Dental Laboratory Procedures PDF

Slides notes in black Doctor's notes in Red Written by: Mohammad Abushehab Corrected by: Yazeed Al-jammal Dr. Islam Abd Alraheam DDs, MS American Board of Operative Dentistry Jordanian Board...

Slides notes in black Doctor's notes in Red Written by: Mohammad Abushehab Corrected by: Yazeed Al-jammal Dr. Islam Abd Alraheam DDs, MS American Board of Operative Dentistry Jordanian Board of Restorative Dentistry  Communicating with the dental laboratory After completing the preparation, taking the impression, and sending it to the lab, we now focus on the laboratory side of fixed prosthodontics. Effective communication between the dentist and the dental laboratory is essential. Both the dentist and the technician need to be on the same page regarding material science and the design of the restoration you are planning. When writing the prescription, you must include all necessary details, and in some cases, direct communication with the dental laboratory may be required to ensure clarity. Understanding laboratory procedures is crucial because there are certain limitations that may prevent the technician from executing your request precisely—sometimes it is simply not feasible. While dentists are not expected to perform the laboratory work themselves, they must have a clear understanding of how it is done. 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. Communicating with the dental laboratory 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, ponticand substructure design, and any additional information. What should you include in the prescription? Simply, you should specify: The material you plan to use. Your preferences for the occlusal and proximal contacts (e.g., heavy occlusion, medium occlusion, light occlusion, or out of occlusion; super tight or medium tightness). The shade. The design you want (e.g., Ridge lab, modified Ridge lab, etc.). The delivery date. Any additional comments or specifications.  Definitive casts and dies -After taking the impression, the first thing the lab will do is to pour it and make the 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 accurately. 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 precise articulation of the opposing casts. (all surfaces that are involved in occlusion or in articulation, must be very clear so we can articulate the models very easily and accurately). 4. The relevant soft tissues should be reproduced in the definitive cast. (any soft tissue relevant to our work should be recorded very well in the impression so we can reproduce it in the diagnostic model).  Definitive casts and dies Now, we are going to look at the dye. The dye means the prepared tooth in the model. The die for the fixed restoration must meet specific requirements: 1. It must reproduce 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. If you recall, when taking impressions, we used retraction cords to open the gingival sulcus and retract the tissue. This allows us to capture the finish line and a portion of the structure beyond it. In the impression, this area is referred to as the cuff. When the impression includes a cuff, the die produced in the lab will accurately duplicate the area beyond the finish line. However, if the impression lacks a cuff, this critical area will not be represented on the die. In other words, there will be no structure beyond the finish line for reference. As a result, the technician will not have a proper guide to contour the restoration accurately and will instead build it arbitrary Therefore, always ensure that your impression includes a cuff so that this area is clearly replicated on the model. 4. Adequate access to the margin is imperative. Proper gingival displacement during impression taking is crucial. If performed correctly, the finish line on the die will be very clear, allowing easy access to the margin.  Definitive casts and dies The technician will pour the silicone impression twice to create two models: 1.The working model: This is the model on which the die will be sectioned. 2.The solid model: This serves as a reference. Once the lab finishes the restoration, it will be tested on the solid model to ensure it fits accurately. If the final restoration does not fit the solid model, it indicates there is an issue that needs to be corrected. This process is beneficial because it eliminates the need to bring the patient back to identify problems—the solid model acts as a guide to detect any discrepancies. 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 )  Materials Science The cast and die materials should have two crucial characteristics, dimensional accuracy and resistance to wear. What are the materials that are available to produce the cast and the dies? 1. Gypsum: most common material used, type 3, 4, and 5 are used for cast and die fabrication. The difference between gypsum types 3, 4, and 5 lies in their expansion properties. The technician must be aware of the exact percentage of expansion to ensure the restoration is duplicated accurately. Higher gypsum expansion results in a looser fit of the restoration in the patient’s mouth. Gypsum Type 1 is a plaster primarily used for fabricating diagnostic models of edentulous patients and for taking impressions of their arches. 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 used) Conventionally, gypsum is used for model fabrication. However, with digital techniques, alternative materials such as resin are utilized. When fabricating a crown or bridge, the dental technician must separate the prepared tooth from the entire arch to ensure precise work and accurate replication of the margins. To achieve this, a die (a positive replica of the prepared tooth) is created, allowing it to be repeatedly removed and repositioned. It is crucial that the die returns to its exact original position without any rotation or displacement. We will discuss systems that facilitate the separation of the die from the model and ensure its accurate repositioning after sectioning: 1st system: (Dowel pins) are placed in the base and extend through the model. They are not completely rounded; one of their three sides is flat, providing an anti-rotational feature that acts as a reference to prevent rotation. Dowel pins 2nd system: ( Pindex dowel system) A popular system that uses a machine equipped with a laser pointer to indicate where the holes will be drilled in the model. After drilling, the pins are placed these pins consist of two joined pins, providing an anti-rotational feature. Pindex(whaledent) dowel Left Image: Full-mouth restoration planning is shown, with pins placed on all teeth. Right Image: The machine with a laser pointer is displayed. 