Posterior Tooth Preparation -

 Posterior Tooth Preparation -

 CONTENTS:

    Introduction of full veneer crowns

    Rotary Instruments used for tooth preparation

    Indications, contraindications, advantages, and disadvantages.

    Preparation for full veneer crowns

              a. Complete cast metal crown

              b. Posterior metal ceramic crown (wing prep.)

               c. Cast ceramic crown    

q  Introduction of partial veneer crowns

q  Rotary Instruments used for tooth preparation

q   Indications, contraindications, advantages, and  disadvantages

q  Preparation for partial veneer crowns

       a. Three-quarter crown

       b. Modified three-quarter crown

       c. Seven-eighth crowns

       d. Proximal half crowns

q       Resin-bonded prosthesis

q      References.

Introduction:

                Tooth preparation is a far more important phase of fixed partial denture prosthesis treatment although some practitioners never realize this. It must be done with skill and meticulous attention to detail, for everything else that follows – pulpal vitality, periodontal health, a good esthetic result, proper occlusion, protection of remaining tooth structure, and the longevity of the restoration itself – will depend on it.

Rotary instruments used for full veneer preparations:

Shape	Use
Round end tapered diamond	1.      Depth orientation grooves 2.      Occlusal reduction 3.      Functional cusp bevel
Torpedo diamond	1.      Axial reduction 2.      Chamfer finish line
Short needle	1.      Initial interproximal axial reduction in posterior teeth
Long needle	1.      Initial proximal axial reduction in anterior teeth
Small wheel diamond	1. Lingual reduction in anterior teeth
Tapered fissure bur (171L)	Seating groove Proximal groove (posterior teeth Smoothing and finishing Occlusal and incisal bevels
Tapered fissure burs (169L & 170L)	Initial groove alignment Angles of proximal boxes Smoothing and finishing Occlusal and incisal bevels
End cutting bur	Conventional shoulder finishing
Torpedo bur	Axial wall finishing Chamfer finishing
Flame bur	1. Flare and bevel finishing

COMPLETE CAST METAL CROWNS:

·         Complete cast metal crowns can be used where the break down of tooth structure is severe, to the extent that is has been described in operative dentistry as “the final attempt to preserve the tooth”. The terms “full crown”, “full cast crown” and “complete crown” can be used interchangeably with full veneer crown to describe a restoration entirely made of cast metal.

·         Clinicians have long considered full veneer crowns to be the most retentive of veneer preparations

·         Controlled laboratory studies have shown that when compared with partial veneer designs, the full veneer crown exhibits superior retention and resistance.

·         It does not mean that it must be used in every case

·         Instead should be used on those teeth whose restoration demands maximum retention.

·         Selection of full veneer retainer becomes mandatory when the abutment tooth is small or when the edentulous space is long.

Indications:

•                    Extensive destruction from caries or trauma.

•                    Endodontically treated teeth.

•                    Existing restoration that needs the use of a more conservative restoration

•                    Necessity for maximum retention and strength.

•                    To provide contours to receive a removable appliance.

•                    Other re-contouring of axial surfaces (minor corrections of mal inclinations).

•                    Correction of occlusal plane.

Contraindications:

•                    Should not be used in mouths with uncontrolled caries

•                    Less than maximum retention necessary.

•                    Esthetics.

Advantages:

·         Strong.

·         High retentive qualities.

·         Usually easy to obtain adequate resistance form.

·         Option to modify form and occlusion.

·         Contact areas can be conveniently developed.

·         Embrasure areas can be enhanced for periodontally compromised dentition.

Disadvantages:

·        Removal of large amount of tooth structure.

·         Adverse effects on tissue.

·         Vitality testing not readily feasible.

·         Display of metal.

TOOTH PREPARATION FOR COMPLETE CAST CROWN:

     Planar occlusal reduction is done using round end tapered diamond and no: 171 bur. Depth orientation grooves are made on the triangular ridges and primary developmental grooves. The depth orientation grooves should be 1.5mm deep on functional cusps and 1mm deep on non-functional cusps. The tooth structures between the orientation grooves are removed following cuspal contours.

