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Fixed prosthodontic treatment in the form of teeth supported fixed dental prosthesis (TSFDP) is hugely a popular treatment in many countries. The treatment usually helps in restoration of missing natural tooth or teeth by way of crown preparation of potential existing natural teeth called abutments which shall be covered by retainers and the pontic shall replace the missing dentition.  Numerous biological,mechanical, technical and esthetic factors should always be considered before concluding on the treatment plan. Proper treatment planning shall consist of  selection of abutment teeth, their periodontal status, number of abutments, angulation, relative parallelism, mode of preparation of abutments, selection of pontic, design, nature of residual ridge, choice of  restorative material, shade selection, choice of luting agent. Unless  due attention is given to the factors mentioned above , the  failure in a fixed prosthesis is always a possibility. This article describes the  various failure modes of FDP ( fixed dental prosthesis) in an orderly manner  and possible reasons for the same.
FIXED DENTAL PROSTHESIS Important  signs and symptoms which  points out  development of problems or failures in a fixed dental prosthesis which had been given recently to the patient or which had been given  several months or years had been pointed out below  SIGNS AND SYMPTOMS OF FAILURES  IN FDP Looseness of the FDP Rocking on chewing & during function Continued Ingress of food and saliva Caries under the FDP Increased Gingival inflammation under the pontic/retainer Progressive Gingival recession Periapical inflammation of abutment Food impaction Tooth mobility Fracture or loss of facing Discoloration Perforation of metal frame Pain on percussion or Sensitivity of abutment Outright fracture of FDP Supra eruption/mesial drifting of adjacent teeth   Charles etal1 Described the Incidence of Failures  in His Article Single Crown Complications Duration-1 to 23 years. (studies) Incidence of complications 11% FDP complications Duration-1 to 20 years. (studies) Incidence of complications 27 % All Ceramic Complications Duration-1 month to 14 years. (studies) Incidence of complications 8 % Resin Bonded prosthesis Complications Duration-1 month to 15 years. (studies) Incidence of complications 26 %  CLASSIFICATION  - Bennard G. N. Smith Loss of retention Mechanical failure of crowns or bridge components Porcelain fracture Failure of solder joints Distortion Occlusal wear and perforation Lost facings Changes in the abutment tooth Periodontal disease Problems with the pulp Caries Fracture of the prepared natural crown or root Movement of the tooth Design failures Under-prescribed FPDs Over-prescribed FPDs  Inadequate clinical or laboratory technique a. Positive ledge b. Negative ledge c. Defect d. Poor shape and color  Occlusal problems Oginni2  described the failures of FPD fabricated in a Nigerian dental school. John J Manapallil3 described  a classification system for conventional crown and fixed partial dentures failures. He described  it  based  on the  increasing severity  from class 1 to class 6. Grading of failures based on severity  Class I   - Cause of failure is correctable without replacing restoration Class II  - Cause of failure is correctable without replacing restoration; however, supporting tooth structure or foundation requires repair or reconstruction Class III  - Failure requiring restoration replacement only. Supporting tooth structure and/or foundation acceptable. Class IV  - Failure requiring restoration replacement in addition to repair or reconstruction of supporting tooth structure and/or foundation Class V  - Severe failure with loss of supporting tooth or inability to reconstruct using original tooth support. Fixed prosthodontic replacement remains possible   through use of other or additional support for redesigned restoration. Class VI  - Severe failure with loss of supporting tooth or inability to reconstruct using original tooth support. Conventional fixed prosthodontic replacement is not  ossible.  Selby4  reviewed the important aspects of fixed prosthodontic failure. For ease of understanding  problems and failures of FDP , in this article the following description had been given.  TYPES OF FAILURES IN FDP Cementation failure Mechanical failure Gingival and periodontal breakdown Caries Pulp degeneration Biomechanical failure Esthetic failure  1.    CEMENTATION FAILURES CEMENTATION FAILURE  Can be broadly divided into: Cement failure Retention failure Occlusal problems Distortion of FDP  Cement failure Causes of cement failure  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  finger pressure causing incomplete seatings  Complications due to incomplete seating  Creation of premature contacts Alteration of contact areas with adjacent teeth Reduction in crown retention by 19-32% Discrepancies in the marginal fit of the crown Cement wash out at the margins ill fitting margins- expose large amounts of cements to oral fluids- increasing rate of deposition of plaque.   