PRINCIPLES OF TOOTH PREPARATION

PRINCIPLES OF TOOTH PREPARATION

Pierre Fauchard (1747) was the first person to describe the process by which roots of maxillary anterior teeth were selected for the restoration of single and multiple teeth.

Principles of tooth preparation

1. Abutment Evaluation – withstand occlusal forces

2. Biomechanical considerations – tooth preparation design, form, & occlusal relationship of the prosthesis

3. Mechanical considerations – integrity and durability of FPD

ABUTMENT EVALUATION

A restoration must be able to withstand the constant occlusal forces.

The forces are transmitted – pontic, connectors and retainers to the abutment teeth.

These teeth are therefore called upon to withstand the forces normally directed to the missing teeth in addition to these usually applied to the abutments.

Hence the abutments and their supporting tissues should be evaluated for the following factors:

  1. Crown – root ratio
  2. Root configuration
  3. Periodontal ligament area or root surface area
  4. Mobility
  5. Endodontic status
  6. Root proximity
  7. Tilt
  8. Caries

Crown-root ratio

The optimum crown- root ratio for a tooth to be utilized as a FPD abutment is 2:3.

 A ratio of 1:1 is the minimum ratio that is acceptable.

Root Configuration

Multi rooted posterior teeth with widely separated roots offer better support than those that converge, fuse or generally present a conical configuration.

          A single rooted tooth with evidence of irregular configuration or with some curvature in apical third of root offers better support than that of a perfectly tapered root.

          Tooth with conical root can be used as an abutment for a short span FPD if all other factors are optimal.

Root surface area / periodontal ligament area

Ante’s law (1926) – “The total periodontal membrane area of the abutment teeth should equal or exceed that of the teeth to be replaced.”

Mobility   

Mobility itself could be either because of occlusal trauma or periodontal disease. Occlusal trauma is usually reversible and given the fact that the dentist is going to construct a restoration on the tooth in question, there is ample opportunity to correct the situation.

Endodontic status

Endodontically treated teeth – as abutment

But since these teeth are weakened by the loss of significantly supporting dentin, a post and core is usually required.

The prognosis is poor for a pulpless tooth with an extremely short root or with a canal that cannot be negotiated to place a post.

These teeth are contraindicated as abutments to support cantilever FPDs.

Endodontic treatment may be necessary for a supraerupted or malaligned teeth to improve the arch relationship with a post and core which facilitates fabrication of a cast restoration with a more favorable arch position and occlusion.

TILT

This could happen to any tooth adjacent to an edentulous space if left unrestored for a long period of time. 

The most common situation – missing mandibular first molar and the second molar has tilted mesially into the space.

It is impossible to prepare the abutment teeth for a FPD along the long axes of the respective teeth and achieve a common path of insertion. 

   A proximal half crown can be used as a retainer on the distal abutment.

A telescopic crown and coping can also be used as a retainer on the distal abutment.

Caries

 Caries on enamel, dentin & root surfaces should be checked.

 If it is deep – vitality testing done

Abutments should also be evaluated for wear facets, abrasions & hypoplasia. Once the abutment is evaluated and selected, it is prepared taking into consideration biologic and mechanical factors.

BIOLOGIC CONSIDERATIONS

Procedures involving living tissues must be carefully executed to avoid unnecessary damage. 

Poor preparation leads to inadequate marginal fit or deficient crown contour, and plaque control around these restorations.

Prevention of Damage during tooth preparation

Damage to the adjacent tooth

A damaged proximal contact area even if reshaped and polished – susceptible to dental caries

This is because the original surface enamel containing higher fluoride concentrations and the interrupted layer is more prone to plaque retention.  The preferred method is to use the proximal enamel of the tooth being prepared for protection of the adjacent structures.

Teeth are 1.5-2mm wider at the contact area than at the CEJ (Cemento Enamel Junction) and a thin tapered diamond is used to do interproximal reduction without causing excessive tooth reduction or undesirable angulations of rotary instrument.