                                                         

Functional cusp bevel is done using round end tapered diamond and no: 171 bur. Depth orientation grooves are placed across the facial occlusal line angle of the mandibular molar. The bevel should parallel the inward facing inclines of the cusps of the opposing tooth, at a depth of 1.5 mm usually forming a 45° angle with the axial wall.

                                                     

   Facial and lingual axial reduction is done with a torpedo diamond producing a definite chamfer finish line at the same time. The facial and lingual reduction is carried as far as possible into the interproximal embrasures without nicking the adjacent teeth.

                                          

    Mesial and distal axial reduction.  A short thin tapered diamond is placed against the facial surface of the remaining  interproximal tooth structure. It is held upright and moved up and down, directing it lingually with light pressure. Once sufficient space has been produced, sweep the short thin diamond back and forth planing the surface to smoothness.

                                             

  Chamfer finishing.

  Torpedo bur is used to produce a distinct finish line and to round of the angles of the preparation.

   Seating groove is made on the axial surface using no: 171 bur. The groove should be cut to the full diameter and it should extend gingivally to a point 0.5 mm above the chamfer.

          

         Chamfer finishing                                                   Seating groove

             

Features of full veneer crown preparation and the function served by each

                       

METAL CERAMIC RESTORATION:

         The use of porcelain fused to metal restorations has grown from the development of the first commercially successful porcelain/ gold alloy restoration by Weinstein et al in 1950’s. While a porcelain-fused to metal crown can serve as a strong and esthetic restoration, patients too often receive this type of restoration for minor irregularities that could have been better handled by conservative treatment or none at all.

Indications:

·         Extensive tooth destruction – caries, trauma or existing previous restorations

·         The need for superior retention and strength

·         An endodontically treated tooth in conjunction with a suitable supporting structure (a post and core)

·         To recontour axial surface or correct minor malinclinations

·         Esthetics 

·         If all ceramic crown is contraindicated

Contraindications:

·         Patients with active caries or untreated periodontal disease

·         Young patients with large pulp chambers

·         When more conservative retainer is technically feasible

·         Intact buccal wall

Advantages:

·         Superior esthetics as compared to complete cast crown.

·         Strength of cast metal is combined with the esthetics of an all ceramic crown by reinforcing a brittle, more cosmetically pleasing material through support derived from the stronger qualities are excellent.

·         Excellent retentive qualities

Disadvantages:

·         Removal of substantial tooth structure.

·         Subject to fracture because porcelain is brittle.

·         Difficult to obtain accurate occlusion in glazed porcelain.

·         Shade selection can be difficult.

·         Inferior esthetics compared to all ceramic crown.

·         Expensive

POSTERIOR PORCELAIN FUSED TO METAL CROWN PREPARATION

  Planar occlusal reduction is done using round and tapered diamond and no: 171 bur. Depth orientation grooves are made on the triangular ridges and primary developmental grooves. The depth orientation grooves should be 1.5 to 2.0 mm in occlusal areas where porcelain coverage is required. The tooth structures between the orientation grooves are removed following cuspal contours.

                                        

 Functional cusp bevel is done using round end tapered diamond and no: 171 bur. Depth orientation grooves are placed across the lingual incline of the maxillary lingual cusp. The bevel should parallel the inward facing inclines of the cusps of the opposing tooth, at a depth of 1.5 mm usually forming a 45° angle with the axial wall.

                                     

 Depth orientation grooves

    A flat end tapered diamond is first aligned with the occlusal portion of the facial surface and three vertical cuts are made to the full diameter of the diamond, fading out at the “break” where the curvature of the facial surface is the greatest. Two similar grooves are made maintaining the same instrument parallel to the gingival segment of the facial surface. A single depth orientation groove may also be used.

                                   

Facial reduction, occlusal half: A flat end tapered diamond is used to remove the tooth structure remaining between the orientation groove in the occlusal portion of the facial surface.

   Facial reduction, gingival half:  A flat end tapered diamond is used to reduce the gingival segment and extend well into the proximal surface. 1.2 mm to 1.4 mm is the accepted reduction for porcelain fused to metal restoration.

 

 Proximal axial reduction:

         Short needle diamond facilities interproximal reduction with out nicking the adjacent tooth. Once separation between the teeth is achieved the needle diamond is used to plane the proximal axial wall.

   Lingual axial reduction:  A torpedo diamond is used for lingual axial reduction and to round over the corner created at the line angle with the proximal surfaces.

