MANDIBULAR IMPRESSION PROCEDURES

MAKING THE PRELIMINARY IMPRESSION An edentulous stock tray is selected that will

provide for approx 5 mm of bulk of impression material over the entire basal seat area.

Posteriorly the retromolar pads should be covered by the tray.

The tray is raised anteriorly for observation of the relation between the lingual flanges and the lingual slope of the lower residual ridge.

Any areas of under extension need to be corrected using soft boxing wax before the impression is made.

Before making the preliminary impression, it is advisable to practice placing the tray in position and to rehearse the patient.

Patient is asked to open the mouth halfway, and the tray is rotated into the mouth in the horizontal plane using the handle until it is centered over the residual ridge, with the tongue raised slightly so that it is positioned in the tongue space of the tray.

Tray is seated gently by alternating pressure from an index finger on either side of the tray in the first molar region and the tongue in relaxed position.

IRREVERSIBLE HYDROCOLLOID Tissue surface and borders of the tray are painted

with an adhesive material to ensure that the irreversible hydrocolloid adheres to the tray.

The loaded tray is then loaded positioned in the mouth in a manner similar to that during the practice session.

Once the material has set, the cheeks and lower lip are lifted away from the borders of impression and the tray is removed from the mouth in one motion.

IMPRESSION COMPOUND Impression compound is used for making the

priliminay impression, the technique is the same except the borders of the tray are not modified by wax.

No need to use a tray adhesive. Impression compound has high viscosity and thus

it is easy to displace the mylohyoid muscle while making the impression.

Preliminary impression has been removed from the mouth the borders of the custom tray should be identified.

CONSTRUCTION OF CUSTOM TRAY Wax spacer approx 1mm thick is placed over the

crest and slopes of residual ridge. The buccal shelf on each side is left uncovered so

that the custom tray contacts the mucous membrane in the region of the buccal shelves.

It helps in positioning the tray correctly in the mouth and to place additional pressure on this primary stress-bearing area when final impression is made.

The custom tray should be 2 to 3 mm thick with an anterior handle centered over the labial flange in the position of anterior teeth.

Achieved by using either a stepped handle of sufficient height to avoid distortion of the lower lip or a straight handle approx. 20mm in height.

Anterior handle is used to carry the tray into the mouth and position it over the residual ridge.

Posterior handle is used as finger rests to complete the placement of the tray and to stabilize its correct position.

REFINING THE CUSTOM TRAY When the custom tray is removed from the

preliminary cast, the wax spacer is left inside the tray which allows the tray to be positioned correctly on the residual ridge.

For the border moulding to be carried out successfully space must be created for the border moulding material.

Flanges of the custom tray should be reduced until they are approx. 2mm short of limiting structure.

Border moulding can be achieved using either an Incremental technique with stick tracing compound or a One-step technique with a rubber material such as polyether impression material.

When using an incremental technique stick tracing compound is added and molded initially along the border of the labial flange, followed in turn by each buccal flange.

Lingually the same sequence is followed, anterior lingual border is molded first, followed by the left and right posterior lingual extension including the retro molar pads.

Irrespective of the method used the following border molding movements are carried out by the dentist:

1) Labial flange is molded by lifting the lower lip outward, upward and inward.

2) The region of the buccal frenum, the cheek is lifted outward, upward, inward, backward and forward to stimulate movement of frenum.

3) Posteriorly the cheeks are pulled buccaly to ensure that the cheek is not trapped under the tray and then the cheek is moved upward and inwards.

4) Anterior lingual flange is molded by asking the patient to protrude the tongue and to push the front part of the palate.

5) Protruding the tongue determines the length of the lingual flange of the tray in this region.

6) Protruding the tongue activates the mylohyoid muscle, which raises the floor of the mouth. This helps the dentist to determine the length and scope of lingual flange in molar region.

7) If the border molding builds up on the inside of the tray, it must be removed otherwise it may interfere with the action of mylohyoid muscle.

8) Distal end of lingual flange is molded by asking the patient to protrude the tongue. This action activated superior constictor muscle which supports the retromylohyoid curtain.

9) The patient is then asked to close as dentist applies downward force on the impression tray. This records the contraction of medial pterygoid muscle on retromolar curtain.

10) Finally the patient is asked to open wide. If the tray is too long a notch is formed at the posteromedial border of retromolar pad, indicating encroachment of tray on pterygomandibular raphe and is adjusted accordingly.

11) Final border molded tray should be so formed that it supports the cheeks and lip in the same manner as the finished denture will do.

12) The lingual surface of the tray should be shaped so that it guided the tongue in the same position it will occupy in the final denture.

PREPARING THE TRAY TO SECURE THE FINAL IMPRESSION

Space now must be provided for the final impression material-otherwise the borders will be over extended and the mucous membrane displaced.

Wax spacer is removed from the inside of the tray along with any border molding compound flow over it.

Any excess material on the outside of the tray is removed and approx. 0.5mm of border molding material is removed from around the border.

Finally small holes can be drilled through the tray, approx 10mm apart, in the center of the alveolar groove and over retromolar pads, this will provide the escape ways for the final impression material and relieve the pressure over the crest of alveolar ridge and retromolar pads when the final impression is made.

MAKING THE FINAL IMPRESSION A good final impression cannot be made unless a

properly fitting tray is in correct position on the residual ridge.

This procedure must be practised before making the final impression.

Tray is rotated into the mouth in horizontal plane using the anterior handle until its over the residual ridge.

The patient is asked to raise the tongue slightly and the tray is moved downwards towards its final position.

The index finger of each hand are placed on top of the posterior handles and with alternating gentle pressure the tray is seated until the buccal flange come in contact with the mucosa covering the buccal shelf.

Tray held steadily and not moving on residual ridge the borders of the impression are formed.

Once the dentist and the patient is familiar with the procedure the final impression material of choice(zinc oxide eugenol paste) is mixed according to the manufacturer’s instructions and evenly distributed within the tray.

All borders must be covered.The tongue must be kept forward touching the

upper lip while the impression material sets.When the final impression has set the tray is

removed from mouth and inspected for acceptability.

If it needs to be remade the impression material is removed carefully preserving the border molding.

BOXING IMPRESSION AND MAKING THE CAST

A wax is developed around the borders of final impression to preserve the shape of the periphery and to simplify making casts, this procedure is called Boxing.

The technique is same as in the upper impression with the addition that tongue space is filled with a sheet of wax that is attached to the superior surface of boxing wax.