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J Dent Res 85(3):272-276, 2006
© 2006 International and American Associations for Dental Research

RESEARCH REPORT
Biomaterials & Bioengineering

The Strengthening Mechanism of Resin Cements on Porcelain Surfaces

G.J.P. Fleming1,*, F.R. Maguire2, G. Bhamra1, F.M. Burke2, and P.M. Marquis1

1 Biomaterials Unit, School of Dentistry, University of Birmingham, St. Chad’s Queensway, Birmingham B4 6NN, UK; and
2 Department of Restorative Dentistry, Cork University Dental School & Hospital, Wilton, Cork, Republic of Ireland

* corresponding author, Materials Science Unit, Division of Oral Biosciences, Dublin Dental School & Hospital, Trinity College Dublin, Ireland, garry.fleming{at}dental.tcd.ie


   ABSTRACT
TOP
 ABSTRACT
INTRODUCTION
 MATERIALS & METHODS
 RESULTS
 DISCUSSION
 REFERENCES
 
All-ceramic crowns bonded with resin cements have increased performance, and two theories have been proposed. Marquis (1992) suggested that the resin modified defects by crack healing, while Nathanson (1993) proposed that resin polymerization shrinkage strengthened porcelains. Both theories imply a sensitivity of strengthening to defect size. The hypothesis tested was that resin strength enhancement is independent of defect severity. We ground 200 porcelain discs to remove imperfections and indented 120 to create a large defect. Discs were tested dry, wet, and after being coated with 75–100 µm of resin cement in bi-axial flexure. Disc strength with and without indentations was increased significantly when coated with 2 resin cements. Both cements significantly increased the strength independent of defect population, and the hypothesis was accepted. It is proposed that the combination of surface pre-treatment and cement moved the fracture origin from the porcelain/cement interface to the cement surface, consistent with resin strength enhancement independent of defect severity.

KEY WORDS: aluminous porcelain • bi-axial flexure strength • surface flaw distribution


   INTRODUCTION
TOP
 ABSTRACT
 INTRODUCTION
MATERIALS & METHODS
 RESULTS
 DISCUSSION
 REFERENCES
 
Considerable differences of opinion exist among practitioners and materials scientists as to which all-ceramic core material offers superior clinical performance. Investigations on all-ceramic crown performance have varied from axisymmetric finite-element analysis of molar crowns (Anusavice and Hojjatie, 1992) to quantitative fractography of failed restorations (Kelly et al., 1989, 1996; Kelly, 1999; Quinn et al., 2005). Interestingly, the fracture-initiating sites are controlled by the location and size of the critical flaw on the inner surface of the crown (Kelly et al., 1989, 1996; Anusavice and Hojjatie, 1992; Thompson et al., 1994; Kelly, 1999; Quinn et al., 2005). Laboratory load-to-failure tests that simulate the failure mechanisms responsible for the clinical failure of all-ceramic crowns have been used to determine performance. The potential longevity of alumina-reinforced discs was influenced by the condensation procedure (Fleming et al., 1999a), storage condition (Marquis, 1992), surface roughness (De Jager et al., 2000; Bhamra et al., 2002), and cement lute (Fleming et al., 1999b; Fleming and Narayan, 2003).

In the dental literature, the apparent resistance of all-ceramic crowns bonded with a resin cement was increased compared with that of crowns bonded with acid-base cements (Rosenstiel et al., 1993; Malament and Socransky, 1999a,b; Pagniano et al., 2005) and resulted in higher survival rates for crowns cemented and bonded with resin cements (Malament and Socransky, 1999a,b). Two theories have been proposed for the apparent improvement. Marquis (1992) suggested that the resin cement modified the surface flaw population by a process of crack healing, which increased the resistance to fracture. In contrast, Nathanson (1993) proposed that the polymerization shrinkage of resin cements ‘stresses’ the molecules together, which strengthens the porcelain. However, the apparent strengthening mechanism behind the proposed theories has not been proven, even though each of the theories may lead to differences in the choice of cement used to maximize clinical performance. The aim of the current study was to test the validity of the proposed resin-strengthening mechanisms to facilitate an improved understanding of the possible mechanisms involved.


   MATERIALS & METHODS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS & METHODS
RESULTS
 DISCUSSION
 REFERENCES
 
Specimen Condensation
Discs were condensed with 1.1 g of Vitadur-Alpha core porcelain powder (lot no. 62167: Vita, Bad Säckingen, Germany) and 0.33 mL of Vita Modelling Fluid (lot no. 5060: Vita, Bad Säckingen, Germany) into a perspex mold (12 mm diameter and 2 mm thickness) secured with burnished aluminum (Fleming et al., 1999a). Specimens were fired in a vacuum furnace (Vita Vacumat 40, Vita Zahnfabrik, Bad Säckingen, Germany) at 1120°C, air-cooled, and stored in a desiccator to prevent hydrolysis.

