Philipp Kohorst

Low-temperature degradation of different zirconia ceramics for dental applications

The aim of this investigation was to determine the influence of simulated ageing on the tetragonal-to-monoclinic phase transformation and on the flexural strength of a 3Y-TZP ceramic, compared to alumina toughened zirconia (ATZ) and ceria-stabilized zirconia (12Ce-TZP). Standardized disc specimens of each material were hydrothermally aged in steam at 134°C and 3bar for 0, 16, 32, 64 or 128h. The phase transformation was determined by X-ray diffraction (XRD) and atomic force microscopy. Scanning electron microscopy was performed to estimate the depth of the transformation zone. The flexural strength was investigated in a biaxial flexural test. XRD revealed a significant increase in the monoclinic phase content for 3Y-TZP and ATZ due to ageing, although this increase was less pronounced for ATZ. In contrast, the monoclinic phase content of 12Ce-TZP was not influenced. For 3Y-TZP and ATZ, a transformation zone was found of which the depth linearly correlated with ageing time, while for 12Ce-TZP no transformation zone could be observed. Changes in flexural strength after ageing were heterogeneous: while 3Y-TZP showed a significant decrease in strength - from 1740 to 1169 MPa - with ATZ there was a considerable increase - from 1093 to 1378 MPa. The flexural strength of 12Ce-TZP remained unaffected at the low level of about 500 MPa. These results indicate that both alumina and ceria, as stabilizing oxides, reduce the susceptibility of zirconia to hydrothermal degradation; the alternative use of these oxides may enhance the clinical long-term stability of dental zirconia restorations.

Influence of cooling rate on zirconia/veneer interfacial adhesion.

Slow cooling firing schedules have recently been introduced by some manufacturers to reduce chipping complications in zirconia-based core/veneer composites. The aim of this study was to test the hypothesis that these firing schedules may influence the bond strength between the zirconia core and veneering ceramic. Four different veneering ceramics recommended for zirconia (Lava Ceram, Triceram, VM9 and Zirox) were fired onto rectangular shaped Y-TZP specimens (Lava Frame) and cooled using a rapid or a slow cooling rate. The resulting bilayer specimens were notched, loaded in a four-point bending test and load-displacement curves were recorded. The critical load to induce stable crack extension at the core/veneer interface was determined, in order to calculate the strain energy release rate (G, Jm⁻²). Additionally, dilatometric measurements of the veneering ceramics were performed to determine the coefficient of thermal expansion (α, ppm.K⁻¹) between 50 and 450°C (α₁) and in the temperature region above the glass transition temperature (α₂). Discrepancies between α₂ and α₁ (Δα) were calculated. For all core/veneer compositions G values were lower for the slowly cooled specimens than for the rapidly cooled specimens. Significant differences with respect to the firing schedule were found in the Triceram and VM9 groups (P<0.05). The reductions in G values correlated with Δα. The bond strength between the zirconia core and the veneer decreased with the slow cooling rate. These results indicate that slow cooling of zirconia restorations may increase the risk of adhesive delamination failures between the core and veneer.

Influence of hydrothermal and mechanical conditions on the strength of zirconia.

Low temperature degradation and mechanical and thermal cycling may decrease the strength of zirconia and jeopardize the long-term success of dental restorations made of this material. The objective of this study was to reveal the influence of different environmental and loading conditions on the strength of 3 mol.% yttria-stabilized polycrystalline tetragonal zirconia (3Y-TZP). A total of 144 disk specimens were produced from each of two 3Y-TZP materials, and subjected to one of the following conditions: (A) no further treatment (control); (B and C) 10⁶ and 5×10⁶ mechanical cycles, respectively, with an upper load limit of 100 N; (D) 10⁴ thermal cycles between 5 and 55°C; (E) 200 days storage in water at 36°C; (F) a successive combination of conditions B, D and E; (G) storage in water at 80°C for 64 days; (H) storage in an autoclave at 134°C for 8 h. Monoclinic phase content was evaluated by X-ray diffraction (XRD) analysis. Specimen strength was determined in a biaxial bending test. The two ceramics exhibited average strengths of 995 and 1239 MPa, respectively. No statistically significant influence of any treatment on strength was demonstrated for either material. However, XRD measurements revealed a substantial increase in monoclinic phase content, from an initial 2% (control) to up to 10%, according to storage conditions. As a consequence of hydrothermal loading a tetragonal to monoclinic phase transformation took place at the surface of the 3Y-TZP materials investigated, but, like thermal and mechanical cycling, this did not lead to significant changes in bulk strength.

