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Aesthetic and Adhesive Cementation for Contemporary Porcelain Crowns

In the past decades, dentistry (and aesthetics in particular) has undergone an evolution. Innovative dental materials have led to the development of contemporary restorative systems and preparation designs that require different cementation materials and methods. Conven­tional cements, with acid-base reactions, are adequate for restorations that necessitate mechanical and macromechanical retention and limited aesthetic expectations. Resin cements, with polymerization reactions, have been formulated as alternatives to conventional cements. Associated with the conditioning of substrates, the use of resin cements is primarily based on micromechanical interlocking and intended for aesthetic results. This allows the use of less invasive preparations, as in partial metal-free restorations, since bond strength is less dependent on the mechanical retention of the preparation design and relies more on adhesion, which reaches values of 30 MPa to 40 MPa. This bond strength is essential for porcelain laminate veneers and ceramic inlays and onlays, where conventional cementation is contraindicated. Considering the respective limitations of conventional and resin cements, resin-modified glass-ionomer cements have been created in order to combine the advantages of glass-ionomer cements and resin cements.

Full-coverage crowns, too, have acquired different features. Prior to the introduction of adhesive luting agents and metal-free restorations, conventional metal-ceramic crowns were almost the only option to achieve aesthetics in anterior and posterior regions. These restorations traditionally featured a metal collar at the cervical margin. Despite the use of different preparation designs, the presence of this metal substructure has somewhat limited natural light transmission in the cervical area.

The development of dental materials for all-ceramic restorations has allowed clinicians to successfully treat many anterior cases, yet there are some clinical situations where a metal substructure is still indicated:

  • Severely discolored dentin substrates.
  • Metal cast posts and cores.
  • Posterior teeth.
  • Virtually all fixed partial dentures or splinted restorations.
  • Patients with parafunctional habits.
  • Low-value teeth.

The presence of metallic posts and cores has often been an aesthetic concern in endodontically treated teeth, and root fractures have been attributed to the corrosive by-products of these posts (Figures 1-2-3-4-5).1 Innovative materials (eg, zirconium and glass fiber posts) have been introduced for use with either cast posts or posts and cores. While fiber posts have many mechanical advantages due to their modulus of elasticity, they are used exclusively for direct post and core placement. Zirconium posts can be layered with a pressed ceramic core or a plastic material such as composite resin core for a direct technique. Advantages of these modern posts are their aesthetic substrates and ability to eliminate corrosive products that damage the root surfaces. Since these posts are bonded to root dentin, less microleakage is also expected with these post systems. Ideally, intraradicular cementation is performed with a self-polymerizing resin cement.2

The treatment of the tooth substrate must also be considered. While partial restorations rely on adhesion to enamel surfaces, full-coverage crowns are cemented primarily to dentin surfaces. Enamel or cementum may be present at the margins, but full-coverage crown preparations will result in the exposure of more dentin tubules. Achieving an adequate seal of the latter will help clinicians reduce postoperative sensitivity by minimizing bacterial invasion and improving the biocompatibility of the restorations. Penetration of primer and adhesive inside the collagen network seals dentin tubules and creates the “hybrid layer.” While adhesion to dentin has been studied for many years, it remains a challenging subject to researchers.

New techniques (eg, electroforming and capillarization) for the laboratory fabrication of cast-free metallic frameworks have introduced benefits that include pure gold copings with reduced thickness (0.2 mm) and warm metallic shades.3,4 These characteristics allow natural results to be achieved for metal-ceramic crowns, particularly when there is a need to mask altered substrates. Another important feature of modern metal-ceramic crowns is the vertical reduction of the metal framework (Figures 6-7-8-9), which results in full-ceramic margins in the cervical region. When compared to conven­tional metal-ceramic crowns, this reduced substructure — used in conjunction with fluorescent margin ceramics — is advantageous due to improved light transmission in the cervical third. While there was no aesthetic inter­ference of the cement in conventional metal-ceramic crowns, light transmission with modern metal-ceramic frameworks, as for all-ceramic crowns, dramatically changes the aesthetic requirements for dental cements.

When one considers that both all-ceramic crowns and advanced metal-ceramic crowns have all-ceramic margins that contribute to the aesthetic result, it is obvious that zinc-phosphate cements are obsolete in modern aesthetic dentistry. This raises one important question: which cements are suitable for cementation of contemporary porcelain crowns?

