Dimensional accuracy of stone casts made from silicone-based impression materials and three impression techniques.
The aim of this study was to measure and compare the dimensional accuracy of stone casts of a partially edentulous mandibular arch made by two condensation silicones, two addition silicones and three impression techniques (1-step putty/light-body, 2-step putty/light-body and monophase technique). A partially edentulous standard stainless steel mandibular arch cast was obtained with reference points on the permanent mandibular right and left canines, and permanent mandibular right and left second molars. The anteroposterior distances (between the mandibular left canine and second molar and between the mandibular right canine and second molar) and transverse distances (between the mandibular left and right canines and between the mandibular left and right second molars) of stainless steel cast were measured by a microscope at 30× magnification and 0.5 µm accuracy. All impressions were made with condensation or addition silicones. The 1- and 2-step putty/light-body impressions were accomplished with putty and light-body materials and the monophase impressions with light-body material only. After the impression procedures, accuracy of each material and technique was assessed measuring the stone casts poured (n=5) from the impressions, by the same microscope. The differences between the values of stone cast and stainless steel cast were calculated, presented as percentages and analyzed statistically by two-way ANOVA and the means compared by Tukey's test (α=0.05). All distances measured on stone casts showed altered dimensions with significantly negative linear changes (shrinkage) as compared to those of the stainless steel cast. The stone casts made from the addition silicones were dimensionally more accurate. No differences were found among the impression techniques.
Analysis of linear dimensional change of different materials used for casting dental models: plaster type 4, nanocomposites carbon nanostructures, polyurethane resin and epoxy resin
Prosthetic dentistry is key to restoring function in patients with missing teeth. During the prosthetic process, controlling dimensional changes of the materials used is paramount for treatment success. Therefore, working models should have dimensions as close as possible to those of the teeth. Dental casting is a critical stage in the process of prosthetic rehabilitation.1 The development of materials and techniques that ameliorate both impressions and cast models are therefore essential for clinical success. The fabrication of an accurate casting is dependent on many factors including impression material and technique, accuracy of die material, and waxing and casting procedures.2 Plasters routinely used to make working models include synthetic or natural plaster type 4 and plaster type V.3 Plaster products are widely used in prosthetic dentistry, despite some limitations4 such as5 low fracture resistance, dimensional instability, technical sensitivity and low wear resistance. Authors have claimed that dental plasters are not able to reproduce some details observed in elastomers.6 Many types of materials have been investigated in the last three decades for use as die materials.7 Certain mechanical features of resin-based die materials have focused increased attention upon these substances as potential substitutes for gypsum casts.4,8 Polyurethane resin models have been introduced9 as a viable alternative to stone plasters models and a possible alternative to plaster type IV.10 The need for quality and durable prosthetic work has encouraged the specialty of prosthodontics to research better materials in the process of making high quality aesthetic and functional appliances. This paper presents a comparison between models made from two types of plaster, epoxy resin and a polyurethane resin in terms of linear dimensional change.
Mississippian Barnett Shale: Lithofacies and depositional setting of a deep-water shale-gas succession in the Fort Worth Basin, Texas
The Mississippian Barnett Formation of the Fort Worth Basin is a classic shale-gas system in which the rock is the source, reservoir, and seal. Barnett strata were deposited in a deeper water foreland basin that had poor circulation with the open ocean. For most of the basin's history, bottom waters were euxinic, preserving organic matter and, thus, creating a rich source rock, along with abundant framboidal pyrite. The Barnett interval comprises a variety of facies but is dominated by fine-grained (clay- to silt-size) particles. Three general lithofacies are recognized on the basis of mineralogy, fabric, biota, and texture: (1) laminated siliceous mudstone; (2) laminated argillaceous lime mudstone (marl); and (3) skeletal, argillaceous lime packstone. Each facies contains abundant pyrite and phosphate (apatite), which are especially common at hardgrounds. Carbonate concretions, a product of early diagenesis, are also common. The entire Barnett biota is composed of debris transported to the basin from the shelf or upper oxygenated slope by hemipelagic mud plumes, dilute turbidites, and debris flows. Biogenic sediment was also sourced from the shallower, better oxygenated water column. Barnett deposition is estimated to have occurred over a 25-m.y. period, and despite the variations in sublithofacies, sedimentation style remained remarkably similar throughout this span of time.