CLINICAL FAILURES IN FIXED PARTIAL DENTURE
CLINICAL FAILURES IN FIXED PARTIAL DENTURE
AND ITS MANAGEMENT
AND ITS MANAGEMENT
INTRODUCTION: INTRODUCTION:
A
A comcompliplicatcatioion n has has beebeen n defdefineined d as as ‘A ‘A secseconondardary y disdiseasease e oror condition developing in the course of a primary disease or condition’. condition developing in the course of a primary disease or condition’. Although complications may be an indication that clinical failure has Although complications may be an indication that clinical failure has oc
occucurrrreded, , ththis is is is tytypipicacalllly y nonot t ththe e cacasese. . It It iis s alalso so popossssibible le ththatat com
complplicaicatitions ons mamay y refrefleclect t subsubststandandard ard carcare. e. BuBut t onconce e agaagain in thithis s isis usually not true. Most of the time, complications are conditions that usually not true. Most of the time, complications are conditions that occur during or after an appropriately performed fixed prosthodontic occur during or after an appropriately performed fixed prosthodontic treatment procedures.
treatment procedures.
An objective evaluation of an existing restoration is necessary An objective evaluation of an existing restoration is necessary before coming to a conclusion that it is defective and requires either before coming to a conclusion that it is defective and requires either replacement or repair.
replacement or repair.
What constitutes a failure? What constitutes a failure?
Are failures absolute or are there degrees of failures? Are failures absolute or are there degrees of failures?
There are of course minor failures, which are a matter of opinion There are of course minor failures, which are a matter of opinion and could be possibly left without immediate repair or replacement, and could be possibly left without immediate repair or replacement, and there are obvious failures where repair or replacement is essential and there are obvious failures where repair or replacement is essential to avoid further damage to
to avoid further damage to the dentition.the dentition.
Failure may occur at any time. Hence it is important to be aware Failure may occur at any time. Hence it is important to be aware of
of obvobvioious us and and subsubtle tle indindicaicatitions ons of of prprostostheshesis is faifailurlure e and and havhave e aa working knowledge of the procedures that are necessary to remember working knowledge of the procedures that are necessary to remember the situation.
It is natural that dramatic mechanical failure such as fracture It is natural that dramatic mechanical failure such as fracture attracts attention, but it must be remembered that failures can be attracts attention, but it must be remembered that failures can be biologic and esthetic in nature.
biologic and esthetic in nature. II.. BBiioollooggiic c ffaaiilluurree
IIII.. MMeecchhaanniiccaal fl faaiilluurree IIIIII.. AAeesstthheettiic c ffaaiilluurree
I. BIOLOGIC FAILURE: I. BIOLOGIC FAILURE: 1. Caries:
1. Caries: It is the
It is the most common biologimost common biologic failure. Caries may affect c failure. Caries may affect a bridgea bridge in
in seseveveraral l wawaysys, , eieithther er didirerectctly ly at at ththe e mamargrginins s of of ththe e reretataininerer, , oror indirectly by starting elsewhere on the tooth and spreading to the fit indirectly by starting elsewhere on the tooth and spreading to the fit surfaces of the casting
surfaces of the casting or it may follow cementation failure.or it may follow cementation failure. Detection:
Detection:
-- ViVisuasual inl inspespectiction (Don (Disciscololororatiation aron arounound mard margingins)s)
-- PrProbiobing mang margrgins oins of resf restortoratiations wons witith a shh a sharp earp explxploreorerr -- RaRadidiogograraphphs fos for inr inteterprproroxiximamal cal caririeses
Causes: Causes:
-- DefecDefective tive margmargins ins (sup(supragiragingivngival al prefpreferred erred over over subgsubgingiingival)val) -- LooLoose rse retaetaininers ers thathat alt allow low grogross lss leakeakage age to oto occuccurr
-- IncIncomomplplete rete remoemoval val of cof cariaries pes pririor tor to reo restostoratratioionn -- PoPoor dor desesigign len leadadining to fg to fooood acd accucumumulalatitionon
It is natural that dramatic mechanical failure such as fracture It is natural that dramatic mechanical failure such as fracture attracts attention, but it must be remembered that failures can be attracts attention, but it must be remembered that failures can be biologic and esthetic in nature.
