The impact of luting agents and stiffness of implant-abutments on marginal adaptation, chipping, and fracture resistance of zirconia crowns.

journal of the mechanical behavior of biomedical materials 39 (2014) 279–291

Available online at www.sciencedirect.com

www.elsevier.com/locate/jmbbm

Research Paper

The impact of luting agents and stiffness of implant-abutments on marginal adaptation, chipping, and fracture resistance of zirconia crowns Lina Scha¨fera,n, Cornelia Winklerc, Genoveva Brandlb, Sebastian Eckla, Verena Preisa, Michael Behra a

Department of Prosthetic Dentistry, Regensburg University Medical Center, Franz-Josef-Strauss-Allee 11, 93053 Regensburg, Germany b Private Practice, Dr. Jünger, Lam, Germany c Department of Cranio-Maxillo-Facial Surgery, Regensburg University Medical Center, Regensburg, Germany

art i cle i nfo

ab st rac t

Article history:

Objectives: This in vitro study evaluated the impact of cements and implant analogs with

Received 10 June 2014

different e-moduli on marginal adaptation, chipping, and the fracture resistance of

Accepted 21 July 2014

zirconia crowns.

Available online 4 August 2014

Methods: 80 crowns (Cercon, DeguDent) were manufactured for 40 polyoxymethylene

Keywords: Implant Zirconia Cement Fracture resistance Marginal adaptation Chipping e-modulus Stiffness

(POM) and 40 titanium (Ti) one-piece implant analogs and divided into 10 groups: A, zinc oxide phosphate (Hoffmann's Cement, Richter&Hoffmann, Berlin, D); B, zinc oxide eugenol (Temp Bond, KerrHawe, Bioggio, CH); C, resin (Variolink II, Ivoclar-Vivadent, Schaan, FL); D, zinc oxide without eugenol (Temp Bond NE, KerrHawe, Bioggio, CH); E, glass ionomer (Ketac Cem, 3M ESPE, Seefeld, D). All samples were thermally mechanically loaded (1.2 10 (6) 50 N; 3000 5 1C/55 1C). Marginal adaptation was semiquantitatively evaluated before and after ageing with a scanning electron microscope. After ageing, intact samples underwent a fracture resistance test. Results: The best sealed margins before ageing were achieved with resin and zinc oxide cement and with resin after ageing. Zinc oxide samples showed the most discontinuously sealed margins after ageing and the difference between POM and Ti samples was significant only for zinc oxide. The numbers of samples failing during TCML were as follows: A(Ti POM) ¼0 1; B(Ti POM) ¼ 0 5; C(Ti POM) ¼ 1/1; D(Ti POM) ¼2 2; E (Ti POM) ¼0 2. Fracture resistance test [N]: A(Ti POM) ¼1181 801; B(Ti POM) ¼ 1469 1517; C(Ti POM) ¼1704 1408; D(Ti POM) ¼ 1992883; E (Ti POM) ¼ 27501015. Conclusions: TCML reduced the number of perfectly sealed samples and increased the

Abbreviations: Ti, titanium; POM,

polyoxymethylene; PMMA,

polymethylenemethacrylat; TCML, thermo-cycling and mechanical

load; SEM, scanning electron microscope n Corresponding author. Tel.: þ49 941 944 6056; fax: þ49 941 944 6171. E-mail address: [email protected] (L. Schäfer). http://dx.doi.org/10.1016/j.jmbbm.2014.07.021 1751-6161/& 2014 Elsevier Ltd. All rights reserved.

280

journal of the mechanical behavior of biomedical materials 39 (2014) 279 –291

number of chipped samples cemented onto POM implants with zinc oxide. This study could not show any significant impact on the fracture resistance of zirconia when different cements and implant analogs were used. & 2014 Elsevier Ltd. All rights reserved.

1.

Introduction

Veneer chipping or delamination of zirconia-based restorations have been frequently discussed in recent literature reports. Kollar et al. found a higher risk of chipping or facing failures for implant crowns than for crowns on human teeth (2008). Under occlusal load, zirconia-based reconstructions are subjected to bending or stretching forces originating from elastic or plastic deformation of the foundation. Sources of any deformation of the foundation may be the dental luting agent as well as the abutment. Dental implants and their abutments are considered to be stiff and stable foundations; hence deformation of the luting agent may be the source of stress for all-ceramic restorations under occlusal load (Lu et al., 2013). On the other hand, occlusal load absorption could be advantageous for cements or abutments to develop more elastic properties. To show the impact of the luting agent or the abutment on veneering failure, we used stiff titanium implant analogs as well as “elastic” polyoxymethylene implant analogs to determine the risk of veneer chipping and to calculate fracture resistance after thermal cycling and mechanical loading representing oral service. Both types of implant analogs were combined with five different luting agents consisting of cements with different mechanical properties under load. Zinc oxide phosphate and glass ionomer have low compressive strength and are brittle in comparison to resin-based composite cements such as RelyX Unicems, Variolink IIs, or Panavias (Behr, 2003). Under load, bending or elastic deformations of these cements are expected to be less than those of zinc oxide phosphate or glass ionomer. A provisional type of cement, such as zinc oxide eugenol, was added to the cements because many dentists prefer the retrievability of restorations luted onto dental implants (Behr, 2007; Behr et al., 2009). The mechanical properties of provisional cements are poor. Therefore, even zirconia restorations are expected to show a higher rate of veneer chipping or failure than restorations luted with any of the other cements mentioned above (Behr et al., 2003). In our study, we analyzed the impact of different dental foundations with various degrees of elasticity on the risk of veneer chipping or failure of zirconia restorations. Additionally, we investigated the marginal adaptation of these cements on abutments with various degrees of stiffness. We hypothesized that zirconia restorations require stable and stiff foundations to avoid veneer chipping or failure and to achieve sufficient marginal adaptation.

2.

