Impression Procedures for Partially Dentate Patients SDS 321 McCracken’s Removable Partial Prosthodontics. Chapter 15.

An impression, of a partially dentate arch must record accurately the anatomic form of the teeth and surrounding tissues so that ..

.. a definite path of insertion can be selected, and …

.. support, retention and stability can be planned.

Materials that could permanently deform on removal from the undercuts should not be used, e.g., rigid and thermoplastic.

Impression Materials (A review) Rigid: Impression plaster Metallic oxide (ZnO Euginol) paste Thermoplastic: Modeling compound Impression waxes Elastic: Reversible Hydrocolloid Irreversible Hydrocolloid Rubber base (Marcaptan, Silicon, Polyether)

Hydrocolloid Materials (A reminder) These materials are elastic enough to be withdrawn from the undercuts without permanent deformation or distortion. One piece impression is possible. No separating medium is needed for pouring. Extremely accurate in tissue details when handled properly.

Reversible Hydrocolloid (Agar – Agar) The gelation of Agar is primarily a physical change in which a latticework of fibrils forms as the temperature is lowered. The gel can be dispersed by heating. It is liquefied at a temperature compatible to oral tissues and then solidified to a firm – yet elastic gel at about 100˚ F (38˚C). The gelation is accomplished by means of special water cooled impression trays.

Irreversible Hydrocolloid (Alginate) These are not thermally reversible. The gelation is induced by a chemical reaction. Powder: Sodium alginate & Calcium sulphate. Add water for reaction to form a latticework of fibrils of insoluble Calcium alginate. Alginate is dimensionally stable for a brief period after its removal from the mouth, hence should be poured immediately or stored for up to 15 minutes in a saturated atmosphere.

Procedure of Impression Making 1. Select a suitable size perforated tray for obtaining an initial impression. 2. Add wax or modeling compound in the tuberosity & palatal area for maxillary, or in the lingual flanges for a mandibular impression. The edentulous areas may also need modifications of the tray with impression compound.

For the Final Impression A better extended custom tray is normally fabricated on the diagnostic cast.

Why to have Tissue stops The custom tray is fabricated to have tissue stops for accurate relocation of the tray and for achieving even thickness of the impression material. These stops are placed on the teeth other than abutments and on the edentulous ridge distally.

Tissue stops can be added intra-orally by applying green stick compound to the chosen location on the tray and then seating it in the mouth under gentle pressure.

For obtaining an Impression Seat the patient in an upright position and check the tray border extensions. For Class I & II it should cover the retromolar pad or maxillary tuberosity. Any overextensions should be relieved by trimming the tray, especially in the frenum areas.

Over-extended Under-extended What to do ! Over-extended Under-extended

is carried out in the areas of under extension of the tray and Border molding is carried out in the areas of under extension of the tray and for the distal extension saddles. The commonly used border molding material is the tracing (green stick) compound.

Choice of an Impression material Any of the elastic material (alginate, agar, polysulphide, polyether, or silicone) can be used. However, in the presence of deep undercuts (bilateral lingually tilted premolars) alginate should not be used as it may be stressed beyond its elastic limit on removal.

Before making the impression Remove the excessive saliva from the mouth with an ejector and dry the rest seat preparations and dimpling (if any) with an air syringe. An excessive drying should be avoided as the material may adhere to the dried tooth surface.

.. also open inter-dental spaces should be occluded with soft wax prior to inserting the loaded tray, otherwise the impression material will flow beneath the contact points and lock the impression in place and distortion will occur upon removal.

Mix the impression material By adding alginate powder to the desired amount of water – spatulate rapidly against the walls of the bowl for one minute, or By mixing the catalyst and base components of the rubber base material uniformly. Place the material in the tray, try to avoid entrapping air and lock the alginate in the perforations – for rubber base materials, an adhesive should be applied to the tray before mixing the material. Do not over – load the tray as the excess material may initiate the gag reflex.

For a bubble – free impression Alginate should be wiped over the teeth and into the deep sulci with a finger immediately prior to inserting the loaded tray. The rubber base materials can be poured over the teeth using a syringe.

