Prosthodontics Sheet 12 PDF

12 Maha Al-junide Ahmad brains Rasha-Al Amoush 1|Page  Flasking, packing and curing cycles Topics to be covered in this lecture: o Flasking o Denture base materials overview o Packing procedure o Curing cycles ▪ Complete dentu...

12 Maha Al-junide Ahmad brains Rasha-Al Amoush 1|Page  Flasking, packing and curing cycles Topics to be covered in this lecture: o Flasking o Denture base materials overview o Packing procedure o Curing cycles ▪ Complete denture passes through clinical and laboratory steps a) First, we take a primary impression — to construct primary cast and special tray. b) Secondary impression and cast — for Record bases construction c) Jaw relation in the clinic and mounting in the lab. d) Teeth setting and wax-up, and then try in with patient. e) Flasking, packing and curing (final processing procedure of the Denture, done in the lab,to make the denture ready for insertion) ❖ Flacking ✓ Flask: Is a metal case or tube used in investing procedures or can be defined as a sectional metal case in which a sectional mold is made of artificial stone or plaster of Paris for the purpose of compressing and processing dentures. ✓ Flasking: The process of investing the cast and a waxed denture in a flask to make a sectional mold used to form the acrylic resin denture base, it is the last step in the complete denture ▪ Flask components: o Most processing flasks contain a lid, an upper and lower component (shaped appropriately according to whether it is used for the upper or lower denture) and a bottom knockout disc. o The lid and the two main sections have an ID number. ▪ Flasking techniques: 1. Compression moulding technique (most common technique) 2. Injection moulding technique 3. Polymerization via microwave energy 4. Fluid resin technique 5. Light-activated denture base resins 2|Page 1. Compression moulding technique (Detailed steps will be discussed in this lecture and the next lecture) ✓ We use: Heat-activated denture base resins are shaped via compression moulding. ✓ Method: 1. The master cast with the waxed trial denture base is secured in the lower half of the flask with gypsum investment material (plaster or stone) 2. Then the upper half of the flask is put in place, and we add gypsum investment material up to the occlusal surfaces of the teeth. 3. Finally, the top portion of the flask is poured with investment, and the lid is placed on the flask. 2. Injection moulding technique o Require a special flask and equipment. o The wax pattern is sprued and the material is injected into the mold. ✓ Advantage of this technique: Allows injection of further material during process/polymerization. So, if there is any shrinkage it will compensate by adding material. 3. Microwave technique: very quick This technique takes 3 mins curing in microwave, fast, easy to use, and produces denture in short processing time but expensive. 4. Fluid resin technique: Low viscosity resins (pourable), not widely used. 5. Light-activated denture base resins: Easy and less polymerization shrinkage but it has inferior mechanical and physical properties and poor bonding with teeth. ❖ Formation and preparation of the mold: - Before we start flasking, we should do de-mounting: a. Seal the periphery of the denture flange to the inner edge of the land of the cast. b. Remove the casts from the mountings (soak in water). - Remember in the lab we did indices on the cast so we can detach the cast from the plaster - At the end of flasking procedure, we produce sectional mold. a. Mold with the master cast in the lower compartment of the flask. b. The artificial teeth will be in the upper caste fixed in the investment material. 3|Page ▪ Flasking process: 1) Place the cast in their respective flasks o The maxillary flask rim will be parallel with the base. o Mandibular flask rim will be inclined from back to front. 2) The distance between the teeth of a denture wax-up and the top of the flask should be 3-6 mm (too short a distance cause tooth to shift during the packing procedure) 3) The base of the cast must be reduced if it is too thick. (may destroy the mounting index, preventing a laboratory remount, so processing error is not corrected) → To facilitate the deflasking process (opening the flask): o Lubricate the denture casts on all exposed surfaces with petroleum jelly. o A piece of tinfoil can be adapted to the base to allow easy separation when deflasking the processed denture. o Prepare the appropriate flask by lubricating all internal surfaces with petroleum jelly. 