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Whether adhesive zirconia ceramic removable partial denture attachments are feasible with current technology is unclear.
Purpose
The purpose of this finite element analysis and in vitro study was to evaluate the effect of the lever arm, tooth preparation, and aging on the loading of the tooth-zirconia attachment interface.
Material and methods
Three different finite element analysis (FEA) models allowing for the loading of an adhesive attachment either directly or through a removable partial denture were used. Two models represented a human tooth with 2 different types of attachments, while the third model also included a removable partial denture. For the evaluation of bond strength, a combination of shear and hydrostatic stress was used. In addition, composite resin teeth were fabricated, and zirconia bars were bonded to them with varying tooth preparations and lever arm lengths. In 1 group the influence of aging was analyzed. Fracture load was determined by using a universal testing machine. Statistical analysis was based on the Shapiro-Wilk normality test, ANOVA, and Games-Howell test (α=.05).
Results
The maximum stress of 65 MPa occurring in the bonding area was reduced to 37 MPa by adding a retainer to the attachment. Loading of the denture resulted in a maximum stress of 9 MPa. Mean fracture loads ranged from 33.6 N to 209.1 N. Preparing a flat bonding surface showed a nonsignificant increase (P=.197), whereas aging led to a nonsignificant decrease in fracture load (P=.075). A lever arm extended by 2 mm significantly reduced fracture load (P=.002). The addition of an occlusal-distal (OD) cavity led to a nonsignificant increase (P=.186), which became significant when a mesial-occlusal-distal (MOD) preparation was applied (P=.001).
Conclusions
Adhesive zirconia attachments should use a MOD cavity and have a cross section of at least 2.5×2.5 mm. The attachment should not extend more than 3 mm.
Clinical Implications
Adhesive attachments made from zirconia ceramic may be feasible if a minimum of 3 tooth surfaces can be used for retention. This approach may lead to more conservative tooth preparation than the conventional complete crown preparation.
Retaining removable partial dentures without compromising esthetics with metal clasps
Such restorations are technically complex, resulting in high dental laboratory costs, and their longevity has been reported to be lower than fixed or implant-supported restorations.
However, sufficient bonding area, discoloration of abutment teeth, and space requirements in the vertical dimension are limitations which lead to unfavorable survival rates.
In vitro evaluation of shear bond strengths of resin to densely-sintered high-purity zirconium-oxide ceramic after long-term storage and thermal cycling.
After tooth preparation and intraoral scanning, male attachments could be CAD-CAM fabricated from zirconia ceramic, while current resin materials might be used to generate the female attachment in the removable partial denture.
The purpose of this finite element analysis (FEA) and in vitro study was to evaluate basic parameters that may affect bond stability as a potentially limiting factor for a zirconia attachment. The null hypotheses tested were that the parameters tooth preparation, length of the lever arm, and aging would not affect loading of the tooth-zirconia attachment interface.
Material and methods
Using a computer-aided design program (Pro/Engineer Wildfire 3.0 Student Edition; PTC Inc), 2 attachment models with 2 different types of adhesive attachments and an attachment tooth plus a removable partial denture model were generated. They were subsequently imported into an FEA program (Ansys Workbench 12.0; Ansys Inc) and meshed by using tetrahedron elements. As the strength of adhesive joints cannot be evaluated solely by considering von Mises equivalent stress, a comparative stress was chosen that combines shear stress and hydrostatic stress for displaying tension and compression in the bonding layer (Schlimmer M. Anstrengungshypothesen für Polymerwerkstoffe, Rheol Acta 1981; 20:542-7.; Diercks N. Validierung und ANSYS-Implementierung des Schlimmer-Mahnken Materialmodells zur Festigkeitsberechnung von Klebeverbindungen. Neubiberg: Universität der Bundeswehr München; 2009. p. 1-60.). Adaptive mesh refinement was implemented and aborted if deviations were lower than 3.5% compared with the previous simulation were reached. The models consisted of 25 378 elements and 40 729 nodes.
In a first step, the 2 attachment models that schematically represented a canine with a Roach spherical attachment (sphere diameter 2.5 mm; diameter of the base 3.9 mm; width of semicircular retainer 4.0 mm; thickness of adhesive layer 0.2 mm) were considered (Fig. 1). In both models, the teeth were apically fixed without modeling a root or periodontium. The material parameters applied are detailed in Table 1. They were loaded with a combination of a 100-N vertically intrusive force and a 50-N horizontal force acting in a distal direction.
Figure 1Schematic canine teeth with Roach spheres adhesively attached on distal surfaces. A, Bonding surface consisted of base of Roach sphere only. B, Semicircular retainer added.
