Search PubMed⌕ Search

Biomedical subjects

R G Craig

Publications and source records attributed to R G Craig.

At least 73 records · Page 4Linked to original sources
PubMed1988

Force-displacement properties of a modified cross-linked silicone compared with facial tissues.

A dimethyl siloxane triacetoxy terminated silane, Type A adhesive, was modified with the base component of a medium grade polydimethyl siloxane with vinyl groups, MDX 4-4210, to produce a more pliable maxillofacial prosthesis. Force-displacement curves of various ratios of Type A adhesive/MDX 4-4210 were determined and compared with force-displacement curves obtained from human subjects. Within the force range of 0.4 to 1.2 N, the force-displacement slopes obtained on human subjects compared favourably with some of the Type A adhesive/MDX 4-4210 test specimens. The force-displacement slopes for the elastomers and humans were not in agreement for forces below 0.4 N.

Biocompatible Materials↗
PubMed1988

Finite element analysis of a mandibular model.

A two-dimensional finite element model of a mandibular quadrant was used to examine the stresses and displacements resulting from a 100 N load placed as follows: (i) distributed on the second molar, (ii) concentrated at 30 degrees to the vertical on the second molar, and (iii) distributed on the second premolar and second molar. Each tooth was supported by periodontal ligaments, cortical and cancellous bone. The modulus and Poisson's ratios for each material were selected from accepted values. The principal stresses were determined throughout the model, with special emphasis being placed for elements in the immediate vicinity of the teeth mentioned above. It was found that for the case where the load was uniformly distributed on the second molar the stresses sigma min were mostly compressive (-) in nature while sigma max were mostly tensile (+). In the case where the load was concentrated and at 30 degrees to the vertical, the magnitude of the stresses were three to five times greater and the pattern less uniform than those resulting from the distributed load. The concentrated load at 30 degrees to the vertical resulted in higher bending stresses than the distributed load and hence higher sigma max stresses.

Bicuspid↗
PubMed1988

Physical properties of addition silicones as a function of composition.

The effects of molecular weight of the base silicone, filler and copolymer concentrations on compression set, strain, creep and dynamic elastic modulus were determined. The molecular weight of the base silicone had the least effect on these properties although low molecular weights showed better properties. Chain extenders were detrimental to physical properties of creep, compression set and dynamic elastic modulus. Increased copolymer concentrations decreased compression set, strain and creep and increased the dynamic elastic modulus. The useful copolymer/base functional group ratio was from 1:1 to 3:1. Filler affected properties to a lesser extent than copolymer ratio. Compression set, strain, and creep decreased and the dynamic elastic modulus increased with increasing filler with the properties being directly proportional to the weight fraction. All properties were interrelated with ln strain linearly related to ln compression set, creep proportional to set, and the dynamic elastic modulus was inversely correlated with strain and creep.

Chemical Phenomena↗
PubMed1988

Cytotoxicity of dental composites and other materials in a new in vitro device.

Several in vitro methods have been developed over the years to improve correlation between screening tests and usage tests of dental restorative materials. Pulpal responses to restorative materials in usage tests are usually less severe than are responses of cells either in vitro screening tests or in implantation tests where there is direct contact between experimental materials and vital cells. In the "in vitro pulp chamber" device tested in the present study, dentin disks were interspersed between composite resins (as well as other dental materials) and the medium which served as the nutrient source for the Balb/c 3T3 test cell system. The dentin restricted the diffusion of materials into the "pulp chamber" to clinically relevant levels. The light-cured composite resins (Fulfil and P30), caused depression of protein synthesis only during the first 24 h postpolymerization, when placed on 0.5 mm but not 1.5 mm thick dentin dishes. The chemically-set composites (SILAR and P10) caused no significant inhibition of protein synthesis (as compared to negative controls) at any postpolymerization time on either 0.5 mm or 1.5 mm dentin disks.

Animals↗
PubMed1988

Reaction of fibroblasts to various dental casting alloys.

The cytotoxicity of a series of dental casting alloys in the as-cast and polished condition was determined with cell culture techniques involving phase contrast microscopy to examine cell morphology and the succinic dehydrogenase histochemical reaction to measure any ring of inhibition of Balb/c 3T3 cellular respiration around alloys. Crown and bridge casting alloys and a nickel- and a cobalt-base alloy were biocompatible in the polished condition, but less so in the as-cast condition. The only two exceptions were casting alloys containing 50-60 wt% Cu. Porcelain-fused-to-metal alloys were biocompatible in either the as-cast or polished condition. This direct contact method appeared satisfactory for evaluating biocompatibility of dental casting alloys, especially since these materials are in contact with gingival tissues.

Alloys↗
PubMed1988

Correlation of parameters used to estimate monomer conversion in a light-cured composite.

The sensitivities of Fourier transform infrared spectroscopy, Knoop hardness, water sorption, and resin leaching were compared for their ability to distinguish differences between composite samples cured through different thicknesses of overlying resin. The method developed allowed samples of light-cured composite to be made with controlled conversion for parameter testing, and eliminated effects of resin lost to slurry during polishing or an increase in conversion as a result of heat generated during grinding. Sensitivity to differences was greatest and equal for FTIR spectroscopy and Knoop hardness, while resin leaching proved to have moderate sensitivity, and water sorption none. The ability of these parameters to predict monomer conversion as measured by FTIR spectroscopy was also determined. Knoop hardness proved the best conversion predictor, resin leaching the next best, and water sorption the worst. Water sorption values did not vary with changes in specimen conversion.