Material and methods

40 identically shaped one-piece polyoxymethylene (POM) and 40 identical titanium (Ti) implants were placed in

polymethylenemethacrylate (PMMA) resin (Palapress Vario; Kulzer, Wehrheim, Germany). The analogs were based on the Straumann standard tissue level implant and its conventional conically formed abutment (Biomechanical Department of the University of Regensburg). This abutment measured 5.5 mm in height, had a conical angle of 61, and was constructed for luting the dental reconstruction. Polyoxymethylene implants were used to evaluate the impact of various Young's moduli of implants on the fracture resistance of all-ceramic zirconia crowns. The implant analogs, which were copy-milled in the biomechanical laboratory of the University of Regensburg, corresponded to the wellestablished implant system Straumann. After scanning the identically shaped titanium and POM implants (cercons eye, DeguDent, Hanau), individualized copings shaped in the form of a premolar were virtually designed (cercons artv2.3, DeguDent, Hanau, D); 80 yttria-stabilized zirconia copings were milled (cercons brain, DeguDent, Hanau, D) from white zirconia blanks (cercons base DeguDent, Hanau, D) and enlarged by 30% to compensate for sintering shrinkage. Afterward, the copings were sintered to their final dimensions (cercons heat, DeguDent, Hanau) at 1350 1C for 6 h. The frameworks were cleaned and degreased with alcohol (70%) and air-abraded (Sandstrahlgeraet P-G 360/3, Harnisch und Rieth, Winterbach, D) with corundum (120 mm, 2 bar). All copings were coated by hand with glass ceramic (dentine/ incisal; cercons Ceram Kiss, DeguDent, Hanau, D) as recommended by the manufacturers. To gain three-point contacts, the occlusal areas of all crowns had been adjusted to steatite beads that were used as antagonists during artificial ageing and subsequently glazed. The crowns were randomly assigned into 10 groups of 8 samples each. Five groups were cemented onto polyoxymethylene implants and five groups onto titanium implants, each with five different cements. With the exception of the automatically mixed glass ionomer cement Ketac Cem Aplicap, all luting agents were handmixed. Further information is listed in Table 1.

2.1.

Artificial ageing

An artificial ageing procedure (TCML) simulated 5 years of oral service (Rosentritt et al., 1997, 2001, 2009a, 2009b, 2009c). The following loading parameters were used: 1,200,000 mechanical loads with 50 N and simultaneous thermal cycling with distilled water between 5 1C and 55 1C (3000 times for 2 min per cycle) (Gröger et al., 2003; Krejci et al., 1990; Rosentritt et al., 2001, 2009a, 2009b, 2009c). Steatite beads were used as antagonists. After ageing, the crowns were checked for failures (fracture, chipping) using a light microscope (Wild Heerbrugg, SUI), and photos were taken


281

Table 1 – Luting agents used in this study. Zinc oxide phosphate (Hoffmann's Phosphate Cement)

Zinc oxide eugenol (Temp Bond)

Resin composite (Variolink II)

Zinc oxide without eugenol(Temp Bond NE)

Glass ionomer cement (Ketac Cem)

Manufacturer

Richter & Hoffmann, Berlin, GER

KerrHawe, Bioggio, SUI

KerrHawe, Bioggio, SUI

3 M ESPE, Seefeld, GER

Batch # Mixing ratio Curing time

1104C09/1116D01 .1.8:1

3434300/3434298 1:1

IvoclarVivadent, Schaan, FL M59920/M52129 1:1

3410562/3306552 1:1

386192 aplicap

5 min

7 min

4 40 s

7 min

min

Upper margin between crown and cement (u)

Crown (ceramic)

Cement

Lower margin between cement and implant (l)

Implant (POM)

Fig. 1 – Picture of scanning electron microscope (Stereoscan 240, Cambridge) showing a perfect margin; zinc-oxide phosphate cement after artificial ageing.

(microscope camera Leica EC3, Leica mikrosystems GmbH, Wetzlar, GER).

2.2.

Marginal adaptation

To examine the crowns' marginal adaptation, both the restoration–cement-interface and the cement–implant-interface were semiquantitatively analyzed with a scanning electron microscope (SEM; magnification: 100 ; working distance: 23 mm; voltage: 10 kV) Stereoscan 240 (Cambridge Instruments, Cambridge GB) before and after aging (Roulet et al., 1989). After taking impressions (Permadyne, 3M Espe, G) before and after TCML, replica (Araldit CW; Ciba SC AG, Basel, CH) of the marginal areas were produced. After sputtering the replicas with gold (600 s, 130–180 V, 30 mA, Sputter SCD 040, Balzers Union, Balzers, FC), both interfaces were classified as “perfect margin” (no interruption of continuity) and “marginal gap” (interruption of continuity) with the SEM ( Figs. 1 and 2) (Roulet et al., 1989).

2.3.

Fracture resistance

After ageing, each crown was loaded with the testing machine until failure (Zwick 1446; Zwick, Ulm, Germany, crosshead speed v ¼1 mm/min) to allow examination of its fracture resistance. Crowns that already showed chipped veneering before the fracture resistance test were listed and rejected. Force was applied by using a steel ball (d¼ 12 mm),

and a tin foil (0.4 mm) between restoration and antagonist prevented force peaks. Crowns failed either because of chipping of the veneering ceramic or because of total fracture that included the core. The mean values of the results of the fracture resistance test and the SEM analysis were statistically analyzed with the statistic software (SPSS 17.0 for Windows, SPSS, Chicago, USA). Means plus standard errors were calculated. The Oneway ANOVA and Tukey Post-hoc test were used to investigate statistical differences. A p-value of less than 0.05 was considered significant.

3.

Results

3.1.

Evaluation of marginal adaptation

As shown in Fig. 3, we compared the mean values of perfect, continuously sealed margins as the percentage of the total circumference of all samples before and after artificial ageing. We also evaluated the upper margin (u) between crown and cement as well as the lower margin (l) between cement and implant. Table 2 lists the mean values of perfect continuous margins as the percentage of the total circumference of the samples before and after TCML, regardless of the type of luting agent used. The difference of the values of the titanium samples before and after ageing at the upper margin was statistically

282


Marginal gap (cement loss)

Fig. 2 – Picture of scanning electron microscope (Stereoscan 240, Cambridge) showing a marginal gap caused by the loss of cement; glass ionomer cement before artificial ageing.

Fig. 3 – All samples, mean values of perfect, continuously sealed margins as the percentage [%] of the total circumference before and after artificial ageing with a standard deviation of 0.699; BSu¼ before 5-year simulation upper margin; BSl¼ before 5-year simulation lower margin; ASu ¼ after 5-year simulation upper margin; ASl¼ after 5-year simulation lower margin.

significant (po0.000). The mean values of the POM samples were statistically significantly higher before than after artificial ageing, for both the upper margin (po0.000) and the lower margin (p r0.000). The difference of the values of the titanium samples before and after ageing at the lower margin was not statistically significant different. All other comparisons in this evaluation affecting deterioration via simulation did not show any significant differences.

