Surface roughness of impression materials and dental stones scanned by non-contacting laser profilometry.

OBJECTIVE: To analyze differences in the way dental materials digitize on a non-contacting laser profilometer (NCLP). METHODS: Three Type IV dental stones and 15 impression materials were mixed according to the manufacturer's instructions and expressed against a glass block to record its surface characteristics. From each material an area of 6 x 40 mm was scanned on the NCLP and the Ra, Rq and Rt roughness values measured from 20 randomly selected transverse profiles. The surface of the impression materials was subsequently poured in Moonstone (Bracon Ltd., Etchingham, England) dental stone and the same roughness values obtained from these casts. Differences in roughness values from the dental materials were compared using ANOVA and differences in roughness between impression materials and the Moonstone casts compared using paired t-tests. RESULTS: There were significant differences in roughness values between individual materials within each type (impression material or dental stone) (p<0.05). The roughness of the dental stones varied between Ra=0.87 and 0.99 microm, Rq=1.09 and 1.23 microm, and Rt=5.70 and 6.51 microm. The roughness values of the impression materials varied between Ra=0.75 and 4.56 microm; Rq=0.95 and 6.27 microm and Rt=4.70 and 39.31 microm. Darker materials showed higher roughness values compared to lighter materials (p<0.05). The roughness of the Moonstone casts varied between Ra=0.80 and 0.98 microm; Rq=1.01 and 1.22 microm, and Rt=5.04 and 6.38 microm. Roughness values of some impression materials were statistically significantly lower when the surface was reproduced in Moonstone (p<0.01). SIGNIFICANCE: Digitization of dental materials on optical profilometers was affected by color and transparency.

Effect of GC METALPRIMER II on bond strength of heat-cured acrylic resin to titanium alloy (Ti-6Al-4V) with two different surface treatments.

This study investigated the bond strength of heat-polymerized acrylic resin to titanium alloy using a proprietary bonding agent (GC Metalprimer II). Two surface treatments (sandblasted or roughened with a tungsten carbide bur) were compared for their effect on bond strength with or without thermal cycling. Eighty specimens of heat-polymerized acrylic resin bonded to titanium alloy (Ti-6Al-4V) were prepared: 20 specimens (control) and 60 used a bonding agent. Four-point bend testing was used to record the load at failure. A chemical bond between heat-polymerized resin and alloy was achieved using GC Metalprimer II with both surface treatments. Reduced failure loads were recorded after thermal cycling. The predominant mode of failure was cohesive. GC Metalprimer II was effective in achieving a chemical bond with either the sandblasted or roughened surfaces.

The constitution, physical properties and biocompatibility of modified accelerated cement.

OBJECTIVES: The aim of this study was to determine constitution and physical properties of a prototype material based on Portland cement and assess biocompatibility compared with glass-ionomer cement by evaluating cell morphology. MATERIALS AND METHODS: Analysis of the material was performed using energy dispersive analysis (EDAX) and X-ray diffraction (XRD) analysis. Compressive strength and the effect of changing the mixing and curing conditions on the compressive strength of the materials were evaluated. Dimensional stability was evaluated by measuring water uptake of the materials. Biocompatibility was assessed at 1 and 28 days using a cell-culture technique and semi-quantitative cell morphological evaluation was performed by SEM. RESULTS: Analysis of the material showed that it was primarily composed of tricalcium silicate and dicalcium silicate. The compressive strength of the prototype cement and variants was comparable to Ketac Molar (47.98 N mm(-2) after 1 day, P>0.05). Vacuum mixing did not improve the compressive strength of the prototype cements at any age. Wet curing was detrimental to the neat cement at 1 day (35.98 N mm(-2), P=0.011) and 7 days (44.08 N mm(-2), P=0.025). The filler-replaced cement prototypes were more stable and less susceptible to changes in compressive strength by varying the curing method (P>0.05). The prototype material took up more water (0.9%) than glass-ionomer cement (1.7%) with P=0 after 1 day. Curing at 100% humidity resulted in a net loss of weight for all the materials tested. The test materials were less biocompatible than glass-ionomer cement at 1 day but their biocompatibility improved as the material aged. CONCLUSIONS: The constitution of the prototype material was broadly similar to that of mineral trioxide aggregate. The prototype cement could be a potential dental restorative material as its compressive strength compared well to an established restorative material. However, the material did not support cell growth, with biocompatibility being similar to that of glass-ionomer cement.