    Axial finishing: All axial surfaces to be veneered with metal are finished using a torpedo finishing bur producing the chamfer finish line. The facial surface and those areas of the proximal surfaces to be veneered with porcelain are smoothened with the no: 171 bur. Lingual to the proximal contact, the transition from the deeper facial reduction to the relatively shallower lingual axial reduction result in a vertical wall or “wing” of tooth structure.

 Shoulder finishing:  No: 957 bur is used to finish the shoulder and is planed with a sharp 1.0 m wide chisel.

  

Gingival bevel: Flame diamond and finishing bur are used to produce a narrow bevel, no wider than 0.3 mm.

The features of a posterior porcelain fused to metal crown and the function served by each.

                        

CAST CERAMIC CROWN PREPARATION:

Ø  Occlusal reduction:  A large round end tapered diamond is used to place depth orientation grooves on triangular ridges and major grooves. The final occlusal reduction should be 1.5 mm to 2.0 mm deep. Remove the tooth structure remaining between the depth-orientation grooves with the large round-end tapered diamond.

Ø  Functional cusp bevel:  The large round end tapered diamond is used to produce depth orientation grooves in the facial incline of the facial cusp. A minimum of 1.5 mm of clearance is necessary.

Ø  Facial and lingual axial reduction:  The large round end tapered diamond is used to obtain axial reduction ranging from 1.0 to 1.5 mm by making depth orientation grooves and removing the tooth structure between them. The axial reductions are carried out as far as possible into the proximal embrasures.

Ø  Complete axial reduction:  A short needle diamond is used to begin the proximal axial reduction without touching the adjacent tooth. The axial reduction interproximally is complete by running the round end tapered diamond.

Ø  Preparation finishing: round end tapered carbide bur is used to finish the axial surfaces and the functional cusp bevel.

The features of a cast ceramic crown preparation for an all ceramic crown on a posterior tooth and the function served by each.

                            

PARTIAL COVERAGE CROWNS – POSTERIORS

Ø  INTRODUCTION
An extracoronal restoration that covers only part of the clinical crown considered to be a partial veneer crown. It can also be referred to as a partial coverage restoration.

Ø  DEFINITION

·         A restoration that restores all but one coronal surface of a tooth, usually not covering the facial surface.(GPT- 8)

·         A partial veneer crown is a restoration covering two or more surfaces of a tooth (Tylman)

TYPES OF PARTIAL COVERAGE CROWNS

For posterior teeth-1) Three-quarter crown

                               2) Modified three-quarter crown

                               3) Seven-eighth crowns

                               4) Proximal half crowns                                                              

INDICATIONS:

·         Partial veneer crowns are a conservative measure and are preferable to the complete veneer restoration.

·         Intact or minimally restored teeth.     

·         Teeth with crown length that is average or that exceeds the average.

·         Teeth with normal anatomic crown form, i.e., without excessive cervical constriction

CONTRAINDICATIONS:

·         High caries rate.

·         Teeth with extensive restoration

·         Deep cervical lesion – difficult to establish finish line, if area is sensitive complete coverage crown is indicated.

·         Teeth with short clinical crown – difficult to achieve resistance and retention form.

·         Thin teeth – preparation of retention grooves is difficult.

·         Poor alignment of teeth in the dental arch.

ADVANTAGES:

Ø  Partial veneer crowns have several advantages over complete crowns:

·         The tooth reduction is conservative.

·         The esthetics surpass the complete veneer cast crown.

·         Margin accessibility for finishing and cleaning is improved.

·         Complete seating of the casting is more easily verified.

·         Complete seating of the casting during cementation is enhanced by diminished hydraulic pressure.

·         Electric pulp testing can be conveniently accomplished on the intact enamel surface.

DISADVANTAGES:

·         The partial veneer crown is not as retentive as a complete veneer crown.

·         Skillful preparation is critical to avoid metal display.

·         The partial veneer crown preparation is limited to fairly intact teeth with normally shaped crown and an average length clinical crown.

Fundamental principles of design:

1) To impart strength to the remaining portion of the tooth crown.

2) To provide maximum resistance and retention form to the finished restoration.

3) To preserve esthetics by conservation of tooth structure, thus limiting the display of gold on buccal and labial surfaces.

