Fixed Prosthodontics – Interim Fixed Restoration

 

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WHAT IS INTERIM PROSTHESIS?

Interim prosthesis means a fixed or removable dental prosthesis, or maxillofacial prosthesis, designed to enhance esthetics, stabilization and/or function for a limited period of time, after which it is to be replaced by a definitive dental or maxillofacial prosthesis.

Often such prostheses are used to assist in determination of the therapeutic effectiveness of a specific treatment plan or the form and function of the planned for definitive prosthesis.

It’s synonyms are provisional prosthesis, provisional restoration.

 

A temporary restoration is not expected to last a long time. A provisional restoration is expected to last for two to six weeks; but, provisional is not only temporary but designed to be replaced by something definitive(permanent) —a definitive restoration in this case.

Interim (provisional or temporary) crown or fixed partial denture is a restoration applied to the prepared tooth temporarily to protect it and to keep the patient comfortable during fabrication of the definitive (permanent) restoration.

It influences the ultimate success of the final restoration.

 

Functions of interim restoration

  1. Protection
  2. Positional stability
  3. Mastication
  4. Esthetics

 

1) Protection

  • The interim restoration protects the pulp, the periodontium and the prepared tooth.
  • It protects the pulp from thermal and chemical irritation caused by foods, drinks.
  • It protects the peridontium from injury by food impaction due to loss of contact and gingival recession due to loss of normal buccal and lingual contours.

Interim restoration must be cleansable in order to maintain gingival health.

 

2.Positional stability

  • To maintain the tooth position and prevent mesial, distal drift or over eruption which will change the relation with the surrounding teeth.
  • To maintain the gingival tissue contour, prevent gingival hyperplasia or gingival recession.

 

3- Mastication

  • To maintain the function of the prepared teeth and enables the patient to use them in mastication satisfactorily.

 

4- Esthetics

  • To restore and maintain esthetics.

 

Mechanical Requirements

  1. The restoration should be strong enough to withstand occlusal forces without fracture.
  2. It should be retentive to avoid displacement.
  3. Easy removal for reuse without being damaged.

 

Factors to be considered in making an interim restoration

The dark red area represents the optimum, in which biologic,  mechanical, and esthetic requirements are adequately met.

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Ideal requirements of Interim restoration materials:

1. Ease of handling, adequate working time, easy moldability. and rapid setting time.

2. Biocompatibility – non toxic, non allergic, non exothermic.

3. Dimensional stability during solidification.

4. Ease of contouring and polishing.

5. Adequate strength and abrasion resistance.

6. Good appearance, color control and color stable.

7. Ease of adding to repair or correct.

8. Chemical compatibility with temporary luting cements.

 


 

Types of interim restorations

A) Prefabricated (crowns)

  • Aluminum crowns
  • Anatomical metal crown forms
  • Clear celluloid shells
  • Tooth coloured polycarbonate crown forms

 

B) Custom (crowns or fixed partial dentures)

  • Material (variety of resins)
  • Technique (direct or indirect)

 

A) Prefabricated Crowns

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These preformed crown forms are commercially available; they can not satisfy the requirement of an interim restoration, so they must be lined with autopolymerizing resin. They are available in a variety of tooth types and sizes.

 

Polycarbonate crown

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Suitable for anterior teeth as it is constructed from a color stable resin, but available in only one shade, this can be modified to a limited extent by the shade of the lining resin. They are supplied in incisor, canine and premolar tooth type.

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Aluminum / and Tin-silver Crowns

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Nickel-chromium crowns

Used mainly for deciduous teeth. They are trimmed and adapted with contouring pliers and cemented with high strength cement. They are longer-term interim restoration due to their hardness.

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Custom made interim crowns and bridges

May be constructed by Indirect or direct method using resin material.

 


 

Indirect Technique

In this technique the interim restoration is constructed outside the mouth so it has the following advantages over the direct technique:

1-There is no contact of free monomer with the prepared tooth or gingiva, which might cause tissue damage or sensitization.

2-The prepared tooth is not subjected to heat created from the exothermic reaction of resin which might cause irreversible pulp damage.

3-  The marginal fit of indirectly constructed restoration is better due to its complete polymerization undisturbed on the stone cast.

4-The indirect technique reduced the chair time.

 

Steps:

* The study cast is constructed from alginate impression before preparation.

* If the tooth or teeth to be restored has any obvious defect, it should be corrected on the study cast with red utility wax.

* Fill all the embrasures with wax or putty to eliminate undercuts.

* Construct the rubber base index for the tooth to be prepared or the index may be constructed from the patient mouth.

* Upon completion of the preparations, make alginate impression for them and pour it in fast-setting plaster.

*  Coat the cast with separating medium.

*  Mix the temporary acrylic resin in a dappen dish and put some on the protected areas of the cast, such as interproximal spaces and in grooves and boxes.

* As the resin begins to lose its surface gloss and become slightly dull, fill the index, place it over the cast.

*  Put them in pressure pot if available or warm water to accelerate polymerization (hot water causes boiling of the monomer porosity).

* The restoration is then removed from the cast, if it is not easily removed from the cast; break the cast with a heavy laboratory knife.

* The interim restoration is then finished using acrylic burs, sand paper discs with different grits.

Finally the restoration is polished with pumice, rag wheel and rubber cups to be ready for cementation.

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The connectors of an interim FPD are purposely overcontoured to improve strength. This should be limited by aesthetics and maintenance of periodontal health.

 

Indirect-direct Procedure

In this technique, the indirect component produces a “custom-made preformed ESF” external surface form”. In most cases the practitioner uses a custom ESF with an underprepared diagnostic cast as the TSF. “tissue surface form”.  The resulting mold forms a shell that is lined with additional resin after tooth preparation. This last step is the direct component of the procedure.

The indirect-direct approach offers these advantages:

  1. Chairside time is reduced.
  2. Less heat is generated in the mouth.
  3. Contact between the resin monomer and soft tissues is minimized compared to the direct procedure and there is a reduced risk of allergic reaction.

However, adjustments are frequently needed to seat the interim completely on the prepared tooth. This is the primary disadvantage of the indirect-direct procedure.

 

Step-by-step Procedure

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  • ESF “external surface form” can be prepared using silicon material or vacuum-formed polypropylene sheet.
  • Prepare the abutment teeth on accurately mounted diagnostic casts. The diagnostic preparation should be more conservative than the eventual tooth preparation and should have supragingival margins.FP_147
  • Apply resin into the ESF and complete the interim restoration.
  • Seat the newly completed interim restoration (called now custom pre-formed ESF) on cast and refine occlusion by articulator. Finish and clean then send it to dentist.

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Step-by-step Procedure – CLINICAL STEPS

  • After tooth preparation, try-in the custom pre-formed ESF.
  • To make the TSF, fill the interim with resin and seat it over prepared teeth.
  • Confirm the marginal fit and occlusion, refinish and polish, then cement the restoration.

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