E.J. Sutow

Dental soft polymers: plasticizer composition and leachability

Abstract The objective of this investigation was to identify the type and quantify the leachability of plasticizer constituents from dental soft polymer gel systems. The clinical implications are that some plasticizers are known to be potentially toxic and in addition, loss of plasticizer results in changes in physical properties of dental soft polymers. A total of 7 different constitutents were quantitatively identified in the 8 solutions analysed by GC and GC-MS. Specimens of gel material (plasticized polymer) were evaluated for loss of plasticizer while subjected to both static and cyclic tests (1 Hz, 5 min, ± 12 N) in H 2 O at 37°C. With one exception, cycled specimens did not yield greater extraction of ester. The use of 1- 14 C EtOH indicated complete diffusion of ethanol from the gel within 24 h. The data indicate that a typical denture soft lining would leach between 10–40 times the level of ester that would normally be obtained from food and the environment, but would leach about one-tenth of the most stringent ADI value.

An in vivo and in vitro Study of the Loss of Plasticizer from Soft Polymer-gel Materials

Polymer-gel materials used as short-term denture soft linings are blended with plasticizers to lower the glass transition temperature (Tg). A lower Tg allows for greater polymer chain mobility, thus producing a more flexible material. The present work evaluated the loss of plasticizers due to leaching both in vivo and in vitro. Two commercial denture soft-lining materials (A and B) were tested. These were both poly(ethyl methacrylate) polymers, blended with alcohol and phthalate esters. A clinical study was conducted in which patients wore, sequentially, dentures bearing (on separate occasions) each of the two soft-polymer lining materials. The two materials A and B were randomly assigned for each of ten patients and were worn for 14 and 30 days, respectively. With one exception, patients wore dentures with both lining materials, for a total of 19 clinical evaluations. The plasticizer loss occurring during the clinical trial was determined by GC analysis from the initial and terminal day sampling of plasticizer content of the soft polymer-gel materials. The results of this analysis were compared with results obtained from an in vitro leachability study by use of sink conditions in water at 37°C for the same two commercial soft polymers conducted over the same time periods of 14 and 30 days. The results indicated that a higher loss of plasticizer occurred in vivo, compared with the in vitro tests for 17 of the 19 clinical evaluations. The average plasticizer lost in vivo from material A at 14 days was 122 ± 58 mg/g, and for material B at 30 days it was 33 ± 27 mg/g. This can be compared with the in vitro loss that was 13.41 ± 1.11 mg/g for material A at 14 days, and 8.47 ± 0.73 mg/g for material B at 30 days. It is not known how much of the plasticizer lost from the denture soft lining in the in vivo trial was ingested by the patient. Patients were cautioned against soaking the denture in any liquid; however, it is not known how much was lost due to cleaning of the denture by the patients.

Chemical and Molecular Weight Analyses of Prosthodontic Soft Polymers

A number of studies have been made investigating the properties of dental prosthodontic soft polymer materials. In such materials, which are used as short-term denture soft liners, the polymer component is complexed with one or more plasticizers. This lowers the glass transition temperature, Tg, of the polymer by allowing greater chain mobility and produces a more flexible material. Little information is available relating to the polymer components of such soft-lining materials. The chemical composition and molecular-weight distribution of a polymer help to determine its ability to form a plasticized soft polymer-gel. In the present work, both pyrolysis depolymerization/gas chromatography (GC) and size exclusion chromatography (SEC) have been used to characterize 11 commercial polymer powders. By use of SEC and polymer standards having a narrow molecular-weight range, it has been possible for accurate molecular-weight distributions to be determined for all 11 powders. The peak molecular weights were found to be between 1.79 x 105 and 3.28 x 105. These values are above the molecular weight at which chain entanglement occurs. The polydispersity ratios (Mw/Mn) were between 2.09 and 4.48. Methacrylate polymers can be readily decomposed thermally to their constituent monomers, which, once recovered, can be analyzed by gas chromatography. The simple pyrolysis apparatus used in the present study was easily assembled and was capable of producing both qualitative and quantitative results. None of the commercial polymers analyzed gave methacrylate decomposition products with pendant chains greater than ethyl (-CH2-CH3). All of the commercial polymers successfully studied by the pyrolysis test method proved to be poly(ethyl [100-79%]/methyl [0-21%] methacrylate) polymers or co-polymers.

