Impact of filling on tooth strength
Preparation of a depulped tooth for a porcelain-fused-to-metal crown requires significant reduction of hard tissues, which leads to a substantial weakening of the prepared core.
Main causes of strength loss:
- The tooth cavity is replaced with filling material, the strength of which is many times inferior to natural dentin.
- Due to depulping, metabolic processes are disrupted, which additionally reduces the resistance of the remaining tissues to masticatory loads.
As a result, the formed core with a shoulder decreases in size and has low resistance to pressure. This creates a high risk of tooth fracture along with the installed prosthesis.
Indications and contraindications for porcelain-fused-to-metal restorations
The main condition for installing a porcelain-fused-to-metal restoration is sufficient tooth wall thickness and prominent clinical crowns. This allows tissue reduction without the risk of accidental exposure of the pulp chamber.
Indications:
- Small bounded defects (anterior and anterior-lateral regions).
- Allergy to plastic veneers.
Absolute contraindications:
- Prosthetics in children and adolescents with vital pulp.
- Thin-walled, short, or flat clinical crowns.
- Large defects (absence of more than 3–4 teeth), as elastic deformation of a long fixed dental prosthesis leads to porcelain chipping.
Relative contraindications (according to V.I. Bulanov):
- Deep overbite in malocclusions.
- Lower incisors with intact pulp and short clinical crown.
- Increased tooth wear.
- Masticatory muscle parafunctions (bruxism).
Protection of vital and preparation of depulped teeth
In young patients (20–30 years old), it is recommended to keep teeth vital due to the wide pulp cavity and high risk of pulp trauma. For vital teeth, the use of temporary (provisional) crowns is mandatory. They protect operated tissues, prevent pulp reaction, prevent shifting of prepared teeth, and provide aesthetic rehabilitation.
If the clinical crown is short, the depulped tooth is reinforced with:
- A metal post (insertion to 2/3 of the canal length).
- A cast core post-and-core.
- Splinting of several adjacent teeth.
Errors at the preparation and impression stages
The shape of the core after preparation should be uniform for any type of crown. The diameter of the coronal part cannot exceed the diameter of the tooth neck.
Preparation stages:
- Planning the amount of reduction in a surveyor.
- Disocclusion with antagonists (occlusal surface).
- Separation of contact surfaces.
- Reduction of the equator and smoothing of edges.
- Preparation of the cervical region.
To accurately reproduce the prosthetic field, a double impression is used. The base (high-viscosity) paste creates the preliminary relief, while the wash (low-viscosity) paste fills fine details. Before the final impression, gingival retraction is performed (medicamentous with cords, physical with warm air, or using temporary crowns).
Framework try-in and ceramic correction
The cast coping is checked on the plaster tooth die. The planned thickness of the ceramic coating is 0.5–1.72 mm. If space is insufficient, the reason may be an inadequate amount of reduced tissue or inaccurate framework fit.
To identify areas hindering seating in the oral cavity, wet articulating paper is placed under the coping. Interfering areas are ground down with diamond burs.
At the final stage, an obstacle may be the excess of ceramic mass on proximal surfaces or the coping margin. Excesses are identified with articulating paper and carefully ground down with finishing burs, after which occlusal contacts are thoroughly verified in all positions.
Briefly: answers to topical questions
What gingival retraction methods are used in prosthetic dentistry before taking an impression?
Before taking the final impression, medicamentous, physical, mechanical, and chemical methods of gingival retraction are used, as well as the use of temporary crowns.
- Medicamentous method — introduction of cotton cords into the gingival sulcus for 15–20 minutes, moistened with solutions of xylometazoline, naphazoline, or orostat.
- Physical method — the least traumatic widening of the sulcus with a stream of warm air from a dental handpiece.
- Mechanical and chemical methods — mentioned as alternative options.
- Use of temporary crowns — crowns made of fast-curing acrylic are filled with wash paste and placed on the teeth, while the crown margin widens the gingival sulcus.
What typical errors are made directly during the filling of carious cavities?
During the filling of carious cavities, errors may occur related to improper base placement, absence of a contact point, and formation of overhanging margins.
- Extension of the insulating base onto the cavity margins — leads to dissolution of the base, disruption of marginal adaptation, loss of the filling, or secondary caries.
- Absence of a formed contact point — considered improper filling when working with amalgam on adjacent teeth.
- Creation of an overhanging filling margin — causes irritation from pressure, leading to inflammation of the gingival papilla and marginal gingiva.
Class and exam questions
Why are temporary crowns necessary on vital teeth?
They provide biological protection of the pulp from the external environment, prevent tooth shifting after preparation, and hide aesthetic defects.
What is the minimum post length for tooth reinforcement?
The post must be inserted into the root canal of the tooth by at least 2/3 of its total length.
What else to study on this topic
- Clinical and laboratory sequence of creating a porcelain-fused-to-metal prosthesis
- Methods of medicamentous and physical retraction of the gingival sulcus
- Determination of ceramic coating shade and firing stages
- Evaluation of artificial crown quality on a plaster cast
- Specifics of using silicone impression materials of different viscosities
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Other topics in "Chapter 9. Restoration of anatomical form and function of the tooth"
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