ROTARY INSTRUMENTS USED FOR TOOTH PREPARATIONS:

·         Round end tapered diamond,

·         Torpedo diamond,

·         Short needle,

·         Long needle,

·         Flame diamond,

·         Small wheel diamond,

·         Tapered fissure burs

·         End cutting bur

Steps for preparation of partial veneer crowns:

·         Outline Form

·         Lingual reduction

·         Incisal reduction

·         Lingual axial reduction

·         Proximal axial reduction

·         Proximal box or groove placement

·         Occlusal or incisal offset placement

·         Facial bevel

·         Finishing the preparation.

Maxillary posterior three quarter crowns

Occlusal reduction:

·         No. 171 or round-end tapered diamond is used

·         Depth orientation grooves are made on the triangular ridges and the developmental grooves 1.0mm on the non functional and 1.5mm on the functional cusp

·         At the occluso-facial line angle groove is made only 0.5mm deep to minimize the display of metal.

·         Tooth structure from between the grooves is removed according to cuspal contours

Functional Cusp Bevel:

·         Should be parallel to the inward facing inclines of the cusps of the opposing tooth, at depth of 1.5mm creating 45˚ angle with the axial wall.

·         No. 171 bur

Lingual Axial Reduction:

·         Torpedo diamond creating a definite chamfer all over.

·          Over inclination of the lingual wall frequently occurs.

·         Check at all times for over inclination.

Proximal Reduction:

·         Initially thin needle diamond is used in sawing motion to just break the contact with the adjacent tooth

·         Once contact is broken, torpedo diamond is used to create a definite chamfer finish line and complete the axial and proximal reduction.

Axial Finishing:

·         Torpedo bur is used to finish the axial walls and round of the margins of the preparation.

Proximal Groove:

·         Parallel to the incisal two thirds of the facial surface.

·         Orientation cut is made using no.170 bur depth 1.0mm or less.

·         Groove is continued apically to about 0.5mm above the chamfer finish line.

·         Grooves on mesial and distal side are made parallel to each other.

Proximal Flares:

·         Flame diamond and flame bur is used to produce the proximal flares that is a flat geometric plane.

·         When access is good a medium grit sandpaper disc may be used.

Occlusal Offset:

·         No. 171 bur used to create a 1 mm ledge on the lingual incline of the facial cusp connecting the grooves and assuming an inverted V shape. No.957 end cutting bur is used to define the margins

Facial bevel:

·         Flame diamond is used to bevel the occluso-facial line angle keeping it perpendicular to path of insertion. The bevel should not be more than 0.5mm wide.

Features of the Maxillary Posterior Three Quarter Crown Preparation:

              

·         A common variation of the three quarter crown preparation, employed when caries or previous restorations are present on the proximal surfaces, is one in which boxes are substituted for grooves.

·         However boxes are very destructive of tooth structure, so their use can be justified only when tooth structure has been destroyed by caries

·         A less destructive alternative for augmenting retention and resistance is a three quarter crown preparation utilizing two grooves on each proximal surface.

·         There is no significant difference between retention afforded by the four grooves and that available from two boxes.

   Mandibular Posterior Three Quarter Crown:

Occlusal Reduction:

·         No. 171 or torpedo diamond is used

·         Depth orientation grooves are made on the triangular ridges and the developmental grooves 1.0mm on the non functional and 1.5mm on the functional cusp

·         At the occluso-facial line angle groove is made only 0.5mm deep to minimize the display of metal.

·         Tooth structure from between the grooves is removed according to cuspal contours.

Occlusal Shoulder:

·         No: 171 bur is used to form the occlusal shoulder along the termination line of the functional cusp bevel facially. End cutting bur is used to plane the shoulder (1.0 mm wide).

Lingual Axial Reduction:

·         Torpedo diamond creating a definite chamfer all over. Over inclination of the lingual wall frequently occurs.

·         Check at all times for over inclination.

Proximal Reduction:

·         Initially thin needle diamond is used in sawing motion to just break the contact with the adjacent tooth

·         Once contact is broken, torpedo diamond is used to create a definite chamfer finish line and complete the axial and proximal reduction.

Axial Finishing:

·         Torpedo bur is used to produce a distinct finish line and round of the angles of the preparation.

