Failures in Fixed Partial Dentures Dr Rohit Fernandez

Failures in Fixed Partial Dentures Dr. Rohit Fernandez

Fixed prosthodontic treatment can offer exceptional satisfaction for both patient and dentist. It can transform an unhealthy, unattractive dentition with poor function into a comfortable, healthy occlusion capable of giving years of further service while greatly enhancing esthetics. To achieve such success, however, requires meticulous attention to every detail from initial patient interview, through the active treatment phase, to a planned schedule of follow-up care. Failure to achieve the desired specifications of design for function and esthetics would result in failure of the prosthesis.

The causes of FPD failures were summarized as early as in 1920 when Tinker wrote “ Chief among the causes for such disappointing results have been: First : Faulty, and in some cases, no attempt at diagnosis and prognosis. Second: Failure to remove foci of infection in attention to treatment and care of the investing tissues and mouth sanitation. Third: Disregard for tooth form Fourth: Absence of proper embrasures Fifth: Inter-proximal spaces Sixth: Faulty occlusion and articulation”

CLASSIFICATION

Bennard G. N. Smith 1. Loss of retention 2. Mechanical failure of crowns or bridge components a. Porcelain fracture b. Failure of solder joints c. Distortion d. Occlusal wear and perforation e. Lost facings 3. Changes in the abutment tooth a. Periodontal disease b. Problems with the pulp c. Caries d. Fracture of the prepared natural crown or root e. Movement of the tooth

4. Design failures a. Under-prescribed FPDs b. Over-prescribed FPDs 5. Inadequate clinical or laboratory technique a. Positive ledge b. Negative ledge c. Defect d. Poor shape and color 6. Occlusal problems

John F. Johnston 1. Discomfort a. Malocclusion or premature contact b. An oversized or poorly positioned mastication area, with retention of food by pontics or retainers. c. Torque produced from the seating of the bridge or from occlusion d. An excess of pressure on the tissue e. Plus or minus contact area f. Over protected or under protected gingival and ridge tissue. g. Thermal shock

2. Looseness of FPD a. Deformation of the metal casting on the abutment b. Torque c. Technique of cementation d. Solubility of cement e. Caries f. Mobility of one or more abutments g. Lack of full occlusal coverage h. Insufficient retention in the abutment preparation i. Poor initial fit of the casting.

3. Recurrence of caries a. Over extension of margins b. Short castings c. Open margins d. Wear e. A retainer becoming loose f. Pontic form that fills the embrasure g. Poor oral hygiene h. Use of wrong type of retainer, which will promote caries susceptibility i. Permanent displacement of the gingiva due to temporary protection

4. Recession of supporting structure a. Length of the span b. Size of the occlusal table c. Embrasure form d. Few extensions of the cervical margins e. Impression technique can also stimulate recession of the gingiva. 5. Degeneration of Pulp 6. Fractures of bridge components a. A faulty solder joint b. Incorrect casting technique c. Overwork of the metal due to length of the span or parts that are too small.

7. Loss of veneers a. Little retention b. Badly designed metal protection c. Deformation of the protecting metal d. Malocclusion e. Improper fusing or technique 8. Loss of function a. They don’t function in occlusion b. They have no contact with opposing teeth c. They have permanent contact d. Over carved or under carved occlusal surface may impair efficiency e. Loss of opposing or approximating teeth

9. Loss of teeth tone or form a. Pontic design b. Position and size of the joints c. Embrasure form d. Over contouring or under contouring of retainers e. Oral hygiene practiced by the patient 10. Failure to seat a. The abutment preparations may not be near parallel b. Soldering assembly may have been incorrect, or relationship of the retainers may have been altered during soldering.

TYPES OF BRIDGE FAILURE I. Cementation failure II. Mechanical failure III. Gingival and periodontal breakdown IV. Caries V. Necrosis of pulp VI. Biomechanical failure VII. Esthetic failure

I. CEMENTATION FAILURE

Cementation failures can be broadly divided into: 1. CEMENT FAILURE 2. RETENTION FAILURE 3. OCCLUSAL PROBLEMS 4. DISTORTION OF FPD

1. CEMENT FAILURE 2. The primary function of the luting agent is to provide a seal preventing marginal leakage and pulp irritation. The luting agent should not be used to provide significant retentive and resistive forces. An ideal luting agent would have the following properties: 1. Adequate working time 2. Adhere well to both tooth structure and metal surface 3. Provides a good seal 4. Non toxic to the pulp 5. Have adequate strength properties 6. Be compressible into thin layers 7. Have low viscosity and solubility 8. Exhibit good working time and setting properties