Damage to the soft tissues

Damage to tongue and cheeks – prevented by careful retraction with an aspirator tip, mouth mirror or flanged saliva ejector

Pulpal degeneration

Occur many years after tooth preparation

Extreme temperature, chemical irritation or microorganisms can cause an irreversible pulpitis, especially when they occur on freshly sectioned dentinal tubules.

Considerable heat is generated by friction between a rotary instrument and the tooth surface. Excessive pressure higher rotational speeds, type, shape and condition of cutting instrument may all increase heat generated.

 Even with the lightest touch the tooth will be over heated unless a well directed water spray is used.

It’s better to prepare grooves or pinholes at low speed because the coolant cannot reach the cutting edge of the bur.

 The chemical action of certain dental materials (bases, restorative resins, solvents and cutting agents) can cause pulpal damage.

Cavity varnish or dentin bonding agent will form an effective barrier in most cases, but their effect on the retention of a cemented restoration is controversial.

 Conservation of tooth structure

The thickness of remaining dentin has been shown to be inversely proportional to the pulpal response.

Hence amount of dentin removed while preparing vital teeth is important as is conservation of the tooth structures.

This is done by

  • Use of partial rather than complete coverage retainers.
  • Preparing teeth with minimum taper.
  • Preparing of occlusal surface so reduction follows the anatomic planes.
  • Preparations of axial surfaces so tooth are prepared evenly.
  • Selection of a conservative margin where indicated.
  • Avoidance of unnecessary apical extension. 

FACTORS AFFECTING FORM AND TERMINATION OF THE FPD

Axial reduction

This is important in securing space for an adequate thickness of restorative material.

If restorations are made with inadequate axial reduction they have thin walls that are subjected to distortion.

 If the technician compensates by over contouring, gingival inflammation results as it becomes more difficult for patients to maintain plaque control around the gingival margin.

Particularly in interproximal and furcation areas, sufficient tooth structure must be removed to allow development of correct axial contours, as periodontal disease often begins in these areas.

Margin integrity

This is dependent on where the margin is placed, margin adaptation and configuration/ geometry.

Margin Placement

The dictum is to place margins supragingivally whenever possible as this generally places the restorations on enamel.

Subgingival margins have been a major etiologic factor in periodontitis.

The deeper the restoration margin is in the sulcus greater is inflammatory response.

 Although a lot of workers have reported no difference in placing margins either way, it has been demonstrated that subgingival margins can be very difficult to evaluate.

 But there are concrete indications for subgingival preparations:

Indications for Subgingival margin placement

Additional retention, esthetics, root sensitivity, modifications of axial contour, proximal contact extending to gingival crest, caries, erosion or restorations extending Subgingival.

 Caution should be exercised if conditions require margin placement any closer to alveolar crest than 2mm, which is the combined dimension of epithelial and connective tissue attachments.

Placements here result in gingival inflammation, loss of alveolar crest height and formation of periodontal pocket.

 Crown lengthening may be done in such situations.

Margin adaptation

The junction/ space between a cemented restoration and teeth is always a potential site for recurrent caries or periodontal disease because of dissolution of luting agent and inherent roughness.

 Hence a smooth and even margin is the beginning of various steps – Tissue displacement, Impression making, Die formation, waxing finishing, casting, involved in making a restoration fit better with least space.

Margin geometry/ configuration

Any configuration of margin design should possess the following characteristics:

  1. Ease of preparation
  2. Acceptable adaptation
  3. Adequate contour
  4. Sufficient strength
  5. Conservation of tooth structures

FINISH LINES:

 Chamfer – cast metal restoration

 Shoulder – All ceramic crowns

 Radial shoulder with rounded gingival axial angle – All ceramic crowns

Deep chamfer or heavy chamfer – All ceramic crowns and metal ceramic crowns

For the facial margin of metal ceramic crowns we can use a shoulder with bevel.

