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Lec Prosthetic Dentistry Dr Farah Nabeel

Impression of Complete Denture

Impression:

In complete denture prosthesis, impression is defined as a –ve registration of the denture

bearing, stabilizing and border seal area that is poured with plaster or stone to produce a

primary cast or a master cast.

The basic requirement of impression making:

1 Knowledge of oral anatomy.

2 Knowledge of basic reliable technique.

3 Knowledge and understanding of materials.

4 Skill.

5 Patient management.

Objectives in impression making:

1. Stability: is defined as the resistance to the force which tries to dislodge the denture in

a horizontal direction. It is very important because it will give us a denture stable inside the

patient mouth during eating, speech.

Factors affecting the stability:

Quality of soft tissue.

Contour of polished surface.

Teeth arrangement.

Height of the ridge.

Occlusal plane.

2. Retention: is defined as the resistance to dislodgement of the denture in a direction

opposite to its path of insertion.

Retention of complete denture depends upon several factors:

Physical factors such as cohesion, adhesion, interfacial surface tension, peripheral

seal and atmospheric pressure.

Anatomical factors (size and quality of the denture bearing area).

Oral and facial musculature.

Physiological factors (saliva and its quality)

Mechanical factors (undercuts and denture adhesive).

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3. Support: is the resistance of the denture to the force which tries to dislodge it in a

tissue ward direction. To give a good support to the denture we should cover the primary

stress bearing area during the primary impression and we should exert the correct amount

of pressure on these areas during impression taking.

4. Esthetic: esthetic consideration in complete denture should begin from the primary

impression. This depends on correct extension of the flange in vertical direction and

correct thickness of the flange in the impression.

5. Preservation of the tissue: This is the most important objective and it is the end result

of the previous objectives since we are dealing with the residual ridge, so we should

preserve this residual ridge.

Primary Impression:

It is an impression made for the purpose of diagnosis, treatment planning and construction

of special tray. It is the first impression made for the patient and from which the study cast

was produced. These impressions are obtained by a stock tray.

When the primary impression is made, the objectives are to record all areas to be covered

by the impression surface of the denture and the adjacent landmarks with an impression

material that is accurate.

The maxillary impression should include the hamular notches, fovea palatina, frenum

attachments, palate, and the entire labial and buccal vestibules.

The mandibular impression should include the retromolar pad, the buccal shelf areas, the

external oblique ridges, frenum attachments, sublingual space, retromylohyoid space, and

the entire labial and buccal vestibules.

Materials used to making primary impression:

*Impression compound. *Alginate *Putty silicon rubber base material.

Impression compound is a thermoplastic material, which

means that it becomes soft when subjected to heat and is

hard at room temperature. This material because it is

thermoplastic can be reused and corrected inside the

patient mouth. It becomes soft between (65-70c° hot water

temperature) and care must be taken to avoid burning the

patient, we can detect the temperature suitable by the back

of our hand before inserting the tray inside the patient’s

mouth. Impression compound

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Alginate is an irreversible hydrocolloid which changes to get through a chemical reaction

for setting.

Stock tray:

It is an impression tray serves to carry the impression material to the mouth and support it

in the correct position while it is hardening. This type of trays can be used for making

primary impression. It makes from different materials such as aluminum, stainless steel or

plastic, in variety of shapes, sizes to fit different mouth.

Steps of making primary impression:

1. Selection of the stock tray:

In primary impression of completely edentulous patient, the stock tray should have certain

criteria:

A. Stock tray for the edentulous patient should have short flanges and narrow floor with

L-shape handle.

B. Type of impression material. For each type of impression materials we need a certain

type of tray. In case of impression compound we use a non-perforated stock tray since the

material stick to the tray. For alginate we use a perforated stock tray because this material

dose not stick to the tray, so those perforations will give mechanical means of retention of

the material to the tray.

C. Size of the arch: the stock tray should cover the whole anatomical landmarks which

are needed in complete denture construction and also should follow the form of the

arch. The stock tray should cover the denture bearing, stabilizing and border seal area

and should have a sufficient room for the impression material (2-3 mm clearance

between the tray and the ridge and 4-5 mm clearance for impression compound).