4) Each denture is first invested in the lower half of the respective flask. o Make a mix of plaster and place it into the flask. then Position the cast in the plaster allowing excess plaster to squeeze upward around the periphery. 5) Trim off excess plaster so that it is flush with the land area of the cast and clear of the flask lip (the land area of the master cast shouldn’t be covered by plaster) ✓ The mix should be flushed with: - The land area (red arrow) not covering the flask. - Lip (yellow arrow) to ensure proper closure of the flask. 6) Apply separating medium into the exposed plaster and stone surfaces (petroleum jelly or a tinfoil substitute). ✓ Types of separating: - Tinfoil substitute is painted on with a brush until a shiny surface is attained and allowed to dry. - Vaseline is rubbed on with the fingers or a brush and the second pour can be done immediately. 4|Page 7) Apply a small amount of stone over the teeth with the fingers to ward off any bubbles that are formed during the second pour. Place the upper half of the flask over the lower flask without the lid. 8) we place the second half and make a mix of plaster (or a 50/50 stone/plaster mixture) and vibrate it into the flask (to get rid of the bubbles) to a point just over the occlusal surfaces of the teeth as a second layer, this is called the second pour o The amount of plaster needed depends upon the size of the denture. 9) We add final mix (third mix) of plaster or stone on the second mix. 10) Then we add the lid (top of the flask) and press to allow any excess plaster to expelled through holes on the top. ✓ This picture summarizes the flasking process: We have the lower portion, we add the first layer of the investment material up to the land area of the cast, then we add the second layer up to the occlusal surface of the teeth, then the final layer just covering the teeth, then place the lid, finally compress the flask lid to let the excess plaster escape through the holes in the lid. ❖ Wax Elimination: - After we make the flasking process, we close the flask tightly and place it in boiling water for 4-6 min to soften the wax. - Wax is then removed, and residual wax is washed out with a stream of boiling water. - In this process we eliminate the wax from the flask - Water is drained from the flask, then it is washed again with boiling water containing a powdered detergent, then finally with clean boiling water. - We open the flask (de-flasking) End result: we will have 2 parts or sectional molds for the denture base material. 5|Page → As we said before: a. Mold with the master cast in the lower compartment b. The teeth will be in the upper cast fixed in the investment material - Remove the flask from the water and carefully try to open the flask halves. - Peel away / remove the base plate and unmelted portions of the wax. - Rinse both sections of the flask with hot water - Flush the flask repeatedly with clean boiling water - Use a detergent to clean out the mold from any wax or remaining denture base plate. - Allow the flasks to cool to room temperature and paint all surfaces with tinfoil substitute twice (separating medium), except the teeth (residue of separating medium on the teeth is the main cause for adhesive failure), until they appear shiny. - Exception for teeth — because the bottom part of the teeth has to be bounded to the denture base material. - Allow the tinfoil substitute to fully dry before beginning the packing process. - The end result of flasking step: The denture teeth are imbedded in one side of the flask and the master casts are imbedded in the other side. Now the cast and the teeth are ready for packing. - More Mechanical retention can be placed at this stage in the bottom surfaces of teeth. Especially in porcelain teeth because they don’t bind chemically to the denture base material. Unlike acrylic teeth that forms a chemical bond with the denture base (they are the same material chemically). - We can use a small round bur, to drill some holes (to improve the Retention). ❖ Denture base materials Any denture base material must have minimum requirements because the complete denture is a definitive treatment, it's not a temporary treatment, so it must be rigid, it must resist fractures and to stay for as a much time as possible. 6|Page  Mechanical requirements: 1) High value of modulus of elasticity (i.e. rigid). 2) High value of elastic limit. 