FINAL IMPRESSIONS-Impression techniques Flashcards
Impression Techniques:
Introduction:
- Soft denture bearing tissues are displaceable at varying degrees
- Displaceability is determined by several factors, such as location, histology & anatomy, thickness, condition etc.
- At rest, the soft tissue contours are in the so-called ‘anatomic form’
- In function, occlusal loads are transferred through a mucosa supported denture base to the soft tissues, which are compressed into the so-called ‘functional form’
Impression Techniques:
Types:
- Muco-static impression technique
- Muco-compressive impression technique
- Sélective pressure impression technique
Impression Techniques:
Muco-static impression technique:
Aim: to record the soft tissues in their ‘anatomic form’
- Spaced impression tray
- Material of choice: impression plaster*
- Other low viscosity materials (e.g. light bodied silicone)
- No border moulding
* Impression plaster is not any more used
Impression Techniques:
Anatomic form impression materials:
- The anatomic form is recorded by a soft impression material, such as a metallic oxide impression paste if the entire impression tray is uniformly relieved
- Depending on the viscosity of the particular impression used and rigitity of the impression tray the ridge form can also be recorded by mercaptan ruber, silicone and hydrocolloid impression materials
Functional form impression materials:
- Distortion and tissue displacement by pressure may result from confinement of the impression material within the tray and from insufficient thickness of impression material between the tray and the tissues, as well as from the viscosity of the impression material.
- Therefore, the recorded form of the residual ridge which is achieved by exerting some loading whether this is done by occlusal loading, finger loading or by special designed individual trays, or depending on the recording medium is called the functional form
Muco-compressive impression technique:
Aim: to record the soft tissues in their ‘functional form’
- More viscous impression material
- Close fitting custom tray
- Material of choice: zinc oxide and eugenol paste or impression wax
- Final impression may be recorded with the denture teeth set up and in occlusion (closed mouth technique)
- Custom tray is uniformly spaced to equalize compression of the denture bearing tissues throughout the area of the denture base
Selective pressure impression technique:
Selective Pressure Theory:
- Combines the principles of both pressure and minimal pressure techniques
- Tissue preservation + mechanical factor achieving retention with minimum pressure, which is within the physiologic limits of tissue tolerance
Selective pressure impression technique:
Philosophy of the Selective Pressure Theory:
- Certain areas of the maxilla and mandible, are by nature better adapted for withstanding extra loads from the forces of mastication
- These tissues can be recorded under slight placement of pressure while other tissues must be recorded at rest
Selective pressure impression technique:
Aim: to compress the soft tissues only in the main load bearing areas
- Viscous impression material
- Spacing of the custom tray is different depending on location
- Material of choice: zinc oxide and eugenol paste
- Alternatively: impression wax, polyether
Impression Objectives:
- Preservation: with the loss of the stimulation of the natural dentition the alveolar ridge will atrophy or resorb, the process can be hastened or reatarded by local factors.
- Support: maximum coverage provides the “snow-shoe” effect
- Stability: close adaptation to the underlyong mucosa is most important to reduce the horizontal movement of the denture.
- Esthetics: border thickness should be varied to restore facial contour and proper lip support
- Retention: atmospheric pressure, ashesion, cohesion (depends on peripheral seal) mechanical locks, muscle control.
What is pressure in the impression technique reflected as?
Pressure in the impression techniqie is reflected as pressure in the denture base and results in soft tissue damage and bone resorption.
Which are the basic differences in techniques for final impressions?
The basic differences in techniques for final impressions for final impressions can be resolved as those that record the soft tissues in a functional position and those that record the soft tissues in the undisplaced or rest position.
Impression techniques:
Tissues recorded in displaced position:
Soft tissues that are displaced and recorded in this position will attempt to return to the undisplaced position when the forces are released.
- The dentures will be unseated from their bases by this tissue action
- When tissues are held in a displaced position, the pressure limits the normal blood flow. When normal tissues are deprived of their blood supply, the result is resorption
In which position should the tissues be recorded?
Which is the exception?
We attempt to record the tissues at rest. The only exception is the posterior palatal seal area.
Selective pressure technique:
The selective pressure technique is a combination of extension for maximum coverage within tissue tolerance with light presure or intimate contact with the movable, loosely attached tissues in the vestibules. The impression is refined with minimum pressure utilizinf a wash of light body impression material.
Final Impression Appointment
- After establishing the health of the denture bearing area final impression are made
- Try in custom impression tray and adjust the length of the flanges 2-3 mm short of the vestibule depth.
