al. (1979) who found that only 2 out of 66 partial denture castings were defect-free. Defects were detected using conventional dental X-ray equipment and film. The idea for such non-destructive testing is not new, but they had undertaken a detailed exercise to optimize parameters. Defects above 10 per cent of the thickness in 06 to 3-O mm section Co-Cr (or 0.1 mm in thin section) were detectable. Pores were most dense in saddle and saddle bar regions, but (fortunately) few in number in clasps. This apart, the aim of the project was to develop a cheap and rapid standard X-ray technique using clinical equipment which produces results which dental technicians could interpret. Radiographic examination of partial denture castings using dental equipment has also been reported in brief by Wise and Kaiser (1979). Mohammed and his co-workers have previously established that the inclusion of iron as a major alloying element in partial denture casting alloys based upon the ternary Co-Cr-Ni system improves ductility. Pursuing this, Mohammed and Shen (1979) found that ductility was at a maximum when the quaternary composition was such that the (IIphase was increased (at the expense of y phase) and the brittle u phase absent. The incidence of these phases has been related to the Fe : Cr ratio (Shen and Mohammed, 1979), who showed that at equiatomic proportions the volume of u phase was at a maximum and ductility at its lowest. When the ratio was increased ductility improved as the QI phase increased, and reached a maximum at 2.5 : 1, after which ductility decreased as y phase replaced (Y.A further part of this programme has been reported by Mohammed and Vasudev (1979) who took the nickelfree alloy which produced the maximum ductility, 30Co-50Fe-20Cr, and made additions of Ta to improve its strength. The optimum addition was 4 per cent which increased the proof stress by 40 per cent whilst the ductility remained high at 16 per cent. Beyond 5 per cent Ta the formation of a u phase between Fe, Cr and Ta drastically reduced ductility. However, it may be possible to make further Ta additions if the Cr content is reduced whilst maintaining the Fe : Cr ratio at 2.5 : 1. As cast, both cobalt and nickel base partial denture alloys exhibit a cored dendritic structure and an interdendritic structure which contains carbide plates in the Ni base alloys or particles in the Co base alloys. The segregation of elements within such structures has been measured by electron microprobe analysis (Lewis, 1979a). Chromium was found segregated to a greater degree than the other major alloying elements within the dendrites * in the Ni base alloy, but surprisingly little of this very stable carbide former was found in the lamellar interdendritic regions. Contrary to optical metallographic features, little segregation was found within dendrites in the Co base alloy although the composition of the carbide was distinct from that of the matrix and contained considerably more MO and Cr. Lewis (1979b) has also looked at the effect of carbon content upon the properties of a 80Ni-20Cr alloy. As the carbon content increased so did the precipitation of carbide which is claimed to be M,,Cb, where M is an individual, or a mixture of carbide-forming metallic elements. This precipitation occurred both intergranularly and interdendritically, and increased the proof strength but decreased elongation. The pick-up of carbon from an oxygas flame is therefore of interest and importance. The increase in carbon content was found to be significant in cast Ni base dental alloys when compared with the as-received material (Baran, 1979).

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The use of heat treatment for cast alloys is often advocated to enhance the properties by controlling the distribution of precipitating phases. The optical metallography of Goodall and Lewis (1979) has shown that the carbide distribution in a cast Ni base dental alloy was changed by heat treatment. The structures produced at low temperatures showed extreme segregation at higher temperatures but these gave way to rod-like matrix particles with fragmentation and coarsening of the grain boundary lamellar carbides. The strength of base alloys (Co-Cr, Ni-Cr, Fe-Cr) fell when 15minute heat treatments at 7OO”C-1200°C were applied to castings (Morris et al., 1979). The widely accepted view that elongation improves when proof strength is reduced by heat treatment was not confirmed. Each alloy exhibited an individual and different response to each treatment. From this it was concluded that heating during soldering, welding or grinding should be kept to a minimum. Co-Cr-Mo alloys for medical prostheses are very closely related in composition to cobalt base partial denture casting alloys; consequently the effects noticed by Clemow and Damell (1979) during the solution treatment of such alloys is of direct interest. As cast the Cr content in the alloys examined varied across the dendrite arms, which is contrary to that reported by Lewis (1979a). Annealing caused a transformation of the interdendritic carbide M2aC6 + M6C and above 1210°C carbides began to dissolve as the average composition of the alloy entered the single-phase region of the pseudo-quaternary phase diagram. The greater carbon content within the interdendritic regions caused incipient melting at temperatures as low as 123O’C. This incipient melting was inhibited by rapid heating which caused M2sC6 to transform to the u phase, which is a transformation to be avoided. This led them to question the level of the carbon content in these alloys (approx. 0.28 per cent) since its reduction would facilitate the solution treatment and prevent incipient melting or u formation. Amongst other properties, base metal alloys, palladium alloys and low gold alloys intended as substitutes for conventional dental gold alloys must be as biologically compatible as the latter. No significant adverse effects were seen by Eiliero et al. (1979) on the cheek pouches of hampsters after a range of alloys (provided by two manufacturers) had been in place for 14 days. The positive acceptability of low gold alloys on the basis of their mechanical properties (similar to type III gold), castibility and corrosion resistance has been reported (Sturdevant et al., 1979). Finally, the research reported here on improving the accuracy of casting and the properties of the cast alloy will only be of academic interest unless the production laboratories are able to reproduce results similar to those from the research laboratories. Morris, Asgar and Brudwick (1979) found that there was a ‘drastic difference’ in the properties of both Co and .Ni base alloys not only between commercial and research laboratories (in America) but also within the laboratories. This, they have stated, has shown a need for routine quality control monitoring.

Metal joining In contrast with the publications on casting alloys those for metal joining deal almost exclusively with joints involving noble alloys. The paper from Earnshaw and Wheatley (1979) is the exception. They have examined some of the technical variables which could exist when soldering wrought wires (both base and noble metal alloys) to a cast Co-Cr alloy for clasp purposes. Although in practice the strength of all joints proved satisfactory, they recommended using 2 mm of wire soldered to the base and half recessed into it.

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Optical

metallography has shown without doubt that solidification of gold solder in a by the gold-based alloys being joined (Saito and Santos, 1979). Epitaxial growth followed nucleation, and as a consequence the grain size of the solder reflected that of the underlying metal. Intergranular cracks were found in those joints where the components had been set up with a gap between them. Thermal contractions within the metal/ solder/investment entity were thought to have caused these cracks. Such cracking was also reported by Earnshaw and Wheatley (1979), but exclusively in butt joints. By considering the triaxial state of stress within the soldered joint, Rasmussen et al. (1979) have explained how the soldered joint can attain strengths several times the strength of the solder itself. This increase has been related to narrowing the soldered gap. However, their experimental results have shown that the surface tension of the solder used was related to the melting point and this determined the filling of the gap and therefore the soundness of the joint. Thus, the optimum size for the gap was found to depend upon the particular solder employed. Support for soldered assemblies over the total length significantly reduced distortion through ‘sagging’ (creep) during a post-soldering heat treatment (degassing prior to the application of porcelain, for example) and thus is recommended for all heat treatments where creep could occur (Bryant and Nicholls, 1979). Interestingly, there was little difference in the sag of soldered assemblies and those cast in one piece. Casting gold directly onto precision attachments has been suggested to overcome the difficulties encountered when soldering these to gold alloy castings (Tsuka et al., 1979a). Using identical casting procedures in each case, they found that the union between a type IV casting gold and five commercial precision attachments was excellent in three cases which showed limited interfacial diffusion and no porosity in the case alloy; acceptable in one case, but poor in the other where interfacial pores were present, possibly from oxidation or breakdown of the investment. These differences were thought to have arisen from differences in composition and size of the attachments. In a second paper, Tsuka et al. (1979b) reported that the quality of the union improved as the mould temperature increased and the thickness of the embedded piece decreased. However, if the mould temperature is excessive the strength of the metal could be reduced as a result of recrystallization. On the other hand, if it is too low, considerable porosity could occur at the interface.

joint

is nucleated

Orthodontic

wires

The ADA specification for orthodontic wires, No. 32, introduced in 1977 allows the bending deformation to be measured either by a torque meter or a stiffness tester. Brantley and his co-workers have compared these two methods for large diameter wires, 040-O-76mm (Brantley and Blake, 1979) and finer wires, O-18-0.25 mm (Brantley and Meyers, 1979). In the former, differences between moduli obtained by these methods were attributed to deviations from idealized loading conditions and experimental testing problems. In the more detailed latter publication the torque meter was shown to give moduli approximately 16 per cent greater than those given by a stiffness tester, which gave values similar to those obtained from tensile tests. It was concluded that both tests yield essentially similar data although careful calibration is required. Increasing the span in cantilever specimens increased the elastic modulus but decreased the flexural yield strength. This is a variable not taken into account in current formulae but desirable since smaller spans gave more accurate results for fine wire.