Influence of the veneering process on the marginal fit of zirconia fixed dental prostheses

Distortions in the marginal region during manufacture are detrimental to the long-term success of fixed dental prostheses (FDPs). The aim of the present in vitro study was to evaluate changes in marginal fit because of the veneering process of four-unit FDPs made from different zirconia materials. Two groups of FDPs with 10 specimens each were machined from white body zirconia blanks (VITA In-Ceram YZ Cubes, KaVo Everest ZS-Blanks) by means of computer-aided design/computer-aided manufacturing systems and subsequently sintered to their final density. The marginal and internal gaps of the frameworks were determined using a replica technique. Afterwards, frameworks were veneered with the recommended ceramics (VITA VM9, GC Initial Zr), and fitting accuracy was evaluated again. Statistical analyses were performed by Students t-test with the level of significance chosen at 0.05. With one framework/veneering system (Everest ZS-Blanks/Initial Zr), both marginal gap (P = 0.019) and internal gap (P = 0.001) at the premolar retainer were significantly affected by the veneering process. The resulting distortions were directed towards the lumina of the retainers and the mean values of the measured gaps decreased by about 23.8 mum (marginal gap) and 27.3 mum (internal gap). For the other system (In-Ceram YZ Cubes/VM9), no distortions because of the veneering were found. The veneering of zirconia restorations may lead to significant changes in the marginal fit, while the combination of core and veneering material used influences the resulting distortions.

Stresses and distortions within zirconia-fixed dental prostheses due to the veneering process

Changes in the marginal and internal fit during manufacture are detrimental to the long-term success of fixed dental prostheses (FDPs). Hence, the aim of this study was to investigate the distortion of four-unit zirconia bridges induced by the veneering process with in vitro and finite element analyses (FEA). Ten all-ceramic FDPs with zirconia frameworks were prepared. The marginal and internal fit of the restorations were determined prior to and after veneering by means of a replica technique. Additionally, a three-dimensional finite element model of the restoration was constructed and cooling after the veneering process was virtually simulated. Statistical analysis revealed significant changes in the marginal and internal fit due to the veneering process. FEA verified these observations and displayed tensile stresses (up to 65MPa) within the framework and compressive stresses (up to 10MPa) within the veneering layer. The present study showed that stresses and distortions, occurring due to the veneering process, may influence the marginal and internal fit and therefore the clinical success of dental restorations.

Effect of Implant–Abutment Connection Design on Load Bearing Capacity and Failure Mode of Implants

PURPOSE In this in vitro study, six implant-abutment connection designs were compared and evaluated regarding load bearing capacities and failure modes. MATERIALS AND METHODS Five implants of Astra Tech, Bego, Camlog, Friadent, Nobel Biocare, and Straumann were separately embedded in stainless steel tubes using polyurethane, for a total of 30 specimens. Specimens were statically loaded under an angle of 30° with respect to the implant axis in a universal testing machine using a test setup according to ISO 14801. Failure was indicated by a load drop of 100 N in force. Load-displacement curves were analyzed, and maximum force and force at which permanent deformation occurred were recorded. Statistical analysis was performed using one-way ANOVA with the level of significance set at 0.05. RESULTS Statistical analysis revealed that the type of implant-abutment connection design has a significant influence on load bearing capacity (p < 0.001). The mean maximum forces ranged between 606 N (Straumann) and 1129 N (Bego); the forces where plastic deformation set in ranged between 368 N (Friadent) and 955 N (Bego). Failure modes differed between the various implant-abutment connection types tested. CONCLUSIONS Implant-abutment connection design has a significant influence on load bearing capacity and failure mode of implants; however, all implant-abutment connection designs tested would be expected to withstand clinically relevant forces.

Effect of thermal expansion mismatch on the Y-TZP/veneer interfacial adhesion determined by strain energy release rate

PURPOSES The aim of this study was to assess the effect of differences in the thermal expansion behaviour of veneering ceramics on the adhesion to Y-TZP, using a fracture mechanics approach. METHODS Seven veneering ceramics (VM7, VM9, VM13, Lava Ceram, Zirox, Triceram, Allux) and one Y-TZP ceramic were investigated. Thermal expansion coefficients and glass transition temperatures were determined to calculate residual stresses (σ(R), MPa) between core and veneer. Subsequently, the veneering ceramics were fired onto rectangular shaped zirconia specimens, ground flat and notched on the veneering porcelain side. Then specimens were loaded in a four-point bending test and load-displacement curves were recorded. The critical load to induce stable crack extension at the adhesion interface was evaluated to calculate the strain energy release rate (G, J/m(2)) for each system. RESULTS Residual stresses ranged from -48.3±1.5MPa (VM7) to 36.1±4.8MPa (VM13) with significant differences between all groups (p<0.05). The strain energy release rate of the Y-TZP/veneer specimens ranged from 8.2±1.7J/m(2) (Lava Ceram) to 17.1±2.8J/m(2) (VM9). Values for G could not be obtained with the VM7, Allux and VM13 specimens, due to spontaneous debonding or unstable crack growth. Except for Triceram and Zirox specimens, strain energy release rate was significantly different between all groups (p<0.05). CONCLUSION Thermal residual stresses and strain energy release rates were correlated. Slight compressive stresses in the region of -20MPa were beneficial for the Y-TZP/veneer interfacial adhesion. Stresses higher or lower than this value exhibited decreased adhesion.

