In this chapter:
■ Patient demands
■ Esthetic requirements – prerequisites
■ Amount and quality of tooth substance
■ Amount and quality of soft tissues
■ Occlusal and functional requirements
The selection of the restorative material is a crucial step within the rehabilitation of patients with fixed restorations, as the material is of high importance for the esthetics and the long-term performance of the fixed tooth- and implant-supported restorations. The restorative team, ie, the dentist and dental technician, need to be aware of the indications, advantages and limitations of all restorative materials in order to select the most appropriate alternative for the specific patient situation (Part I, Chapter 1).
Today, the selection of the material also includes the selection of the fabrication technology and, consequently, the efficiency (time, costs) and efficacy (predictability of outcomes) of the treatment.
In this chapter, the clinically relevant patient-based factors for the material selection will be discussed.
The thorough evaluation and definition of the patient expectations at the beginning of the treatment is possibly the most important Part of a prosthetic rehabilitation.The patient demands for dental improvement can focus on the following factors:
■ occlusion and function
■ intraoral health.
A patient may have only one priority or may desire to improve several or all of these factors with the proposed treatment. Esthetics is probably the most complex patient demand for a prosthetic rehabilitation when considering the material selection. Some esthetic prosthetic materials exhibit low stability (see Part I, Chapter 1), feldspathic ceramics being a good example. Besides good esthetics, the restoration has to withstand the conditions of the oral environment for long-term periods. With this in mind, the selection of material for esthetic rehabilitation needs to include such parameters as the position in the jaws (anterior, posterior), occlusal and functional scheme, expected occlusal forces, and quality of substrate for adhesive cementation (abutment tooth, implant abutment).
Dental esthetics has generally become a very important Part of well-being today1,2. Patients research social media and the internet on methods of self-improvement.3 It has been shown that the improvement of dental esthetics contributes to self-improvement independent of culture or gender1,2. Frequently, patients approach the dental office with a clear treatment goal in mind after having researched the internet. They won’t hesitate changing dentist to receive the desired treatment, if their current dentist is not willing to perform it due to medical/dental or other reasons. This can complicate the professional relationship with the patients today. A thesis, focusing on factors that influence the demands of patients for restorative treatment, demonstrated the importance of the relationship of confidence and competence between clinicians and dental technicians and patients4.
Communication of the treatment goal and the individual treatment steps is crucial for the establishment of confidence between the restorative team and the patients. The discussion and the decision-making process between patients and the restorative team is simplified if the foreseen treatment goal has been visualized in the diagnostic phase, either by a conventional, manually made wax-up and set-up, or by virtual wax-ups and set-ups using CAD5.
Hence, prior to any prosthetic treatment the patient-oriented treatment goal needs to be defined through comprehensive pre-treatment diagnostics (see also Part I, Chapter 4). Diagnostics is the key element to the execution of the treatment, the selection of the restorative material, and shared decision-making.
From an esthetic point of view, the material selection depends on the color of the substrate, ie, the abutment tooth or the implant abutment, and the optical properties of the remaining dentition (technical factor, see Part I, Chapter 3).
In situations with no or only slight discoloration, all restorative materials can be used. The selection of the respective material depends more on factors like the available space and the region of the jaw (see decision trees, Part I, Chapter 12).
In situations with discoloration, however, the selection of the restorative material becomes more complex (Fig 1-2-1). Dental ceramics are translucent materials, and depending on the type of microstructure their translucency varies. Glass-ceramics are rather translucent and have low masking ability. In order to improve their masking ability, the thickness of the ceramic needs to be increased. Oxide ceramics have more opaque properties and, therefore, better masking quality (Fig 1-2-2).
To mask the same discoloration different amounts of space may be required depending on the ceramic. It has been shown that a thickness of at least 1.2–1.5 mm of glass-ceramic is needed to cover dark discolorations6. To mask discolorations with this type of ceramic induces a need for more invasive preparations. The same applies also to oxide ceramics like zirconia; however, this ceramic is also influenced by the color of the substrate and the cement used for the fixation of the restorations at thicknesses below 1 mm7,8. In situations with intense dark discoloration, metal-ceramics may be the most appropriate material combination for the masking.
The amount and the quality of the abutment tooth substance influence the selection of the restoration type (eg, overlay or crown), and of the respective restorative material. The conventional prosthetic treatment concepts are in transition today, shifting from full fixed/removable partial prostheses to less invasive, defect-oriented restoration types like veneers, overlays, partial crowns, or resin-bonded prostheses, where applicable.
One reason for this change of concepts is, that conventional tooth preparation for full crowns has been shown to lead to the of loss of abutment tooth vitality at rates between 2% to 4% after 5 years up to 10% at 10 years of restoration function9–12 (for further details see Part III). Hence, on the one hand biologic limitations exist at the tooth preparation and the amount of the tooth substance needs to be considered. On the other hand, the minimal material thickness values recommended for the respective restorative materials have to be taken into consideration. Table 1-1-1 in Chapter 1 summarizes the indications of the different types of restorative materials for conventional fixed partial dentures. Another reason for the change of concepts is, that significant improvements of the adhesive cementation means and methods have been made, widening the indications for esthetic materials that need adhesive cementation for good long-term stability13.