Proximal Groove:

·         Parallel to the incisal two thirds of the facial surface.

·         Orientation cut is made using no.170 bur depth 1.0mm or less.

·         Groove is continued apically to about 0.5mm above the chamfer finish line.

·         Grooves on mesial and distal side are made parallel to each other.

Proximal Flares:

·         Flame diamond and flame bur is used to produce the facial flare that is a flat geometric plane. When access is good a medium grit sandpaper disc may be used.

·         The flare is wider at occlusal than at the gingival with a definite finish line.

Facial Bevel: 

·         Flame diamond and no: 170 bur is used to bevel the occlusal shoulder.

Features of the Mandibular Posterior Three Quarter Crowns:

         

Reverse Three – Quarter Crowns:

·         Complete coverage of facial surface is used only occasionally and is usually reserved for one or two situations

·         It is ideal for restoration of molars that are inclined lingually.

·         It can also be used on molars that have suffered destruction of the facial surface, but not of the lingual

Maxillary Posterior Seven Eighth Crown:

Occlusal reduction:

·         No. 171 or torpedo diamond is used

·         Depth orientation grooves are made on the triangular ridges and the developmental grooves 1.0mm on the non functional and 1.5mm on the functional cusp

·         At the occluso-facial line angle groove is made only 0.5mm deep to minimize the display of metal.

·         Tooth structure from between the grooves is removed according to cuspal contours.

Functional Cusp Bevel:

·         Done using round end tapered diamond and no: 171 bur. Depth orientation grooves are placed across the lingual inclines of the lingual cusps.

·         The bevel should parallel the inward facing inclines of the opposing tooth, at a depth of 1.5 mm usually forming a 45° angle with the axial wall.

Facial and Lingual Axial Reduction:

·         Is done with a torpedo diamond producing a definite chamfer finish line at the same time.

·         The lingual reduction is carried as far as possible into the interproximal embrasures with out nicking the adjacent teeth.

·         The disto axial reduction should end 1.0 mm mesial to facial groove.

Proximal Axial Reduction:

·         A short thin tapered diamond is placed against the lingual surface of the remaining interproximal tooth structure and a sawing motion is used. Continue toward the facial surface until contact with adjacent teeth is barely broken.

·         Once sufficient space has been produced, a torpedo diamond is used for axial reduction and to produce chamfer finish line.

Axial Finishing:

·         Torpedo bur is used to produce a distinct finish line and round of the angles of the preparation.

Grooves:

·         No: 171 bur is used to prepare a mesial and a facial groove correctly aligned with the path of insertion.

Flares:

·         A flame diamond and carbide bur is used to make the mesial and facial flares which are narrow at the gingival end and wider occlusally.

Occlusal Offset:

·         No: 171 bur is used to produce a 1.0 mm wide ledge on the lingual incline of the facial cusp connecting the grooves, assuming the shape of an inverted V. Its floor is perpendicular to the path of insertion

Occlusal Finishing Bevel:

·         A flame diamond or no: 170 bur is used for creating an  occluso facial finishing bevel. The bevel is rounded up to the proximal occlusal line angle to blend with the proximal flares.

Features of the Seven-Eighth Crown Preparation:

                           

Proximal Half Crowns

Occlusal Reduction:

·         Round end tapered diamond and no: 171 bur. 

·         Depth orientation grooves on the distal aspect of the occlusal surface 1.5mm deep. Since the mesial segment of the tooth is dropped below the occlusal plane the grooves may not be as deep.

·         The tooth structures between the orientation grooves are removed following cuspal contours.

Functional Cusp Bevel:

·         Is done using round end tapered diamond and no: 171 bur. Depth orientation grooves are placed across the facial occlusal line angle of the mandibular molar. The bevel will be shorter and shallower on the mesial cusp than on the distal.

Mesial Axial Reduction:

·         Torpedo diamond is used for the axial reduction by making the path of insertion of mesial surface parallel with the long axis of the premolar abutment.

·         The end of the diamond will eventually make contact with the tooth in the gingival area of the mesial surface and produce a chamfer finish line.

Facial and Lingual Axial Reduction:

·         Is done with a tropedo diamond producing a definite chamfer finish line at the same time. End the extension 1.0 mm or more mesial to the disto-facial embrasure.

