Stefan Rues

In vitro fracture load of monolithic lithium disilicate ceramic molar crowns with different wall thicknesses

ObjectivesThe objective of this in vitro study was to assess the effect of wall thickness on the fracture loads of monolithic lithium disilicate molar crowns.Material and methodsForty-eight extracted molars were prepared by use of a standardized preparation design. Lithium disilicate crowns (e.max CAD, Ivoclar/Vivadent, Schaan, Liechtenstein) of different wall thicknesses (du2009=u20090.5, 1.0, and 1.5xa0mm; nu2009=u200916 for each series) were then constructed and milled (Cerec MC-XL, Sirona, Bensheim, Germany). After placement of the teeth in acrylic blocks (Technovit, Heraeus Kulzer, Hanau, Germany), the crowns were adhesively luted (Multilink, Ivoclar Vivadent). In each series, eight crowns were loaded without artificial aging whereas another eight crowns underwent thermocycling (10,000xa0cycles, THE-1100, SD Mechatronik) and chewing simulation (1.2xa0million cycles, Willytec CS3, SD Mechatronik, Fmaxu2009=u2009108xa0N). All specimens were loaded until fracture on one cusp with a tilt of 30° to the tooth axis in a universal testing machine (Z005, Zwick/Roell). Statistical assessment was performed by use of SPSS 19.0.ResultsCrowns with du2009=u20091.0 and 1.5xa0mm wall thickness did not crack during artificial aging whereas two of the crowns with du2009=u20090.5xa0mm wall thickness did. The loads to failure (Fu) of the crowns without aging (with aging) were 470.2u2009±u200980.3xa0N (369.2u2009±u2009117.8xa0N) for du2009=u20090.5xa0mm, 801.4u2009±u2009123.1xa0N (889.1u2009±u2009154.6xa0N) for du2009=u20091.0xa0mm, and 1107.6u2009±u2009131.3xa0N (980.8u2009±u2009115.3xa0N) for du2009=u20091.5xa0mm. For aged crowns with du2009=u20090.5xa0mm wall thickness, load to failure was significantly lower than for the others. However, differences between crowns with du2009=u20091.0xa0mm and du2009=u20091.5xa0mm wall thickness were not significant.ConclusionsFracture loads for posterior lithium disilicate crowns with 0.5 mm wall thickness were too low (Fuu2009<u2009500xa0N) to guarantee a low complication rate in vivo, whereas all crowns with 1.0 and 1.5 mm wall thicknesses showed appropriate fracture resistances Fuu2009>u2009600xa0N.Clinical relevanceThe wall thickness of posterior lithium disilicate crowns might be reduced to 1xa0mm, thus reducing the invasiveness of the preparation, which is essential for young patients.

Fracture resistance of glazed, full-contour ZLS incisor crowns

PURPOSEnTo compare the failure behaviour of zirconia-reinforced lithium silicate (Celtra Duo, DeguDent) with that of lithium disilicate (IPS e.max CAD, Ivoclar Vivadent) and feldspar (Mark II, VITA) ceramics.nnnMETHODSnThree groups of sixteen glazed maxillary incisor crowns were produced. The inner surfaces of the crowns were etched, then luted to metal dies with self-adhesive cement. Single load-to-failure tests were performed before and after thermo-mechanical ageing. To simulate clinical conditions, the specimens were thermocycled (10,000 cycles between 6.5°C and 60°C) and underwent chewing simulation (1,200,000 cycles, Fmax=86N). Statistical analyses were performed by use of non-parametric Kruskal-Wallis and Mann-Whitney U-tests.nnnRESULTSnBefore ageing, all the monolithic incisor crowns fractured at test forces >285N. Mean fracture loads were highest for zirconia-reinforced lithium silicate (725N, SD 162N), slightly lower for lithium disilicate (701N, SD 276N), and lowest for feldspar (554N, SD 190N). The differences between the results were not statistically significant. After ageing, fracture resistance decreased for all materials except zirconia-reinforced lithium silicate. Mean fracture loads were highest for zirconia-reinforced lithium silicate (766N, SD 98N) and significantly lower for both lithium disilicate (485N, SD 64N) and feldspar (372N, SD 116N).nnnCONCLUSIONnMonolithic restorations fabricated from zirconia-reinforced lithium silicate retain high fracture resistance after extensive thermo-mechanical ageing. This preclinical study suggests that cohesive failures of monolithic anterior crowns produced of these ceramics will not be a major problem in dental practice.