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The properties of alloys are often obtained by machining specimens from the bulk alloy to a size which allows a simple and accurate testing method to be used. The erroneous extrapolation of such ‘bulk’ values to orthodontic wires has been demonstrated by Coquillet et al. (1979) who fatigued austenitic stainless steel wire in saliva. Surface scratches, produced during manufacture or subsequently during bending, considerably reduced the fatigue limit. The fatigue limit was increased by the work hardening resulting from wire drawing (during manufacture), but not as effectively as the UTS. Consequently an empirical index, applied extensively in this study, u fatigue limit/u UTS, gave poor values for the heavily drawn orthodontic wire. The necessity or desirability of a stress-relieving heat treatment for austenitic stainless steel wire has been questioned (Lane and Nikolai, 1979). Such treatment increased both the stiffness and activation range. Since to increase the stiffness is an inherently negative procedure and the range was stated to be sufficient already, it has been deemed unnecessary. On the other hand, Fillmore and Tomlinson (1979) have shown that a short precipitation heat treatment (5 minutes at 482 “C) increased the ‘proof strength’ of stainless steel wire by 139 per cent. This substantial improvement was also found in all four tempers of Elgiloy (40Co-20Cr-15Ni-15Fe-7Mo-2Mn-O*15C). It is interesting that they found little difference between the specimens formed from the intermediate tempers (yellow and green) for this alloy; an effect they have attempted to explain by differences in the work hardening behaviour of the alloy in these two conditions during bending of the looped specimens. A feasibility study on the use of &Titanium alloys for orthodontic wires has been reported (Goldberg and Burstone, 1979). These alloys are attractive since they combine a proof stress (u,, up to 1380 MN. m-‘) in the same order as that for stainless steel but with a lower modulus (E, 55 to 110 GN rn-‘) which produces one of the greatest maximum elastic deflections (-uv/E) known. For the single alloy evaluated (78TI-11.3Mo-6*6Zr-V3Sn) optimum values were obtained with as-drawn wire and after precipitation of cu-Ti. Of equal importance with this property were the possession of good formability (even after considerable cold work), environmental stability and biocompatibility. An application for another relatively new orthodontic alloy, Nitinol, has been proposed by Andreasen et al. (1979). It is suggested that the wire could be extended and bent into an arch form below its transition temperature range (TTR). This would then be attached to the molars and heated through the TTR. Recovery to the original length would produce a force of between 1.12 and 184N per tooth, which could be used to close spaces. Arguments have been advanced for its use in preference to conventional methods for applying force (e.g. elastics or springs); however, a clinical study will be required to assess the clinical practicality of this proposal. Finally, the application of coatings to orthodontic wires was found to reduce the coefficient of friction from 0.162 to O-973 when that coating was a metal/polymer composite or 0.028 if it was PTFE (Greenberg and Kusy, 1979). Not only should this reduce wear but it will also result in more efficient transmission of forces to the teeth.

Magnetic alloys The intensity of magnetization within intermetallic compound hard magnets (particularly SmCo5), which produces large attractive forces in relation to weight, has resulted in a variety of uses for both prosthetic retention and tooth movement in orthodontic appliances. In an

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extensive laboratory investigation Tsutsui et al. (1979) have shown that the magnetic properties of SmCos were superior to those of other ferromagnetic alloys, particularly when fabricated as flat magnets. Attractive forces suitable for dentistry (0.1 to 2N) can be obtained from pairs of magnets no greater than 80 mm3 in size (O-65 g). However, corrosion resistance in dilute acids was poor and it was shown that Co but not Sm is cytotoxic, therefore Cr plating (5 m thick) has been recommended. Gillings and Cerny (1979) reported that overlay dentures had been retained satisfactorily in the short term (4 months) with such magnets, exerting 2N retentive force per tooth (i.e. per intraradicular/denture pair of magnets). Greater detail of a clinical technique for constructing an overlay denture retained by magnets cemented into the roots of endodontically treated teeth has been given by Moghadan and Scandrett (1979). This left the magnets exposed directly to oral fluids and since they have not stated which magnets were used caution should be exercised in the light of the findings of Tsutsui et al. This apart, they put forward a strong case for magnetic retention as an economic and functional alternative to precision attachments. On this point Gilhngs and Cerny have quoted a price of US $4 per unit and stated that no specialist skiIl or equipment is required. In a few cases the use of such magnets may be limited by their preformed shape, consequently the development of a ferromagnetic casting alloy based upon Pd-Co is an with conventional important development (Kinouchi et al., 1979). Used in conjunction SmCo5 magnets retentive forces of a few newtons were obtained. The successful application of these in two part prostheses as well as overlay dentures has been reported (Sasaki et al., 1979). The attractive force of up to 2N exerted by small SmCo5 magnets with a flat geometry (2 X 6 X gmm) allowed such magnets to replace orthodontic wires and elastics to achieve smooth and rapid movement of canines and other teeth (Kawata and Matsuga, 1979).

Miscellaneous This section contains miscellaneous publications which are loosely linked by the theme of working, cutting and finishing procedures. The quantitative assessment of burnishing cast alloys has been attempted on an empirical basis to relate remedial work for a given deficiency to metallurgical parameters (Hamerink and Asgar, 1979). Few data were given in this abstract and we look forward to a full report. The flow of a type III gold, produced by a variety of conventional fmishing techniques has been shown to be insignificant. (Lofstrom, Asgar and Myers, 1979). Consequently such techniques would be incapable of making good any marginal deficiency in a type III gold casting. It has been claimed that polishing significantly changed the behaviour of clasps cast in a nickel base alloy (Morris et al., 1979) although with the limited data available in this abstract the degree of improvement is not clear. Relief of the internal fitting surface of a casting required to facilitate seating with a thin cement lute can be produced either by electrochemical polishing or by acid dissolution. These techniques have been compared in terms of rate of removal, uniformity, reproducibility and surface finish (Cherberg and Nicholls, 1979). Both were found to produce a uniform, reproducible rate of removal with that for electrochemical removal exceeding that for the chemical process. Since the former imparted a smooth polished surface to the casting, sand blasting has been suggested as an aid to the mechanical retention of the cement. The widespread use of both high-speed cutting instruments and composite restorative

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materials have lead Pines and Schulman (1979a) to reexamine the in vivo wear of conventional, fluted WC dental bum. The wear when cutting composite surfaces was very much in excess of that for either enamel or amalgam. Although dulling of the cutting edges, as perceived by an operator, was primarily caused by the abrasive nature of the filler, adhesion of the resin matrix tended to clog the flutes and thus reduce the cutting efficiency. They concluded that the most efficient bur design for cutting one material is not necessarily that for all others. In particular, a different design is required for cutting composites to that used for amalgams. In a second report (Pines and Schulman, 1979b) they compared the fluted carbide bur with a diamond bur. The efficiency of the latter when cutting composite was maintained for a greater number of surfaces, although frequent cleaning was still required. DENTAL