biologic and esthetic in nature. II.. BBiioollooggiic c ffaaiilluurree
IIII.. MMeecchhaanniiccaal fl faaiilluurree IIIIII.. AAeesstthheettiic c ffaaiilluurree
I. BIOLOGIC FAILURE: I. BIOLOGIC FAILURE: 1. Caries:
1. Caries: It is the
It is the most common biologimost common biologic failure. Caries may affect c failure. Caries may affect a bridgea bridge in
in seseveveraral l wawaysys, , eieithther er didirerectctly ly at at ththe e mamargrginins s of of ththe e reretataininerer, , oror indirectly by starting elsewhere on the tooth and spreading to the fit indirectly by starting elsewhere on the tooth and spreading to the fit surfaces of the casting
surfaces of the casting or it may follow cementation failure.or it may follow cementation failure. Detection:
Detection:
-- ViVisuasual inl inspespectiction (Don (Disciscololororatiation aron arounound mard margingins)s)
-- PrProbiobing mang margrgins oins of resf restortoratiations wons witith a shh a sharp earp explxploreorerr -- RaRadidiogograraphphs fos for inr inteterprproroxiximamal cal caririeses
Causes: Causes:
-- DefecDefective tive margmargins ins (sup(supragiragingivngival al prefpreferred erred over over subgsubgingiingival)val) -- LooLoose rse retaetaininers ers thathat alt allow low grogross lss leakeakage age to oto occuccurr
-- IncIncomomplplete rete remoemoval val of cof cariaries pes pririor tor to reo restostoratratioionn -- PoPoor dor desesigign len leadadining to fg to fooood acd accucumumulalatitionon
Prevention: Prevention:
-- MetMeticuiculolous orus oral hyal hygiegiene mune must be a rst be a routoutinine proce proceduedure fore for patr patienientsts with a high caries index and particularly for those who have a with a high caries index and particularly for those who have a history of developing carious
history of developing carious lesions around restorations.lesions around restorations. -- FlFluoruoride ide concontaitainining dng dententifrifriceices ans and md moutouthwahwasheshess
Management: Management:
1)
1) If carious lesion is smallIf carious lesion is small conservative operative proceduresconservative operative procedures can be performed
can be performed
-- GooGood fod foil iil is ths the mae materterial ial of chof choicoice foe for mar margirginal nal carcariesies
-- AAmmaallggaam m iis s pprreeffeerrrreed d oovveer r ggoolld d bbeeccaauusse e oof f iitts s lloonng g tteerrmm marginal seal and into areas of limited accessibility
marginal seal and into areas of limited accessibility
-- In aIn aestesthethetic aic areareas – Rs – Resiesin man materterialials or Gs or GIC mIC may be ay be useused.d.
2) If caries is on the proximal surfaces, prosthesis has to be removed 2) If caries is on the proximal surfaces, prosthesis has to be removed to gain access. If the lesion is small the preparation can be extended if to gain access. If the lesion is small the preparation can be extended if a large amalgam restoration may be required before fabrication of a a large amalgam restoration may be required before fabrication of a prosthesis.
prosthesis.
2. Pulp Degeneration: 2. Pulp Degeneration: Clin
Clinical ical featurfeatures: es: - - PostiPostinsernsertion tion pulppulpal al senssensitivitivity ity in in the the abutabutmentment te
teeteth h ththat at dodoes es nonot t susubsbsidide e wiwith th titimeme; ; inintetensnse e papain in or or pepeririapapicicalal abnormali
abnormality that ty that are detected are detected radiographiradiographically.cally.