Besides an inadequate retainer, failure can also occur because of a poor cementation technique. This maybe due to the wrong choice of material, failure to observe the manufacturer’s mixing instructions, the use of old or contaminated material, an inadequate powder/liquid ratio, or the insertion of the prosthesis when the cement has started to set. An inadequately cemented restoration may cause 1. An increased vertical dimension of occlusion 2. A loosening of the crown or FPD after a relatively short time 3. Leakage and decay under the abutment 4. The unsightly appearance of a metal margin where originally the metal was concealed under the gingiva 5. Sensitivity to sweets or brushing due to exposure of the cervical end of the tooth

Causes of cement failure 1) 2) 3) 4) 5) 6) 7) 8) 9) 10) Cement selection Old cement Prolonged mixing time Thin mix Cement setting prior to seating Inadequate isolation Incomplete removal of temporary cement Thick cement space Inclusion of cotton fibers Insufficient pressure

2. RETENTION FAILURE For a restoration to accomplish its purpose, it must stay in place on the tooth. No cements that are compatible with living tooth structure and the biologic environment of the oral cavity possess adequate adhesive properties to hold a restoration in place solely through adhesion. The geometric configuration of the tooth preparation must place the cement in compression to provide the necessary retention and resistance. CAUSES FOR RETENTION FAILURE 1) Excessive taper 2) Short clinical crowns 3) Mis-fit 4) Misalignment

Excessive taper : As a cast metal or ceramic restoration is placed on or in the preparation after the restoration has been fabricated in its final form, the axial walls of the preparation must taper slightly to permit the restoration to seat Theoretically, the more nearly parallel the opposing walls of the preparation are, the greater should be the retention. Recommendations for optimal axial wall taper of tooth preparations for cast restorations ranged from 10 to 12 degrees. Tooth preparation taper should be kept minimal because of its adverse effect on retention, but Mock estimates that a minimum taper of 12 degrees is necessary just to insure the absence of undercuts.

Short clinical crown : Cement creates a weak bond largely by mechanical interlocks between the inner surface of the restoration and the axial wall of the preparation. Therefore, the greater the surface area of the preparation the greater is its retention. The preparations on large teeth are more retentive than preparations on small teeth. A short, over-tapered or short clinical crown would be without retention as there would be many paths of removal. For the restoration to succeed, 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. A shorter wall cannot afford this resistance. The walls of short preparations should have as little taper as possible.

Clinical conditions with excessive taper and short clinical crowns should be treated with : - 1. In case of excessive taper: a. Incorporation of proximal grooves. b. Additional retentive grooves (should be along with the path of insertion). c. Additional pins 2. In case of short crowns: a. Crown lengthening procedure b. Modification of supra-gingival margin to sub-gingival margin c. Additional retentive grooves and proximal box d. Incorporation of pins e. Addition of extra abutments

Misfit : The fit of casting can be defined best in terms of the “misfit” measured at various points between the casting surface and the tooth. The measurement of misfit at different locations and geometrically related to each other and defined as : 1. Internal gap 2. Marginal gap 3. Vertical marginal discrepancy 4. Horizontal marginal discrepancy 5. Over-extended margin 6. Under-extended margin

Causes for misfit : a. Expansion of the metal substructure b. Improper water / powder ratio c. Improper mixing time d. Improper burnout temperature e. Distortion of the margins (towards the tooth surface) f. Distortion of the metal substructure g. Metal bubbles in occlusal or marginal regions i. Inadequate vacuum during investing ii. Improper brush technique iii. No surfactant h. Porcelain flowed inside the retainer i. Excessive oxide layer formation in inner side of the retainer (due to contaminated metal or repeated firing of porcelain) j. Tight contact points k. Thick cement space l. Insufficient pressure during cementation procedure

Misalignment : In case of the fixed FPD, it is more difficult to differentiate whether a FPD is not seating because of a faulty fit, or the alignment of the retainers relative to each other is incorrect. The only difference which may sometimes be apparent is that, in the case of misalignment the FPD will have some ‘spring’ in it and tend to seat further on pressure due to the abutment teeth moving slightly, whereas in the case of a defective fit, the resistance felt will be solid.