Influence of Plasticizer on Soft Polymer Gelation

An evaluation has been made of the soft polymer gel systems of the type used as prosthodontic short-term denture-lining materials. The purpose of the study was to determine the effect upon gel strength and gelation time produced by variations in the composition of the plasticizer liquids. Poly(ethylmethacrylate) co-polymers were mixed with combinations of ethyl alcohol and the esters dibutyl phthalate (DBP), butylphthalyl butylglycolate (BPBG), and benzyl salicylate (BS). The relative rate of gelation of plasticizer polymer compositions was obtained by means of a reciprocating rheometer. The gelation rate was found to increase rapidly with an increase in the ethyl alcohol content. Significantly shorter gelation times were also found with liquids containing the lower-molecular-weight esters. A linear relationship was found between the ethyl alcohol content and the log of the gelation time. The higher-molecular-weight esters produced the strongest gel. A linear relationship was found between the gelation (log) times and the seven-day puncture strength. The ratio of polymer to plasticizer liquid was found to have a very significant influence upon the puncture strength and gelation time. Molecular weight or molecular volume of the ester plasticizers was found to have a greater influence on gel formation than the cohesive energy parameter. The strong polar bonding of ethyl alcohol was found to have a significant influence on both the rate of gel formation and the subsequent gel strength. The data indicate that controlled variations in the characteristics of plasticity and gel strength of dental soft polymer systems can be made to suit different clinical prosthodontic requirements.

Clinical implications of resilient denture lining material research. Part I: Flexibility and elasticity

The clinical use of resilient denture liners is based on their flexibility and elasticity. The nine commercial resilient denture liners measured were less flexible during and after simulated chewing and clenching test sequences. The elasticity of the intraoral-cure liners reached maximal values at 24 hours. Although interpretations require caution, the results imply that the dentist should (1) use maximum liner thickness in the denture base when conditioning abused supporting tissues because of the materials tendency to stiffen under function, and (2) leave a functional impression in the mouth for at least 24 hours before pouring the cast.

Materials Science The Response of Dental Amalgam to Dynamic Loading

A conventional (γ 2-containing) dental amalgam was fatigue-tested at 1800 and 80 cycleslmin, employing uni-axial, sinusoidal loading, with R = -8. Compressive, tensile, and creep tests were conducted to characterize the alloys static mechanical behavior. Tests were performed at 37°C on specimens which were aged for seven days, at 37°C. Fatigue-tested specimens were microscopically examined for fracture surface appearance and crack path. The amalgam demonstrated a frequency dependence and a significant reduction in fracture strength due to fatigue loading. The fatigue crack path was primarily intergranular in the γ 1 phase and inclined at approximately 45° to the principal stress axis. These observations are characteristic of some metals when subjected to low-frequency, elevated temperature testing where significant grain boundary sliding occurs, and therefore suggest a creep-fatigue interaction for this alloy.A conventional (gamma 2-containing) dental amalgam was fatigue-tested at 1800 and 80 cycles/min, employing uni-axial, sinusoidal loading, with R = -8. Compressive, tensile, and creep tests were conducted to characterize the alloys static mechanical behavior. Tests were performed at 37 degrees C on specimens which were aged for seven days, at 37 degrees C. Fatigue-tested specimens were microscopically examined for fracture surface appearance and crack path. The amalgam demonstrated a frequency dependence and a significant reduction in fracture strength due to fatigue loading. The fatigue crack path was primarily intergranular in the gamma 1 phase and inclined at approximately 45 degrees to the principal stress axis. These observations are characteristic of some metals when subjected to low-frequency, elevated temperature testing where significant grain boundary sliding occurs, and therefore suggest a creep-fatigue interaction for this alloy.

Improved fit of maxillary complete dentures processed on high expansion stone casts.

PURPOSE Although relatively well-fitting dentures are produced at low cost and with easy-to-manipulate material, current denture base materials are not ideal. Because acrylic resin complete and removable partial dentures change dimensionally as a result of polymerization and thermal contractions, a high expansion stone was tested in this study to determine its ability to compensate for some of the dimensional change. MATERIAL AND METHODS Maxillary complete dentures were processed on type III dental stone and high expansion stone casts. The accuracy of fit along the posterior palatal border was measured and compared after the prostheses were trimmed and polished and after their immersion in water at 23 degrees C at 1-day and 1-week intervals. RESULTS Statistical analysis of the data revealed that at every measurement interval the maxillary complete dentures processed on high expansion stone had posterior palatal border openings that were significantly smaller when compared with type III dental stone (p < 0.001).

