Final impression for removable partial dentures

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Clinical Removable Prosthodontics
Forth Year

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Final impression for removable partial dentures

  1. 1. MAKING FAINAL IMPRESSION FOR REMOVABLE PARTIAL DENTURES Lecture Outline Introduction Materials used - Irreversible hydrocolloid - Reversible hydrocolloid - Polysulfide - Silicone Impression procedure Reasons for rejecting impression Introduction For the production of an accurate master cast the impression technique far outweighs the selection of the impression material. No available knowledge of the person making the impression material will produce results greater than the skill and knowledge of the person making the impression. Good technique will indeed result in better treatment and improved patient care. All major classes of impression materials irreversible hydrocolloid, reversible hydrocolloid, polysulfide rubber, and silicone rubber - are capable of producing accurate results if respect is shown for the properties of the particular material. No single impression material can be used for all impressions. The operator must be in complete control of all aspects of the impression procedure: the position and intraoral condition of the patient, the size and position of the tray, and the selection of material and technique. IMPRESSION MATERIALS IRREVERSIBLE HYDROCOLLOID (ALGINATE) IRREVERSIBLE HYDROCOLLOID (ALGINATE) Alginate impression material is the most widely used and possibly the most versatile of the impression materials available in the practice of removable partial prosthodontics. The ease of handling, the relative inexpensiveness, the dimensional accuracy, the lack of need of additional items of equipment, and the cleanliness of the material contribute to its popularity. The inability to store the impression safely is a true limiting factor in the use of alginate impression material.
  2. 2. Alginate impressions must be poured within 12 minutes after being removed from the mouth. The impression should not be stored in anything during the 12-minute period. Custom made tray is not required in making most impressions. The only time a custom-made tray is normally used is when the size of the arch will not accommodate a standard tray. When a standard rim-lock or metal perforated tray is used for a final impression for a removable partial denture, the tray must be modified to reduce the bulk of alginate. The simples and most accurate method of modifying the tray is by the use of modeling plastic or cake compound. Wax is occasionally used to modify an impression tray. This is a hazardous method because if the wax is compressed during the impression procedure, it will rebound once the pressure is removed. This can cause an inaccurate impression. Clinical Use The impression technique for a final impression for a removable partial denture is the same as for primary impressions, with the exception that special care must be shown in recording the rest seat preparations that have been made in the abutment teeth. Reversible Hydrocolloid (Agar-Agra) Reversible Hydrocolloid (Agar-Agar) Agar hydrocolloid was the first successful elastic impression material to be used in dentistry. To this day it remains one of the most accurate and cleanest elastic materials. Its popularity among general practitioners has declined primarily because of the additional equipment required for its clinical application. Because of this additional equipment, the cost per impression is significantly higher than that of alginate. Like alginate impressions, agar hydrocolloid impressions must be poured immediately. If storing is necessary, it must be done in 100% humidity. This does not prevent dimensional change, only reduces it slightly. The storage should not be more than 10 minutes. Tends to sag as the gel state is approached. Sagging of the impressions of the palate will produce a cast that is inaccurate. It is possible to modify the impression tray for an alginate impression to compensate for this sagging, but not for an agar hydrocolloid impression. Clinical Use
  3. 3. The tray has two tubes incorporated within the metal that transport the cooling medium. Must be connected to the entering and exit tubes. Roomtemperature water 21° to 22°C is preferred to ice water. The selection of the size of impression tray for an agar agar hydrocolloid impression is the same as for an alginate impression except the tray cannot be modified safely. Any modification material will also serve as an insulator and interfere with the formation of the gel, influencing the accuracy of the impression. The chairside impression technique remains the same except the mucosa does not need to be dried; however, pools of saliva should be removed or controlled. The syringe with the injectable material is used to deposit the lighter agar into these areas. The impression tray must be seated quickly after the injectable material is deposited to prevent this lighter agar from starting to gel before it combines with or is displaced by the heavier tray material. . Once the tray is seated, a pause of 30 seconds should be allowed before the cooling water starts to flow to cool the hydrocolloid. With 21°C (70°F) water flowing rapidly, a total of 3 minutes should be allowed to thoroughly cool and gel the tray material. The water is stopped, and the tray held steadily for an additional 30 seconds. This time allows the surface of the impression to return to body temperature and prevents the release of stress. The impression should be removed from the mouth with a single, quick, definite movement to prevent tearing or distorting the material. After the impression has been poured with a minimal expansion dental stone, the cast should be separated within 45 to 60 minutes after initial set of the stone POLYSULFIDE RUBBER BASE The impression material is supplied in two tubes of paste. One tube contains the catalyst, or accelerator, and the other the base. The material is produced in several types and varies as to the viscosity and flow characteristics. The types are classified as being light, regular, or heavy bodied. The light bodied is normally used in syringe for injection purposes or for complete denture impression where little tissue displacement is desired. The heavy bodied is used in combination with light bodied injection material to capture the surface details essential for fixed prosthodontics or restorative dentistry. For removable partial prosthodontics the regular bodied or a mix of regular with light bodied is used for impression Clinical Use
