Primary Impression and Related Procedures PDF

PRIMARY IMPRESSION AND RELATED PROCEDURES By : Dina Mohamed EL-Shokafy Lecturer at Fixed Prosthodontics department Agenda Diagnostic casts Tray selection Impression making Evaluation PRIMARY IMPRESSION 2 DIAGNOSTIC CASTS & RELATED PROCEDURES...

PRIMARY IMPRESSION AND RELATED PROCEDURES By : Dina Mohamed EL-Shokafy Lecturer at Fixed Prosthodontics department Agenda Diagnostic casts Tray selection Impression making Evaluation PRIMARY IMPRESSION 2 DIAGNOSTIC CASTS & RELATED PROCEDURES 3  PRIMARY IMPRESSION DIAGNOSTIC CASTS Accurate diagnostic casts are essential in planning fixed prosthodontic treatment. This enables examination of static & dynamic relationships of teeth without interference from protective neuromuscular reflexes, & unencumbered views from all directions reveal aspects of the occlusion not always easily detectable intraorally (e.g., the relationship of the lingual cusps in the occluded position). 4 PRESENTATION TITLE 5  PRIMARY IMPRESSION DIAGNOSTIC CASTS Other critical information not immediately apparent during the clinical examination includes the occlusocervical dimension of edentulous spaces. On an articulator, these are readily assessed in the occluded position & throughout the entire range of mandibular movement. 6 7 DIAGNOSTIC CASTS Articulated diagnostic casts enable a detailed analysis of the occlusal plane & occlusion. Diagnostic procedures can be performed for a better diagnosis & treatment plan. PRIMARY IMPRESSION 8  PRIMARY IMPRESSION Tooth preparations can be “rehearsed” on the casts 9  Diagnostic waxing procedures allow DIAGNOSTIC CASTS evaluation of the eventual outcome of the proposed treatment PRIMARY IMPRESSION 10 IMPRESSION MAKING FOR DIAGNOSTIC CASTS PRESENTATION TITLE 12  PRIMARY IMPRESSION Accurate digital scans or traditional impressions of both dental arches are required. Saliva or foreign debris on occlusal surfaces of optical scans are interpreted as hard contours of the teeth & result in errors in articulation. 13  PRIMARY IMPRESSION Flaws in the traditional impressions result in inaccuracies in the casts that easily multiply. For instance, a small void in impression caused by trapping of an air bubble on one of occlusal surfaces results in a nodule on the occlusal table. 14  If it is not recognized & carefully removed, it leads to an inaccurate articulator mounting & diagnostic data are incorrect.  As long as the optical scan or traditional impression extends several millimeters beyond the cervical line of the teeth, the borders of diagnostic impressions are usually not of great concern for fixed prosthodontic purposes, unless a removable prosthesis is also to be fabricated.  Properly manipulated irreversible hydrocolloid (alginate) is sufficiently accurate & offers adequate surface detail for planning purposes.  Irreversible Hydrocolloid Alginates, are essentially sodium or potassium salts of alginic acid ( therefore water soluble). They react chemically with calcium sulfate to produce insoluble calcium alginate. 18  These materials contain other ingredients: Diatomaceous earth (strength & body) Trisodium phosphate (Na3PO4), & similar compounds to control the setting rate as they react preferentially with calcium sulfate. When this reaction is complete & the retarder is consumed, gel formation begins. 19  The clinician can control the reaction rate by varying temperature of the mixing water. Colder water retards setting, while warmer water hastens setting. Use cold water if you need to increase working & setting time 20  Because irreversible hydrocolloid is largely water, it readily absorbs (by imbibition) as well as gives off (by syneresis) liquid to the atmosphere causing distortion of the impression. 21  PRIMARY IMPRESSION Diagnostic Impression Technique Armamentarium Impression trays Modeling plastic impression compound Mixing bowl Mixing spatula Gauze squares Irreversible hydrocolloid American Dental Association type IV or V stone Vacuum mixer Humidor Disinfectant 22 PRIMARY IMPRESSION Tray Selection Impression materials must be retained in impression tray. This can be accomplished for irreversible hydrocolloid with an adhesive or by means of perforations or undercuts around the rim of the tray. 24  PRIMARY IMPRESSION All types of trays are capable of producing impressions with clinically acceptable accuracy, although casts may be produced more accurately by rigid plastic trays than by perforated metal trays. 25  The largest tray that will fit comfortably in patient’s mouth should be selected. A greater bulk of material produces a more accurate impression (i.e. A bulky impression has a more favorable ratio of surface area to volume & is less susceptible to water loss or gain & therefore unwanted dimensional change). In contrast, elastomeric impression materials work well with a relatively tightly fitting custom impression tray in which a uniform thin layer of material is used. This produces the most accurate impression. 26  Distortion of irreversible hydrocolloid can occur if : Any part of impression is unsupported by the tray There is movement of the tray during setting. For these reasons, the tray may need to be extended & its perimeter modified with modeling plastic impression compound. 27 Tray should 1-Extend facially to include all teeth as well as the musculature & vestibule. 2. Extend distally approximately 2 to 3 mm beyond the last tooth in the arch to include the retromolar area. 3. Provide a 2- to 3-mm depth of alginate beyond the occlusal surface & incisal edge. 4. Be comfortable for the patient. 