CERAMICS

The acceptance of new techniques for improving the strength of porcelain crowns now enables the published information on dental ceramics to be divided into three closely allied sections. These are porcelain itself, porcelain fused to cast metal substrates (PFM) and porcelain bonded to foil (PBF). Porcelain

The American National Standards Institute (1979) has reduced the limit for the amount of uranium salts that are permitted in dental porcelain, in order to produce fluorescence from not more than 0.05 per cent to not more than 0.03 per cent by weight. Whilst the ANSI statement suggested that no satisfactory substitute for the uranium is yet available, Peplinski et al. (1979) have shown that bismuth oxide together with various rare earth oxides (such as cerium and europium oxides) produce a range of luminescence with wavelengths between 400 and 49Omn. This compares favourably with natural teeth which show luminescence at wavelengths between 400 and 450nm. The use of hydrofluoric acid to remove remnants of the platinum foil matrix from aluminous porcelain jacket crowns has been shown by Hussain et al. (1979) to produce a rough surface due to the dissolution of the glass matrix. The result was a weakened crown and this procedure is clearly contra-indicated. However, the tensile, transverse and compressive strength of porcelain has been found by Katsura et al. (1979) to increase by two and a half to three times, and the hardness has been raised 50 per cent, as a result of low temperature ionic crowding induced by treating porcelain specimens in molten potassium nitrate at 500 “C for between 5 and 10 hours. The cement film contour underneath porcelain jacket crowns and porcelain-fused-to-metal crowns was reported by Walton (1979) who suggested that the irregularity of that under the former probably contributed to their unpredictable mechanical behaviour and occasional catastrophic failure. Porcelain fused to cast metal substrates

Apart from a report by Allard et al. (1979) on a new technique which coats metal surface with an aluminium oxide layer by evaporating aluminium through plasma of argon and oxygen, and which produces such an excellent bond to both porcelain that failures occur within the oxide layer itself, the majority of the

the as-cast a reactive metal and published

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information

in this section

deals with interpreting

the mysteries

of the porcelain/metal

interface. An intriguing and conflicting report has come from Ghalili and Pameijer (1979) who used scanning electron microscopy and dispersive X-ray analysis to examine 100 noble and non-precious metal alloy castings which had been subjected to various treatments prior to the deposition of porcelain. Their results suggest that 110 significant ion exchange took place at the interface between the alloys and the porcelain, and that the bond strength was predominantly dependent on the total surface area contact. The scanning electron microscope was also used by Golestaneh et al. (1979) to quantify the amount of marginal distortion which occurs to PFM restorations during the application of porcelain. Their results suggest that it was not the shrinkage of the porcelain which distorted the margins but the heating cycles of the alloys prior to the application of porcelain. Whilst it now looks as if no ion exchange occurs at the interface, Susz et al. (1979) reported that oxidation of high gold, indium-containing alloys under vacuum produced localized areas with greatly increased indium concentrations, whereas oxidation in air followed by acid pickling produced a heterogeneous distribution of alloying elements with no indiumenriched areas. When the high indium zones were present the thermal expansion was raised and the bond strength lowered. Conversely, when they were absent the bond strength was high. Oxidation studies by Espevik and @ilo (1979) have shown that whilst PFM alloys which contained 75 to 98 per cent of noble metals had a small increase in weight with their oxides concentrated 5 to 10~ beneath the surface, PFM alloys with low noble metal contents produced four times as much oxide in the same period and these oxides were distributed in net or finger-like extensions up to 50~ beneath the surface. In seeking to explain what happens at the interface between porcelain and base metal alloys, Carter et al. (1979) provided details of the results obtained by their torsional adherence test, in which a porcelain-coated alloy plate is twisted about its long axis until the porcelain separates or cracks. Their results suggest that grit blasting produced not only a greater surface area and hence improved mechanical retention, but also a greater weight of oxide and this too gave better adherence of the porcelain to the alloy. By the use of various etchants and electrolytic-etching techniques, Baran (1979) showed that depletion of the alloying elements at the interface between porcelain and base metal alloys occurred during the oxidation and sand blasting procedures and not during the bake of the porcelain. He also reported that phase transformations had taken place in the metal and Ringle et al. (1979) also noted the presence of second- and/or third-phase structures near the interfacial reaction zone. They suggested that these phases need to be taken into consideration when interpreting the interaction zone behaviour. A further factor which should be considered is the presence of residual stresses at the interface due to the thermal expansion mismatch between the porcelain and the alloy, and Bertolotti and Shelby (1979) used a modified beam-bending viscometer to show that porcelains intended for PFM systems had a wider softening range than typical glasses with similar softening points, due, they suggest, to the large fraction of non-melting crystalline material present in the porcelains. The problem of the in viva repair of porcelain which has fractured or become detached from PFM units was reviewed by Dent (1979). He listed the three basic methods as the use of (i) silane-bonded polymers and composites, (ii) non-precious metal overcastings with porcelain bonded to them and (iii) pm-retained castings (when porcelain is present as a labial

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surface veneer only). In a laboratory assessment of the first method, Highton et al. (1979) fractured porcelain in a transverse test and repaired the fragments with either self-cured poly(methy1 methacrylate) or a dimethacrylate composite. Whilst after 24 hours, the fragments repaired with the composite had regained 48 per cent of the original strength and those repaired with acrylic 28 per cent, after storage for 3 months in saline solution at 37°C both had deteriorated the composite system to 25 per cent and the acrylic to 39 per cent of the original transverse strength. A variation on the second method of repairing a PFM unit which had sustained a fracture to its porcelain was reported by Bruggers et al. (1979). They recommended the removal of the facial, incisal and lingual porcelain and the preparation of a shoulder upon the metal substrate to produce a l-mm space. A McLean-Seed crown (porcelain bonded to an oxidized, tin-plated platinum foil) should then be made and cemented in place. Porcelain bonded to metal foil The value of oxidizing the tin plating on tincoated platinum foils was emphasized by Sarkar et al. (1979), who showed that, whilst tin enters the porcelain from a nonoxidized surface and produces atomic contact between the porcelain and the foil, oxidation produces a substrate full of micro-irregularities, into which porcelain flows and the bond is thus further strengthened by mechanical retention. A new technique, which is not strictly a foil nor is it a casting, was described by Rogers (1979). He created matrices by electro-forming pure gold from alkaline cyanide solution. The gold matrix can be tin-plated and has no tinman’s joint. During the firing of porcelain, the silver colour of the tin becomes deep gold due to diffusion and oxidation and this adds brightness and vitality to the crown, whose margin (in gold) can be burnished and thus closely adapted to the preparation.

DENTAL IMPLANTS In the 1978 Review it was reported that the pace in dental implant research had slackened a little with the literature containing few papers on fundamental studies of dental implants. This trend was again obvious in the 1979 literature with, if anything, even fewer papers published on the subject. There could be two reasons for this, either than dental implants are so successful that there is little need to publish results of experimental work or that ‘long-term success is harder to achieve and that investigators are finding it increasingly difficult to sustain novelty and creativeness in the search for solutions to the undoubtedly difficult problems. Although there are some practitioners who, by careful patient selection and technique, achieve good results with both endosseous and subperiosteal implants, the evidence favours the latter explanation. Vitreous carbon appears to have become a very popular material in recent years with extensive clinical use. However, Al-Salman et al. (1979) studied the response to the commercially available vitreous carbon endosseous implants in dogs and found that although the implants were well tolerated by the alveolar process, they were not accepted by the gingival margins and mucosa. Continuous gingival recession was noted with no epithelial attachment at the neck. Thus, in contrast to earlier findings by other workers with these implants,