The amount and the quality of the tooth substance have to be evaluated during treatment planning as Part of the decision-making process, influencing the type of restoration and the restorative material (including its fixation).
Amount of tooth substance
A histologic investigation has examined the minimal thickness of dentin needed to avoid pulpal damage at different amounts of abutment tooth preparations15. The researchers included vital teeth with poor prognosis in this investigation foreseen for extraction, and full-crown preparations were performed. Thereafter, the teeth were extracted and the pulpal tissues histologically examined. A correlation between the degradation of the pulpal tissues and the remaining thickness of the dentin after tooth preparation was performed. The study showed that a minimum remaining amount of dentin of 1 mm was crucial in order to avoid pulpal damage at tooth preparation. As a consequence, to avoid damage to the pulp a maximum of 0.7 mm (young patients) and up to 2.1 mm (elders) of (sound) tooth substance may be removed for restorative purposes. These values apply for sound healthy teeth which under normal conditions would not need to be restored. Still, the biologic limitations need to be considered when performing tooth preparations for restorative purposes.
It was assumed that for esthetic all-ceramic crowns less invasive tooth preparations were needed than for conventional metal-ceramic crowns, as the color of the ceramics already resembles the color of the tooth substance. Consequently, less loss of vitality of abutment teeth supporting all-ceramic crowns would be expected. This, however, does not apply for all dental ceramics. A recent review of the literature has shown that with weaker ceramics, like glass-ceramic, the incidence of loss of abutment tooth vitality was even higher than with metal-ceramics9,10. Indeed, a laboratory study demonstrated that the amount of removed tooth substance for all-ceramic and metal-ceramic anterior and posterior crowns is rather similar. Both are the most invasive types of fixed restorations16.
An overview of the different types of preparations for the different restorations is given in detail in Part I, Chapter 6.
Quality of tooth substance
The quality of the tooth substance influences the predictability of adhesive fixation of the restoration material to the abutment tooth substance. Materials for minimally invasive restorations like composites and ceramics depend on the adhesive fixation to the enamel and/or dentin in order to obtain sufficient stability for good clinical performance17. Numerous studies have demonstrated that adhesively cemented ceramic crowns exhibited better clinical survival rates than conventionally cemented ceramic crowns13. Veneers, onlays, and resin-bonded fixed/removable dental prostheses rely entirely on the adhesive fixation, as they have no or only little geometric retention to the abutment teeth. For good adhesion, the amount and the quality of enamel and/or dentin are crucial18. In case of lack of enamel/dentin for predictable adhesive cementation, the conventional treatment protocols with conventionally cemented restorations shall still be considered.
With tooth- and implant-supported restorations, the initial examination should include the evaluation of the patient- and site-specific soft tissues in addition to the previously discussed tooth-related factors.
The amount and the quality of the soft tissues play an important role for the selection of the restorative material. The thickness and the type of the soft tissues vary between patients. At approximately 80% of the population thin, delicate, and rather translucent soft tissues can be found, whereas at 20% of the population thick and resistant soft tissues are observed19. This difference plays an important role at treatment planning as it influences the selection of the restorative material. The soft tissue color may be positively or negatively influenced by the restorative material, most specifically in the marginal area of tooth- or implant-supported restorations20–22.
A recent study demonstrated that soft tissue color changes are perceived by dental professionals (dentist, dental technician) and laypeople to similar extent23. This study tested the threshold value for the visibility of soft tissue color changes using photographs of ideal anterior dentitions with non-discolored soft tissues as test objects. The photographs were introduced into a specific software (Adobe Photoshop), and by means of this software the gingiva and the teeth were separated into two layers. Thereafter, the color parameters (Lab values) of the gingival layer were gradually changed to a 1–6% range of higher and lower Lab values, increasing or reducing the brightness and shifting the soft tissue color within the color spectrum (either to more red and yellow, or to more green and blue). The modified gingival layers were merged back with the tooth layers, resulting in 12 color-changed pictures of the respective clinical case and one original picture. In another software (Keynote), the changed and the unchanged pictures of each of the clinical cases were combined in the presentations in a way that half of each image was of original color and the other half was color-modified. These presentations were then separately examined by 3 groups of 10 observers each – 2 professional groups (dentists, technicians) and 1 group of laypeople. The observers had to determine whether or not they perceived a difference in soft tissue color between the unmodified and the modified sides at the 13 images per patient, and if yes, whether the color change was lighter or darker. With the aid of the color difference ∆E, calculated between the modified and unmodified soft tissues for each picture, and the evaluations of the different groups of observers, the respective threshold values for the perception of soft tissue color changes were assessed23. The study showed that our human eyes are sensitive to soft color differences, and soft tissue discolorations are equally perceived by professionals and laypeople23. Consequently, soft tissue discoloration caused by the restorative material can lead to the esthetic failure of the restoration and the restorative material has to be selected carefully.
The soft tissue color can be a critical factor for the esthetic outcome of a restoration both at non-vital discolored abutment teeth and at implant-supported restorations. In some situations, the restorative material is not capable of improving pre-existing discolorations. It has been shown that at non-vital abutment teeth, the soft tissue color was not related to the restoration material, ie, the post-and-core material or the prosthesis24. The soft tissue color could not be influenced, neither positively by white root post nor negatively by dark root posts24