Pesticdies: transatlantic movements in the northeast trades.

Concentrations of chlorinated hydrocarbons in airborne dust carried by the trade winds from the European-African land areas to Barbados range from less than 1 to 164 parts per billion. The lower limit of the average content of 1 cubic meter of air is 7.8 x 10-(14) gram. The contributions of river-borne and atmospherically transported pesticides to parts of the marine environment are calculated approximately and comtpared. The amnounts of pesticides contributed to the tropical Atlantic by the trade winds appear to be comparable to those carried to the sea by major river systems.

Hepatic neoplasms in the mummichog Fundulus heteroclitus from a creosote-contaminated site.

High prevalences of idiopathic hepatic lesions were found in mummichog, Fundulus heteroclitus, from a site in the southern branch of the Elizabeth River, VA, contaminated with polycyclic aromatic hydrocarbons. Grossly visible hepatic lesions occurred in a total of 93% of the individuals from this site and 33% of these fish had hepatocellular carcinomas. Hepatic lesions were not detected in fish from two less contaminated sites. Lesions included foci of cellular alteration, hepatocellular adenoma, early and advanced hepatocellular carcinomas, and cholangiocellular proliferative lesions. Advanced carcinomas exhibited several distinct cellular patterns and some livers contained multiple neoplasms occupying up to 80% of the hepatic parenchyma. Sediments from the contaminated site contained extremely high concentrations (2200 mg/kg dry sediment) of polycyclic aromatic hydrocarbons, which are believed to originate from an adjacent wood treatment facility that has used creosote. Concentrations were 730- and 35-fold higher than those at the two other sites. These findings indicate a strong positive association between exposure to creosote-contaminated sediments and the high prevalence of hepatic neoplasms in a feral population of mummichog and support the putative role of polycyclic aromatic hydrocarbons in fish hepatocarcinogenesis. Additionally, they suggest that the mummichog may be a useful indicator of exposure to carcinogens in aquatic environments.

Stereochemistry of poly(methyl methacrylate) acrylic resin denture base material.

Nuclear magnetic resonance (NMR) 13C has been used to determine the tacticity of poly(methyl methacrylate) (PMMA) denture base materials. Curing cycle has no effect on tacticity. A tendency towards a mainly syndiotactic arrangement is shown. Industrially produced PMMA showed the same tacticity as the dental products.

Evidence of aberration of the natural cytotoxic cell activity in Fundulus heteroclitus (Pisces: Cyprinodontidae) from the Elizabeth River, Virginia.

There is a general agreement that exposure to high concentrations of polynuclear aromatic hydrocarbons (PAH) in sediments is associated with high frequencies of neoplasms in feral fish species. Since PAH modulate the activity of murine and amphibian natural cytotoxic (killer) cells, a leukocyte subpopulation that is believed to play an important role in immunosurveillance, we wished to determine if fish exposed to PAH could have an altered natural cytotoxic cell (NCC) activity. In the present study, mummichog (Fundulus heteroclitus L.) were collected from two sites in the Elizabeth River, VA that are heavily contaminated with PAH, and from a relatively unpolluted reference site in the York River, VA. The cytotoxic activity of anterior kidney and splenic leukocytes was tested against the tumor cell line K562. The leukocytes from Elizabeth River fish displayed a significant depression of the in vitro tumorilytic activity as compared with leukocytes from the York River fish. Analysis of leukocyte-tumor cell conjugates indicated that Elizabeth River fish leukocytes were unable to recognize and subsequently bind to the tumor target cells. This suggests an aberration in the early events of the cytotoxic mechanism. By keeping the fish in cleaner York River water for up to 28 weeks the suppressed NCC activity was reversed totally in one site, which is slightly contaminated, and partially in the other site, which is heavily polluted with creosote from an operating wood treatment plant. This indicates that the decreased NCC activity was related, at least in part, to exposure to the chemical pollutants in the Elizabeth River sediments.

Effect of cross-linking on sorption properties of a denture-base material.

Cross-linking of the polymer of the interstitial matrix of a heat-cured resin denture base is said to provide craze resistance for the denture base. The mechanism of crazing can be related to both water sorption and solvent attack. The present study shows that the addition of ethylene glycol dimethacrylate in concentrations of 0-100% of monomer volume has little effect on water sorption properties but is an efficient method of providing solvent resistance.

The effect of esthetic fibers on the flow properties of an acrylic resin denture base material.