 The bevel or sloped shoulder can also be used.

Occlusal considerations

To provide adequate bulk of restorations, there should be adequate occlusal reduction of 1-2mm depending on the type of restoration and the functional cusp.

The preparation should duplicate the basic inclined plane pattern of the occlusal surface. This aids in cusp placement and in avoiding the pulp. Flat plane may be acceptable when inter occlusal relationships are worn out in older patients.

A functional cusp bevel provides space for an adequate bulk of restoration in an area of heavy occlusal contact.

Lack of this may cause perforation, over contouring with deflective contact or over inclination of axial surface.

The number and area of occlusal contacts have a profound influence on the distribution of occlusal forces.

The larger the total area of contact over which a given occlusal force is applied, the less stress is concentrated at any point. 

As total number of occlusal contacts increases in an occlusal scheme, the force is applied over a greater number of locations, also reducing the localized stress.  Large number of contacts results in more cutting or
grinding surfaces to facilitate mastication.

MECHANICAL CONSIDERATIONS:

           Tooth preparation design for fixed prosthodontics must adhere to certain mechanical principles; otherwise, the restoration may become dislodged or may distort or fracture.

 Divided into 3 categories

  1. Retention form
  2. Resistance form
  3. Deformation of the restoration

Retention Form

This is the ability of the restoration to withstand forces acting in the same direction as path of withdrawal. 

Sticky foods & chewing gum are known to remove restorations in the line of draw. 

Only dental caries and porcelain failure outrank lack of retention as a cause of failure of fixed partial dentures.

Retention depends on following factors

  1. Magnitude of dislodging forces
    • Geometry of tooth preparation
    • Roughness of fitting surface of casting
    • Materials being cemented
    • Film thickness of luting agent

Magnitude of dislodging forces

Forces that tend to remove a cemented restoration along its line of draw are small & rare. 

This can happen by pulling with floss under connector, and sticky food.

 The magnitude of these forces depend on – stickiness of food, surface area of contact, and texture of restoration being pulled.

Geometry of tooth preparation

No cement possesses adequate adhesive properties to hold a restoration in place solely through adhesion.

 The geometric configuration of tooth preparation must place the cement in compression to provide necessary retention and resistance.

The grains of the cement only prevent two surfaces from sliding, although they do not prevent one surface from being lifted from another.

 Cement is effective only if the restoration has a single path of withdrawal.

Mechanism of retention

The relationship between two bodies, one (tooth prepared) restraining movement of the other (a cemented restoration), has been studied and is known in analytical mechanics as a closed lower pair of kinetic elements.

 The relevant sliding pair is formed by two cylindrical surfaces constrained to slide along one another.

The elements are constrained if the curve that defines the cylinder is closed or shaped to prevent movement at right angles to the axis of the cylinder.

A tooth preparation will be cylindrical if the axial surfaces are prepared by a cylindrical bur held at a constant angle.

 The gingival margin of the preparation becomes the fixed curve of mathematical definition, and the occluso axial line angle of the tooth preparation should be a replication of the gingival margin geometry.

 The curve of a complete crown preparation is closed whereas the grooves of a partial crown preparation prevent movement at right angles to the long axis of the cylinder.

However if one wall of the complete crown preparation is overtapered it will no longer be cylindrical, and the cemented restoration will not be constrained by the preparation because restoration has multiple paths of withdrawl.

The cement particles will tend to lift away from rather than slide along the preparation, and the only retention will be a result of the cements limited adhesion.

FACTORS AFFECTING RETENTION
Taper

Theoretically the more nearly parallel the opposing walls of a preparation, greater should be retention.

 This was first demonstrated by Jorgenson in 1955.

 The relationship was found to be hyperbolic, with retention rapidly becoming less, as taper increased.

But parallel walls are impossible to create in the mouth without producing preparation undercuts.

Taper is necessary to visualize preparation walls, prevent undercuts, compensate for inaccuracies in the fabrication process, and permit more nearly complete seating of restorations during cementation.

