Fixed Prosthodontics Concepts

Laboratory Procedures

• The slides are in black ink, doctors' notes are in red.
• Mohammad Abushehab wrote the notes, Yazeed Al-Jammal corrected them.
• Dr. Islam Abd Alraheam is the presenter, and he has qualifications from the American Board of Operative Dentistry and the Jordanian Board of Restorative Dentistry.

Communicating with the Dental Laboratory

• Effective communication between dentist and lab technician is essential for fixed prosthodontics.
• The dentist's prescription must include all necessary details, and sometimes direct communication with the lab is needed.
• Lab technicians have limitations, so understanding these limitations is crucial for the dentist to make clear requests.
• Everyone on the dental team needs to understand what each member can reasonably expect.
• Clinicians need in-depth understanding of lab work to make appropriate clinical decisions.
• Key elements to include in the prescription are: the material, occlusal/proximal contact type (e.g., heavy, medium, light, or out of occlusion), shade, design specifications (e.g., ridge, modified ridge), and delivery date.

Definitive Casts and Dies

• After taking an impression, the lab first makes the definitive cast.
• The cast needs precise reproduction of both prepared and unprepared teeth (including adjacent teeth).
• Adjacent unprepared teeth should not have voids.
• Surfaces related to anterior guidance and occlusion should allow for precise articulation.
• Soft tissues relevant to the work should be recorded well in the impression.
• The die specifically needs to reproduce the prepared tooth precisely, with no bubbles or voids.
• Easily visible structure (0.5-1mm) immediately around the finish line should be apparent on the die.
• The die should have a clear finish line that allows good access to the margins.
• The lab should create a working model and a solid model.

Materials Science

• The ideal cast and die materials display dimensional accuracy and resistance to wear.
• Gypsum is the most common material for cast and die fabrication (types 3, 4, 5).
• Gypsum expansion properties vary; technicians must be aware of these properties for accurate restorations.
• Resin (epoxy resin) is another material, which is less compatible with some impressions and slightly tighter fitting than those made with gypsum.
• Electroplated dies are used less frequently.

Fabricating a Crown or Bridge

• Separating the prepared tooth from the rest of the arch is essential for precise work and margin replication.
• A die (positive replica of the prepared tooth) enables repeated removal and repositioning without any rotation or displacement.

Systems for Separating and Repositioning the Die

• Dowel pins: placed in the base, extending through the model with one flat side for anti-rotational reference.
• Pindex (whaledent) dowel: uses a machine with a laser pointer to drill the holes for the pins, each pin consists of two joined ones.
• Di-Lok (DentiFax/Di-Equi): utilizes a pre-made plastic base for the model.
• DVA model system: uses multiple plastic parts with the gypsum model on top for die repositioning.

Ditching the Dies

• Trimming the area below the finish line with a round bur helps to give emphasis to the finish line.
• The finish line is marked (red/blue marker).

Mounting Casts on Articulator

• Accurate bite registration is essential for precise mounting on an articulator.
• Final outcome heavily depends on the quality of the bite registration.
• Significant occlusal adjustments might be required if the bite is inaccurate.

Before Starting the Waxing Procedure

• Correct any undercut imperfections with wax, to prevent final restorations from locking in die, so that areas with undercuts are identified and blocked, ideally, there should be no undercut defects.
• Adequate luting agent space (20-40 micrometers) is essential to allow proper seating for the restoration.

Luting Agent Space

• A space between the casting and the prepared tooth, except immediately adjacent to the margin.
• Ideal space is 20-4 micrometers.
• Too narrow: improper seating during cementation.
• Too wide: reduced restoration form and longevity.
• Various factors influence the luting agent space (e.g., impression material, investment/casting alloys thermal and polymerization shrinkage, expansion of the investment mold).

Wax Patterns

• Full-contoured crown wax-ups (except for upper framework only).
• Accurate occlusal contacts on the wax pattern are crucial for correct reproduction in the final restoration.

Cutting Back

• Preparing the veneering area by cutting back the full anatomical wax-up for porcelain layering.
• Removing wax to create space for layering porcelain.
• The core material might occasionally be zirconia (radiopaque) rather than metal (translucent).

Lost Wax Technique

• Surrounding the wax pattern with heat-resistant investment material, heating to melt the wax, and injecting molten metal into the mold through channels called the sprue.
• Small variations can affect the quality of the final restoration.

Casting Machine

• Features a rotating arm to introduce molten metal into the cavity.
• The arm rotates clockwise a set number of times.
• Modern machines might eliminate the need for direct flame.

Metal Casting

• The wax pattern is attached to the sprue.
• The sprue serves as the channel for molten metal.
• The casting ring has a base, upon which the casting ring is placed.
• The upper part of the image shows the final casting emerge.
• The sprue needs to be cut off.
• The metal (e.g., gold) can be remelted and reused.
• The finished casting should be smooth, without nodules.

Pressed Ceramics

• Similar steps to metal casting, but ceramic material is pressed into the investment mold instead of casting metal.
• The sprue channel is cut.

Evaluation of the Casting

• Possible casting defects include generalized roughness, nodules (gas trapped), fins (cracked/filled investment), and incompleteness.
• Excessive burnout temperature, incorrect investment materials, gas formation in investment mix, distortion during removal of wax, are some causes of issues.
• Proper investment and casting technique are crucial, as dimensional issues result from factors like shrinkage/expansion of investment and the presence of die spacers.

Finishing and Polishing the Casting

• Polishing all casting surfaces (occlusal, buccal, lingual).
• applying glaze for shiny finish (after polishing).
• stains (optional) to enhance incisal translucency in anterior restorations.

Porcelain Layering over Metal Core

• Layering porcelain over the (metal) core using a powder and liquid form.
• Opaque layer, body dentine porcelain, and enamel shade are layered.
• Proper layering technique is crucial for creating an aesthetically pleasing outcome.

Porcelain Layering over Zirconia Core

• Similar to layering over metal cores.
• Zirconia core materials are layered with porcelain.
• This creates a restoration closely resembling natural teeth.

Porcelain Firing (Crystallization)

• Porcelain needs to be solidified or crystallized in an oven with appropriate precise temperature and time settings.
• The restoration is allowed to cool down after firing.

References

• Contemporary fixed prosthodontics textbook, Chapters 16, 17, 18, 22, and 29.

Restoration Evaluation on the Solid Model

• Final evaluation of the restoration involves checking the fit on a solid model.
• The solid model replicates the patient's mouth precisely.
• Adjustments might be necessary based on the model's evaluation and any needed modifications.