Interim Fixed Prosthesis

Dr. Leslie Laing
Authored by:
Dr. Leslie Laing

Introduction

An interim fixed prosthesis includes a temporary crown and a temporary fixed partial denture (FPD). It is essential in fixed prosthodontic treatment such that it provides provisional coverage for teeth from the time of initial preparation until the delivery and placement of the definitive prosthesis. According to the Glossary of Prosthodontic Terms (GPT-8), an interim prosthesis is defined as “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.” 1 

Requirements

The requirements of an interim prosthesis are similar to those for the definitive one, although the longevity and colour attributes may not be as strict.2 They may be classified as biologic, mechanical, and aesthetic, and, in addition, they may provide a diagnostic potential.

Biologic Requirements

It is essential that an interim prosthesis provides protection to the pulp and to the periodontal tissues. 

Pulpal Protection

While it is inevitable that a certain degree of pulp trauma will occur during tooth preparation, due to the sectioning of dentinal tubules, the provisional prosthesis must be able to seal and insulate the prepared tooth surface thereby preventing sensitivity and further irritation to the pulp. The internal adaptation and marginal integrity of the interim prosthesis, and the provisional luting agent help to protect healthy pulp from microleakage of bacteria, as well as from both chemical and thermal irritation. For teeth that have been endodontically treated, the provisional prosthesis must maintain the biologic seal and prevent coronal microleakage.3

Periodontal Health

In order to maintain good gingival health and to facilitate plaque removal, the interim fixed prosthesis must provide marginal integrity, proper gingival contours and emergence profiles, and a smooth, highly polished surface. Gingival irritation and increased plaque retention, leading to inflammation and subsequent recession of the marginal gingiva4, 5 may result from over-extended and over-contoured gingival contours.  Inappropriate emergence profiles and proximal contours may have adverse effects on interdental papilla form. In addition, the health and aesthetic contours of the gingival tissues overlying the edentulous ridge may be jeopardized if the pontic form impinges on the soft tissues. This may be detected initially as tissue blanching, but if not corrected, a localized inflammation or necrosis may develop.6

Occlusal Stability and Tooth Position

The interim fixed prosthesis must either establish or maintain optimal proximal and occlusal contacts. By achieving these contacts, the positions of the prepared teeth, and, where applicable, those of the opposing teeth, will be maintained and the inter- and intra-arch relationships will be stabilized. Mesial or distal drifting and supra-eruption will be avoided. This is especially crucial in the time intervening between the final impression procedure and the placement of the definitive prosthesis.7

Prevention of Enamel Fracture

In situations where a partial-coverage design has been planned, the provisional prosthesis should protect teeth that have been weakened as a result of the preparation. This would be the case where the margin of the preparation has been placed in close proximity to the occlusal surface of the tooth which could be damaged during the course of chewing. Once the final impression has been made, any chipping of the enamel prior to placement of the definitive prosthesis would result in costly remakes.6

Mechanical Requirements

Interim fixed prostheses must be able to withstand the functional forces during chewing without fracture or displacement.

Function

Fracture of an interim fixed prosthesis is more likely than that of the definitive one, since the strength of the interim material, e.g., poly (methyl methacrylate) (PMMA) resin, is approximately one-twentieth that of metal-ceramic alloys.6,8 Nevertheless, assuming that the tooth has been adequately reduced, breakage of a full-coverage interim crown is usually not a problem. However, there are at least two situations where fracture of a provisional fixed prosthesis may occur: a partial-coverage design, and an interim FPD. Since the provisional restoration in a partial-coverage design does not completely encircle the tooth, thereby weakening the provisional, fracture of the interim prosthesis is more likely to occur. In the case of an interim FPD, it is frequently at the sites where the abutment teeth are connected to the pontic(s), i.e., the connectors, which are the sites of failure.6,9 To minimize the risk of failure, the connectors are often purposely over-contoured in comparison to those of the definitive prosthesis. In such a manner, not only is greater strength achieved by reducing the depth and sharpness of the embrasures, but the cross-sectional area of the connectors is increased, while reducing the stress concentration associated with shaper internal line angles.6 It should be kept in mind that the extent of over-contouring in the anterior regions is limited by the aesthetic requirements. Although the aesthetics are less restrictive in the posterior area, the prosthesis should not be over-contoured near the gingiva to the extent where periodontal health is jeopardized. 

