ABSTRACT
STATEMENT OF PROBLEM: Off-axis, external forces with a moment arm on posterior restorations have not been investigated in experimental studies. PURPOSE: The purpose of this proof-of-concept study was to determine the interaction between occlusal force directed toward cuspal incline angulations with variations in base width and preparation vertical heights. Torque forces on a single crown restoration in simulated premolar and molar tooth forms were calculated for 3 different rotational axes. MATERIAL AND METHODS: Trigonometric calculations were made to determine the amount of torque generated in a simulated-crown restoration in premolar and molar tooth forms. Restorations with different cuspal incline angulations were loaded with an off-axis force of 200 N. This force was applied to 5 different cuspal incline angulations in both tooth forms at varying preparation heights. Right triangles were used to enable trigonometric computations of the resulting moment arms that accompanied the 3 rotational axes. RESULTS: The total torque values were calculated with a range from 7.5 to 372.8 Ncm. The highest levels of torque were generated in the 5-mm-high molar tooth form with a rotational axis located within the root form, perpendicular to the 45-degree cuspal incline. In general, large moment arms were generated with steep cuspal incline angulations and mid-root axis locations; the lowest torque values in all cuspal incline angulations were found in the rotational axis locations at the crown finish line. The torque values at the rotational axis finish line location were found to be greatest in the largest vertical tooth form height category (5 mm) in both tooth model sizes. CONCLUSIONS: The crown restoration cuspal incline angulations, vertical preparation heights, base widths, and rotational axis locations all played a role in the torque force levels generated, probably influencing restoration stability.
ABSTRACT
PURPOSE: The purpose of this study was to compare the dimensional accuracy, translucency, and biaxial flexural strength of milled zirconia (MZ) versus 3D-printed zirconia (PZ) discs. MATERIALS & METHODS: A circular disc measuring 14.0 mm in diameter and 1.20 mm in thickness was designed using computer-aided design (CAD) software. The resulting standard tessellation language (STL) file was used both as a control and to fabricate 36 zirconia (3Y-TZP) disc specimens (n = 36): 18 were milled (group MZ) and 18 were 3D-printed (group PZ). The diameter and thickness of each disc were measured using a digital caliper. Translucency was evaluated using a calibrated dental colorimeter. The flexural strength was determined using the piston-on-three-ball biaxial flexure test. All measurements were done by one blinded examiner. The statistical significance level was set to α = 0.05. RESULTS: The MZ discs had significantly more accurate dimensions than the PZ discs in both diameter and thickness when compared to the control CAD software-designed disc. The MZ discs exhibited significantly higher translucency (translucency parameter (TP) = 16.95 ±0.36 vs. 9.24 ±1.98) and biaxial flexural strength (996.16 ±137.37 MPa vs. 845.75 ±266.16 MPa) than the PZ discs. Finally, MZ possessed a significantly higher Weibull modulus relative to PZ. CONCLUSIONS: The results showed that the milled specimens achieved better dimensional accuracy and were more translucent, stronger, and less prone to failure than printed specimens.
ABSTRACT
AIM: The aim of this study was to digitally measure the dimensional changes in an irreversible hydrocolloid impression material (alginate) resulting from varying storage times under optimal storage conditions. MATERIALS AND METHODS: A single type V dental stone control cast was used to make 25 alginate impressions using perforated stock trays. The impressions were randomly assigned into five groups of five samples each (n=5 per group) with varying storage times: Group 1, poured at 15 minutes; Group 2, poured at one hour; Group 3, poured at 24 hours (one day); Group 4, poured at 72 hours (three days); Group 5, poured at 168 hours (seven days). All impressions were stored in sealed Ziploc® plastic bags with a wet paper towel (100% relative humidity) at room temperature and stored according to the assigned group storage times. All impressions were poured in type V dental stone according to the manufacturer's instructions. The casts were scanned with a digital 3D desktop scanner and saved as electronic stereolithography (.stl) files. Each .stl file of the scanned casts were superimposed on the .stl file of the control cast using Geomagic® Control X™ software. Three preselected fixed comparison measuring points (CMP) on each cast were compared to the control cast. Point one (CMP1) was on the midfacial surface of central incisor. Point two (CMP2) and point three (CMP3) were on the mesiobuccal proximal marginal ridge areas of third molars. The discrepancies between the files at each point were analysed with colour maps, and quantified (Table 1). The tolerance was set at ±10µm. CMP scores were analysed using one-way analysis of variance (ANOVA) and Kruskal-Wallis (K-W) non-parametric H tests. RESULTS: Average gap distances across groups ranged from 0.04mm (seven-day group) to 0.06mm (one hour and 24-hour groups). Colour maps indicated increased dimensional change with increased storage time up to one day. After three days, shrinkage up to 139µm was measured. ANOVA results for CMP1 (F[4,20] = 1.65, p = 0.020) and CMP3 (F[4,20] = 0.44, p = -0.78) were not statistically significant. Similarly, K-W results for CMP2 were not significant (χ2= 3.62, p = 0.46). CONCLUSIONS: Under optimal storage conditions, there were no significant dimensional changes in casts poured from alginate up to seven days.