3rd system: Di-Lok (DentiFax/ Di-Equi) This system uses a pre-made plastic base on which the model fits. After fitting, the model is sectioned with a saw. The plastic base ensures that the dies return to their exact original position after sectioning. 4th system: DVA model system This system consists of multiple plastic parts with the gypsum model placed on top. The gypsum is fixed onto the plastic components, and sectioning is performed. The plastic base ensures the dies are accurately repositioned after sectioning. 5th system: Zeiser (Girrbach) This system features a wide plastic base, similar to the previous one.  Ditching the dies After taking the definitive impression and creating the cast, the die is sectioned to separate the prepared tooth from the rest of the arch. It is carefully indexed to ensure it can return to its original position accurately. At this stage, we begin working on the die itself, a process known as ditching. Ditching with round bur Ditching involves trimming the area below the finish line using a round bur to make the finish The cuff on the impression line more prominent. refers to the area beneath the finish line. This area will be Afterward, the finish line is marked with a red reproduced on the die and or blue marker to enhance visibility, making it then ditched during trimming easier to identify and create the margin to make the finish line more accurately. prominent. Ditching involves trimming the area below the finish line to make it more prominent. The finish line is then marked, typically with a red or blue marker, to enhance its clarity and ensure precise work.  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 be articulated using the bite registration provided by the dentist. When done correctly, this ensures minimal chairside adjustments. However, the outcome heavily depends on the quality of the bite registration. If the bite is inaccurate or has issues, significant occlusal adjustments will be required.  Before starting the 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 defects on the die, such as undercuts, should be corrected. These undercuts must be blocked with wax or another material to prevent the final restoration from being locked into the die. Ideally, the preparation should not have any undercuts; however, dentists may sometimes miss areas where undercuts exist. The lab will identify these on the die and should block them accordingly. 2. Provision of adequate luting agent space (die spacer) You should imagine that your restoration is designed to fit onto the tooth. If there is no space for the cement or luting agent, the restoration will not seat properly. To avoid this issue, it’s essential to provide a space between the die and the restoration for the luting agent, typically around 20-40 micrometers (a small gap). However, if the space is too wide, retention and resistance will be compromised. Therefore, it's important to find the right balance in the space needed for proper seating and stability. luting agent space: A space should exist between the internal surface of the casting and the prepared surface of the tooth everywhere 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.  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: (the space between the restoration and the die is bigger because of): 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). 4) Removal of metal from fitting surface by grinding, airborn particle abrasion....etc. If you remove metal from the fitting surface of the restoration using burs or airborne particle abrasion, you will create more space for the luting agent. This image shows how to apply die spacer. It is similar to applying nail polish on the die, and it provides the desired thickness for the cement. Once the die is prepared—by correcting defects, blocking any undercuts, and applying the die spacer—it is then ready for the wax pattern. WAX PATTERNS In all these pictures, we have full-contoured crown wax-ups, except for the upper one, which shows a wax-up for the framework only. For the pictures on the left and right lower, we will cast the full-contoured crown. However, for the upper picture, we will cast only the framework. It is crucial to ensure that the occlusal contact on the wax pattern is correct, as it will be reproduced in the final restoration. If there is high occlusal contact on the wax, the final restoration will also have high occlusal contact.  Cutting Back The veneering area should be cutback from the full anatomical wax up to layer it later on with feldspathic porcelain. This means that if you are planning to make a PFM (Porcelain-Fused-to-Metal) restoration and have done a full-contour wax-up for the entire restoration, some of the wax must be removed to create space for the porcelain. The core material can sometimes be zirconia (not always metal). In this case, the full contour is translucent, while the core is radiopaque (as shown in the figure below). "Cut back" refers to the process of removing some surface wax from the full-contour wax-up to create space for layering the porcelain. Lost wax technique Casting ring and liner: The ring serves as a container for the investment while it sets and restricts the setting expansion of the mold. The liner allows for more expansion of the castings. (larger casting) The technique of converting the wax pattern into a metal or all-ceramic restoration to fit the patient's mouth involves several steps. After taking the impression and creating the definitive cast, the die is prepared, and the wax pattern is made. The wax pattern is then attached to the casting ring, which contains a liner and an empty space (shown as the light blue area in the picture D). The wax is secured to the base, and then the investment material (a type of gypsum material) is mixed and poured around the wax model. Next, the entire assembly is placed in the oven, where the wax melts, leaving a mold space. This space is then filled with metal or all-ceramic material. After the material solidifies, the casting is broken open, and the casted crown is removed. In the past, asbestos was used as a liner for the casting ring, but it is no longer used due to the health risks associated with asbestos fibers. Why do we need a liner? The investment material undergoes expansion during the setting process. To control this expansion, we use liners. The liner allows the investment to expand, as it is flexible, while the casting ring is rigid and restricts the expansion. If the expansion is not controlled, the space created will be too large, and the cast crown produced will be bigger, resulting in a larger luting agent space. This will cause the crown to fit loosely on the die. To avoid unwanted expansion, we remove the liner. If more expansion is needed, the liner is used to control and accommodate the expansion.  