Too large a taper will decrease the retention

 Lot of studies has recommended the ideal taper. In recent years it has ranged from 3-5 degrees, 6 degrees and 10-14 degrees.

 A taper of 6 degrees has been suggested as optimum to minimize stress in the cement interface between the preparation and restoration.

 There is only a slight increase in stress as taper is increased from 0-15o degrees.

 However at 20 degrees, the stress concentration was found to increase sharply.

Surface area

Cement creates a weak bond, largely by mechanical interlocks, between the inner surface of the restoration and axial wall of preparation.

Therefore, greater the surface area of preparation, greater is retention. Simply, preparations for large teeth are more retentive than preparation on small teeth.

  Molar crowns are more retentive than premolar crowns of similar taper.

Surfaces where the crown is essentially being pulled away from rather than sliding, such as occlusal surface, do not add much to the total retention.

Surface area can be increased somewhat by adding boxes and grooves.

 Freedom of displacement

Retention is improved by geometrically limiting the number of paths along which a restoration can be removed from the tooth preparation.

 Maximum retention is achieved when there is only one path.

Limiting freedom of displacement from torquing or twisting forces in a horizontal plane also increases resistance. Grooves and proximal boxes can be used for this.

 But, when the internal wall of a groove or box meets the external axial wall at an oblique angle, resistance is reduced. A definite wall perpendicular to the force is required.

Type of preparation

Different types of preparation have different retentive values when other factors are kept constant.

Thus a complete crown is more retentive than partial coverage restorations.

Roughness of the surfaces being cemented

The internal surface of casting is most effectively prepared by air- abrading with 50μm of alumina. O’Conner(1990) showed that air abrasion increased retention by 64%.

     Failure rarely occurs at the cement- tooth interface. Therefore, deliberately roughening the tooth preparation hardly influences retention and is not recommended, because roughness adds to difficulty of impression making and waxing.

Types of luting agent:

Adhesive resin cements seem to be most retentive. However, the decision regarding which agent to use is also based on other factors.

RESISTANCE FORM

          Resistance prevents dislodgement of restoration by forces directed in an apical or oblique direction and under occlusal forces. 

          Resistance depends on

  1. Magnitude and direction of dislodging forces
  2. Geometry of preparation
  3. Physical properties of luting agent

Magnitude and direction of dislodging forces

Magnitude varies with individual patients.

 In a pipe smoker or bruxer, it may be difficult to prevent fairly large oblique forces from being applied to a restoration.

Geometry of Tooth Preparation

 Occlusogingival length is an important factor.

 Short tooth preparations with large diameters were found to have very little resistance form.

 The length must be great enough to interfere with the arc of the casting pivoting about a point on the margin on the opposite side of the restoration.

          Hence the walls of shorter preparations should have as little taper as possible to increase resistance.  It can also be increased placing grooves in the axial walls.

Teeth with short diameter and short walls have better resistance than teeth with larger diameter but short walls.

The preparation on the smaller tooth will have a short rotational radius for the arc of displacement, and the incisal portion of axial wall will resist displacement.

          The larger rotational radius on the larger preparation allows for a move gradual arc of displacement, and the axial wall does not resist removal.

Physical Properties of luting agent

Modulus of elasticity and compressive strength of the luting cement have an effect on resistance to deformation.

 Glass ionomers and resin cements have higher compressive strengths.

Alloy Selection

          Although type I and type II gold alloys are satisfactory for intracoronal restorations.  They are too soft for crowns and FPDs; for which type III and type IV alloys are chosen.

 Nickel – Chromium alloys are considerably harder and can be chosen for long span FPD’s with some limitations. Hence the right alloy has to be chosen for a particular restoration.

SPECIAL CONSIDERATIONS:

Span length

In addition to the increased load placed on the periodontal ligament by a long span FPD, longer spans are less rigid. 

Bending or deflection varies directly with the cube of length and inversely with the cube of occlusogingival thickness of pontic.