OBJECTIVES: This study was conducted to measure the effect of viscose rayon esthetic fibers on the flow properties of an acrylic resin denture base material. METHODS: Two testing methods using a needle penetrometer and an oscillating rheometer were employed on weighed proportions (0.1%, 0.2%, 0.8%, and 0.3%) of fiber. RESULTS: The results show that the fibers affect the flow properties of the polymer/monomer mix. Increasing the percentage of fibers decreases the flow of the material and reduces the doughing time. With increasing amounts of fiber, a rapid increase in the viscosity occurs with a corresponding adverse effect on flow properties. SIGNIFICANCE: It is concluded that these properties are important for the successful handling of acrylic materials since they can influence the accuracy and quality of the molded denture.

Rapid curing of acrylic denture-base materials.

A rapid-cure polymerization of cure, including a terminal boil, with respect to transparency, residual monomer, mechanical properties, and generation of gaseous porosity. It was found that in order for porosity-free resin to be produced during rapid curing, a low concentration (approximately 0.26%) of benzoyl peroxide initiator in the powder component is vital, and a very small concentration (approximately 0.025%) of the chemical activator dimethyl-p-toluidine in the monomer component is an advantage. Mechanical properties were only influenced where substantial porosity existed, but it was recognized that a material "tailor-made" for rapid curing would be most welcome.

Evaluation of analytical techniques for measurement of denture-base acrylic resin glass-transition temperature.

The glass-transition temperature of a range of acrylic resin materials used in prosthetic dentistry was determined. The techniques used to make the measurement included: thermal mechanical analysis, dynamic mechanical thermal analysis, and differential scanning calorimetry. It was found that the measuring techniques used yielded very similar results, and as a consequence it was concluded that: familiarity and easy availability of thermal mechanical analysis lead to the recommendation that this technique should be employed as the standard glass-transition evaluation technique for denture-base acrylic resins.

Effect of water and artificial saliva on mechanical properties of some denture-base materials.

Two artificial saliva samples selected on the basis of being chemically and physically similar to natural saliva were used as saturation media so that we could investigate their effects on mechanical properties of four denture-base materials, and the results were compared with the effect of water on the same materials. Diffusion coefficients in the more viscous saliva were different from those in water but, in general, mechanical properties were similarly affected in all liquids, indicating that the aqueous phase of the artificial medium was a major factor influencing the results.

Rheology of acrylic denture-base polymers.

The aim of this study was to investigate the changing rheological behavior of a denture-base polymer from mixing to setting. In addition, monomer evaporation and exothermic behavior of the mix were evaluated. The results show that the material behaves as a pseudoplastic fluid. It is shown that the viscosity increases at different rates with respect to lapsed time, and increases with higher temperature. Also, it is shown that polymerization and monomer evaporation both play a part in dough formation.

Effect of the curing cycle on residual monomer levels of acrylic resin denture base polymers.

It has been shown previously that high levels of residual monomer have a deleterious effect on the properties of denture base polymers. Levels of residual monomer were determined on a homopolymer and a copolymer using gas-liquid chromatography. A wide range of recommended and 'short cut' curing cycles were then investigated which produced values ranging from 0.56 to 18.46%. From the ranges examined an optimum cycle of 7 h at 70 degrees C and 1 h at 100 degrees C was established which was used to polymerize 23 currently available synthetic denture base polymers. Only small differences were found between the materials tested with a range from 0.54 to 1.08% of residual monomer.

Measurement methods used for the determination of dimensional accuracy and stability of denture base materials.

The literature relating to methods of measurement of dimensional accuracy and stability of denture base materials is reviewed. The papers are presented chronologically together with information on the measurement technique used and the reported level of accuracy. Most authors utilized optical measuring apparatus, with the use of calipers being the second most popular method.

Determination of doughing and manipulation times of acrylic resin denture base material and a proposal for a specification for a packing plasticity test.

The doughing and manipulation times of a range of acrylic resin denture base polymers are presented in this study. The methods evaluated were: (i) probing following American Society for testing materials specification, F451-76; (ii) hole penetration following British Standard, 2487: 1989 and International Standard Organisation, 1567: 1988; (iii) probing using a penetrometer employing a 20 g load and 1 mm diameter needle; (iv) oscillating rheometer apparatus used at a fixed frequency of 1 Hz by sweeping through a programmed linear increase in strain. The results indicate that each of the methods is capable of evaluating the doughing and manipulation times of the unprocessed polymers. It is concluded that the Bohlin VOR Oscillating Rheometer is excellent for investigating the flow properties of denture base polymers but it cannot be recommended for a standards specification test because of the high cost. The penetrometer method has been shown to be superior to the current standards specification test and would be ideal for a packing plasticity standards test.

Stress analysis techniques in complete dentures.