Methods to improve marginal fit:  Venting Internal relief  (clinically acceptable â 20- 40µm) Selection of luting agent 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 should have the following properties:  Adequate working time Adhere well to both tooth structure  and metal surface Provides a good seal Non toxic to the pulp Have adequate strength properties Be compressible into thin layers Have low viscosity and solubility Exhibit good working time  and setting properties  B.   Retention failure For a restoration to accomplish its purpose, it must stay in place on the tooth. The geometric configuration of the tooth preparation must place the cement in compression to provide the necessary retention and resistance.  Causes for Retention Failure  Excessive taper Short clinical crowns Mis-fit Misalignment  Excessive taper: The axial walls of the preparation must  taper slightly to permit the restoration to  get seated . Recommendations for optimal axial wall taper of tooth preparations for cast restorations ranged from 10 to 12 degrees.  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 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. 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:  Incorporation of proximal grooves. Additional retentive grooves Additional pins 2. In case of short crowns:  Crown lengthening procedure Modification of supra-gingival margin  to sub-gingival margin Additional retentive grooves  and proximal box Incorporation of pins Addition of extra abutments  Misfit: The measurement of misfit at different locations and geometrically related to each other and defined as :  Internal gap Marginal gap Vertical marginal discrepancy Horizontal marginal discrepancy Over-extended margin Under-extended margin  A. Causes for misfit  Defective casting Porcelain flowed inside the retainer Excessive oxide layer formation in inner side of the retainer (due to contaminated metal or repeated firing of porcelain) Tight contact points with abutment teeth Incorrect manipulation of luting agents Insufficient pressure during cementation procedure  B. Misalignment  It is more difficult to differentiate whether a FDP 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 FDP will have some âspringâaction 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  Abutment displacement due to improper temporization. Distortion of wax pattern while sprueing and investing. Casting defects Distortion of metal frameworks in porcelain firing. Porcelain flow inside the retainers. Misalignment of soldering points. Insufficient pressure in cementation. Thick cement film. Excessive metal or porcelain in tissue surface (ridge lap) of pontic prevents the proper seating of FDP and open margin (can be detected by observing the blanching of the tissue or patient may complain of pressure on the pontic region).  C.  Occlusal problems Following the placement of a dental restoration, a patient might report discomfort ranging from a feeling of âlamenessâ to âsevere and constant painâ. 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 Immediate problems Occlusal interference Marginal ridges at different levels Supra eruption of the opposing tooth Parafunctional habits  2 . Delayed problems  Wearing of occlusal surface Loss of occlusal contacts Perforation of occlusal surface due to Porcelain Vs  resin  or Porcelain Vs gold Food lodgement due to plunger cusp Fracture of facing due to defective occlusal contact Periodontal or gingival breakdown due to improper  occlusal contacts Tenderness due to food lodgement  D. Distortion of FDP  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 of distortion Casting defects- Distorted margin, Rough castings, Bending of the FPD due to improper care  taken during wax pattern making, Investing and casting procedures. Bending of long span FPDs due to Thin crown, Soft metal, Heat treatment not being done, Porosity in the metal Distortion of the metal substructure during the porcelain firing.  2.  MECHANICAL FAILURES 1.    Retainer failure 2.    Pontic failure 3.    Connector failure  RETAINER FAILURE Perforation Marginal discrepancy Facing failure Fracture       Wen Kou etal5 conducted a study to examine the fracture mechanism and process of  ceramic fixed partial denture (FPD) framework under simulated mechanical loading using a recently developed  numerical modeling code, the R-T2D code, and also to evaluate the suitability of R-T2D code as a tool for this purpose. Based on the findings in the study, the R-T2D code seems suitable for use as a complement to other tests and clinical observations in studying stress distribution, fracture mechanism and fracture processes in ceramic FPD frameworks.  Wearing Discoloration A. Perforation  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                                                                                    Burak etal6 conducted a study  to evaluate the clinical performance of crowns and fixed partial dentures (FPDs) made with the Empress 2 system over a 2-year period.  U.S. Public Health Service criteria showed 100% Alpha scores concerning recurrent  caries for both crowns and FPDs. No crown fractures were observed during the 2-year followup, however, 10 (50%) catastrophic failures of FPDs occurred. Five (25%) failures occurred within the 1-year clinical period and the others (25%) within the second year.  