Treatment Planning in Fixed Prosthodontics

When restoring a damaged single tooth, the preservation of remaining tooth structure is considered while compensating for lost tissue using cast metal, ceramic, or metal-ceramic restorations. The success of the planned restorations depends on treatment planning, appropriate material selection, and correct design. The decision to use cast metal or ceramic...
dentist and patient discussion about planned teeth treatment in dental clinic office room.
Treatment Planning in Fixed Prosthodontics
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When restoring a damaged single tooth, the preservation of remaining tooth structure is considered while compensating for lost tissue using cast metal, ceramic, or metal-ceramic restorations. The success of the planned restorations depends on treatment planning, appropriate material selection, and correct design. The decision to use cast metal or ceramic restorations instead of amalgam or composite restorations depends on the extent of tooth destruction, aesthetic expectations, plaque control, cost, and retention factors. If the restoration is in the anterior region or if the patient has high aesthetic expectations, the cosmetic effect of the restoration should be considered. When the tooth is extensively damaged and the restoration needs to provide resistance, cast metal or ceramic restorations are preferred over amalgam or composites. The success of cemented restorations depends on implementing and maintaining a good plaque control program by the patient. If there is widespread plaque, decalcification, and caries in the mouth, the application of any type of crown restorations should be carefully evaluated. Retention is crucial for teeth with short crown lengths and when the restored crown will support removable prostheses.

Intracoronal Restorations

  • Glass ionomer
  • Composite resin-indirect inlays
  • Amalgam
  • Metal inlays and onlays
  • Ceramic inlays and onlays

When there is not enough tooth structure to retain the restoration intracoronally, normal or modified crowns must be applied. Crowns are types of fixed prostheses that restore the shape, function, and aesthetics of the natural tooth. Although crowns can be classified in various ways based on the materials used, the areas covered, the surfaces of the tooth they cover, and their temporary or permanent use, we can primarily classify crowns under two main headings:

  1. Full Crowns

  • Full metal crowns
  • Acrylic or porcelain veneer crowns
  • Porcelain-fused-to-metal crowns
  • Telescope crowns
  • Full (metal-free) porcelain crowns
  1. Partial Crowns

  • 3/4 crowns
  • 4/5 crowns
  • Modified partial crowns
  • Porcelain laminate crowns

Indications

  • Caries
  • Fractured teeth
  • Attrition, abrasion, erosion
  • Hereditary and acquired dental defects
  • Positional disorders
  • Occlusal disorders
  • Aesthetics
  • Splinting
  • Abutment for bridges
  • Clinical crown shortness
  • In partial dentures

Full Metal Crown: Entirely made of cast metal, it meets biomechanical requirements completely. The major disadvantages are the lack of aesthetics and the inability to conduct vitality tests. It is applied to upper 2nd and 3rd molars, and lower 1st, 2nd, and 3rd molars. Can be used as a single crown or as a bridge abutment. Less tooth structure needs to be removed during preparation compared to designs that carry aesthetic material.

 

Acrylic or Porcelain Veneer Crowns: A modified full crown type with one or more surfaces covered in a natural tooth-colored aesthetic material, using acrylic or porcelain. The aesthetic material does not contact the opposing or adjacent teeth. While having all the advantages of full metal crowns, their aesthetic nature makes them suitable for application in anterior teeth. They require deeper and different tooth preparations to accommodate the aesthetic material on the metal infrastructure.

 

Telescope Crowns: The term ‘telescope’ refers to the parts consisting of interlocking upper and lower structures. The base structure is the cap cemented on the prepared tooth as an abutment, and the upper structure is the second part, the crown itself. It’s a retaining system that provides parallelism for a tooth deviated from its normal axis to be used as an abutment. The inner crown with parallel outer walls is not anatomically shaped. The outer crown is prepared in anatomical forms.

 

Porcelain-Fused-to-Metal Crowns: These crowns are based on the principle of reinforcing the glassy structure of porcelain with metal to create a more resistant structure. Teeth prepared for metal-supported porcelain crowns require more reduction from occlusal and incisal surfaces. They offer superior retention like full metal crowns and meet aesthetic expectations very well. They can be used as fixed prosthesis abutments in all areas of the mouth.

 

Full Porcelain Crowns: A design that can be used when desiring superior aesthetic results in addition to enveloping the entire tooth. They are recommended for use in areas with medium or low load bearing due to their lower fracture resistance compared to metal-ceramic restorations. While reinforced ceramics can be used for posterior teeth restorations, other types of full ceramic crowns are usually used for anterior teeth. A significant amount of tooth preparation is required to achieve sufficient thickness for ceramic structure.

