Crown & Bridge (Sheet 10) PDF

University of Jordan Dentistry 019 Crown & Bridge Written by: Dana ballout Corrected by: Doctor: islam abd alraheam Sheet# 10 Slides in blue 1|Page Today lecture Is about laboratory pro...

University of Jordan Dentistry 019 Crown & Bridge Written by: Dana ballout Corrected by: Doctor: islam abd alraheam Sheet# 10 Slides in blue 1|Page Today lecture Is about laboratory procedure in fixed prosthodontics Communication with the dental laboratory #To make a high quality fixed prosthesis, all members of the dental team must understand what they can reasonably expect from each other. #Clinicians who take the time to develop an in depth understanding of laboratory work make better clinical decisions because of their understanding of applicable technical and material science limitations. So after finishing the tooth preparation , taking an impression and send it to the lab … now the laboratory side of fixed prosthodontics start It is very important for the dentists to communicates well with the dental laboratory , and very important to be in the same page , they know the material science , they should know what is the design of the restoration that you are planning for So in the prescription you should write everything and sometimes you need to communicate directly Usually the clinicians who understand the lab work are better than who doesn’t understand it , simply because there are a lot of things that are not applicable ( like foe example , you might ask the technician that you need this tooth thinner or more palatal , and if you understand the laboratory work you can understand that there is some indentations that the lab cannot do what you are asking for because it is not duable )…. So understanding lab work is essential for any clinician do fixed prosthodontic work ( these steps you will not do them but you need to understand them ) Q: what will you write in the prescription ? Dentist should write the laboratory prescription (work authorization) to identify the material to be used, how the occlusion and contacts should be, the shade and shape, pontic and substructure design, and any additional information. So you should write the material you are planning to use , how would you need the occlusal and proximal contact to be ( heavy occlusion , light occlusion , medium occlusion , out of occlusion , super tight or medium tight ) ,the shade , the shape , pontic design ( like hygienic design , ridge lab , modified ridge lab ) and any additional comments + the delivery date when you are planning to see the ptn again for tha lab to bring the work on time. 2|Page Definitive casts and dies After sending the impression , the first thing the lab to do is to pour the impression and make definitive cast. The cast to be used to fabricate fixed dental prosthesis must meet specific requirements: 1) It must reproduce both prepared and unprepared tooth surfaces accuretly. 2) The unprepared teeth immediately adjacent to the preparation must be free of voids. 3) All surfaces of any teeth involved in anterior guidance and the occlusal surfaces of the unprepared teeth must allow for presice articulation of the opposing casts. ( all surfaces that are involved in occlusion and articulation must be very clear so we can articulate the models easily ) ( defect- free occlusal surfaces are essential for articulation) 4) The relevant soft tissues should be reproduced in the definitive cast. After we get good diagnostic cast , we need to look into the dies What is the die ? the prepared tooth on the model The die for the fixed restoration must meet specific requirements: 1) It must reproduced the prepared tooth exactly. 2) All surfaces must be accurately duplicated, and no bubbles or voids can be accepted. 3) The remaining unprepared tooth structure immediately cervical to the finish line should be easily discernible on the die, ideally with 0.5 to 1 mm visible to help the technician establish the correct cervical contour of the restoration. This is very important , in which the remaining unprepared tooth structure cervical to the finish line should be easily discernible on the die ‫ يمكن تمييزها بسهولة‬to establish correct cervical contour of the restoration Remember that we used the retraction cord to open the gingival sulcus and to retract the soft tissue , so we expose the finish line and the tooth structure below the finish line and this called on the impression : cuff If the impression has a cuff , so that the die will have area below finish line duplicated If there is no cuff on the impression , the die will lack this area below the finish line and the contour of the restoration will not be accurate because the technician has not a guide to build the restoration and he will build it arbitrary 3|Page Cuff on impression =area under the finish line of the die in the cast So the impression should contain the cuff 4) Adequate access to the margin is imperative. So if you do gingival displacement properly when you take the impression , the finish line on the die will be very clear and you will be able to access the margin easily ( so gingival displacement is very important during impression taking ) The impression should be poured at least twice to have a working model (the dies are sectioned on this model) and a solid model to serve as a reference (the final restoration should fit on this model ) The technician will pour the silicon impression that you sent twice to have 2 models 1- The working models that you will take / section the die on