Chemical Phenomena↗
PubMed1987

Properties of a modified cross-linked silicone for maxillofacial prostheses.

A dimethyl siloxane-triacetoxy terminated silane, Type A adhesive, can be modified with the base component of a medium grade polydimethyl siloxane with vinyl groups, MDX 4-4210, to produce more pliable maxillofacial prostheses. The mechanical properties of samples with various Type A adhesive/MDX 4-4210 ratios were determined. Addition of the MDX 4-4210 base decreased the hardness, modulus, and ultimate tensile strength, while the percentage elongation increased. The initial tear strength was relatively constant for the various ratios except for the 50/50 mix, where a 50% decrease was observed. The ability to obtain different mechanical properties by using various ratios of Type A adhesive and MDX 4-4210 could result in the production of maxillofacial prostheses which will more closely stimulate the properties of facial tissues.

Adhesiveness↗
PubMed1986

Accuracy and bond strength of combination agar/alginate hydrocolloid impression materials.

The accuracy and bond strength of several combinations of agar and alginate hydrocolloid impression materials were assessed. In general, there was little difference in accuracy among agar hydrocolloids. Of the alginate hydrocolloids, products J and CA produced the most accurate casts in combination with any of the agar hydrocolloids. For all product combinations, however, distortion of the diameter of the improved stone die was evident. The buccal-lingual diameter was 0.32% larger, whereas the mesial-distal diameter was only 0.06% larger. Regarding bond strength, alginate hydrocolloids P, CA, and VA in combination with agar hydrocolloids DB and DG produced the best results of all combinations tested. Thus, considering both bond strength and accuracy, product CA in combination with either DB or DG produced the best overall results.

Agar↗
PubMed1986

Accuracy of addition silicones as a function of technique.

Both addition and condensation silicones produced stone dies that were larger in diameter and shorter in height than the tooth preparation. For both materials, there was little change in the distance between stone die preparations compared with the standard. The most significant difference between types of silicone was that condensation silicones produced significantly shorter dies (-0.24% to -0.37%) than addition silicones (-0.08%). Among addition silicones, Cinch produced more than twice as much vertical change (-0.16%) than the other three products (-0.06%). Since castings made from a short die will not seat completely on the prepared tooth, these results support the use of three of the four addition silicones tested. The same accuracy of impressions was achieved for all techniques (that is, putty/wash, single mix, and double mix) when addition silicones were used, whereas the putty/wash technique produced the most accurate dies for the condensation silicones. The custom tray produced dies that were much more accurate in vertical dimensions (-0.03%) than the stock trays (-0.15% to -0.21%). Thus the custom tray is the impression tray of choice, even for addition silicones, which produce relatively little polymerization shrinkage and are dimensionally stable.

Chemical Phenomena↗
PubMed1985

Quantitative evaluation of proximal tooth movement effected by wedging: a pilot study.

The major purpose of this pilot study was to develop a method to accurately measure proximal tooth movement. A preliminary clinical evaluation of anterior and posterior tooth movement was performed with five patients. The patients were carefully chosen to eliminate as many variables as possible. The controlled patient criteria included periodontal health, proximal contacts, the age of the patients, and the teeth selected for anterior and posterior displacement. The small sample size dictates that additional experimentation must be performed before definitive conclusions can be drawn.

Adult↗
PubMed1985

Accuracy of four types of rubber impression materials compared with time of pour and a repeat pour of models.

The accuracy of four types of elastomeric impression materials was studied as a function of model location, time of pouring, and repetition of pouring. There was little change in dimension among abutment preparations for all materials, for all times of pour, and with a repeat pouring. It appears that measurements between posts are not sensitive to detecting differences between materials and that other dimensions should be examined to discern where differences might lie. The diameter of the stone post was larger than the standard for the AS and PS materials and was unchanged or smaller than the standard for the CS and the PE. The vertical dimensions of stone posts were, in general, shorter than the standard for all materials, with the AS demonstrating the smallest change. The AS and CS products demonstrated the best recovery from undercuts and the least change in dimensions between an initial and second pour of an impression. The AS and PE were the least affected with delays of 1, 4, and 24 hours in pouring the impression.

Crowns↗
PubMed1985

Evaluation of an automatic mixing system for an addition silicone impression material.

The automatic mixing system of an addition silicone impression material yields mixes and set material that comply with ADA specification no. 19 when testing is started at 0.5 minute rather than the specified 1.5 minutes. This change is reasonable because there is zero mixing time rather than the usual 45-60 second needed for standard two-paste rubber impression materials. Uniform mixing of base and catalyst occurs with the automatic system, with a fourth to a fifth as many bubbles in the mix as for comparable mixes obtained by hand spatulation. The automatic feature of the system simplifies mixing and nearly eliminates the training of assistants in the mixing of rubber impression material. The automatic mixing system is economical as it wastes only a third as much material as a typical hand-dispensing and mixing system. The properties and accuracy of the system are excellent and typical of addition silicones, including excellent recovery from deformation, low dimensional change on setting, and low flow. A wash or two-phase impression technique may be used with equal clinical accuracy.

Dental Impression Materials↗