Comparability of clinical wear measurements by optical 3D laser scanning in two different centers

OBJECTIVEnThe purpose of this study was to compare the use of different variables to measure the clinical wear of two denture tooth materials in two analysis centers.nnnMETHODSnTwelve edentulous patients were provided with full dentures. Two different denture tooth materials (experimental material and control) were placed randomly in accordance with the split-mouth design. For wear measurements, impressions were made after an adjustment phase of 1-2 weeks and after 6, 12, 18, and 24 months. The occlusal wear of the posterior denture teeth of 11 subjects was assessed in two study centers by use of plaster replicas and 3D laser-scanning methods. In both centers sequential scans of the occlusal surfaces were digitized and superimposed. Wear was described by use of four different variables. Statistical analysis was performed after log-transformation of the wear data by use of the Pearson and Lin correlation and by use of a mixed linear model.nnnRESULTSnMean occlusal vertical wear of the denture teeth after 24 months was between 120μm and 212μm, depending on wear variable and material. For three of the four variables, wear of the experimental material was statistically significantly less than that of the control. Comparison of the two study centers, however, revealed correlation of the wear variables was only moderate whereas strong correlation was observed among the different wear variables evaluated by each center.nnnSIGNIFICANCEnModerate correlation was observed for clinical wear measurements by optical 3D laser scanning in two different study centers. For the two denture tooth materials, wear measurements limited to the attrition zones led to the same qualitative assessment.

Effect of impact velocity and specimen stiffness on contact forces in a weight-controlled chewing simulator.

OBJECTIVESnChewing simulators are used for preclinical evaluation of newly developed dental restorative materials. To guarantee the independence of test conditions, contact forces during chewing simulation should be independent of the specimen. Because of its mode of operation, i.e., impact of an antagonist, this requirement is not met for a widely used chewing simulator (Willytec/SD Mechatronik, Feldkirchen-Westerham, Germany). This study was therefore intended to clarify the extent to which specimen stiffness affects maximum contact force at different impact velocities. Possible differences between the forces in the eight test chambers were also of interest.nnnMETHODSnFrom each of five dental materials differing in Youngs modulus, eight cylindrical disks were manufactured and embedded in specimen holders. Alumina spheres were used as antagonists. During chewing simulations with different impact velocities and dental materials, vertical acceleration was recorded and contact forces were estimated on the basis of these measurements.nnnRESULTSnSpecimen stiffness and impact velocity had a substantial effect on maximum contact force. The force overshoot relative to the static load ranged from 4% for small specimen stiffness and low impact velocity to values greater than 200% for high specimen stiffness and high impact velocity. Large differences between the chambers were also detected.nnnSIGNIFICANCEnWeight-controlled chewing simulations should be performed either with a low impact velocity or with a spring-damper system (placed between mass and specimen) which efficiently reduces the effects of contact force variation. Influence of specimen stiffness on contact forces must be considered at data interpretation.