2. Seating of the patient:

The patient should be seated in an upright position, so that the back of the patient

perpendicular on the floor. The head rest receives both occipital protuberances. The

mandible should be parallel to the floor when the mouth is opened. In this position the

muscles which control the position of the mandible is suitable situation for impression

taking procedure, so we can registrar all the anatomical landmarks.

3. Position of the dentist:

When taking a lower impression we should stand infront of the patient while when taking

an upper impression we should stand behind the patient, this will facilitate the manipulation

of the tray and impression taking procedures.

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Notes:

The amount of material loaded inside the tray is important to be considered in

taking the impression. We should load sufficient amount because insufficient

amount of the impression material will give under extension, while excessive

material will give over extension.

The centralization of the tray inside the patient mouth is also important. We should

centralize our loaded tray in both directions (anteroposterior direction and

mediolateral direction) inside the patient mouth. The handle of the tray will help us

in the centralization of the tray.

Production of study model or primary cast:

The primary cast is produced by pouring the primary impression with plaster which is the

positive reproduction of the oral tissues.

Advantages of primary cast:

1. To see the problems with that kind of ridges (undercuts in the premaxilla, buccal

tuberosity and lingual vestibule in the lower).

2. To show the patient the problems associated with his future denture.

3. To see the laboratory difficulties and how to overcome these problems.

4. To indicate the difficulties during taking final impression that will face the dentist.

5. To eliminate the under cuts and to proper extension of the border (mucobuccal fold,

and posterior palatal seal area) and area buccal to maxillary tuberosity.

6. Also in some cases may choose a final impression material that will be suitable for

each case depending on the form of the ridge and overlying mucosa.

7. To construct a special tray (custom tray).

Patient sits in an upright position

Dentist stands behind the patient while taking upper

impression

Dentist stands in front of the patient while taking

lower impression

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Special tray:

Is a device to carry and confine final impression material, also it called individual tray or

custom tray which is constructed or specified for specific patient on a study cast in order to

get a final impression.

The important of special tray:

It gives a correct adaptation to the tissue.

It gives equal and adequate amount of space between the tissue and the tray.

It gives uniform space of the impression material as giving a thickness of 1-2mm in

the whole area while the stock tray gives about 10 mm so shrinkage or deformation

will occur (Even thickness will lead to equal pressure on the tissue of the denture

bearing area).

Characteristics of special tray:

It should be rigid and strong to hold heat, insertion and border molding which

means to withstand manipulation. (Resist heat and pressure).

Adjustable: you can add or remove from the tray.

Comfortable: no injury or pain during insertion or removal to the patient (that’s

obtained by well roundining of flanges and good polishing).

Material used in construction of special tray:

Cold cure acrylic is the most common material because it will lead to a strong and

adjustable tray.

Shellac base plate (not common) leads to fracture easily.

Wax (in certain technique) flexible.

Light cure resin.

Spacer:

One layer, two or three layers of base plate

wax depend on the technique and the quality

of tissue where adapted on the cast to have

space between the inner surface of the tray

and the tissue.

Special tray Acrylic

Spacer

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Correction of special tray:

The correction of the special tray should be done in to two steps:

A. Laboratory correction: This is done in the lab by

providing a smooth, well rounded, well-polished

borders, this can be checked with the little finger. The

flange should be 2 mm away from the mucobuccal fold.

Also there should be enough room for the labial frenum

and buccal frenum.

B. Clinical correction: this is done inside the patient mouth as following:

Maxillary special tray:

1. Insert the tray obliquely inside the patient’s mouth and do visual examination of the

extension of the special tray in the anterior region. There should be a sufficient room for

the labial frenum and 2 mm shorter border than the mucobuccal fold.

The borders of the tray should be checked by pulling the upper lip down ward if there is

displacement of the special tray this mean that there is an over extension in this region

which should be corrected.