it 2- The solid models that act as a reference ( so once the lab finish the restoration , he will try it on this model and it must fit ) so if the final restoration does not fit the solid model that mean there is something wrong and he need to fix it and this is good because we don’t have to bring the patient to see if there is any problem ( the solid model can tell us if there is any problem ) material science what are the materials that are available to produce the cast and the dies ? The cast and die materials should have two crucial characteristics, Dimensional accuracy and resistance to wear. We will talk about some materials and what is better based on these 2 characteristics 1-Gypsum: most common material used, type 3, 4, and 5 are used for cast and die fabrication. The difference between 3,4 and 5 types is how much expansion the gypsum will have and the technician should really know the percentage of expansion that happen so he can take an idea about the fitting of the restoration (the higher the expansion of the gypsum , the loose fitting of the restoration inside patient mouth ) 4|Page  - Gypsum type 1 is a plaster which used mainly to fabricate the diagnostic models of edentulous patient 2- Resin: epoxy resin, harder than gypsum, not compatible with polysulfide impression material, the fit of the restoration is tighter than the one made on gypsum dies. 3- Electroplated dies ( rarely ) Extra : electroplating is used to overcome the poor abrasion resistance of gypsum , this technique involves the deposition of a coat of pure silver or copper on the impression and then it is supported with type 4 stone or resin Usually we use gypsum , but with digital we use other types like resin ( so the conventional is the gypsum ) When the lab will work to fabricate a crown or bridge , he really need the prepared tooth separated from the whole arch ( make a die) to do accurate work and to replicate the margins accurately.. and to do this we have to make this die ( positive replica of the prepared tooth ) to move it in and out and every time w move it we should be able to return it back into its exact position without any rotation and without any change in the position We will explain some systems that help us to separate the die and return it in the model exactly in the same position was before sectioning A- Dowel pins : It is like a pin you place it on the base and go through the model. It is not rounded instead have one of the3 sides flat so that it act as a reference ( antirotational feature of the pin ) 5|Page B- Pindex ( whaledent ) dowel - ( popular system ) - In which there is a machine with laser pointer that point where the hole will be , then we make the holes in the model and then we put the pins which is actually 2 pins together ( anti-rotational feature ) At the pic on left : they are planning for full mouth restoration ( put the pins on all teeth) Pic on right : show the machine with laser pointer C- Di-Lok (DentiFax/ Di-Equi) - Have readily made plastic base and the model fit on the plastic base and then it is sectioned with a saw and the plastic base gurantee that the dies will go in its place as it was before sectioning D- DVA model system Composed of multiple plastic parts and the gypsum model go on the top of plastic parts 6|Page  We fix the gypsum on the plastic part and do sectioning and then the plastic will guarantee the re placement accuretly E- Zeiser ( girrbach) : Wide plastic base , similar to previous one 7|Page  Ditching the dies After we take impression definitive cast section the die ( to separate the prepared tooth from the rest of the arch ) and it is index well to return it in its position accurately …. Now it is the time to work on the die itself and this called ditching the die Ditching mean : trimming the area below the finish line with round bur to make the finish line more prominent And then we mark the finish line with red or blue marker so it is easier to the eye t catch it and make the margin correctly Ditching with round bur the cuff on impression (area behind the finish Line ) and this area will be produced on the Die and ditched Ditching and marking the finish line to make it more clear Mounting casts on articulator The bite should be accurate to enable precise mounting if lengthy chairside adjustment is to be avoided. Now the casts are ready to articulate it using a bite that the dentist has taken and this promises that the chairside adjustment will be minimal if it done right And all depends on the bite that you send , if the bite was bad and has problem so we will need a lot of occlusal adjustment 8|Page Before starting waxing procedure 1) Correct defects on the die. Any undercut should be blocked out as long as the defect doesn’t extend to within 1 mm of the cavity margin. - any defect on the die should be corrected so any undercut must be blocked it with wax or any other material so the final restoration will not be locked into the die - usually the ideal preparation should not have any undercut but sometimes the dentists can easily miss the area where the undercut is there , so the lab can see them on the die and he should block them 2) Provision of adequate luting agent space ( die spacer ) A space should exist between the internal surface of the casting and the prepared surface of the tooth every where except immediately adjacent to the margin. Ideally the space is 20-4- micrometer. Too narrow the casting doesn’t seat properly during cementation Too wide the casting is loose on the tooth, resistance