Preparation and opening of the tooth cavity
Access to the pulp chamber begins with the preparation of the carious cavity or trepanation of an intact crown. In intact teeth, a round diamond bur is moved from the side closest to the pulp horn.
For lower intact molars, opening is performed in the middle third of the longitudinal fissure. The bur is directed toward the anterior buccal cusp. Widening the cavity in the buccolingual direction is a gross clinical error; access must be formed strictly in the anteroposterior direction (the exception is obliteration, where movement toward the posterior canal is permitted).
The opening of the pulp chamber itself is performed with an ISO size 011 round bur (or #1). A reliable sign of success: upon probing, the instrument characteristically drops into the cavity, or a pinpoint drop of blood appears. Afterward, the formed cavity is treated with weak antiseptic solutions and thoroughly dried with an air stream.
Devitalizing pastes: arsenic and paraformaldehyde
Two main types of active substances are used to kill the pulp:
- Arsenious anhydride (Acidum arsenicosum). Causes direct death of cellular elements due to protein denaturation and blockade of enzyme systems (cytochromes), which disrupts tissue respiration. The drug also blocks sympathetic nerve synapses. Due to the loss of tone, the blood vessels of the pulp dilate, leading to thrombosis and complete cessation of blood circulation.
- Paraformaldehyde. Acts more slowly. At oral cavity temperature, it gradually depolymerizes, releasing formaldehyde. This leads to deep dehydration and mummification of the pulp.
Crucial rule: paraformaldehyde pastes must be mixed strictly with oil (eugenol, phenol). The presence of water is unacceptable, as paraformaldehyde will turn into an aqueous formaldehyde solution too quickly, and mummification will not occur.
Both pastes include a local anesthetic (tetracaine, cocaine, or benzocaine) as auxiliary components for rapid pain relief, a strong antiseptic (thymol, carbolic acid) to suppress microflora, and camphor to reduce the impact of decaying tissue toxins.
Application rules and temporary filling
The standard dose of arsenious paste is 0.0006–0.0008 g, which visually corresponds to the size of a #1 round bur head. The preparation is applied pinpoint-style with a dental probe directly onto the exposed pulp horn. A loose, squeezed cotton pellet soaked in an analgesic liquid and antiseptic (e.g., camphor-phenol) is placed on top.
The final stage of devitalization is placing a temporary filling made of water dentin, which is mixed on a rough glass slide for 30 seconds. The material is applied in a single portion with a plastic instrument and compacted with a cotton pellet. The filling must provide absolute hermetic sealing.
Drug exposure times are strictly regulated:
- Arsenious paste: 24 hours in single-rooted teeth, 48 hours in multi-rooted teeth.
- Prolonged arsenious pastes: 1–2 weeks.
- Non-arsenic (paraformaldehyde) pastes: 7–10 days.
Establishing endodontic access
After pulp devitalization, it is necessary to properly open the tooth cavity. The main goal is to ensure straight-line access of the endodontic instrument to the root canals. The file must not bend when introduced into the orifice.
To achieve this, the roof of the tooth cavity (the ceiling of the pulp chamber) must be completely removed. The walls of the formed cavity should be made vertical, transitioning smoothly into the walls of the root canals without ledges. Overhanging enamel and dentin edges are unacceptable.
Frequent clinical errors:
- In the anterior group (incisors): attempting to enter the canal directly through the carious cavity (e.g., on the proximal surface) without creating a trepanation opening on the lingual or palatal side. This leads to a severe bend of the instrument, the inability to treat the apex, and the risk of file fracture. Access is always performed from the oral surface.
- In molars: incomplete removal of the pulp chamber roof. Overhanging dentin blocks access (especially to the medial canals). The instrument bends, and remnants of necrotic pulp under the overhanging edges subsequently cause crown discoloration and maintain infection.
Briefly: answers to topical questions
What materials are used to place a temporary filling after introducing a devitalizing paste?
Water dentin is used to place a temporary filling after introducing a devitalizing paste. It is mixed for 30 seconds on a rough glass surface, introduced into the cavity in a single portion, compacted with a cotton pellet, and shaped into a filling. The temporary filling must hermetically seal the cavity.
What complications arise when the exposure time of arsenious paste is exceeded?
In sources, medicinal periodontitis is listed as a complication of arsenious paste penetrating into the periodontal tissues. The exposure time of arsenious paste is 24 hours in single-rooted teeth and 48 hours in multi-rooted teeth. The mechanism of the paste's toxic action is associated with a direct cytotoxic effect, blockade of cytochromes, disruption of cellular respiration, and cell death.
What antidotes are used for the toxic action of arsenious anhydride in the event of periodontitis development?
Sodium thiosulfate is used as an antidote for arsenic poisoning. It enters into a chemical reaction with arsenic and forms non-toxic sulfites.
Which endodontic instruments are used for the extirpation of devitalized root pulp?
Pulpextractors and reamers are used for root pulp extirpation in readily patent canals.
- Pulpextractor — introduced to the full length of the canal without pressure and rotated by 90-180°. The pulp is torn away from the periodontal tissues as a single strand, leaving a jagged wound.
- Reamer — performs reciprocating or rotational movements. When used, the root pulp is sliced, resulting in a clean-cut wound.
The procedure is performed under the cover of an antiseptic solution.
The arsenic exposure rule: "One root — one day, multiple roots — two days." And the paste portion size is always equal to the size of the smallest round bur (#1).
Class and exam questions
When probing the floor of the cavity, the instrument characteristically "drops" into the pulp chamber, or a drop of blood appears in the created point-like opening.
When probing the floor of the cavity, the instrument characteristically "drops" into the pulp chamber, or a drop of blood appears in the created point-like opening.
Why must paraformaldehyde paste categorically not be mixed on a water base?
In the presence of water, paraformaldehyde turns into a formaldehyde solution too quickly. Because of this, the process of gradual dehydration and mummification of the pulp does not occur.
In which direction should the cavity of intact lower molars be opened?
Opening is performed in the middle third of the longitudinal fissure, and the bur is directed toward the anterior buccal cusp. Access goes strictly in the anteroposterior direction; moving buccolingually is a clinical error.
What else to study on this topic
- Topography of root canals in the anterior group of teeth
- Anatomy of the coronal cavity of upper and lower jaw premolars
- Structural features and number of canals in molars
- Criteria and methods for clinical control of tooth cavity opening
- Formulations and classical recipes of arsenious pastes
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Other topics in "Chapter 7. Endodontics"
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