Chapter 3 Impression Trays PDF

Chapter III Impression Trays Impression tray : A receptacle or device used to carry the impression material to the mouth, confine the material in apposition to the surfaces to be recorded, and control the impression material while it sets to form the impression. REQUIREMENTS FOR IMPR...

Chapter III Impression Trays Impression tray : A receptacle or device used to carry the impression material to the mouth, confine the material in apposition to the surfaces to be recorded, and control the impression material while it sets to form the impression. REQUIREMENTS FOR IMPRESSION TRAYS. 1- The tray should be rigid but not overly thick. 2- It should retain its shape throughout the construction and pouring of the impression. 3- The method of construction should be simple enough so that an acceptable impression tray can be made in a minimal amount of time at a reasonable cost. 4- It should be possible to trim or thin the tray readily with a bur, mounted stone or scissors. 5- The tray should be smooth because sharp edges may injure oral tissues. IMPRESSION TRAY MATERIALS: Materials used to make impression trays are special autopolymerizing acrylic resin impression tray material , conventional autopolymerising acrylic resin , thermoplastic resin sheets used in cacuum or pressure adapting devices and thermoplastic shellac baseplate materials. AUTOPOLYMERIZING RESIN IMPRESSION TRAYS : These are materials easy to use , require no special equipments and when manipulated properly make excellent impression trays. They can be thin but rigid, modified easily by grinding and smoothed and polished. The Two methods of using these materials are the sprinkle-on method and the finger adapted dough method.  THE SPRINKLE-0N METHOD This method is commonly used for constructing acrylic resin baseplates but it is also used in constructing resin impression trays. The dentist specifies the amount and placement of relief depending on the impression material to be used and sharp bones or hyper plastic tissues in the mouth. Procedure : 1- Make an outline of the impression tray on the cast usually short of the vestibular reflections of the cast and posteriorly between the hamular notches 2mm distal to the fovea palatina. 2- block out the severe undercuts and adapt a layer of baseplate wax to the cast for relief on the prefixed outline.(2mm shorter of the cast outline). make tissue stops on the relief wax by removing squares of 4mm2 surface. 3- Paint tinfoil substitute on the stone cast and the relief wax. 4- sift powder polymer on the cast and relief wax then saturate it with liquid monomer from an eye dropper until there is an uniform layer approximately 2mm thick. 5- cure the impression tray under an inverted bowl or in a pressure pot. 6- mix more resin in a cup , and when it is in the dough stage form a handle and adapt it to the impression tray. The handle should be 4mm thick and 8mm long. 7- wet the resin at the point of attachment with liquid monomer to facilitate the bonding with the tray. 8- after setting remove the tray from the cast and trim it with a bur. 9- finish and polish the tray to eliminate all areas which can cause discomfort for the patient. 10- store the finished tray on the cast. PROBLEM AREAS : The principal problem with the sprinkle-on method is the difficulty in controlling the thickness of the impression tray. Tilting the cast during the process can compensate for the flow characteristics. If severe undercuts are present on the cast the tray risk to be broken.finger adapted dough method is better for the modified acrylic tray resins. Table 3-1 page 32  FINGER ADAPTED DOUGH METHOD This method is used extensively for making resin impression trays. The resin tray material can be formed into a dough that can be thinned readily and adapted to the cast. PROCEDURE : 1- place the outline for impression tray on the cast and bead the outline with a sharp instrument. Beading on the cured resin tray helps in trimming. 2- place the outline for areas of relief on the cast 3- Block out undercut areas and adapt relief wax to the cast then remove 4-mm squares of the relief wax to expose the cast thereby providing tissue stops. 4- Paint tinfoil substitute on the cast and relief wax. 5- Proportion the impression tray material according to the manufacturer instructions and mix in a suitable container. 6- check the consistency of the resin periodically and remove it when it reaches the dough stage. Roll the resin to achieve the desired thickness. 7- Hand adapt the material to the cast carefully to avoid over-thinning the resin on the convex portions of the cast. It’s easy to over-thin the impression tray by applying too much finger pressure. 8- Remove excess tray material from the cast borders. 9- Form the excess material into handles and adapt them to the tray making sure to smooth all the area surrounding the handle. Use liquid monomer on the point of attachement to improve the bonding. 10- Make handles small in order to take minimum time of finishing. 11- Continue finger adaption to prevent the resin from rebound. 12- Cure the impression tray on the bench or under an inverted bowl or in the pressure pot. 13- After setting remove the tray from the cast and begin trimming it. 14- Smooth all rough areas and store on the cast. PROBLEM AREAS : Problems with the finger adapted dough method are related to finger adaption. There is a tendency to over-thin the impression tray material by finger pressure over the convex or ridge portions of the cast. The resin should be in dough stage when applied to the cast. Failure to roll the resin to a uniform thickness will result in thick trays leading to spending more time and cost to finish it. Table 3-2 page 36  THE VACUUM ADAPTED METHOD Vacuum or pressure-formed thermoplastic sheets can make good impression trays but requires special equipment to adapt the sheet to the cast. The materials are available in different color and thickness as well as different degrees of flexibility. PROCEDURE : 1- place the outline for impression tray on the cast and bead the outline with a sharp instrument. 