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unfavourable results were obtained and the clinical use of such implants questioned. It is worth noting that no inflammation or foreign body reaction to the material was seen, but rather a response to the presence of the implant itself. On the other hand, two preliminary reports on carbon-coated implants contained promising results. Michieli et al. (1979) evaluated vapour-deposited carbon-coated Vitallium tooth root implants in dogs, with satisfactory early results and Leake et al. (1979) from the same research group, published a clinical discussion of similarlycoated subperiosteal implants. Their limited results showed a healthy mucosa around the posts with no signs of inflammation or erosion. Porous materials continue to receive attention. Duff and Grant (1979) have produced porous calcium phosphate implants by sintering compacted hydroxyapatite and substitute apatites in air at 14OO’C for up to 24 hours. These compacts showed good resistance to attack when stored in aqueous solutions for several months provided the pH did not fall below 4. Such implants were histologically inert towards rats. Hodosh et al. (1979) published further studies of porous vitreous carbon-poly(methy1 methacrylate) tooth root implants, showing better results with 6 per cent porosity than with 3 per cent. One of the dangers with porous materials was highlighted by Pedersen et al. (1979) when infection, dehiscence and exposure of the implants resulted in their three clinical cases of trans-mucosal implantation of a porous ceramic/titanium composite structure. In contrast, Young et al. (1979) studied porous titanium endosseous implants in rhesus monkeys, using microradiography and histology, and confirmed that bone growth into the porous surface coatings provided a good method of dental implant furation. Their material was Ti-6% A-4%V and the root was fabricated by sintering a 0.5mm-thick coating of spherical powder (250pm diam.) to a solid core. Of 29 implants placed in healed mandibular premolar extraction sites, 3 were lost and 4 rated as failures on the basis of histological evaluation. Failure was primarily due to lack of bone in the cortical plate. Elsewhere there have been some attempts to improve endosseous dental implants by a variety of methods. Plekavich (1979) used a section of an extracted anterior tooth to envelop the neck of an endosseous implant in an attempt to prevent oral epithelial migration around the implant. Some success was achieved although resorption of the tooth substance was shown to take place. Zarb et al. (1979) investigated the possibility of cementing endosseous implants in place within the bone, using acrylic cement in much the same way as components of hip prostheses are cemented into femur and acetabulum. The experiments were performed with conventional titanium blade vent implants in dogs, but poor results were obtained with only 1 firm implant being present at 12 months, out of 30 (15 cemented, 1.5 control uncemented), the majority being either mobile or already exfoliated. The cement provided no improvement in the epithelial seal. Restoration of infra-bony defects was studied by Strub et al. (1979) who compared tricalcium phosphate implants with frozen allogenic bone grafts. By 1 year there were only minor differences between these two situations but although the storage and handling characteristics of the tricalcium phosphate were superior, slightly greater bone apposition and reduction in pocket depth were found with the allogenic bone. On a similar subject, Den&en and de Groot (1979) discussed animal and clinical studies involving tooth root implants of synthetic, dense calcium hydroxyapatite. These are placed in fresh extraction sockets to retain a stable alveolar ridge. In the animal experiments, alI implants were retained, with no exfoliation, and showing close adaptation to the surrounding bone. One hundred

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implants were placed in 20 patients and good results were achieved, the implants functioning as ankylosed natural roots. The most extensive and significant papers were published by Brunski et al. (1979a, b). Their work was related to the nature of the implant-tissue interface in the deeper regions of the bone and the correlation between the characteristics of this interface with clinical performances. It was noted that three types of interface may develop, type 1 have direct bone-implant apposition, type 2 consist of dense, fibrous connective tissue with varying degrees of collagen orientation and type 3 in which there is a substantial amount of epithelial tissue between implant and bone. Although there is some evidence that the latter type usually leads to clinical failure, the authors could not determine any concensus as to the relative acceptability of types 1 and 2. In the experimental work, titanium blade vent implants were placed in dogs. Two implants per animal were employed and bridgework was constructed so that one implant sustained functional loads and the other remained non-functional. It was clearly shown that the presence or absence of apical forces on the implants determined whether or not a fibrous capsule formed. The forces applied to the functioning implants resulted in relative motion which caused fibrous encapsulation. However, it was shown that the very large histological differences between functional and nonfunctional implants were not reflected by clinical performance and both implants with and without fibrous encapsulation may be clinically successful. The authors concluded that the actual biomechanical stresses transmitted to the tissues, the acceptable range of stress in the tissues, the magnitude of the relative motion between the implant and the tissues and the determining factors of the gingival interface response are but a few of the variables which must be investigated and related to implant success.

CORROSION

The information published on corrosion during 1979 can be divided into that concerning dental amalgam, casting alloys, dental implants or orthodontic wires and pins. Dental amalgam

The basic physiochemical factors responsible for both creep and corrosion have been reviewed by Sarkar (1978) who also analysed all the available experimental data and clinical observations. He was able to confirm that JBrgensen’s hypothesis is correct and that it is the corrosion of the Sn-Hg phase which is responsible for the marginal fracture of conventional (low copper) dental amalgams. Creep, he suggests, follows as a result of the presence of yr but corrosion is the responsibility of the y2. It is just a coincidence that they both occur together due to the concurrent crystallization of both phases. In animal studies, Ellender et al. (1979) showed that amalgam corrosion products from subcutaneous implants stimulated a prolonged inflammatory response with delayed granulation tissue formation and slow faulty collagen formation, and they speculate on the possible effects of such contamination on the gingival tissues in man. Of interest to all who have to place amalgam restorations are two papers by Boyer and his co-workers. In the first, Boyer et al. (1978) determined the anodic polarization curves in Ringer’s solution of both high copper dispersion and ternary alloys after one of three possible

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finishing procedures-as carved, burnished or polished. Their results were in agreement with their own previous studies on conventional amalgam, in that the carved amalgam corroded the most and the burnished or polished the least. There was no difference in behaviour between the high copper dispersion and ternary amalgams. From the results of their microleakage studies, Boyer and Torney (1979) suggest that corrosion is not a significant factor in the formation of a marginal seal between tooth and an amalgam restoration. They found no difference between the leakage patterns of restorations in either conventional or high copper amalgams. Just like creep, which is known to accelerate if the mercury content of the set amalgam is allowed to increase, Marek and Mahler (1979) showed that the corrosion susceptibility of high copper amalgams also suffered a sharp increase. They note that corrosion causes both the transformation of yr into PI and the breakdown of the Cu6Sn5 into soluble copper and insoluble tin corrosion products. According to Marek and Okabe (1978), who studied the corrosion of high copper amalgam using a crevice cell and a saline electrolyte, the least corrosion resistant phase (in the absence of yZ) is the Ag-Hg-Sn matrix phase (rr ), but its deterioration was not uniform. They plated mercury onto a high copper amalgam alloy tablet and after a long exposure in the corrosive environment new particles were seen to develop. These had X-ray spectra close to that of the yZ phase and they suggest that these must have been produced as a result of the release of mercury from a deteriorating yr phase, but they are mystified as to why the new yZ did not immediately dissolve as previously observed. Chloride solutions are proving useful in the continuing studies of amalgam corrosion, and by using a Sn ring-conventional amalgam disc electrode rotating at 2000 rpm Do Due and Tissot (1979) produced soluble species which were then precipitated in solution and ejected along streamline pattens on the disc after only 2 minutes. They suggest that these were probably the chloro-complex of tin. Mueller (1979) reported that conventional amalgam when subjected to potentiostatic polarization in chloride solution exhibited several layers of differing colours and he analysed these for metal ions together with the precipitate which formed in solution. The metal content of this precipitate was almost completely tin, with just a trace of silver and mercury. Potential sweep and intensiostatic pulse studies by Do Due et al. (1979) showed that anodic fums produced in chloride solution were less adherent on amalgam and on the yZ phase than on pure tin. By using electron microprobe analysis they confirmed the presence of Sn(OH)* and SnOHC1.H2 0 in a 15-m thick layer which formed on pure tin after an hour of electrolysis. X-ray diffraction also indicated the presence of both the hydrated tin oxide 5Sn0.2Hz 0 and Sn4(OH), Cl*. Whilst Do Due et al. suggest a mechanism for the passivation of tin in a chloride medium, Finkelsten and Greener (1979) by using potentiostatic scans at different temperatures in aerated Ringer’s solution concluded that the chloride ion is capable of disrupting the protective film of Sn(OH)2 which forms on y2. Tarnish is often a precursor to corrosion and Gettleman (1979) in an animal study of both amalgam and a wide range of implant alloys has measured tarnish using tristimulus calorimetry. He was able to show, amongst other things, that silver darkened in vivo but not in vitro, probably due to the absence of the sulphide ion in his chloride solutions. Vaidyanathan and Gowda (1979) showed that tarnished regions on amalgam acted as more active electrochemical phases than the rest of the surface and they also reported that their SEM studies of amalgam, which had been corroded by controlled potentiostatic means, showed voids on the surface where the yZ phase had dissolved.