3) Should have sufficient flexural strength (to resist fracture). 4) Base material should have adequate fatigue life. 5) Should have high impact strength. 6) Should be abrasion resistant. 7) High craze and creep resistance. Chemical requirements: 1) Chemically inert (not chemically reactive). 2) Should be insoluble in oral fluids. 3) Should not absorb water or saliva. Biological requirements: 1) Not harmful to the technician. 2) Biocompatible, readily cleansable. 3) The set material should be non-toxic and non-irritant to the patient. 4) They should be impermeable. Other: 1) High thermal conductivity. 2) Low density. 3) High softening temperature, dimensionally stable and accurate. 4) Superior aesthetic, color stability and Radiopaque. 5) Good adhesion with teeth and denture liners. 6) Ease of fabrication, inexpensive and easily repaired. ❖ Classification of denture base polymers → According to iso-classification: There are 5 types, Types 1 and 2 (chemically active) are the most widely used products. (chemically- activated and Heat-activated). 7|Page  Heat-activated denture base materials: We need to know the composition of this material, because this is what we use most of the time. ** for heat activated denture base materials the activator is heat, meanwhile, for the chemically activated denture base materials it’s the NN-dimethyl-p- toluidine. Heat-cured PMMA Polymethyl methacrylate: Has favorable working characteristics, acceptable aesthetics, physical and mechanical properties, easy to fabricate, inexpensive. → BUT: - Undergoes water absorption and loss - Has low thermal conductivity - Deforms under load with time (low creep resistance). Reduced by the addition of cross- linking agents. - Does not bond chemically with non-acrylic teeth. - Water absorption causes colonization of candida. NOTES: ✓ Polymerization reaction passes through activation and initiation, propagation and termination. ✓ Most denture bases used today are formed of heat-cured PMMA and copolymers (PMMA with polystyrene-butadiene rubber). Rubber improves impact strength. ✓ Polymers with chemical bonds between different chains (cross- linked). Improved rigidity, craze resistance and reduce solubility. Other types of bases: Injection moulding technique: The resin is injected in a closed sprued mould under continuous pressure resulting in minimal polymerization shrinkage. (not commonly used for denture bases, however it has certain advantages over the heat cured denture base materials). Chemically activated resins: Tertiary amine initiates the reaction. Not frequently used for denture bases, polymerization in 30- 45 min, have higher residual monomer of 3-5%, compromised biocompatibility and mechanical properties, higher solubility, inferior color stability, higher creep rates but less polymerization shrinkage. 8|Page  Microwave activated: Same composition, reaction in 3 min, comparable properties but more expensive. Light-activated bases (also not commonly used for denture): Copolymers of urethane dimethacrylcate and an acrylic resin copolymer. Curing with light at 400-500 nm. Photoinitiator: camphoroquinoneamine. Can be used with monomer-sensitive patients. Has reduced shrinkage, but slightly reduced mechanical properties and poor bonding with teeth. Modified acrylic materials: Mechanical reinforcement of resins by fibres (glass, carbon, kevlar and others) or metal inserts (wires, plates, fillers). But problems in tissue irritation from fibres, poor aesthetics, poor bonding with fibres, stress concentration from inserts, increased working time and handling difficulties. → Improved radiopacity with the addition of salts such as barium sulfate and bismuth. Problems: Cytotoxicity, reduced strength and aesthetics. ▪ Comparison between heat-cured and chemically- cured resins: - Heated cured, has more advantages but It has a high percentage of volumetric contraction or polymerization shrinkage. - But it is still favorable because it has less amount of unreacted monomers so it’s more biocompatible. - Mechanical properties in heat activated resins are superior to the chemically activated resins ❖ Compression moulding technique - Most common flasking technique, using temperature and pressure for polymerization. - Polymerization reaction is exothermic, should be carefully controlled to avoid exceeding boiling point of unreacted monomer (100.8 C). - If we exceed → Gaseous porosity due to rapid heating and evaporation of monomer, appears as fine pores in thick portions of dentures. - Other causes of porosity: Insufficient pressure during flask closure, insufficient amount of dough, improper P/L or improper mixing. - Porosity affects physical properties, aesthetics, hygiene. - Distortion is reduced when the resin is processed at or below the manufacturer’s recommended temperature. 