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Biomedical subjects

M J Larsen

Publications and source records attributed to M J Larsen.

At least 37 records · Page 2Linked to original sources
PubMed1994

Human experimental caries models: intra-oral environmental variability.

In situ caries models serve purposes other than just being a simpler way to obtain data than running a clinical trial. However, variation in information obtained not only among individuals but also, in particular, depending on different locations of the models within the oral cavity have so far been given little attention. In the present review, the aim has been to characterize the different designs of in situ caries models and to describe some important factors which may vary within the oral cavity and thus influence the outcome of the way the different in situ models are used. Advantages and disadvantages of in vivo models vs. in situ models are discussed. In the latter case, the distinction is made between dental appliance models and so-called "single tooth" models. The review concludes that in situ models differ distinctly with regard to their "biological potential". Because of regional differences in salivary film velocity, pH, and composition of the microflora, results obtained by the various models are not likely to be immediately comparable. Moreover, local factors in relation to specimen environment, such as degree of "protection" and plaque thickness, may further add to the differences. It is suggested that these observations are important in considerations of the relevance of substituting clinical trials with in situ studies. Because of the pronounced intra-oral variation in certain parameters thought to be important for caries lesion development, we conclude that no in situ model can, by itself, fulfill the role as the "model of choice". Finally, although in situ models are useful adjuncts in attempts to estimate the relative effects of new anticaries methods or compounds on caries initiation, the selection of in situ study model design will strongly depend on the aim and purpose of the study.

Dental Caries↗
PubMed1993

Notes on the dissolution of human dental enamel in dilute acid solutions at high solid/solution ratio.

The aim of the present study was to elucidate the nature of the relationship between enamel apatite and lesion fluid during demineralization. Powdered enamel in samples of 1.0 g was suspended in 3 ml of 10, 30, 50, or 70 mmol/l HCl under gentle agitation for up to 24 h at 20 degrees C. After 20 min and 24 h, pH and the concentrations of calcium and phosphate were determined and the degree of saturation with respect to various calcium phosphates calculated. The experiments were replicated 15 times using the same enamel samples. Twenty minutes after the start of dissolution, both pH and concentrations of calcium and phosphate had increased, and the solution became supersaturated with respect to hydroxyapatite, and in some runs also with respect to brushite. During the subsequent 23 h and 40 min, pH continued to increase, despite the supersaturation with respect to apatite, whilst the concentrations of calcium and phosphate decreased due to formation of apatite and, occasionally, brushite mineral. The data indicated that release of carbonate from enamel and its conversion to H2CO3 caused the increase in pH and thus, played a major role in the dissolution-reprecipitation process.

Calcium↗
PubMed1993

Defluoridation of drinking water by co-precipitation with apatite.

The two-step brushite saturation, apatite precipitation method which has previously been shown to be effective for the partial defluoridation of water containing 10 ppm F has now been tested at lower concentrations. The method also reduced F concentrations in the range 1-5 ppm, but was less efficient at lower F levels. Although co-precipitation as fluorapatite is the intended mechanism here, experiments indicated that F adsorption on the seed material added to the defluoridation effect. When the contribution of co-precipitation was isolated it was found that this mechanism could, on its own, account for a reduction of 2.2-2.6 ppm F in water initially containing 3-5 ppm F. A computer program which determines equilibrium F concentrations when supersaturated solutions are induced to precipitate fluorapatite was used to estimate the maximum possible F removal from water by co-precipitation. Using saturation with brushite as a starting point for precipitation, F reductions similar to those found experimentally were predicted. Computer simulations also suggest that any desired reduction in F concentration may be achieved by adding sufficient Ca and phosphate and/or by raising the initial pH.

Adsorption↗
PubMed1993

Defluoridation of water at high pH with use of brushite, calcium hydroxide, and bone char.

