Bond Strength of Luting Cements to Core Foundation Materials

Patients are retaining their natural dentition for longer periods because of improved dental care and successful contemporary endodontic treatment. Dentists are therefore faced with restoring tooth surfaces compromised by caries, trauma, and endodontic treatment.^1,2 For many patients, this involves the placement of a foundation material to replace lost tooth structure, which then receives an indirect restoration that is cemented into place with a luting cement.

Retention of a complete coverage restoration is a function of tooth preparation, specifically the axial taper, height, surface area, and surface roughness, which limits the paths of displacement of the restoration.^3,4 Frequently, destruction of tooth structure as a result of caries, fractures, and root canal treatment leaves little tooth structure to allow for retentive preparations. To restore the tooth, a clinician may therefore choose from various options, which include cast post and cores, coronal radicular foundations, pin-retained core foundations, and prefabricated posts with core foundations.^5,6 Of these, the use of prefabricated posts with amalgam, composite resin, or glass ionomer foundations has become widespread.^5,7 Their popularity is because of reduced chairside time, ease of manipulation, and a reduced cost when compared with cast post and core.

The availability of adhesive luting cements, which bond tooth substance and/or core foundation substrates to the definitive restoration, offers the potential for increased crown retention that is independent of preparation geometry. The role of cements in improving the durability of restorations has been shown in several studies.^8-13 Resin cements used with dentin bonding agents have been shown to provide increased crown retention compared with other classes of cements⁶ and have been recommended as the luting agents of choice for bonding ceramic, metal, and indirect composite restorations.¹⁰ It has been reported that all-ceramic crowns that were etched and cemented with resin cement had a significantly higher survival rate after 16 years compared with crowns using cements that function through mechanical retention only.¹¹ This finding has been confirmed by studies that show bond strength between tooth, cement, and restorative materials is the major contributing factor to the improvement.^6,8-10 In light of these reports, a strong bond between luting cement and core foundation material is expected to be beneficial.

Retention of luting cements to foundation materials can be affected by water uptake, leading to hygroscopic expansion and dissolution at restoration margins. It is reported that water-based cements show greater erosion in acidic storage media while resin-based cements demonstrate a hygroscopic expansion.¹⁴ Immersion in lactic acid has been used effectively to evaluate the effect of acidic media on cements.^14-17 The purpose of this study, therefore, was to compare the shear bond strength of a glass ionomer luting cement, a resin-modified glass ionomer (RMGI) luting cement, and a resin luting cement to the following foundation (core) substrates: a spherical alloy amalgam, composite resin, RMGI, and glass ionomer. Because cements undergo chemical breakdown in the oral cavity,^14-17 the effect of immersion in aqueous lactic acid solution on the bond strengths also was investigated. The hypothesis of this research is that foundation materials and cements that are similar in chemistry will provide higher bond strengths than chemically different materials.

Disks 8 mm in diameter and 2 mm in thickness were fabricated in a metal mold from each of the following foundation substrates: amalgam (Tytin^®, Kerr Corporation, https://www.kerrdental.com), an auto-polymerizing composite resin (CompCore, Premier Products Company, https://www.premusa.com), a tri-polymerized RMGI (Vitremer^™ Core Buildup, 3M^™ ESPE^™, https://www.3MESPE.com), and a traditional glass ionomer foundation (Ketac^™ Molar, 3M ESPE).

The disks were stored at 37ºC and 100% relative humidity for 1 week. Specimens were then embedded with Type III dental stone (Lincoln Dental Supply, https://www.lincolndental.com) in copper rings that were 2.5 mm in diameter and 2.5 mm in height. The top surfaces of the disks, stone, and copper rings were parallel and in the same plane. The surfaces were ground on 320-grit silicon carbide paper (Leco Corp, https://www.leco.com) until a flat surface was obtained for bonding.

Cylinders of the test cements measuring 2 mm in diameter and 4 mm in length were fabricated in a metal mold. The cement cylinders were bonded to the foundation materials by expressing mixed cement onto the foundation substrate and pressing on the cement cylinder under a 500-g load. The cements used (Table 1) were a resin luting cement (Calibra^®, DENTSPLY International, https://www.denstply.com), RMGI luting cement (RelyX^™ Luting Plus, 3M ESPE), and a traditional glass ionomer luting cement (Ketac^™ Cem, 3M ESPE). Manufacturers’ instructions were followed for each material. A total of 20 specimens of each core foundation material/cement pair was stored at 37ºC and 100% relative humidity for 24 hours. A second set of 20 specimens was stored at 37ºC in a 0.01M lactate buffer solution at a pH of 4 for 24 hours. Shear bond strength was determined using a mechanical testing machine (Instron^®, https://www.instron.com) at a crosshead speed of 0.5 mm/min. The peak load at fracture was recorded and the shear bond strength calculated. Bond strengths of luting cements to the foundation substrates were compared using a three-way analysis of variance (ANOVA). Foundation material, luting cements, and immersion were the independent variables, and shear bond strength was the dependent variable. Multiple pairwise comparisons were performed using the Holm-Sidak method to identify groups that were significantly different (P < .05). The fractured specimens were examined with low power magnification to determine whether the fracture occurred within the luting cement or foundation substrate.

