Glass-Ceramic Crowns: Material Selection

The glass-ceramic crown has become the default restoration for the anterior and the many posterior teeth, and the material selection within the family is the decision that determines the strength, the aesthetics, and the longevity of the result. The lithium disilicate, the leucite, and the zircon...

The glass-ceramic crown has become the default restoration for the anterior and the many posterior teeth, and the material selection within the family is the decision that determines the strength, the aesthetics, and the longevity of the result. The lithium disilicate, the leucite, and the zirconia-reinforced ceramics each carry the different balance of the translucency and the fracture resistance, and the clinician who selects by the case rather than by the habit gives the patient the restoration that the tooth and the occlusion require. This article reviews the materials, the properties, the selection criteria, and the cementation that the glass-ceramic crown demands.

The Glass-Ceramic Family

The glass-ceramic is the material that the controlled crystallisation produces from the glass, and the crystal content determines the optical behaviour and the mechanical strength. The feldspathic ceramic carries the highest translucency and the lowest strength and suits the veneer and the highly aesthetic anterior unit. The leucite-reinforced ceramic offers the moderate strength and the good aesthetics for the single unit. The lithium disilicate provides the higher strength with the acceptable translucency, and the zirconia-reinforced lithium silicate combines the strength with the improved optical properties for the posterior and the anterior use.

The Properties That Guide the Choice

The flexural strength, the fracture toughness, and the wear of the opposing enamel are the properties that the clinician weighs, and the material that resists the fracture must also preserve the opposing tooth. The high-strength ceramic that is too hard wears the natural antagonist, which is the consideration that the selection should not ignore.

The Clinical Selection

The selection follows the site, the occlusion, and the aesthetic demand, and the clinician who identifies the high load before the preparation decides whether the glass-ceramic is the correct material at all. The posterior unit under the heavy parafunction and the short bruxer may need the zirconia or the metal-ceramic rather than the glass-ceramic, because the material that fractures under the load fails the patient regardless of the beauty.

The Preparation and the Cementation

The glass-ceramic requires the preparation with the adequate reduction and the finish line that the material can support, and the adhesive cementation is the step that the lithium disilicate and the zirconia-reinforced silicate depend on for the strength. The clinician who etches the intaglio with the hydrofluoric acid, silanates the surface, and uses the resin cement achieves the bond that the material needs. The cementation of the glass-ceramic with the non-adhesive cement leaves the restoration weaker than the material specification promises, which is the reason the adhesive protocol is not the option but the requirement.

The Occlusion and the Loading

The occlusion that the restoration receives determines the survival, and the clinician who adjusts the contacts in the centric and the eccentric movements protects the ceramic from the load that the interference concentrates. The glass-ceramic resists the compression well and the tension poorly, which means that the design of the occlusal surface and the thickness of the material at the contact points matter more than the nominal strength of the material. The night guard that the clinician prescribes for the parafunctional patient protects the restoration and the opposing teeth together.

The Failure Modes

The chipping, the fracture, the debonding, and the wear of the antagonist are the failures that the glass-ceramic restorations present, and the cause that the clinician identifies guides the repair or the replacement. The chip at the incisal edge of the anterior unit often allows the intraoral repair with the composite, while the fracture that extends through the body of the crown requires the remake and the review of the loading. The debonding points to the cementation protocol, and the wear of the opposing enamel points to the surface that the material and the polishing have produced.

The repair of the chipped ceramic with the composite requires the etch and the silane on the fractured surface, and the bond that the protocol produces allows the clinician to postpone the remake where the fracture is the local and the aesthetic is the acceptable. The clinician also reviews the opposing tooth and the occlusal scheme at the follow-up, because the failure that the material alone cannot explain usually traces back to the load that the occlusion delivers.

The Maintenance

The ceramic does not decay, but the tooth and the margin that the crown covers still need the hygiene that the patient maintains, and the plaque that collects at the margin produces the caries and the gingival inflammation that the restoration cannot prevent alone. A soft electric brush such as the BrushO cleans the margin of the restoration at the controlled pressure without the abrasion of the ceramic glaze, and the interdental cleaning that the clinician recommends protects the adjacent units. The patient who keeps the recall interval allows the clinician to polish the margin and to inspect the contacts that the wear changes.

Clinical Key Points

- Select the material by the site, the load, and the aesthetic demand rather than by the habit.

- Use the leucite or the lithium disilicate for the aesthetic anterior unit.

- Use the lithium disilicate for the single posterior crown and the short bridge.

- Follow the adhesive cementation protocol that the glass-ceramic requires.

- Protect the restoration with the occlusal adjustment and the night guard.

Conclusion

The glass-ceramic crown offers the practice the aesthetic and the conservative restoration, and the selection within the family determines the result. The clinician who matches the material to the case, respects the cementation, and controls the occlusion delivers the restoration that serves the tooth for the years rather than the months.