Merits And Demerits Of Various Impression Techniques

Merits And Demerits Of Various Impression Techniques

Question. Define impression. List the merits and demerits of various impression techniques.
Answer:

Impression

An impression is the negative form of the teeth and other tissues of the oral cavity, made in a plastic material that becomes relatively hard or set while in contact with these tissues.

• An impression is made in order to produce a positive form or cast of the recorded tissues. A complete denture impression is a negative registration of the entire denture bearing, stabilizing, and border seal areas present in the edentulous mouth.

Impression Techniques Types of Impression Techniques

1. Definite Pressure Impression or Mucocompressive Technique or Closed Mouth Technique.
2. Minimum Pressure Impression or Mucostatic Technique as the open mouth or Passive Technique.
3. Selective Pressure Technique.
4. Controlled Subatmospheric Pressure.

Read And Learn More: Complete Dentures Question and Answers

1. Definite-pressure Impressions or Mucocompressive Technique

These are closed-mouth techniques that record the tissues in their functional position. The patient applies pressure by closing against teeth or occlusion rims, that are attached to the impression trays which in turn exerts pressure and executes muscle actions such as swallowing, grinning, or pursing the lips while the impression material flows.

Impression techniques  Impression materials used for this technique

1. Impression compound.
2. Waxes that flowed at mouth temperature.
3. Soft liners.

The significance of the closed mouth technique (Macmillan, 1947) is that this technique is capable of adequately trimming the lingual borders of the lower as the tongue movements are more forceful when the teeth are together than when the mouth is opened and the tongue is protruded.

Impression techniques  Disadvantages

1. Soft tissues that are displaced and recorded tend to return to their undisplaced position when the forces are released hence unseating the denture.
2. As the denture is constantly held in a displaced position, the pressure limits the normal blood flow.
3. Excessive bone resorption is due to a lack of proper blood flow.
4. Inadequate retention.
5. Often the dentures made with closed-mouth impressions are overextended and must be arbitrarily trimmed. Soft liners are good for refitting complete dentures and also possess the advantage of retaining compliance for many weeks, good dimensional stability, and excellent bonding to the resin denture base.

2. Mucostatic, Open mouth, Passive or Minimal-Pressure Impressions

• This technique was proposed by Richardson and popularised by Harry Page hence also referred to as the Page technique.
• Open mouth techniques are those that record the tissues in their undisplaced position hence also referred to as a non-pressure or mucositis technique.
• The main point of the mucositis principle concerned Pascal’s law which states that the mucosa, containing a percent of water, will react like a liquid in a closed vessel and thus cannot be compressed.
• The impression is made in an oversized or spaced tray with no border seal or border refining. The completed denture borders are usually reduced to the stress-bearing mucosa depriving the denture of maximum coverage within tissue tolerance.
• Hence results in close adaptation of the denture base to the undistorted mucosa for stability but lacks in retention as no border seal is present. The only advantage was that the dentist can see whether muscle trimming is done properly or not.

Minimal-Pressure Impressions Disadvantages

1. The theory that mucosa is a closed vessel is not true as tissue fluids can easily escape under the border of a denture.
2. According to the principle of mucositis, the impression material should record, every detail of the mucosa without distortion so that the completed denture would fit all minute elevations and depressions. Chrome alloys that were used did not copy all details as accurately as Gold alloys.
3. The mucosal topography is not static over a 24-hour period. There is a difference between the mucosal contours just after rising in the morning and that which exists after 12 hours in the upright position again losing all the infinite details achieved in the impression.
4. The adherents of the mucositis principle considered interfacial surface tension as the only important retentive factor in complete dentures.
   • They did not use conventional flanges because these did not resist vertical displacement, which was the only movement interrupting surface tension. The lack of flanges did not resist lateral displacement.
5. The mucositis principle ignores the value of dissipating masticatory forces over the largest possible basal seat area.
6. Mucostatic denture minimized the retentive role of the musculature, as described by Fish.

3. Selective Pressure Technique

• The selective pressure technique is a combination of extension for maximum coverage within tissue tolerance with light pressure or intimate contact with the movable, loosely attached tissues in the vestibules.
• The impression is refined with a minimum of pressure. This technique combines pressure over certain areas and little pressure over others.
• It utilizes a preliminary compound impression that is generously relieved over the midline and incisive papilla areas.
• The final impression is taken in plaster, which acts as a wash and also records the relieved areas with minimal pressure while the ridge areas are undergoing considerable pressure.
• Thus the midline and papilla sections of the denture will not make contact with the mucosa when the denture is not in function, but by the same token, they will not bear heavily when the patient is chewing.
• This principle of impression-making is based on the belief that the mucosa over the ridge is best able to withstand pressure, whereas that covering the midline is thin and contains very little submucosal tissue (Boucher, 1951).

Selective Pressure Technique Disadvantage

This technique cannot be done on flabby ridges. This technique demands firm, healthy mucosal covering over the ridge.

4. Controlled Subatmospheric Pressure

Controlled subatmospheric pressure is a technique using individualized impression trays to which are attached to small rubber hoses. Through these hoses, a vacuum is created in the tissue side of the tray. The borders are refined, using an impression material such as Adaptol.

Controlled Subatmospheric Pressure Materials Used

Impression materials used in complete denture construction are elastic materials such as alginates, reversible hydrocolloids, rubber bases, and silicone compounds, and inelastic materials such as plaster, compound, wax, and metallic oxide pastes.