Final Impression in Prosthodontics

1. FINAL
IMPRESSION
Shivani Deshpande

2. An impression that represents completion of registration of the surface or object,
made for the purpose of fabricating a prosthesis; also referred to as ‘secondary
impression’ or ‘master impression’.
 Classification of impressions
Depending on purpose of impression making
◦ Diagnostic impression
◦ Primary/Preliminary impression
◦ Final/Secondary/Master impression
Depending on theories of impression making
◦ Mucostatic /passive/ nonpressure/ minimal pressure impression technique
◦ Mucocompressive /pressure impression technique
Depending on impression technique
◦ Open mouth
◦ Closed mouth

3. OBJECTIVES OF IMPRESSION MAKING
• Retention.
• Stability.
• Support.
• Aesthetics.
• Preservation of remaining structures.

4. Final impressions are made using the following :
1. Custom trays
2. Stock trays
3. Record bases with occlusal rims

5. Custom trays
Checking the custom trays intraorally:
• The custom tray is designed with a spacer adapted over
The arch. The tray should be 2 mm short of the sulcus and
border moulding should be carried out to record the sulcus in dynamic function
• The posterior extension of the maxillary tray should cover the hamular notch and
extend up to the posterior vibrating line. This is checked by palpating for the notch with
a ball or ‘T’ burnisher, and drawing the posterior vibrating line in the mouth and
transferring it to the tray.
• The mandibular tray should cover the retromolar pads posteriorly.
• If a spacer is placed, it should be removed only after border moulding

6. • The spacer is scraped off before making the impression.
• The area where the relief wax was present, a pressureless impression is recorded.
• The area where there was no relief wax, the tissues are recorded under pressure.
This concept of applying pressure on certain area and relieving the others is called
selective pressure impression making
Tray compound
• A primary impression is made using the tray impression compound. This impression is
refined and trimmed.
• A metal wire is attached to the tray compound to act as a
handle.
• Now the tray compound acts as the custom tray

7. Making final impression
Material selection:
• When gross tissue undercuts exist, elastomeric impression materials are indicated.
• When using a custom tray with relief (close fitting tray), ZOE impression paste is
preferred.
• When using a custom tray with spacer, medium/regular body or monophase
elastomeric impression material is preferred.
• When using a custom tray with window, impression plaster is used for the displaceable
tissues, in combination with ZOE paste. Lightbody elastomeric impression material can
also be used

8. Secondary Impression Procedure for maxilla
This includes,
Border moulding
Tray preparation
Making the Impression
Recording the PPS
Checking error in the
PPS

9. Border moulding
◦ It is also termed as ‘peripheral tracing’.
Purpose - to create a peripheral seal.
Methods of manipulating the peripheral tissues
1. Active method: The patient performs various functions related to
the concerned areas to manipulate the borders.
2. Passive method: The dentist physically manipulates the tissues to
mould them.
Techniques of border moulding
1. Incremental or sectional border moulding
2. Single-step or simultaneous border moulding

10. Tray Preparation
The wax spacer is removed to provide space for the impression material.
• 0.5 to 1 mm of the tracing material is removed from the outer, inner and top surfaces
of the border
• The material over the posterior palatal seal is not removed because:
— It enhances the posterior palatal seal.
— It serves as a guide for positioning the tray.
— It prevents aspiration of the impression material
• The thickness of the flanges and the border should be 2.5 to 3 mm.

11. Tray
Preparation
• Holes are drilled on the
impression tray to allow
escapement of the
impression material.
These holes are placed in
the areas of the
midpalatine raphe,
anterolateral and
posterolateral regions of
the hard palate and the
residual ridge region.
These holes prevent
tissue displacement
during impression making

12. Making the Master or Wash Impression
The impression material chosen for the secondary impression should be of low
viscosity to record the structures accurately. The amount of material loaded onto the
tray should be able to form a uniform, thin layer.
The materials of choice for a secondary impression are zinc oxide eugenol (ZnOE)
impression paste or medium-bodied elastomeric impression materials.
• The impression material is manipulated and loaded onto the tray.
• The patient is made to sit in an upright position.
• The operator should stand towards the rear or the side of the patient.
• The tray is placed in the patient’s mouth using the labial notch as a guide.