Causes:--- ExcExcess ess heaheat t gengeneraeratition on durduring ing prepreparparatiationon -- EExcxcesess ts tooooth th rrededuuctctiioonn
-- PiPin pon poinint ext expoposusure wre whihich mch may gay go uno unnonotiticecedd -- OOcccclluussaal l ttrraauummaa
Prevention: - Use of varnish or dentin bonding agent form an effective Prevention: - Use of varnish or dentin bonding agent form an effective barrier and prevents underlying pulp from toxic effects of cement and barrier and prevents underlying pulp from toxic effects of cement and core materials.
core materials.
Management:--- AcAccecess to ss to ththe e pupulp reqlp requiuireres s a a hohole in le in ththe e prprososththesesis this throrougughh which the necessary treatment is completed.
which the necessary treatment is completed.
-- PerPerforforatiation cron createated can bed can be rese restortored wied with goth gold fold foil or ail or amamalgalgam.m. -- If thIf the retae retaininer caser castinting becog becomemes loos loose or pose or porcercelailain fracn fracturture occue occursrs during access cavity
during access cavity preparationpreparation remake of prosthesis.remake of prosthesis.
-- DuDuriring endng endodoodontintic treac treatmetment an asnt an assessessmsment sent shouhould be mld be made oade of f the quality and quantity of tooth structure remaining for support the quality and quantity of tooth structure remaining for support and retention of restoration. If it is decreased reinforcement with and retention of restoration. If it is decreased reinforcement with post and core may be required.
post and core may be required.
-- TTeeeeth th ththat at wewerre e ssatatiisfsfacactotorriilly y rroooot t fifilllled ed wwhhen en tthe he crcroowwn n oorr br
brididge ge wewere re mmadade e mmay ay lalateter r gigive ve trtrououblble. e. In In susuch ch sisitutuatatioionsns apicectomy is the solution. Care must be taken not to shorten the apicectomy is the solution. Care must be taken not to shorten the root of the abutment tooth more than absolutely necessary so that root of the abutment tooth more than absolutely necessary so that maximum support for the bridge can be
maximum support for the bridge can be maintained.maintained.
Note: - Indirect pulp capping is not recommended as its failure may Note: - Indirect pulp capping is not recommended as its failure may jeopardize the existing prosthesis.
jeopardize the existing prosthesis.
3. Periodontal Breakdown: 3. Periodontal Breakdown: Clin
Clinical ical FeatuFeatures: res: - - GingGingival ival recesrecessionsion, , furcafurcationtion, , pockepocket t formformatioation,n, mobili
This can be either a generalized periodontal breakdown of the whole mouth which may be associated with the drifting of teeth or may be localized to the bridge abutment.
Causes:-- Inadequate instructions in prosthesis hygiene or its poor implementation by the patient.
- Prosthesis that hinders good oral hygiene
o Poor marginal adaptation
o Overcontouring of axial surfaces
o Large connectors that restrict cervical embrasures
o Pontics that contact too large an area on the edentulous
ridge
o Prosthesis with rough surfaces which promote plaque
accumulation - Traumatic occlusion
- Insufficient number of abutment selected
Prevention: - Proper oral hygiene instructions Review appointments
Preparation design: - Proper axial contours flat axial contour are better than overcontouring as they are easy to maintain and avoid plaque accumulation.
Treatment:-- If less severe scaling and proper plaque control - Increased severity – flap surgery, bone graft etc.
- If prognosis of abutment teeth has decreased than the crown or bridge and the tooth may have to be removed.
4. Occlusal Problems:
Clinical Features: - Large wear facets, mobility, tender on percussion, open contacts, perforation, cusp fracture, tenderness of the masticatory muscles involved. Radiographically-widened periodontal ligament is seen.
Interfering centric or eccentric occlusal contact can cause excessive tooth mobility. If this is detected early, the interferences can be eliminated by occlusal adjustments without permanent damage.
However, traumatic occlusion on teeth previously weakened by periodontal disease or long term presence of occlusal interferences on teeth with normal bone support can lead to mobility which cannot be reduced or eliminated through adjustment of the interfering area. The prosthesis may have to be removed and teeth bilaterally braced with RPD. Many a times it requires extraction of abutment teeth.