B. Marginal discrepancy  Causes Selection of  finish margin Improper preparation and failure to establish the margin properly Failure to do gingival retraction prevents definite margin location and subsequently in impression Selection of the impression material             i. Shrinkage in material (condensation silicone)             ii. Distortion of material (alginate)  Improper impression procedures Voids in the impression Variation in pressure application in wash technique Delayed pouring of die material  Distortion of wax patterns at margins Impressions  FINAL IMPRESSION PROCEDURES I)    Selection of tray â correct size Under extended â insufficient coverage Over extended â distortion II)   Moisture control â successful impression making III) Adequate tissue retraction IV) Do not remove the impression before it sets V)   Voids, incomplete details are the usual errors made with hasty handling of the impression material VI) An acceptable impression must include sufficient unprepared tooth immediately adjacent to the margins for the dentist and lab technician to identify the contour of the tooth and all the prepared tooth surfaces VII)   Particular attention to the lingual contour of the anteriors as they  influence the anterior guidance VIII) Impression defects- visible flaws IX)    Finish line not visible Gingival inflammation and bleeding Subgingival finish line Localized Gingival overgrowth Retraction cord displaced X)      Air bubbles in critical places XI)    Voids and drags XII)   Unset impression material â latex contamination XIII)  Impression defects- Invisible flaws,  restoration fit on the die , but not on the mouth XIV) Tray and impression recoil  Detachment of impression from the tray XV)   Permanent deformatiom K)  Insufficient flow of metal L)  Shrinkage of metal M)  Nodules in margins and inner side of coping       i. Due to inadequate vacuum during investing       ii. Improper brushing technique       iii. No surfactant  N)  Excessive sand blasting O)  Distortion due to degassing procedure P)  Open margins due to porcelain shrinkage (opaque porcelain) Q)  Thick mixing of luting agent R)  Cement setting prior to seating S)  Insufficient pressure application during cementation  C . Facing failure    Types of veneer failures  a) Fracture       Panida etal7  conducted a study to  test the hypothesis that fracture toughness of the veneers in clinically failed zirconia-based fixed partial dentures (FPDs) is not significantly different from that of the in vitro group and to determine the potential reasons for their  failures. The  study showed that Fractal analysis is shown to be an alternative analytic tool for clinically failed ceramic restorations, especially for those with fracture origins chipped off during mastication and hence could not be analyzed using other techniques, such as fractography.  b) Wearing of facing (resin veneers) c) Discoloration  PONTIC FAILURE Pontic is  the articial tooth which replaces the natural missing tooth or teeth  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  What factors should be considered when choosing a pontic? Tissue contact Post insertion hygiene Pontic design Ridge lap pontic Modified ridge lap pontic Sanitary pontics  3. CONNECTOR FAILURE The connector is that part of the FPD or splint that joins the individual components (retainers and pontics) together.  Causes for connector failure  Improper selection of connector Thin metal at the connector Incorrect selection of solder Solder gap â narrow or wide Porosity Insufficient metal around Defective occlusal contacts over thin connectors  Garry etal 8  conducted a  study to assess the effect of core to dentine thickness ratio on the bi-axial flexure strength and fracture mode and failure origin  using bilayered ceramic specimens as an in vitro assessment for all-ceramic crowns and the connector area of fixed partial dentures(FPDs). The fracture mode and failure origin in bilayered ceramics tested to represent the failure mode of all-ceramic crowns and FPDs was dependent upon the core to dentine thickness ratio employed. However, the conventional wisdom regarding bilayered ceramic specimens with core thicknesses greater than 1mm are not followed when the core thickness was reduced to 1mm since the fracture resistance was not dependent on the core to dentine thickness ratio.  3.    GINGIVAL AND PERIODONTAL PROBLEMS FINISH MARGIN 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. 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 Failure to  reproduce the margin of the preparation in the impression leads to failure in the marginal integrity of the restoration. Using of gingival retraction technique in case of sub gingival preparation is mandatory. However, all displacement techniques have the potential to 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.  