Axial Finishing:

·         Torpedo bur is used to produce a distinct finish line and round of the angles of the preparation.

Occlusal Isthmus and Countersink:

·         No: 171 bur is used to make an occlusal isthmus and a countersink in the distal fossa to supplement retention, resistance and structural durability.

Facial and Lingual Grooves:

·         No: 171 bur is used to make parallel grooves on the facial and lingual surfaces with in 1.0 mm of the vertical distal extension of preparation.

·         The grooves should be parallel to the mesial surface of the tooth and the long axis of the other abutment.

Distal Occlusal Offset:

·         A V-shape offset 0.5 to 1.0 mm from the distal occlusal finish line connecting the lingual groove to the countersink to the facial groove is made using no: 171 bur.

Flares and Occlusal Bevel:

·         A flame diamond is used to place flares distal to the facial and lingual grooves.

·         A bevel is cut along the distal marginal ridge with the flame diamond taking care not to extend into the distal occlusal embrasure.

Features of the Proximal Half Crown Preparation:

              

 

RESIN BONDED FIXED PARTIAL DENTURE:

Definition:

Resin bonded prosthesis:                                                                                                                                                  A prosthesis that is luted to tooth structure, primarily enamel, which has been etched to provide mechanical retention for the resin cement. -GPT-7.

Indications:

•          Replacement of missing anterior teeth in children and adolescents.

•          Short span.

•          Unrestored abutments.

•          Single posteior tooth.

•          Significant crown length.

•          Excellent moisture control.

Contraindications:

·         Para functional habits.

·          Long edentulous spans.

·         Restored or damaged abutments.

·         Compromised enamel.

·         Deep vertical overlap.

·         Nickel allergy.

Advantages:

•          Minimal removal of tooth structure

•          Minimal potential for pulpal trauma

•          Anaesthesia not usually required

•          Supragingival preparation

•          Easy impression making

•          Provisional not usually required

•          Reduced chair time

•          Reduced patient expense

•          Rebond possible.

Disadvantages:

•      Reduced restoration longevity

•      Enamel modifications are required.

•      Space correction is difficult

•      Good alignment of abutment teeth is required.

•      Esthetics is compromised on posterior teeth.

Fabrication:

•          In the fabrication of resin‑retained FPDs, following three phases is necessary for predictable success:

1. Preparation of the abutment teeth

2. Design of the restoration

3. Bonding

Preparation of the abutment teeth:

•          Common principles dictate tooth preparation design.

–   A distinct path of insertion must exist, proximal undercuts must be removed to provide "planes of metal" on the lingual and proximal surfaces,

–   occlusal rest seats and proximal groove slots must provide resistance form, and a definite and distinct gingival margin should be established wherever possible

.

•          The tooth preparation includes:

Ø  axial reduction and

Ø  guide planes on the proximal surfaces with a slight extension onto the facial surface to achieve a faciolingual lock.

Ø  should encompass at least 180 degrees of the tooth to enhance the resistance

Ø  extended as far as possible to provide maximum bonding area

Ø  should be a finish line which is placed about 1.0 mm supragingivally.

–    Occlusal clearance is needed on very few teeth: 0.5 mm is needed on maxillary incisors

–    Vertical stops are placed on all the preparations consisting of two or three flat countersinks on the lingual surface of an incisor, a cingulum rest on a canine or an occlusal rest seat on a premolar or molar.

–    Wilkes found rests to be the dominant feature in a preparation, contributing to both resistance and rigidity. The occlusal rest directs the applied force from the pontic to the abutments.

–     Barrack strongly recommends the use of two rests.

–    The resistance features used in a tooth preparation for an acid etched resin‑bonded retainer will normally be grooves.

–    31% to 77% in  study by saad etal and  81% in study by burgess et al found grooves were found to increase resistance to displacement on anterior preparations.

–    If there is an existing amalgam, all of the amalgam, or at least all of its surface, is removed so that the box form can be utilized.

–     BARRACK  et al in his study proposed that the entire occlusal outline of the existing amalgam restoration is included within the outline of the retainer's occlusal rest.

–    HEMBREE et al in his study stated if the retainer margins cross over an amalgam‑enamel margin; there is a high probability of leakage occurring around that margin.