In patients with bruxism night guards or occlusal splints may be given. A slightly flatter anterior ramp is preferred in clenchers than ordinarily given.
Neuromuscular discomfort related to improper occlusion can result in prosthesis failure, hence selective reshaping of defective contacts and restoring or replacing teeth in more favorable position should be done to accommodate occlusal forces.
5. Tooth Perforations:
Pin holes or pins used in conjunction with pin retained restorations can be improperly located and may perforate the tooth laterally.
• If perforation is located occlusal to PDL – it is often possible
to extend the tooth preparation to cover the defect.
• If perforation extends into the PDL – perform periodontal
surgery to smooth off the projecting pin or place a restoration into the perforated area.
• If area is not accessible – lead to extraction of tooth.
• Perforations may not be detected initially, becoming
apparent only after insertion of the prosthesis.
Endodontic treatment is required when pinholes or pins perforate into pulp chamber.
II. Mechanical Failure: 1. Loss of Retention:
This occurs mainly due to leverage and unequal occlusal loads on different parts of the bridge. Loose retainers cause rapid destruction of the abutment tooth. Saliva and plaque and pumping action of loose retainer are responsible for caries leading to rapid destruction of abutment teeth.
Clinical Features: - Patient may be aware of looseness or sensitivity to temperature or sweets. Also there may be a recurring bad taste or odour, which must be differentiated from similar symptoms caused by
Detection:-- Sometimes the patient is aware of movement developing in the bridge.
- Diagnostic test is to examine the bridge carefully without drying the teeth, pressing the bridge up and down (occlusocervically) and with a curved explorer – looking for small bubbles in the saliva at the margins of the retainer.
- When more than 2 abutment teeth are involved in prosthesis, it is difficult or impossible to detect a single loose retainer.
Management:-- If retainer becomes loose prosthesis must be removed so that the abutment teeth can be evaluated.
- If the restoration can be dislodged from the prepared teeth without damage and no caries is present, it is possible to recement the prosthesis. Improper cementation procedures, such as contamination with moisture or increased cement space may have caused the problem.
- If the prosthesis reveals loss of adequate retention, teeth should be modified to improve the retention and resistance form. Additional retention by cross pinning, grooves, boxes etc. Alternatively it may be necessary to include additional abutment to increase overall retention or change the design in some other way (i.e. use of full coverage instead of partial coverage). In case of grossly destructed teeth, core build up may be done to support the
retainer or surgical exposure of crown can also be done. After all this a new prosthesis is fabricated.
- Sometimes FPD come loose even when maximally retentive preparation have been developed. This problem is caused by excessive span length or heavy occlusal forces – A RPD may be the only satisfactory solution.
- “It is better for teeth to have no cover than loose cover”.
- Because there is usually less permanent damage or plaque is not retained against the surface of preparation and the patient is obviously aware of the problem and seeks treatment quickly.
2. Connector Failure/ Solder Joint Failure:
There are several points to watch if a breakdown of the solder joint is to be avoided.
i) Adequate width and depth to resist occlusal stress ii) A sufficient bulk of gold
Causes:-- Connector failure can occur under occlusal load. When fracture occurs pontic is placed in an cantilever relationship with the retainer casting which may lead to excessive forces on abutment teeth. Hence prosthesis should be removed and remade.
- A flaw / inclusion in solder itself (porosity) - Failure to bond to surface of metal
- Joint not be sufficiently large for the condition in which it is placed.
- Improper flow of metal due to decreased width between joining parts. Minimum width for solder to flow properly is 0.25mm.
Treatment:-- Fracture connectors are difficult to detect in an abutment teeth with no mobility. Wedges are placed beneath the connector to separate the FPD components to confirm diagnosis. Occasionally an inlay like dovetail preparation can be developed in metal to span the fracture site and casting can be cemented to stabilize the prosthesis.
