Repairing Fractured and Chipped Ceramic Crowns: From Identifying the Ceramic Type to Choosing the Protocol
Fractures and chipping of ceramic crowns still occur in the clinic despite the high success of these restorations. Factors such as parafunction and the patient's occlusal pattern, inadequate restoration design, insufficient bonding, and internal defects of the ceramic contribute to them. In most cases, intraoral repair is a more conservative alternative to replacing the entire crown. It requires no further tooth preparation and does not compromise tooth structure or pulp vitality.
But not all fractures can be repaired with a single protocol. The first and decisive step is to know which ceramic is exposed at the fracture site.
1. First, identify the ceramic material
In terms of surface preparation, ceramics fall into two main groups. The first group is ceramics with a glass phase, namely feldspathic porcelain, leucite-reinforced ceramics, and lithium disilicate, which respond to hydrofluoric acid (HF) etching. The second group is zirconia and alumina, which have no glass phase and on which HF has no effect.
A third situation arises when two ceramics are exposed together. A common example is a veneered zirconia crown in which the fracture has passed through the veneering porcelain and reached the zirconia core.
2. Glass-phase ceramics: HF and silane
For this group, HF etching followed by silane has the strongest support. HF dissolves part of the glass phase and prepares the surface for micromechanical retention, and silane establishes the chemical bond between ceramic and resin.
Silane must be applied as a separate step. The silane contained in a universal adhesive does not replace this step, because mixing it with other components in one bottle reduces its effect. Phosphoric acid is not a substitute for HF either, because it has no effective action on the glass-ceramic surface. Air abrasion alone gives a weaker bond than HF and may create surface cracks in glass ceramics. Self-etching ceramic primers such as Monobond Etch & Prime have sometimes performed similarly to HF and sometimes worse, and they cannot yet be considered equivalent to HF.
There is also an important point about zirconia-reinforced lithium silicate (ZLS) ceramics. Despite the word zirconia in their name, these ceramics contain a large amount of silica, respond to HF, and their protocol is the same HF and silane.
3. Zirconia and alumina: air abrasion and MDP
Because HF has no effect on zirconia and alumina, mechanical surface preparation is done with air abrasion. But air abrasion alone is not enough, and chemical bonding requires an MDP-containing primer, which reacts with the oxide surface of zirconia. Silane has little effect on zirconia, because its surface does not contain enough silica.
Air abrasion with silica-coated particles has given better results than conventional aluminum oxide particles in several studies. But this silica only sits superficially on zirconia and may detach over time.
4. When porcelain and zirconia are exposed together
In this situation, the most logical approach is to prepare each surface with its own protocol: porcelain with HF and zirconia with air abrasion. The problem is that when preparing two adjacent surfaces, there is a risk of cross-contamination, and no method to prevent it has been reported. For this reason, there is still no definitive protocol for this type of fracture.
5. A few practical points
Intraoral HF etching should only be done under a rubber dam, because HF is harmful to soft tissue and rubber dam isolation has also been associated with better bonding. Intraoral air abrasion also requires a rubber dam.
Laser is not a suitable alternative. In glass ceramics it has usually produced a weaker bond than HF, and in zirconia it does not match air abrasion.
The type of repair composite has not produced a significant difference in bond strength in studies.
6. A good bond is not the whole story
A composite that matches the ceramic at the time of repair may lose its match over time due to staining or changes in translucency. Its color match may also not look the same under different lighting.
Mechanically as well, in one study the fracture resistance of all repaired crowns decreased after cyclic loading, regardless of how their surface had been prepared. So a good initial bond does not mean guaranteed durability.
Clinical summary
Before repair, identify the exposed ceramic. For feldspathic porcelain, leucite, lithium disilicate and ZLS, HF etching followed by silane in a separate step; for zirconia and alumina, air abrasion with an MDP-containing primer. Both should be done under a rubber dam. For fractures involving both porcelain and the zirconia core, there is still no definitive protocol.
These recommendations rely almost entirely on laboratory studies, and there are no clinical trials in this field. They are therefore a good guide for choosing the surface preparation method, but they do not guarantee the clinical durability of the repair.