13. • The tray is seated into position by applying alternating pressures on the right and left
molar regions using the index fingers.
• Tray placement is complete only when the posterior border of the tray rests in the
hamular notch and is in contact with the palate
• The tray is maintained in this position by placing a finger in the palatal region of the
tray, immediately anterior to the posterior palatal seal.
• Passive movements similar to those performed during border moulding are repeated.
• After the material is set, the tray is removed in a single jerk.
• The impression is inspected for deficiencies and voids and is repeated if necessary

14. Recording the Posterior palatal Seal
Recording the posterior palatal seal is very important for the retention of the denture.. A
good posterior palatal seal is essential as it
 Aids in retention by maintaining constant contact with the soft palate
 Reduces gag reflex by preventing gap formation
 Prevents food accumulation at the posterior border
 Compensates for Polymerisation shrinkage

15. Classification of Posterior palatal Seal
Class I
Butterfly
shaped
Class II
Arch
shaped
Class III – Narrow
Band

16. Size of the PPS varies according to its classification
> Anteroposterior width is 8mm at the widest point (5-12mm range)
> Average inter hamular notch distance is 37.1mm (males) and 35.6mm (females)
The methods used to mark the postdam are:
• Conventional approach.
• Fluid wax technique.
• Arbitrary scraping of the master cast.
> Boucher’s technique
> Bernard levin’s technique
> Applebaum-Winkler’s technique
> Silverman’s technique
• Extended palatal technique

17. Errors in Recording Posterior palatal seal
Underextension
• When the denture does not cover the fovea palatina
• The dentist may intentionally leave the posterior borders underextended in order to
reduce gagging.
• Improper delineation of the anterior and posterior vibrating lines.
• Excessive trimming of the posterior border of the cast by the dental technician
Overextension
Overextension of the denture base can lead to ulceration of the soft palate and painful
deglutition. Covering of the hamular process can lead to sharp pain in that region

18. Underpostdamming
This can occur due to improper head-positioning and mouth positioning. This will
produce a space between the denture base and the tissues.
Overpostdamming
This commonly occurs due to excess scraping of the master cast. It occurs more
commonly in the hamular notch region.
• Mild overdamming in the hamular notch region can lead to tissue irritation of the
mucosa and excessive postdamming produces downward displacement of the denture
posteriorly

19. Secondary impression Procedure for mandible
Impression procedures for the mandible include:
• Border moulding (Fournet and Tuller’s technique).
• Tray preparation after border moulding.
• Making the wash or master impression.
Tray Preparation (after Border Moulding)
It is similar to the procedure done for maxillary impression.
But the escape holes should be placed 10 mm apart in
the alveolar ridge region and over the retromolar pad.

20. Making the Final or Master or Wash Impression
• Tray placement should be practiced before making the final impression
• Dry gauze should be placed in the floor of the mouth to remove the saliva.
• The gauze should be removed before making the impression.
• The impression material is manipulated and loaded onto the tray.
• The tray is rotated in a horizontal plane and inserted into the mouth using the anterior
handle.
• The tray is seated completely by applying alternating pressure over the posterior
handles. The patient should be asked to touch his upper lip with his tongue while
making the impression. Passive movements similar to those performed during border
moulding should be repeated.
• After the material is set the impression is removed and examined for any defects.

21. Final impression using custom tray with window
• This is indicated for flabby or displaceable tissues.
• A custom tray with a window in the displaceable area is made as described previously.
• Border moulding is completed as described previously with greenstick low fusing
impression compound.
• A final impression is made with ZOE impression paste
• The tray is reinserted and impression plaster is injected
onto the window
• The tray is removed after the plaster sets

22. Inspecting the Impression
The impression made is inspected for air inclusions and voids. The surface is inspected
to make sure, that all the landmarks are recorded accurately. Small voids can be
rectified by filling them with wax.
DISINFECTING THE IMPRESSION
The impression is disinfected using iodophor or 2% gluteraldehyde. It should be left
undisturbed for 10 minutes.

23. REMAKING THE IMPRESSION
The impression is remade if there is
• improper positioning of the impression tray.
• Large voids
• Improper consistency of impression material
• Movement of the tray during the setting of the impression material.
• Inadequate scrapping of the border moulding material.
• Using too much or too little impression material.

24. Digital Impressions
◦ Fabrication of final dental restorations through conventional practices involves a
complicated process the fabrication of final dental restorations. A comparatively new
approach employs Computer-Aided Design/Computer-Aided Manufacturing
(CAD/CAM) technology such as to take a digital impression intra orally, fabricate the
master model, and design as well as produce the final restoration
The main digital impression systems those are available on the market include
◦ CEREC
◦ Lava C.O.S. system,
◦ iTero
◦ E4D
◦ TRIOS

25. ◦ CAD/CAM systems are having three main parts:
(1) A data acquisition unit, which gathers the information or data from the prepared
teeth and the adjacent structures and then converted into visual or optical
impressions which are created directly or indirectly at the same time
(2) Different softwares are used for the designing of the final restorations which are
secured in optical impressions and prepared for the milling parameters
(3) A computerized milling system for the final manufacturing of the restoration with
solid blocks of the appropriate restorative material.
The first two parts of the system are associated in the CAD phase, while the third one
is the CAM phase