Fixed Restorations. Irena Sailer
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
The mean thickness of the enamel and dentin, eg, at sound central incisors, varies between 1.8 mm for 10–19-year-old patients to 3.1 mm for 60-year-old patients14 (Fig 1-2-3).
Fig 1-2-3a to 1-2-3c Cross-sections of anterior and posterior teeth, displaying the differences of the amounts of dentin and enamel surrounding the pulpal chambers.
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.
1.2.5 Amount and quality of soft tissues
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