Finite element analysis of a four-unit all-ceramic fixed partial denture.

All-ceramic restorations are known to be prone to brittle fracture. However, a previously performed in vitro study indicates that four-unit fixed partial dentures (FPDs) with a zirconia framework are sufficiently strong to withstand occlusal forces in the posterior region. The aim of this study was to determine the stress distribution in such a four-unit FPD made of yttria-stabilized polycrystalline tetragonal zirconia (Y-TZP), under an occlusal load. A three-dimensional finite element model was constructed and a stress analysis performed with a force of 1630 N applied at the centre of the middle connector area. The location of maximum tensile stress according to finite element analysis coincided with the fracture origin of all 10 specimens fractured within the previous in vitro study. The maximum tensile stress in the area of the middle connector amounted to 633 MPa. It increased with the load being applied from the oral towards the buccal side (648 MPa) and decreased with the load being applied from the buccal towards the oral side (570 MPa). These stresses are of the same order as the flexural strength of Y-TZP, determined under standardized test conditions to be 600-1000 MPa. The model presented is intended to be used for further investigations, including thermally induced stresses during veneering.

Influence of the interface design on the yield force of the implant–abutment complex before and after cyclic mechanical loading

PURPOSE The aim of this in vitro study was to evaluate the implant-abutment assembly of Astra Tech (AST), Bego (BEG), Camlog (CAM), Friadent (FRI), Nobel Biocare (NOB) and Straumann (STR) with respect to yield force before and after cyclic fatigue, using a static overload test with a test set-up according to ISO 14801. METHODS Ten specimens of each type were split into two homogenous groups: one half was tested for static yield force without any further treatment (control), whereas the other one underwent one million cycles of mechanical loading with 100N as the upper load limit. For load-to-failure testing, specimens were then placed in a stainless steel jig and loaded in a universal testing machine under an angle of 30° with respect to the implant axis until failure. Load-displacement curves were analyzed and the yield forces at which non-linear behaviour set in (Fp) were recorded. Statistical analysis was performed using one-way ANOVA and t-test, respectively, with the level of significance set at 0.05. RESULTS Statistical analysis revealed that the type of implant-abutment connection has a significant influence on Fp (p<0.001). Furthermore, dynamic loading proved to significantly influence Fp of BEG and CAM (p<0.001). CONCLUSION None of the implant-abutment types tested would be expected to fail under clinically relevant forces, but the type of implant-abutment connection significantly influences the yield force Fp.

Influence of lubricant on screw preload and stresses in a finite element model for a dental implant

STATEMENT OF PROBLEM Loosening or fracture of the abutment screw are frequent complications in implant dentistry and are detrimental to the long-term success of the restorations. However, little is known about the factors influencing the stability of the screw-abutment complex. PURPOSE The purpose of this study was to investigate the influence of lubricant action during implant assembly on screw preload and stresses in a dental implant-abutment complex. MATERIAL AND METHODS A dental implant was modeled for finite element stress analysis. Different friction coefficients (μ=0.2 to 0.5) were chosen for the interfaces between implant components to simulate lubricant action or dry conditions. The stress analyses were each divided into 2 load steps. First, the abutment screw was virtually tightened with a torque of 25 Ncm. This was achieved by applying an equivalent preload calculated according to the different friction coefficients chosen. Second, the construction was externally loaded with a force of 200 N inclined by 30 degrees relative to the implant axis. RESULTS The screw preload increased with the decreasing friction coefficient. In all components, stresses increased with decreasing friction coefficient. Plastic deformation was observed at the implant neck in an area that expanded with decreasing friction coefficient. No plastic deformation occurred in the abutment. CONCLUSIONS The results of this study indicated that screw preload should be included in the finite element analysis of dental implants for a realistic evaluation of stresses in the implant-abutment complex. The friction coefficient significantly influenced the screw preload value and modified the stresses in the implant-abutment complex.