Causes for misalignment a. Abutment displacement due to improper temporization. b. Distortion of wax pattern while sprueing and investing. c. Casting defects. d. Distortion of metal frameworks in porcelain firing. e. Porcelain flow inside the retainers. f. Misalignment of soldering points. g. Insufficient pressure in cementation. h. Thick cement film. i. Excessive metal or porcelain in tissue surface (ridge lap) of pontic prevents the proper seating of FPD and open margin (can be detected by observing the blanching of the tissue or patient may complain of pressure on the pontic region).

3. OCCLUSAL PROBLEMS Following the placement of a dental restoration, a patient might report discomfort ranging from a feeling of ‘lameness’ to ‘severe and constant pain’. Sensitivity, in most cases, is due to pulp irritation from traumatic contact or greater leverages. When the occlusion has been adjusted, each type of discomfort may be relieved almost instantly and should disappear shortly.

Causes in occlusal problems 1. Immediate problems Ø Occlusal interference Ø Marginal ridges at different levels Ø Supra eruption of the opposing tooth Ø Parafunctional habits 2. Delayed problems Ø Wearing of occlusal surface Ø Loss of occlusal contacts Ø Perforation of occlusal surface due to • Porcelain Vs resin • Porcelain Vs gold Ø Food lodgment due to plunger cusp Ø Fracture of facing due to defective occlusal contact Ø Periodontal or gingival breakdown due to improper occlusal contacts Ø Tenderness due to food lodgment

4. DISTORTION OF FPD Margin Integrity The completed restoration should go into place without binding of its internal aspect against the occlusal surface or the axial walls of the tooth preparation. In other words, the best adaptation should be at the margins. If the indirect procedure is handled properly, there should be no noticeable difference between the fit of a restoration on the die and that in the mouth.

Causes for failure in marginal integrity: 1) Bending of FPD (wax patterns and metal substructure) • In waxing stage • Removal from the die • Spruing stage • Investing stage (thick mix of investment distort or displace the wax pattern) 2) Incomplete casting • Wax patterns too thin • Incomplete wax elimination • Cold mold or melt • Inadequate metal

3) Rough casting • Improper finishing of wax pattern • Excess surfactant • Improper water powder ratio • Excessive burnout temperature • Improper devesting (direct hit on the metal framework) 4) Bending of long span FPDs • Thin crown • Soft metal • Heat treatment not being done • Porosity in the metal • Distortion of the metal substructure during the porcelain firing • Contaminated metal

Inadequate FPD design Designing FPDs is difficult. It is neither a precise science nor a creative art. It needs knowledge, experience and judgment, which takes years to accumulate. So it is not surprising that some designs of FPDs, even though well intentioned and consciously executed, fail. A simple classification of these failures is as; under-prescribed and over-prescribed FPDs. Under-prescribed FPDs – these include designs that are unstable or have too few abutment teeth. E. g. , a cantilever FPD carrying pontics that cover too long a span or a fixed movable FPD where again the span is too long or where abutment teeth with too little support has been selected. Another ‘under-design’ fault is to be too conservative in selecting retainers. E. g. , intra coronal inlays for fixed FPDs. With these design faults, little can be done other than to remove the FPD and use another type of replacement.

Over-prescribed FPDs – cautious dentists will somewhere include more abutment teeth than necessary, and fate usually dictates that it is the unnecessary retainer that causes fault. Some use the upper canines and both premolars on each side in replacing the four incisor teeth. As well as being destructive, this gives rise to unnecessary practical difficulties in making the FPD. This, in turn, reduces the chance of the FPD being successful.

Several suggestions have been proposed scientifically to explain the distortion resulting in metal frameworks after the various stages of the porcelain firing schedule; these include: a) b) c) d) e) f) g) h) i) Contraction of the porcelain with subsequent metal deformation Contamination of the casting, reducing its melting temperature Grain growth of the alloy, constricting the diameter of the crown Plastic flow and creep of the porcelain gold alloy at high temperatures Reduction in the resiliency of the metal due to the rigidity of porcelain Improper support of the framework during firing Inadequate framework design at the gingival level inadequate design of the framework as a whole Shillingburg stated that ceramic metals require a certain amount of j) bulk in the cervical area to resist distortion when subjected to the

II MECHANICAL FAILURES

In the design of fixed partial denture pontics, if insufficient attention is given to mechanical principle, the prognosis will be compromised. Mechanical problems might be due to poor diagnosis and treatment plan, improper choice of materials, poor framework design, poor tooth preparation, or poor occlusion. These could lead to fracture of the prosthesis or displacement of the retainers. It is therefore important to evaluate the likely forces on a pontic and to design it accordingly. For example, a strong all metal pontic may be needed in situations of high stress rather than a metal ceramic pontic which could be more susceptible to fracture.