- If this is not possible, and a remake cannot be rapidly accomplished, the pontics should be removed by cutting through the intact connectors. A temporary RPD can be inserted to maintain the existing space and satisfy esthetic requirements. - It is better whenever possible to join multiple unit bridges by
solder joint in the middle of pontics before porcelain is added. This gives much larger surface area for the solder joint and it is
also strengthed by porcelain covering.
Effect of connector design on the fracture resistance of all ceramic FPD. JDP 2002; 87
- The results of this study showed that the occlusal embrasure can be designed as sharp as is practical for the aesthetics of an all ceramic 3 unit FPD; provided that the gingival embrasure has a increased ratio of curvature to increase the fracture resistance.
Heavy chewing forces, clenching or bruxism can produce accelerate occlusal wear of a prosthesis.
Clinical Features:- Attrition of opposing teeth, polished facets on the retainers/ pontics, gingival recession or inflammation.
Causes:-- Faulty preparation were occlusal clearance for metal is inadequate.
- Even with normal attrition, occlusal surfaces of posterior teeth wear down substantially over a period of time.
- Gold crowns made with 0.5mm or so of gold occlusally may wear through a period of 2-3 years.
- There perforations allow leakage and caries to occur which leads to prosthesis failure.
Management:- If perforation is detected early, a gold or amalgam restoration can be placed.
Other materials – resin, composite and GIC
o If perforation is over amalgam core, leave it untreated and
check it periodically.
o If metal surrounding perforation is extremely thin a new
prosthesis should be fabricated.
o If occlusal surfaces are covered with porcelain, wear of
ceramic is not a problem, instead the opposing natural teeth shows dramatic wear of enamel. This problem is exacerbated by heavy chewing forces, clenching or bruxism and often requires the
restoration of abraded teeth. The same occurs when porcelain opposes metallic restoration.
So, in mouths in which occlusal wear is anticipated, it is better to place metal over occluding surfaces to minimize wear and maintain the integrity of natural teeth.
4) Tooth Fracture: a) Coronal fracture:
Coronal tooth fracture can be dramatic, resulting in considerable loss of tooth structure, or it can be minor with little significant damage.
Causes:-- Caries of abutment teeth
- Excessive tooth preparation which may leave insufficient tooth structure to resist occlusal forces.
- Preparation may have been composed mainly of restorative material which was not retained in sound dentin with pins.
- Presence of interfering centric and eccentric occlusal contacts or even heavy occlusal loads.
- Fracture can also occur when attempts to forcibly seat an improperly fitting prosthesis/ unseat a cemented bridge incorrectly.
Management:-- If defect is small it is restored with amalgam, gold foil or resin to provide additional years of service.
- If there is a question regarding the integrity of the remaining tooth structure or restoration, a new prosthesis should be fabricated so that it encompasses the fractured area.
- Large coronal fracture around partial coverage retainers, then full coverage restorations may be fabricated. Tooth may require separate pin retained restoration to serve as core and provide support and retention.
- If fracture causes exposure of pulp, endodontic treatment along with post and core; abutment preparation should involve placement of bevels to increase resistance form.
- Abutment tooth fracture under full coverage restoration usually occur horizontally at the level of finish line. This necessitates removal of prosthesis. Endodontic treatment post and core
new prosthesis.
b) Root
Fracture:-Causes:- Most often due to trauma
- During endodontic treatment, forceful seating of post - Attempts to fully seat an improperly fitting post
Fracture may not be immediately apparent and only become detectable with time.
Root fracture are located well below the alveolar bone, so it must be extracted and new prosthesis fabricated.
Occasionally fracture terminates at or just below the alveolar bone, in such cases it may be possible to perform periodontal surgery, remove bone and expose the fracture site so that it can be encompassed by new prosthesis.
c) Pontic fracture/
failure:-Mechanical failure of the pontic may occur because of inadequate strength. Thus an all porcelain occlusal pontic should never be used unless the occlusion is favorable.
