The influence of the impression tray on the accuracy of impressions for crown and bridge work--an investigation and review

OBJECTIVE: To evaluate the accuracy of working casts for crown and bridge restorations made from twin mix putty/wash silicone elastomeric impression materials using different types of stock tray. DESIGN: A master cast was made from ivorine teeth in a mandibular model. Three teeth were prepared for full crown restorations to simulate a single unit and a three unit bridge. Impressions were taken using four different types of impression tray and two different viscosities of silicone putty, together with a wash. Full metal crowns were constructed and seated on duplicate master casts and measurements made of the marginal discrepancies. SETTING: The work was carried out in a dental technology laboratory in a university department of restorative dentistry in 1995/1996. RESULTS: Metal and rigid plastic impression trays showed the least discrepancy (about 50 microns), the latter slightly greater with the normal putty than the soft one. Impressions taken with flexible plastic trays produced considerable discrepancy with both putties, approximately 210 microns and 180 microns respectively. CONCLUSIONS: Metal and rigid plastic stock trays give greater accuracy in the putty/wash silicone twin mix impression technique compared with flexible plastic ones for crown and bridge work. Those using the latter should be aware of their shortcoming in this respect.     

塑料模具标准件顶杆整体淬火用65MnV钢的组织与性能

分析了塑料模具标准件顶杆的工作条件和性能要求,研究了顶杆整体淬火用钢６５ＭｎＶ的热处理工艺,组织和力学,工艺性能,并与目前国内常用顶杆用钢进行了比较,同时探讨了６５ＭｎＶ钢的强韧化机制。     

Computer-based production of bite mark comparison overlays

Bite mark comparison protocols include measurement and analysis of the pattern, size, and shape of teeth against similar characteristics observed in an injury on skin or a mark on an object. The physical comparison of tooth position often depends upon transparent acetate overlays to detect similarities or differences between the teeth and the bite mark. Several methods are used to produce life-sized comparison overlays. The perimeter of the biting edges of the anterior teeth are usually recorded to produce facsimile images called hollow volume overlays. Some investigators hand-trace these outlines from dental study casts, or from bite exemplars produced in wax, styrofoam, or similar materials. Some use hand-traced outlines from xerographic images produced with office photocopiers that are calibrated to produce life-sized final images. Others use radiographic images and toneline photography of wax exemplars filled with radio-opaque materials, such as metal filings or barium sulfate. Dependence upon subjective input by the odontologist to trace these images manually is considered problematic. This is because the errors incorporated at any production stage are increased in the final product. The authors have developed a method to generate accurate hollow volume overlays using computer-based techniques. A PowerPC Macintosh computer, flatbed scanner, and Adobe Photoshop (a popular graphical interface application) are used to acquire, select, arrange and export detailed data from class and individual characteristics of a suspect's teeth to acetate film loaded in a high-resolution laser printer. This paper describes this technique to enable the odontologist to produce high-quality, accurate comparison overlays without subjective input.     

A modified occlusal registration and implant transfer technique

The technique for the transfer of implant and abutment position to a working cast has been hindered by multiple transfers and record reproductions. These serve only to delay completion of the patient's prosthetic requirements. A transfer technique that uses custom impression trays fabricated from surgical templates allows for a single-visit transfer of centric occlusion, vertical dimension, tooth position, and implant or abutment location in one procedure. This streamlines treatment and allows for quicker delivery of final prostheses.     

Mutations in the N-terminal globular domain of the type X collagen gene (COL10A1) in patients with Schmid metaphyseal chondrodysplasia

STATEMENT OF PROBLEM: Improved dental stone has been widely used for producing dies for the fabrication of restorations with the lost-wax technique. Improved dental stone is used for several reasons, but it is selected most often because of its reasonable cost, ease of use, and ability to produce consistent results. PURPOSE: This study evaluated the ability of an epoxy resin die material and a type IV dental stone to dimensionally reproduce a custom-fabricated metal die. MATERIAL AND METHODS: Dies were fabricated and measurements were made from three reference lines. Measurements were repeated three times for the master die and for the specimen dies. A mean measurement and percent relative change for each dimension was calculated. RESULTS: A significant difference in the relative change for die height was found between the groups studied (p < 0.003). CONCLUSIONS: This epoxy die system will provide a degree of dimensional accuracy comparable to gypsum when used with addition silicone impression material.     