The aim of this study was to improve the efficiency of the bone-char method of water defluoridation by pre-treating the water with brushite and calcium hydroxide. Various amounts of brushite, calcium hydroxide, and bone char were suspended batchwise in 100 mL of distilled water containing 0.53 mmol/L fluoride for 24 h under gentle agitation. At suitable intervals, pH and the concentrations of fluoride, calcium, and phosphate in the water were determined and, when possible, the degrees of saturation with respect to brushite, hydroxyapatite, and fluorapatite calculated. Bone char used alone took up fluoride slowly and inefficiently. The addition of brushite and calcium hydroxide resulted in high concentrations of calcium and phosphate, making the solutions highly supersaturated with respect to fluorapatite, and led to a 20-fold increase in fluoride removal from the water. The combined use of all three salts left low concentrations of phosphate in solution and optimized the fluoride uptake capacity. Repeated use of the same bone char for 18 consecutive runs demonstrated that uptake of fluoride by the bone char was improved by repeated use, provided that brushite and calcium hydroxide were added. Therefore, addition of the two salts to the water may prolong the life of the bone char indefinitely, ensure the removal of fluoride, and thus avoid the problem of determining when the bone char is exhausted. In conclusion, we show that the bone-char defluoridation technique can be improved by addition of brushite and calcium hydroxide to the water. The problem of high terminal pH remains, however, and further work is required to improve potability.

Animals↗
PubMed1993

Surgical wound dressing in outpatient paediatric surgery. A randomised study.

A comparison of two different wound dressings was carried out in a series of 88 paediatric patients. The patients were randomised to either Steristrip plus Cutiplast, a conventional absorbent wound dressing, or to DuoDERM, an occlusive adhesive dressing. The study, set up to show which was the most convenient dressing for minor outpatient surgery, focused on the psychological aspects of operative treatment of infants and children. The results showed that the DuoDERM group had fewer bandage changes than the Steristrip & Cutiplast group, although this was not significant. Significantly less pain was experienced on removal of the wound dressing in the DuoDERM group. When dressed with DuoDERM, children could be bathed in the immediate postoperative period without complication. In conclusion, an occlusive wound dressing (DuoDERM) seems to minimize the physical and psychological trauma to the infant or child and reduce the disruption to the child's and the parents' daily routines.

Adolescent↗
PubMed1992

Some notes on the diffusion of acidic and alkaline agents into natural human caries lesions in vitro.

The aim was to obtain information on the penetration of ions, in particular H+ and OH-, into a wide variety of natural caries lesions in human teeth. Thirty-seven permanent teeth were slit in half and soaked in a 1% neutral (blue) litmus solution for at least a week. The cut face of the tooth was painted with a transparent, colourless nail varnish before the tooth was immersed in either of 0.1 M hydrochloric acid or 0.1 M phosphoric acid. In a reverse experiment the samples were soaked in acid (red) litmus solution and then immersed in 0.1 M sodium hydroxide after varnishing the cut face. All solutions were saturated with enamel apatite to avoid the possibility that the solutions might etch their way into the enamel. The colour change was recorded photographically. Several days of external acid/alkali exposure were required to change the litmus colour in the body of most of the lesions, suggesting that diffusion of acidic and alkaline species through the natural surface into the body was slow. The presence of a well-mineralized surface layer covering the lesion postponed the colour change. In contrast, the colour change of two root surface lesions was rapid, occurring within an hour. Thus, because the lesion body is relatively isolated and because lesion fluid is in intimate contact with the enamel apatite, we conclude that lesion fluid is close to saturation with respect to this mineral. Further, a pH change of lesion fluid in vivo paralleling that in plaque after a sucrose intake is unlikely.(ABSTRACT TRUNCATED AT 250 WORDS)

Dental Caries↗
PubMed1992

Partial defluoridation of drinking water using fluorapatite precipitation.