Deformation of phosphate-bonded investment materials at elevated temperatures.

OBJECTIVES: To determine deformation in phosphate-bonded investment (PBI) materials for "in-service conditions" and to investigate the influence of air bubble pores on deformation at 900 degrees C. METHODS: Surviving high and low strain-rate disc-rupture strength test samples were assessed for deformation. A dental centrifugal casting machine and a dental superplastic-forming machine were used to apply the loads. An indirect technique was used to measure the deformation of investment diaphragms with both a wide pore size distribution and with very low porosity for four investment materials. A total of 128 high strain-rate and 29 low strain-rate samples were investigated. A one-way analysis of variance was carried out. RESULTS: All but one of the high strain-rate test samples showed measurable deformation. All of the low strain-rate test samples containing pores showed measurable deformation. Six out of 14 samples, in which pores had been eliminated, showed no deformation. There was no statistically significant difference in deformation between samples with no pores and those with a large pore size distribution for either the high or low strain-rate tests (P>0.05). SIGNIFICANCE: PBI must survive all forming procedures without deformation if the resultant prosthesis is to fit. One hundred micrometres is generally recognized as the acceptable tolerance of fit for removable metal-based prostheses. The vast majority of deformations measured during this investigation were considerably greater than 100 microm.

Characterization of Portland cement for use as a dental restorative material.

OBJECTIVES: The aim of this study was to evaluate the suitability of fast-setting cement formulations based on Portland cement as dental core build-up materials using two different methods of testing compressive strength and evaluation of setting times. METHODS: Four fast-setting cements based on Portland cement and their four respective densified with small particle (DSP) mortars were tested for setting time, constitution of cement by EDAX, and compressive strength using International and British Standards. Ordinary Portland cement (OPC) was used as a control. RESULTS: All the fast-setting cements had a similar elemental composition to OPC and the setting times were less than 7 min. The compressive strength of OPC was different between the two methods (P<0.001). All the fast-setting cements tested showed no difference in compressive strength regardless of the method of testing at 1 and 7 days (P>0.05), but the cylinders showed a lower compressive strength at 28 days (P<0.05). The OPC DSP mortar showed poorer compressive strength than OPC (P<0.01) at all times for cube testing but not for cylinder testing, where no difference was observed. The fast-setting DSP mortars had a lower compressive strength at 1 day (P<0.005) with both methods. At later times, there was no difference between the cements and DSP mortars for the cubes. SIGNIFICANCE: The pure fast-setting cements set in <7 min and were not susceptible to changes in the compressive strength testing procedure at 1 and 7 days but at 28 days all the fast-setting cements had a significantly higher strength with the test using cubes (P<0.05). A reduction in strength was observed at 28 days in cylinder testing. Most of the cements tested did not show encouraging strengths, however, one of the prototype cements tested could be a prospective dental restorative material.

The constitution of mineral trioxide aggregate.

OBJECTIVES: The aim of this study was to determine the constitution of a commercially available root-end filling material, mineral trioxide aggregate, (MTA) (ProRoot MTA, Tulsa Dental, Tulsa, OK, USA). The surface morphology of the material with various treatment conditions was also evaluated. METHODS: The constitution of two commercial versions of MTA was determined before and after mixing with water. The unset material was analysed using Energy Dispersive Analysis by X-ray (EDAX) in a scanning electron microscope (SEM) and X-ray diffraction (XRD). The first technique identified the constituent elements while XRD analysis identified the compounds or phases present. The set material was evaluated using EDAX. The surface morphology of the material stored under various conditions (100% humidity, immersion in water, or immersion in phosphate solution) was evaluated using SEM. RESULTS: The EDAX showed the white MTA to be composed primarily of calcium, silicon, bismuth and oxygen, with the gray MTA also having small peaks for iron and aluminum. The XRD analysis showed gray MTA to be composed primarily of tricalcium silicate and dicalcium silicate. The surface morphology of the materials differed under the various conditions, particularly following immersion in phosphate solution with crystal formation. SIGNIFICANCE: The commercial versions of MTA were shown to have broadly similar constitution to ordinary Portland cement except for the addition of bismuth compounds. The white MTA did not contain iron.