Placement of multiple and different stent types for very long dissections during coronary angioplasty

A coordinate measurement machine with laser probe was used to measure the vertical distortion of the casts produced by use of three types of impression materials (irreversible hydrocolloid, condensation silicone, and addition silicone) and two types of trays (stock and custom trays). Results indicated that all impression groups showed positive vertical distortion (ranging from 0.00566 to 0.30299 mm) at the edentulous ridges and palatal area. The amount of the vertical distortion was greatest at the palatal area and was followed by the high edentulous ridge and the low edentulous ridge. Addition silicone, with either custom tray or stock tray, was the most accurate impression material. Condensation silicone was more accurate than irreversible hydrocolloid in custom tray impression. However, in stock tray impression the irreversible hydrocolloid was more accurate than the condensation silicone. The results suggest that, with careful manipulation, irreversible hydrocolloid with stock tray impression may provide a satisfactory cast for fabricating the framework of a distal extension removable partial denture.     

Hyphenated history: Osgood-Schlatter disease

The dental technician must have an accurate impression of gingival contour, in order to apply porcelain correctly to a metal-ceramic restoration. This article describes a technique for making an accurate model of the soft tissues around a restoration. The technique involves taking a transfer impression and making a soft tissue model.     

Fumonisin B1 in maize for food production imported in The Netherlands

STATEMENT OF PROBLEM: When compared with waxes and prefabricated components made of dental alloys, processing resins is difficult and often results in a less accurate fit. The exact linear dimension is hard to realize, as the materials required for a dental casting are adjusted to dental waxes, and usually there are no special processing instructions for these resins. PURPOSE: The surface structure of 16 residue-free burning resins was determined to find which could enlarge the range of application for waxes in the dental casting technique. MATERIAL AND METHODS: A total of 480 specimens (30 castings for each of 16 resins) were marked with an individually cut pyramid, which left an impression in the resin. This was distinctly visible in the metal after casting in "the lost form." The surfaces of these 480 specimens were measured in three dimensions with a positioning aid "before casting in resin" and "after casting in metal" by the Perthometer micrometer. All specimens were treated identically by the lost-wax process by using the rotation symmetry of the muffle. The differences in the surface data before casting in resin and after casting in metal characterize the quality of the particular resin. RESULTS: Surface roughness differed between the 16 resins in the range of micrometer. CONCLUSIONS: With the dental casting technique, it is recommended that some resins replace waxes or wax-resin compositions.     

Clinical significance of surface activation of silicone impression materials

OBJECTIVES: Surface activation of silicone impression materials enhances impression accuracy as well as the quality of stone casts. This study examined the relation between impression quality and the hydrophilicity of silicone impression materials used in a two-stage putty and wash technique. METHODS: The study included 14 silicone impression materials with and without surface activation. The contact angles of the light body materials were measured using deionised water. Free surface energies were calculated. A total of 140 impressions of wet intact permanent molar teeth were examined for definition of the gingival sulcus reproduction and then classified in a quality ranking. The percentage of the sulcus reproduction ability of each material was calculated from the sulcus depths of cross-sectioned resin casts from the impressions and clinically measured sulcus depths. RESULTS: Free surface energies (mJ/m2) of hydrophobic materials (contact angle > 90 degrees) ranged from 16.8 to 26.8 mJ/m2. Surface activated materials (contact angle < 90 degrees) exhibited free surface energies from 38.1 to 55.7 mJ/m2. Not all materials described as surface activated showed hydrophilic properties. Medians of impression quality ranking ranged from 1.75 (best) to 13.25 (worst), mean sulcus reproduction ranged from 30 to 52%. Significant differences (P < 0.05) in quality ranking and sulcus reproduction were found between certain materials but correlations were found neither between sulcus reproduction and free surface energy nor between quality ranking and free surface energy. CONCLUSIONS: For clinical impression recording, surface activation seems to be of minor relevance, at least in the two-stage putty and wash technique investigated and is superimposed by other material properties.     

Infectious complications after OKT3 induction in liver transplantation

TEN PATIENTS WHO REQUIRED two or more anterior teeth extractions were utilized in this study. Extraction procedures were carried out with a full thickness surgical flap approach. After flap reflection, teeth were removed with a minimum of trauma to the surrounding bone. Following extraction silicone-based impression techniques were used to produce a model of the alveolar process and small metal pins were placed in the alveolus to be used as fixed points to make measurements of ridge dimensions. One socket was covered with an expanded polytetrafluoroethylene (ePTFE) barrier membrane (experimental site); the other socket was a conventional control. The soft tissue flaps were then mobilized using periosteal releasing incision and the wound closed with ePTFE mattress sutures. Six months following extraction, patients were treated with flap surgery to expose both extractions sites to remove the ePTFE membranes and to measure ridge dimensions using the pins as fixed points. Clinical and model measurements have shown statistically significant better ridge dimensions at experimental sites than at control (P < or = 0.05). Three patients with exposed membranes had similar dimensional changes as controls. Results from this study suggested that this improved technique offers a predictable alveolar ridge maintenance enhancing the bone quality for dental implant procedures and esthetic restorative dentistry.     