 Preparation of posterior tooth:

•          The basic framework for the posterior resin­retained FPD consists of three major components:

–   occlusal rest (for resistance to gingival displace­ment),

–   retentive surface (for resistance to occlusal displacement), and

–   proximal wrap and proximal slots (for  resistance to torquing forces).

•          A spoon‑shaped occlusal rest seat is placed in the proximal marginal ridge area of the abutments adjacent to the edentulous space. An additional rest seat may be placed on the opposite side of the tooth.

•          Proximal and lingual axial surfaces are reduced to lower their height of contour to approximately 1 mm. from the crest of the free gingiva.

•          Proximal surfaces are prepared so that parallelism results

•          Occlusally, the framework should be extended high on the cuspal slope, well beyond the actual area of enamel recontouring

•          A single path of insertion should exist. The alloy framework should be designed to engage at least 180 degrees of tooth structure when viewed from the occlusal.

•          This proximal wrap enables the restoration to resist lateral loading by engaging the underlying tooth structure and is assisted in this re­gard by grooves in the proximal just lingual to the buccal line angle. Distal to the edentulous space, the retainer resistance is augmented by a groove at the lingual proximal line angle.

•           Moving a properly designed resin‑bonded FPD in any direction except parallel to its path of insertion should,not be possible, nor should it be possible to displace any tooth to the buccal from the framework

•          Preparation differs between maxillary and mandibular molar teeth only on the lingual surfaces. The lingual wall of the mandibular tooth may be prepared in a single plane. The lingual surface of the maxillary molars requires a two‑plane reduction due to occlusal function and the taper of these functional cusps in the occlusal two thirds.

•          Lingually inclined  mandibular molars (this may require a two­plane modification)

•      Preparation for mandibular 1 premolar:

Ø  Placement of rest seat would leave very little area in the lingual cusp of premolar lingual cusp coverage when it does not interfere in occlusion is an excellent means of incerasing surface area and reinforcing the retainer

Posterior resin bonded fixed partial denture framework configurations:

Standard -

•          2 grooves one near the facioproximal angle adjacent to the edentulous space and one at the opposite linguoproximal corner, with 180° of axial wall convergence.

•          Two rests: this variation suggested by barrack, has axial coverage on both proximal walls and the rest seats located near the central groove at the mesio-occlusal and disto-occlusal. They resist displacement by occlusal forces.

•          Loops: these features are formed by occlusal inlays being joined to a groove on a lingual or proximal surface. They brace the arms.

•          Lingual cusp coverage: lingual cusps of mandibular molars and premolars can be covered to bolster the retainer againist deformation.

•          Tilted molars: mesial and particularly mesiolingual tipped molars are out of occlusion can be covered to improve occlusion and remove sublingual food traps.

Other design concepts:

•          Occasionally, a combination restoration can be used. This type of FPD includes a resin‑bonded re­tainer on one of the abutment teeth and a conven­tional cast restoration on the other

•          Periodontal splinting is the most demanding of the restoration designs; splints and splint‑FPD combinations require care in designing adequate mechanical retention

•          The posterior FPD splint uses multiple rests and distinct mechanical retention of the abutment in the retainer, which can be important when the abutment is the most distal tooth in the arch.

•          The anterior splint must engage as much enamel as possible to aid in retention and is more demanding in tooth alignment and preparation design.

REFERENCES:

•                  Fundamentals of fixed prosthodontics-H.T.Shillingberg

•                  Contemparary fixed prosthodontics-S.F.Roseintiel.

•                  Tylman’s theory of practice of fixed prosthodontics.

•                  Fundamentals of tooth preparations -H.T.Shillingberg, Jacobi, Brackett

•                  Tjan AHL and Miller GD. Biometric guide to groove placement on three quarter crown preparations. J Prosthet Dent. 1979; 42: 405-410.

•                  Kishimoto M, Shillingburg T, and Duncanson MG. Influence of preparation features on retention and resistance. Part II: Three quarter crown. J Prosthet Dent. 1983; 49: 188-192.

•                  Potts RG, Shillingburg HT Jr, Duncanson MG Jr Retention and resistance of preparations for cast restorations.J Prosthet Dent. 1980 Mar; 43(3):303-8.