Classification of mechanical failure 1. Retainer failure 2. Pontic failure 3. Connector failure

1. RETAINER FAILURE 1) Perforation 2) Marginal discrepancy 3) Facing failure Fracture Wearing Discoloration

1) Perforation 2) a) b) c) d) e) f) g) h) i) j) k) Causes Insufficient occlusal reduction Insufficient occlusal material High points in opposing dentition (plunger cusp) Premature contacts Contaminated metal Porosity in metal work (subsurface, back pressure, suck back) Due to improper melting temperature Improper pattern position Improper sprue (too thin) Improper location Parafunctional habits

2) Marginal discrepancy Causes a) Selection of margin b) Improper preparation and failure to establish the margin properly c) Failure to do gingival retraction prevents definite margin location and subsequently in impression d) Selection of the impression material i. Shrinkage in material (condensation silicon) ii. Distortion of material (alginate) e) Improper impression procedures f) Voids in the impression g) Variation in pressure application in wash technique h) Delayed pouring of die material i) Distortion of wax patterns at margins

j) Insufficient flow of metal k) Shrinkage of metal l) Nodules in margins and inner side of coping i. Due to inadequate vacuum during investing ii. Improper brushing technique iii. No surfactant m) Excessive sand blasting n) Distortion due to degassing procedure o) Open margins due to porcelain shrinkage (opaque porcelain) p) Thick cement q) Cement setting prior to seating r) Insufficient pressure application during cementation

3. Facing failure Types of veneer failures a) Fracture b) Wearing of facing (resin veneers) c) Discoloration

Causes for veneer fracture: i. Too little retention (mechanical) ii. Badly designed metal protection iii. Deformation of the protecting metal iv. Malocclusion v. Micro-leakage between metal and facing vi. Improper curing or fusing technique vii. Excessive oxide layer formation

Cause of wearing of facing: i. Improper curing or fusing technique ii. Deep bite (decreased overbite in lower anteriors) iii. Acrylic veneering opposing porcelain teeth iv. Faulty brushing techniques and flossing v. Parafunctional habits

Causes of discoloration: i. Absorption of oral fluids ii. Absorption of artificial food colouring agents through micro-cracks or microleakage in metal and facing interfaces iii. Tarnish of underlying metal and facing (greening of porcelain in silver alloys) iv. Micro-cracks due to malocclusion

2. PONTIC FAILURE Factors affecting selection and failure of pontics 1) Pontic space 2) Residual ridge contour 3) Biological consideration a. Ridge relation b. Dental plaque c. Gingival surface of pontic (Contact with mucosa) i. Mucosal contact ii. Non mucosal contact 4) Pontic ridge relationship 5) Pontic material 6) Biocompatibility 7) Occlusal forces 8) Metal substructure support

1) Pontic space 2) One function of an FPD is to prevent tilting or drifting of the adjacent teeth into the edentulous space. If such unwanted movement has already occurred the space available for the pontic may be reduced and its fabrication may be complicated. Under these circumstances it is often impossible to create an acceptable appearance without repositioning the abutment teeth orthodontically where aesthetics is important. 3) Even with a less aesthetic requirement, as for posterior teeth, overly small pontics are unacceptable because they trap food and are difficult to clean. When orthodontic repositioning is not possible, it may be better to increase the proximal contours of adjacent teeth than to make an FPD with undersized pontics.

2) Residual ridge contour The contour texture of the edentulous ridge should be carefully evaluated during the treatment planning phase. An ideally shaped ridge will be smooth, for this is the easiest to maintain plaque free. Unfortunately, many patients present with irregular hypoplastic tissue, particularly where an ill fitting RPD has been in place, and under these circumstances , surgical removal of the excess fibrous tissue may be recommended. Some patients suffer severe bone resorption following tooth loss, particularly if the loss occurred due to trauma. These patients can present a significant aesthetic challenge. Surgical ridge augmentation (e. g. , with hydroxyl apatite) may be one solution. Another surgical procedure that has been proposed is to create a role of soft tissue labial to the pontic site, which will enhance the illusion that a tooth is growing out of the gingival tissue.