 

Modern Requirements for Cementation

From a clinical perspective, successful cementation has the following requirements (Table A):

  • Aesthetics compatible with the restorative material.
  • Absence of postoperative sensitivity.
  • Strong adhesion and an adequate dentin seal.
  • Sufficient marginal bond.
  • Washout resistance.
  • Minimum film thickness.
  • Radiolucency.

 

Table A: Clinical Sequence for Crown Cementation

  1. (Preparation session) Complete crown preparation and reline provisional crown.
  2. (Preparation session) Before taking impression, condition tooth surface and hybridize dentin surface.
  3. (Cementation session) Properly treat internal crown surface (See Table C).
  4. (Cementation session) Apply luting agent to internal crown surface and to preparation with microbrush.
  5. (Cementation session) Insert crown with occlusal pressure.
  6. (Cementation session) Remove excess luting agent prior to final setting.
  7. (Cementation session) Apply glycerin gel to cervical margins of the crown
  8. (Cementation session) Remove excess of luting agent with a scalpel after its setting reaction.

Full-coverage crown preparations exhibit some characteristics that can result in postoperative sensitivity, as the surface area of dentin tubules exposed is much greater than those exposed in partial preparations. Moreover, metal-free feldspathic ceramic-based restorations can also require greater depth of preparation to achieve proper strength. Manufacturers, however, are developing aluminum oxide and zirconia ceramic substructures to achieve stronger mechanical resistance with less invasive (0.4 mm) preparations.

The first step for a biologically successful cementation is to eliminate or reduce postoperative sensitivity. Once the preparation is performed, pulp tissue is exposed, and a biological wound is created. Open tubules allow the penetration of bacteria and other factors (eg, chemical substances) that irritate the exposed pulp. Adequate chemical treatment of this wound is required to minimize penetration of bacteria into dentin tubules and disinfect them. This can be obtained by application of a 0.1% to 0.2% chlorhexidine solution.

Once the prepared tooth is disinfected, its surface must be treated. At present, each prepared dentin surface should receive a seal to prevent fluid movements and postoperative sensitivity. Since bonding became an important step in modern dentistry, numerous investigations have been conducted to improve comprehension of the involved processes and to more effectively treat the surfaces to be bonded. Two surfaces must be considered: tooth and restoration (Tables B and C). Enhancing mechanical retention will also improve cementation and requires 1) the creation of microretentions on bonded surfaces to augment surface energy and 2) bonding agents capable of penetrating even the thinnest microretentions created.

 

Table B: Tooth Surface Treatment

  1. Disinfect surface with chlorhexidine solution
  2. After rebasing provisional crown and before taking the impression, etch tooth surface with 37% phosphoric acid for 10 to 15 seconds.
  3. Thoroughly rinse to remove acid.
  4. Reapply chlorhexidine solution and remove excessive moisture.
  5. Apply adhesive system of choice.
  6. Repeat steps 1 and 3; reapply a thin layer of primer in case sensitivity is present prior to final cementation.
(Continued from page 1 )

While bonding to enamel has been well established,5 dentin treatment is more challenging. Adhesion to dentin can be obtained chemically, but it is primarily accomplished by mechanical means. Interlocking will be obtained by resin penetration into dentin, and better surface preparation results in more effective mechanical retention. Three diameters of dentin tubules are essentially encountered clinically6:

  • Major branches: diameter ranges from 0.5 µm to 1 µm and localized peripherally.
  • Fine branches: diameter of 0.3 µm to 0.7 µm and primarily localized at root dentin or at coronal dentin where tubule/intertubular ratio is low.
  • Micro branches: diameter ranges from 50 nm to 100 nm and located anywhere in dentin but primarily at the crown.

Demineralization of dentin allows the penetration of resin by widening these tubules and exposing the collagen matrix. Collagen itself is not a very reactive surface,7,8 so adhesion is primarily obtained by mechanical retention rather than chemical bonding.7,9 Macromechanical retention is formed by intratubular resin penetration, while micromechanical retention is obtained by intertubular retention. The presence of a smear layer will harm this interlock by creating a weaker link, so its removal by etching is essential.7,8,10 This can be achieved through the 10- to 15-second application of a 37% phosphoric acid that must be rinsed thoroughly for complete removal. Once the smear layer is removed and the surface is conditioned, resin will penetrate into widened tubules and the intertubular dentin, which constitutes the hybrid layer.