The shear bond strength of luting cements to core foundation materials after 24 hours in 100% humidity at 37ºC is shown in a bar graph in Figure 1. Figure 2 shows the bond strengths obtained after immersion in lactic acid. Statistical analysis showed significant differences (P < .001) for all three factors investigated: foundation material, luting cement, and immersion status. The resin cement showed significantly (P < .05) higher bond strength to amalgam, composite resin, and RMGI foundation materials compared to RMGI and glass ionomer luting cements. The bond between the foundation materials, amalgam, and RMGI, and the RMGI luting cement and glass ionomer luting cement did not differ significantly. The glass ionomer luting cement showed the lowest bond strength to composite resin and glass ionomer foundation materials. The bond strengths of RMGI and glass ionomer luting cements to amalgam and RMGI foundation were not statistically different from each other. The RMGI luting cement had the highest bond strength to glass ionomer foundation, although not significantly different from the resin luting cement.

After immersion in the lactate buffer solution, all specimens recorded significantly lower shear bond strengths compared to those stored in 100% humidity, except for the composite resin luting cement/composite resin foundation material pair, which showed a bond strength increased by 26%. The highest bond strength after immersion was obtained between the composite resin foundation and resin luting cement followed by the RMGI foundation and resin cement pair. The lowest bond strengths were obtained between amalgam and glass ionomer and also glass ionomer foundation material and glass ionomer luting cement. The bond strength between these pairs of materials was not measurable. A statistically significant interaction was observed between foundation materials, luting cements, and immersion state (P < .001). Evaluation of interactions between foundation material and cement throughout levels of immersion showed a significant interaction among nonimmersion (P < .001) as well as immersion groups (P < .001).

After shear testing was done, the fracture surfaces were examined to determine the failure site and its characteristics. Table 2 shows the types of failure observed at interfaces after fracture. All luting cement specimens bonded to amalgam (100%) produced adhesive failure between the cement and amalgam. For the composite resin foundation material, bonds with RMGI luting cement and glass ionomer cement produced 100% adhesive failures at the interface. However, most (90%) composite resin foundation material specimens bonded to resin luting cement produced cohesive failure within the composite resin foundation material. For the RMGI foundation, adhesive failure was observed for bonds with the resin luting cement and RMGI luting cement. Thirty percent of the bonds between RMGI foundation and the glass ionomer luting cement were adhesive and 70% were cohesive in the luting cement. The failures of bonds of the glass ionomer foundation material to resin and glass ionomer luting cements were adhesive. Seventy percent of bonds between glass ionomer foundation material and RMGI luting cement failed within the luting cement.

This study compared the shear bond strengths of three luting cements to four commonly used foundation materials. The bond strengths after immersion in a 0.01M lactate buffer solution were also compared. The results show that the resin cement had the highest overall bond strength to most of the foundation materials investigated. This finding confirms the results of a recent investigation of the retentive force of crowns cemented onto foundation materials.⁶ Furthermore, the bond between the composite resin foundation and resin cement materials was the most durable of the materials investigated after storage in acidic media.

Two factors contribute to bond strength: chemical adhesion and mechanical retention.^18-21 Combinations of materials of similar composition, such as the resin luting cement and composite resin foundation material and the pairs RMGI foundation/RMGI luting cement and glass ionomer foundation/glass ionomer cement, were expected to bond by both mechanical retention and chemical adhesion. However, these pairs did not show the highest bond strengths. Because all the cements bond to amalgam by mechanical retention only, weaker bond strengths to amalgam were expected compared to the other foundation materials. This was the case for RMGI luting cement and glass ionomer luting cements. The bond between the resin luting cement and amalgam was higher than the bond between some material pairs of similar chemical composition, such as glass ionomer foundation and glass ionomer luting cement. This low bond strength between a glass ionomer foundation material and a glass ionomer luting cement is in agreement with a previous investigation of retentive strengths of dental cements.²¹ The bond between RMGI foundation and resin luting cement was better than RMGI foundation to RMGI luting cement.