Fig. 4 compares the deterioration by the ageing simulation in consideration of the different luting agents, implants, and margins. The mean values, standard error of mean, and standard deviation are listed in Table 3. The area of contact between the crown and the zinc oxide cement without eugenol on titanium implants showed significantly (po0.000) fewer sealed margins after the aging simulation than before. The combination of zinc oxide phosphate


283

Table 2 – The mean values (with a standard deviation of 0.699 and a standard error of mean of 15.8) of perfect, continuously sealed margins as the percentage of the total circumference of the samples before and after TCML; Ti¼ titanium, POM¼ polyoxymethelene, BSu¼ before 5-year simulation upper margin; BSl¼ before 5-year simulation lower margin; ASu¼ after 5-year simulation upper margin; ASl¼ after 5-year simulation lower margin. Sample

Perfectly sealed margin, mean, percentage of total circumference [%]

Standard deviation

Standard error of mean

BSu, Ti BSl, Ti ASu, Ti ASl, Ti BSu, Pom BSl, Pom ASu, POM ASl, POM Total

93.6 94.0 84.7 90.2 93.6 93.1 75.3 78.7 87.9

11.3 11.8 14.3 11.0 10.4 10.6 20.5 20.0 15.8

1.4 1.5 1.8 1.4 1.3 1.3 2.6 2.5 0.699

Fig. 4 – The mean values (standard error of mean as well as standard deviation are listed in Table 3) of perfect, continuously sealed margins as the percentage of the total circumference of the samples before and after simulation [%] in consideration of the different cements; BSu¼ before 5-year simulation upper margin; BSl¼ before 5-year simulation lower margin; ASu¼ after 5-year simulation upper margin; ASl ¼ after 5-year simulation lower margin.

(p¼ 0.025) or zinc oxide cement without eugenol and POM implants yielded significantly fewer sealed margins after the ageing simulation. However, these differences became significant only after the ageing simulation. In comparison to titanium samples, POM samples cemented with zinc oxide phosphate (p¼ 0.022/p¼ 0.003) or glass ionomer cement (po0.000/po0.000) showed significantly fewer sealed upper margins and lower margins. Therefore, use of zinc oxide eugenol as a luting agent or use of POM instead of titanium limited the performance of the cement in the marginal gap after the ageing simulation. Fig. 5 shows all mean values before ageing and compares the results of the different luting agents and implant analogs without differentiating between upper and lower margins. All significant differences and their values of significance are listed in Table 5. Before ageing and after the removal of all surplus remains of luting agents, the margins were evaluated. The samples of POM and titanium implant analogs cemented

with zinc oxide phosphate and the samples of POM fixed with glass ionomer cement showed a high rate of unsealed areas at the margins. Table 4 lists all mean values of perfect, continuously sealed margins as the percentage of the total circumferences of all samples before ageing. Fig. 6 shows all mean values after ageing and compares the results of the different luting agents and implant analogs without differentiating between upper and lower margins. All mean values of perfect, continuously sealed margins as the percentage of the total circumferences of all samples after ageing are listed in Table 6. The samples cemented onto POM implant analogs luted with zinc oxide phosphate and glass ionomer seemed to show a higher rate of unsealed margins caused by the ageing simulation, whereas resin seemed to remain unaffected by ageing in combination with both titanium and POM. All significant differences are listed in Table 7.

284


Table 3 – The mean values of perfect continuous margins as the percentage of the total circumference of the samples before and after TCML in consideration of the various cements and implant-dummies as well as of the upper and the lower margin; BSu¼ before 5-year simulation upper margin titanium; BSl¼ before 5-year simulation lower margin titanium; ASu¼ after 5-year simulation upper margin titanium; ASl¼ after 5-year simulation lower margin titanium;, BSu2¼ before 5-year simulation upper margin polyoxymethelene; BSl2¼ before 5-year simulation lower margin polyoxymethelene; ASu2¼ after 5-year simulation upper margin polyoxymethelene; ASl2¼ after 5-year simulation lower margin polyoxymethelene. Cement

Sample

Perfectly sealed margin, mean, percentage of total circumference [%]


Standard deviation

Zinc oxide phosphate

BSu BSl BSu2 BSl2 BSu BSl BSu2 BSl2 BSu BSl BSu2 BSl2 BSu BSl BSu2 BSl2

86.5 85.3 89.7 88.3 93.1 95.2 99.8 99.6 98.4 98.7 97.5 98.2 96.3 96.7 87.4 86.2

3.6 4.5 3.1 3.2 3.4 2.6 0.1 0.4 0.9 0.8 0.8 0.8 1.6 1.3 3.4 3.1

14.6 18.0 12.3 12.7 13.4 10.5 0.5 1.6 3.6 3.2 3.1 3.0 6.4 5.3 13.5 12.3

Resin composite

Zinc oxide

Glass ionomer

Table 4 – Mean values of perfect, continuously sealed margins as the percentage of the total circumferences of the samples before aging [%] with standard error and standard deviation. Cement

Perfect margin [%]; mean Ti/POM

Standard error of mean Ti/POM

Standard deviation Ti/POM

Zinc oxide phosphate Resin Composite Zinc oxide Glass ionomer

85.9/89.0 94.1/99.7 98.6/97.8 96.5/86.8

2.8/2.1 2.1/0.2 0.6/0.5 1.0/2.3

16.1/12.3 11.9/1.2 3.4/3.0 5.8/12.7

The samples cemented with zinc oxide onto titanium as well as the samples cemented with zinc oxide phosphate, zinc oxide, composite, and glass ionomer onto POM were significantly affected by ageing. Table 8 shows the significance of the differences in this evaluation.

3.2.

significantly (Table 10) higher fracture resistance than samples cemented onto titanium with zinc oxide phosphate cement or samples cemented onto POM implant dummies with zinc oxide phosphate, zinc oxide, or glass ionomer cement.

Chipping due to the ageing simulation

4. 14 out of the 80 samples showed chipped ceramic after the ageing simulation, of which 11 were cemented onto POM implant analogs. Thereof, 5 samples were cemented with zinc oxide eugenol. The t numbers of failed crowns are shown in Fig. 7. Samples cemented onto POM with the provisional cement zinc oxide eugenol showed more chipping than samples of the other groups. Chipped veneering was absent only in the groups cemented onto titanium with zinc oxide phosphate, glass ionomer, and zinc oxide eugenol cement.

3.3.