Investment Materials 1) Gypsum bonded investments: used for casting made from ADA type II, III, and IV gold alloys. It does not tolerate high melting temperature. ( 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. Vacume mixing of the investment material is necessary to avoid gas formation. Why? Any gas entrapment or bubbles can result in inaccuracies in the casting process. For optimal results, high-quality mixing of the investment material is essential.  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.  Casting machine This is an image of a casting machine, which features an arm. The investment material with the wax pattern is placed inside the machine, and then it is put into the oven. As the wax melts, it creates an empty space within the investment where the wax was. Once the space is created, molten metal is injected into this cavity. The arm of the machine then rotates clockwise two or three times, depending on the type of metal being used. After the metal is melted and injected, the casting is ready to be released. The concept of casting is similar to the separation of red blood cells during centrifugation, where rotational movement helps introduce the molten metal into the investment. This is a picture of another casting machine, a more advanced and safer version, as it eliminates the need to handle direct fire. It is a more sophisticated version of the casting machine.  Metal Casting If you remember the wax pattern we made for the gold onlay, the wax is attached to the sprue. The sprue serves as the channel through which the molten metal flows to fill the space within the investment. In the first picture, we see the base of the casting ring, with the ring placed on top of it. The upper-right picture shows the final casting emerging from the investment after the casting process. Once the casting is complete, the sprue is cut off using burs and discs. Since the metal used here is gold, it can be remelted and reused. In the lower-right picture, you can see the fitting surface of the casting, which is smooth with no nodules. Dr. Islam Abd Alraheam  Pressed ceramic In pressed ceramics, the same exact steps are followed, except instead of casting metal into the investment mold, we press the ceramic material into the mold. Afterward, the sprue channel is cut. Defect in the casting: Evaluation of the casting 1) Roughness-----generalized casting roughness may indicate a breakdown on the investment from excessive burnout temperature. Remember, some investment materials can tolerate high temperatures, while others cannot. It is crucial to know the melting temperature of the metal you are using and choose an investment material that can withstand that temperature. If the investment material cannot tolerate the temperature, the casting will turn out 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 refer to cracks within the investment material, which can cause the molten metal to fill these cracks during the casting process. 4) Incompleteness------If an area of wax is too thin or inadequate heating of the metal, incomplete wax elimination, excessive cooling of the mold, insufficient casting force, not enough metal. incompleteness refers to areas that are not properly cast, resulting in poorly filled sections of the mold.  Evaluation of the casting nodules fins incompleteness  Evaluation of the casting Defect in the casting: 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 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 are many factors that can result in dimensional inaccuracies, such as shrinkage, expansion, die spacers, and others. Therefore, it’s essential to balance these factors to ensure a great fit for the restoration.  Finishing and polishing the casting This is the final gold onlay we created—finished and polished Dr. Islam Abd Alraheam Porcelain layering over metal core In PFM restorations, after casting, we’re not quite finished yet. While the core is ready, the next step is to layer the porcelain. The porcelain comes in powder and liquid form, which we mix together. We begin with an opaque layer to mask the color of the metal core, followed by the body dentine porcelain and then the enamel shade. The layering technique is crucial because it determines the aesthetic outcome of the restoration. If the lab doesn’t have enough space for porcelain, they won’t be able to layer properly, resulting in a less attractive restoration. However, with sufficient space, the lab can layer the porcelain correctly (opaque, then body dentine, then enamel), leading to a beautiful and more aesthetic final restoration. Porcelain layering over zirconia core Sometimes the core is not metal, but zirconia. In this case, the process is the same: we layer the porcelain to cover the opaque zirconia and create a final restoration that closely resembles natural teeth. Porcelain firing (crystallization) After we finish layering, we need to solidify or crystallize the porcelain by placing it in the oven. Each material requires a specific program with precise temperature and time settings. Once the restoration comes out of the oven, we must wait for it to cool down before proceeding with staining and polishing. Here we have a polishing kit for porcelain. We need to polish all surfaces (occlusal, buccal, and lingual). After polishing, we apply the glaze and stain if necessary. If we need to apply stains, this is the stage where it happens. Stains are especially important for anterior restorations to create incisal translucency. Additionally, if you want to add a glaze (a porcelain material applied to give the restoration a shiny finish), it must be done after polishing the surface. Restoration evaluation on the solid model Finally, remember that we have a solid model to try the restoration on. This model is not sectioned because it needs to replicate the patient's mouth exactly. If the restoration fits, the lab will send it to you. If it doesn’t fit on the model, the lab should continue working on it until it does, and then send it. References Contemporary fixed prosthodontics textbook. Chapters 16, 17, 18, 22and 29.