  4. 4. Equal lengths of the base and catalyst are extruded on a paper mixing pad. The two lengths should not be in contact. A stiff spatula is needed to make the mix. If an injection with light-bodied material is to be made, the two mixes should be started simultaneously, so at least one assistant is needed. The mixing is accomplished with a circular sweeping motion. Because the base material is white and the catalyst colored, mix completion is evidenced by the absence of streaks and the uniformity of the color. This should take about 45 seconds. The injection material is deposited into rest seat preparations and other critically areas carefully with the syringe to avoid trapping air. The loaded tray is seated by using the same technique as for any other impression. The tray must be held steady until the final set of the material has taken place, usually about 8 minutes. After removal from the mouth the impression is inspected for completeness and accuracy and cleansed by rinsing with cool tap water. The moisture should be removed by shaking the impression or by gentle blast of air. The impression should be immersed in a disinfectant solution for ten minutes before pouring. The pouring technique is the same as for alginate impression. Recovery of the cast from the impression should be delayed longer than when one of the hydrocolloids is used. Rubber will not damage the cast surface, and allowing extra time before recovering the cast will produce a harder cast. Polysulfide rubber is not as elastic as alginate or agar, and tooth breakage is not uncommon. SILICONE RUBBER MATERIALS The material is supplied as a base and a catalyst, or accelerator. The base paste contains a low-molecular-weight silicone liquid, dimethylsiloxane, which has the reactive OH groups. Silica or other agents are added to give the paste the proper consistency and provide stiffness to the set rubber. The catalyst, a tin octoate suspension and an alkyl silicate, is supplied as a liquid usually, but may be provided as a paste by the addition of thickening agents. The silicone pastes are supplied in the same consistency as the polysulfide rubber - light, regular and heavy-bodied - with the new addition of a very heavy bodied paste called a putty. The molecular weight of the dimethylsiloxane and concentration of silica determine the consistency of the paste. The higher the molecular weight, the heavier the past.
  5. 5. Normally the various consistencies have different colors so they can be easily distinguished. This may be an important consideration when injectable material is used followed by a tray material. The mixing technique is the same as that for polysulfide rubber. The mixing is continued until the material is free of streaks, normally 45 seconds. As a final step the mix should be spread in a thin layer to eliminate any trapped air bubbles. The syringe or tray is loaded, and the impression procedure followed as described for the polysulfide rubber material. The use of the putty is seldom if every indicated for an impression for a removable partial denture. The putty is used to make loose fitting impression tray generally of a segment of an arch. The light-bodied, or injection, material is then used as a wash within the putty to refine the impression, usually of a crown or fixed partial denture. The pouring of the impression should not be delayed. The material is compatible with improved stone. The tear strength for silicone rubber is less than that of the polysulfides, but care must be taken in recovering the cast to prevent breaking the teeth. IMPRESSION PROCEDURE Position the patient in the chair properly for the impression to be made. After the tray has been modified, rehearse the operator position and the tray placement several times so that the patient will be aware of his responsibilities. Prepare the mouth by having the patient rinse, and then pack with gauze pads. Measure and mix the impression material according to directions, and load the tray in small increments to avoid trapping air. Remove the packing and patient or inject impression material in critical areas: rest preparations, hard palate, and peripheral extensions. Seat the tray as rehearsed, being careful not to over seat. Clear the lips and cheeks. For the mandibular impression be sure that the patient’s tongue is raised and protruded gently. Hold the tray steady until the material has set. Never leave a patient unattended with an impression in the mouth. Remove the tray with a sudden movement in a direction as near as possible to the long axis of the teeth. Wash or clean saliva from the impression.
  6. 6. Examine the impression critically to determine whether all details are recorded accurately. If there is any doubt, reject the impression. Disinfect the impression. REASONS FOR REJECTING IMPRESSION REASONS FOR REJECTING IMPRESSION Bubbles or voids in and around rest preparations. Contact of cusps with the tray, especially when the teeth are involved in the framework design. Show through between teeth and modeling plastic or modeling plastic and hard palate if the tray has been modified for a alginate impression. Voids or bubbles in palatal vault when palatal major connectors are to be constructed. Peripheral underextension when a denture base has been designed and a corrected cast impression is not planned. Interproximal tearing of the impression material when coverage of those teeth has been designed. Lack of detail on the impression surface. Any doubt as to the accuracy of the impression.
  7. 7. Examine the impression critically to determine whether all details are recorded accurately. If there is any doubt, reject the impression. Disinfect the impression. REASONS FOR REJECTING IMPRESSION REASONS FOR REJECTING IMPRESSION Bubbles or voids in and around rest preparations. Contact of cusps with the tray, especially when the teeth are involved in the framework design. Show through between teeth and modeling plastic or modeling plastic and hard palate if the tray has been modified for a alginate impression. Voids or bubbles in palatal vault when palatal major connectors are to be constructed. Peripheral underextension when a denture base has been designed and a corrected cast impression is not planned. Interproximal tearing of the impression material when coverage of those teeth has been designed. Lack of detail on the impression surface. Any doubt as to the accuracy of the impression.

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