Results of fracture resistance test

After ageing, all samples without any signs of chipping were subjected to a fracture resistance test. Table 9 lists all mean values of fracture resistances. Samples cemented onto titanium implant analogs with glass ionomer cement showed

Discussion

We were unable to prove our hypothesis that zirconia restorations require stable and stiff foundations to obtain sufficient marginal adaptation and to avoid veneer chipping. Several factors have already been discussed in the literature with regard to reducing the risk of failures of full ceramic crowns, particularly adequate substructure design (Rosentritt et al., 2009a, 2009b, 2009c; Komine et al., 2010, Quinn et al., 2010a, 2010b), uniform thickness of the veneering (De Jager et al., 2005), and application of an adequate cooling protocol during the veneering process (Rues et al., 2010; Guazzato et al., 2010). The present study evaluated the influence of different luting agents on marginal adaptation, fracture resistance, and chipping behavior. Gaps in the cement at the margin between crown and implant may lead to plaque accumulation and subsequently


285

Fig. 5 – Mean values (standard error of mean and standard deviation values are listed in Table 4) of perfect, continuously sealed margins as the percentage of the total circumferences of all samples before aging [%]. Table 5 – p-values of significant differences between samples before the ageing simulation in consideration of the different cements and implant-dummies; t¼ titanium, POM¼ polyoxymethylene. Significant differences before the ageing simulation

Zinc oxide phosphate t

Zinc oxide phosphate POM

Glass ionomer POM

Resin composite t Zinc oxide t Glass ionomer t Composite POM Zinc oxide POM

0.020 0.000 0.001 0.000 0.000

– 0.003 0.049 0.000 0.009

– 0.000 0.003 0.000 0.000

to peri-implantitis and thus to the complete failure of a dental restoration (Li and White, 1999; Roulet et al., 1989; Behr et al., 2004; Behr, 2007; Heitz-Mayfield and Lan, 2010). On that account, Roulet et al. claimed that the quality of a margin sealing is crucial for the durability of an entire restoration (1989). We chose a semiquantitative investigation technique with the SEM (Roulet et al., 1989; Rosentritt et al., 2007) to evaluate such margins. Values before ageing show the sealing of a marginal gap after the removal of surplus cement. Zinc oxide phosphate cement on both types of implant dummies and the glass ionomer cement on POM appear to stick significantly less to the marginal gap than any other luting agent. In previous studies, both cements have shown good results compared to other luting agents (Li and White, 1999; Rosenstiel et al., 1998). This effect may be due to the antibacterial and

cytotoxic effect of zinc oxide phosphate and glass ionomer cement, in which little gaps are potentially tolerable (De la Macorra and Pradies, 2002; Lewinstein et al., 2005; Li and White, 1999; Rosenstiel et al., 1998; Welker et al., 1984). However, the extent to which this effect is detectable and its duration are yet unclear. Adhesive cement appeared to be less affected by either of the two parameters tested than by all the other cements under investigation. In the literature, composite cement has been described as being less soluble and has thus shown good results (De la Macorra and Pradies, 2002; Behr et al., 2004; Gerdolle et al., Panighi; Rosenstiel et al., 1998). According to Tinschert et al., the benefits of adhesive cementation are esthetics, the stabilization of full-ceramic crowns, high retention, and low abrasion of the cement gap (Tinschert et al., 2001; Pospiech et al., 2004). Disadvantages of composite cements

286


Fig.6 – Mean values (standard error and standard deviation are listed in Table 6) of perfect, continuously sealed margins as the percentage of the total circumferences of all samples after aging [%]. Table 6 – Mean values of perfect, continuously sealed margins as the percentage of the total circumferences of the samples after aging [%] with standard error of mean and standard deviation. Cement

Perfect margin [%]; mean Ti/POM

Standard error of mean Ti/POM

Standard deviation Ti/POM

Zinc oxide phosphate Resin Zinc oxide without eugenol Glass ionomer

88.1/70.6 89.5/96.0 78.7/69.8 93.5/71.5

2.8/2.1 2.1/0.2 0.6/0.5 1.0/2.3

10.2/14.0 12.5/5.5 15.6/24.7 6.1/19.0

Table 7 – p-values of significant differences between samples after the ageing simulation in consideration of the different cements and implant-analogs; t ¼ titanium, POM¼ polyoxymethylene. Significance of differences after the ageing simulation

Zinc oxide phosphate Ti

Composite Ti

Glass ionomer POM

Glass ionomer Ti

Composite POM

Zinc oxide phosphate POM Zinc oxide POM Glass ionomer POM Resin POM Zink oxide phosphate Ti Zinc oxide Ti

0.000 0.000 0.000 0.000 – –

0.000 0.000 0.000 0.000 0.000 –

– 0.000 – 0.000 – –

0.000 0.000 0.000 –

0.000 0.000 0.000 – – 0.000

are sensibility against application errors, time-consuming management, shrinkage, the danger of peri-implantitis caused by non-polymerized composite, and the difficulty in removing any surplus cement (Behr, 2007; De la Macorra and Pradies, 2002;

0.000

Pospiech et al., 2004). Hence, composite cements should be used only if they are essential for the stability or the retention of ceramic crowns (Behr, 2007; De la Macorra and Pradies, 2002). During the ageing simulation, the POM implant-dummies


seemed to slightly bend under the pressure of the simulator, so that the brittle cement broke leaving a gap. Flexile abutments or zinc oxide cement caused more marginal gaps as well as chipping in our study, though not significantly. In vitro studies have already shown that zinc oxide eugenol cement is inferior to other cements with regard to retention and the development of secondary caries (Li and White, 1999). The main reason for temporary cementation is the option to remove a crown without having to destroy it, but full-ceramic crowns may chip during wear or removal (Behr, 2007). Because of strong atomic linkage energy the ceramic atom patterns cannot slide or deform, making ceramics brittle. Ceramics are sensitive toward tensile and flexural stress and thus highly resistant to pressure. One of the main problems of full ceramic zirconia crowns is chipping due to chewing forces (Kollar et al., 2008; Pospiech et al., 2004; Raigrodski et al., 2012). Quinn did not find any significant difference between the chipping behavior of full ceramic and metal–ceramic crowns (Quinn et al., 2010a, 2010b). In this study, we simulated 5 years of wear. Despite factors, such as fabrication process, abutment design, and the three-dimensional fit of the core, the type of luting cement may influence the type of failure and the failure rate of zirconia-based restorations. All crowns were luted onto the abutments with different types of cements that were applied according to the manufacturers' instructions so that their behavior would not be influenced by application discrepancies (Rosenstiel et al., 1998). Samples cemented onto POM with the temporary cement zinc oxide eugenol showed more Table 8 – p-values of significant differences between the samples before and after the ageing simulation in consideration of the different cements and implant-dummies; t¼ titanium, POM¼ polyoxymethylene). Significance of differences due to the ageing simulation Zinc oxide Ti Zinc oxide phosphate POM Zinc oxide POM Composite POM Glass ionomer POM

