Dentistry 019: Crown & Bridge

Crown & Bridge

• Presented by Dana ballout, corrected by Islam abd alraheam
• Course: Dentistry 019, University of Jordan
• Subject matter: Communication with the dental laboratory and procedures for fixed prosthodontics
• Importance of clear communication and understanding between clinicians and laboratory personnel is essential for high-quality prostheses

Communication with the Dental Laboratory

• Clinicians need in-depth understanding of lab work, technical, and materials science for informed clinical decisions
• Communication is critical between dentists and lab technicians for success
• Detailed prescription is necessary to guide the lab, including materials, occlusion, shade, shape, pontic and substructure designs, and any additional info
• The dentist and the lab technician need to be on the same page in terms of material science and the restorations being planned
• Accuracy for the prepared teeth needs to be accurate and precise, with no voids
• Communication with the lab includes the necessary information about design and the restoration planning

Definitive Casts and Dies

• The lab's first task after receiving an impression is to produce a definitive cast
• The definitive cast must meet specific requirements, including accurate reproduction of prepared and unprepared tooth surfaces, absence of voids, and precise representation of occlusal surfaces for articulation
• Diagnostics are created to ensure the diagnostic cast and the die accurately represent the prepared tooth
• The die should accurately duplicate all surfaces of the prepared tooth, including cervical areas.
• Smooth surfaces are essential for articulating dental models

Material Science

• Gypsum is the most common material for creating casts and dies (types 3, 4, and 5 are used for fabrication
• Other materials, like resin, are sometimes used, are generally tighter fitting when made on resin when compared to those made using gypsum
• Important characteristics for these materials are dimensional accuracy and resistance to wear
• Different types of gypsum have different expansions, and technicians must be aware of these differences to avoid problems with the fit of the restoration

Separating and Returning Dies in Model

• Methods like dowel pins (Pindex / Whalerent) create precise placements for separating and returning dies to the original position
• Di-Lok (DentiFax/Di-Equi), involves a plastic base that ensures accurate positioning of the dies
• Additional techniques like the Zeiser (Girrbach) system use a wide plastic base

Ditching the Dies

• Dies must be prepared by cutting away areas below the finish line. This is important for precise placement in the mold.
• Proper marking of the finish line is extremely important

Mounting Casts on Articulator

• Accurate bite recording is key for precisely mounting casts on an articulator.
• The cast's bite records the relationship between the upper and lower teeth
• Avoiding adjustments during mounting ensures precise reproduction of the patient’s occlusion.

Before Starting Waxing Procedure

• Correcting defects in the die(undercuts) is crucial before waxing
• Provision of adequate space for cement or luting agent is essential
• The space between the internal casting surface and the tooth must be enough for the cement to fill nicely
• Space measurements should be 20-40 µm between the restoration and the tooth's surface
• It should be noted that too little or too much space can affect the longevity of the restoration

Metal Casting

• The correct metal is selected according to the needed alloy temperatures, and a mold is created from the investment material
• The molten metal is introduced into the space, and after cooling, the casting is removed from the mold.
• The finished casting will need additional machining
• The proper procedure for using these different methods, along with any specific requirements, is critical to producing a high quality restoration
• Pressing ceramic techniques are similar, but the ceramic material is pressed into the investment mold.
• The castings are tested, and any defects noted

Evaluation of the Casting

• Casting defects can occur due to investment errors and temperature issues.
• Identifying roughness, nodules, fins, incompleteness, voids, and dimensional inaccuracies can require corrections and adjustments.
• Evaluating the casting involves checking both the quality of the mold and the casting procedure.

Porcelain Layering over Metal Core/Restoration Finishing

• Layering porcelain over the casting is a crucial step in fixed prosthodontics.
• Proper layering technique is critical for a good-looking, fit restoration. Different materials such as Zirconia may replace metals
• A specific material is used for porcelain firing(crystallization)

Vacuum Mixing

• Proper mixing of investment materials is important to avoid gas/bubble formation

Lost Wax Technique

• Lost wax method is a common technique in casting metal prosthetics
• The process involves making a mold from refractory material that will be heated during casting.
• The wax pattern is enclosed in a mold and after solidifying the refractory material, the wax is melted, leaving a cavity where metal can be inserted