 

Partial Crowns: If full crown indications are not required in terms of retention and coverage, partial crowns requiring the least amount of material loss can be considered. They are crown types that achieve maximum retention with minimal tooth removal and least risk to the normal tooth pulp. Their retention is not as high as full metal crowns. Suitable for use as a single crown or bridge abutment, but not recommended for long bridges. Suitable for conducting vitality tests. Should be applied in mouths with a low risk of caries. Contraindicated when the clinical crown length is insufficient, especially in teeth with labial surface caries, and when the angles between the tooth crown axis and the path of insertion created by the prepared grooves are not suitable.

3/4 Crown

  • These are cast metal crowns that cover three out of the four surfaces of the natural tooth crown.
  • They never extend to the labial surface.
  • Minimal tooth removal and loss of substance from the teeth, due to the cut remaining within the enamel boundaries, provide an advantage to this crown.

4/5 Crown

  • Partial veneer crowns that cover four out of the five surfaces of small and large molar teeth.
  • The cutting depth can be significant, providing the restoration with the necessary resistance.

Modified Partial Crown

  • Known as ‘Mc Boyle crowns’, these modified partial crowns have a metal structure perfectly adapted to the lingual, mesial, and distal surfaces, extending to the gingiva on the mesial and distal sides, and staying away from the free gingival margin on the lingual side.
  • If necessary, a pinhole can be added to the incisal surface or just to the cingulum area to increase retention.

Porcelain Laminate Crowns

  • Applied to the labial surfaces of anterior teeth, these restorations encompass the gingival enamel-cement junction, contact points at the approximals, and the cutting edge at the incisal.
  • They can provide highly conservative and aesthetic solutions.
  • Suitable for application in cases of tooth discoloration, diastema, enamel defects, abrasions, and malpositioned teeth.
  • Oral habits and insufficient enamel presence are contraindications.
  • Traditional methods, as well as high-resistance ceramics, are used in their production.

Treatment Planning for Replacing Lost Teeth

  • The dental arch, consisting of supporting teeth, is a whole with a dynamic balance. When a tooth is missing, the structural integrity of the arch is disrupted, and a new balance emerges.
  • The teeth adjacent to and/or opposite the toothless gap tend to move towards the gap. This movement, especially in the teeth distal to the gap, can lead to bodily displacement or often appears as tilting.
  • The teeth opposite the gap tend to elongate towards the gap, and in cases of severe elongation, there may not be enough space to restore the gap. In such cases, it might be necessary to restore the elongated opposing posterior tooth as well to achieve full function.

Choosing the Type of Prosthesis

  • Lost teeth can be restored with one of three different types of prostheses: Removable partial dentures, Tooth-supported fixed partial dentures, Implant-supported fixed partial dentures.
  • Factors to consider when choosing the appropriate type of prosthesis: Biomechanical situation, Periodontal condition, Aesthetics, Financial situation, Patient’s preferences.

Removable Partial Dentures

  • Generally indicated in cases with more than two missing teeth in the posterior area and more than four incisors in the anterior area.
  • A gap without a distal abutment generally requires a removable prosthesis.
  • In cases with multiple edentulous spaces, even though each could be restored with a fixed partial prosthesis, technical difficulties and financial concerns may necessitate the use of a removable partial denture.
  • Teeth with weak periodontal support generally provide better support to a well-planned removable prosthesis than fixed prostheses.
  • Teeth with short clinical crown length usually do not provide good support for short edentulous spaces. Insufficient number of support teeth may also necessitate the choice of removable partial dentures over fixed prostheses.
  • Excessive tissue loss in the edentulous area may require removable prostheses to restore both function and aesthetics.

Conventional Tooth-Supported Fixed Partial Denture

  • For the construction of classic bridges, it is necessary to use the teeth on both sides of the edentulous area as prosthetic supports.
  • If these supporting teeth are periodontally sound, the bridge is well-designed, and the edentulous area is short, the prosthesis can serve the patient for a long time.
  • There should not be significant soft tissue loss in the edentulous area considered for a fixed prosthesis.
  • A dry mouth is not a suitable environment for fixed prostheses because the margins of the supporting teeth will be at high risk of secondary caries development.

Resin-Bonded Tooth-Supported Fixed Partial Dentures

  • Composite resin-bonded prostheses are a conservative restoration type that minimizes damage to supporting teeth, typically used for missing incisors or small molars.
  • For these applications, supporting teeth must be present both distally and mesially to the edentulous area.
  • These prostheses, having a standard pontic form capable of filling moderately resorbed edentulous areas, are not successful in cases of large tissue losses.
  • Adhesive bridges, which require only a shallow cut within the enamel boundaries, can be preferred for young patients with large pulp chambers for tooth preparation.
  • Resin-bonded prostheses cannot restore anterior region tooth deficiencies with excessively covered occlusions.
  • Even though resin-bonded bridges can be considered for use as periodontal splints, the indication must be carefully evaluated, and modifications like well-prepared long grooves to increase the retention of supporting teeth should not be overlooked.