In some situations, a heat-processed resin or cast metal interim fixed prosthesis may be warranted. These include patients undergoing complex fixed prosthodontic treatment over a prolonged period of time; those requiring a long-span posterior FPD; those who are unable to avoid excessive forces on the prosthesis; those with above-average masticatory muscle strength; and those encountering a history of frequent breakage.6

Displacement

In order to avoid pulpal irritation or tooth movement, a provisional fixed prosthesis that has inadvertently been removed or broken, should be replaced or recemented as soon as possible. When there is excessive space between the tooth/teeth and the provisional prosthesis, it is not appropriate to rely on the strength of the luting agent alone to retain the prosthesis, since provisional cements have a lower strength than regular luting cement. The best prevention of displacement is proper tooth preparation and an interim prosthesis with a closely adapted internal surface.6

Removal for Reuse

An interim fixed prosthesis may frequently need to be removed and re-inserted at various times during the treatment until the final prosthesis has been placed. As a result, the interim prosthesis should be removed as atraumatically as possible. Under usual circumstances, it does not break upon removal, provided that the cement is sufficiently weak, that the interim prosthesis has been well fabricated, and that no undercuts are present on the prepared tooth/teeth.

Aesthetic Requirements and Diagnostic Potential

It is important that the aesthetics be considered when treatment is required for anterior teeth, and possibly premolars. The patient’s opinions should be taken into account as the prosthesis will have a profound effect on the patient’s self-image. The colour of the prosthesis should match that of the adjacent teeth, at least initially. However, some resins discolour intraorally with time.2, 10 Through duplication of the diagnostic wax-up, the interim fixed prosthesis helps to determine whether the proposed prosthodontic therapy satisfies the patient’s functional requirements and aesthetic expectations. If properly adapted and contoured, an accurate interim fixed prosthesis can determine if there is adequate retention and resistance form for the functional requirements of the final prosthesis. It is also a practical way of obtaining specific feedback for the design of the definitive prosthesis, including changes in tooth length, contour, incisal edge position, lip support, incisal or occlusal plane, and tooth colour.2, 6, 7, 11-14

Provisional prostheses may be useful when adjunctive therapies are required, such as in the case of periodontal therapy. When properly designed, they may serve as excellent guides to periodontal surgical procedures, such as crown lengthening, ridge augmentation, and pontic site development procedures.  Upon removal, they may facilitate the access for periodontal control procedures and aid in the diagnosis and decision-making of periodontally compromised teeth.2, 15

Once the dentist and patient are mutually satisfied with the interim fixed prosthesis in terms of function, phonetics, and aesthetics, a duplicate of the provisional may be used as a guide for the laboratory technician in the fabrication of the final prosthesis. As a result, a more predictable, functional, aesthetic, and satisfying definitive prosthesis can be achieved.2, 7

Materials

The majority of materials used in the fabrication of interim fixed prostheses, whether they are single- or multiple-unit, are resin based. They consist of pigments, monomers, filler, and an initiator. They differ with respect to polymerization method, filler composition, and monomer type. The pigments provide the appearance of natural teeth, with a variety of shades. The other ingredients contribute to the handling, setting, and final characteristics of the provisional prosthesis. However, it is the ability of the primary monomer to convert to a polymer that allows the material to set into a solid that is strong enough to withstand the ravages of the oral environment for the desired interim period.6, 7

