Effect of connector design on fracture resistance in all-ceramic fixed partial dentures for mandibular incisor region

Designing anterior cantilever resin-bonded fixed dental prostheses based on finite element analysis

PURPOSE

A resin-bonded bridge (RBB) is a minimally invasive prosthetic treatment option for intact adjacent teeth. However, it is contentious as to whether the mesial or distal adjacent tooth should be used as an abutment. This study aimed to investigate the potential of finite element analysis (FEA) for the selection of abutment teeth and the determination of the optimal design for anterior cantilever RBBs.

METHODS

Three types of RBBs were designed to simulate loss of the maxillary left lateral incisor. The fixed-fixed RBB (FF-RBB) had one retainer each for the left central incisor and canine. Distal- and mesial-cantilever RBBs (D-CRBB and M-CRBB) had a single retainer on the central incisor and canine, respectively. Three-dimensional models for FEA were generated from computed tomography slices and dental casts. FEA was performed for each RBB to evaluate stresses in the intercuspal, protrusive, and lateral mandibular positions.

RESULTS

The maximum principal stresses on the bridges in the intercuspal position were 141.9 MPa, 93.6 MPa, and 45.7 MPa, for the FF-RBB, D-CRBB, and M-CRBB, respectively. The stress in the D-CRBB position was greater than in the M-CRBB position in the intercuspal, protrusive, and lateral mandibular positions.

CONCLUSIONS

Based on the results, M-CRBB on the canine had a higher clinical performance than D-CRBB on the central incisor. FEA was useful for determining the optimal design of RBB for each patient.

Strain analysis of anterior resin-bonded fixed dental prostheses with different thicknesses of high translucent zirconia

Background/purpose

High translucent zirconia has been used as a new monolithic zirconia prosthesis, which has the potential to make anterior resin-bonded fixed dental prostheses (RBFDPs) without veneering porcelain. However, it is unclear whether the RBFDPs retainer can be thinned as much as conventional zirconia RBFDPs. The aim of this study was to assess the usability of high translucent zirconia RBFDPs with a thin retainer thickness by evaluating differences in retainer thickness on the surface strain.

Materials and methods

A model with a missing upper lateral incisor was used. The abutment teeth were upper central incisor and canine. Three types of RBFDPs were fabricated as follows: metal RBFDPs with a retainer thickness of 0.8 mm (0.8M), and high translucent zirconia RBFDPs with a retainer thicknesses of 0.8 and 0.5 mm (0.8Z, 0.5Z) (n = 10). The fitness of the margins was evaluated by the silicone replica technique. The surface strain of each retainer under static loading was measured and statistically analyzed using a t-test with Bonferroni correction.

Results

The marginal fitness of all RBFDPs was under 76.1 μm, which was clinically acceptable. Each strain of the 0.8Z and 0.5Z groups was significantly lower than that of the 0.8M (p < 0.05). There was no difference in strain of the zirconia RBFDPs even if the retainer thickness was changed.

Conclusion

Our results suggest that the high translucent zirconia RBFDPs can be manufactured with a retainer thickness of 0.5 mm, which reduces the amount of tooth preparation compared to the metal RBFDPs.

Influence of additional reinforcement of fixed long-term temporary restorations on fracture load

PURPOSE

In implant dentistry, temporary restorations (TR) might often be required for up to one year. The aim of this in vitro study was to evaluate the long-time performance of four-unit TRs in the posterior region based on different materials and reinforcement methods.

METHODS

One hundred and forty four TRs were manufactured on 16 models simulating an oral situation of two missing posterior teeth. With a computer-aided-design/computer-aided-manufacturing (CAD/CAM) workflow, a TR was fabricated (CAD; Telio CAD), which served as a template for other subgroups. With a vacuum-formed template, unreinforced and reinforced TRs [glass fibres (g; EverStick); polyethylenefibres (p; Ribbond original) and TRs with increased connector area (c; 27.5-35mm²)] were manufactured. Two different composite materials were used (C1: Luxatemp, C2: Protemp). Altogether, 16 subgroups with 8 specimens each were tested. After temporary luting (Temp Bond NE) and artificial-aging [1600 thermo-cycles (5-55°C), 240,000 chewing-cycles (50N)], all specimens were tested until fracture in a universal testing machine.

RESULTS

After artificial aging, mean fracture loads (N) were: (C1)201.2±109.7, (C1c)1033.0±173.1, (C1p)90.0±40.0, (C1g)75.9±25.9, (C2)108.6±58.6, (C2c)1363.3±148.6, (C2p)104.7±54.7, (C2g)50.0±0.0 and (CAD)232.5±19.1. The one-factor ANOVA analysis showed significant differences for the factors temporary material (p<0.047), reinforcement (p<0.0001) and artificial-aging (p<0.0001).

CONCLUSIONS

The study indicated that both CAD/CAM TRs and TRs with increased connector areas are suitable for long-term use of one year. No enhancement of fracture load was observed for fibre-reinforced TRs except for the fact that fractured TRs were not totally separated.

Fatigue resistance of all-ceramic fixed partial dentures - Fatigue tests and finite element analysis

OBJECTIVE

To estimate the fatigue resistance of a new translucent zirconia material in comparison to lithium disilicate for 3-unit fixed partial dentures (FPDs).