CONTOUR  Overcontoured restoration - plaque accumulation and gingival inflammation Buccally and lingually crown should follow the outline of the tooth Interproximally - slightly concave to permit optimal plaque control without compromising aesthetics  INTERPROXIMAL EMBRASURES  Must allow access for plaque control Axial reduction must allow for thickness of restorative material and oral hygiene May have to compromise in anterior region due to black triangles leading to poor aesthetics   CONTACT POINTS  Contact points are required to prevent food packing. Position varies depending which tooth contact is made;  upper central incisors - incisal third upper central and lateral incisors - middle third upper laterals and canines - gingival third  PRESERVATION OF PERIODONTIUM  When the margin of the restoration intrudes into the biologic width, the inflammation and osteoclastic activity are stimulated Bone resorption will continue until the alveolar crest is at least 2mm from the restoration margin.  4. CARIES CAUSES Iatrogenic (dentistsâ role)  Failure to identify caries in abutments Incomplete removal of caries in abutments Marginal discrepancy with subsequent plaque accumulation and microleakage Subgingival marginal placement in inaccessible areas or regions Burning of root dentin or cementum in electro surgical technique (leads to damage or rough surface and causes plaque retention) Over contouring of the cervical thirds of crowns or bridges prevents the physiologic tooth cleaning by tongue or muscles Thick cement space in margins leads to cement dissolution. Narrow embrasures (inaccessibility to maintain hygiene) Wide connector  Patient role  Systemic factors Xerostomia Due to radiation therapy Drug induced Endocrine disorders Epilepsy (difficult to maintain the oral hygiene) Rheumatoid arthritis Local factors Improper brushing and flossing Dietary habits Failure to understand importance of oral hygiene.  5. PULP DEGENERATION Pulp  reactions to various procedures should always be considered Each step in full crown preparation is hazardous, to the pulp. In general, heat desiccation or chemical injury or over preparation with less than 1mm of reaming dentin.  The result may be pulpitis or even necrosis.  Preservation of tooth structure Devan â âpreservation of what remains is most important than the restoration what is lostâ Use of partial coverage rather than the complete coverage Preparation of teeth with minimum convergence angle (taper) between the axial  walls of Preparation. Anatomic reduction of the occlusal surface, so reduction follows the anatomic planes to give uniform thickness in the restoration .  6 . BIOMECHANICAL FAILURES Causes: Failure in selection of right abutment Lack of retention and resistance form Incorrect design of FPD Wrong material selection Such failures can be avoided in the following ways  Retention and Resistance Form given in  abutment helps in  Degree of Taper - parallelism Minimise shear forces - grooves, boxes Large surface area of cement Preparation length and width Path of insertion  Structural Durability achieved by Adequate Anatomic Occlusal Reduction Functional Cusp Bevel  - Given  on the functional cusps Lingual  inclines of the maxillary lingual cusp Buccal inclines of the mandibular buccal cusp  7 . ESTHETIC FAILURES REASONS FOR ESTHETIC FAILURE Failure to identify patient expectations regarding esthetics Improper shade selection Excessive metal thickness at incisal and cervical regions Thick opaque layer application Surface blistering (chalky appearance) Over glazing or too smooth a surface Metal exposure in connector, cervical and incisal regions (anteriors)  Kugel etal9 also described  restoration of edentuluous anterior maxilla using alumina and zirconia based cad cam restorations.  Failure to produce incisal and proximal translucency Improper contouring Failure to harmonize contra lateral tooth morphology Contour Color Position Angulation Dark space in cervical third due to improper pontic selection Discoloration of facing  Esthetical  Considerations in Tooth Preparation  Facially inclined tooth  â overcutting of the mesiofacioocclusal corner â display of metal Lingually inclined tooth â facial surface intersects lingual-shorter preparation, may encroach on pulp Under preparation results in poor aesthetics or an over built crown (dotted line) with periodontal and occlusal consequences. Conversely over preparation can be compensated by making a thicker and perhaps very aesthetic crown, but the strength and pulpal vitality of the underlying tooth may be compromised. In reality, preparations should be planned according to each individual case and in each case the  existing situation will be different.  Goodacre etal10 conducted a MEDLINE search, 50-year literature review of survival and  failure modalities of FPD. Fixed partial dentures failures: caries (18% of abutments and 8% of    prostheses), endodontic treatment(11% of abutments and 8% of prostheses), loss of retention   (7% of prostheses), esthetics ( 6% of prostheses),periodontaldisease (4% of prostheses), tooth   fracture (3% of prostheses), and prosthesis/porcelain fracture (2% of prostheses). Lindquist etal11  also conducted a study on success and failure retes of FPD after 20 years in service.
It is imperative to understand that  a successful fixed prosthodontic practice requires Knowledge of sound biological and mechanical principles involved in abutment selection and subsequent preparation techniques. Growth of desirable and acceptable manipulative skills to implement the treatment plan identified for the particular patient. Development of a critical eye and judgment for assessing details of the treatment and subsequent prognosis.
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