Patients with epilepsy who die suddenly have cardiac disease

This paper describes the design and fabrication of a soft liner-retained, implant-supported overdenture used in 10 patients over a period of 1 to 6 years. A final complete denture impression is made of the dental arch, including implant abutments extending 5 mm above the gingival tissue. The master cast is fabricated with actual abutments placed in the impression. After the final wax try-in, the denture is flasked in the usual manner with the metal abutments in place. Before packing with acrylic resin, plastic tubing 1 to 2 mm thick is placed around each abutment. The denture is trial packed and allowed to bench cure over night. The plastic tubing is then removed and a soft denture lining material is placed in these areas only. The flask is closed and cured according to the manufacturer's recommendation. The denture is delivered as a conventional overdenture.     

Impression management ability in psychiatric hospital samples and normals.

As a partial test of the view that schizophrenia is essentially a manipulatory phenomenon, the abilities of normal, neurotic, alcoholic-character disorder, and single and married schizophrenic groups (N = 180) to manage the impressions they make on others were compared. Ss were administered a semistructured interview twice-once each under instructions to make as bad and as good impressions as possible. All groups proved capable of managing the impressions they made to some degree. However, the single schizophrenics produced higher scores than any of the patient samples. Results argue against the hypothesis that schizophrenics are peculiarly capable impression managers. Moreover, the impression management scores of the groups did not change differentially in response to threat, casting some doubt on the view that schizophrenics are peculiarly inclined to produce spurious abnormal behavior to escape painful stress. Results contradict the view that the behaviors characterized as schizophrenic arise from special impression management tendencies rather than cognitive deficits. (15 ref) (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Indirect Methods of Detecting and Evaluating Inclusions in Steel A Review

Thenonmetallicinclusionisasignificantprob lemincaststeelsleadingtoexcessivecastingrepairs orrejectedcastings.Inclusionsinsteelandinclu sions relateddefectsinsteelproductshavebeenre viewedelsewhere[1].Measurementtechniquesofin clusionsrangefromdirectmethods,whichareaccu ratebutcostly,toindirectmethods,whicharefast andinexpensive,butareonlyreliableasrelativein dicators.Indirectmethodsincludetotaloxygen measurement,nitrogenpickup,slagcomposition measurement,andsoon.Anextensivereviewon indirectmet…     

Determination of interfacial heat transfer coefficients in casting and quenching using a solution technique for inverse problems based on the boundary element method

Both casting and quenching are processes during which several physical phenomena like heat transfer, fluid flow, phase transformation,etc. interact in a complex manner. To obtain a nu-merical model which is capable of accurately simulating the actual process, one has to be able to quantify all the parameters affecting the process. One parameter which substantially influ-ences heat transfer in these processes is the heat transfer coefficient at the interface between the mold and the metal in casting and that between the metal and the quenchant in quenching. The heat transfer coefficient could vary on the surface of a casting or a quench metal both spatially and with time. Its accurate determination is imperative for a realistic simulation of these processes. In this work, an algorithm based on the boundary element technique is proposed to solve for the interface heat transfer coefficient. The problem is cast as one of inverse heat conduction in two dimensions where some of the boundary conditions, namely, the previously mentioned heat transfer coefficients, are unknowns. Since it is the boundary properties that are being determined, the boundary element method (BEM) is the most suitable technique to use. The algorithm uses experimentally measured temperature data inside the domain to determine the interface heat transfer coefficient. The technique is outlined in detail and some casting and quenching examples are presented to demonstrate its capability.     

Marginal accuracy of indirect provisional restorations fabricated on poly(vinyl siloxane) models

Indirect interim restorations have been traditionally fabricated on stone casts. Recently, flexible models fabricated from poly(vinyl siloxane) impression material have been suggested in place of stone casts. This study compared the marginal accuracy of indirect single-unit provisional restorations fabricated on stone casts with units made on poly(vinyl siloxane) impression material models. The mean marginal discrepancy of provisional restorations was compared for restorations fabricated from stone, low-viscosity poly(vinyl siloxane), and medium-viscosity poly(vinyl siloxane). Ten restorations were also fabricated directly on a master metal model (control). The mean marginal discrepancies were: control = 0.161 mm; stone model = 0.180 mm; low-viscosity poly(vinyl siloxane) = 0.245 mm; and medium-viscosity poly(vinyl siloxane) = 0.157 mm. The mean marginal discrepancy for the low-viscosity poly(vinyl siloxane) models was significantly higher than the other three groups. No significant difference was found between the control, stone cast, and medium-viscosity poly(vinyl siloxane) model groups.     