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From a 2-year clinical study of three high copper amalgams and one conventional amalgam Larson et al. (1979) were able to show statistically that the marginal integrity of either high copper dispersant phase or ternary alloy amalgams was improved and that surface discoloration was also much reduced. However, Sarkar and Nanda (1979) recovered a tarnished high copper amalgam restoration after 1 year of clinical service and examined it by both SEM and energy dispersive X-ray analysis. They observed a discontinuous tarnish film, severe corrosion at the yl-grain boundaries, a moderate amount of corrosion of the Cu-Sn phase and the formation of a new phase, possibly /3r, within the yr grams themselves. Casting alloys Using a potentiodynamic polarization technique, Sarkar et al. (1979a) claim to have found a useful, quick and convenient method for testing chloride corrosion resistance of the casting alloys which are currently being suggested as alternatives to the certified gold alloys. The ‘low golds’, which contain between 45 and 60 per cent of gold, platinum and palladium, showed an inferior chloride corrosion resistance and this, they suggest, is a result of the presence of Ag-rich and Cu-rich microsegregations or precipitates. In similar tests of silverbase casting alloys Sarkar et al. (1979b) were again able to blame silver- and copper-rich regions for the poor chloride corrosion resistance of silver-palladium alloys containing various combinations and quantities of gold, indium, copper or zinc. When more gold was present the corrosion resistance was always improved. Whilst the presence of gold is always a good way of preventing tarnish, Vaidyanathan et al. (1979) showed that when copper was present in alloys with high silver contents the tarnish resistance was very poor. Not everything about electrochemical dissolution is disadvantageous as Tanaka et al. (1970) showed by using the corrosion pits, produced on the surface of a Ni-Cr-Cu alloy by electrolysis in a rapidly stirred saline solution, as a means of retaining acrylic resin facings to castings in this alloy. The retention and sealing was superior to that produced by either spherical powder or beads added to the surface of the wax pattern prior to investment. Dental implants A technique for short-term comparative tests of implant alloys has been reported by Sutow and Jones (1979) who developed an occluded corrosion cell to measure crevice corrosion when a piece of 316L stainless steel was held in contact with a Teflon plate. The steel suffered crevice corrosion in Ringer’s solution at 37 “C after only 100 minutes. The corrosion susceptibility of alloys used as mechanical substrates for carbon coated implants was outlined by Thompson et al. (1979). They employed in vitro potentiostatic polarization tests in buffered, de-aerated saline solution at 37°C on either 316L stainless steel or Ti-6A14V alloy galvanically coupled to carbon. Whilst the titanium alloy showed no breakdown potential even up to 1.2V, the stainless steel showed corrosion potentials in the breakdown range of this alloy when the carbon/metal ratio was between 10 : 1 and 100 : 1. They were thus able to predict that pitting corrosion might be expected in stainless steel/carbon couples but not in titanium alloy/carbon couples under such conditions. Orthodontic wires and pins The electrochemical polarization of orthodontic wire joints made from stainless steel, Elgiloy or Co-Cr wires soldered by either silver or gold dental solders in various commercial

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Brown et al.: Review of dental materials

oxygenating perborate cleansers was reported by Mueller et al. (1979). They observed that gold-soldered couples were more resistant to corrosion in solutions with both high pH and CT ion concentrations than those soldered with silver. Sarkar, Redmond et al. (1979) have also studied the corrosion resistance of a number of different orthodontic alloy wires including p-titanium and Nitinol. They noted that Nitinol was not as corrosion-resistant as the other types of wire. In animal studies and in vitro tests, Mobasherat and Pameijer (1979) reported that if you have to use retentive pins for amalgam or composite restorations, then for maximum corrosion-resistance pure-gold pins are to be recommended. However, more realistically, stainless steel or type 2 gold pins are unlikely to cause many problems. REFERENCES

Impression materials Coleman R. M., Hembree J. H. and Weber F. N. (1979) Dimensional stability of irreversible hydrocolloid impression material. Am. J. Orthod. 75, 438. Eames W. B., Sieweke J. C., Wallace S. W. et al. (1979a) Elastomeric impression materials: effect of bulk on accuracy. J. Prosthet. Dent. 41, 304. Eames W. B., Wallace S. W., Suway N. B. et al. (1979b) Accuracy and dimensional stability of elastomeric impression materials. J. Prokthet. Dent. 42, 159. Luebke R. J., Scandrett F. R. and Kerber P. E. (1979) The effect of delayed and second pours on elastomeric impression material accuracy. J. Prosthet. Dent. 41, 517. Mack P. J. (1979) Inhalation of alginate powder during spatulation. Br. Dent. J. 146, 141. Meiners H. and Dittmer R. (1979) Warmertonung und Reaktionswarme elastomerer Abformmaterialien. Dtsch. Zahnaerztl. Z. 34,678. Nayyar A., Tomlins C. D., Fairhurst C. W. and Okabe T. (1979) Comparison of some properties of polyether and polysulfide materials. J. Prosthet. Dent. 42, 163. Norling B. K. and Reisbick M. H. (1979) The effect of non-ionic surfactants on bubble entrapment in elastomeric impression materials. J. Prosthet. Dent. 42, 342. Razek M. K. A. (1979) Assessment of tissue conditioning materials for functional impressions. J. Prosthet. Dent. 42, 376. Rehberg H. J. (1979) Fliessverhalten von Abformwerkstoffen bei planer Ausbreitung. Dtsch. Zahnaerztl. Z. 34, 670.

Shigeto N., Kawazoe Y., Hamada T. and Yamada S. (1979) Adhesion between copperplated acrylic tray resin and a polysulfide rubber impression material. J. Prosthet. Dent. 42, 228.

Stannard J. G. and Craig R. G. (1979) Modifying the setting rate of an addition-type impression material. J. Dent. Res. 58, 1377.

silicone

Model, die and investment materials Boswell P. G. and Stevens L. (1979) A comparison between the diametral and double punch testing of a dental gypsum. Aust. Dent. J. 24, 238. Brukl C. E., Norling B. K. and Kalmus S. R. (1979) Wettability of addition silicone impression materials modified by non-ionic surfactants. J. Dent. Res. 58 (Special Issue A) Abstr. 201. Hosoda H. and Ohsawa M. (1979) Evaluating dimensional accuracy of stone models. J. Dent. Res. 58, 1352.

Jdrgensen

K. D. and Finger W. (1979)

Stand. J. Dent. Res. 87, 73.

Die spacing technique

by diffusion

precipitation.

