Crowns
Crowns
Partial crowns or full crowns replace the outer layer of teeth, at least the enamel section, but also often to some extent large areas of dentine. If they are mainly fabricated for replacing lost tooth structure (tooth conservation), they can be considered a part of conservative dentistry but they often function as a component of restorations.
Crowns can be fitted on natural teeth (preparation) or the intraoral sections and abutments of implants. Crowns are retained by cementation and/or adhesion; it is also possible to fabricate screw-retained restorations on abutments. The long-term seal of the crown margin (marginal integrity) is very important for maintaining the health of the surrounding marginal periodontium and prevention of secondary caries.
Anterior crown preparations
Implant-borne metal-ceramic crown
Crowns are fabricated using a single material (full cast crown, all-ceramic crown) or by the combination of different materials (e.g. bonded crown). Very different procedures are used for this, e.g. casting (cast crown), milling, trimming, sintering (CAD-CAM), pressing (press-ceramic crown) or electroforming (electroformed crown). With a metal-ceramic crown a coping (framework) is first fabricated using metal and then coated with ceramic (veneered) to achieve optical similarity with the tooth. Historical techniques include soldering of crown parts (banded crowns).
Three metal full cast crowns
Prefabricated steel crowns can be used for treating severely destroyed deciduous teeth. Temporary (short, medium or long-term) custom-adapted prefabricated, frequently intraorally (direct) or extraorally fabricated temporary crowns (mainly using resins) bridge the period until a permanent crown is fitted.
Crowns fulfil various functions. As protective crowns they absorb masticatory forces to prevent teeth with weakened structures breaking apart and/or seal endodontically treated teeth against penetration of bacteria. They also protect the tooth against chemical and thermal irritation.
Anchor or abutment crowns retain bridges whereas support crowns anchor retentive and support elements such as rests and clasps. Connectors, e.g. attachments can also be integrated into the crown (attachment crowns).
Crowns can be fabricated individually (single crowns) or connected with other crowns (splinted crowns).
Upper anterior crowns
In the specific case of the telescope crown (conical telescope crown, parallel telescope crown) the crown consists of a fixed inner crown (primary crown) and an outer crown (secondary crown), which fits on the primary crown in the removable restoration. If a telescope crown is fabricated using the electroforming technique, it can be adhesively retained later in a tertiary crown fabricated over it.
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Word of the day
| English | German |
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| right-hand twist | Rechtsdrall |
Focus text of the month
Sleep apnoea Sleep apnoea Compared with healthy sleep, there is a great increase in breathing cessations (AHI = apnoea/hypopnoea index > 10 per hour), in the form of complete apnoea or reductions in airflow (hypopnoea). Central nervous system causes are rarer than the much more common OSA (obstructive sleep apnoea) of local (peripheral) origin. This itself causes central cessations, so mixed forms often occur (approx. 90%). OSAS (obstructive sleep apnoea syndrome) is nearly always associated with heavy snoring that disturbs other sleepers. The interaction of a number of factors, such as supine sleeping position, slack pharyngeal musculature, especially involving the soft palate and uvula and also the hypopharynx (e.g. due to alcohol consumption, sleeping pills), physiological and pathological anatomy (obstructed nasal breathing, enlarged tonsils, macroglossia, obesity) causes temporary complete obstruction of the airways during inspiration. This causes a drop of about 4% in the blood's oxygen saturation, which is normally 95% to 100% so the oxygen supply to the tissues is impaired. If the episode lasts longer, hypercapnia (a rise in carbon dioxide level) may occur. This then triggers frequent "micro-arousals" via chemosensors, which often do not lead to full awakening; sleep is disturbed with shortened deep sleep phases and an overall reduction in sleep quality with lower relaxation and recovery, which can lead to an increased need for sleep and increased daytime tiredness, daytime somnolence (hypersomnia) and a tendency to fall asleep. In the long term, apart from quality of life and energy levels, life expectancy is also affected as OSA is associated with high blood pressure, cardiac arrhythmias and depression and there is an increased tendency to stroke, heart attack and accidents. The diagnosis can be confirmed by recording relevant parameters during sleep at home (cardiorespiratory polygraphy) or in hospital (polysomnography when the patient spends the night in the sleep laboratory); these include ECG, pulse oximetry, EEG, video recording, etc.
Various approaches are used in treatment, alone or in combination:
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Polysomnogramme with hypnogramme