Siegbert Witkowski

Shear bond strengths between different zirconia cores and veneering ceramics and their susceptibility to thermocycling.

OBJECTIVES The purpose of this study was to evaluate the shear bond strength between various commercial zirconia core and veneering ceramics, and to investigate the effect of thermocycling. METHODS The Schmitz-Schulmeyer test method was used to evaluate the core-veneer shear bond strength (SBS) of three zirconia core ceramics (Cercon Base, Vita In-Ceram YZ Cubes, DC-Zirkon) and their manufacturer recommended veneering ceramics (Cercon Ceram S, Vita VM9, IPS e.max Ceram). A metal ceramic system (Degudent U94, Vita VM13) was used as a control group for the three all-ceramic test groups (n = 30 specimens/group). Half of each group (n = 15) was thermocycled (5-55 degrees C, 20,000 cycles). Subsequently, all specimens were subjected to shear force in a universal testing machine. Fractured specimens were evaluated microscopically to determine the failure mode. RESULTS The initial mean SBS values in MPa+/-S.D. were 12.5+/-3.2 for Vita In-Ceram YZ Cubes/Vita VM9, 11.5+/-3.4 for DC-Zirkon/IPS e.max Ceram, and 9.4+/-3.2 for Cercon Base/Cercon Ceram S. After thermocycling mean SBS values of 11.5+/-1.7 MPa for DC-Zirkon/IPS e.max Ceram, 9.7+/-4.2 MPa for Vita In-Ceram YZ Cubes/Vita VM9, and 9.6+/-4.2 MPa for Cercon Base/Cercon Ceram S were observed. Neither the differences between the SBS values of the all-ceramic test groups nor the influence of thermocycling on all groups were statistically significant. Irrespective of thermocycling the metal ceramic control group (27.6+/-12.1 MPa, 26.4+/-13.4 MPa) exhibited significantly higher mean SBS than all three all-ceramic groups tested. The all-ceramic groups showed combined failure modes as cohesive in the veneering ceramic and adhesive at the interface, whereas the metal ceramic group showed predominately cohesive fractures. SIGNIFICANCE The results indicated that the SBS between zirconia core and veneering ceramics was not affected by thermocycling. None of the zirconia core and veneering ceramics could attain the high bond strength values of the metal ceramic combination.

Influence of framework configuration on the marginal adaptation of zirconium dioxide ceramic anterior four-unit frameworks

Objective. To evaluate the influence of the framework configuration on the marginal adaptation of four-unit anterior fixed partial denture (FPD) frameworks made of partially sintered zirconium dioxide (ZrO2) ceramics. Material and methods. Forty-eight standardized partially sintered ZrO2 ceramic four-unit FPD frameworks were fabricated using three different CAD/CAM systems: Cercon Smart Ceramics (group CE), Vita YZ/Cerec In-Lab (group YZ/CL), and Xawex (group XA). Two different framework configurations (straight and curved) were manufactured for each group. The marginal adaptation of the frameworks was measured at 60 different points across the entire circumferential margin using a stereomicroscope. Marginal discrepancy values were compared between the two framework designs and between the three test groups using the t-test. The overall level of statistical significance was 5% after correcting the p-values using the Bonferroni–Holm method. Results. The following geometrical means of the marginal discrepancies were obtained for the curved/straight design: group CE, 120.0 μm/88.0 μm; group YZ/CL, 96.8 μm/86.5 μm; and group XA, 147.3 μm/113.4 μm. Significant differences were detected between the straight and curved designs for groups CE (p=0.001) and XA (p=0.003), but not for group YZ/CL (p=0.225). For both designs, the marginal discrepancies were significantly smaller in group YZ/CL than in group XA. For the curved design, the marginal discrepancies in group YZ/CL were also significantly smaller than those in group CE. Conclusions. Within the limitations of this study, the framework configuration influences the marginal adaptation of anterior four-unit FPD frameworks that are manufactured from partially sintered ZrO2 ceramics independently of CAD/CAM system.

Influence of Preparation and Wall Thickness on the Resistance to Fracture of Zirconia Implant Abutments

BACKGROUND Studies about the effect of grinding procedures as well as material thickness on the resistance of zirconia implant abutments are in short supply. PURPOSE This study evaluated the effect of wall thickness as well as preparation on the resistance of zirconia implant abutments. MATERIALS AND METHOD Sixty-four implants received titanium (group Ti) and zirconia abutments (groups Zr-8, Zr-18, and Zr-1). The abutments of group Zr-8 had a 0.8-mm wall thickness, whereas the wall thickness of group Zr-18 was reduced by preparation from 1 mm to 0.8 mm. The abutments of group Zr-1 had a wall thickness of 1 mm. Standardized maxillary central incisor metal crowns were cemented on all abutments. All specimens were then tested in a universal testing machine for their resistance to fracture before and after masticatory simulation (n = 8). RESULTS The median resistance to fracture values (N) before and after aging were, respectively: group Ti: 500-504; group Zr-8: 487-491; group Zr-18: 490-451; and group Zr-1: 519-480. No significant effects of group, aging, or combinations were found (p > .05). CONCLUSION All tested abutments have the potential to withstand physiologic occlusal forces in the anterior region (> 200 N). The applicability of the results to other implant systems should be verified.