2. Then the posterior area (buccal flange) should be checked by asking the patient to open

his mouth if there is a downward movement of the special tray this means over extension

in this region, so we correct it until there is no downward movement of the special tray on

opening the mouth.

3. Also checking the extension of the special tray in the posterior palatal seal area, this

can be checked by drawing the vibrating line, and the special tray should be 2 mm behind

that line.

Procedure of adding tracing compound and border molding for upper special tray

Border molding is the process by which the shape of the border of the tray is made to

conform accurately to the contours of the labial and buccal vestibules in order to ensure an

optimal peripheral seal.

1. This process begins with the buccal flange; either right or left by adding tracing stick

compound on the border of the special tray in this region (Any tracing compound in the

inside of the special tray should be removed; otherwise the position of the special tray will

be different and wrong). Then tempering the tracing in water, inserting the special tray

inside the patient’s mouth, and doing the border molding.

2. After finishing one buccal region, the other side of the buccal flange should be done

followed by the labial region.

Mucobuccal fold

2 mm short

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3. In the posterior palatal seal area, the tracing compound is added inside the special tray.

4. At the end of this procedure, the special tray should be retentive so when we try to

remove it from the patient’s mouth, we should feel resistance. In addition to that the

special tray should be stable when the patient opens his mouth.

Mandibular special tray:

1. Checking the extension of the special tray in the anterior region by visual examination.

The labial flange of the special tray should be 2 mm away from the labial vestibule; also

there should be a sufficient room for the labial frenum.

2. Checking the extension of the buccal flange in the buccinator muscle region by asking

the patient to open his mouth but not widely. We put our fingers on the special tray and

ask the patient to open his mouth, if there is an upward movement of the special tray this

mean an over extension in the buccinator muscle region which should be corrected.

3. Correction of extension of the buccal flange in the masseter muscle region, by putting

our fingers on the tray and ask the patient to open his mouth widely, if there is an upward

movement this means an overextension in the masseter muscle region.

4. In the retromolar pad area, the special tray should cover the whole extension of the

retromolar pad without interference with the pterygomandibular raphae. So we ask the

patient to open his mouth a very wide opening. By this opening we can find the

pterygomandibular raphae and when the special tray go upward so there is an over

extension in this area.

Recommended sequence of maxillary border molding starting with

A. posterior buccal region. B. buccal region C. anterior region and D. posterior palatal seal area

Final result retentive tray

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5. Lingual correction of lower special tray can be checked by:

a) (Anterior region or sublingual region)

Inserting the special tray obliquely inside the patient’s mouth and asking the patient to

raise his tongue while we are supporting the tray. If there is movement (dislodgement)

this means an overextension which needs to be corrected.

b) (Mylohyoid region or middle region):

Ask the patient to protrude his tongue which should be limited just to humidify the

upper lip with the tip of the tongue then move his tongue to the left side. If there is an

over extension, movement will occur on the right side as the mylohyoid muscle will

elevate.

c) (Retromylohyoid region):

Ask the patient to move his tongue as far as he can in the lateral direction with a

protrusion as much as he can. If there is an over extension, this will lead to special

tray dislodgement so we correct the opposite region.

d) Lingual frenum:

Ask the patient to put the tip of his tongue in the posterior part of the palate. If there is

an overextension or insufficient space for the frenum there will be dislodgement of the

special tray.

Procedure of tracing and border molding of the lower special tray:

The border molding is either done as one step or step

by step (It is more difficult in the lower than that of the

upper special tray).

The direction of the extension of the lingual flange

should be medially to provide some sort of seal with the

tongue and floor of the mouth.

The extension of the flange in the molar region is very

important as the anatomy of the mylohyoid muscle. The

direction of the muscle in the posterior area is oblique,

while in the anterior region is in horizontal direction.

Therefore we should do relief in the posterior region at

the mylohyoid line.

The step by step border molding, starting by heating

and applying of tracing sticks compound to the border

of the special tray and as showing in the figure

Recommended sequence of mandibular border molding

starting with A. buccal region. B. posterior buccal region

C. anterior region D. anterior lingual

E. Posterior lingual

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Note: One step border molding is less time consuming and if

the patient in hurry.