In a second step, the configuration with the semicircular retainer was used to design the denture model, which represented a removable denture attached to the Roach sphere (Fig. 2). The canine was again fixed apically, and loading was simulated as a combination of a 200-N vertical load and a 50-N horizontal load in a distal direction, which could be exerted on the first premolar or first molar. The attachment system allowed for vertical movement and for rotation around the buccolingual axis. The canine and the denture body were modeled as stiff structures, modeling zirconia ceramic.
Figure 2Denture model configuration comprising canine tooth with adhesively bonded Roach sphere and removable denture.
A typodont canine tooth (Modellzahn; KaVo Dental GmbH) served as the basis for the subsequent in vitro investigation. Duplicating silicone (Adisil blau 9:1; SILADENT Dr Böhme & Schöps GmbH) was used to generate a mold, which allowed the fabrication of standardized canine teeth from a dual-polymerizing composite resin (Grandio Core Dual Cure; VOCO GmbH).
Seven experimental groups with 10 specimens each were established as per Table 2. Simple rectangular bars made from presintered zirconia ceramic (AM 40/25 Premium Zirkonoxid; DEGOS Dental GmbH) were fabricated by using a diamond band saw (EXAKT 300; EXAKT Advanced Technologies GmbH) and a grinding and polishing machine (Tegra-Pol 31; Struers GmbH). After sintering (Everest Therm; KaVo Dental GmbH), the bars had a final size of 2.5×2.5×10 mm. These bars were used in all groups except for the occlusal-distal (OD) and mesial-occlusal-distal (MOD) groups (Fig. 3). For these groups, master teeth were prepared and duplicated as described previously, followed by waxing restorations consisting of a retentive element and a bar, which was then manually copy-milled (Ceramill Multi-X; Amann Girrbach AG) with the same zirconia material (Fig. 4). Before adhesive fixation (Panavia F 2.0; Kuraray Europe GmbH), the zirconia bars were silica/silane coated (Rocatec Pre; Rocatec Plus; Espe Sil; 3M Deutschland GmbH). Subsequently, static fracture tests
were performed in a universal testing machine (Z020; ZwickRoell GmbH & Co KG), recording the maximum force applied (Fig. 5). Statistical analysis was based on the Shapiro-Wilk normality test, ANOVA, and Games-Howell test (α=.05).
Table 2Characteristics of experimental groups established for in vitro study (n=10)
Figure 3Master teeth for experimental groups showing preparations used as additional anchorage for adhesive attachments. A, Occlusal-distal. B, Mesial-occlusal-distal.
With the attachment models, maximum comparative stress in a range up to 70 MPa occurred in the bonding area when only the base was used to bond the Roach spherical attachment (Fig. 6A). The addition of the semicircular retainer led to a reduction in comparative stress up to 30 MPa on the flat bonding surface and nearly 37 MPa at the borders of the retainer (Fig. 6B).
Figure 6Results of simulated Roach sphere loading with combined 100-N vertical force and 50-N horizontal force. A, Base only. B, Semicircular retainer.
With the denture model, force application in the molar area caused vertical displacement of the removable denture nearby the attachment because of the simulated resilience of the gingiva. In the bonding area, maximum stress up to 9 MPa was observed (Fig. 7A). This was comparable with the stress situation resulting from loading in the premolar area (Fig. 7B), which also led to an intrusive movement of the denture in the proximity of the Roach sphere.
Figure 7Results of simulated denture loading with combined 200-N vertical force and 50-N horizontal force. A, Acting on first premolar. B, Acting on first molar.
The mean fracture loads ranged from 33.6 N for Group 4 mm to 209.1 N for Group MOD (Table 3). In no instance did the zirconia ceramic fracture – the failure always occurred at the zirconia-resin interface. Based on the Shapiro-Wilk normality test, the measurement values could be assumed to be normally distributed (Table 4). Because of the variation in standard deviations ranging from 6.98 N to 45.63 N, the assumption of homogeneity of variances, a prerequisite for ANOVA, was violated. As ANOVA is robust against such violation if the sample size is constant, 1-way ANOVA was nevertheless performed, indicating significant differences among groups (P<.001). Table 5 gives the results for the comparisons among the different experimental groups.