3) Biological consideration The biologic principles of pontic design pertain to the maintenance and preservation of the residual ridge, abutment and opposing teeth and supporting tissues. Factors of specific influence are: a) Pontic ridge contact b) Removal of dental plaque c) Gingival surface of the pontic

a)Pontic ridge contact b) Pressure free contact between the pontic and the underlying tissues is indicated to prevent ulceration and inflammation of the soft tissues. If any blanching of the soft tissue is observed at try in, the pressure areas should be identified with pressure indicating paste and the pontic re-contoured until tissue contact is entirely passive. c)b)Dental plaque The chief cause of ridge irritation is the toxins that are released from microbial plaque, which accumulates between the gingival surface of the pontic and the residual ridge causing tissue inflammation and calculus formation. Unlike a RPD, a FPD cannot be taken out of the mouth daily for cleaning. To enhance plaque control, the patient must be taught to perform efficient oral hygiene techniques, with particular emphasis on cleaning the gingival surface of the pontic. The shape of the gingival surface, its relation to the ridge, and the materials used in its fabrication will influence the success of these measures.

c) Gingival surface of the pontic Where aesthetics is of concern in the anterior region of the mouth, the pontic should contact the gingival tissue on the labial or buccal aspect to give an appearance of ‘emerging from the tissue’. In the posterior region, like the mandibular premolar and molar areas more attention should be given to occlusion, function and hygiene. Considering these aspects, pontic contacts may be classified into different groups: mucosal and non mucosal contacts based on the shape of the gingival surface and its relationship with the underlying tissue. Normally, where tissue contact occurs, the gingival surface of a pontic is inaccessible for cleaning with a tooth brush. Therefore, the patient must develop excellent hygiene habits and the use of devices such as proxibrushes, pipe cleaners and dental floss.

A pontic with a concave fitting surface that overlaps the residual ridge bucally and lingually is called a saddle. This is avoided because the gingival surface cannot be easily cleaned. An egg shaped or bullet shaped pontic is probably easiest for the patient to keep clean. It should be made as convex as possible, with only one point of contact at the center of the residual ridge. This design is recommended for the replacement of mandibular posterior teeth because aesthetics is of less concern here.

4) Pontic ridge relationship Since 1918 it has been a popular concept that the tissue surface of a mandibular posterior pontic should sometimes be left well clear of the residual ridge. This design was often called ‘hygienic’ or ‘sanitary’. The hygienic design permits easier plaque control by allowing gauze strips and other cleaning devices to be passed under the pontic and seesawed in shoeshine fashion. There are disadvantages to the design as well. Food particles tend to become trapped, which may lead to tongue habits that are annoying to the patient. The hygienic design also is contraindicated if minimum vertical space exists and where esthetics is important; tissue proliferation can occur when the pontic is too close to the residual ridge, forgoing the originally intended advantages.

5) Pontic material Any material chosen to fabricate the pontic should provide good aesthetic results where needed, biocompatibility, rigidity and strength to withstand occlusal forces, and the desired longevity. FPDs, during mastication or parafunction, may impinge upon the gingiva and also the veneering material may fracture. In the fabrication of metal-ceramic FPDs, the porcelain on the occlusal surfaces should be carefully evaluation. Porcelain is a brittle material and may fracture easily. When a metal-ceramic restoration is chosen, it is of paramount importance to design the metal substructure properly if flexure and porcelain fracture is to be avoided. Occlusal contacts should not fall on the junction between metal and porcelain during centric and eccentric contacts.

6) Biocompatibility Glazed porcelain is generally considered to be the most biocompatible of the available pontic materials and clinical data tends to support this opinion, although the critical factor seems to be the material’s ability to resist accumulation of plaque rather than the material itself. Highly glazed porcelain is relatively easy to clean, making plaque removal from it easier than from other materials. For ease of plaque removal, it is recommended that the tissue surface of the pontic be made in glazed porcelain whenever possible. Well-polished gold is smoother, less prone to corrosion, and less retentive of plaque than an unpolished or porous casting.

7) Occlusal forces Reducing the buccolingual width of the pontic by as much as 30% has long been suggested as a means of lessening occlusal forces on abutment teeth. Narrowing the occlusal table may actually impede or even preclude the development of a harmonious and stable occlusal relationship. Like a malposed tooth, it may cause difficulties in plaque control as well as fail to provide proper cheek support. For these reasons, pontics with normal occlusal widths are generally recommended. Mechanical failure of the pontic may occur because of inadequate strength. Thus an all-porcelain occlusal pontic should never be used unless the bite is favourable.

8) Compromised metallic substructure Causes a. Limited edentulous space occluso-cervically due to supraeruption of opposing tooth. b. Limited space mesiodistally due to migration or drifting of adjacent tooth.

