Objectives of Impression making in complete denture

An impression is made for the purpose of achieving a few properties and characteristics in the denture to be fabricated. There are a few principles of making a good impression, but the main factors that are responsible for the long-term satisfactory results of the denture, during impression making are –
1. Retention
2. Stability
3. Support
4. Aesthetics
5. Preservation of Remaining Structures

Retention

It is defined (accd. to GPT) as “That quality inherent in the prosthesis which resists the force of gravity, adhesiveness of the foods, and the forces associated with the opening of the jaws”.
There are several forces that try to displace the denture away from the path of insertion. Retention is the ability to resist all these forces.

The factors that affect retention are –

Anatomical factors like size of the denture bearing area, and quality of the denture bearing area. Retention increases with increase in the size of the denture bearing area. Maxillary denture bearing area is usually around 24 sq.cm. and mandibular area is 14 sq.cm. So maxillary dentures always has greater retention when compared to the mandibular dentures. Displacement of the tissues also affect the retention. Those tissues which were displaced during impression making can rebound while using dentures, leading to loss of retention.

Physiological factors like Saliva. Viscosity of saliva has been a great factor for determining the amount of retention of dentures. Thick and ropy saliva gets accumulated between the tissue surface of the denture and palate, leading to loss of retention. Thin and watery saliva also produces compromised dentures. Cases with ptyalism can lead to gagging. Dentures in patients with the Xerostomia (reduced salivary flow in mouth) condition can produce soreness and irritation.

Physical factors like Adhesion, Cohesion, Interfacial surface tension, capillarity, atmospheric pressure and peripheral seal.

• Adhesion is the physical attraction of unlike molecules to one another. Saliva plays a major role in adhesion. It wets the tissue surface of the denture, and the mucosa, and a thin film is formed between the two surfaces and it helps to hold the two surfaces to one another. In xerostomia, there is no role played by saliva for adhesion.
• Cohesion is the physical attraction of like molecules to one another. The cohesive forces act within the thin film of saliva, and the effectiveness of these forces increases with increase in denture bearing area. Watery saliva produces thinner film and more cohesion, when compared to thick mucous saliva.
• Interfacial surface tension is the tension or resistance to separation possessed by the film of liquid between two well adapted surfaces. This acts with the air-liquid interface acting between two surfaces where a thin film of liquid holds the surfaces on the either sides. Thin film of saliva resists the displacing forces, and this aids in retention. For retention to happen effectively, there needs to be a thin film of saliva, and as there is excess saliva in the borders of a mandibular denture, there is minimal interfacial surface tension seen.
• Capillarity is that quality or state, because of the surface tension causes elevation or depression of the surface of the liquid that is in contact with a solid. Closeness of adaptation of denture base to soft tissue, and greater surface area can help in a good capillarity.

Mechanical factors include undercuts, retentive springs, magnetic forces, denture adhesives, suction chambers.

• Undercuts on one side can help in retention, but bilateral undercuts would require surgical correction as they can interfere with denture insertion.
• Intramucosal magnets can be placed for retention in highly resorbed ridges.
• Denture adhesives can be used where retention is needed, and should be coated on tissue surfaces before wearing the dentures.
• Suction chambers creates areas of negative pressure, which help in retention (these are avoided now, as they are found to creating palatal hyperplasia).

Stability

It is defined (accd. to GPT) as “The quality of a denture to be firm, steady or constant, to resist displacement by functional stresses and not to be subject to change of position when forces are applied.”
Its the ability of the denture to withstand horizontally acting forces, and the various factors that affect stability of the denture are –

• Vertical height of the residual alveolar ridge should be good enough. Highly resorbed ridges offers the least stability.
• Quality of the soft tissue over the ridge should be firm and resilient. Tissues that are flabby with excessive submucosa offers poor stability.
• The impression made should be as accurate as possible, and smooth, duplicating all the details of the tissues accurately.
• Occlusal plane should be oriented parallel to the ridge, and if these forces are inclined, it can lead to shifting of the sliding forces towards the denture, leading to reduced stability.
• Teeth in the denture should be placed in the neutral zone.
• Contour of the polished surface of the denture should be in harmony with the oral structures, and should not interfere with the normal action of the oral musculature and tissues.

Support

It is defined (accd. to GPT) as “The resistance to vertical forces of mastication, occlusal forces applied in a direction towards the denture-bearing area.”
For a proper support, the denture should be covering as much tissue as possible and this helps in distributing the forces over a wider area. This is termed as snowshoe effect. Support will improve if the forces of occlusion are localized to stress bearing areas, and other areas are relieved.

Aesthetics

It is always a matter of concern in complete denture thickness. It is to be seen that the denture and teeth color are in good harmony with the surrounding tissues of the mouth, and the thickness of denture flange is also important. Thick denture flanges usually help in long-term edentulous patients for the required mouth fullness.

Preservation of Remaining Structures

Muller de Van in 1952 stated that “The preservation of that which remains is of utmost importance and not the meticulous replacement of that which has been lost.” While recording the impressions, the tissues should be subjected both to stress and relief, so that during the fabrication of the denture, it can be seen that the tissues are not under excess pressure while dentures are being used.
The peripheral tissues should be recorded accurately, so that the over-extension of the denture is prevented, thereby preventing the tissue irritation.

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References:
Textbook of Prosthodontics: Nallaswamy
Syllabus of Complete Dentures: Charles M. Heartwell, Jr. & Arthur O. Rahn