2-primary imp.pdf
- 1. Preliminary Impression and Diagnostic Cast 1 Preliminary Impression Definition: An impression is a negative copy (likeness) of the teeth and adjacent structures. An impression for removable partial dentures (RPDs) should provide accurate record of the teeth and surrounding structures. RPD cast should be an accurate replica of the mouth to ensure that the RPD framework will fit accurately the corresponding supporting structures in the patient's mouth. In constructing a RPD there are two main impression procedures: 1-Primary impression procedure, this impression is made to have the diagnostic cast. 2-Final impression procedure, this impression procedure is made to produce the master cast on which the steps of RPD construction are completed (N.B. either prior or after framework construction). The Preliminary Impression First of all and before making any impression, the patient should be motivated to a proper oral hygiene. Scaling should be done and the remaining teeth should be polished to remove any existing debris. Tray selection and modification: Aluminum stock trays can be used, having the advantage that they can be easily altered, bent and trimmed. Upper and lower trays are selected that approximate the arch size of each patient. The stock trays should be altered to suit as much as possible the patient’s arch size. The tray can be trimmed around the periphery with crown shears if necessary. Trimming allows better adaptation and fit of the tray in the mouth, especially in
- 2. Preliminary Impression and Diagnostic Cast 2 the area of frenal attachments. In the trimmed areas wax can be applied on the sharp cut edges, to protect the soft tissues from lacerations. To improve the fit of the tray it may be necessary, sometimes, to bend the tray backward in the upper anterior region. Also, wax can be applied inside the borders to confine the impression material and to aid in producing good adaptation in the vestibule. A small amount of wax is added in the palatal area, especially in patients with high vaults to control the thickness of the impression material in that area. The wax is then warmed slightly over a flame, and the tray is replaced in the mouth for better adaptation. Fig.1 : Type of impression material and tray should be selected during the Treatment Planning Appointment. a b Fig.2 a-b: An ideal size and shape of stock tray is not available, Because the range of stock trays is limited. Select the suitable stock trays (adapted, fitted and well extended).
- 3. Preliminary Impression and Diagnostic Cast 3 Fig.3: wax or compound to improve adaptation. . Fig.4: Provide for a relatively even thickness of impression material. Test tray in mouth, should not impinge or bind anywhere. Check sizing for fit. Check for tray length, use mouth mirror. Beading or periphery wax to improve adaptation. Bend or cut tray flange if necessary.
- 4. Preliminary Impression and Diagnostic Cast 4 Fig.5: Lingual border moulding of the compound is achieved by the patient first pushing the tongue to contact the upper lip-and then thrusting the tongue into each corner of the mouth, in turn . Fig.6: Compound that has contacted the teeth should be cut away as it may prevent accurate reinsertion of the impression tray and provide enough space for the alginate . Fig.7: Underextension in the labial region can be corrected by the addition of either impression compound or modeling wax or silicone putty (if water bath is not available) to compensate for the deficient areas of the tray. Warm wax for fitting, then chill with water prior to impression. If aluminum tray is used Bend or trim tray flange if necessary.