0.000 0.000 0.000 0.000 0.000

287

chipping than samples of the other groups. No chipped veneering was observed in the samples cemented onto titanium with zinc oxide phosphate, glass ionomer, or zinc oxide eugenol cement. Use of the more flexible POM abutments, particularly in combination with temporary cementation, seemed to increase the rate of chipping of the coating ceramics due to mastication forces. This finding may indicate that chipping of coating ceramics could possibly be reduced by choosing a solid type of abutment and cement, although the differences were not significant. Previous studies found that abutments with lower e-moduli were destructive for fullceramic crowns (Kelly et al., 1990; Malament and Socransky, 2001; Scherrer and De Rijk, 1993; Rosentritt et al., 2000). The German Association for Prosthetic Dental Medicine and Biomaterials (DGPro) recommends that glass ceramic crowns should be cemented with adhesive luting agents. Such luting agents used to be the state of the art for full-ceramic crowns for a long time. Nevertheless, long-term clinical studies have shown the benefits of oxide ceramic crowns cemented with conventional cements. Oedman et al. described a 10-year survival rate of 92.2% (2001). However whether zirconia crowns should be provisionally cemented onto implants is not yet clear although, in our study, provisionally cemented zirconia crowns luted onto titanium implants did not show any significantly higher fracture rates than the other samples. The less solid POM abutments seemed to bend due to mastication forces during the ageing simulation and may therefore have resulted in higher chipping rates as shown in other studies (Kelly et al., 1990; Malament and Socransky, 2001; Scherrer and De Rijk, 1993; Rosentritt et al., 2000). However, these differences were not significant in our study. None of our samples showed any dissolving of coating ceramics from the core after the ageing simulation or damage of the zirconia framework as described in the literature (Kollar et al., 2008). Jokstad reported that zinc oxide phosphate and glass ionomer cement did not have any significantly different influence on survival rates of crowns in clinical trial lasting 8.5 years (Jokstad, 2004). In that study, samples cemented with zinc oxide phosphate, glass ionomer, or composite cement showed less chipping than the other

Fig. 7 – Number of samples with chipping due to the ageing simulation.

288


Table 9 – Mean values (with standard error and standard deviation) of fracture resistance in consideration of the different cements and implant-analogs; t ¼ titanium, POM¼ polyoxymethylene. Material

Mean fracture resistance [N]

Standard deviation


Zinc oxide phosphate, Ti/POM Zinc oxide eugenol, Ti /POM Composite, Ti/POM Zinc oxide no eugenol, Ti/POM Glas sionomer, Ti/ POM

1180.8/801.6 1469.1/1516.7 1704.3/1408.4 1992.4/882.5 2749.8/1015.2

1081.4/634.9 516.7/914.2 1302.6/877.9 819.9/576.9 625.8/582.0

382.3/240.0 182.7/527.8 492.3/331.8 309.9/235.5 221.2/132.2

Table 10 – p-values of significant differences between mean values of fracture resistance in consideration of the different cements and implant-dummies; t¼ titanium, POM¼ polyoxymethylene. Significance of differences between mean values of fracture resistance

Glass ionomer, Ti

Zinc oxide phosphate Ti Zinc oxide phosphate POM Zinc oxide POM Glass ionomer POM

0.012 0.001 0.004 0.010

samples. Samples on titanium implants showed generally less chipping and better survival rates, which conforms to the results obtained for non-implant supported crowns (Kelly et al., 1990; Malament and Socransky, 2001; Scherrer and De Rijk, 1993). Ceramics are sensitive toward plastic deformation. If implant dummies can be deformed by pressure as well as by bending and extending forces, the fracture resistance of ceramic may be decreased (Pospiech et al., 2004). The only significant differences found were between samples cemented onto titanium with both glass ionomer and zinc oxide phosphate and samples cemented onto POM implant dummies with zinc oxide phosphate, zinc oxide, and glass ionomer. The reason why the glass ionomer samples on titanium showed such a result may be explained by possible errors in the experimental setup. In contrast to TCML, fracture testing, during which crowns were loaded to failure in one single stroke, does not seem to be clinically relevant, although the results may provide helpful data for comparing specimens. During oral simulation, superficial ageing or wear effects cause deterioration of the material and reduce its fracture strength. Subsequent fracture testing could show the various influences of the ageing simulation on the materials and their weak points. The mean fracture strength of zinc oxide phosphate was 801.6 N on POM samples and 1180.8 N on titanium. Maximum chewing forces have been reported to range between about 100 and 900 N depending on several factors, such as the type of measurement, the sex and age of a patient, and others (Varga et al., 2011; Waltimo and Könönen, 1995). Samples on POM may not always survive without any damage. Load-bearing capabilities of at least 500 N are required in molar regions. Samples cemented onto both POM or titanium are able to withstand such forces (Koerber 1983). Two samples showed visible cracks, gained against speculation not the lowest results. According to the literature, cracks reduce the fracture strength of full ceramic crowns (Bhowmick et al., 2007;

Pospiech et al., 2004). Some samples may have had unrecognizable cracks, which reduced their fracture strength. When analyzing metal-free restorations, Tinschert reported that zirconia crowns had the highest stability in the beginning and also the best long-term durability (Tinschert et al., 2001; Pospiech et al., 2004). Rosentritt et al. found fracture resistance values between 1111 N and 2038 N for titanium samples luted with zinc oxide phosphate (Rosentritt et al., 2009a, 2009b, 2009c). The minimum values are rather different, probably because of a different study design. The mean value of the POM samples cemented with glass ionomer was 1051.2 N. One sample with a visible crack showed the highest result and finally broke including its framework. Again, the crack did not lead to the premature failure of the crown. The mean value of the titanium samples cemented with zinc oxide eugenol was 1992.4 N. Lim et al. reported fracture resistance values of about 741.21 N þ/- 41.95 N for zirconia crowns cemented onto titanium implants with zinc oxide eugenol (Lim et al., 2010). In their study, the mean value of titanium samples cemented with zinc oxide eugenol was 1992.4 N. Only three POM samples cemented with zinc oxide eugenol reached the fracture resistance test because of chipping during the ageing simulation. On that account, the low number of these samples led to a high standard deviation, thus the mean value of 1516.7 N of that group is insignificant. The mean fracture resistance of the composite POM samples was 1408.4 N and that of the composite titanium samples 1704.3 N, and the difference between these two values was insignificant. These two groups showed the highest number of samples, and these specimens broke including the zirconia framework. As soon as the veneering was stable enough to refrain from failing at the beginning, the entire ceramic crowns and their frameworks broke by the use of high forces, which may indicate that the veneering was strengthened by the composite cement as described in the literature (Öedman and Andersson, 2001) and able to tolerate such high forces. Kerler et al. did not confirm this finding in their study (Kerler et al., 2005). Rosentritt et al. reported fracture resistance forces between 1181 N and 2295 N for zirconia crowns in combination with adhesive cement (Rosentritt et al., 2009a, 2009b, 2009c) and also no significant difference to conventional cements. Lim et al. reported fracture resistance forces of 1560,78 N for zirconia crowns in combination with adhesive cement that significantly differed from the results of crowns cemented with zinc oxide eugenol in our study (Lim et al., 2010). Literature reports generally describe variable values of fracture resistance for zirconia crowns. For the fracture resistance of zirconia frameworks only,