Inserting the tray in the mouth Stand behind (for maxillary) and in front of the patient (for mandibular) impression. Retract the cheek with the tray and with your free hand and rotate the tray in the mouth.

Then, Retract the lip (upper or lower) and seat the tray anteriorly, centering it according to the frenum and the sulcus, and then posteriorly. Let the lip drape naturally and complete the border molding. For lower impression – ask the patient to elevate the floor of the mouth, e.g., by licking his lips.

Muscle trimming of the borders

While waiting for the material to set Hold the tray immobile for at least 3 minutes with a light finger pressure applied over the left and right pre – molar areas. Any movement of the tray can cause an inaccurate impression.

Once the material is set Release the surface tension by lifting the lip (up or down) away from the tray. Remove the impression quickly (snap removal) in line with the long axis of the teeth. Rinse it thoroughly under running tap water – free it from saliva or blood. Examine the impression carefully for acceptance and disinfect it before pouring.

An acceptable impression

Another acceptable one !

Edentulous Vs Distal extension

Now the unacceptable ones Incorrect seating of the impression tray laterally.

Not Acceptable In-complete seating of the impression tray – unsupported impression at the borders.

Repeat the Impression Under extended tray – thin edge of unsupported impression of the posterior lingual sulcus.

Re-do border molding also Over-extended border molding resulting in the distorted sulcus reflection in the impression.

Finally, an acceptable impression A maxillary impression taken with combination of impression compound in the distal extension areas and overall alginate impression. Alginate has been cut off on one side to show its thickness.

Managing the Gag reflex

Management of Gag reflex Exaggerated gag reflex can frustrate both the patient and the dentist. It can also compromise the treatment plan. Its effective management is based on experience with combinations of clinical techniques, prosthodontic management, medication, and psychologist referral, or a specialist referral. Some simple measures that usually works: Reassurance and kind handling. Upright position of the patient. Avoid overloading the tray. Distract patient’s mind – talking, raising one foot etc.

Care of Alginate Impression

Care of Alginate Impression Should not be exposed to air (dehydration). Should not be immersed in water (imbibition). Should be stored in a humid atmosphere by wrapping in a damp paper towel or in 100% relative humidity (humidor). Should be poured immediately, or immersed in a solution of accelerator (Pot. Sulphate) to avoid syneresis - giving off a mucinous exudate that has retarding effect on gypsum and results in a soft or chalky cast surface.

Forming the Cast (Without Boxing)

Procedure of Forming a Cast (Without Boxing) 1. Remove the alginate impression from the damp paper towel and shake out any moisture. 2. Add 100 gm of stone (powder) into 28 ml of water and spatulate for one minute. Place the bowl on the vibrator to escape the entrapped air. double pour technique

3. Hold the impression tray against the vibrator and add a small amount of mixed stone to the distal area. Let it vibrate around the arch - from tooth to tooth. Continue to add small increments until impressions of the teeth have been filled. 4. Continue to add stone in larger portions until the impression is completely filled. double pour technique

5. Let the stone reach its initial set. 6. The base of the cast can then be formed with a new mix of stone. The base should be at least 15 mm thick. double pour technique

Forming the Cast (With Boxing)

Forming the Cast (With Boxing) Apply beading wax to the periphery of the impression. It should be placed 2-3 mm from the borders of the impression and should be 4 mm wide.

Alternately, beading can be done with a 50 – 50 mix of plaster and pumice.

Box the Impression with Boxing wax sheet

Hold the boxed impression on a vibrator and add mixed dental stone in small increments until completely filled.

After the stone has completely set & the exo-therm completed, peal off the boxing wax and remove the beading wax.

Remove the impression gently off the cast with a plaster knife Remove the impression gently off the cast with a plaster knife. For the border molded impression, the cast should be dipped in warm water for easy removal.

Trimming the Cast The sides of the cast are trimmed to be parallel - any stone blebs or nodules are carefully removed. The base can be trimmed for either orthodontic specifications (for a record cast) or to remove excess stone only ( for a master or a working cast ).

Land area of the cast should be trimmed with a sharp knife to have uniform height of the sulcus reflection ( 2-3 mm ).

it should be 3-4 mm wide to provide necessary strength to the cast. and it should be 3-4 mm wide to provide necessary strength to the cast.

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