Objectives Of Impression Making

Objectives Of Impression Making

An impression is made for the purpose of recording the anatomical landmarks and then to reproduce them on the cast to be poured.

The main 5 objectives of impression making are: PRESS
P – Preservation of remaining tooth structures
R – Retention
E – Esthetics
S – Stability
S – Support
Retention:

It is defined as “That quality
inherent in the prosthesis which resists the force of gravity,
adhesiveness of foods, and the forces associated with the opening of
jaws”
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

Itis defined 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

It is defined as:*“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.
All these objectives should be kept in mind to get a perfect impression which can replicate the oral tissues in the best form.