Christopher C.K. Ho
Ceramics are widely used in dentistry due to their ability to re-create natural optical characteristics. They have been developed to enhance bio-compatibility and durability; however, fractures have become an increasing complication due to the advent in popularity of tooth-coloured materials and subsequently less use of metal. The mode of fracture can occur with the veneering porcelain or a framework fracture. In glass ceramics, inceram and alumina restorations, framework fracture has often been the mode of failure, while in zirconia restorations, veneering porcelain chipping or even delaminating has been identified.
The majority of ceramics consist of crystalline fillers that vary in proportion, from relatively low concentrations in glass-based ceramics to higher concentrations in polycrystalline high-strength materials, resulting in superior mechanical properties.
The failure of ceramic can occur due to several factors, including the following:
- Trauma – from micro-trauma (i.e. parafunctional forces) and macro-trauma (i.e. physical trauma).
- Preparation design – sharp line angles, grooves and knife-edge margins may predispose a restoration to more stress with subsequent fracture.
- Ageing – low-temperature degradation due to liquid penetration leading to phase transformation of zirconia materials has been shown to reduce mechanical properties.
- Thermal conductivity – zirconia has a low thermal conductivity with a different coefficient of thermal expansion to many of the veneering porcelains. Developments have been made to more closely match the thermal coefficients as well as understanding the heating and cooling times required.
- Framework design – appropriate design of the framework to properly support veneering porcelain while also understanding the ratio of framework thickness to veneering porcelain thickness. Moreover, anatomically designed copings are better able to reduce chipping.
- Inappropriate adjustment of ceramics chairside – with induced micro-cracks leading to failure.
There may be several options for repairing the restoration, depending on the size, position and type of fracture that has occurred:
- Small chips – these may be satisfactorily contoured and polished, especially in non-aesthetic areas and where food or plaque will not accumulate and predispose patients to caries or periodontal disease. Furthermore, polishing should be performed to minimise any surface flaws that may lead to subsequent failures.
- Direct repair:
- Bonding the fractured portion of ceramic back onto the restoration with resin cement
- Directly repairing with composite resin
- Preparing the fractured ceramic restoration for a veneer or an over-crown, and bonding a new restoration to the existing restoration
- Indirect repair – removal of restoration and re-firing or replacing veneering porcelain by a dental ceramist. This may be possible with a screw-retained implant restoration, but in cemented conventional crown and bridgework it may be difficult to carry out without damaging or distorting the cemented restoration during its removal.
The repair of ceramic requires an understanding of how retention can be attained by providing micro-mechanical retention and enhancing the bond with silane primers or phosphate monomers.
- Air abrasion/intra-oral sandblaster