METHODS

Eighteen 3-unit FPDs (replacement of first upper molar) with a connector size of 4mm×4mm were dry milled with a five-axis milling machine (Zenotec Select, Wieland, Germany) using discs made of a new translucent zirconia material (IPS e.max ZirCAD MT, Ivoclar Vivadent). Another 9 FPDs with a reduced connector size (3mm×4mm) were milled. The zirconia FPDs were sintered at 1500°C. For a comparison, 9 FPDs were made of IPS e.max Press, using the same dimensions. These IPS e.max Press FPDs were ground from a wax disc (Wieland), invested and pressed at 920°C. All FPDs were glazed twice. The FPDs were adhesively luted to PMMA dies with Multilink Automix. Dynamic cyclic loading was carried out on the molar pontic using Dyna-Mess testing machines (Stolberg, Germany) with 2×10⁶ cycles at 2Hz in water (37°C). Two specimens per group and load were subjected to decreasing load levels (at least 4) until the two specimens no longer showed any failures. Another third specimen was subjected to this load to confirm the result. All the specimens were evaluated under a stereo microscope (20× magnification). The number of cycles reached before observing a failure, and their dependence on the load and on the material, were modeled, using a Weibull model. This made it possible to estimate the fatigue resistance as the maximum load for which one would observe less than 1% failure after 2×10⁶ cycles. In addition to the experimental study, Finite Element Modeling (FEM) simulations were conducted to predict the force to failure for IPS e.max ZirCAD MT and IPS e.max Press with a reduced cross-section of the connectors.

RESULTS

The failure mode of the zirconia FPDs was mostly the fracture of the distal connector, whereas the failure mode of the lithium disilicate FPDs observed to be the fracture of the connectors or multiple cracks of the pontic. The fatigue resistance with 1% fracture probability was estimated to be 488N for the IPS e.max ZirCAD MT FPDs (453N for repeated test), 365N for IPS e.max ZirCAD MT FPDs with reduced connector size and 286N for the e.max Press FPDs. All three IPS e.max ZirCAD groups statistically performed significantly better than IPS e.max Press (p<0.001). On the other hand, no significant difference could be established between the two IPS e.max ZirCAD MT3 groups with a 4mm×4mm connector size (p>0.05). The allowable maximum principal stress (σ_max) which did not lead to failure during fatigue testing for IPS e.max ZirCAD MT3 was calculated between 208MPa and 223MPa for FPDs with 4mm×4mm connectors for 2×10⁶ cycles. This value could also be verified for the FPDs of the same material with 3mm×4mm connectors. On the other hand fatigue strength in terms of σ_max at 2×10⁶ cycles of IPS e.max Press was calculated to be between 78 and 90MPa.

SIGNIFICANCE

The fatigue resistance of the translucent zirconia 3-unit FPDs was about 60-70% higher than that of the lithium disilicate 3-unit FPDs, which may justify their use for molar replacements, provided that a minimal connector size of 4mm×4mm is observed. Even with a limited number of specimens (n=9) per group it was possible to statistically differentiate between the tested groups.

Effect of geometry on deformation of anterior implant-supported zirconia frameworks: An in vitro study using digital image correlation

PURPOSE

To evaluate the effect of geometry on the displacement and the strain distribution of anterior implant-supported zirconia frameworks under static load using the 3D digital image correlation method.

METHODS

Two groups (n=5) of 4-unit zirconia frameworks were produced by CAD/CAM for the implant-abutment assembly. Group 1 comprised five straight configuration frameworks and group 2 consisted of five curved configuration frameworks. Specimens were cemented and submitted to static load up to 200N. Displacements were captured with two high-speed photographic cameras and analyzed with video correlation system in three spacial axes U, V, W. Statistical analysis was made using the nonparametric Mann-Whitney test.

RESULTS

Up to 150N loads, the vertical displacements (V axis) were statistically higher for curved frameworks (-267.83±23.76μm), when compared to the straight frameworks (-120.73±36.17μm) (p=0.008), as well as anterior displacements in the W transformed axis (589.55±64.51μm vs 224.29±50.38μm for the curved and straight frameworks), respectively (p=0.008). The mean von Mises strains over the surface frameworks were statistically higher for the curved frameworks under any load.

CONCLUSION

Within the limitations of this in vitro study, it is possible to conclude that the geometric configuration influences the deformation of 4-unit anterior frameworks under static load. The higher strain distribution and micro-movements of the curved frameworks reflect less rigidity and increased risk of fractures associated to FPDs.

Basic Finite Element Analysis of Para-periodontal Ligament in All-ceramic Zirconia Fixed Partial Denture

The purpose of the present study was to investigate the validity of incorporating a para-periodontal ligament in the test mold used in a basic fracture test of a zirconia all-ceramic fixed partial denture (FPD). A simplified three-dimensional finite element analysis model was designed based on the three-unit FPD fracture test. Two types of model, one with and one without a para-periodontal ligament between the abutment and base mold, were fabricated. Microfocus CT of the missing first molar area in a dry human mandible was performed. A three-dimensional model was then fabricated based on the data obtained. A load of 600 N was applied to the center of the pontic and stress distribution observed. The model with the para-periodontal ligament showed stress dispersion to the dental root with rotation of the abutment mold. Stress distribution in the finite element analysis model with a para-periodontal ligament showed greater similarity with that in the mandibular model than with that in the other two models without a para-periodontal ligament.