Ion adsorption and ion exchange are two methods commonly used in small home units to treat drinking water to bring the fluoride concentration to within acceptable limits. However, the necessary flowthrough system is often difficult to arrange where there is no piped supply and gradual exhaustion of the active agent is not easily detected. In an attempt to overcome these problems a defluoridation method based on the precipitation of a sparingly soluble fluoride salt, fluorapatite, has been studied. Samples of simulated high-fluoride drinking waters, approximately 10 ppm F, were saturated with brushite, resulting in a state of supersaturation with respect to fluorapatite. Subsequent seeding with hydroxyapatite caused a lowering of the calcium, phosphate, and fluoride concentrations in solution, indicative of fluorapatite precipitation. Repeating the process had an additive effect. Bone char was a less effective seed than hydroxyapatite with water containing fluoride only, but was a more effective seed with simulated Kenyan borehole water containing additional salts. Sixty-minute brushite saturation and apatite seeding steps were generally more effective than 10-min steps. The results suggest that apatite coprecipitation may be a convenient low-technology way to defluoridate drinking water, although prior testing might be useful to ensure adequate removal of fluoride.

Apatites↗
PubMed1991

Release of fluoride from light cured lining materials.

Fluoride was extracted from 0.3 ml samples of five different light cured lining materials. From two brands, fluoride concentrations ranging from 5 to 10 ppm were quickly established and maintained. From three other brands only very small amounts of fluoride were released. The fluoride released was related to the possible caries preventive effect.

Acrylic Resins↗
PubMed1991

Effect of a diffusion layer on the nature of enamel demineralization.

The aim of the present study was to evaluate the significance of diffusion barriers in the development of artificial caries-like lesions. A 30-mm2 enamel surface area was covered with a layer of filter paper and exposed to a gently agitated 50 mM acetate buffer, pH 4.5, containing either 0.1 or 1.0 ppm fluoride for up to 48 h at 20 degrees C. Similar enamel surfaces, but unprotected by filter paper, served as controls. It was found that below the filter paper an approximately 60-microns-deep caries-like lesion was developed exhibiting a preserved surface layer, the mineral content of which depended on the fluoride concentration of the buffer. The lesions of the control teeth without a filter paper coverage were erosion-like surface lesions. In the filter paper calcium and phosphate released from the enamel combined with fluoride from the bulk solution had established a supersaturation with respect to fluorapatite while the undersaturation with respect to hydroxyapatite was maintained, which caused a caries lesion to develop. The lesions produced in noncovered enamel by bulk solution unsaturated with respect to both apatites were erosion-like. It was concluded that a diffusion barrier can significantly favor the establishment of supersaturation with respect to fluorapatite and thus facilitate the development of a subsurface caries-like lesion at the expense of the erosion.

Apatites↗
PubMed1991

On the chemical and physical nature of erosions and caries lesions in dental enamel.

The aim of the present paper was to study the relation between the development of enamel erosion and caries lesion. Intact teeth were exposed to a gently agitated 50 mM acetate buffer, with or without fluoride for various periods of time. Further, intact teeth were exposed to an agitated 0.2 M HCl solution. It was found that irrespective of experimental conditions the aqueous phase was initially unsaturated with respect to both hydroxyapatite and fluorapatite during which period fluorhydroxyapatite was dissolved. Histological examination showed that the dissolving apatite originated from the enamel surface and the resulting lesion exhibited the signs of a dental erosion. The depth of the erosive lesion was found to be a direct function of the amount of mineral dissolved before the aqueous phase became saturated with respect to fluorapatite. The calcium, phosphate and fluoride dissolved made the aqueous phase first saturated and later supersaturated with respect to fluorapatite and a reuptake of fluoride in the enamel began, now under development of a caries-like lesion with preservation of the surface layer over a subsurface demineralizing zone. The demineralization depended on the volume of the aqueous phase. In conclusion, in a closed system with a limited amount of unsaturated solution a double lesion, an erosion over a caries lesion, will develop according to the changing saturation with respect to fluorapatite.

Apatites↗
PubMed1991

Detection of occlusal caries without cavitation by visual inspection, film radiographs, xeroradiographs, and digitized radiographs.