Specimen Surface Modification
Using an alcohol-based lubricant (DP-Blue, Struers, Glasgow, Scotland), we ground 200 specimens on a Dap-7 Pedemin grinding and polishing machine (Struers, Glasgow, Scotland) with graded silicon carbide (SiC) abrasive papers (P500, P800, P1200, P2400, and P4000). Specimens were affixed to aluminum blocks (30 mm diameter) with cyanoacrylate adhesive (RS Components, Corby, England) before being ground for 30 sec at a force of 10 N, followed by a change of grinding paper and two 30-second intervals at 20 N, with a change of grinding paper, for each SiC paper grade (Bhamra et al., 2002). The ground disc specimens were separated from the blocks and stored in a desiccator. One hundred twenty ground specimens were indented to produce an indent of 30–40 µm (Fig. 1aGo), by means of a Vickers Micro-Hardness Indenter (Vickers Instruments Ltd., York, England), for 5 sec at 50 N to the centers of the specimens..


Figure 1
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  		Figure 1. Representative profilometry traces of the controlled defect populations, namely, (a) the indented (group B) and (b) the ground (group A) aluminous core porcelain surfaces examined in the current study.

 
Twenty ground (group A) and 20 indented (group B) specimens were stored in a desiccator as ‘dry’ controls. Twenty ground (group C) and 20 indented (group D) specimens were placed in a water-bath (37 ± 1°C) for 24 hrs as ‘wet’ controls. Four groups (E–H) were coated on the prepared surface with encapsulated resin cements. Compolute® Aplicap® (3M ESPE, Seefeld, Germany) was utilized for ground (group E) and indented (group F) specimens, while Unicem® Aplicap® (3M ESPE, Seefeld, Germany) was utilized for groups G and H (ground and indented, respectively). The specimens were etched with Mirage SuperEtch (Chameleon Dental Products Inc., Kansas City, KS, USA) for 90 sec and washed thoroughly with distilled water. The etched surface was primed with Monobond-S (Vivadent, Schaan, Liechenstein) for 60 sec and air-dried. The capsules were activated and placed in a Rotomix® (3M ESPE, Seefeld, Germany), for 8 sec for the Compolute® Aplicap®, and for 10 sec for the Unicem® Aplicap®, in accordance with the manufacturers’ instructions. The cement was extruded onto a glass cover-slip, the prepared disc surface was pressed lightly onto the cement, and the porcelain disc/cement/cover-slip assembly was transferred to a penetrometer (Fleming et al., 1999b). A force of 9.81 N was applied until a cement film thickness of 75–100 µm was produced. The cement was light-activated (Optilux 501, SDS Kerr, Danbury, CT, USA) for 20 sec at a light intensity of 740 mW cm–2, with a 13-mm-diameter curing tip to ensure complete cure of the cement. Two additional already-indented groups determined the impact of etching (group I) and etching and priming (group J) by acting as further controls, and all samples were placed for 24 hrs in the water-bath (37 ± 1°C).

Bi-axial Flexure Strength Testing
We determined the bi-axial flexure strength by centrally loading the unprepared surface on a thin rubber film on a 10-mm-diameter ring-support with a 4-mm-diameter spherical ball indenter at 1 mm/min. The bi-axial flexure strength was calculated from the following equation (Timoshenko and Woinowsky-Krieger, 1959):


Formula

where {sigma}max was the maximum tensile stress, P the measured load at fracture, a the radius of the knife-edge support, v the Poisson’s ratio of 0.25 (Zeng et al., 1998), and h the specimen thickness. When the bi-axial flexure strength was calculated, the thickness used did not include the cemented layer. Multiple comparisons of group means were made by one-way analysis of variance (ANOVA) and Tukey’s multiple-range tests at P < 0.05. The biaxial flexure strength data were ranked in ascending order, and a Weibull analysis (Weibull, 1951) was performed. The confidence limits for groups subjected to Weibull analysis were considered to be significant when the confidence intervals did not overlap.

Profilometry
We used a Profilometer (Taylor Hobson, Form TalySurf Series 2, Leicester, UK) to examine the surface roughness of the ground and indented porcelain control surfaces (Fig. 1Go). The analysis method used the Ra value, which is the arithmetic mean of the absolute departures of the roughness profile from the mean line. Three readings were taken across the center of each specimen, with a ‘traveling distance’ of 2 mm.