Retention behavior of double-crown attachments with zirconia primary and secondary crowns

OBJECTIVEnTo investigate whether adequate retention of zirconia conical crown (CC) attachments can be achieved, and to investigate their long-term retention.nnnMETHODSnSixteen individual zirconia primary crowns were produced with convergence angles of 1° or 2° (eight of each). After determination of the convergence angles of the primary crowns, monolithic zirconia secondary crowns were manufactured. To evaluate the retention behavior of all-zirconia CC, the crowns were fitted with forces from F=12.5-100 N. Force magnitudes during the loosening process (L) were then measured. L/F ratios were recorded and the coefficient of friction (μ0) was calculated. Long-term retention was tested with up to 50,000 cycles of denture integration at a speed of 30 mms(-1) and a fitting force magnitude of 53 N.nnnRESULTSnEven when primary crowns were manufactured with the utmost care, the real convergence angles were greater than the nominal angles of the standardized burs (1° and 2°) by 0.28° (SD 0.11°). Without mechanical aging, mean L/F was 0.632 (SD 0.038) for 1° samples and 0.526 (SD 0.022) for 2° samples, indicative of high retentive forces of up to 63.2% of the fitting forces. When all the test results were used, best-fitting curves gave μ0=0.117 for new samples and μ0=0.126 for samples after 50,000 integration cycles.nnnSIGNIFICANCEnWhen the correct milling and sintering parameters are chosen, the retention behavior of zirconia CCs is adequate and stable. This innovative type of attachment is appealing because of the beneficial properties of zirconia and the efficient CAD/CAM-based manufacture.

Two-Body Wear of CoCr Fabricated by Selective Laser Melting Compared with Different Dental Alloys

AbstractThe purpose of this study was to assess the two-body wear of cobalt–chromium (CoCr) samples fabricated by selective laser melting (SLM) in contrast to cast samples and enamel. The following materials were chosen for comparison with SLM-CoCr: a CoCr alloy for use in the lost-wax technique, four gold alloys with different hardness, titanium (grade 1), and human enamel. Seven specimens of diameter 12xa0mm and thickness 1xa0mm were produced from each material and polished with emery paper. Al2O3 ceramic balls served as antagonists in the biaxial chewing simulator. Multiple chewing cycles were applied with a static load of 50xa0N. Wear was analysed laser-optically by the use of cast replicas. Statistical analyses were performed with Kruskal–Wallis test and subsequent post hoc Mann–Whitney U tests at a significance level of 0.05. SLM-CoCr showed less wear than any of the other materials. The difference was statistically significant between SLM-CoCr (vertical loss −54.4xa0±xa06.9xa0µm after 300,000 cycles) and cast CoCr (vertical loss −117.2xa0±xa018.6xa0µm after 300,000 cycles). Greatest vertical wear was observed for titanium. SLM-CoCr exhibits a great resistance to wear and might represent an alternative for the manufacture of prosthetic restorations even in occlusal loading areas.n Clinical relevance Clinical investigations on wear of SLM-CoCr are needed to assess its in vivo behaviour.

Fracture resistance of zirconia‐based all‐ceramic crowns after bur adjustment

Intra-oral grinding is often required to optimize occlusion of all-ceramic restorations. The effect of burs of different grit size on the fracture resistance of veneered zirconia crowns was investigated in this study. Forty-eight standardized zirconia copings were produced. The ceramic veneer was designed with a positive ellipsoidal defect on the palatal aspect of the crowns. To simulate adjustment of dental restorations by burs, this palatal defect was removed by use of three different diamond-coated burs with grit sizes 46, 107, or 151xa0μm (fine, medium, or coarse, respectively). Each different grit size of bur was used to grind 16 crowns. All crowns were then polished and surface roughness was measured. Half of the specimens underwent thermomechanical aging (10,000 thermocycles between 6.5°C and 60°C) and 1.2 million cycles of chewing simulation (Fxa0=xa0108xa0N). A linear regression model was computed to test the effect of aging and grinding grit size at a level of significance of αxa0=xa00.05. Fracture loads increased with decreasing grit size. Grit size and aging had a significant effect on the fracture resistance of the crowns. Use of fine and coarse burs for intra-oral adjustments resulted in different fracture resistance of veneered zirconia crowns. Coarse burs should be avoided in the final stage of grinding before polishing.