The study compared visual inspection, conventional film radiographs, xeroradiographs, and digitized radiographs for the detection of caries in occlusal surfaces without cavitation. 166 extracted premolars and permanent molars without macroscopic cavitation were included. Eight observers assessed five grades of occlusal caries by visual inspection and by the three imaging techniques. Histologic sections (700-1,000 microns) served as validating criterion for the presence and depth of carious lesions, 82 teeth being found carious (27 with deep dentinal involvement) and 84 being scored as sound. Approximately 20% of the dentinal carious lesions were detected by visual inspection and 40% by conventional film radiography (both with a minimal number of false-positive observations). By xero- and digital radiography, an increasing number of false-positive scorings was seen. By digital radiography the true-positive detection rate was similarly increased to 60%. The predictive values of a positive test were, on average, 0.85 for visual inspection, 0.89 for film, 0.78 for xero-, and 0.82 for digital radiography and for a negative test 0.56, 0.61, 0.61, and 0.69, respectively. However, digital radiography was able to detect over 70% of deep dentinal lesions in contrast to 45% by the other two imaging methods, without an increase in false-positive diagnoses of truly caries free teeth, compared with the other techniques.

Bicuspid↗
PubMed1991

Posteruptive changes in human dental fluorosis--a histological and ultrastructural study.

The aim of the present study was to describe the structural features characterizing the severe grades of human fluorotic enamel (TF scores 5-9) with particular emphasis on the posteruptive changes in severely fluorosed teeth. Dental fluorosis is a subsurface hypomineralized lesion deep to a well-mineralized outer enamel surface, which in severe cases breaks apart shortly after eruption. Early signs of posteruptive changes comprise small defects corresponding to the opening of striae of Retzius. The enamel pits which develop after eruption in more severe cases exhibit an increase in mineral content at their base which correspond to the exposed subsurface hypomineralized lesions. Likewise, the extensive removal of surface enamel in the most severe cases of human fluorosis results in a highly varying uptake of mineral into the exposed subsurface hypomineralized lesions. The uptake varies greatly within apparently similar degrees of hypomineralized lesions. In approximal abrasion facets, however, where the subsurface lesions are also exposed, no evidence of mineral uptake was found. At the ultrastructural level, the well-mineralized surface zone consists of large hexagonal enamel crystals separated by rather large intercrystalline spaces in which numerous irregular small crystals are observed. Moreover, the large crystals may exhibit central and peripheral dissolution. In addition, mineral appeared to be deposited into such defects as well as along the side of the crystals, often with the lattices being continuous from the original crystal into the apparently posteruptive formed crystal material. It is concluded that a substantial mineral uptake can take place in exposed porous hypomineralized fluorotic enamel after eruption, but is most likely to be associated with the presence of microbial deposits, the metabolic activity of which may play a keyrole in mineral exchange.

Crystallography↗
PubMed1990

Release of fluoride from conventional and metal-reinforced glass-ionomer cements.

Fluoride was extracted by 1 or 50 ml deionized water from 0.3 ml samples of two conventional and three metal-reinforced glass-ionomer cements. The fluoride concentration was determined by electrode. It was found that the fluoride concentration around the glass-ionomer cements ranged 2-3 ppm after 15 min and 3-5 ppm after 45 min. The maximum fluoride concentration attained within 24 h varied from 15-21 ppm, while the total amount of fluoride released during 100 days varied from about 2 mg fluoride to 12 mg fluoride per ml glass-ionomer cement. An alloy reinforced cement and its conventional counterpart released the highest amount of fluoride whereas a glass-cermet type released the lowest amount. Within the premises of this study it was concluded that composition and handling of the cements are of importance for the release of fluoride, and that a caries inhibitory fluoride concentration can be established in the surroundings of all of the cements.

Cermet Cements↗
PubMed1990

Sensitivity to and Degradation of Pentachlorophenol by Phanerochaete spp.