Table 2 shows the failure site of the bond between cements and foundation materials. When failure of the bonded unit is cohesive in one material rather than at the bond interface, it can be concluded that the interface is sufficiently strong to resist the local tensile stress.^22,23 The bond strength reported is then likely to be lower than the true bond strength. Studies of the stress distribution at the interface of the shear bond test show that the stresses are predominantly tensile. Substrates of low tensile strength are therefore likely to show cohesive failure. It can be concluded that the bond between composite foundation and resin luting cement is probably stronger than the value reported in this investigation. Correspondingly, the bond between RMGI foundation/glass ionomer luting cement and the glass ionomer foundation/RMGI luting cement may be stronger than the intrinsic tensile strength of the foundation substrates.

After immersion in the lactate buffer solution, the shear bond strengths decreased because of erosion of the materials and a susceptibility to degradation of the bond by lactic acid.¹³ In general, the glass ionomer cement showed the lowest bond strength after immersion (Figure 2). This could be due to a a high degree of susceptibility to degradation.^13-17,24 The bond between the composite foundation and resin luting cement, however, was higher after immersion in lactic acid. This is likely because of the reported increase in strength of composite resins on immersion in liquids that has been attributed to a leaching out of residual monomer.^25,26 The behavior of the RMGI luting cement lay between the extremes provided by the resin and glass ionomer cements. This intermediate behavior has been attributed to the similarities between RMGI luting cement and resin cement and glass ionomer cement. This conflicts with the findings of Czarnecka and Nicholson,¹⁵ who concluded that the presence of the resin phase in resin-modified glass ionomers makes little or no difference to the overall interaction with aqueous media.

Other criteria should be considered when selecting a foundation material. It is now generally recognized^5,27 that the margin of the final restoration should extend beyond the core material–dentin junction to provide a “ferrule effect.” In this way, the restoration mechanically grasps the root, not just the foundation material. This effect, which usually extends as a collar around the tooth surface, aids in the transmission of shear and tensile stresses to the coronal tooth structure and prevents root fractures. It has been shown that when adequate tooth structure is present with at least a 2-mm ferrule apical to the margin of the core foundation, any foundation material is acceptable. However, there are times clinically when remaining tooth structure is minimal and the margins of the crown must be placed at or just apical to the core.⁵ It is under these conditions that the choice of the foundation material and luting cement becomes critical.

The interaction between foundation material and cement was demonstrated in a study in which the fatigue life of three core materials under full cast crowns prepared 0.5 mm to 1 mm below the margins of the foundation and cemented with zinc phosphate cement were compared.⁵ It was observed that all the teeth restored with glass ionomer foundations failed under the conditions of the study and the material did not have adequate strength to withstand occlusal loading when the majority of the forces were borne by the core material. The composite resin foundations had adequate strength (no core fractures were observed); however, 83% eventually failed. Forty percent of the failures resulted from post fractures and 50% were from failure of the zinc phosphate cement at the crown–core interface. The high number of cement failures for composite resin cores was associated with the material’s apparent low modulus of elasticity. Flexure of the composite resin core placed significantly higher tensile stresses on the rigid zinc phosphate cement interface. These results imply that the choice of luting cement with a similar modulus of elasticity to the core material may reduce the incidence of cement failure. Glass ionomer cores, though less flexible than composite resin cores, allowed significantly more flexure than the amalgam cores.⁵ Based on these reports, the authors expect that the high bond strength between resin cement and composite resin core material will enhance the integrity and durability of the restoration.

Until now, the choice of core foundation materials and luting cements has largely been left to practitioner’s preference and has been dictated by ease of manipulation, past experience with these materials, cost of the materials, and the number of visits required. In view of the results, the bond strength between a core material and cement should be considered in the selection of materials for restoring broken down and endodontically treated teeth. The durability of cements as measured by resistance to dissolution or disintegration is also an important consideration.^13,28,29 Acid erosion is of particular clinical significance because acidic conditions can occur in the oral cavity either by ingestion of acidic foods and drinks or by the degradation of polysaccharides. The results of this investigation show that composite resin foundations bonded to resin cements provide the most durable combination in the presence of an acid environment. Bonds involving glass ionomer foundations and luting cements undergo the highest degree of degradation in an acid environment.

Within the limitations of this study, the following conclusions were made: 1) the resin luting cement has the highest shear bond strength to most foundation substrates investigated; 2) the bond between the composite foundation and resin luting cement was also the most durable on immersion in lactic acid; and 3) glass ionomer luting cement showed low bond strengths to foundation substrates investigated and was the most susceptible to breakdown on immersion in lactic acid.

Sandra Hewlett, BDS
Department of Restorative Dentistry
University of Ghana Dental School
Korle-bu, Ghana

Rose O. Wadenya, BDS, MS, DMD
Department of Preventive and Restorative Sciences
University of Pennsylvania
Philadelphia, Pennsylvania

Francis K. Mante, PhD, DMD
Department of Preventive and Restorative Sciences
University of Pennsylvania
Philadelphia, Pennsylvania