Impression Making in Complete Denture

Impression Making in Complete Denture

Question. What are the objectives of impression-making? Explain various impression procedures in CD fabrication.
Answer:

There are five main objectives in impression-making They are:

1. Preservation:

• Preservation of the remaining residual ridges
• As the natural teeth are lost there is a loss of stimulation resulting in alveolar ridge atrophy or resorption.
• Local factors such as occlusion, interocclusal distance, centric relation in harmony with centric occlusion, impression technique, and the impression material all have their own effect on the continued health of both the soft and hard tissues.
• Pressure in the impression technique leads to pressure in the denture base and results in soft tissue damage and bone resorption.

Read And Learn More: Complete Dentures Question and Answers

2. Support:

Support is defined as, “The resistance to vertical forces of mastication, occlusal forces and other forces applied in a direction towards the denture-bearing area.” Maximum coverage provides the “snowshoe” effect, which distributes applied forces over wide an area as possible. This helps in preservation, stability, and retention.

3. Stability:

• Stability is defined as, “Resistance against horizontal movement and forces that tend to alter the relationship between the denture base and its supporting foundation in a horizontal or rotatory direction.”
• The size and form of the basal seat, the quality of the final impressions, the form of the polished surfaces, and the proper location and arrangement of the artificial teeth help in maintaining the stability of the dentures. Close adaptation to the undistorted mucosa increases stability.

4. Retention:

• Retention 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 the jaws”–GPT or it is the ability of the denture to withstand displacement against its path of insertion.
• If the other objectives are achieved, retention will be adequate. Atmospheric pressure, adhesion, cohesion, muscle control, and patient tolerance affect retention.
• Atmospheric pressure: Atmospheric pressure depends on the peripheral seal. To ensure this seal, denture borders should extend into, but not to the extent to damage movable tissue.
• Adhesion: Adhesion is defined as, “The physical attraction of unlike molecules to one another”. Adhesion is the attraction of saliva to the denture.
• Cohesion: Cohesion is defined as, “The physical attraction of like molecules for each other”. The cohesive forces act within the thin film of saliva. Cohesion is the attraction of molecules of saliva to each other.
• Muscle control and patient tolerance: Dentures are often retained in the mouth not because of the accuracy of conforming to the support but because of patient tolerance and the adaptability of the muscles of the lips, tongue, and cheeks.

5. Aesthetics:

Border thickness should be varied with the needs of each patient depending on the extent of residual ridge loss. The vestibular fornix should be correctly filled to restore facial contour.

6. Steps in complete denture impression making:

1. Ensure the health of tissues that comprise the denture-bearing surfaces:
   • Leave the denture out of the mouth.
   • Adjust the patient’s present denture by tissue conditioning, and occlusal adjustments.
   • Prescribe pre-prosthetic surgery as in flabby tissues and prominent bony projections, etc.
2. Preliminary impression:
   • Use a rim-locked stock tray and irreversible hydrocolloid impression material.
   • Identify and mark the preferred peripheral outline of the custom tray. Make a laboratory stone cast.
3. Custom tray fabrication:
   • Outline the stress-bearing surface for the denture and cover it with a wax spacer.
   • Make an acrylic resin tray that extends just past the identified junction of the attached/ unattached mucosa.
4. Border Molding:
   • Try the tray in the mouth and correct for any overextensions.
5. Final Impression:
   • Use the preferred final impression material.
6. Working cast preparation:
   • Box and pour the final impression.
   • Trim the cast.

Mandibular

Mandibular Preliminary Impression:

After assessing the space available in the mouth, the general form, size, and health of the basal seat are evaluated. An edentulous metal stock tray is selected that will provide a 6 mm bulk of impression material over the entire basal seat area and all supporting tissues including the retromolar pads.

Mandibular Material of choice:

1. Alginate or
2. Impression compound.
   (Depending on the clinician’s choice.)

Mandibular Wax spacer design:

Mandibular: A wax spacer about 1 mm thick is placed over the crest and slopes of the residual ridge. The buccal shelf on each side and the retro mylohyoid spaces on the cast are left uncovered.

Mandibular Tray:

• Autopolymerizing acrylic resin tray approximately 2 to 3 mm thick with an anterior resin handle centered over the approximate position of the anterior teeth without interfering with the position of the lip.
• Two posterior handles in the molar region are used as finger rests to complete the placement of the tray on the residual ridge and to stabilize the tray in the correct position. The flanges of the tray should be contoured like the flanges of the completed denture.

Mandibular Preparing the final impression tray:

The acrylic resin tray is removed along with the spacer from the preliminary cast. The buccal and lingual flanges of the tray are marked in pencil and reduced until the borders are short of the limiting anatomic structures.

Mandibular Border molding:

1. Modelling compound is added to the lingual borders in the molar regions on both sides of the tray between the mylohyoid and post-mylohyoid eminences. This area of the compound is heated and tempered and the patient is asked to protrude the tongue.

This develops the slope of the lingual flange in the molar region to allow for the action of the mylohyoid muscle.