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Norling B. K. and Reisbick M. H. (1979) The effect of non-ionic surfactants on bubble entrapment in elastomeric impression materials. J. Prosthet. Dent. 42, 342. Presswood R. G., Choate D. and Choate J. (1979) Modification of stone dies for fabrication of porcelain jacket crowns. J. Prosthet. Dent. 41,470. Probst R. T. and Duncanson M. G. (1979) Fractography of die stones: L/P ratios and water substitution effects. J. Dent. Rex 58 (Special Issue A) Abstr. 199. Smith C. D., Nayyar A. and Koth D. L. (1979) Fabrication of removable stone dies using cemented dowel pins. J. Prosthet. Dent. 41, 579. Stackhouse J. (1979) Linear dimensions of electrodeposits and impression material substrates. J. Dent. Res. 58 (Special Issue A) Abstr. 195. Vermilyea S. G., Huget E. F. and Wiskoski J. (1979) Evaluation of resin die materials. J. Prosthet. Dent. 42, 304.

Waxes Grajower R. (1978) A new method for determining

the thermal expansion

of dental waxes.

J. Dent. Res. 57, 659.

Acrylic resins Alexieva C. (1979) Character of the hard tooth tissue-polymer bond I. Study of the interaction of calcium phosphate with N phenyl glycine and with N phenyl glycineglycidyl methacrylate adduct. II. Study of the interaction of human tooth enamel and dentine with N phenyl glycine-glycidyl methacrylate adduct. J. Dent. Res. 58, 1879. Antonucci J. M., Crams C. L. and Termini D. J. (1979) New initiator systems for dental resins based on asorbic acid. J. Dent. Res. 58, 1887. Barsby M. J. and Braden M. (1979) A hydrophilic denture base resin. J. Dent. Res. 58, 158 1. Bowen R. L. (1979) Adhesive bonding of various materials to hard tooth tissues XVIII: synthesis of a polyfunctional surface-active comonomer. J. Dent. Res. 58, 110 1. Brauer G. M., Dulik Dianne, M., Antonucci J. M. et al. (1979a) New amine accelerators for composite restorative resins. J. Dent. Res. 58, 1994. Brauer G. M., Jackson J. A. and Termini D. J. (1979b) Bonding of acrylic resins to dentine with 2cyanoacrylate esters. J. Dent. Res. 58, 1900. Budtz-J$rgensen E. (1979) Materials and methods for cleansing dentures. J. Prosthet. Dent. 4,619.

Dulik Dianne, M. (1979) Evaluation of commercial and newly synthesized amine accelerators for dental composites J. Dent. Res. 58, 1308. Duran R. L., Powers J. M. and Craig R. C. (1979) Viscoelastic and dynamic properties of soft liners and tissue conditioners. J. Dent. Res. 58, 1801. Gay D. W. and King G. E. (1979) An evaluation of cure of acrylic resin by three methods. J. Prosthet. Dent. 4,437.

Hargreaves A. S. (1979) Equilibrium

water uptake and denture base resin behaviour. J. Dent.

6, 342.

Harrison A., Huggett R. and Handley R. W. (1979) A correlation between abrasion resistance and other properties. J. Biomed. Mat. Res. 13, 23. Hedergsd B. (1979a) Fluid acrylates. Some views and facts from the literature. Tandlakartidningen, 70, 158. Hederg&d B. (1979b) Pour acrylates. Points of view and data from the literature. Tandtekniken 47, 19 1.

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293

Hikage S., Tanaka T., Atsuta M. et al. (1979) A white band and a transparent zone observed at an interface between a polymerizing polyfunctional monomer and a poly(methy1 methacrylate) rod J. Dent. Res. 58,497. Keng S. B., Cruickshanks-Boyd D. W. and Davies E. H. (1979) Processing factors affecting the clarity of a rapid-curing clear resin. J. Oral Rehabil. 6, 327. Lorton L. and Phillips R. W. (1979) Heat-released stress in acrylic dentures. J. Prosthet. Dent. 42, 23. Manley T. R., Bowman A. J. and Cook M. (1979) Denture bases reinforced with carbon fibres. Br. Dent. J. 146, 25. Mom K. (1979) An evaluation of shear strength measurements of unfilled and filled resin combinations. Am. J. Orthod. 74, 531. Paddon J. M. and Wilson A. D. (1978) Stress relaxation studies of dental materials. 2. Simple restorative resins. J. Dent. 6, 321. Stafford G. D. and Griffiths D. W. (1979) Investigation of the strain produced in maxillary complete dentures in function. J. Oral. Rehabil. 6, 24 1. Denture base polymers Faraj S. A. and Ellis B. (1979) The effect of processing temperatures on the exotherm, porosity and properties of acrylic denture bases. Br. Dent. J. 147, 209. de Gee A. J., ten Harkel E. C. and Davidson C. L. (1979) Measuring procedure for the determination of the three dimensional shape of dentures. J. Prosthet. Dent. 42, 149. Lorton L. and Phillips R. W. (1979) Heat-released stress in acrylic dentures. J. Prosrhet. Dent. 42, 23. Manley T. R., Bowman A. J. and Cook M. (1979) Denture bases reinforced with carbon fibres. Br. Dent. J. 146, 25. Soni P. M., Powers J. M. and Craig R. G. (1979) Comparison of the accuracy of denture bases by a non-parametric method. J. Oral. Rehabil. 6, 35. Soft lining materials and tissue conditioners Douglas W. H. (1979) Resilient soft materials in dentistry. Northwest Dent. 58, 116. Duran R. L., Powers J. M. and Craig R. G. (1979) Viscoelastic and dynamic properties of soft liners and tissue conditioners. J. Dent. Res. 58, 1801. Ellis B., Al-Nakash S., Lamb D. J. and McDonald M. P. (1979) A study of the composition and diffusion characteristics of a soft liner. J. Dent. 7, 133. Murray C. G. (1979) A resilient lining material for the retention of maxillo-facial prostheses. f. Prosthet. Dent. 42, 53. Razek M. K. A. (1979) Assessment of tissue conditioning materials for functional impressions. J. Prosthet. Dent. 42, 376. Cast and wrought dental alloys Casting alloys Baran G. (1979) Casting effects on carbon contents of Ni alloys. J. Dent. Res. 58, B196. Barreto M., Goldberg J., Nitkin D. et al. (1979) Investment and technic variable effects in casting high fusing alloys. J. Dent. Res. 58, B 194. Clemow A. J. T. and Daniel1 B. L. (1979) Solution treatment behaviour of Co-Cr-Mo alloy. J, Biomed. Mater. Res. 13, 265. Cole W. A. and Vincent G. R. (1979) Casting accuracy of non-precious dental alloys. J. Dent. Res. 58, B194. DeWald E. (1979) The relationship of pattern position to the flow of gold and casting completeness. J. Prosthet. Dent. 41, 531.

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Goodall T. G. and Lewis A. J. (1979) The metallography of heat treatment effects in a nickel-base casting alloy. A preliminary report. Amt. Dent. J. 24, 235. Huget E. F., Vermilyea S. G., Kuffler M. J. et al. (1979) Accuracy of base metal crowns. J. Dent. Res: 58, B3 14.

Lewis A. J. (1979a) An electron microprobe investigation on the extent of microsegregation occurring in base metal dental casting alloys. Amt. Dent. J. 24, 17 1. Lewis A. J. (1979b) The effect of carbon on the metallography of a nickel base removable partial denture casting alloy. Amt. Dent. J. 24, 90. Lorey R. E., Morris H. F., and Wagner J. D. (1979) Progress report. Four-year clinical evaluation of base metal restorations. J. Dent. Res. 58, B116. Mohammed H. and Shen C. (1979) Microstructural transformations due to iron substitution for nickel in Co-Cr-Ni. J. Dent. Res. 58, B195. Mohammed H. and Vasudev U. (1979) Intermetallic strengthening of Co-Fe-Cr alloys. J. Dent. Res. 58, B195. Morris H. F., Asgar K. and Brudwick J. (1979) Simple method to monitor quality of removable partial denture castings. J. Dent. Res. 58, B117. Morris H. F., Asgar K., Rowe A. P. et al. (1979) The influence of heat treatments on several types of base-metal removable partial denture alloys. J. Prosthet. Dent. 41, 388. Ogura H., Raptis C. and Asgar K. (1979) The inner surface roughness of full cast crowns made by centrifugal casting machines. J. Dent. Res. 58, B195. Piliero S. J., Carson S., LiCalzi M. et al. (1979) Biocompatibility evaluation of casting alloys in hamsters. J. Prosthet. Dent. 41, 220. Pines M., Vaidyanathan T. K., Tielsen J. et al. (1979) Castibility analysis of dental alloys. J. Dent. Res. 58, B197. Pulskamp F. E. (1979) A comparison of the casting accuracy of base metal and gold alloys. J. Prosthet. Dent. 41, 272-6.