How to avoid: a. The framework must provide a uniform veneer of porcelain (approx 1. 2 mm). excessive thickness of porcelain contributes to inadequate support and predisposes to eventual fracture. This is often true in the cervical portion of an anterior pontic. A reliable technique for ensuring uniform thickness of porcelain is to wax the fixed prosthesis to complete anatomic contour and then accurately cut back the wax to a pre-determined depth. b. The metal surfaces to be veneered must be smooth and free of pits. Surface irregularities will cause incomplete wetting by the porcelain slurry, leading to voids at the porcelain metal interface that reduces bond strength and increases the possibility of mechanical failure.

c. Sharp angles on the veneering surface should be rounded. They produce increased stress concentrations that could cause mechanical failure. d. The location and design of external metal porcelain junctions need particular attention. Any deformation of the framework at the junction can lead to chipping of the porcelain. For this reason occlusal centric contacts must be placed at least 1. 5 mm away from the junctions. Attention must be paid to excursive eccentric contacts that might deform the metal ceramic interface.

3. CONNECTOR FAILURE The connector is that part of the FPD or splint that joins the individual components (retainers and pontics) together. Requirements of solders are their ability to resist tarnish and corrosion, to be free flowing, to match the colour of the units to be joined and to be strong. These factors also depend on the chemical composition of the solder. Casting can make a rigid connection as part of a multi unit wax pattern or by soldering which involves the use of an intermediate metal whose melting temperature is lower than that of the parent metal. The parts being joined are not melted during soldering, but they must be thoroughly wettable by liquefied solder. Dirt or surface oxide can reduce wetting and impede successful soldering.

Causes for connector failure a. Improper selection of connector b. Thin metal at the connector c. Incorrect selection of solder d. Solder gap – narrow or wide e. Porosity f. Insufficient metal around g. Defective occlusal contacts over thin connectors

III GINGIVAL AND PERIODONTAL PROBLEMS

Margins are one of the most important and weakest links in the success of FPD restorations. One of the prime goals of restorative therapy is to establish a physiologic periodontal health. A successful prosthesis depends on a healthy periodontal environment and periodontal health depends on the continued integrity of the prosthodontic restoration. All displacement techniques have the potential damage gingiva, attachment apparatus and bone, especially if anatomic forms are weak or if disease is present. In healthy patients, properly used cord displacement or copper band methods have proved to be atraumatic.

The margin is one of the components of the cast restoration most susceptible to failure, both biologically and mechanically. Most of the investigative proof shows that supragingival margins are kinder to the gingiva than are subgingival margins. However, practicality dictates that supragingival margins are not always usable There are three locations in which to prepare crown margins: Ø Supragingival Ø At the crest of the gingiva Ø Subgingival

SUPRAGINGIVAL Vs SUBGINGIVAL MARGINS: Whenever possible, the margin of the preparation should be supragingival. Subgingival margins of cemented restorations have been identified as a major factor in periodontal diseases, particularly where they encroach on the epithelial attachment. Supragingival margins are easier to prepare accurately without trauma of the soft tissues. They can usually also be situated on hard enamel, whereas subgingival margins are often on dentin or cementum.

SUPRAGINGIVAL MARGINS ADVANTAGES: Ø They can be easily finished Ø They are more easily cleaned Ø Impressions are more easily made, with less potential for soft tissue damage Ø Restorations can be easily evaluated at recall appointments DISADVANTAGE: Ø Aesthetically not indicated for anterior region Ø Metal can be seen Ø Not indicated in short clinical crowns Ø The proximal contacts extend to the gingival crest Ø In case of root sensitivity

SUBGINGIVAL MARGINS SPECIFIC DEMANDS FOR SUBGINGIVAL MARGINS: Ø Aesthetic demands Ø Caries removal ØTo cover existing subgingival restorations Ø To gain needed crown length Ø To provide more favourable crown contour DISADVANTAGES: Ø Difficult for preparation Ø Gingival management should be perfect Ø Prone for soft tissue trauma Ø More prone for gingival and periodontal pathosis Ø Difficult to maintain oral hygiene Ø Metal margins can be seen thru the gingiva

SOFT TISSUE PORBLEMS: GENERALIZED (Not due to bridge) LOCALISED (May be due to bridge) Causes for soft tissue problems: Ø Over / under contouring Ø Narrow embrasures Ø Over / under extended crowns Ø Pressure of pontic over tissue Ø Loss of contact Ø Horizontal food impaction due to plunger cusp in the opposing arch Ø Marginal ridges at different levels Ø Wide occlusal table Ø Trauma from occlusion Ø Parafunctional habits Ø Acrylic facing in contact with gingiva