The fracture of acrylic resin dentures remains an unresolved problem. It is known that eventual fracture of an appliance occurs due to crack initiation and propagation from areas of high stress concentration. In order to understand and overcome the problem of fracture, it is important to identify the regions of stress concentration. A number of different methods are used in stress analysis. However, the finite element method, a numerical technique, appears to overcome most of the problems associated with the earlier experimental methods. This article reviews the different techniques and their application to examining stresses in dentures.

Denture fracture--a survey.

The fracture of dentures is an unresolved problem. Despite increasing costs incurred by the nation on the repair of these prostheses, very little has been documented on the type of fracture encountered. This survey was carried out to determine the prevalence of type of fracture by the distribution of questionnaires to three different laboratories. Results obtained showed that 33% of the repairs carried out were due to debonded/detached teeth. Twenty-nine per cent were repairs to midline fractures, more commonly seen in upper complete dentures. The remaining 38% were other types of fractures, the majority of which constituted repairs to upper partial dentures. The latter involved detachment of acrylic resin saddles from the metal in metal based dentures and the fractures of connectors in the all-acrylic resin partial dentures.

Cross-infection control in dental laboratories.

The attitudes to cross-infection control of 800 dental laboratories registered with the Dental Laboratories Association have been surveyed. Considering the topicality of the subject material and the need for careful cross-infection control within and beyond the dental surgery, the response rate of 22% (175) was disappointingly low. As a result of the low response rate, the results should be seen as a guide only. Forty-nine percent of respondents had a cross-infection policy and of those with no policy 64% intended to implement one in the future. Thirty percent of laboratories receive known undisinfected work from the dental surgery. Of those items disinfected on arrival at the laboratory, those most frequently disinfected are dental impressions (77%) and dentures (51%). The most popular chemicals used for disinfection are household bleach, chlorhexidine and glutaraldehyde. Forty-four percent of the respondents generally (90% or more of the time) wear gloves when handling dental work received and opened in the laboratory. Seventy-four percent wear protective eye spectacles when trimming or polishing prostheses. Sixty-one percent used no disinfectant in the pumice and 93% did not disinfect the polishing instruments, eg wheels and mops. Forty-six percent had a policy for immunisation of staff against Hepatitis B.

Complete denture construction in general dental practice: an update of the 1970 survey.

A survey was carried out on materials and techniques used for complete denture construction in general dental practice, by means of a postal questionnaire sent to a random sample of 1000 dentists whose names were taken from the BDA membership. There was a 71.5% response. The results obtained are compared with a similar survey undertaken in 1970. The majority of practitioners still provided dentures totally or mainly within the NHS. Whilst the demand for complete dentures was reported as having remained stable, there has been considerable reduction in the numbers of dental technicians employed by general dental practitioners. Alginate remains the impression material of choice and the majority of impressions were sent to a commercial laboratory for pouring. Although still the most popular, the use of the plane-line hinge articulator has reduced and there has also been a continued reduction in the use of porcelain teeth. The importance of a cross-infection policy is highlighted. There were conflicting opinions expressed on the role of implants in general dental practice. Many respondents took the opportunity to comment on the status of dental prosthetics under the NHS and most criticised the fee structure.

A survey of complete denture prosthetics for the elderly, the handicapped and difficult patients.

A survey was carried out to establish information on the facilities and management of the elderly and the handicapped and to evaluate the treatment of the difficult edentulous patient in general dental practice. A postal questionnaire was sent to a total of 1090 dentists in three regions of England and Wales. There was a 50.4% response. Ninety-nine per cent of practitioners were able to treat the ambulant elderly, whereas only 72% offered treatment if the patient was wheelchair dependent. Ninety-six per cent of all respondents undertook domiciliary visits. All practitioners provided prosthetic treatment (complete and partial dentures) for the elderly, whereas fewer felt able to treat the physically and the mentally handicapped. There was less conservative and periodontal treatment provided than prosthetic treatment. A similar pattern was obtained in all three regions. Forty-seven per cent preferred to treat difficult complete denture patients privately, 44% under the NHS and 6% not at all. There were regional variations in the referral patterns of these difficult denture cases. The majority of respondents considered that there should be more opportunities for postgraduate education in prosthetic dentistry.

The effect of thermocycling on the hardness of soft lining materials.

This investigation studied the effect of thermocycling on the hardness of six commercially available soft lining materials. The apparatus used to measure hardness measured the difference between depth of penetration of an indentor into a specimen under a primary contacting load and a secondary penetrating load. Control specimens were stored in water at 20 +/- 2 degrees C and trial specimens were thermocycled between 18 +/- 1 degree C and 53 +/- 1 degree C for periods up to 28 days. It was found that thermocycling had a deleterious effect on the softness of all six materials. Further, it was shown that chairside provisional soft lining materials demonstrate a rapid deterioration at temperatures greater than 50 degrees C.