Measurement of the accuracy of dental working casts using a coordinate measuring machine.

Background/Aim: Dental impressions present a negative imprint of intraoral tissues of a patient which is, by pouring in gypsum, transferred extraorally on the working cast. Casting an accurate and precise working cast presents the first and very important step, since each of the following stages contributes to the overall error of the production process, which can lead to inadequately fitting dental restorations. The aim of this study was to promote and test a new model and technique for in vitro evaluation of the dental impression accuracy, as well as to asses the dimensional stability of impression material depending on the material bulk, and its effect on the accuracy of working casts. Methods: Impressions were made by the monophasic technique using the experimental master model. Custom trays with spacing of 1, 2 and 3 mm were constructed by rapid prototyping. The overall of 10 impressions were made with each custom tray. Working casts were made with gypsum type IV. Measurement of working casts was done 24 h later using a co-ordinate measuring machine. Results: The obtained results show that the working casts of all the three custom trays were in most cases significantly different in the transversal and sagittal planes in relation to the master model. The height of abutments was mainly unaffected. The degree of convergence showed certain significance in all the three custom trays, most pronounced in the tray with 3 mm spacing. Conclusion: The impression material bulk of 1­3 mm could provide accurate working casts when using the monophasic impression technique. The increase of the distance between abutment teeth influences the accuracy of working casts depending on the material bulk. [Projekat Ministarstva nauke Republike Srbije, br. TR 35020: Research and development of modelling methods and approaches in manufacturing of dental recoveries with the application of modern technologies and computer aided systems]

A Proposed Methodology to Assess the Accuracy of 3D Scanners and Casts and Monitor Tooth Wear Progression in Patients.

PURPOSE: The aim of this study was to detail and assess the capability of a novel methodology to 3D-quantify tooth wear progression in a patient over a period of 12 months. MATERIALS AND METHODS: A calibrated stainless steel model was used to identify the accuracy of the scanning system by assessing the accuracy and precision of the contact scanner and the dimensional accuracy and stability of casts fabricated from three different types of impression materials. Thereafter, the overall accuracy of the 3D scanning system (scanner and casts) was ascertained. Clinically, polyether impressions were made of the patient's dentition at the initial examination and at the 12-month review, then poured in type IV dental stone to assess the tooth wear. The anterior teeth on the resultant casts were scanned, and images were analyzed using 3D matching software to detect dimensional variations between the patient's impressions. RESULTS: The accuracy of the 3D scanning system was established to be 33 µm. 3D clinical analysis demonstrated localized wear on the incisal and palatal surfaces of the patient's maxillary central incisors. The identified wear extended to a depth of 500 µm with a distribution of 4% to 7% of affected tooth surfaces. CONCLUSION: The newly developed 3D scanning methodology was found to be capable of assessing and accounting for the various factors affecting tooth wear scanning. Initial clinical evaluation of the methodology demonstrates successful monitoring of tooth wear progression. However, further clinical assessment is needed.

Evaluation of physical-mechanical properties of 3 brands of plaster of Paris in the Kenyan market.

In this study, 3 brands of plaster of Paris that are available in the Kenyan Market for prosthetics and dental rehabilitation uses were subjected to physical-mechanical tests. The particles in two of the brands that had been imported were finer than a locally made plaster since less than 1.41% of their particles were larger than 150 microns compared to 15.2% in the local plaster. The range of their setting times was between 7.5 and 27.5 min. and was within the International Standards Organisation (ISO) recommendation. The water:power ratios required to produce plaster mixes of standard consistency for the locally made plaster was about 1.5 times of other brands. In compression, the mean 1-hour strength of local plaster was 2.5 MN/m2 (Mega Newtons per square metre) compared to 6.81 MN/m2 and 8.95 MN/m2 for the other brands. The differences between these strength values were highly significant (p less than 0.01). There were no significant differences in deformation of fracture for the 3 brands whose range was 1.32% to 1.71%. In order to obviate the need to import plaster, strict manufacturing techniques of locally made plaster are recommended.

A comparison of the accuracy of polyether, polyvinyl siloxane, and plaster impressions for long-span implant-supported prostheses.

PURPOSE: The purpose of this study was to compare the capacity of different impression materials to accurately reproduce the positions of five implant analogs on a master model by comparing the resulting cast with the stainless steel master model. The study was motivated by the knowledge that distortions can occur during impression making and the pouring of casts and that this distortion may produce inaccuracies of subsequent restorations, especially long-span castings for implant superstructures. MATERIALS AND METHODS: The master model was a stainless steel model with five implant analogs. The impression materials used were impression plaster (Plastogum, Harry J Bosworth), a polyether (Impregum Penta, 3M ESPE), and two polyvinyl siloxane (PVS) materials (Aquasil Monophase and Aquasil putty with light-body wash, Dentsply). Five impressions were made with each impression material and cast in die stone under strictly controlled laboratory conditions. The positions of the implants on the master model, the impression copings, and the implant analogs in the subsequent casts were measured using a coordinate measuring machine that measures within 4 µm of accuracy. RESULTS: Statistical analyses indicated that distortion occurred in all of the impression materials, but inconsistently. The PVS monophase material reproduced the master model most accurately. Although there was no significant distortion between the impressions and the master model or between the impressions and their casts, there were distortions between the master model and the master casts, which highlighted the cumulative effects of the distortions. The polyether material proved to be the most reliable in terms of predictability. The impression plaster displayed cumulative distortion, and the PVS putty with light body showed the least reliability. CONCLUSIONS: Some of the distortions observed are of clinical significance and likely to contribute to a lack of passive fit of any superstructure. The inaccuracy of these analog materials and procedures suggested that greater predictability may lie in digital technology.

