Considerations in the Design of a Pontic Crown in Dental Fabrication

What are the biological, mechanical and esthetic considerations in designing a pontic? Add a note on fabrication of pontics.

Pontic Design

Pontics are fixed partial denture components that replace missing teeth and restore function and appearance compatible with continued oral health and comfort.

Prerequisites

• Proper diagnosis and treatment planning phase
• Proper diagnostic waxing procedures
• Pontic space.

Read And Learn More: Fixed Partial Denture Short Essay Question And Answers

Biologic Considerations Considerations in Designing a Pontic

Pontic design needs to maintain and preserve:

• Residual ridge
• Abutment
• Opposing teeth
• Supporting tissues.

Factors affecting biologic considerations are:

• Pontic ridge contact
• Removal of dental plaque
• Direction of occlusal forces
• Surfaces of pontic.

Pontic ridge contact

Pressure-free contact between the pontic and the underlying tissues is indicated to prevent ulceration and inflammation of the soft tissues. Passive contact should be present only on keratinized attached tissue.

Dental plaque

Dental plaque releases toxins that cause tissue inflammation and calculus formation.

This occurs in the area between the tissue surface of the pontic and ridge.

Non-mucosal contacts have better cleansing space than mucosal contacts – Spheroidal and sanitary design.

Factors influencing plaque accumulation:

• Design of the pontic
• Materials used in its fabrication
• Oral hygiene measures.

Design

Mandibular posteriors

• An egg-shaped or bullet-shaped pontic is easiest to keep clean
• It should be as convex as possible with one point contact at the center of the residual ridge.

Maxillary posterior and all anterior pontics

The lingual surface is still made convex and the embrasures are exaggerated on the lingual side to facilitate cleaning.

The tissue surface of the pontic must contact the ridge passively along the faciocervical line-angle and on attached keratinized tissue.

Pontic materials

Should possess good biocompatibility, rigidity, strength and longevity.

Materials which can be used include:

• Porcelain
• Metal
• Gold
• Glazed porcelain.

Occlusal contacts should not fall on the junction between metal and porcelain during centric or eccentric tooth contacts. Glazed porcelain is the most biocompatible of the above pontic materials. Well-polished gold is smoother, less prone to corrosion, and less retentive of plaque than an unpolished or porous casting.

Occlusal forces

• If the buccolingual width of the pontic is reduced by 30% the amount of occlusal forces transferred to the abutment teeth is reduced
• Other forces, such as biting on a hard object, parafunctional activities, load the abutment teeth even if the occlusal table is narrowed.

Pontic surfaces

• Tissue surface
• Occlusal surface
• Buccal surface
• Lingual surface
• Interproximal surface.

Tissue Surface

The tissue surface of the pontic should have only minimal passive contact with the ridge. Pontic should not be placed on movable mucosa and should not blanch the tissue.

Occlusal Surface

• The occlusal table should be reduced to limit forces transferred on to the abutment teeth
• It should be placed within the neutral zone concept and provide a stable vertical dimension support
• The maxillary buccal cusps and mandibular lingual cusps should not be altered. Altering of pontic is done when there is lack of space or to create a favorable occlusal relationship.

Buccal and Lingual Surfaces

• The pontic contours of these surfaces are determined by esthetic, functional and hygienic requirements
• Esthetically, the pontic needs to be contoured facially with proper axial alignment and length
• Embrasures on the lingual side are wider than the buccal or facial side.

Interproximal surface

Should be contoured physiologically to maintain oral hygiene. For proper contouring vertical clearance should be adequate.

Mechanical Considerations in Designing a Pontic

Factors influencing mechanical aspects:

• Improper choice of materials
• Poor frame work design
• Poor tooth preparation
• Poor occlusion.

Pontic materials

• Metal
• Porcelain
• Metal and porcelain
• Resin-veneered pontics
• Bis-GMA resin.

Resin-veneered pontics

Resin-veneered pontics wear and discolor easily as water absorption and thermal fluctuations cause leakage at the metal–resin interface.

Resin – veneered pontics Advantages

• Easy to manipulate and repair
• Do not require high-melting range alloys.

Microfilled composites have better physical properties than resin veneers.

Prefabricated porcelain pontics

As slotted facings, long pin facings. Harmony facings and reverse pin facings were used earlier.

Metal – ceramic pontics

Metal – ceramic pontics are strong, easy to clean and natural appearing.

Causes of failure in metal-ceramic facings:

• Uniform thickness of porcelain is required for longer durability
• The metal surfaces to be veneered must be smooth and free of pits
• Sharp angles on the veneering surface increases stress concentrations causing mechanical failure
• Metal–porcelain junction must be placed at least 1.5 mm away from the centric contact junction.