form is reduced, longevity is compromised. You should imagin that your restoration is come to fit on the tooth , so that if there is no space for the cement or the luting agent then the restoration will not seat properly so to avoid that problem we must provide space between the die and the restoration for the luting agent and this space is usually 20-40 micro meters ( not a lot space) While if we have too wide space , the retention and resistant will be reduced so we need to balance how much space we need Luting agent space The precise amount of luting agent space obtained depends on the materials and techniques used in the indirect process, particularly the choice of impression material, investment, and casting alloys. Factors increase the luting agent space: 1) Increased thermal and polymerization shrinkage of the impression material 2) Increase expansion of the investment mold 3) Use of die spacer ( material coated on the die) 9|Page  4) Removal of metal from fitting surface by grinding, airborn particle abrasion..etc. ( if you remove metal from the fitting surface of the restoration by burs or airborne particle abrasion …. You will have more luting agent space ) this image shows how the add die spacer , it is basically like nails polish on the die and it will provide the thickness that you want for your cement Once the die is ready ( we correct the defects and block out any undercut and apply die spacer …. Then it is the time to do wax pattern ) In all these pictures , we have a full contoured crowns wax up but the picture on upper right it is wax up for the framework only So the picture on the left and on lower right…. We will cast the full contour crown But the pic on the upper right … we will cast the framework only And we have to make sure that the occlusal contact on the wax pattern is good because it will be reproduce on the final restoration , so if you have high occlusal contact on the wax you will have high occlusal contact on your final restoration 10 | P a g e  Cutting back technique The veneering area should be cutback from the full anatomical wax up to layer it later on with feldspathic porcelain. Which mean that if you are planning to do PFM and you did a wax up for the whole restoration ( full contoured wax up) , we need to remove some of the wax to create space for the porcelain The core can be zirconia sometime ( not always metal)so here we have the full contour is the translucent and the core is the radiopaque ( look to the figure below ) So cut back mean removing of some surface wax of full contoured wax up to create a space to layer the porcelain Lost wax technique The technique of converting the wax pattern into metal or all ceramic restoration to fit in the patient mouth So after taking an impression and make the definitive cast and the die is ready , we did the wax pattern and now the wax pattern now is attached to the casting ring that have a liner and empty space ( light blue area in the pic D ) , and we have the wax attached to the base and then we will mix the investment material (a type of gypsum material ) and pour it around the wax model , then we will put 11 | P a g e everything in the oven so the wax will be molten and leave a space and then we will inject the metal or all ceramics material in this created space and after solidifying we can break everything and take the casted crown In the past , they use asbestos as a liner for the casting ring but nowadays it ia not used anymore because of the health risk associated with asbestos fiber Why we need the liner ? Because the investment material undergo expansion and to control the expansion we use the liners ….. so that the ring act as a container for the investment will it sets and restricts its expansion and the liner will allow the investment to expand because it is flexible , but the container(ring) is rigid and restricts the expansion And when the investment expands ,the space created will be bigger and the cast crown produced will be bigger ( luting agent space will be larger ) and the crown will be loose on the die So if we don’t want the expansion , we don’t want the liner And if we need more expansion , we put the liner We talked about the investment but what are the investment materials we have? Investment material 1) Gypsum bonded investments: used for casting made from ADA type II, III, and IV gold alloys. It does not tolerate high melting temperature. ( so we only use it with soft alloys like type 2,3,4 gold alloys ) 2) Phosphate bonded investments: recommended for metal ceramic framework. Tolerate high melting temperature. Higher expansion. 3) Silica bonded investments: used for high melting base metal alloys used in casting partial removable dentures. 12 | P a g e Vacume mixing of the investment material is necessary to avoid gas formation.