- 5. Preliminary Impression and Diagnostic Cast 5 Primary impression making The irreversible hydrocolloid (Alginate) is the material of choice for making primary impressions. To get the best results from any material the manufacturer's directions for the type of alginate used should be followed when manipulating it. The proper amount of water is measured by the water measure, and put into a dry and clean rubber bowel. The powder is also measured by the powder measure, and poured into the water. The material is mixed for one minute, during that time the mixture is repeatedly flattened and pressed against the wall of the rubber bowel. Upper impression To prevent air from being trapped around the teeth, small amount of the mixed material is placed into the occlusal surfaces of the upper teeth with the index finger. The upper tray is then filled and placed in the patient’s mouth by retracting the cheek on the left side with the index finger of the left hand. Insert one side of the tray, removing the left hand, and then rotating right side through angle of the mouth, which is retracted by the little finger of the left hand. Once the tray is in, it is slowly seated upward the backward with a slight side movement. The tray is held tightly in place with the index fingers of the hands. The remaining fingers and palms of the hands should rest against the patient’s cheeks. The impression material is left in the mouth until gelation (about three minutes). At this time it loses luster and thickens. Also, if a hard instrument presses the material it will go back to its original position after releasing the pressure. These signs indicate that time is suitable to remove the impression. Remove the impression, after breaking the seal with the index finger pressing against the tissues in the mucolabial fold. Then, a sudden downward pressure on the tray handle is used to remove the impression. The impression is examined for accuracy, washed under running water and then wrapped in moist cotton. Lower impression The lower impression is made in a similar manner like the upper. After loading the tray the patient is asked to lift the
- 6. Preliminary Impression and Diagnostic Cast 6 tongue above the lingual flanges of the tray as it is being seated. Once the tray is seated, it is held lightly by putting the index fingers of both hands on the top of the tray and the thumbs beneath the mandible while the operator is standing in front of the patient. After gelation of the material and breaking the seal the tray is removed by sudden upward pull (in a “snap” movement). Inspect the pressure areas, carefully rinse the impression with cold tap water in order to remove saliva from the surface of the impression. Failure to do so will result in a cast with a soft or chalky surface. Then disinfect the impression. Before pouring the cast remove all moisture with a gentle stream of air. Be careful not to overly dry the impression. Never box an alginate impression with wax or a mixture of plaster and pumice. The casts should be poured immediately to prevent distortion of the impression material. (Imbibitions: distortion by water absorption. Or Synersis: loss of water and shrinkage distortion). Stone plaster should be used for casting the study casts to avoid abrasions and possible breakage from frequent handling. Control of Gagging • Thicker mix of Alginate • Mixing the alginate out of sight of the patient. • In lower impression Contact with tongue can be unavoidable. Proper fit of tray is important, shorten un-necessary areas • In Maxillary impression: Bend head forward this cause lift of the soft palate. Beading wax to reduce alginate posterior flow. • Seating the tray in the posterior first does not improve the situation and causes many technical problems. • Tell patient, try and relax and breathe through your nose. (Try not to move tongue). • Remove the viscous material present on the soft palate.
- 7. Preliminary Impression and Diagnostic Cast 7 Fig.8 a, b: Block out large embrasures and inter-proximal spaces to prevent tearing of the impression material on removal. Dry teeth Pack arch with gauze. f ig.9 a-d: For maximum accuracy, The impression material should be thoroughly mixed. Mix alginate in a vigorous manner using sweeping, Rapid stroke against the wall of the bowl. Look for a thoroughly mixed creamy consistency. Fig.10: To avoid bubbles use your finger to apply alginate impression material to into critical areas. Fig. 11: The alginate impression material is wiped over the teeth and into deep sulci with a finger immediately prior to inserting the loaded tray.
- 8. Preliminary Impression and Diagnostic Cast 8 Fig.12: The tray is loaded with the suitable amount, inserted, and seated in the mouth and border molding completed. Fig.13: Seat the tray slowly into position Fig.14: Taking an Alginate Impression Explain the procedure to the patient: The material will feel cold, there is no unpleasant taste, and the material will set quickly. Breathe deeply through your nose to help you relax and be more comfortable. Use hand signals to communicate any discomfort. b A Fig.15 a and b: – Carefully rinse the impression with cold tap water in order to remove saliva from the surface of the impression. Failure to do so will result in a cast with a soft or chalky surface. – Saliva can be identified on the cast by sprinkling stone on the impression and gently rinsing it away with tap water. Before pouring the cast remove all moisture with a gentle stream of air. Be careful not to overly dry the impression. – Thoroughly examine the impression to determine its acceptability. – Remove excess alginate that extends beyond the back of the tray. – Cut carefully and towards the tray to avoid pulling the alginate away from the tray.