Shen C. and Mohammed H. (1979) Influence of the ratio Fe : Cr on properties and structure of cobalt-base alloys. J. Dent. Res. 58, B196. Sturdevant C. M., Leinfelder K. F. and Kusy R. P. (1979) Clinical laboratory evaluation of low gold alloys. J. Dent. Res. 58, B197. Wictorin L., Julin P. and Mollersten L. (1979) Roentgenological detection of casting defects in cobalt-chromium alloy frameworks. J. Oral Rehabil. 6, 137. Wise H. B. and Kaiser D. A. (1979) A radiographic technique for examination of internal defects in metal frameworks. J. Prosthet. Dent. 42, 594.

Metal joining Bryant R. A. and Nicholls J. I. (1979) Measurement of distortions in fixed partial dentures resulting from degassing. J. Prosthet. Dent. 42, 5 15. Eamshaw R. and Wheatley D. J. (1979) Soldering wrought wires to cobalt-chromium alloy. J. Dent. Res. 58, 1212. Rasmussen E. J., Goodkind R. J. and Gerberich W. W. (1979) An investigation of tensile strength of dental solder joints. J. Prosthet. Dent. 41, 418. Saito T. and Santos J. F. F. (1979) Studies of microstructure of gold soldered joints. J. Dent. Res. 58, 1108. Tsuka T., Hamada T. and Yamada S. (1979a) Casting a gold alloy to embedded precision attachment metals. J. Prosthet. Dent. 42, 262. Tsuka T., Hamada T. and Yamada S. (1979b) Evaluation of unions between casting dental alloys and embedded metals. J. Dent. Res. 58, 1289.

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Orthodontic wires AndreasenG. F., Bigelow H. and Andrews J. G. (1979) Fifty-five Nitinol wire: force developed as a function of ‘elastic memory’. Aust. Dent. J. 24, 146. Brantley W. A. and Blake R. E. (1979) Comparison of stiffness and transverse bending tests for orthodontic wires. J. Dent. Res. 58, B2.55. Brantley W. A. and Myers C. L. (1979) Measurement of bending deformation for small diameter orthodontic wires. J. Dent. Res. 58, 1696. Coquillet B., Vincent L. and Guiraldenq P. (1979) Influence of cold working on fatigue behaviour of stainless steel used for prosthesis: application to the study of wires with small sections. J. Biomed. Mater. Res. 13, 657. Fillmore G. M. and Tomlinson J. L. (1979) Heat treatment of cobalt-chromium alloys of various tempers. Angle Orthod. 49, 126. Goldberg J. and Burstone C. J. (1979) An evaluation of p-titanium alloys for use in orthodontic appliances. J. Dent. Res. 58, 593. Greenberg A. R. and Kusy R. P. (1979) A survey of specialty coatings for orthodontic wires. J. Dent. Res. 58, B98.

Lane D. F. and Nikolai R. J. (1979) Effects of stress relief on the mechanical orthodontic wire loops. J. Dent. Res. 58, B97.

properties

of

Magnetic alloys Gillings B. R. D. and Cerny R. (1979) High strength cobalt/rare earth magnets as denture retention aids. J. Dent. Res. 58, 1212. Kawata T. and Matsuga M. (1979) A study of new orthodontic treatment by means of magnets. J. Dent. Res. 58, 1292. Kinouchi Y., Sasaki H., Tsutsui H. et al. (1979) Applicability of Pd-Co casting ferromagnetic alloys to dentistry. J. Dent. Res. 58, B246. Moghadan B. K. and Scandrett F. R. (1979) Magnetic retention for overdentures. J. Prosthet. Dent. 41, 26. Sasaki H., Kinouchi Y., Tsutsui H. et al. (1979) Case studies of sectional dentures using Sm-Co magnets. J. Dent. Res. 58, B314. Tsutsui H., Kinouchi Y., Sasaki H. et al. (1979) Studies on the Sm-Co magnet as a dental material. J. Dent. Res. 58, 1597. Miscellaneous Cherberg J. W. and Nicholls J. I. (1979) Analysis of gold removal by acid etching and electrochemical stripping. J. Prosthet. Dent. 42,638. Hamerink H. A. and Asgar K. (1979) The influence of mechanical properties on burnishability of cast crowns. J. Dent. Res. 58, B195. Lofstrom L. H., Asgar K. and Myers G. E. (1979) SEM evaluation of conventional cast gold finishing procedures. J. Dent. Res. 58, B279. Morris H. F., Popescu E., Brudvick J. et al. (1979) The influence of polishing on clasp behaviour. J. Dent. Res. 58, B27?. Pines M. S. and Schulman A. (1979a) Characterisation of wear of tungsten carbide burs. J. Am. Dent. Assoc. 99, 831.

Pines M. S. and Schulman A. (1979b) Comparison cutting efficiencies. J. Dent. Res. 58, B359.

of diamond

and carbide

bur in vivo

Dental ceramics Allard L. F., Asgar K., Bigelow W. C. et al. (1979) A promising technique for porcelainmetal bonding: reactive ion plating of intermediate layers. J. Dent. Res. 58, (Int. Assoc. Dent. Res., New Orleans) Abstr. 694.

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American National Standards Institute (1979) ADA specification number 52 for the uranium content in dental porcelain and porcelain teeth. J. Am. Dent. Assoc. 98, 755. Baran G. R. (1979) Phase changes in base metal alloys along metal-porcelain interfaces. J. Dent. Res. 58, 2095.

Bertolotti R. L. and Shelby J. E. (1979) Viscosity of dental porcelain as a function of temperature. J. Dent. Res. 58, 2001. Bruggers H., Jeansonne E. E. and Grush L. (1979) Repair techniques for fractured anterior facings. J. Am. Dent. Assoc. 98, 947. Carter J. M., Al-Mudafar J. and Sorensen S. E. (1979) Adherence of a nickel-chromium alloy and porcelain. J. Prosthet. Dent. 41, 16’7. Dent R. J. (1979) Repair of porcelain-fused-to-metal restorations. J. Prosthet. Dent. 41, 66 1. Espevik S. and @lo G. (1979) Oxidation of noble metal alloys for porcelain veneer crowns. J. Dent. Res. 58, (Int. Assoc. Dent. Res., New Orleans) Abstr. 693. Ghalili N. and Pameijer C. H. (1979) Microprobe analysis of porcelain bonded to precious and non-precious metals. J. Dent. Res. 58, (Int. Assoc. Dent. Res., New Orleans) Abstr. 695. Golestaneh R., Asgar K. and Loftstrom L. H. (1979) An SEM study of marginal distortion in porcelain-fused-to-metal restorations. J. Dent. Res. 58, (Int. Assoc. Dent. Res., New Orleans) Abstr. 100. Highton R. M., Caputo A. A. and Matyas J. (1979) Effectiveness of porcelain repair systems. J. Prosthet. Dent. 42, 292.