- Establish the 3D contours of the denture borders by border molding the custom tray utilizing a thermoplastic “compound” material.
- Final impression with a light body material to achieve “mucostatic__“ final impression.
Instruct the patient to leave out their dentures for 24 hrs prior to the final impression appointment
Custom impression Trays:
Design Objectives:
- Well adapted to tissues with only slight wax blockout of undercuts to allow for consistent and repeatable seating and accurate impressions
- 2-3 mm thickness
- Border extensions should be 2-3 mm short of the depth of the vestibule when the intraoral tissues are at rest.
- Handle design should not impigne on the vestibule nor disort lips
- Finger rest in the 1st molar and 2nd premolar region so the finger should not disort the vestibule when border molding and making the mandibular master impression
- Fabricated utilizing “tray acrylic” which has a higher % filler material-more accurate, less shrinkage
Final impression-Custom Trays:
How should tray extension be adjusted?
Adjust tray extension:
- 2-3 mm short of the depth of the vestibule
Final maxillary impressions:
Areas requiring special attention:
- Posterior palatal seal area
- Incisive papilla
- Buccal and labial vestibule
Final maxillary impression:
Special consideration:
- Vibrating line
- Hamular notch
The tray must extend 2-3mm beyond the vibrating line
Border molding:
Area “A” has been refined. Note that the denture extension in this area is thinner and flatter. What structure limits the thickness and length of the denture border in this region?
Coronoid process
Border molding the labial flange-Area “C”:
What are the anterior areas molded by?
The anterior areas are molded by the following:
- Massage the upper lip with lateral motion
- Instruct the patient to pucker and smile
- Check the flange thickness for proper lip support
Don’t pull down on the lip. This maneuver will foreshorten the denture flange.
Developing the Posterior Palatal Seal-Area “D”:
- Place 2-3 mm of compound on top of the tray in a butterfly configuration to displace the tissues in the posterior palatal seal area.
- Seat the tray firmly. After the tray has been in position for 10 seconds ask the patient to swallow. Remove the tray and chill.
Border Molding-Cut Back:
Why?
- With the edge of your knife blade scrape away a thin layer of compound from the border molded periphery. This will create space for your impression material ad avoid undesiravlke tissue
- The areas of the periphery overlying the frenums should be relieved more aggressively.
What is the purpose of the vent hole?
- To permit proper seating of the loaded master impression tray while making the final impression
- To relieve the pressure over the incisive papilla and the rugae.
- To prevent entrapment of air bubbles in the impression
Caution: Do not drill the palatal relief hole(s) in the maxillary tray until are borders have been molded and the peripheral seal demonstrated.
Apply Tray Adhesive:
Apply a thin layer of tray adhesive and permit it to dry. Note that adhesive is applied 2-3 mm onto the external border of the tray.
Completed Maxillary Impression:
Remove the impression. Examine it carefully. What factors make for a good impression?
- Smooth well defined peripheries
- Maximum extension
- Even pressure distribution (there should be no areas where the underlying tray or compound shows through)
- There should be intimate tissue contact
Master Mandibular Impression:
Area requiring special attention:
- Retromolar pad
- Retromylohyoid space
- Buccal sheld
- Vestibules
Mandibular Sequence of Border Molding:
Area “A”:
- Dry the tray. Slowly heat the compound and apply to area “A” on one side of the tray.
- Insert the tray with compound being caresully to retract the cheek with a mouth mirror or your index finger.
- Be careful to seat the tray evenly.
- Define the tray extension by molding the lateral border “A” by massaging the cheek and having the pt. pucker and smile.
- Remove tray from the mouth and chill the compound
- Trim the excess compound that has flowed onto the tissue surface or the external surfaces.
- This is done with care using a red handled knife
- Section”A” on one side is complete. This defines the proper tray extension for this area.
Completed Buccal Flange:
What does section “A” define?
Section “A” on one side is complete. This defines the proper tray extension for this area.
Border molding the labial flange-Area “C”:
- Apply compound to area “C”. Temper, insert and gently massage the lower lip. Do not pull up the lip for it will foreshorten the labial vestibule
Border molding the lingual flange-Area “D”:
- Temper, insert and mold area “D” by instructing the patient to push their tongue against your thub placed in the lower incisor area.
- Proper extension into area “D” will create seal for the mandibular denture in selected patients with favorable tongue position and floor of mouth posture.
Apply tray adhesive:
After the compound is cut back apply a thin layer of polysulfide tray adhesive to the surface of the tray. Be sure to apply the adhesive 3-4mm beyond the border
