The selection of the best cement for an E.max crown, or any lithium disilicate restoration, significantly impacts the restoration’s strength and longevity. E.max is a popular, high-strength lithium disilicate glass-ceramic used for crowns, veneers, inlays, and onlays due to its excellent aesthetic qualities and durability. The technique used to affix this restoration is crucial, as the method of cementation determines how the restoration resists forces encountered in the mouth, influencing its long-term success.
Why E.max Requires Adhesive Bonding
The long-term performance of lithium disilicate restorations relies heavily on adhesion, which is a chemical and micromechanical union, rather than simple luting. Luting cement essentially fills the gap between the restoration and the tooth, relying on the preparation’s geometry to mechanically hold the crown in place. Adhesive bonding creates a strong link between the ceramic and the tooth, which is especially important for restorations that are thin or lack inherent mechanical retention.
Adhesive techniques are important for E.max because they dramatically increase the fracture resistance of the ceramic. By bonding the restoration to the tooth, the resin cement penetrates the microscopic irregularities of both surfaces, forming a strong hybrid layer that reinforces the ceramic material. This bonding process can increase the ceramic’s strength significantly compared to conventional cementation, making it the preferred method for thin veneers or minimally invasive onlays.
Achieving this powerful adhesion requires a mandatory two-step preparation process on the internal surface of the E.max restoration. First, the surface must be etched using hydrofluoric (HF) acid, typically 5%, for a short duration. This acid creates a porous, rough surface for micromechanical retention.
After rinsing and drying, the second step involves applying a silane coupling agent. One end of the silane molecule chemically reacts with the silica in the etched ceramic, while the other end chemically bonds with the resin cement. This chemical bridge facilitates the durable, high-strength adhesion. Skipping the silane application, or failing to properly etch the ceramic, significantly compromises the integrity of the bond.
Essential Categories of Cementation Materials
The materials used to secure E.max restorations fall into three primary categories, distinguished by their composition and bonding mechanism.
Conventional or Total-Etch Resin Cements
These pure resin-based systems achieve the highest bond strength but require separate, multi-step application protocols. The tooth structure must be etched with phosphoric acid, followed by a separate adhesive or bonding agent before the cement is placed. This approach creates a strong connection to both enamel and dentin but is highly sensitive to moisture and technique.
Self-Adhesive Resin Cements
These cements combine the etching, priming, and bonding steps directly into the cement paste. They contain acidic monomers that condition the tooth surface as the cement sets, eliminating the need for a separate phosphoric acid etch and bonding agent on the tooth. While offering a simplified clinical workflow, their bond strength is generally moderate compared to total-etch systems.
Resin-Modified Glass Ionomers (RMGIs)
RMGIs are primarily luting agents, relying on a chemical bond to the tooth’s calcium ions and mechanical retention from the preparation. They offer a lower bond strength than resin cements. RMGIs release fluoride but should only be used in specific, low-stress applications due to their weaker mechanical properties.
Selection Criteria Based on Clinical Factors
The determination of the best cement depends entirely on the specific clinical parameters of the restoration and the prepared tooth structure.
Restoration Thickness and Type
For thin restorations, such as veneers, inlays, onlays, or crowns less than 1.5 millimeters thick, Total-Etch Resin Cements are the required standard. This cement type provides the maximum adhesive strength necessary to reinforce the ceramic, which is inherently weaker in thin sections. The high strength and superior aesthetics of these light-cure resin cements are paramount for such applications.
Preparation Retention
The retention form of the tooth preparation describes the height and taper of the walls. If the preparation is short or lacks adequate retention, the cement must compensate by providing maximum adhesion to prevent dislodgement. In these cases, the Total-Etch Resin Cement protocol is necessary to maximize the bond strength to both the ceramic and the tooth.
For full-coverage crowns on preparations with excellent retention (tall, parallel walls and a minimum thickness of 1.5 to 2.0 millimeters), the requirements for adhesive strength are reduced. Self-Adhesive Resin Cements offer a balance of strength and simplified technique, making them a suitable choice. Resin-Modified Glass Ionomers are only considered for thick crowns with exceptional preparation retention where aesthetics are not the primary concern.
Step-by-Step Adhesive Cementation
The most rigorous and reliable protocol, using Total-Etch Resin Cement, begins with establishing absolute isolation of the working area, often accomplished with a rubber dam. This prevents contamination from saliva and blood, which compromises the bonding process.
The E.max restoration, having been pre-etched with hydrofluoric acid, is then treated with the silane coupling agent. The silane is allowed to react for the manufacturer-specified time, usually 60 seconds, and then gently air-dried.
Preparation of the tooth structure requires careful application of 35% to 37% phosphoric acid etchant. The etchant is applied to the enamel margins for 15 to 30 seconds to create a porous surface for micromechanical retention. If dentin is exposed, the application time is reduced to 10 to 15 seconds to avoid over-etching. Following a thorough rinse and gentle drying, a separate bonding agent is applied to the tooth surface and often lightly cured.
The resin cement is loaded into the internal surface of the restoration. The restoration is seated onto the tooth with firm pressure to ensure complete seating and cement flow. A brief “tack cure” of one to three seconds is applied to gel the excess cement, allowing for easy removal of the material with an instrument and dental floss. The final step involves a complete light cure from all accessible directions, typically for 20 seconds or more per surface, to ensure the entire cement layer is fully polymerized for maximum strength.