Resin penetration is dependent on wettability of the substrates and viscosity of adhesive.9 Enamel can be completely dried to receive the resin, but dentin must be moist in order to avoid collagen collapse and to promote hybridization; desiccation of dentin will harm the formation of the hybrid layer. Collapsed collagen can prevent resin to flow into these tubules.6 Blot drying with absorbent paper will remove excessive moisture without desiccation, and chlorhexidine 0.1% to 0.2% can be reapplied with a microbrush. Use of primers to chemically remove excess fluids from the etched dentin surfaces will transform a hydrophilic into a hydrophobic surface. Therefore, most primers require an acetone or ethanol to volatize all the surface moisture, which allows penetration of the monomers into tubules, their branches, and the collagen network.

 

Table C: Restorative Surface Treatment

Internal Surface – Surface Treatment

Metal -- Sandblasting (50 µm to 250 µm Al2O3)

Metal (Optional) -- Chemical retention system

Feldspathic porcelain; leucite reinforced porcelain; lithium disilicate -- 9.6% flouridic acid and silanization

Glass-infiltrated aluminaSandblasting only

Machined aluminaAlcohol, sandblasting, and silanization

Zirconia to improve wettability onlySilanization only

 

Proper diffusion of this monomer and correct formation of a hybrid layer protects the cut dentin and prevents the damage of the pulp tissue. Independently of the system chosen, one can assess clinically that a hybrid layer has been formed by the shiny appearance of the dentin surface and lack of sensitivity. Multiple- and one-step systems have been formulated in an attempt to simplify these clinical procedures. If adhesion is easily obtained on enamel, however, the dentin surface may require multiple applications of a one-bottle system to achieve hybridization and reduce postoperative sensitivity.3 Hybridization should be performed when preparations and provisional restorations are completed and prior to impressions. Over time, the clinician can verify that the hybrid layer was removed if vital teeth become sensitive. In the authors’ opinion, this procedure should be repeated only if sensitivity is present. This means that the hybrid layer has been disturbed, which can occur, for instance, with multiple try-in sessions. Unnecessarily repeating the hybridization procedure could result in a thick seal, however, and prevent full seating of the crown. If there is a need to rehybridize, one should clean the surface, re-etch in the same manner, and use a fourth-generation adhesive system. It is critical to obtain a very thin hybrid layer at this stage, as thickness will increase the vertical discrepancy of the crown.10

If restorative surfaces can have their wettability improved via surface treatment, metal substructures have to be sandblasted with 50 µm to 250 µm aluminum oxide in order to clean and augment microretention.11 Chemical retention to metal has been introduced in many systems to enhance the resin-to-metal bond. These systems are indicated primarily for veneering with resin. As with dentin surfaces, however, improving the wettability of metal surfaces to be bonded will also enhance the interlocking of cements or luting agents and contribute to better interfaces and reliable cementation.11

In all-ceramic crown restorations (feldspathic, leucite-reinforced, lithium disilicate, slip-cast aluminum oxide, densely sintered aluminum oxide, or zirconia), there are many possible surface treatments, which include sandblasting, etching, and silanization.12 Leucite-reinforced and lithium disilicate ceramic are ideally conditioned with 10% hydrofluoric acid.11 When possible, etching is the most effective surface treatment for increasing surface area and energy.12 These changes will create greater wettability and consequently a better interlock with a resinous luting agent. While sandblasting is an effective means of cleaning surfaces to be bonded and augmenting surface energy, it has little effect on micromechanical retention and can increase the potential of iatrogenic damage to thin margins. Sandblasting with 50µm aluminum oxide is the only option for treating the surfaces of certain crowns,13 which cannot be etched or silanated. Similarly machined aluminum oxide and zirconia require internal surface cleaning with ethanol and sandblasting with 50µm aluminum oxide particles prior to cementation or luting.11

Silanes are responsible for bonding to ceramic surfaces by covalent and hydrogen bond formation. The ceramic surface is thus transformed into a hydrophobic surface that prevents hydrolytic degradation. When a luting agent is applied to this surface, its wettability is increased, and it is transformed again into an organophilic surface. For complete silanization, it is important that multiple layers be applied. Microscopic undercuts associated with silane application result in reduced tensile stresses at the adhesive joint and consequently less debonding.14

As previously stated for tooth surfaces, one could conclude that greater bond strength can be achieved through treatment of the internal aspect of the crowns. This promotes micromechanical retention and a better chemical reaction among different surfaces (eg, metal, ceramics, and bonding agents). It was also proven that the resistance of all-ceramic crowns is greater when both dentin and ceramic are treated prior to bonding.14

A proper marginal seal is also desirable. For full-coverage crown preparations, the margins can be placed on enamel, dentin, or cementum. Lack of dentin tubules (as in cementum margins and peripheral dentin) and changes in dentin structure (as found in sclerotic dentin) can complicate this seal. Therefore, one can expect to encounter seals of differing quality depending on which structure the margin is placed. Following the removal of excess cement, a protective glycerin gel is applied to the cervical margins in order to reduce oxygen inhibition to polymerization or water sorption.