Esthetic Considerations in Designing a Pontic

Pontics should copy the form, contours, gingival margin, incisal edge, gingival and incisal embrasures and color of the adjacent teeth.

Factors affecting esthetics

• Alveolar bone resorption and remodelling
• Incorrect visual perception
• Abnormal mesiodistal width.

In excessive bone loss cases

• Recontouring the gingival half of the labial surface
• Pink porcelain to simulate the gingival tissues
• Ridge augmentation procedures using hydroxylapatite
• RPD is better than an FPD.

Abnormal mesiodistal width

• Can be corrected by orthodontic treatment
• Space can be adjusted between retainers.

Pontic Fabrication

Available materials

• Metal – ceramic pontics
• Resin – veneered
• All metal
• Unidirectional reinforced composite.

Metal – ceramic pontics

Steps in metal – ceramic pontics

• Wax pattern formation.
• Cut-back design.
• Investing and casting.
• Metal preparation.
• Application of porcelain veneer.

Wax pattern formation

• Inlay wax is softened and shaped to the desired pontic shape, by waxing up the internal, proximal, and axial surfaces of the retainers.
  The basic pontic shape can either be waxed up, or an impression of the provisional restoration can be made and duplicated in wax or a prefabricated pontic shape can be used.
• After the pontic is waxed up it is connected to retainers.

Cut-back design

• After outlining the area to be veneered with porcelain, make depth cuts in the wax pattern and complete the cut-back as far as access will allow with the units connected. The porcelain–metal junction should be placed as lingual as possible for good esthetics.
• Separate each retainer and complete the cut back till there is a distinct 90° porcelain–metal junction.

After cutting back reflow and finalize the margins. Do each retainer one at a time and join the entire unit back.

Variation

In some cases the gingival surface of the pontic is cut back in the metal.

Investing and casting

• Sprue the units and invest and cast.

Metal preparation

• The casting is recovered from the investment, the sprue is cut off and the remaining investment material in the casting is removed by sand blasting with aluminum oxide
• The gingival surface of the pontic is finished without over reducing.

Application of porcelain veneer

• A separating agent needs to be applied on the residual ridge of the cast to prevent porcelain powder from sticking to the stone.
• The metal is prepared and opaquer is applied.
• Cervical porcelain is applied to the gingival surface of the pontic with the castings seated on the master cast.
• Porcelain is built up layer-by-layer with the appropriate cervical body and incisal shades and condensed.
• After condensing, section between the units with a thin razor blade to prevent porcelain from pulling away from the framework due to firing shrinkage.
• A second application of porcelain is done to correct deficiencies caused by firing shrinkage (additions are needed proximally and gingivally on the pontic).

Contouring gingival surface

• After application of a porcelain separating liquid to the stone ridge, the desired tissue contact is achieved and gingival surface is contoured (as convex as possible).
• The porcelain on the tissue surface is made as smooth as possible.
• Pontic is ready for try-in, staining, glazing, finishing and polishing.
• The metal framework should be highly polished and gingival embrasures well contoured.

Alternate Fabrication Technique

In case of partial-coverage retainers made of conventional gold alloys, the retainers must be soldered to the pontic after porcelain application, final staining and glazing.

Resin-veneered pontics

• Resins have low abrasion resistance and bonding between the resin and the metal framework is poor
• New resins have better physical properties than the older ones.
  • Waxing and cut-back are similar to those for metal–ceramic restorations
  • Mechanical undercuts to retain acrylic resin can be formed by retention grooves, acrylic beads to the wax pattern, wax loops, or using an electrolytic etchant
  • Acrylic beads are placed on the entire metal surface to be veneered. The cast metal is also air abraded with an aluminum oxide
  • An opaquer is applied to the metal to mask the metal color and body shade of resin is added with a modelling instrument and polymerized under pressure in a heated water bath. A light-curing resin can be used as an alternative
  • The body resin core is ground to the desired shape, before adding the incisal shade
  • Incisal resin is applied and polymerized
  • The finished restoration is polished.

All-metal pontics

• A wax pattern is made of the desired contour
• The completed wax pattern is finished, sprued and casted
• Casting is retrieved, finished and polished.

Unidirectional reinforced composite

• An impression is made after tooth preparation, and a cast is poured
• After application of a special separating medium, glass fibers are placed on the groove in the cast
• The pontic is built in indirect composite resin, finished, polished and cemented on to the prepared teeth.