Evaluation of factors affecting the accuracy of impressions using quantitative surface analysis

Impression material goes from a plastic to an elastic state during setting. Movement of the impression and excessive seating pressure during this transition can cause distortion in the impressions. The purpose of this study is to determine if the impression distortion is related to movement during setting or to distortion of the putty phase in the two-step impressioning technique. A master model of a maxillary quadrant of teeth was impressed using four different procedures: 1) one-step technique without movement (1S-NM); 2) one-step technique with movement (1S-M); 3) two-step technique without movement (2S-NM); and 4) two-step technique with movement (2S-M). An artificial oral environment and surface analysis technique of the Minnesota Dental Research Center for Biomaterials and Biomechanics were used to produce the impressions and measure their accuracy. A digitized image of the first premolar of the master model was aligned with a digitized image of the first premolar of each epoxy model using AnSur. The root mean squared difference (RMS) between the aligned images is a measure of the distortion. The corresponding RMS values for the different methods were: 1S-NM = 23.7 +/- 9.21; 1S-M = 20.4 +/- 3.9; 2S-NM = 20.5 +/- 7.7; 2S-M = 21.3 +/- 4.4. Statistical analysis using a two-way analysis of variance showed no difference at the 0.05 level of significance. Pairwise comparison using the Tukey method showed that neither technique (one-step vs two-step) nor movement is a significant factor. These results showed that low seating pressure will not cause any greater distortions in the two-step impression technique than in the one-step technique, and minor movement during the setting of the impression material will no cause distortion.     

Lessons Learned from Field and Laboratory Testing of a DBR Project

Where faulting takes place due to the absence of dowel bars and inadequate subbase support in jointed concrete pavement (JCP), dowel bar retrofit (DBR) is used to improve load transfer efficiency (LTE) and to prevent further faulting of slabs at transverse joints. Even though DBR generally improves LTEs and overall performance of JCPs, not all DBR projects have been successful. Faulting reoccurred within 2?years after DBR treatment on US59 in Texas. An investigation from the cores taken in the project revealed excessive voids under a dowel bar due to poor consolidation of the grouting material. A laboratory investigation was performed to determine the most critical factors for adequate consolidation of grouting materials in DBR. Typical rapid-setting grout materials widely used in DBR were selected and full-scale specimens were made for evaluations. Four testing variables for consolidation performance were investigated: time of placement after mixing, vibration time, slot width, and maximum aggregate size. Maximum aggregate size and slot width were not critical factors for consolidation performance of grout. The most significant factor was vibration time. Twenty s of vibration is recommended. Placement time was also an important factor, with grout materials placed after initial set performing poorly. Delayed placement of grout materials without vibration led to the most voids under the dowel bars.     

A numerical simulation of the D.C. continuous casting process including nucleate boiling heat transfer

The steady-state thermal problem associated with the direct-chill continuous casting of A6063 aluminum cylindrical ingots is solved using the numerical finite element technique. Excellent correlation is demonstrated between the numerical model and experimental data from ingots cast at two different speeds. By application of the model, effective heat transfer coefficients are calculated as a function of vertical position on the outside surface of the ingot. It is shown that direct application of these coefficients to the modeling of different casting situations will produce substantial errors in the region in which heat transfer is by nucleate boiling. Using theories of nucleate boiling with forced convection and film cooling, a method is developed to calculate the external boundary conditions in the submold region of the ingot, thus making it possible for the first time to define explicitly all of the thermal boundary conditions associated with this casting configuration. These theories are incorporated into the numerical model, and a subsequent simulation shows excellent agreement with experimental data from a third ingot.     

Semi‐Solid Rheoforging of Steel

To produce steel components with complex shapes excessive machining is necessary frequently since high pressure die casting of steel is not industrially applied. Forming steel in the semi‐solid state can in principle produce new components and geometries which cannot be realised by conventional closed die forging. Semi‐solid forging of steel combines the possibility of producing geometries not conventionally forgeable in one forming operation and of adding further functions during the same operation. In previous investigations on thixoforming of steels, the semi‐solid steel was generated by reheating precursor material billets. An alternative approach for generating semi‐solid steel from the liquid state with subsequent forging operation is presented in this paper for the first time. The steel grades X210CrW12 cold work tool steel and 100Cr6 bearing steel are molten and driven into a globular semi‐solid state using a cooling slope and a cup. By cooling the steel into the semi‐solid range instead of heating it, the required process temperatures are lower than in the process route via heating. Therefore, the load on the dies in a semi‐solid forging operation is decreased. Suggestions for the respective layout of the process are made for both steel grades. Future potentials and challenges to be solved are discussed, showing advantages especially in the field of high melting point alloys such as steels. This technique enables to produce pre‐shaped semi‐solid billets to optimise the materials flow and the homogeneity of the mechanical properties.