Table 3Mean fracture loads (N) and standard deviations recorded for experimental groups
The preparation of a flat bonding surface did not significantly increase fracture load (Group 2 mm compared with Group 2 mm control; P=.197), whereas water storage led to a significant decrease in fracture load (Group 2 mm compared with Group 2 mm aging; P=.001). Increasing the lever arm by 1 mm led to a significant reduction in fracture load (Group 2 mm compared with Group 3 mm; P=.002 and Group 3 mm compared with Group 4 mm; P=.013) as did increasing the lever arm by 2 mm (Group 2 mm compared with Group 4 mm; P=.001). The use of an OD cavity (Fig. 3A) for retention instead of a flat surface (Group 2 mm compared with Group OD, P=.753) or an unaltered tooth surface (Group 2 mm control compared with Group OD; P=.186) did not significantly affect fracture load. Maximum fracture load was observed when an MOD preparation (Fig. 3B; Fig. 4) was made with all statistical comparisons revealing significant differences (P=.001).
Discussion
Based on the findings of this finite element analysis and in vitro study, the null hypotheses that the parameters tooth preparation, length of the lever arm, and aging would not affect loading of the tooth-zirconia attachment interface were rejected. Improvements in adhesive dentistry, the advent of zirconia ceramic, and the availability of intraoral scanning and CAD-CAM fabrication
Both parts of the current investigation indicated that bonding surface and bodily engagement of the abutment tooth are critical factors for adhesive attachments. This finding was consistent with that of a recently published study
led to a reduction in the fracture load, while using an MOD preparation for additional retention resulted in increased fracture load. These findings indicate that an adhesive attachment requires an inlay-like preparation, a cross-sectional area of 2.5×2.5 mm and a less than 3-mm lever arm.
Depending on the type of resin cement used, the stresses simulated in the FEA exceeded critical magnitudes, requiring an attachment system be designed to avoid vertical loading, allowing the denture to move freely in the vertical direction. This, however, might lead to occlusal interferences, as the occlusal plane may be interrupted and the denture might require more frequent relining.
Limitations of the present study included that the length of the lever arm was limited to 4 mm in the in vitro study, reflecting the dimensions of a standard Roach sphere. The data shown here preclude the rigid fixation of longer lever arms, which means that the removable part of the restoration must not exert forces on the tooth-attachment interface. In addition, simplistic FEA models lacking periodontal structures and exact geometries were applied, not simulating the deformation of the denture or abutment tooth. Numerous possible combinations of zirconia pretreatment and bonding agents could have been used. Applying silica/silane coating and a universal adhesive cement as performed here only reflects one option which, however, has been advocated.
Copy-milling of the restorations used in groups OD and MOD was challenging and required a learning curve for achieving specimens with an adequate level of fit. These problems led to the decision to limit the sample size to 10 specimens per experimental group. However, better results may be achieved by using CAD-CAM technology.
In addition, it has to be taken into account that only a simple bar design was used as a model for an extracoronal attachment. The dual-polymerizing composite resin used for fabricating standardized teeth did not reflect clinical reality where bonding to enamel would have to be performed. Based on the assumption that the enamel-resin interface would show greater bond strength as compared with the resin-zirconia interface, this approach seemed feasible for a pilot investigation.
In vitro evaluation of shear bond strengths of resin to densely-sintered high-purity zirconium-oxide ceramic after long-term storage and thermal cycling.
Static overload is not a frequently occurring failure mode clinically, where fatigue phenomena limit the service time of restorations; consequently, this testing method has been questioned.
as the most straightforward method of aging led to a large reduction in bond strength. More sophisticated experiments are required before drawing clinically relevant conclusions with respect to long-term applicability.
Based on the findings of this in vitro study, the following conclusions were drawn:
1.
Adhesive attachments fabricated using contemporary zirconia ceramic materials appear to be feasible.
2.
The basic problems with respect to bonding surface and adhesive strength seem to remain.
Acknowledgments
The experimental part of this study was carried out by Kamran Orujov in partial fulfillment of the requirements for the degree Dr med dent at Saarland University, Homburg, Germany.
René Höller, MSc, is acknowledged for carrying out the finite element analysis as part of his studies in mechanical engineering at the University of Erlangen-Nuremberg (Institute of Applied Mechanics; Head: Prof Dr P. Steinmann).
The authors thank Dr Friedrich Graef, Professor emeritus, Department of Mathematics, University of Erlangen-Nuremberg for statistical data analysis.
CRediT authorship contribution statement
Kamran Orujov: Data curation, Formal analysis. Werner Winter: Investigation, Methodology. Matthias Karl: Conceptualization, Writing - original draft, Conceptualization, Supervision. Siegfried Heckmann: Project administration, Writing - review & editing.
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Aesthetic considerations for the treatment of partially edentulous patients with removable dentures.
In vitro evaluation of shear bond strengths of resin to densely-sintered high-purity zirconium-oxide ceramic after long-term storage and thermal cycling.
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