9|Page - Monomer remaining in the cured resin clearly affects the degree of cytotoxicity of the denture base material. - Powder-to-liquid ratio is important to produce a workable mass and reduce volumetric contraction. This is 3:1 by volume (2.5:1 by weight). A shrinkage of 7% results (about 0.5% linear shrinkage). - This ratio will provide sufficient monomers to wet the powder that is not in excess to cause shrinkage. Polymer monomer interaction Upon mixing, the material passes through 5 physical stages: - Sandy stage: grainy mix. - Stringy stage: The mass becomes sticky when handled with a spatula. - Dough like stage: The material is no longer tacky or adheres to the walls of the mixing jar.( workable stage, and the stage where we do packing) - Rubbery or elastic: In clinical use the mass rebounds when compressed or stretched - Stiff: The mixture appears very dry and resistant to mechanical deformation. Mixing and curing of heat-activated materials: - Powder and liquid mixed in proper proportions to produce a workable ‘dough’. ✓ Polymerization shrinkage: o Polymerization of monomers results in about 21% decrease in the volume of the material. o Therefore, to reduce shrinkage, the monomer is mixed with pre-polymerized beads of the material. ✓ Dough-forming time: o The time required to reach the dough like stage. o It should not exceed 40 minutes from the start of mixing. o Most materials on the market have a dough-forming time of less than 10 minutes. ✓ Working time: o This is the time duration the material remains in the dough like stage. o It should be 5 minutes at least. ❖ Packing the mould - Packing is adding the denture base material in the mould sufficiently and doing the trail closure to make sure there is no excess material. 10 | P a g e - One of the most critical steps in denture base fabrication - Monomer is a sensitizer that can cause an allergic contact eczematous reaction on the skin or mucous membrane. (wear rubber gloves and work under proper ventilation). - When the mixture has reached a doughy consistency, it is placed between two plastic sheets and formed into a roll that is flattened to about 14 inch (6 mm) thick. - This is the dough stage, As you can see we placed two plastic sheets and formed a roll that was flattened to about 14 inch (6 mm) thickness - Over-packing results in excessive thickness and increased vertical dimension at occlusion. (extra material, bigger denture base) - Under-packing results in porosity. (less material) → to avoid over packing or under packing we do a trial closure: o Trial closure ensures proper packing of the resin. o Therefore, a well-defined sequence should be followed. For the trial closure, place an amount of excess acrylic into the half of the flasks containing the teeth with two sheets of plastic placed between the two halves before closure. - Trial closure: Slowly compress the flask in a press, allowing excess acrylic to slowly squeeze out between the flask halves. - The slightest discrepancy in closure of the two halves of the flask will cause an error in the occlusion. (We repeat the trial closure till no flush occurs) - The flask is transferred to a spring clamp for processing. - The clamp is closed tightly but not fully compressed. - This will allow the resin to expand upon processing and then finally contract while still under pressure. If a mold is underpacked, add more Make sure that the expressed acrylic acrylic and repeat the packing totally covers the flasked denture process until the flash is expressed and investment surfaces to ensure around the entire border of the flask that it is a good pack. In this figure: its not sufficient and we need to add more acrylic 11 | P a g e  To ensure that the acrylic is sufficiently dense, press it until it feels rubbery to the touch, and remove all flash. Paint the cast side of the flask with tinfoil substitute again, and then make the final closure. ❖ Polymerization cycles (curing cycles): final stage of processing the denture - heating process used to control the polymerization reaction of denture material - This process should be