Accuracy of three implant impression techniques with different impression materials and stones.

The purpose of this study was to compare the accuracy of casts made using three different impression techniques to obtain an accurate definitive cast for fabrication of multiple-implant prostheses. Twelve experimental groups were formed combining the following conditions: three impression techniques, two impression materials, and two cast materials. The main effects of the three factors were analyzed by three-way analysis of variance using the full factorial general linear model between factors. The results showed that there were no significant differences in mean values for the transferred dimensions between the control and experimental groups. None of the measurements in the horizontal plane of the definitive casts demonstrated significant differences among the impression techniques with different impression and cast materials (P > .01).

Distribution of rare earth elements in marine sediments from the Strait of Sicily (western Mediterranean Sea): evidence of phosphogypsum waste contamination.

Concentrations of rare earth elements (REE), Y, Th and Sc were recently determined in marine sediments collected using a box corer along two onshore-offshore transects located in the Strait of Sicily (Mediterranean Sea). The REE+Y were enriched in offshore fine-grained sediments where clay minerals are abundant, whereas the REE+Y contents were lower in onshore coarse-grained sediments with high carbonate fractions. Considering this distribution trend, the onshore sediments in front of the southwestern Sicilian coast represent an anomaly with high REE+Y concentrations (mean value 163.4 µg g(-1)) associated to high Th concentrations (mean value 7.9 µg g(-1)). Plot of shale-normalized REE+Y data of these coastal sediments showed Middle REE enrichments relative to Light REE and Heavy REE, manifested by a convexity around Sm-Gd-Eu elements. These anomalies in the fractionation patterns of the coastal sediments were attributed to phosphogypsum-contaminated effluents from an industrial plant, located in the southern Sicilian coast.

[Effect of porosity on strength and hardness of dental stone (author's transl)].

The strength and hardness of dental stone were found to have better a correlation with porosity than with water-powder ratio. Within the range of porosity of less than 34 vol-%, the strength and hardness varied logarythmically with a linear change in porosity.

[Study of the linear variation in certain dental plasters and self-polymerizing resins]. / Etude de la variation linéaire de quelques plâtres et résines autopolymérisantes

Dental plasters and self-curing acrylic resins are tested. One must pay attention to the setting expansion of plasters and to the setting contraction of acrylic resins which may be very important if these materials are used without care.

[Comparative material testing of dental impression and articulation plasters according to German Industrial Standard DIN 13911]. / Vergleichende Untersuchungen an zahnarztlichen Abdruck- und Artikulationsgipsen nach DIN 13911.

The purpose of the present investigation was a comparison of three special articulation plasters and three impression plasters. As DIN 13911 does not refer to articulation plasters, the inspection as well as the testing of pouring time, setting time, setting expansion and compressive strength using standard and manufacturer's consistence as carried out conforming to the guidelines for impression plasters. None of the tested plasters could meet the requirements of inspection in all points. Considering the physical parameters all plasters are up to standard, while Alpa quickstone and Arti-Plaster exhibited very low expansion and high compressive strength values. Although the compressive strength standard needs redefining, the value of a standardized testing technique for dental plasters is evident.

[Abrasion in modeling materials]. / Abrieb an Modellwerkstoffen.

Ten frequently used hard plasters (Types 3 and 4 in accordance with DIN 13911) and 5 current modelling plastics have been investigated. Abrasion with testing forces of 1, 2 and 5 N were determined on samples 2 mm broad and about 12 mm long. The amount of abrasion could be calculated from the loss in weight. Only Kol-Dur and the plastics Diemet, Impredur and Alpha-Die are sufficiently reliable not to become damaged by abrasion on wax modelling (abrasion less than 5 micrometer). Hard plasters are not hard enough to resist abrasion. Taking into account the bending strength and modulus of elasticity, only Kol-Dur and Diemet can be recommended. Because of the improved dimensional stability of the hard plasters, Alca Dental Stone, Begolith, Duralit, Tewstone or Ferrodur can be recommended if the bending properties and a possible slight abrasion are taken into consideration. With the hard plasters, damage of more than 10 micrometer is easily possible.

[Comparative material studies on dental stones in accordance with DIN 13911]. / Vergleichende werkstoffkindliche Untersuchungen an zahnärztlichen Hartgipsen nach DIN 13911.

It was the object of this investigation to compare nine high-strength stone plasters in terms of visual appearance, pouring time, setting time, setting expansion, compressive strength and detail reproduction in accordance with DIN 13911 and ISO 6873 using both standard consistency as indicated in the industrial standard and the water-to-powder ratio as indicated by the manufacturer. None of the tested stones met the visual requirements in all points. Regarding their physical properties, six tested stones were up to standard. Three stones failed to meet the DIN and ISO requirements. Following the manufacturer's mixing instructions resulted in a significant improvement in material properties as against standard consistency. Disregarding some minor improvements required, DIN 13911 and ISO 6873 are useful standards for a standardized comparison of dental plasters.

Type III gold alloy complete crowns cast in a phosphate-bonded investment.

A phosphate-bonded investment and a calcium sulphate-bonded investment were evaluated for the surface smoothness and marginal fit they impart to type III gold castings. The phosphate-bonded investment was used with burnout temperatures of 900 degrees F and 1,300 degrees F. The results of this study lead to the following conclusions: 1. The marginal fit with the phosphate-bonded investment method was superior to that obtained with the calcium sulphate-bonded investment. 2. Surface roughness was greater for the castings made with the phosphate-bonded investment when measured with SEM photographs and visual observation. 3. The phosphate bonded-investment increased the incidence of nodules on the surface of the castings. 4. No correlation was demonstrated between marginal fit and surface roughness or the incidence of nodules on the castings.