The most commonly used resins include auto-polymerizing and dual curing ones, such as poly (methyl methacrylate) (PMMA) e.g., Jet, Duralay; poly (R’ methacrylate) (PR’MA; R’ represents an alkyl group larger than methyl) e.g., Trim, Snap; bisphenol A diglycidylether methacrylate (bis-GMA) resins e.g., Protemp Garant; and visible light cured (VLC) urethane dimethacrylate resins e.g., Unifast LC, Triad. Each of these, or combinations thereof, may be converted to a polymer by free radical polymerization.6, 7 Material selection should be based on clinically relevant properties such as strength, rigidity, reparability, exothermic reaction, polymerization shrinkage, marginal integrity, and colour stability.

Strength and Rigidity

All of the resin-based interim materials have relatively poor strength. However, PMMA has the greatest strength of the methacrylate acrylic resins.7, 16 The bis-acryl composite resins have the highest modulus of rupture and flexural elastic modulus of all provisional material types7, 17, whereas the PMMA resins have higher flexural strength than composite resin.7,18 Enhanced microhardness, and resistance to wear, has been exhibited by the bis-acryl resin composite materials over the traditional PMMA provisional materials.7, 19 Resistance to crack propagation (fracture toughness) is the highest for PMMA, followed by bis-GMA composite resin and PR’MA. 7, 20 Urethane dimethacrylate resin also has high fracture toughness values.7, 21 The bis-GMA composite resin materials are more brittle than PMMA and are therefore more likely to fracture in interim long-span prostheses.7 The repaired strength of all interim restorative materials is considerably less than the strength of the original unrepaired materials. 7, 22

Exothermic Reaction

No matter which material is used in the fabrication of an interim fixed prosthesis, there is a potential for pulpal injury to occur as a result of the heat released during the exothermic setting reaction. In general, PMMA exhibits the greatest temperature release during the exothermic reaction, followed by PR’MA, bis-acryl composite, and VLC urethane dimethacrylate resins7, 16, 23, 24. By choosing an appropriate provisional material, minimizing the volume of material used, and selecting an appropriate technique for fabrication of the provisional prosthesis, the thermal insult to the pulp may be reduced.

Colour Stability

The VLC urethane dimethacrylate resin provisional materials have been shown to be more colour stable than those composed of PMMA, followed by those of PR’MA.7, 16, 25-29 Porous, unpolished surfaces have a tendency to darken more than highly polished interim materials.25 In addition, colour instability may be due to the incomplete polymerization of the material, absorption of oral fluids, surface reactivity, dietary habits, and oral hygiene.7

Procedure

1. External Surface Form (Matrix)

a. Custom Matrix

i.  Vacuum-formed Mould

  1. Prepare a duplicate stone cast of the diagnostic wax-up of the optimized fixed restoration.
  2. Trim the base of the cast to be 3-4 mm thick.
  3. Place a circular opening 1 cm in diameter equidistant in the molar region of the cast.
  4. Heat a thermoplastic sheet (125 x 125 mm of 0.5-mm thickness) in a vacuform matrix machine and adapt it to the stone cast.
  5. Trim the matrix such that it extends the occlusal/incisal length of 1-2 teeth beyond each abutment tooth and ~ 3 mm below the gingival margins.

ii.  Silicone Putty mould

  1. Prepare a duplicate stone cast of the diagnostic wax-up of the optimized fixed restoration.
  2. Mix an equal amount of base and catalyst of flexible silicone putty in clean, un-gloved hands.
  3. Adapt the material to the stone cast such that it extends the occlusal/incisal length of 1-2 teeth beyond each abutment tooth and ~ 3 mm below the gingival margins.
  4. When the material has set, remove from the cast and trim any excess.