Optimisation of the superplastic forming of a dental implant for bone augmentation using finite element simulations.

OBJECTIVES: The phenomenon of superplasticity has made it possible to form complex shapes that require extremely high degrees of ductility in titanium alloy with minimal internal stresses. Combined with the use of an investment casting material as the die material, which makes possible the forming of re-entrant angles, it is possible to produce membranes for ridge augmentation. The aim is to characterise the metal alloy sheet and simulate the superplastic forming process in three dimensions to produce process parameters, namely gas pressure as a function of time, to accurately adapt the titanium sheet to the bone surface. METHOD: The surface of the die was digitised using a 3D laser scanning system (UBM-Keyence LC2450). Ti-6Al-4V sheet of 140 mm diameter was modelled using a grid of triangular membrane elements. This mesh was automatically refined during the simulations. Finite element simulation was carried out using the Superflag software program (University of Wales Swansea) Three different options for gas pressure control were adopted, namely, target flow stress, target strain rate and target energy dissipation. The pressure cycles produced from the simulation were used to form titanium alloy sheet at 900 degrees C using argon gas. The deformed regions of the formed sheet were then examined to determine the regions of contact with the die and to characterise surface damage. RESULTS: Comparison of the simulations with experiment showed that there was good agreement between simulated and experimental thickness distributions in most parts of the sheet that were examined. Interrupted tests showed that in the intermediate positions of the forming sheet the simulations were slightly ahead of the experiment. The target stress option was found to produce the best degree of adaptation and the sheet formed using this cycle showed good surface quality, whereas in highly deformed regions using the other target options, the sheet was found to have formed microcracks. The use of a solid lubricant on the surface of the forming sheet was not found to have a significant influence on the adaptation of the titanium alloy sheet except in areas of high deformation where the sheet perforated. SIGNIFICANCE: The finite element membrane formulation is well adapted to the superplastic forming of a ridge augmentation membrane prosthesis. The simulation accurately describes the evolution of the shape of the prosthesis and its thickness distribution with time, which allows the manufacturer to select an appropriate initial thickness of titanium alloy sheet prior to attempting to form the component. The investment dies are found to have sufficient strength to withstand the forming operation if a suitable orientation of the titanium sheet with respect to the die is adopted. A metal surface of good quality can be produced in the formed prosthesis using the appropriate gas control option.

Sensitivity of a disc rupture strength test to air bubble pores in phosphate-bonded investment materials at elevated temperatures.

OBJECTIVES: To investigate the sensitivity of a modified disc rupture test to variables affecting the strength of four phosphate-bonded investment materials under conditions closely resembling the in-service environment. This study examined the influence of air bubble pores on the strength of phosphate bonded investment materials at a temperature of 900 degrees C. METHODS: A modified disc rupture configuration was used to test a circular investment diaphragm with clamped edges, in tension, initiated by bending. A sequentially-varying molten metal mass was used to apply the load using an electronic centrifugal casting machine. The staircase method was used to determine the increase or decrease of the applied load with the standardized increment of load being no greater than 1 or 2g. The mean load applied to 30 samples defined the transition of survival to failure of the investment disc and was taken as a measurement of investment strength. Two material handling techniques, one giving virtually no pores and one showing a wide pore size distribution were used. RESULTS: The diaphragm clearly either failed or survived the test with the pattern of fracture indicating brittle failure. A statistically significant difference in strength (P<0.050) of diaphragms with and without air bubble pores was demonstrated for three materials. Fewer pores resulted in increased strength of those materials and produced a much reduced scatter of strength in the fourth. SIGNIFICANCE: The modified disc rupture test is sufficiently sensitive to identify variations in strength of phosphate bonded investment materials caused by differing pore size distributions.