 Alloys such as nickel-chromium with high yield strength may be used in these situations.

Double abutments

Use of two adjacent teeth at one or both ends of a FPD joined by a solid connector.

 Indications are

  1. Increase retention of restoration
  2. Splint and stabilize periodontally compromised teeth
  3. Increase area of supporting PDL  & Bone

For this a secondary abutment must meet the following criteria if it is to strengthen the FPD.

  1. It must have as much root surface area and as favorable a crown-root ratio as the primary abutment
    • Retainers as secondary abutments must be atleast as retentive as those on primary abutments
    • There should be sufficient crown length and space between adjacent abutments to prevent impingement on the gingiva under the connector.

Pier abutments

   An abutment with an edentulous space as either side is a pier abutment.

Prostheses with non-rigid connectors should not be used if abutments exhibit mobility.

Cantilever FPDs

          This has an abutment on one end only. It is a potentially destructive design with the lever arm created by the pontic.

          Prospective abutment teeth for cantilever FPDs should be evaluated with an eye toward lengthy roots with favorable configuration, long clinical crowns, good crown root ratios, and healthy periodontium.  Generally they should be used to replace only one tooth.

Indications

  1. Missing maxillary lateral incisors using canine for support
  2. Missing first premolar using II premolar and I premolar for support
  3. Missing I molars using I & II premolars.

Cantilevers also seem to work well with periodontally compromised teeth as they have extremely long clinical crowns.

Resin bonded fixed partial dentures

          This development is based on the principle that grinding away up to 50% of sound tooth structure as in conventional FPDs is not a biological approach and other methods of retention were explored.

All the different designs in this category – Rochette, Maryland, Virginia bridges, to name a few, have certain common features.

1. Enamel etching

2. Resin luting cement

3. Conservative tooth preparation – only one surface (lingual or palatal) is prepared.

They all differ only in their mechanism of obtaining retention between metal and resin.

These bridges are usually indicated for young patients preferably replacing single tooth in the anterior region. As they rely totally on the cement for retention their longevity is uncertain compared to conventional FPDs.

References

  • Rosenstiel SF, Land MF,Fugimoto:Contemporary fixed Prosthodontics,3rd edition,St louis ,2001 Mosby.
  • Shillinburg HT, Hobo S, WhitSett LD, Jacobi R, Brackett SE: Fundamentals of fixed Prosthodontics, 3rd edition, Illinios Quintessence.
  • WFP Malone, DL Koth, E Cavazos Jr, Da Kaiser, SM Morgano: Tylman’s Thoery  and practice of fixed Prosthodontics, 8th edition. Ishiyaku EuroAmerica, inc. publishers. Tokyo. St Louis.
  • Smith BGN: Planing and making of crown and bridges, 3rd edition ,London 1998 , Dunitz.
  • Jepsen A: Root surface measurement and a method for x ray determination of root surface area.Acta Odontol Scand 1963;21:35-46.
  • Nyman S, Ericsson I: The capacity of reduced periodontal tissues to support fixed bridge work,J Clinical Periodontol 1982;9:409.
  • Dowden WE: Discussion of methods and criteria in evaluation of dentinal and pulpal responses , Int Dent J 1970 ;20: 531.
  • Kent WA , Shlillinburg HT, Duncanson MG: Taper of clinical preparations for cast restorations Quintessence Int 1988;19:339-345.
  • Norlander J,Weir D Stoffer W,Ochi S: The taper of clinical preparations for fixed Prosthodontics . J Prosthet Dent 1988; 60: 148-151.
  • Good acre CJ, Campagni WV, Aquilino SA : Tooth preparations for complete crowns ; An art form based on scientific principles. J Prosthet Dent 2001 :85: 363-376.
  • Parker MH, Malone KH, Trier AC, Striano TS: Evaluation of resistance form for prepared teeth. J Prosthet Dent 1991 ; 66: 730-733


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