RESULTS OF IMPROPER CONTACT AREAS Ø Cause displacement of teeth bucally, lingually, mesially and distally. Ø Exert a lifting force on the tooth when placed too high occlusally. Ø Disturb the axial relation of the teeth, resulting in trauma. Ø Cause rotation of the teeth. Ø Cause injury to the investing structures by excessively opening or closing the contact and interproximal embrasures. Ø Disturb the coordination of the inclined planes and cusps causing deflective occlusal contacts. Ø Cause vertical or horizontal food impaction.

OVER EXTENDED CROWN The over extended crown usually encroaches beyond the cut of the preparation on the tooth and the excess beyond the margin of the preparation is usually not in contact with the tooth surface. This overhang impinges the gingival tissue, irritates and often causes edema and proliferation of the gingival tissue, destruction of the marginal alveolar bone and ultimate loss of the tooth. The overextension of the crown is usually due to inaccurate technique and / or the dentists desire to ‘play safe’ by making it long enough to cover the preparation or to extend beneath the gingival margin.

SHORT CROWN The short crown fails to cover the cut surface of the prepared tooth and often does not extend below the gingival margin. This uncovered ground tooth surface is often sensitive to sweets and to temperature changes and invites development of caries and causes gingival irritation. Also, it is usually due to inadequate technique and a willingness of the dentist to accept impressions that are incomplete. CONTOUR The poorly contoured crown is one which may have an excess contour that impinges on the gingival tissue and deflects food over and away from this tissue, thereby depriving it of its normal stimulation; or it may be under contoured and permit the impaction of food into the gingival crevice, thereby stripping the gingival tissue away from the tooth. Either will cause irritation of the surrounding tissue and may lead to the loss of the tooth.

IV CARIES

CAUSES Iatrogenic (dentists role) ü Failure to identify caries ü Incomplete removal of caries ü Rough abutment finishing margins ü Subgingival marginal placement in inaccessible areas or regions ü Burning of root dentin or cementum in electro surgical technique (leads to damage or rough surface and causes plaque retention) ü Overhanging margins ü Rough margins of crowns or bridges ü Over contouring of the cervical thirds of crowns or bridges prevents the physiologic too cleaning by tongue or muscles ü Marginal discrepancy ü Thick cement space in margins leads to cement dissolution. ü Narrow embrasures (inaccessibility to maintain hygiene) ü Wide connector ü Failure to motivate or educate the patient about oral hygiene

Patient role ü Systemic factors ü Xerostomia ü Due to radiation therapy ü Drug induced ü Endocrine disorders ü Epilepsy (difficult to maintain the oral hygiene) ü Rheumatoid arthritis ü Local factors ü Improper brushing and flossing ü Dietary habits ü Failure to understand importance of oral hygiene.

V PULP DEGENERATION

Supporting structures or root length may be lost owing to periapical involvement brought about by the method of preparation, lack of protection of the prepared abutment teeth during construction, hidden caries, and malocclusion. The preparation of the abutment tooth and the building of the FPD, irritation from temporary coverage, lack of temporary coverage, or malocclusion can activate a latent, low grade pulp infection. There is no method of discovering such pulp conditions and discomfort or pulp degeneration may occur due to such infections. A pulp may degenerate because of too rapid preparation of the tooth or because of improper cooling during preparation. Teeth unprotected during the construction of a FPD are exposed to saliva and the resulting irritation. Pulp reactions to various procedures Each step in full crown preparation presents hazards, which may injure the pulp. The result may be pulpitis or even necrosis. Among the many essential procedures that may cause pulp injury are tooth preparation, impression making, temporization and cementation. In general, heat desiccation and / or chemical injury cause the insult.

VI BIOMECHANICAL FAILURE

Every restoration must be able to withstand the constant occlusal forces to which it is subjected. This is of particular significance when designing and fabricating an FPD, since the forces that would normally be absorbed by the missing tooth are transmitted through the pontic, connectors and retainers to the abutment teeth. The abutment teeth are therefore called upon to withstand forces directly to the missing teeth in addition to those usually applied to them. In addition to 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 the length and inversely with the cube of the occluso-gingival height of the pontic. Compared with the FPD having a single tooth span, a two-toothed pontic span will bend eight times as much, and a three-toothed pontic will bend 27 times as much.

