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Dental Tribune Indian Edition

17Dental Tribune Indian Edition - July 2013 Introduction Many types of ceramic materials and processing techniques have been in- troduced throughout the years. As early as 1903, Charles Land patented all-ceramic restorations, using fi- red porcelains for inlays, onlays and crowns.1 Insufficient understanding of material requirements for survival in the oral environment, poor ceramic processing techniques, and the ina- bility of adhesive cementation led to early catastrophic failure. Since then, all imaginable varieties of materials and techniques, from very conser- vative ceramic restorations to very complex porcelain veneered of either metal or high-strength crystalline ceramics, have been introduced and tried with varying levels of success.2 The authors have previously publi- shed two detailed descriptions of, or classification systems for, ceramics used in dentistry, one based on the microstructure of the material and the second on the way in which the mate- rial is processed.3 There is considerable misinfor- mation and a general lack of rational treatment planning guidelines publi- shed regarding the use of different ceramics in dentistry. The literature is replete with various accounts of clinical success and failures of all types of dental treatments. Sadowsky4 published a review of the literature covering treatment considerations using aesthetic materials, for example whether to use amalgam or composite and the success rates of different tre- atments. No recent literature could be found presenting a thorough discus- sion of when to use the various cera- mics, for example when feldspathic porcelains should be used, when ei- ther pressed or machined glass-cera- mics are appropriate, when different types of glass-ceramics should be employed, when a high-strength all- ceramic crown system of either alu- mina or zirconia is ideal, and when metal ceramics are suitable. This article provides a systema- tic stepwise process for treatment planning with ceramic materials and presents specific guidelines for the appropriate clinical conditions for ap- plications of the various systems. Treatment philosophy A treatment philosophy based on cur- rent standards of care that consider the patient’s aesthetic requirements is a prerequisite to making any deci- sion regarding the use of a material or technique. More importantly, this philosophy should be aimed at main- taining the long-term biological and structural health of the patient in the least destructive way. Restorative or aesthetic dentistry should be practised as conservati- vely as possible. The use of adhesi- ve technologies makes it possible to preserve as much tooth structure as feasible while satisfying the patient’s restorative needs and aesthetic desi- res.5 The philosophy today is not to remove any healthy tooth structure unless absolutely necessary. This will reduce dentists’ frustration when or- thodontics would have been the ideal treatment. With restorations, clini- cians should choose a material and technique that allows the most con- servative treatment in order to satisfy the patient’s aesthetic, structural, and biological requirements, and that me- ets the mechanical requirements to provide clinical durability. Each of these requirements could be the topics of individual articles. There are four broad categories or types of ceramic systems: 1. Powder/liquid feldspathic porcelains; 2. Pressed or machined glass-ceramics; 3. High-strength crystalline ceramics; and 4. Metal ceramics. Category 1 Porcelains—the most translucent— can be used the most conservatively, but are the weakest.3, 6 Category 2 Glass-ceramics can be very translu- cent too but require slightly thicker dimensions for workability and aes- thetics than porcelains do. Categories 3 and 4 High-strength crystalline ceramics and metal ceramics, although demon- strating progressively higher fracture resistance, are more opaque and, the- refore, require additional tooth reduc- tion and are thus a less conservative alternative. Based on the treatment goal of being as conservative as possible, the first choice will always be porce- lains, then glass-ceramics, followed by high-strength ceramics or metal ceramics. The decision will be based on satisfying all the treatment requi- rements, that is, if the more-conserva- tive material meets all the treatment requirements then that is the ideal choice. The article will identify the clinical conditions in which treatment requirements dictate the use of a spe- cific category of ceramics. Space required for aesthetics The first consideration is the final 3-D position of the teeth, that is, smile design. There are several resources available for smile design.7, 8 The se- cond consideration is the colour chan- ge desired from the substrate (tooth), since this will dictate the restoration thickness. In general with porcelains, a porcelain thickness of 0.2–0.3 mm is required for each shade change (A2 to A1 or 2M1 to 1M1). For example, A3 to A0 would require a veneer of 0.6–0.9 mm in thickness. Glass-ceramics have the same space requirements as porcelain for effective shade change; however, the authors find it difficult to work with this category and produce the best aesthetic results when the material is less than 0.8 mm in thickness. High- strength all-ceramic crowns require a thickness of 1.2–1.5 mm, depending on the substrate colour, and metal ce- ramics need a thickness of at least 1.5 mm to create lifelike aesthetics. With that in mind, a diagnosis based on to- oth position and colour change will direct treatment planning, as well as the final decision regarding tooth pre- paration design (i.e., total tooth struc- ture reduction) ceramics and whether orthodontic treatment is required to facilitate a more conservative and ae- sthetic outcome. Clinical parameters to evaluate Once the 3-D smile design has been completed, colour change assessed, and adjunctive therapy finished to create an environment that will al- low the least removal of healthy tooth structure, an evaluation of each tooth is needed for ascertaining which ce- ramic system and technique are most suitable. The evaluation of individual teeth for specific material selection involves assessing four environmen- tal conditions in which the restoration will function. Substrate The first consideration is evaluating the substrate to which the material will be attached (Fig. 1). Is it enamel? How much of the bonded surface will be enamel? How much enamel is on the tooth? Is it dentine? How much of the bonded surface will be dentine? What type of dentine will the resto- ration be bonded to (tertiary or sclero- tic dentine exhibits a very poor bond strength, and bonding to this type of dentine should be avoided when pos- sible)? Is it a restorative material (e.g., composite, alloy)? These questions should be addressed for each tooth to be restored, since this will be a major parameter for material selection. It is generally understood and ac- cepted that a predictable and high bond strength is achieved when resto- rations are bonded to enamel, given the fact that the stiffness of enamel supports and resists the stresses pla- ced on the materials in function. Ho- wever, it is equally understood that bonding to dentine surfaces, as well as composite substrates, is less pre- dictable given the flexibility of these substrates. The more stress placed on the bonds between dentine and com- posite substrates and the restoration, the more damage to the restoration and underlying tooth structure is li- kely to occur. Therefore, because ena- mel is significantly stiffer than either dentine or composite and much more predictable for bonding, it is the ideal substrate for bonded porcelain resto- rations. Flexure risk assessment Next is the flexure risk assessment. Each tooth and existing restorations are evaluated for signs of past overt tooth flexure. Signs of excessive to- oth flexure can be excessive enamel crazing (Fig. 2), tooth and restoration wear, tooth and restoration fracture, microleakage at restoration margins, recession, and abfraction lesions. Often, the aetiology is multifactorial and controversial. However, if seve- ral of these conditions exist, there is an increased risk of flexure on the restorations that are placed, which may overload weaker materials. Evaluation of this possibility is also based on the amount of remaining tooth structure. The more intact the enamel is, the less potential there is for flexure. The amount of tooth preparation can directly affect tooth flexure and stress concentration. There is much potential subjectivity in any obser- vational assessment of clinical con- ditions; however, an assessment of flexure potential for each tooth to be restored is needed. A subjective assi- gnment of low, medium, or high risk for flexure is based on the evaluated parameters, as outlined below: Low risk for clinical situations in which there is low wear; minimal to no fractures or lesions in the mouth; and the patient’s oral condition is rea- sonably healthy. Medium risk when signs of occlu- sal trauma are present; mild to mode- rate gingival recession exists, along with inflammation; bonding mostly to enamel is still possible; and there are no excessive fractures. High risk when there is evidence of occlusal trauma from parafunction; more than 50% of dentine exposure exists; there is significant loss of ena- mel due to wear of 50% or more; and porcelain must be built up by more than 2 mm. Excessive shear and tensile stress risk assessment The third parameter is the risk (or amount) of ongoing shear and ten- sile stresses that the restoration will undergo, since the prognosis is more guarded for specific materials. All types of ceramics (especially porce- lains) are weak in tensile and shear stresses.9 Ceramic materials perform best under compressive stress. If the stresses can be controlled, then wea- ker ceramics can be used, for example bonded porcelain to the tooth. The same parameters are evaluated, simi- lar to flexure risk, for example deep overbites and potentially large areas where the ceramic would be cantile- vered (Fig. 3). If a high-stress field is anticipated, stronger and tougher ceramics are needed; if porcelain is used as the aesthetic material, the restoration de- sign should be engineered with such support (usually a high-strength core system) that it will redirect shear and tensile stress patterns to com- pression. In order to achieve that, the substructure should reinforce the veneering porcelain by utilising the reinforced-porcelain system tech- nique, which is generally accepted in the literature as a metal ceramic concept.10 The practitioner can assess and categorise low, medium, or high risk for tensile and shear stresses ba- sed on the parameters and symptoms mentioned above. Bond/seal maintenance risk asses- sment The fourth parameter is the risk of losing the bond or seal of the resto- ration to the tooth over time. Glass- matrix materials, which consist of the weaker powder/liquid porcelains, and the tougher pressed or machined glass-ceramics, require maintenance of the bond and seal for clinical du- rability.11, 12 Owing to the nature of the glass-matrix materials and the absen- ce of a core material, the veneering porcelains are much more suscep- tible to fracture under mechanical stresses and, therefore, a good bond in combination with a stiffer tooth substructure (e.g., enamel) is essential for reinforcing the restoration. If the bond and seal cannot be maintained, then high-strength ceramics or metal ceramics are the most suitable, since these materials can be placed using conventional cementation techniques. Clinical situations in which the risk of bond failure is higher are: • Moisture control problems; • Higher shear and tensile stresses on bonded interfaces; • Variable bonding interfaces (e.g. different types of dentine); • Material and technique selection of bonding agents (i.e., as dictated by such clinical situations as inability to achieve proper isolation for moi- sture control to enable use of adhe- sive technology); and • The experience of the operator (Fig. 4). An assignment of low, medium, or Figure 1: Image of prepared tooth: Signi- ficant dentine is exposed along the propo- sed length, flexure and tensile stress risk is at least medium, and the restoration thickness would be at least 0.9 mm. This was noted in the chart. Figure 2: Image demonstrating excessive enamel crazing, leakage, and staining. Flexure, tensile, and shear sheer risk would be medium to high. The substrate would depend on preparation. Figure 3: Image demonstrating deep over- bite in which shear and tensile stresses would be at least medium. Bonded porce- lain would require maintenance of enamel and an occlusal strategy to reduce levera- ge on the teeth. Cosmetic Ceramics: Rationale for material selection