Sometimes we use different materials as rubber base

impression material (polyether) to cover the whole border of the

special tray.

Then we insert the tray and do border molding by pulling the lip

and cheek downward and backward with rotational movement

(for the upper) and upward and backward movement with

rotational movement (for the lower).

The advantages of one step border molding:

1. It is one step which means that the procedure is carried out at the same time.

2. Less possibility of error if placed correctly.

The disadvantages:

It is difficult and need well trained dentist.

Final impression

It is an impression made for the purpose of fabrication of prosthesis. This impression is

made with individual tray. Final impression must be poured with stone material to produce

the master cast.

Classification of impression technique:

There are several methods for classifying the impression and each method depends upon

certain criteria. The most practical and easiest one depends on the amount of pressure

exerted during the impression taking. Therefore, we have three techniques:

1. Non-pressure technique (muco-static): it is that technique which records the tissues

in their nondisplaced or rest position with minimal pressure (registration of denture

supporting area under static condition). The impression is made in a spaced special tray

and using low viscous impression material such as impression plaster without border

molding.

The resulted denture will be closely adapted to the mucosa of the denture bearing area but

has poor peripheral seal.

2. Pressure impression technique (muco-functional): It is that technique which records

the tissues in their functional position (registration of denture bearing, stabilizing and

peripheral seal area under pressure. The tray used is a close-fit special tray, we should do

border molding by tracing stick compound; here zinc-oxide eugenol is used as an

impression material.

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3. Selective pressure technique: This technique used custom tray constructed with less

relief in the primary stress bearing areas and greater relief in the secondary stress bearing

areas. The tray can exert different amount of pressure on the denture bearing area, the

pressure should be high on the buccal shelf area and minimum on the crest of the alveolar

ridge and. This can be achieved by certain modification to the tray either by use of spacers

or perforations at certain areas. Here we use silicon or rubber base rubber base

impression material.

Procedure of making final impression:

Before taking the final impression, we should be sure that the special tray is retentive and

stable and have enough space for the impression material.

1. The most widely used material is zinc oxide euginol to which a close fit special tray

is needed and dose not need a retention mean. However, if rubber base material is

used then a retention mean is indicated which is either an adhesive material or

perforated special tray.

2. The mixing is done according to manufactures instructions by using equal lengths of

impression material (about 5-6 cm). Mixing is done on the oil resisting pad paper or

mixing slab and when a homogenous mixture is obtained then paint the tray and insert it

in an oblique direction by reflecting the cheek inside the patient mouth.

3. It is important to centralize the tray and seat it with reasonable pressure, and then

we do border molding. Also ask the patient to open his mouth widely and move his

mandible to the right and left side, because the mandible has effect on the distobuccal

part of the maxillary impression by the coronoid process in order to get uniform

thickness of the flange.

4. For the lower impression, border molding is carried out on the lingual flange by

moving the tongue from side to side, putting it in the posterior part of the palate and also

protruding it forward and in lateral direction.

The protrusion of the tongue in forward and in lateral direction is done for the purpose of

doing border molding in the molar region in mylohyoid and retromylohyoid areas. The

placement of the tongue in the palatal part is to provide sufficient room for the lingual

frenum.

5. After setting of the material, the impression is removed by breaking the seal and

inspected for any deterioration or deformation in the flange area, also room for frenum,

and finally length and width of the impression.

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There are many causes for the inaccuracy in final impression making which include:

1. Incorrect centralization of the special tray, so thick flange in one region and thin in the

other.

2. Pressure area especially in the secondary stress-bearing area

3. Large air bubble inside the impression.

4. Removal of the material before set.

5. Movement of the tray during setting of the material.

Note:

ZOE and impression plaster can’t be used when some undercuts are present. Also it is

unwise to use it in a patient with dry mouth (as it will stick to the dry surfaces). Therefore,

rubber base or silicon impression material is indicated. If rubber base is used, the spacer

should be kept until border molding is finished, and then removed before final impression

is taken. If impression plaster is used, then the tray should be spaced all around.