   RESULTS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS & METHODS
 RESULTS
DISCUSSION
 REFERENCES
 
A significant increase was identified between the mean bi-axial flexure strengths of the dry ground (group A) and dry indented (group B) control groups (P < 0.05) when the one-way ANOVA and paired Tukey test comparisons were analyzed (TableGo). Analysis of the profilometry results highlighted that grinding produced a surface roughness with an associated Ra of 0.87 µm, with asperities varying from 4 to –6 µm. The controlled Vickers indent varied between –30 and –35 µm, resulting in an increase in the Ra value to 2.81 µm (Fig. 1Go). Immersion of the specimens in a water-bath for 24 hrs prior to being tested significantly reduced the strength (P < 0.05) when the ground (group C) and indented (group D) controls were compared with their respective dry controls, although the survival probability distributions demonstrated similar symmetries (Fig. 2aGo).


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  		Table. The Influence of Storage Condition and Cement Lute on the Controlled Porcelain Surface Preparations
 

Figure 2
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  		Figure 2. Survival probability distributions of (a) the dry ground and dry indented [{sigma}m = 171.7 (11.7) and 104.6 (11.9), groups A and B, respectively] and wet ground and wet indented [{sigma}m = 105.7 (12.0) and 80.4 (9.8), groups C and D, respectively] control defect populations for n = 20 specimens per group. {sigma}m is the mean fracture strength, and the numbers in parentheses are standard deviations highlighting the significant strength reduction on immersion. Survival probability distributions of (b) the dry, wet, acid-etched and acid-etched and silane indented controls [{sigma}m = 104.6 (11.9), 80.4 (9.8), 79.9 (5.2), and 81.8 (7.0) (groups C, D, I, and J, respectively], highlighting that no effect was evident following acid-etching.

 
No strength reduction was achieved among the indented ‘wet’ (Group D), the indented etched (group I), or the indented etched and primed (group J) controls when the paired Tukey test comparisons or symmetry of the survival probability distributions was analyzed (Fig. 2bGo). A significant increase in the fracture strength of the ground and indented specimens coated with the Compolute® Aplicap® (groups E and F) and Unicem® Aplicap® (groups G and H) resin cements was identified, compared with the wet controls (groups C and D, respectively), with similar symmetries in the survival probability distributions (Figs. 3a, 3bGo).


Figure 3
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  		Figure 3. Survival probability distributions of the ground and indented porcelain surfaces coated with the resin cement (a) Compolute® Aplicap® [{sigma}m = 130.8 (14.1) and 113.9 (10.7), for groups E and F, respectively] and (b) Unicem® Aplicap® [{sigma}m = 180.9 (19.9) and 155.7 (20.7), groups G and H, respectively], highlighting the significant strength increase compared with that of the wet ground and indented controls ({sigma}m is the mean fracture strength, and the numbers in parentheses are standard deviations for n = 20 specimens per group).

 

   DISCUSSION
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS & METHODS
 RESULTS
 DISCUSSION
REFERENCES
 
Problems with reproducing a mechanical testing regime to replicate the in vivo failure mechanism encountered by luted all-ceramic crowns in clinical practice make it difficult for laboratory data to be extrapolated. The utilization of load-to-failure mechanical tests that load the occlusal surfaces of all-ceramic crowns has been reported extensively in the dental literature (Scherrer and de Rijk, 1992; Casson et al., 2001). However, non-standardization of protocols, including loading through either a ball or bar arrangement, markedly influences the test results, so that extrapolation of the data to provide information on clinical performance is problematic. As a result, we used the bi-axial flexure test in the current study, since it replicates the failure mechanism, namely, tensile failure on the inner surface of the crown.

It is proposed that the significant decrease in flexure strength for discs tested ‘dry’ following indentation (group B), compared with ‘dry’ non-indented discs (group A), was attributed to the introduction of the surface flaw, which acted as a stress concentrator (Griffith, 1920). As a result, a decreased applied load was required to initiate crack propagation in indented samples, decreasing the mean bi-axial strength compared with that of the non-indented control. Stress corrosion theory predicts the extension of crack tips by hydrolysis of the Si-O-Si bonds, due to the relative ease of ion exchange between mobile alkali ions (M+) in dental porcelain and the H+ ions in solution (Douglas and Isard, 1949; Budd, 1961). Water or water vapor reacts with the molecules at the crack tip, breaking the Si-O-Si bonds to form a hydroxide group (Charles, 1958,b), such that slow crack growth at the crack tip would advance the crack length following immersion in an aqueous environment, resulting in the strength decreases identified (TableGo).

