The choice of cement, or luting agent, for a crown made of Emax is a determining factor for the restoration’s long-term success. Emax is the brand name for a high-strength, highly aesthetic glass-ceramic material known as lithium disilicate, which is commonly used for crowns, veneers, inlays, and onlays. Unlike traditional metal or metal-fused-to-ceramic restorations that rely on mechanical retention, the longevity of lithium disilicate restorations is highly dependent on achieving a strong chemical bond to the underlying tooth structure. The necessity for adhesive bonding guides the selection of the luting system based on the material’s specific physical properties and the clinical situation.
Material Requirements for Lithium Disilicate
Lithium disilicate glass-ceramic is inherently less resistant to fracture than high-performance materials like monolithic zirconia. This relatively lower flexural strength, typically around 500 megapascals (MPa), means the restoration itself is susceptible to failure under heavy chewing forces unless it is structurally reinforced. Bonding the ceramic to the tooth transforms the restoration from a vulnerable cap into a highly durable, integrated unit.
Adhesive resin cementation significantly increases the overall fracture resistance of the ceramic by distributing functional loads across the entire tooth-restoration interface. Studies show that bonding can improve the strength of the restoration by as much as 50% compared to conventional cementation methods. This bond prevents micro-movement at the margin, which is a major cause of micro-leakage, recurrent decay, and eventual failure of the restoration. The resin cement layer acts as a continuous, high-strength buffer that reinforces the ceramic structure from within.
Primary Types of Resin Cementation
The selection of a resin luting agent for Emax generally falls into two primary categories, each defined by its mechanism for bonding to the tooth. The first category includes Adhesive Resin Systems, which are multi-step protocols. These require the separate application of an etchant, a primer, and a bonding agent to the tooth before the resin cement is applied.
These full adhesive systems, often referred to as total-etch or selective-etch systems, are considered the gold standard for achieving the highest bond strength, particularly to enamel. The trade-off for this superior strength is a higher degree of technique sensitivity and a more complex, time-consuming clinical procedure.
The second category is Self-Adhesive Resin Cements, which are simplified systems. These materials contain acidic monomers that condition the tooth surface while simultaneously polymerizing and acting as the luting agent. This eliminates the need for separate etching and bonding steps on the tooth.
Self-adhesive cements offer a significant reduction in clinical steps and are less sensitive to moisture contamination than their full adhesive counterparts. However, they generally achieve lower bond strengths, especially to unprepared enamel, than the multi-step adhesive systems. For non-retentive restorations like veneers or thin onlays, the maximal bond strength provided by an adhesive resin system is usually required.
Pre-Cementation Surface Treatment Protocols
Achieving a durable bond for lithium disilicate requires a specific protocol for treating the inner surface of the ceramic restoration itself. This preparatory sequence is necessary to create both micromechanical retention and a chemical link to the resin cement. The first step involves Hydrofluoric Acid (HF) Etching, where a low-concentration HF gel, typically 5%, is applied to the internal surface of the crown for approximately 20 seconds.
This acid selectively dissolves the glassy matrix of the lithium disilicate, creating microscopic pores and irregularities. This allows the liquid resin cement to flow into and interlock with the ceramic upon curing. Due to the high toxicity of HF acid, this step is often performed by the dental laboratory before the restoration is delivered, or performed chairside with extreme caution and protective measures. After etching and thorough rinsing, the surface must be meticulously cleaned to remove any residual salts or contaminants, often using specialized ceramic cleaning agents.
The second step is Silanization, which involves applying a silane coupling agent to the etched and cleaned ceramic surface. Silane acts as a chemical bridge, with one end of its molecule bonding to the inorganic silica in the glass-ceramic and the other end co-polymerizing with the organic resin matrix of the luting cement. This dual-action chemical linkage completes the adhesion protocol for the ceramic. Without the proper application of both hydrofluoric acid and silane, the cement will only achieve mechanical retention, which is insufficient for the long-term success of an Emax restoration.
Clinical Considerations for Cement Selection
The final decision on the best cement involves specific clinical factors related to the patient and the restoration design. One factor is Preparation Retention, which refers to the height and parallelism of the prepared tooth walls. If the tooth preparation provides ample mechanical retention, such as a tall crown with slightly convergent walls, a self-adhesive resin cement may be a practical choice due to its simplified procedure.
If the preparation is minimally retentive, such as a thin veneer or a short clinical crown, the practitioner must select a full adhesive resin system to achieve the highest bond strength to prevent dislodgement. Another consideration is Aesthetics and Shade Matching, particularly for restorations in the highly visible anterior region. For thin, highly translucent veneers, light-cure-only resin cements are often preferred.
Light-cure-only cements offer superior color stability over time because they lack the chemical activators necessary for self-curing, which can sometimes lead to slight color shifts. These cements also provide an extended working time for precise placement and cleanup. For thicker, more opaque crowns where light penetration is a concern, a dual-cure resin cement is necessary, as it contains both light-activated and chemical-activated polymerization components to ensure the material sets completely.