Effect of veneering technique on the fracture resistance of zirconia fixed dental prostheses

To compare the fracture resistance of zirconia 3-unit posterior fixed dental prostheses (FDPs) frameworks veneered with different veneering materials and techniques before and after artificial ageing. Forty-eight zirconia 3-unit FDPs, representing a missing first molar, were adhesively cemented on human teeth. The zirconia frameworks were randomly distributed according to the veneering materials and techniques into three groups, each containing 16 samples: group LV (layering technique/Vintage ZR), group LZ (layering technique/ZIROX) and group PP (CAD/CAM and press-over techniques/PressXZr). Half of each group was artificially aged through dynamic loading and thermocycling to simulate 5 years of clinical service. Afterwards, all specimens were tested for fracture resistance using compressive load. An analysis of variance (anova) was used to assess the effect of veneering ceramic and artificial ageing on fracture resistance (P < 0·05). Except for one minor cohesive chipping in group LV1, all specimens survived artificial ageing. The mean fracture resistance values (in Newton) of different non-aged (± s.d.)/aged (± s.d.) groups were as follows: LV0 2034 (± 401)/LV1 1625 (± 291); LZ0 2373 (± 718)/LZ1 1769 (± 136); and PP0 1959 (± 453)/PP1 1897 (± 329). Artificial ageing significantly reduced the fracture resistance in groups veneered with the layering technique (P < 0·05), whereas no significant effect was found in specimens veneered with the CAD/CAM and press-over techniques. All tested systems have the potential to withstand occlusal forces applied in the posterior region. The combination of the CAD/CAM and press-over techniques for the veneering process improved the overall stability after artificial ageing, relative to the layering technique.

Reliability of shade selection using an intraoral spectrophotometer

In this study, we evaluate the accuracy and reproducibility of human tooth shade selection using a digital spectrophotometer. Variability among examiners and illumination conditions were tested for possible influence on measurement reproducibility. Fifteen intact anterior teeth of 15 subjects were evaluated for their shade using a digital spectrophotometer (Crystaleye®, Olympus, Tokyo, Japan) by two examiners under the same light conditions representing a dental laboratory situation. Each examiner performed the measurement ten times on the labial surface of each tooth containing three evaluation sides (cervical, body, incisal). Commission International on Illumination color space values for L* (lightness), a* (red/green), and b* (yellow/blue) were obtained from each evaluated side. Examiner 2 repeated the measurements of the same subjects under different light conditions (i.e., a dental unit with a chairside lamp). To describe measurement precision, the mean color difference from the mean metric was used. The computed confidence interval (CI) value 5.228 (4.6598–5.8615) reflected (represented) the validity of the measurements. Least square mean analysis of the values obtained by examiners 1 and 2 or under different illumination conditions revealed no statistically significant differences (CI = 95%). Within the limits of the present study, the accuracy and reproducibility of dental shade selection using the tested spectrophotometer with respect to examiner and illumination conditions reflected the reliability of this device. This study suggests that the tested spectrophotometer can be recommended for the clinical application of shade selection.

Responding to manuscript CLOI-D-10-00562: Reliability of shade selection using an intraoral spectrophotometer

The questions raised by Sabour and colleagues are justified and require explanations. In our original submission, we calculated general statistics to measure repeatability and reliability. One reviewer asked us to calculate instead the mean color difference from the mean which is a quantity to evaluate measurement uncertainties given the L*, a*, and b* values. This is a specific and supposed to be an established measure in color technology. To give a better picture of the data, we now provide the general measurements such as the intraclass correlation (reliability coefficient estimate) [1], with information of the variance between and within teeth (Table 1). These values show that the spectrophotometer gives repeatable results: the intraclass correlation is close to 1 in all scenarios (ranging from 0.92 to 0.99). In addition, the outcome measures are also reliable since observers 1 and 2 (reflecting changed environmental conditions) provide the same agreement (intraclass correlation ranges from 0.96 to 0.99). This is in line with our conclusion. To the second point, Sabour and colleagues are absolutely right: validity and reliability are two completely different methodological issues. Our study can only give information about repeatability and reliability. For validity, however, one needs to compare the measurements with a gold standard.