No significant strength reduction was achieved for the indented ‘wet’ (Group D) compared with the indented etched (group I) and indented etched and primed (group J) controls. The acid would have been expected to attack the glassy phase in the aluminous core porcelain; however, this was not seen on examination of the survival probability distribution, and the etching effect was probably negated by water storage (static fatigue) (Fig. 2bGo). Previously, Fleming et al.(1999b), using scanning electron microscopy (SEM), showed that it was difficult to detect the morphological changes following acid-etching by SEM, since the glass dissolution rates were too low. It is proposed that the combination of the low glass dissolution rate and the static fatigue on wet storage made it difficult to distinguish the expected strengthening effect from the acid-etching reported consistently in the dental literature (Fleming et al., 1999b; Fleming and Narayan, 2003).

To provide an insight into the role of resin cements in the clinical performance of all-ceramic crowns, we coated specific surface flaw distributions with a 75–100-µm film thickness. Discs with indentations were strengthened by 28% when coated with Compolute® Aplicap® (group F), and by 48% when coated with Unicem® Aplicap® (group H), compared with the wet indented control (group D). Discs with no indentations were strengthened by 20% when coated with Compolute® Aplicap® (group E), and by 42% when coated with Unicem® Aplicap® (group G), compared with the wet ground control (group C). The capacity of the resin cements to modify ground surfaces (groups E and G) and indented surfaces (groups F and H), with no statistical differences between the strength increases for each resin cement type, suggests that the strengthening mechanism was not dependent on the surface flaw population. These findings are not consistent with the proposal, put forward by Marquis (1992), that the apparent resistance of all-ceramic crowns bonded with a resin cement modified the surface flaw population by crack healing. If crack healing was operative, then the apparent resistance to fracture of the resin-coated indented discs would have been markedly increased compared with that of the resin-coated ground discs for both resin cements investigated in the current study. Similarly, the theory proposed by Nathanson (1993) suggested that the greater intermolecular distance across the indent would reduce the intermolecular attraction, such that failure would occur at lower stress levels than in ground specimens, as was confirmed earlier. However, Nathanson (1993) proposed that the polymerization shrinkage of resin-luting materials can increase the apparent resistance of cemented discs by ‘stressing’ the molecules together, rather than away from each other. As a result, the apparent resistance to fracture of the resin-coated indented discs would have been expected to increase compared with that of the resin-coated ground discs for both resin cements investigated, if the author’s theory was entirely correct. Therefore, the proposed hypothesis, that strength enhancement of the resin layer is independent of defect severity, was accepted.

In the pre-cementation treatment, the resin cement required substantial surface preparation (acid-etching and resin priming) for an intimate bond to be achieved between the porcelain surface and the cement lute (McLean and Kedge, 1987). It is proposed by the authors that the combination of the surface preparation and the cement lute acted to move the fracture origin from the porcelain/cement interface to the cement surface. The enhanced energy requirement to fracture the resin-cemented specimens is proposed to be the energy required for a crack to travel through the resin cement to the porcelain/cement interface. The apparent fracture strength at the interface is then dependent upon the surface roughness, and, therefore, the greater intermolecular distance for the indented specimens reduces the intermolecular attraction such that failure occurs at lower stress levels than in the ground specimens (TableGo).

The possible movement of the fracture initiation site from the porcelain/cement interface to the cement surface would mean that both the bilayered equation suggested by Rosenstiel et al.(1993), derived from the work of Timoshenko and Woinowsky-Krieger (1959), and the equation used in the current study are inappropriate, since these equations assume that failure occurs at the porcelain/cement interface. The calculation of the bi-axial fracture strength, according to the bilayered equation suggested by Rosenstiel et al.(1993), highlighted no strength differences compared with the Timoshenko and Woinowsky-Krieger (1959) strength equation used by the authors in the current study. Unfortunately, the mismatch in thickness made it impossible to distinguish the fracture mode clearly (Lawn et al., 2002); however, the results do unambiguously show that a layer of well-bonded cement does substantially increase the fracture resistance of an aluminous core porcelain.


   ACKNOWLEDGMENTS
 
This work was done in partial fulfillment of the requirements for an MPhil degree for F.R. Maguire. The authors acknowledge the support of this research from the National University of Ireland at Cork University Dental School & Hospital Department of Restorative Dentistry, and the University of Birmingham (UK) School of Dentistry Biomaterials Unit.

Received May 27, 2005; Last revision October 10, 2005; Accepted October 20, 2005


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 ABSTRACT
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 MATERIALS & METHODS
 RESULTS
 DISCUSSION
 REFERENCES
 
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