iii. Alginate Mould

  1. This technique may be done either intra-orally or on a cast of the diagnostic wax-up of the fixed restoration.
    1. If the intra-oral technique is used, preoperatively, block in the pontic area by softening pink wax placed in the edentulous area, contour with finger pressure, and have the patient bite down.  
    2. If made on a model, wax up the crown or bridge accurately and flow extra wax over proposed gingival margins.  Soak the cast in water for 2-3 minutes.
  2. Make a full-arch alginate impression to ensure adequate occlusal stops and orientation for re-seating.
  3. For the areas not directly involved in the interim restoration, trim away the impression material leaving only the occlusal surfaces to act as stops.
  4. Using a pencil, mark the midline or other reference point on the mould to allow it to be seated quickly and accurately. Wrap it in moist paper towels until used. Note: When seating an alginate mould, keep in mind that it is compressible; therefore, do not seat it with pressure, since over-seating will produce thin, weak, plastic interim restorations.

iv. Wax Mould

  1. This technique may be done either intra-orally or on a cast of the diagnostic wax-up of the fixed restoration.
    1. If the intra-oral technique is used, preoperatively, block in the pontic area by softening pink wax placed in the edentulous area, contour with finger pressure, and have the patient bite down. Place Vaseline or lubricant over the wax to avoid the wax mould sticking to it.
    2. If made on a model, wax up the crown or bridge accurately and flow extra wax over proposed gingival margins. Soak the cast in water for 2-3 minutes. Place Vaseline or lubricant over the wax to avoid the wax mould sticking to it.
  2. Cut 2 pieces of pink wax the occlusal/incisal length of the interim restoration plus 1-2 teeth beyond each abutment tooth. Using dry heat, fuse the 2 thicknesses together. In some cases, the posterior end may rest on the retromolar area.
  3. Soften the wax under warm water. Place the wax over the area to be temporized, firmly and carefully moulding it to all surfaces that are to be covered by the interim treatment and to the occlusal surfaces of the adjacent teeth.
  4. Chill the wax with cold water and remove. Inspect for accuracy.
  5. Place a thin layer of Vaseline or lubricant in the mould before adding the interim impression material.
  6. Mark the midline or other reference point on the mould to allow it to be seated quickly and accurately. Note: When seating a wax mould, keep in mind that it is flexible; therefore, do not seat it with pressure, since over-seating will produce thin, weak, plastic interim restorations.

b. Preformed Mould

Several preformed moulds are available, including polycarbonate, cellulose acetate, aluminum, tin-silver, and nickel-chromium. They are generally limited to use as crowns or single restorations, and must be lined with autopolymerizing resin. In addition, most require further modification such as internal relief, axial recontouring, and occlusal adjustment.
 

2. Internal/Tissue Surface Form

a. Direct Technique

i. After tooth preparation, try the matrix/mould in the patient’s mouth. Make any necessary adjustments.

ii. Prepare the abutment teeth for the interim restoration by:

  1. Varnishing all exposed dentin.
  2. Washing and gently drying them.
  3. Placing a thin layer of lubricant or Vaseline on them.

iii. Lubricate any surrounding composite resin restorations with lubricant or Vaseline.

iv. Mix the selected provisional material according to the manufacturer’s instructions and place into the matrix, taking care not to incorporate any air bubbles.

v. Seat the matrix containing the provisional restorative material over the prepared teeth and allow the material to polymerize. NOTE #1: Since the reaction is exothermic, it is essential to minimize heat build-up. This may be achieved by flushing the area with water acting as a coolant using the air water syringe until polymerization is complete. NOTE #2: To prevent the restoration from becoming locked into any undercuts on the preparation or adjacent gingival embrasures, the interim restoration may be gently teased off the preparation and re-seated several times while flushing the area with water until polymerization is complete.

b. Indirect Technique

i. After shade selection and tooth preparation, retract the gingival tissues and make an alginate impression of the prepared teeth. Pour the impression in fast-setting plaster or stone.

ii. Fit the matrix to the cast, making any necessary modifications to ensure a passive and complete fit.       

iii. Apply separating medium to the prepared teeth, adjacent teeth, and tissues on the cast.