Implant-Supported Fixed Partial Dentures

Circumstances for Preference:

  • Situations where there are not enough support teeth.
  • Cases where the supports are not strong enough to carry a traditional fixed prosthesis.
  • Presence of free-ending crests.
  • Single tooth missing.
  • Situations where the patient cannot use a removable prosthesis due to preference, social circumstances, or intraoral factors.

    Advantages of Implants:

  • Implants can be a better option compared to long-spanned, complex, and expensive fixed partial prostheses on potential abutment teeth that have undergone post-core, endodontic treatment, periodontal surgery, or root resection.

Evaluation of Abutment Teeth

General Characteristics:

  • Every restoration must be able to resist the chewing forces applied on it.
  • In fixed prostheses, the chewing forces on the missing tooth area are transmitted to the abutment teeth through bodies, connectors, and supports.
  • The potential abutment tooth being vital in every situation is preferred.
  • Endodontically treated, asymptomatic teeth can be used as supports if the canal or canals are well-filled and this is confirmed by radiography.
  • The presence of some amount of healthy coronal structure is important for the longevity of the restoration.
  • Post-core, pin-supported amalgam, or composite resins can be used to complete the missing crown tissue.
  • Before any prosthesis design, the surrounding tissue of the abutment tooth must be healthy and free of inflammation.
  • The abutment teeth should not have mobility.

Evaluation of Roots and Tissues Supporting Fixed Prosthesis

Factors for Evaluation:

  • Crown-root ratio.
  • Root shape.
  • Surface area of the periodontal membrane.

Crown-Root Ratio

Definition and Ideal Ratios:

The ratio of the length of the tooth from the occlusal surface to the alveolar crest compared to the length of the root embedded in the bone.

The ideal crown-root ratio for a tooth to serve as a support for a fixed partial prosthesis is 2/3.

The lowest acceptable crown-root ratio for a tooth to function under normal conditions is 1/1.

In certain special circumstances, ratios greater than 1/1 may also be considered sufficient.

Root Shape

 

Preferred Root Types:

Teeth with roots that are wide in the labio-lingual direction are preferred over those with round root cross-sections.

In posterior teeth to be used as supports, divergent roots are preferred over concave, fused, and conical roots.

Conical-rooted teeth can be chosen as support for fixed partial prostheses with short body lengths if all other conditions are suitable.

In single-rooted teeth, teeth with apical curvature and irregular structure are preferred over those with a conical root shape.

Periodontal Membrane Area

 

General Characteristics:

Larger teeth, having a wider surface area, are more resistant to additional forces.

A reduction in the amount of bone supporting the roots decreases the capacity of teeth to serve as supports.

According to the ‘Ante’s Law,’ the root surface area of the abutment teeth should be equal to or greater than the root surface area of the missing teeth.

Bridges for more than two missing teeth carry a high risk.

Failure is due more to abnormal stresses caused by leverage arms and torque rather than excessive forces.

Biomechanical Factors:

Biomechanical factors play a significant role in the failure of restorations in long edentulous areas.

Biomechanical Considerations in Prostheses

  • Long-spanned fixed partial prostheses increase the load on the supporting teeth and have less rigidity.
  • The deflection or flexure of bridges is directly proportional to the cube of the length of the span and inversely proportional to the cube of the occluso-gingival height of the pontic.
  • Compared to a bridge restoring a single tooth deficiency, bridges restoring two missing teeth deflect 8 times more.
  • Restorations for three missing teeth deflect 27 times more.
  • Halving the occluso-gingival height of the bridge pontic increases its deflection 8 times.
  • In bridge designs, forces aimed at dislodging the restoration are localized in the mesio-distal direction, whereas in single tooth restorations, these forces are more in the bucco-lingual direction.
  • Tooth preparations need to be modified to provide more resistance and structural integrity according to the type of restoration planned.
  • The most common modification is creating various grooves on the buccal and lingual surfaces.
  • In some cases, the use of double abutments to overcome problems caused by insufficient crown-root ratio and long edentulous spaces.
  • The secondary abutment should have at least as much root surface area and crown-root ratio as the primary abutment.
  • The supporting teeth in the secondary abutments should be as retentive as those in the primary abutments.
  • Sufficient crown length and space between abutments placed side by side to prevent soft tissue impingement under the embrasure.
  • The effect of arch curvature on the stresses occurring in fixed bridges.
  • When bridge bodies fall outside the axis line between the supporting teeth, the bridge body starts to act like a lever arm creating a torque movement.
  • To prevent this, additional retention is necessary; the distance between the supporting teeth should equal the length of the lever arm.

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