Luethy et al. measured a value of 755 N (Lüthy et al., 2005) but Tinschert et al. a value of about 2000 N (Lüthy et al., 2005). The results of the fracture resistance test in that study did not show any significantly different influence of the luting agents on the fracture resistance of full-ceramic zirconia crowns. The only group with significantly lower results was the glass ionomer titanium group. These samples resisted lower fracture forces than the zinc oxide phosphate – titanium group, the zinc oxide phosphate – POM group, and the glass ionomer POM group. Therefore, our results are not consistent with the results of the above-mentioned studies and also in contrast to the literature. Possible aberrations in our study may have been caused by the experimental setup and the manufacturing of the crowns. A source of error for the durability of the crowns may have been the sharp edges of the implant analogs (Rosentritt et al., 2011) that could also not be reproduced by the round burs of the zirconia milling machine. As a result, all copings had to be adjusted manually, which in combination with the emerging heat can alter the structure of ceramic lattices (Kosmač et al., 2000; Luthardt et al., 2004; Curtis et al., 2006; Pospiech et al., 2004; Zhang et al., 2004, 2012). Contrary to that assumption, late grinding did neither increase the failure rate nor reduce the fracture resistance of zirconia crowns (Preis et al., 2012). The frameworks were sandblasted with fused aluminum oxide. Zang et al. argued that the fracture resistance of zirconia was reduced by little defects caused by sandblasting (Zhang et al., 2004). Other authors have stated that the fracture resistance of zirconia was not reduced but increased by compression stress during sandblasting (Preis et al., 2012; Karakoca and Yilmaz, 2009; Guazzato et al., 2005; Kosmač et al., 2000). To achieve close-to-reality exposure of the oral cavity necessitates standardized thermo-mechanical simulation of mastication. In comparison to clinical cases, the ageing simulator available at Regensburg showed similar results, though clinical cases are still essential (Behr et al. 2000). Parameters for the thermal and mechanical loading process followed established proposals in the literature with regard to temperature ranges (Marx, 1995; Palmer et al., 1992; Pfeiffer and Marx, 1989; Soh and Selwyn, 1992; Vult von Steyern et al., 2006) and mastication forces (Krejci, 1990; Rosentritt et al., 2009a, 2009b, 2009c; Rosentritt et al., 2001; Groeger et al., 2003; Körber and Ludwig 1983, Eichner 1963). Load-bearing capabilities of at least 500 N are required in molar regions; hence the clinical application of veneered zirconia crowns in posterior regions seems justified (Körber and Ludwig 1983). Neither national nor international norms exist for fracture resistance tests, hence the results can only be seen as an orientation but not as definite material parameters (Pospiech et al. 2004). The direction and the point of contact could have influenced the fracture forces (Rekow et al. 2006). On that account, it was important to place the samples in the same reproducible position in the testing machine. All samples were unique due to layering the veneering by hand as described above. Such placing may have led to differences in the dissemination of pressure, shear, and tensile stress and therefore to spreading of the fracture resistance values. To reduce fracture resistance, a 0.3 mm thin tinfoil was set between the samples and the steal sphere to decrease peaks of stress.

5.

289

Conclusion

Adhesive cementation offered the most perfectly sealed marginal gaps before and after TCML and was not influenced by the type of implant material used. Zinc oxide phosphate and glass ionomer showed good results. Zinc oxide was significantly influenced by the ageing simulation and resulted in marginal gaps. All crowns cemented onto POM had significantly less filled margins after ageing. However, over 69% of the marginal gaps of all samples were still perfectly sealed after the 5-year simulation. During the ageing simulation, most zirconia crowns with chipping had been cemented onto POM implant dummies with zinc oxide. This finding may indicate that use of abutments with lower e-moduli, particularly in combination with temporary cement, seemed to increase chipping. This study cannot give any recommendation for the choice of a luting agent. Every case is different and should be evaluated separately (Behr 2007). In general, the fracture resistance of full ceramic zirconia crowns in our evaluation was not significantly influenced, neither by the choice of cement nor by the choice of implant material.

r e f e r e nc e s

Behr, M., 2007. Adha¨siv oder konventionell zementieren. ZM 97 (10), 40–44. Behr, M., Balkenhol, M., Handel, G., 2009. Tempora¨re Befestigung von festsitzendem Zahnersatz.Wissenschaftliche Mitteilung der Deutschen Gesellschaft fu¨r zahna¨rztliche Prothetik und Werkstoffkunde. DGZPW. Behr, M., 2003. Der Regensburger Kausimulator. Zukunft im Brennpunkt 2, 77–78 (Mu¨nchen: Bayrische Forschungsalianz). Behr, M., Rosentritt, M., Mangelkramer, M., Handel, G., 2003. The Influence of different cements on the fracture resistance and marginal adaption of all-ceramic and fiber-reinforced crowns. Int. J. Prosthodont. 16 (5), 538–542. Behr, M., 2007. Zementierung von festsitzenden Restaurationen auf dentalen Implantaten. ZMK 23, 630–632. Behr, M., Rosentritt, M., Regnet, T., Lang, R., Handel, G., 2004. Marginal adaptation in dentin of a self-adhesive universal resin cement compared with well-tried systems. Dent. Mater. 20 (2), 191–197. Bhowmick, S., Meleńdez- Martıńez, J.J., Zhang, Y., Lawn, B.R., 2007. Design maps for failure of allceramic layer structures in concentrated cyclic loading. Acta Mater. 55 (7), 2479–2488. Curtis, A.R., Wright, A.J., Fleming, G.J., 2006. The influence of surface modification techniques on the performance of a Y- TZP dental ceramic. J. Dent. 34 (3), 195–206. De Jager, N., Pallav, P., Feilzer, A.J., 2005. The influence of design parameters on the FEA-determined stress distribution in CAD-CAM produced all-ceramic dental crowns. Dent. Mater. 21, 242–251. De la Macorra, J.C., Pradies, G., 2002. Conventional and adhesive luting cements. Clin. Oral Invest. 6 (4), 198–204. Gerdolle, D.A., Mortier, E., Loos-Ayav, C., Jacquot, B., Panighi, M. M., In vitro evaluation of microleakage of indirect composite inlays cemented with four luting agents. J. Prosthet. Dent. 93 (6): 563–570. Gro¨ger, G., Rosentritt, M., Behr, M., Handel, G., 2003. Ceramic veneering of Au alloy and CAM zirconia ceramics. J. Dent. Res. 82 (1454), 194. Guazzato, M., Walton, TR., Franklin, W., Davis, G., Bohl, C., Klineberg, I., 2010. Influence of thickness and cooling rate on

