Statement of problem
Wetting of the set gypsum product used for cast articulation may result in additional expansion.
The purpose of this study was to evaluate the linear expansion of 2 type II and 1 type III gypsum products with and without addition of water after setting, over a time period of 120 hours.
Material and methods
The gypsum products tested in this study were Model Plaster, Lab Plaster, and Mounting Stone. All materials were hand mixed for 5 seconds and mechanically mixed under vacuum for another 15 seconds. The mixes were poured into a linear expansion-measuring instrument (n=20). Half of the specimens were wet with 25 ml of water immediately after the setting time recommended by the manufacturer, and at all time intervals, immediately after data collection. No treatment was performed on the remainder of the specimens. The expansion values were recorded with an expansion-measuring device over a period of 120 hours. Collected data were subjected to a 2-way repeated-measures analysis of variance (ANOVA) (α=.05).
The highest expansion values for all gypsum products were recorded at 96 and 120 hours. No significant difference in setting expansion values was recorded between these 2 time intervals for any of the materials and treatment conditions (dry vs. wet). Mean expansion values ranged between 0.08% ±0.06% for dry Mounting Stone specimens at 15 minutes, to 0.57% ±0.02% for wet Lab Plaster specimens at 96 and 120 hours. Significant differences ( P <.001) between different mounting gypsum products and time were identified. A significant difference between wet and dry specimens was not observed.
The expansion of all gypsum products used for articulation purposes was complete at 96 hours. The type III gypsum product demonstrated lower mean expansion values than type II products. Time and material were more important factors than the dry/wet condition when measuring total expansion values. Type II gypsum products demonstrated 80% or more of the total expansion values in the first 45 minutes, while type III stone demonstrated the same expansion after 24 hours. (J Prosthet Dent 2009;102:313-318)
Articulating errors in the laboratory can be minimized by selecting a gypsum product that has low setting and hygroscopic expansion and by minimizing the time for the laboratory procedures.
Diagnosis, treatment planning, and accurate execution of planned restorative procedures require that the diagnostic and the definitive casts be mounted on an articulator. Apart from the techniques used for the articulation of the casts, the use of interocclusal recording media and gypsum products which demonstrate dimensional stability over time are important.
Plaster and stone products used in dentistry are made by calcining calcium sulfate dehydrate. The principal constituent of gypsum-based products is calcium sulfate hemihydrate, which is CaSO 4 . 1/2 H 2 O. Gypsum products are classified by ADA Specification 25 into 5 types : type I: impression plaster; type II: model plaster; type III: stone; type IV: high strength, low expansion stone; type V: high strength, high expansion stone. Of these, types II and III are usually used for articulation purposes.
The articulating technique for both diagnostic and definitive casts is essentially the same. A small inaccuracy in articulated diagnostic casts may provide enough information for diagnosis and treatment planning. However, articulation of the definitive casts is more crucial, since minute errors may cause occlusal discrepancies, which can require lengthy clinical appointments or additional laboratory procedures for correction.
Gypsum products exhibit dimensional changes during setting. A decrease in the true volume of the reactants can be observed in the early phase of setting, while the mix is still fluid. However, as the reaction advances, gypsum crystals start forming and an isotropic expansion is observed. Although ADA Specification 25 requires the linear expansion measurements to be made 2 hours after mixing, authors have reported that delayed linear setting expansion can continue for up to 120 hours. In addition to the setting expansion, hygroscopic expansion is observed when the gypsum products are exposed to additional water while setting. Several theories have been introduced to explain the mechanism for hygroscopic expansion. Nevertheless, the influence of water on set gypsum products used for cast articulation has not been investigated thoroughly. Since plaster and stone used for mounting the casts are often wet with water in the early stages that follow cast articulation, during plaster addition and polishing with wet sandpaper, further investigation is needed. Any dimensional changes that may occur in the mounting plaster or stone can affect the 3-dimensional occlusal contacts of the definitive prostheses.
The purpose of this study was to measure the linear dimensional changes of 3 different gypsum products used for mounting procedures after exposing them to water treatment after setting, and to compare these changes with linear setting expansion values of specimens not exposed to water treatment, at 15 minutes, 45 minutes, and 2, 24, 72, 96, and 120 hours. The null hypothesis was that linear dimensional changes of the examined gypsum products would not be affected by the water treatment.
Material and methods
Three gypsum products used routinely for cast articulation were included in this study: Model Plaster (Heraeus Kulzer GmbH, Hanau, Germany), Lab Plaster (Dentsply Trubyte, York, Pa), and Mounting Stone (Whip Mix Corp, Louisville, Ky). The first 2 gypsum products are classified as plasters (type II) and require a powder (g) to water (ml) ratio of 100:47, while the third is classified as a dental stone (type III) and requires a powder (g) to water (ml) ratio of 100:26. Manufacturers' instructions were followed for the mixing procedures. Therefore, a 5-second hand mixing was followed by a 15-second mechanical mixing. An electronic scale (EC-411; Acculab Sartorius Group, Gotting, Germany) was used to measure the gypsum powder. Distilled water (Holyoke Distilled Water Co, Holyoke, Mass) was first measured and added in a vacuum bowl (Twister Pro; Renfert GmbH, Hilzingen, Germany). The gypsum product was then added. A spatula with a stiff blade (3R; Buffalo Dental Mfg Co, Syosset, NY) was used for a 5-second hand mixing to fully incorporate the powder into the water. Mechanical mixing under vacuum at 25 mm Hg for 15 seconds followed. The mixture was then poured into a V-shaped mold, which was part of an expansion-measuring device (EMI 100; SAM Präzisiontechnik GmbH, Munich, Germany). The mold was lined with a rubber dam (Hygenic Dental Dam; Coltène/Whaledent, Inc, Cuyahoga Falls, Ohio). The dial gauge attached to the device was calibrated to 0.01 mm. Twenty specimens of each material were fabricated to be 100 mm in length and triangular in cross-section (33 × 50 × 33 mm) ( Fig. 1 ). The guidelines of ADA Specification 25 and International Organization for Standardization (ISO) 6873 were followed for the fabrication of the specimens. Half of the specimens were wet with 25 ml of distilled water immediately after the setting time recommended by the manufacturer: after 8 minutes for the Model Plaster (Heraeus Kulzer GmbH) and the Lab Plaster (Dentsply Trubyte), and after 5 minutes for the Mounting Stone (Whip Mix Corp). The same quantity of water (25 ml) was also used for wetting the specimens at all time intervals immediately after data collection. Water was poured slowly over the treated specimens to facilitate full absorption. No treatment was rendered for the remaining specimens.
Measurements for each specimen were made at 7 time intervals: 15 minutes, 45 minutes, and 2, 24, 72, 96, and 120 hours. Specimen treatment and data collection were accomplished by the same operator. The linear dimensional change raw data was converted to a percentage value by the following equation:
(L c /L o ) × 100% where L c represents the specimen's change in length, and L o represents the original specimen length in millimeters. The temperature of the distilled water was 23°C ±2°C. Room temperature (21°C ±1°C) and relative humidity (50% ±10%) were recorded each day throughout the experiment. Two-way repeated-measures analysis of variance (ANOVA) (α =.05) was used to determine statistically significant differences among different gypsum products and different treatment conditions over time.