Crevice Corrosion Products of Dental Amalgam

The objective of this study was to determine the in vitro corrosion products that resulted from crevice corrosion of low-and high-copper dental amalgams. Specimens were potentiostatically polarized in a chloride-containing electrolyte while set against a PTFE surface to form a crevice. After 16 h, corrosion products were examined by light microscopy, SEM, EDS, and XRD. Analysis showed the presence of three previously reported products [Sn4(OH)6Cl2, SnO, and Cu2O] and a new product, CuCl, which formed on high-copper, γ2-free amalgams. Thermodynamic considerations show that CuCI is stable for the reported in vivo potentials of amalgam restorations and the high acidity and high chloride ion concentration associated with crevice corrosion.

Materials Science In vitro Crevice Corrosion Behavior of Implant Materials

The crevice corrosion susceptibility of cold-worked Type 316LVM stainless steel, cast Co-Cr-Mo, wrought Co-Cr-W-Ni, non-nitrided and nitrided Ti-6Al-4V ELI, and c.p. Ti, Grades 1 and 4, was studied in vitro by means of a crevice cell. Occlusion was created by interfacing a disc specimen and a Teflon bar. Specimens were mechanically prepared through 2-4 μm Al2O3 and passivated in 30% HNO3, followed by steam sterilization. Tests were performed in a deaerated Ringers solution, maintained at pH = 7 and 37°C. Anodic polarization was conducted potentiostatically at pre-selected levels, and resultant currents were monitored: stainless steel, 50 and 100 mV (S.C.E.), 450 min; non-stainless materials, 600 mV, 1000 min. Results for the stainless steel demonstrated that a HNO3 passivation treatment reduced its crevice corrosion susceptibility. For the non-stainless steel materials, no crevice corrosion susceptibility was observed, although a dulling and discoloration of c.p. Ti was evident. Recognizing that 600 mV is in excess of the O2 reduction potential in vivo, it was concluded that, in the absence of fretting, implants of these non-stainless steel materials would not experience significant corrosion loss under crevice conditions.The crevice corrosion susceptibility of cold-worked Type 316LVM stainless steel, cast Co-Cr-Mo, wrought Co-Cr-W-Ni, non-nitrided and nitrided Ti-6Al-4V ELI, and c.p. Ti, Grades 1 and 4, was studied in vitro by means of a crevice cell. Occlusion was created by interfacing a disc specimen and a Teflon bar. Specimens were mechanically prepared through 2-4 micron Al2O3 and passivated in 30% HNO3, followed by steam sterilization. Tests were performed in a deaerated Ringers solution, maintained at pH = 7 and 37 degrees C. Anodic polarization was conducted potentiostatically at pre-selected levels, and resultant currents were monitored: stainless steel, 50 and 100 mV (S.C.E.), 450 min; non-stainless materials, 600 mV, 1000 min. Results for the stainless steel demonstrated that a HNO3 passivation treatment reduced its crevice corrosion susceptibility. For the non-stainless steel materials, no crevice corrosion susceptibility was observed, although a dulling and discoloration of c.p. Ti was evident. Recognizing that 600 mV is in excess of the O2 reduction potential in vivo, it was concluded that, in the absence of fretting, implants of these non-stainless steel materials would not experience significant corrosion loss under crevice conditions.

Micrographic and profilometric evaluation of the finish produced by diamond and tungsten carbide finishing burs on enamel and dentin

Abstract This study ranked the surfaces of bevels cut in enamel and dentin by four ultraspeed burs: DTP, MF3, 12F, and 30F. Each bur was used to cut a bevel in enamel and dentin and stereoscopic micrographs were made at X600 magnification. These micrographs were sorted into sets representing the four burs at three separate locations on the margin (entry, exit, and gingival). The micrographs were ranked for surface roughness at two locations: (1) at the margin between the cut and uncut tooth surface and (2) within the body of the cut dentin. In addition, the bevel was examined within the cut dentin with a profilometer. A CLA of roughness was identified for the finish created by each bur. Mean CLA (±SE) values revealed that the DTF bur produced the roughest finish (3.84 ± 0.13 μm) whereas the 30F bur produced the smoothest finish (0.29 ± 0.02 μm) ( p p = .05). Micrographs of the finish within the body of the cut dentin ranked from roughest to smoothest: DTF, MF3, 12F, and 30F ( p p