( why?) Because any gas entrapment or any bubbles will result in an inaccuracy in the casting we are doing ( we need high quality mixing of investment material) Again … Lost wax technique The process consists of surrounding the wax pattern with a mold made of heat resistant investment material, eliminating the wax by heating, and then introducing molten metal into the mold through channel called the sprue. Small variations In investing or casting can significantly affect the quality of the final restoration. This is a pic of casting machine , has an arm and we will put the investment that you did with the wax pattern inside it and then we put it inside the oven and the wax will done because it will be molten and the investment now will have an empty space where the wax was before melting and then we will inject the molten metal in this created space , then the arm of the machine will be rotate clockwise 2 or 3 times depend on the metal type and then we gonna release it after we melt the metal And the concept of the casting resembles the separation of red blood cells in which you are under rotational movement you will introduce the molten metal into the investment This is a picture of other machine ( a fancier casting machine) which is alittle more safe because you don’t deal with direct fire ( fancy version of casting machine ) 13 | P a g e If you remember the wax pattern we did for the gold onlay , so the wax is attached to the sprue and the sprue will be the channel where the molten metal will go to reach the space within the investment In the first picture we see the base of a casting ring in which the ring will come on the top of this And the upper right picture show how the final casting come out of the investment after casting process , then we will cut the sprue by burs and discs And the metal here is gold so we can re melt and re use it again In the lower right pic , look for the fitting surface with no nodules and it is smooth in pressed ceramic , the same exact steps except instead of casting the metal into the investment mold , we will press the ceramic into the investment mold , then we will cut the sprue channel 14 | P a g e Evaluation of the casting Defect in the casting: 1) Roughness generalized casting roughness may indicate a breakdown on the investment from excessive burnout temperature. Remember some of investment materials can tolerate temperature and some don’t , so it is very important to know the melting temperature for the metal you are using and chose the investment material that can tolerate this temperature because if the investment cannot tolerate that temperature , the casting will be very rough 2) Nodules gas trapped between the wax pattern and the investment produce nodules on the casting surface. 3) Fins are caused by cracks in the investment that have been filled with molten metal. Fins mean cracks within the investment and this lead the molten metal to fill these cracks 4) Incompleteness If an area of wax is too thin ( so it will not be produced in casting)or inadequate heating of the metal, incomplete wax elimination, excessive cooling of the mold, insufficient casting force, not enough metal. ( incompleteness : area is not casted well) a pictures of nodules , fins , incompleteness respectively 5) Voids or porosity: caused by debris trapped In the mold 6) Back pressure porosity: may be caused by air pressure in the mold as the molten metal enters. 7) Marginal discrepancies: inaccuracies of fit at the margin can be caused by distortion during removal of the wax pattern from the die. 8) Dimensional inaccuracies: the casting can be too small or. Too large. Attention to. 15 | P a g e Details is essential for an accurately expanded mold. A standardized procedure is needed in regard to L/P ratio, spatulation, the ring liner, the amount of liquid added, and mold heating. There is a lot of factors that could results in dimensional inaccuracy like shrinkage , expansion , die spacers and a lot , so we need a balance between these factors to guarantee great fit of restoration this is the final gold onlay that we do this is the finished , polished gold onlay in PFM , after casting we are not ready yet , in which the core is ready but now we have to layer the porcelain it is a powder and liquid which we are going to mix them then we start with opaque layer to block the color of the metal core and then we add the body dentine porcelain and then enamel shade , so the layering here is very important because the difference between the nice and ugly restoration is the layering technique , in which is the lab doesn’t have enough space for porcelain , he cant layer properly and the final restoration will be ugly but with enough space , he can do layering properly ( opaque then body dentine then enamel ) and the restoration will be nice and more aesthetic sometime the core is not metal , instead it is zirconia , and here the same we will layer the porcelain to cover the opaque zirconia and reproduce the final natural looking restoration 16 | P a g e after we are done with layering , we need to solidify or to crystalline this porcelain , se we need to put it into the oven there is a specific program for a specific material with specific temperature and time and after the restoration come out from the oven we need to wait until it cools down and then we continue with staining and polishing here we have polishing kit for porcelain we have to polish all surfaces ( occlusal , buccal , lingual ) and after we polish them we have to apply the glaze and the stain if needed so now if we need to apply some stains it happens at this stage and it is important to have stains for anterior restoration that you want to create incisal translucency and if you want to add a glaze ( porcelain material that is added to the restoration to make it shiny but we have to polish the surface before) finally , remember that we have a solid model to try the restoration on it and this model we don’t sectioned it because it should mimic exactly the patient mouth if the restoration fit , the lab will send it to you if it doesn’t fit on the model he shoule work on them until it fit and then send it ‫اشهد ان ال اله اال هللا‬ thank you 17 | P a g e 18 | P a g e