carefully regulated to avoid the effects of uncontrolled temperature rise. → We have 2 cycles: (they are similar but the duration is different ) 1) Long cycle: Controlled constant temperature of 74° C for 8 hours or longer, sometimes followed by 2-3 hours of terminal boil at 100° C. (a) 2) Short cycle: constant 74° C for 2 hours then boiling for 1 hour at 100°. (b) - Processing at low temperature or short time causes higher amounts of residual monomer (unreacted monomers), which causes tissue irritation, sensitivity, allergic reactions, reduces strength and dimensional stability. - Ideally we should cure it with long Cycles slowly at room temperature to allow the release of internal stresses and minimize warpages. after we do the curing we should let the flasks cool at room temperature slowly to avoid any release of Eternal stresses and minimize the warpage of the denture base materials. - Compression moulding causes high processing stresses due to polymerization shrinkage (7% by volume), thermal shrinkage as the resin cools, differences in thermal contraction between gypsum and resin. These do not affect denture fit but cause occlusal inaccuracies. 12 | P a g e  Pack the flasks under moderate pressure and avoid compressing the Then cure it under pressure spring in the bottom of the pressure flask until the pressure plate contacts the bottom of the flask. ❖ Bench cooling we allow the flask after packing to cool down this is called bench cooling. - Rapid cooling can result in distortion of the denture base. - To minimize potential difficulties, the flask should be removed from the water and bench cooled for 30 min. - Subsequently, the flask should be immersed in cool tap water for 15 min. - To decrease the probability of unfavorable dimensional changes, the finished denture should be stored in water until it is placed in the oral cavity.(you will notice when you go to the clinic, the denture will be placed in plastic bag with water when it is delivered from the lab for the insertion). ❖ Deflasking the denture After processing the acrylic resin denture and the flask was cooled slowly to room temperature. - Remove the lid of the flask by prying with a knife - Place the flask in an ejector press. -Using minimum pressure eject the mold (denture) from the flask. Remove the flasks and allow Remove the flask lid by them to bench cool wedging an instrument such as a Buffalo knife or chisel in the notch. - Remove the stone cap ( the third layer ) by gently prying it off to expose the occlusal surfaces and incisal edges of the teeth. - With a plaster saw make 4 cuts, 2 in canine region and 2 in the posterior region. 13 | P a g e - With the plaster knife gently pry the sectioned plaster away from the facial surface of the teeth. - After removing the outer sections of stone from the denture, trim the stone away from the lingual surfaces of the teeth before attempting to remove the inner section of plaster and stone. → This will: 1. prevent breaking the teeth. 2. prevent lifting the denture from the cast ❖ During deflasking: - be careful to preserve the cast, also do not left or remove the denture from the casts. - Clean the denture and cast from plaster. - Remove any stone or bubbles from the exposed acrylic resin and from the occlusal surfaces of the teeth. - Remove any stone particles from the base of the cast and index grooves. - Using a stiff brush, soap and water clean the denture and cast before starting the laboratory remount. → To sum up: 1) First, we do flasking to the cast 2) Then we make Wax elimination 3) Then we can improve our mechanical retention. 4) then packing the mould 5) Trial closure 6) Polymerization cycles (curing cycles) 7) Bench cooling 8) Deflasking of the dentures 14 | P a g e **Sometimes after removing the denture you will notice some porosity. Internal gaseous porosity: the most common type that you might see. ✓ Causes: 1) If temperature exceeds boiling point of monomer (100.8 ͦC) they will boil. 2) Because resin is an extremely poor thermal conductor, the heat generated in a thick section of resin can not be dissipated. 3) When heating is poorly controlled, boiling of unreacted monomer takes place in the deep parts of the resin not on the surface. 4) This produces internal porosity within the processed denture. Look at the figure: air bubbles on the thickest part of the denture >>> internal gaseous porosity. End of sheet #12 { ‫ف يُ َرى‬ َ ‫س ْو‬ َ ‫} َوأَ َّن‬ َ ُ‫س ْعيَه‬ 15 | P a g e