The data obtained in this in vitro study indicate that contact with pit and fissure sealants to which NaF has been added in amounts ranging from 2 to 5% substantially increases the fluoride content of the enamel and reduces its solubility in acid. The properties of the materials do not seem to be impaired by the addition of fluoride in these amounts. It thus appears that this approach to providing a backup anticariogenic mechanism may, indeed, be feasible. However, further investigation must be done to confirm the anticariogenic effect and to establish the most efficacious means of fluoride incorporation in the materials.

A two-body, wear-testing method was developed and the test results were used for comparing and ranking the rate of wear for an amalgam, an experimental composite resin, and a commercial composite resin. The ranking of wear found by this method was the same as that shown by clinical research for the rate of wear of amalgam and commercial composite resin.

Ab stractChemical cure resin materia.ls are generally used in the repair of dentures. Different repair resins used may yield different results. The bond strength of three autopolymerizing resins were evaluated using a torsional test method. The results showed that Palapress and Caulk resins had a higher repair strength than Rapid Repair resin .Key words: Denture repair resins, bond strength, torsion tests evaluation.(Received for publication March 1995. Revised June 1995. Accepted July 1995 Int roductionAlthough resins for denture repairs are not as extensively studied as adhesives to dentine, porcelain or enamel, the importance of denture repairs cannot be underestimated. Huggett et al. 1 cited that nearly three-quarters of a million dentures were repaired each year in England and Wales at a cost of £4 .7 million (at 1987 price) to the National Health Service (UK) and an estimated similar amount is spent by patients privately. Most of these repairs are due to failure of polymethylmethacrylate (PMMA) denture bases.Smith 2 noted that denture failure depends on the shape, conditions of loading, inherent residual stresses and mechanical properties of the denture base. Farmer 3 listed various clinical factors such as improperly contoured mandibular occlusal plane, high frenal attachment, incorrect occlusal schemes, heavy occlusal forces, poor adaptation of the denture base to the residual alveolar ridge and denture base thickness as primary causes of denture fractures. Vallittu et al. 4 noted that the highest incidence of denture fractures occurred between 16-36 months after being in service.The clinician must often decide whether to repair or replace the broken denture . To minimize inconvenience to the patient and save costs in the reconstruction of the dentures, quick and reliable denture repairs are often necessary. Success depends on correcting the offending clinical cause and a strong repair junction. Confidence in the repair rests on the repair resin. There is, therefore, a need to fully characterize the bond strength of the denture repair resins available in the market.The purpose of the present study was to use the torsional test proposed by Stewart et al. 5 to assess the relative merits of denture repair resins. It seeks to compare the shear bond strength of three commercially available acrylic resin denture repair materials in vitro.A torsional test was chosen as the stress exerted on the specimen has a significant shear component and is, therefore, closer to the clinical situation. The torsion specimen also has a uniform state of stress at any point on the specimen surface and this state of stress is less dependent on parallelism and specimen geometry than for diametral or three-point bending tests.

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ONE of the important considerations associated with the selection and use of any dental material is its solubility in oral fluids. The most common laboratory test for solubility is the measurement of disintegration in distilled water, as outlined in A.D.A. Specifications 8 and 9. Using this test, one study on silicate cement showed a gradual decrease in solubility with time.' It has been suggested that these observations are not corroborated in the clinical restoration and that durability may be dependent upon a number of unknown factors. Henschel,2 in a clinical survey, noted that the lingual and labial areas of silicate restorations which are exposed to the greatest washing action of the saliva, as well as the most mechanical wear, remain relatively intact, most disintegration occurring adjacent to gingival areas where debris and food plaques collect. It is also recognized that in spite of the low solubility of zinc phosphate cement in distilled water, deterioration of the thin layer surrounding the casting is common.3Even though water is a primary constituent of saliva, other factors appear to contribute to the disintegration of dental cements. Since the greatest solution of silicate restorations does occur in regions susceptible to the formation of food plaques, organic acids could be responsible for at least a portion of the clinical dissolution of these materials. Voelker demonstrated the solubility of different silicate cements to be 3 times greater in a 1 per cent lactic acid solution than in distilled water4 and Rosthj5 noted that silicates were more soluble in lactic than in acetic acid. These studies, involved a limited number of specimens, did not include many variables which may well be associated with solubility, and the composition of the restorative materials, themselves, have been altered considerably since this early work. Hence this present study was designed to investigate the effects of certain of these variables on the solubility of zinc phosphate and silicate cements. PROCEDURE Three commercial brands of zinc phosphate cement and 2 silicates were employed in the study. The consistency at which they were tested was established by a modification of the A.D.A. slump test for silicates and zinc phosphate cement. Various methods of fabricating the specimens were tried and the following procedure evolved.

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