5. Some metal trays can be adapted to the patient by bending the sides of tray.  PRIMARY IMPRESSION Stock impression trays can be readily modified with modeling compound to provide better support for the alginate. The posterior border typically needs extension. If the patient has a high palate, the alginate should be supported here, too, although the compound should not block out the retentive area of the tray. 29 Impression Making For optimum results, teeth should be cleaned & mouth thoroughly rinsed. Some drying is necessary, but excessively dried tooth surfaces cause the irreversible hydrocolloid impression material to adhere. 30  The material is mixed to a homogenous consistency & loaded into the tray. Its surface is smoothed with a moistened gloved finger. 31  Concurrently, a small amount of material is wiped into the crevices of the occlusal surfaces before the tray is seated. In addition, a small amount can be applied by wiping it into the mucobuccal fold. 32 A to D, Making an alginate impression for diagnostic casts. PRIMARY IMPRESSION 33  Minimize Gagging Having the patient breathe through his or her nose while seated in an upright position. 34 PRESENTATION TITLE 35  As the tray is inserted into the patient’s mouth & seated, the patient is instructed to “close gently” on the tray. 36  If the patient continues to stretch the mouth wide open while the tray is being fully seated, impression material is often squeezed out of the mucobuccal fold or from underneath the upper lip. Excessive opening greater than 20mm or protrusion of the mandible may also lead to inaccuracies in a mandibular impression because of mandibular flexure.  A loss of tackiness of the material (gelation) implies initial set. The tray should be removed quickly 2 to 3 min after gelation. Teasing or wiggling the set impression from the mouth causes excessive distortion as a result of viscous flow. In addition, certain irreversible hydrocolloid materials become distorted if held in the mouth more than 2 or 3 minutes after gelation. 38  PRIMARY IMPRESSION STORAGE & DISINFECTION 39  Set alginate undergoes imbibition & syneresis if left in a normal clinical environment. The time before cast-pouring is critical. After being removed from the mouth, alginate impressions should be washed with a water spray, disinfected by means of the practitioner’s choice of disinfection procedures, & dried until the shine just disappears. 40  Alternatively, the impression can be immersed in iodophor or glutaraldehyde disinfectant. The disinfection protocol is an essential precaution for preventing cross infection & protecting laboratory personnel. Irreversible hydrocolloid impressions carry significantly higher numbers of bacteria than do elastomeric materials. 41  For disinfection, spraying with a suitable glutaraldehyde & placement in a self-sealing plastic bag for approximately 10 minutes is recommended, after which it can be poured. 42  To ensure accuracy, pouring should be completed within 15 minutes after the impression is removed from the mouth. Keeping an impression in a moist towel is no substitute for pouring within the specified time. 44  Trimming off gross excess impression material before setting the tray down on the bench top is helpful. 45 Cast fabrication Alginate impressions should be poured using vacuum-mixed stone & vibrator. A thick mix can trap air bubbles. 46  A vacuum-mixed ADA type IV or type V stone is recommended. 47  The stone should be allowed to set in trays with the teeth down. 48  After mixing, a small amount of stone is added in one location (e.g., the posterior aspect of one of the molars). Adding small amounts consistently in the same location helps to minimize bubble formation. 49  PRIMARY IMPRESSION If air is trapped, a small instrument (e.g., a periodontal probe or a wax spatula) can be used to poke the bubbles & eliminate them. While they are setting, the poured impressions must be stored tray side down, not inverted. Inverting freshly poured impressions results in a cast with a rough & grainy surface. 50  If tray is turned upside down onto base of stone, there would be a tendency for water to rise to the highest point (cusp tips). This can result in faulty, very soft cusps on the model. Inverting the tray may also “bend” alginate away from tray if excess material has not been trimmed away prior to pouring. Cast has to be removed immediately after adequate set; otherwise, the model would have “moth-eaten” appearance. 51  Stone is added to create a sufficient base that provides adequate retention for mounting on the articulator. 52 53  To achieve maximum strength & surface detail, the poured impression should be covered with wet paper & stored in a humidor for 1hour. This minimizes distortion of the irreversible hydrocolloid during the setting period. 54  PRIMARY IMPRESSION The setting gypsum cast should never be immersed in water. If this is done, setting expansion of plaster, stone, or die stone doubles or even triples through the phenomenon of hygroscopic expansion. For best results, the cast should be separated from the impression 1hour after being poured. 55 PRESENTATION TITLE 56  Evaluation Questionable impressions & casts should be discarded and the process repeated. Voids in the impression create nodules on the poured cast. These can prevent proper articulation & effectively render useless a subsequent occlusal analysis or other diagnostic procedure. 57 PRESENTATION TITLE 58 PRIMARY IMPRESSION 59  Diagnostic casts must be accurate if they are to articulate properly. A, Occlusal nodules may make proper occlusal analysis impossible. B, Proper technique ensures a satisfactory cast. 60