This research measured mycelial extension rates of selected strains of Phanerochaete chrysorhiza, Phanerochaete laevis, Phanerochaete sanguinea, Phanerochaete filamentosa, Phanerochaete sordida, Inonotus circinatus, and Phanerochaete chrysosporium and the ability of these organisms to tolerate and degrade the wood preservative pentachlorophenol (PCP) in an aqueous medium and in soil. Most of the tested species had mycelial extension rates in the range of </=0.5 to 1.5 cm day, but there were large interspecific differences. A notable exception, P. sordida, grew very rapidly, with an average mycelial extension rate of 2.68 cm day at 28 degrees C. Rank of species by growth rate was as follows: P. chrysosporium > P. sordida > P. laevis > P. chrysorhiza = P. sanguinea > I. circinatus = P. filamentosa. There were also significant intraspecific differences in mycelial extension rates. For example, mycelial extension rates among strains of P. sordida ranged from 1.78 to 4.81 cm day. Phanerochaete spp. were very sensitive to PCP. Growth of several species was prevented by the presence of 5 ppm (5 mug/g) PCP. However, P. chrysosporium and P. sordida grew at 25 ppm PCP, albeit at greatly decreased mycelial extension rates. In an aqueous medium, mineralization of PCP by P. sordida 13 (ca. 12% after 30 days) was significantly greater than that by all other tested P. sordida strains and P. chrysosporium. After 64 days, the level of PCP had decreased by 96 and 82% in soil inoculated with P. chrysosporium and P. sordida, respectively. Depletion of PCP by these fungi occurred in a two-stage process. The first stage was characterized by a rapid depletion of PCP that coincided with an accumulation of pentachloroanisole (PCA). At the end of the first stage, ca. 64 and 71% of the PCP was converted to PCA in P. chrysosporium and P. sordida cultures, respectively. In the second stage, levels of PCP and PCA were reduced by 9.6 and 18%, respectively, in soil inoculated with P. chrysosporium and by 3 and 23%, respectively, in soil inoculated with P. sordida. PCA was mineralized by both P. chrysosporium and P. sordida in an aqueous medium.

Journal Article↗
PubMed1990

Chemical events during tooth dissolution.

The aims of the present survey were to describe some aspects of (a) enamel solubility in relation to pH and to the fluoride concentration, (b) chemical changes which occur during enamel dissolution, and (c) enamel resistance with an attempt to relate chemical events to clinical observations. The solubility of enamel powder increases dramatically with a decrease of pH. As long as the fluoride concentration in solution is high, the amount of enamel dissolved is low. When fluoride occurs only in the solid phase, the solubility is only slightly affected. Two clinically distinct lesions have been described in dental enamel: the erosion lesion, characterized by a dissolution of enamel from the surface; and the caries lesion, in which the enamel surface layer, accumulating fluoride, remains relatively intact, while the subsurface enamel dissolves. It was concluded that any dissolution of enamel is caused by an undersaturation with respect to enamel apatite. When whole enamel dissolves, the saturation with respect to fluorapatite determines whether a well-mineralized surface layer remains, i.e., whether the dissolution results in an erosion or a caries lesion. The degree of supersaturation with respect to fluorapatite determines the uptake of fluoride in the surface layer and its mineral content. It was observed that different areas of the enamel surface possess different degrees of resistance. A satisfactory explanation for the resistance has not been given.

Cariostatic Agents↗
PubMed1989

The hydroxyapatite solubility product of human dental enamel as a function of pH in the range 4.6-7.6 at 20 degrees C.

Samples of 200 mg powdered enamel apatite were suspended in 10 ml of aqueous solutions made from phosphoric acid and calcium hydroxide for 60 days at 20 degrees C. Calcium and phosphate concentrations were determined by spectroscopy; pH and the concentrations of fluoride and carbonate by electrometry. The solubility product of hydroxyapatite varied linearly from 10-56.9 at pH 4.6 to 10-52.8 at pH 7.6. The carbonate concentration in the aqueous phase was too low to account for calcium carbonate complex formation of significance to the solubility product. After the equilibration, X-ray diffraction analysis of the powder showed that the salt was unchanged pure apatite with no trace of a transformation to brushite or any other calcium phosphate. A re-equilibration of some of the powder samples for another 60 days produced similar solubility products. Thus the change of the solubility product was most likely attributable to a pH-induced change of the ionic composition of the enamel crystal surface.

Calcium Carbonate↗