Minimum film thickness is desirable to ensure the integrity of the cement margin and the conservation of occlusion after cementation. Although resin cements and resin-modified glass-ionomer cements have greater resistance to solubility when compared to conventional cements, abrasion from a toothbrush will cause washout of these margins. Guzman et al stated that the larger the marginal gap, the more abrasion from a toothbrush will occur.15 Over time, the process of tooth brushing will cause horizontal discrepancy that results in bacteria pene­tration, recurrent caries, and periodontal inflammation.16-18 It is important to preserve occlusal adjustments obtained prior to cementation. If cement thickness interferes with final occlusion, undesirable adjustments will be necessary on glazed or mechanically polished ceramic surfaces. Film thickness depends on various factors that include margin design/adaptation, the amount of die spacing, and the type of bonding and/or luting agent as well as the cementation technique itself.

The marginal adaptation of dental materials, which are constantly improved by manufacturers,19-21 should obviously be minimal. For ideal precision, crown spacing should ideally range from 25 µm to 40 µm and closely match the thickness of the cement. It has also been stated that the use of varying types of die spacers and their variable layers on dies will result in statistically different internal reliefs.22 In the authors’ opinion, the internal relief of crowns designed via computer-aided design/computer-assisted machining can thus be expected to be most reliable (Figures 10-11-12-13-14-15-16-17-18).

Depending on quantity and diameter of the filler content and the type of solvent in the bonding agent, great film thickness and marginal discrepancy can be expected.10 Despite the availability of formulas with minute filler particles, resin cements have the thickest layer due to the prepolymerization of these particles. Ideally, one should expect that greater filler content without prepolymerization would reduce film thickness, improve cement resistance, and thus reduce washout.23

Glass-ionomer cements might appear to be thick, but their film thickness will be comparable to that of zinc phosphate when cementation occurs under pressure (ie, thixotropy).24,25 One concern with glass-ionomer cements is the possibility of crack formation under all-ceramic restorations due to water sorption and hygroscopic expansion.26 Recent formulations of resin-modified glass-ionomer cements have a wide range of available shades, less expansion, and essentially eliminated concerns regarding crack forma­tion. Although resin cements have higher film thickness when compared to zinc-phosphate and resin-modified glass-ionomer cements, they are the only clinical option when an opacious cement is necessary to mask discolored substrates. A chairside mixture of resin cements and intensive colors allows the clinician to create a customized opacious cement that is well adapted to the range of discoloration.

The aesthetic characteristics of contemporary dental cements are of greatest clinical importance. The range of opacity is certainly more critical than the range of chroma. When used in thin layers, differences in chroma can hardly be distinguished, but variations in opacity can help to conceal discolored substrates and create a transition to the crown. Considering that aesthetic requirements for modern crowns have been updated, similar efforts must be made to enhance the optical properties of dental cements.

Radiolucency is a requirement for dental cements and all-ceramic crown restorations in particular. Ideally, the radiopacity of modern cements should be comparable to the dentin in order to facilitate diagnosis for recurrent caries. This is achieved with resin-modified glass-ionomer cements (Figures 19-20-21-22-23-24).

 

Conclusion

Contemporary cements now have an active role in final aesthetic results and must respect the aforementioned parameters in order to achieve this goal. From conventional zinc-phosphate cements to modern resin-modified glass-ionomer cements, many changes have been introduced to improve cementation techniques — mechanically and aesthetically. Adhesion, hybridization, biomechanical properties, aesthetic colors, and range of opacity are important clinical improvements that can benefit all-ceramic and advanced metal-ceramic crowns. Never­theless, additional clinical experience and research are necessary in order to more precisely assess the respective indications and limitations of modified glass-ionomer and resin cements. It would be of significant clinical interest to develop new cements that possess a chemical bond with high-strength ceramics (eg, alumina and zirconia) and a range of opacity.

 

References:

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  4. Wirz J, Hoffman A. Electroforming in Restorative Dentistry — New Dimensions in Biologically Based Prosthesis. Carol Stream, IL: Quintessence Publishing; 2000:430.
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