The distal end of the lingual flange should extend about 1 cm distal to the end of the mylohyoid ridge. The flange should be so

shaped that it turns laterally toward the ramus below the level of the retromolar pad and mylohyoid ridge. It assumes a typical “S” shape.

The compound on the distal end of the flange is heated and the tray is placed in the mouth and patient is instructed to protrude his tongue to activate the superior constrictor, then he is asked to close as the dentist applies a downward force on the impression tray.

The resulting contraction of the medial pterygoid muscle, acting posteriorly on the retro mylohyoid curtain, can limit the space available for the border of the impression in the retro mylohyoid fossa.

With the lower final impression tray placed in the mouth, the patient should be able to wipe the tip of his tongue across the vermilion border of the upper lip without noticeable displacement of the lower tray.

2. Buccal flange is border molded on both sides when the cheek is moved outward, upward, and inward.

Buccal frenum: The cheek is lifted outward, upward, inward, backward, and forward to simulate the movement of the lower buccal frenum.

3. Labial flange is border molded on both sides when the lower lip is lifted outward, upward, and inward.

4. The compound on the lingual surface from premolar to premolar of the flange, on both sides,is softened and the tray is placed in the mouth and the patient is asked to push the tongue forcefully against the front part of the palate.

This functional activity molds the mylohyoid fossa in the canine-premolar region, the flange becomes longer and extends below the level of the mylohyoid line.

After the tray is placed in the patient’s mouth, the patient is instructed to protrude the tongue which raises the anterior part of the floor of the mouth.

This functional activity molds the lingual frenum, both premylohyoid eminences and the lengths of the lingual flange on each side of the lingual notch will frequently be symmetric.

Mandibular Final Impression:

• The wax spacer is removed from the inside of the tray and several holes about 12.5 mm apart are marked in the center of the alveolar groove and the retromolar fossae of the tray and are cut with a No.6 round bur.
• The modeling compound borders are shortened by approximately 0. 5 to 1 mm to make space for the final impression material.
• Zinc oxide eugenol impression paste is used for the final impression. The refined impression is removed from the mouth, washed with tap water, dried, and boxed.
• Sufficient stone is poured into the boxed impression with a base height of 9 to 15 mm from the deepest portion of the impression. It is separated after 30 minutes.
• Another selective pressure technique in which Polyether is the material of choice. This is a single-step border molding procedure.

Mandibular Maxillary

Maxillary preliminary impression:

For the maxillary impression, the tray should cover all the basal seat area including the hamular notches and maxillary tuberosities. The labial and buccal borders of the tray are observed in relation to the limiting anatomic structures.

Maxillary wax spacer design:

• The cast is made of artificial stone poured into the irreversible hydrocolloid impression or impression compound. The outline for a wax spacer is drawn in pencil on the cast.
• A wax spacer about 1 mm thick is placed over the incisive papillae and median palatine raphae or a full spacer short of posterior palatal seal area and 2 mm short of sulcus depth can be designed depending on the clinical situation.
• Maxillary Impression Tray is fabricated in the same method as mandibular.

Mandibular Border molding procedure:

1. Distal to the buccal frenum, including the hamular notch and across the posterior palatal seal area is molded first. After softening the stick compound ask the patient to open his mouth wide as in yawning, then protrude and move the mandible to the right and left.
   • This combined action develops the distal extent of the denture in the hamular notch and the space between the anterior border of the ramus and the tubercles.
   • The impression compound across the posterior palatal seal area maintains contact with the tissues in this area. This is done bilaterally to ensure proper seating of the tray.
2. Add and soften the stick compound at the buccal frenum attachment (unilaterally). Flame, temper, and seat to place. Gently massage the upper lip in a superior-inferior direction. Remove the tray and chill it.
3. Repeat for the opposite side.
   • Soften impression compound in the buccal frenum area (unilaterally).
4. Add compound in the labial flange area and ask the patient to suck gently on his finger to join the refined buccal border and seat. Repeat for the opposite side
5. Soften the impression compound in the labial frenum area, seat, grasp the upper lip, and extend directly, forward, and to and fro.

Mandibular Final Impression:

1. After the borders have been refined, the tissues are examined and palpated with a ball burnisher and the tray is relieved with a round bur, three holes are placed in the tray. in the anterior section of the rugae area and two holes in the posterior area.
2. Apply petrolatum to the patient’s face, as the impression material is sticky.
3. Mix the impression material according to the manufacturer’s instructions. Load the trays, including the borders, to a uniform thickness.
4. Place the loaded trays in the patient’s mouth; seat them with positive pressure until the impression material is seen to flow from the borders and the posterior palatal seal area and the holes in the rugae area.
5. After which all pressure is released and the index and middle fingers are rested lightly, one on each side. The patient is instructed to flex his head forward and breathe through his nose.
6. When the zinc oxide-eugenol impression paste is thoroughly set, the refined impression is removed from the mouth, washed with tap water, dried with a gentle application of air, and inspected. If needed, the impression can be reseated and checked for retention and stability.