Hussain M. A., Bradford E. W. and Charlton G. (1979) Effect of etching on the strength of aluminous porcelain jacket crowns. Br. Dent. J. 147,89. Katsura H., Kaneko K. and Kameda T. (1979) Studies on the method of strengthening dental porcelain. J. Dent. Res. 58, (Int. Assoc. Dent. Res., Japanese Div.) 1978, Abstr. 61. Peplinski D. R., Wozniak W. T. and Moser J. B. (1979) Spectral studies of new luminophors for dental porcelain. J. Dent. Res. 58, (Int. Assoc. Dent. Res., New Orleans) Abstr. 604. Ringle R. D., Fairhurst C. W. and Anusavice K. J. (1979) Microstructures in non-precious alloys near the porcelain-metal interaction zone. J. Dent. Res. 58, 1987. Rogers 0. W. (1979) The dental application of electroformed gold. l-Porcelain jacket crown techniques. Aust. Dent, J. 24, 163. Sarkar N. K., Hebert C. and Jeansonne E. E. (1979) Bonding mechanism of tin coated platinum to aluminous porcelain. J. Dent. Res. 58, (Int. Assoc. Dent. Res., New Orleans) Abstr. 96. Susz C. P., Meyer J. M. and Orosz P. F. (1979) Prefiring procedures and porcelain failures of ceramo-metallic restorations. J. Dent. Res. 58, (Int. Assoc. Dent. Res., New Orleans) Abstr. 690. Walton T. R. (1979) The cement contour beneath anterior porcelain and porcelain-metal full veneer restorations. J. Dent. Res. 58, (Int. Assoc. Dent. Res., Aust./NZ) Abstr. 11. Dental implants Al-Salman A. H., Sayegh F. and Chappell R. F. (1979) Wound healing of endosteal vitreous carbon implants in dogs. J. Prosthet. Dent. 41, 83. Brunski J. B., Moccia A. F., Pollack S. R. et al. (1979a) The influence of functional use of endosseous dental implants on the tissue-implant interface. I. Histological aspects. J. Dent. Res. 58, 1953. Brunski J. B., Moccia A. F., Pollack S. R. et al. (1979b) The influence of functional use of endosseous dental implants on the tissue-implant interface. II. Clinical aspects. J. Dent. Res. 58, 1970.

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Denissen H. W. and de Groot K. (1979) Immediate dense root implants from synthetic dental calcium hydroxyapatite. J. Pro&et. Dent. 42, 55 l-6. Duff E. J. and Grant A. A. (1979) The preparation of phosphate ceramics for implantation. J. Dent. Res. 58, (Int. Assoc. Dent. Res., British Division) Abstr. 39. Hodosh M., Povar M. and Shklar G. (1979) The porous vitreous carbon-poly(methy1 methacrylate) replica implant-continuing studies. J. Prosthet. Dent. 42, 557. Leake D., Reed 0. K., Armitage J. et al. (1979) Carbon coated sub-periosteal dental implant for fixed and removable prostheses. J. Prosthet. Dent. 42, 327-33. Michieli S., Leake D. L., Freeman S. et al. (1979) Vapor-deposited carbon coated tooth root implants: preliminary evolution of a stylised tooth implant system in dogs. J. Prosthet. Dent. 42, 58.

Pedersen K. N., Haaneas H. R. and Faehn 0. (1979) Subperiosteal transmucosal porous ceramic/titanium implants-clinical experience from three cases. Znt. J. Oral Surg. 8, 349.

Plekavich E. J. (1979) A variation

of the endosseous

blade vent implant.

J. Prosthet. Dent.

41, 644.

Strub J. R., Gaberthull T. W. and Firestone A. R. (1979) Comparison of tricalcium phosphate and frozen allogenic bone implants in man. J. Periodontol. 56,624. Young F. A., Spector M. and Kresch C. H. (1979) Porous titanium endosseous dental implants in rhesus monkeys: microradiography and histological evolution. J. Biomed. Mater. Res. 13, 843.

Zarb G. A., Smith D. C., Levant H. C. et al. (1979) The effects of cemented endosseous implants. J. Prosthet. Dent. 42, 202.

and uncemented

Corrosion Boyer D. B., Kai C. C., Svare C. W. et al. (1978) The effect of finishing on the anodic polarization of high copper amalgams. J. Oral. Rehabil. 5, 223. Boyer D. B. and Tomey D. L. (1979) Microleakage of amalgam restorations with high copper content. J. Am. Dent. Assoc. 99, 199. Do Due H. and Tissot P. (1979) Detection of soluble species produced during anodic polarization of dental amalgam by rotating ring-disc electrode. J. Dent. Res. 58, 1578. Do Due H., Tissot P. and Meyer J. M. (1979) Potential sweep and intensiostatic pulse studies of Sn, SnsHg and dental amalgam in chloride solution. J. Dent. Res. 58,576. Ellender G., Ham K. N. and Harcourt J. K. (1979) The ultrastructural localisation of the corrosion products of dental amalgam. Aust. Dent. J. 24, 174. Finkelsten G. F. and Greener E. H. (1979) Mechanism of chloride corrosion of dental amalgam. J. Oral Rehabil. 6, 189. Gettleman L. (1979) The problem of corrosion and tarnish of dental restorative and implant metals. Biomater. Med. Devices Artif Organs 7, 19 1. Larson T. H., Sabott D., Colley R. et al. (1979) A clinical study of marginal integrity and tarnish behaviour of three Cu-rich amalgam systems. J. Oral. Rehabil. 6, 6 1. Marek M. and Mahler D. B. (1979) The corrosion susceptibility of a high-copper amalgam as a function of the mercury content. J. Dent. Res. 58, (Int. Assoc. Dent. Res., New Orleans) Abstr. 968. Marek M. and Okabe T. (1978) Corrosion behaviour of structural phases in high copper dental amalgam. J. Biomed. Mater. Res. 12, 857. Mobasherat M. and Pameijer C. H. (1979) Corrosive properties of retentive pins. J. Dent. Res. 58, (Int. Assoc. Dent. Res., New Orleans) Abstr. 101. Mueller H. J. (1979) Chemical analysis of NaCl solutions and solid products after dissolution of dental amalgam. J. Dent. Res. 58, (Int. Assoc. Dent. Res., New Orleans) Abstr. 972.

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Mueller H. J., Greener E. H. and Marker B. C. (1979) Corrosion by external polarization of soldered orthodontic wires in cleanser solution. Am. J. Orthod. 76, 555. Sarkar N. K. (1978) Creep, corrosion and marginal fracture of dental amalgams. J. Oral Rehabil. 5,413.

Sarkar N. K., Fuys R. A. and Stanford J. W. (1979a) The chloride corrosion of low-gold casting alloys. J. Dent. Res. 58, 568. Sarkar N. K., Fuys R. A. and Stanford J. W. (1979b) The chloride corrosion behaviour of silver-base casting alloys. J. Dent. Rex 58, 1572. Sarkar N. K. and Nanda C. R. (1979) The in uivo corrosion of a ya-free dental amalgam. J. Dent. Res. 58, (Int. Assoc. Dent. Res., New Orleans) Abstr. 970. Sarkar N. K., Redmond W., Schwaninger B. M. et al. (1979) The chloride corrosion behaviour of four orthodontic wires. J. Dent. Res. 58, (Int. Assoc. Dent. Res., New Orleans) Abstr. 22. Sutow E. J. and Jones D. W. (1979) A crevice corrosion cell configuration. J. Dent. Res. 58, 1358.

Tanaka T., Atsuta M., Uchiyama Y. et al. (1979) Pitting corrosion for retaining acrylic resin facings. J. Prosthet. Dent. 42, 282. Thompson N. G., Buchanan R. A. and Lemons J. E. (1979) In vitro corrosion of Ti-6A1-4V and type 3 16L stainless steel when galvanically coupled to carbon. J. Biomed. Mater. Res. 13, 35.

Vaidyanathan T. K., Ballal H. and Prasad A. (1979) Influence of Pd alloying on the corrosion behaviour of dental gold alloys. J. Dent. Res. 58, (Int. Assoc. Dent. Res., New Orleans) Abstr. 965. Vaidyanathan T. K. and Gowda R. (1979) In vitro corrosion of dental amalgam. J. Dent. Res. 58, (Int. Assoc. Dent. Res., New Orleans) Abstr. 976.