iv. Prepare a syringe to have an orifice opening of 2-3 mm in diameter. Mix autopolymerizing resin (poly-methyl methacrylate, Jet) and carefully load it into the syringe.

v. Start filling the matrix at one end of the restoration space and continue working to the other end. Keep the syringe tip in constant contact with the resin, to avoid bubbles from forming. Fill the matrix only to the level of the gingiva.

vi. Position and firmly seat the matrix over the cast until the matrix has been completely seated.       

vii. Lightly stabilize the matrix with elastic bands.

viii. Place the matrix/cast complex in warm water (40°C) in a pressure pot (0.15 MPa) for 5-10 minutes.

ix. Gently remove the matrix from the cured resin restoration and cast. Examine the interim restoration for thin areas. Add resin to these locations before separating the restoration from the cast.

x. Remove, trim and try the restoration back on the cast.

xi. Remove resin flash with an acrylic-trimming bur and a fine-grit garnet paper disk.

xii. To correct for over-trimming or deficiencies, add a small amount of resin to the external edge of the restoration only. If an attempt is made to reline the interior of the restoration with resin, the restoration will not seat completely.

xiii. Polish the interim restoration lightly using wet pumice and whiting.

xiv. Clean the restoration using appropriate infection control procedures prior to try-in.

c. Indirect-direct Technique

i. Minimally prepare the abutment teeth on articulator-mounted diagnostic casts. 

ii. Apply separating medium to the prepared teeth, adjacent teeth, and tissues on the cast.

iii. Prepare a syringe to have an orifice opening of 2-3 mm in diameter. Mix autopolymerizing resin (poly-methyl methacrylate, Jet) and carefully load it into the syringe.

iv. Start filling the matrix at one end of the restoration space and continue working to the other end.  Keep the syringe tip in constant contact with the resin, to avoid bubbles from forming. Fill the matrix only to the level of the gingiva.

v. Position and firmly seat the matrix over the cast until the matrix has been completely seated.

vi. Lightly stabilize the matrix with elastic bands.

vii. Place the matrix/cast complex in warm water (40°C) in a pressure pot (0.15 MPa) for 5-10 minutes.

viii. Gently remove the matrix from the cured resin restoration and cast. Examine the interim restoration for thin areas. Add resin to these locations before separating the restoration from the cast.

ix. Remove, trim and try the restoration back on the cast.

x. Remove resin flash with an acrylic-trimming bur and a fine-grit garnet paper disk.

xi. To correct for over-trimming or deficiencies, add a small amount of resin to the external edge of the restoration only. If an attempt is made to reline the interior of the restoration with resin, the restoration will not seat completely.

xii. Polish the interim restoration lightly using wet pumice and whiting.

xiii. Clean the restoration using appropriate infection control procedures prior to try-in.

xiv. Try in the preformed custom restoration. If it does not seat completely and the teeth have been adequately reduced, relieve the internal surface of the restoration until the occlusion is acceptable.

xv. Apply a thin coating of lubricant or Vaseline to the prepared abutment teeth, gingival tissues, and external surfaces of the restoration.

xvi. Using a small round bur, make a vent hole on the occlusal or lingual surface of each abutment retainer.

xvii. Fill the retainers with resin. Seat the restoration once the resin has lost its surface sheen. Immediately wipe away any resin that is expressed through the vent holes.

xviii. When the resin has reached the rubbery stage, rock the restoration gently to remove it from the patient’s mouth.

xix. Place the restoration in warm water (37°C) for 3-5 minutes to hasten polymerization.

xx. Mark the margins with a sharp pencil. Trim the excess resin with an acrylic resin-trimming bur. Complete axial shaping with a fine-grit garnet paper disk.

xxi. Confirm the marginal fit and occlusion, refinish, and polish where necessary.

xxii. Cement using a temporary cement (TempBond).

xxiii. Confirm the occlusion.

 

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