290


development of spontaneous cracks in porcelain/zirconia structures. Aust. Dent. J. 55, 306–310. Guazzato, M., Quach, L., Albakry, M., Swain, M.V., 2005. Influence of surface heat treatments on the flexural strength of Y- TZP dental ceramic. J. Dent. 33 (1), 9–18. Heitz-Mayfield, L.J., Lan, N.P., 2010. Comparative biology of chronic and aggressive periodontitis vs. peri-implantitis. Periodontol 53, 167–181. Jokstad, A., 2004. A split-mouth randomized clinical trial of single crowns retained with resinmodified glass-ionomer and zinc phosphate luting cements. Int. J. Prosthodont. 17 (4), 411–416. Karakoca, S., Yilmaz, H., 2009. Influence of surface treatments on surface roughness, phase transformation, and biaxial flexural strength of Y-TZP ceramics. J. Biomed. Mater. Res. B 91, 930–937. Kelly, J.R., Giordano, R., Prober, R., Cima, M.J., 1990. Fracture surface analysis of dental ceramics: clinically failed restorations. Int. J. Prosthodont. 3, 430–440. Kerler, T., Beuer, F., Gernet, W., 2005. In-vitro-Untersuchung zum Einfluss der Verblendkeramik auf die Bruchfestigkeit von Zirkoniumdioxidrestaurationen. Mu¨nchen: Dissertation. Ko¨rber, K.H., Ludwig, P., 1983. Maximale Kaukraft als Berechnungsfaktor zahntechnischer Konstruktionen. Dent. Labor 31, 55–60. Kollar, A., Huber, S., Mericske, E., Mericske-Stern, R., 2008. Zirconia for teeth and implants: a case series. Int. J. Periodont. Rest. Dent 28 (5), 479–487. Komine, F., Blatz, MB., Matsumura, H., 2010. Current status of zirconia-based fixed restaurations. J. Oral Sci. 52, 531–539. Kosmacˇ, T., Oblak, C., Jevnikar, P., Funduk, N., Marion, L., 2000. Strength and reliability of surface treated Y- TZP dental ceramics. J. Biomed. Mater. Res. 53 (4), 304–313. Krejci, I., Albertoni, M., Lutz, F., 1990. In-vitro-Testverfahren zur Evaluation dentaler Restaurationssysteme. Schweiz Monatsschr Zahnmed 100, 1164–1168. Lewinstein, I., Matalon,S., Slutzkey, S., Weiss, E.I. (2005) Antibacterial properties of aged dental cements evaluated by direct contact and agar diffusion tests. J. Prosthet. Dent. 93 (49):364–371. Lim, H.P., Yoo, J.M., Park, S.W., Yang, H.S., 2010. Fracture load of implant-supported zirconia allceramic crowns luted with various cements. Int. J. Prosthodont. 23 (4), 361–363. Li, Z.C., White, S.N., 1999. Mechanical properties of dental luting cements. J. Prosthet. Dent. 81 (5), 597–609. Lu, C., Wang, R., Mao, S., Arola, D., Zhang, D., 2013. Reduction of load-bearing capacity of all-ceramic crowns due to cement aging. J. Mech. Behav. Biomed. Mater. 17, 56–65. Lu¨thy, H., Filser, F., Loeffel, O., Schumacher, M., Gauckler, L.J., Hammerle, C.H., 2005. Strength and reliability of four- unit allceram posterior bridges. Dent. Mater. 21 (10), 930–937. Luthardt, R.G., Holzhu¨ter, M.S., Rudolph, H., Herold, V., Walter, M., 2004. CAD/ CAM- machining effects on Y- TZP zirconia. Dent. Mater. 20 (7), 655–662. Malament, K., Socransky, S., 2001. Survival of Dicor glass-ceramic dental restorations over 16 years. Part III: Effect of luting agent and tooth or tooth-substitute core structure. J. Prosteht. Dent. 86 (5), 511–519. Marx, R. 1995. Auswahl der Werkstoffe. In: T. Kerschbaum (Ed.), Adha¨sivprothetik. Bru¨cken – Attachements – Schienen – Veneers. Mu¨nchen. Urban & Schwarzenberg. ¨ edman, P., Andersson, B., 2001. Procera AllCeram crowns O followed for 5 to 10.5 years: a prospective clinical study. Int. J. Prosthodont. 14 (6), 504–509. Palmer, D.S., Barco, M.T., Billy, E.J., 1992. Temperature extremes produced orally by hot and cold liquids. J Prosthet. Dent. 67 (3), 325–327. Pfeiffer, P., Marx, R., 1989. Temperaturbelastung von Adha¨sivbru¨cken und ihre Auswirkung auf die