Owing to the fact that forces are being applied though the pontics to the abutment teeth, the forces on the castings serving as retainers are different in magnitude and direction from those applied to a single restoration. The dislodging forces on an FPD retainer tend to act in a mesio-distal direction as opposed to the common bucco-lingual direction of forces on single restorations. Preparations should be modified accordingly to produce greater resistance and structural durability e. g. grooves on buccal and lingual surfaces. Double abutments are sometimes used as a means of overcoming problems created by unfavourable crown-root ratios and long spans. A secondary abutment must have at least as much root area and as favourable a crown-root ratio as the primary abutment it is intended to bolster.

VII ESTHETIC FAILURES

REASONS FOR ESTHETIC FAILURE ü Failure to identify patient expectations regarding esthetics ü Improper shade selection ü Excessive metal thickness at incisal and cervical regions ü Thick opaque layer application ü Surface blistering (chalky appearance) ü Over glazing or too smooth a surface ü Metal exposure in connector, cervical and incisal regions ü Dark space in cervical third due to improper pontic selection (anteriors) ü Failure to produce incisal and proximal translucency ü Improper contouring ü Failure to harmonize contra lateral tooth morphology ü Contour ü Color ü Position ü Angulation ü Discoloration of facing

CONCLUSION

Failures in FPD construction for the most part is due to attempted short cuts or positive indifference and inexcusable ignorance on the part of those concerned with building the prosthesis. Also a FPD can just wear out and this cannot be called as failure and no lifetime guarantee can be given. Failures most often occur because of violation of principles either collectively or individually. This may be due to reactions of the soft tissue and reactions of the abutment. It is better to speak of the level of acceptability to the patient and the dentist and consider what needs to be done to improve the treatment. The fundamentals of fixed prosthodontic therapy modality have to be followed strictly, failure of which will lead to the failure of the prosthesis itself.

REVIEW OF LITERATURE

1. I. Gerson (1957) – stated that inadequately cemented restorations may cause an increased vertical dimension, loosening of the crown or FPD after a relatively short time, microleakage and decay under the abutments, exposure of metal margins and sensitivity. 2. Kenneth C. Pruden (1957) – stated that the multiple abutments should be resorted to only where the added support or root length is needed, and only periodontally sound teeth should be included. 3. Leonard I. Linkow (1962) – gave the importance of form, type, shape and position of contact areas. He stated that the contacts which are flat, open, improperly placed, rough, or poorly polished will lead to displacement of teeth and exert a lifting force, disturb, the axial relationship, injure supporting tissues, produce a deflective occlusal contact and vertical or horizontal food impaction.

4. Marvin Reynolds (1964) – reviewed the properties of porcelain, acrylic resin and gold as related to their use in pontics. He stated that acrylic resin does not permit the precise control of color and light refraction as it absorbs oral fluids and is less resistant to wear and abrasion. Gold alloys provide the best strength to withstand the stresses of occlusion and resistance to wear. Porcelain is superior to other materials as it maintains the color, is resistant to abrasion and is dimensionally stable and insoluble to oral fluids. 5. Richter et al (1970) – studied the tensile strength and compressive strength of cements. The results showed the tensile strength of zinc phosphate, hydrophosphate and Zn. OE cements are equal, carboxylate cements had less strength properties. Compressive strength is lowest with carboxylate and highest with zinc phosphate. 6. Guy M Newcomb (1974) – investigated the location of subgingival margins and related that to gingival inflammation. The results showed that the least inflammation is observed when subgingival crown margins are placed at the gingival crest or just into the gingival crevice.

7. Brule et al (1981) – conducted a study on placement of margins in anterior veneer crowns. He concluded that the subgingival margin placement of anterior crown for esthetics might be unnecessary except in some cases like in short clinical crowns. 8. Abraham Revah et al (1985) – discussed the problems with tilted posterior tooth in relation with path of insertion. He advised the mesial half of the crown be with parallel seating grooves for better seating and retention. 9. Aaron H Wilson et al (1994) – examined the relationship between degree of convergence or a machined metal die and retention of its casting. The retention was found to increase from 0 degree convergence to a peak between 6 -12 degree convergences. Convergence angles of less than 6 degrees are not advisable as it causes incomplete seating of the crown due to increase in hydraulic pressure in between the crown and cement.

10. Paulo Baldissara et al (1998) – conducted a comparative study about the marginal microleakage of six cements in fixed provisional crowns. Results showed Zinc phosphate and cavity bas compound cements had the best sealing properties than other cements.