2- block out the undercuts and place the relief on the cast with a material that will not melt during when heating the resin sheet. 3- center the cast on the vacuum adapter plate 4- place a resin sheet of the appropriate color and thickness in the heating frame of a vacuum former machine and rotate the heating unit into position. 5- switch the heating button and continue heating until the specified sag in the material occurs. 6- Lower the frame and resin sheet onto the cast and start vacuum adaptation. After adaptation is complete allow the resin sheet to cool the remove it from the vacuum adapting unit. 7- Trim the excess material with a large bur. 8- Remove the tray from the cast and trim the borders. 9- add handles made with auto polymerizing acrylic or preformed metal handles. 10- Store the tray on the cast. PROBLEM AREAS : This type of resin trays require special material and equipment. Making relief is also challenging due to the difficulty of adapting it. TABLE 3-3 PAGE 40 SHELLAC METHOD : Too old and not effective due to the instability of this material. It is used no more in our days. IMPRESSION TRAYS FOR IMMEDIATE DENTURES The presence of teeth on the cast change the method used to fabricate impression tray for traditional complete dentures. Many dentists remove posterior teeth and after a healing period take an impression of the posterior region , making a denture and then removing anterior teeth and completing the work. Two areas are to be registered edentulous one and remaining natural teeth. Three impression methods are used for immediate dentures. A one-piece full-arch impression tray that covers edentulous ridges and natural teeth. A second method is to use custom-made impression tray for posterior edentulous portion and then make an over-all impression over the posterior tray and natural teeth.The third method is to use two – piece trays. FULL-ARCH IMPRESSION TRAYS. Three methods are used to make full-arch impression trays: the finger-adapted dough, vacuum and sprinkle-on methods. FINGER ADAPTED DOUGH METHOD. 1- place the outline of the tray on the cast. 2- block out the teeth on the cast with base plate wax then adapt a layer of base plate wax over the cast and trim to the relief border. 3 – paint tinfoil on the cast and the relief wax. 4- proportion the auto-polimerizing resin according to the manufacturer recommendations and mix it in a suitable container. Immediately before the dough stage remove some material from the container and place it in the border area of the cast. 5- When the resin reaches the dough stage remove it and roll it into the desired thickness. 6- Adapt the resin over the relieved cast.Always remember to trim excess material from the borders. 7- Continue finger adaptation until the resin begins to set. 8- Permit the resin to cure on the cast then carefully remove it. 9- Finish and polish borders and eliminate all rough surfaces. Perforate the tray with No 8 round bur to create retentions for elastic impression materials. The finger-adapted dough method is preferable to the sprinkle-on method for full arch trays but both are applicable. PROBLEM AREAS : In the finger-adapted dough method maintaining the tray thickness is not always easy. Another problem resides in the overall bulk of the tray. Table 3-5 page 47. VACUUM-ADAPTED METHOD. It is constructed in the same way as the same way of complete denture trays but with the difference of block-out. This method advantage is the speed of the process and its disadvantage is the specialized equipment needed for achieving it. PROCEDURE : 1- Place the outline on the cast. 2- Block out undercuts with a heat stable material. 3- Position a sheet of resin of the proper thickness in the heating frame of the vacuum adapter. 4- Center the blocked out cast in the vacuum former. 5- turn on the heater and when the recommended amount of sag occurs. 6- after the adaptation is complete allow the tray to cool then trim the borders with a large bur. 7- Remove the tray and trim it to the previously deigned outline. 8- Smooth and finish all borders. 9- Use the resin that projects from the anterior teeth as handle. 10- Remove all traces of block out from the interior of the tray. 11- If specified , perforate the tray to improve retention for the impression tray material. 12- Store the tray on the cast. PROBLEM AREAS : Problems with this type of tray is minimal. Sometimes sheets do not adapt to the cast especially on the border reflections. The solution to this problem is to be sure that the recommended amount of sag is reached before starting vacuum adaptation. Sometimes thin sheets result in flexible trays so proper thickness of sheets must be chosen. Table 3-6 page 49 Custom posterior trays. Another way to make impression tray for immediate denture is to make the tray over the edentulous portion of the cast. Another overall impression is made over the teeth and anterior vestibule with a stock tray. It is constructed with the finger adapted dough method and we can also use the sprinkle-on and the vacuum adapted methods. PROCEDURE : 1- place outline on the cast. 2- extend the impression tray outline to the lingual surfaces of anterior teeth. we can also place a 2mm ledge of wax anterior to the incisive edge of anterior teeth. It allows the tray to extend in front of the teeth. 3- block out spaces between the teeth with wax to prevent the resin from entering between them thus preventing teeth from being broken when removing the tray. Then adapt a sheet of base plate wax for relief. 4- paint tinfoil substitute on relief and cast. 5- Mix the resin according to the manufacturer’s recommendation. 6- when the resin reaches the desired state remove the resin roll it and adapt it to the cast. 7- Continue adaptation until the resin begins to set. After setting remove from the cast , trim finish and polish. PROBLEM AREAS: The main problem is when we have spaces between anterior teeth and the resin extrudes between them. Block out and relief will prevent this problem. Table 3-7 page 51. TWO PIECE TRAYS : The first stage of the impression tray is made over the posterior edentulous area of the cast. the second stage of the tray is constructed to be placed over the first tray. Table 3-8 page 55