Verbundfestigkeit der Klebeverbindung. Schweiz Monatsschr Zahnmed 99, 782–786. Pospiech, P., Tinschert, J., Raigrodski, A., 2004. KeramikVollkeramik, Ein Kompendium fu¨r die keramikgerechte Anwendung vollkeramischer Systeme in der Zahnmedizin. 3M ESPE. Preis, V., Behr, M., Hahnel, S., Handel, G., Rosentritt, M., 2012. In vitro failure and fracture resistance of veneered and fullcontoured zirconia restorations. J. Dent. 40, 921–928. Quinn, GD., Studart, AR., Herbert, C., Verhoef, JR., Arola, D., 2010a. Fatigue of zirconia and dental bridge geometry: design implications. Dent. Mater. 26, 1133–1136. Quinn, J.B., Sundar, V., Parry, E.E., Quinn, G.D., 2010b. Comparison of edge chipping resistance of PFM and veneered zirconia specimens. Dent. Mater. 26 (1), 13–20. Raigrodski, A.J., Hillstead, M.B., Meng, G.K., Chung, K., 2012. Survival and complications of zirconia-based fixed dental prostheses: a systemic review. J. Prosthet. Dent. 107, 170–177. Rekow, E.D., Harsono, M., Janal, M., Thompson, V.P., Zhang, G., 2006. Factorial analysis of variables influencing stress in allceramic crowns. Dent. Mater. 22 (2), 125–132. Rosenstiel, S.F., Land, M.F., Crispin, B.J., 1998. Dental luting agents. A review of the current literature. J. Prosthet. Dent. 80 (3), 280–301. Rosentritt, M., Behr, M., Kolbeck, C., Handel, G., 2007. Marginal Integrity of CAD/CAM Fixed Partial Dentures. Eur. J. Dent. 1 (1), 25–30. Rosentritt, M., Behr, M., Lang, R., Kleinmmayer, S., Handel, G., 2001. Fracture strength of tooth coloured posterior fixed partial dentures. J. Dent. Res. 80, 57. Rosentritt, M., Behr, M., Thaller, C., Rudolph, H., Feilzer, A., 2009a. Fracture performance of computer-aided manufactured zirconia and alloy crowns. Quintessence Int. 40 (8), 655–662. Rosentritt, M., Behr, M., Van der Zel, J.M., Feilzer, A.J., 2009b. Approach for valuating the influence of laboratory simulation. Dent. Mater. 25 (3), 348–352. Rosentritt, M., Steiger, D., Behr, M., Handel, G., Kolbeck, C., 2009c. Influence of substructure design and spacer settings on the in vitro performance of molar zirconia crowns. J. Dent. 37, 978–983. Rosentritt, M., Kolbeck, C., Handel, G., Schneider-Feyrer, S., Behr, M., 2011. Influence of the fabrication process on the in vitro performance of fixed dental prostheses with zirconia substructures. Clin. Oral Investig. 15, 1007–1012. Rosentritt, M., Leibrock, A., Lang, R., Schamagl, M., Behr, M., Handel, G., 1997. Regensburger masticator. Mater. Test. 39, 77–80. Rosentritt, M., Plein, T., Kolbeck, C., Behr, M., Handel, G., 2000. In vitro fracture force and marginal adaptation of ceramic crowns fixed on natural and artificial teeth. Int. J. Protsthodont. 13 (5), 387–391. Roulet, J.F., Reich, T., Blunk, U., Noack, M., 1989. Quantitative margin analysis in the scanning electron microscope. Scanning Microsc. 3, 147. Rues, S., Kro¨ger, E., Mu¨ller, D., Schmitter, M., 2010. Effect of firing protocols on cohesive failure or all-ceramic crowns. J. Dent. 38, 987–994. Scherrer, S.S., De Rijk, W.G., 1993. The fracture resistance of allceramic crowns on supporting structures with different elastic moduli. Int. J. Prosthodont. 6, 462–467. Soh, G., Selwyn, M.J., 1992. An evaluation of exposure time and temperature in the thermocycling of dental restorative materials. Clin. Mater. 9 (1), 37–41. Tinschert, J., Natt, G., Mautsch, W., Augthun, M., Spiekermann, H., 2001. Fracture resistance of lithium disilicate-, alumina-, and zirconia-based three-unit fixed partial dentures: a laboratory study. Int. J. Prosthodont. 14 (3), 231–238.


Varga, S., Spalj, S., Lapter Varga, M., Anic Milosevic, S., Mestrovic, S., Slaj, M., 2011. Maximum voluntary molar bite force in subjects with normal occlusion. Eur. J. Orthodont. 33, 427–433. Vult von Steyern, P., Ebbesson, S., Holmgren, J., Haag, P., Nilner, K., 2006. Fracture strength of two oxide ceramic crown systems after cyclic pre-loading and thermocycling. J. Oral. Rehabil. 33 (9), 682–689. Waltimo, A., Ko¨no¨nen, M., 1995. Maximal bite force as its association with signs and symptoms of craniomandibular disorders in young Finnish non-patients. Acta Odontolo. Scand. 53, 254–258.

291

Welker, D., Oehring, H., Erbsmehl, J., Bleischwitz, B., 1984. Antimikrobielle Eigenschaften zahna¨rztlich- restaurativer Werkstoffe. Zahn Mund Kieferheilkd 72 (4), 291–304. Zhang, Y., Lawn, B.R., Rekow, E.D., Thompson, V.P., 2004. Effect of sandblasting on the long- term performance of dental ceramics. J. Biomed. Mater. Res. B Appl. Biomater. 71 (2), 381–386. Zhang, Z., Zhou, S., Li, Q., Li, W., Swain, MV., 2012. Sensitivity analysis of bi-layered ceramic dental restaurations. Dent. Mater. 28 (2), e6–14.

Effect of cements on fracture resistance of monolithic zirconia crowns.

Fracture resistance of monolithic zirconia molar crowns with reduced thickness.

Effect of Resin Bonded Luting Agents Influencing Marginal Discrepancy in All Ceramic Complete Veneer Crowns.

Effect of Veneering Methods on Zirconia Framework-Veneer Ceramic Adhesion and Fracture Resistance of Single Crowns.

Marginal adaptation of castable ceramic crowns.

Correlation between Microleakage and Absolute Marginal Discrepancy in Zirconia Crowns Cemented with Four Resin Luting Cements: An In Vitro Study.

CAM stone types on the marginal gap of zirconia crowns.

Fracture Strength of Aged Monolithic and Bilayer Zirconia-Based Crowns.

Chipping fracture resistance of denture tooth materials.

Evaluation of marginal adaptation of all-ceramic crowns and metal ceramic crowns.

Use of luting or bonding with lithium disilicate and zirconia crowns.

Effect of Different Luting Agents on the Retention of Lithium Disilicate Ceramic Crowns.

Marginal adaptation of four inlay casting waxes on stone, titanium, and zirconia dies.

Effect of thermocycling with or without 1 year of water storage on retentive strengths of luting cements for zirconia crowns.

Effects of cementation surface modifications on fracture resistance of zirconia.

Effect of adhesive luting on the fracture resistance of zirconia compared to that of composite resin and lithium disilicate glass ceramic.

CAM milling, laser sintering, and cast metal ceramic crowns.

Intraoral repair protocol for chipping or fracture of veneering ceramic in zirconia fixed dental prostheses.

Fracture of porcelain-veneered gold-alloy and zirconia molar crowns using a modified test set-up.

Misfit and fracture load of implant-supported monolithic crowns in zirconia-reinforced lithium silicate.

Fracture Load and Phase Transformation of Monolithic Zirconia Crowns Submitted to Different Aging Protocols.

Fracture Strength of Zirconia and Alumina Ceramic Crowns Supported by Implants.

Retention of Implant Supported Metal Crowns Cemented with Different Luting Agents: A Comparative Invitro Study.

Effects of framework design and layering material on fracture strength of implant-supported zirconia-based molar crowns.