Similarly the gold framework must always be of adequate rigidity. Even slight flexion will cause cementation failure or fracture of the porcelain facing.
Probably one of the commonest cause of pontic failure is a faulty occlusion particularly in lateral excursions, which was not corrected when the bridge was placed.
An acrylic facing will wear and discolour quite rapidly. Tissue contact of pontic – extensive area of tissue contact is cited as major cause of failure. Area of contact should be small and convex. Mesial, distal, lingual and gingival embrasure should be wide open to allow easy cleaning.
5) Porcelain Fracture:
Porcelain fracture occur with both metal ceramic an all ceramic crown restoration. The majority of PFM fracture can be attributed to improper design characteristics of the metal framework or to problem related to occlusion.
All ceramic restorations commonly fail because of deficiencies in tooth preparation or presence of heavy occlusal forces.
a) Metal-Ceramic Porcelain Failure: Framework
design:-• Sharp angles or extremely rough and irregular areas over
the veneering area serve as points of stress concentration that cause crack propagation and ceramic fracture. Perforations in the metal can also cause failure for same reason.
• An overly thin metal casting does not adequately support
porcelain, so that flexure and porcelain fracture are allowed.
• Overbuilt porcelain unsupported by metal in PFM may
fracture because of cohesive failure within the porcelain.
• In PFM restoration porcelain fracture result from framework
design that allows centric occlusal contact, on or immediately next to the metal ceramic junction.
• When angle between veneering surface and non-veneered
aspect of the casting is less than 90°. These designs allow occlusal
forces to cause localized burnishing of metal and distortion, which leads to premature porcelain fracture.
Occlusion:-• Heavy occlusal forces or habits such as clenching and
bruxism
• Centric or eccentric occlusal interferences can lead to
which create deflective contact of opposing teeth with the prosthesis.
Metal Handling
Procedures:-• Improper handling of alloy during casting, finishing or
application of the porcelain can lead to metal contamination.
• Bubbles may form at metal ceramic junction, when
porcelain is applied, creating stress or possibly cracks.
• Severe contamination
• Excessive oxide layer on metal, due to improper
conditioning of base metal alloys can lead to separation of porcelain from metal.
Preparation, Impression and Insertion:
Preparation with slight undercut can cause binding of the prosthesis as it is seated, which initiates crack in the prosthesis. This may go unnoticed until premature postinsertion failure occurs.
An impression that is slightly distorted can lead to same problem.
Teeth with feather edge finish line or impression which do not record all finish lines can lead to extension of metal beyond the actual termination of tooth reduction. The thin metal may bind against the tooth and initiate a crack in overlying porcelain.
Good preparation with definite line and impression that record proper detail are prerequisites to acceptable ceramics.
In rare instances, an alloy and porcelain are found to be truly incompatible, and successful bonding without loss of the veneer or cracking is impossible.
Repair of fractured metal ceramic
restorations:-• Best method is fabrication of a new prosthesis
• Resin materials are often used to rebuild the porcelain
form in area where fracture has occurred, adequate to good colour matching can be achieved.
Drawback is lack of longevity and discolouration. Even light cure composites can be used.
Retention of these materials is mainly due to mechanical interlocking so if used in areas of heavy occlusal forces repair often fails shortly after insertion.
• If fracture is due to heavy occlusal forces the contact
should be avoided at the metal ceramic junction, and it should be at least 1.5mm away from the junction.
• A more permanent repair is possible if adequate thickness
of metal available. Steps –
- Removal of remaining porcelain
- Drill several pin hours (4-5) to depth of 2mm and make impression
- Creating pin retained metal casting 0.2 – 0.3mm thickness out of a metal ceramic alloy to fit over exposed metal framework.
- Fusion of porcelain to the pin retained casting and establish normal form
- Cementation of casting in position
- If there is any risk of pontic area flexing, porcelain should be carried on to the lingual side of the pontic to stiffen them further.
Sleeve
Crown:-When a considerable portion of porcelain is lost from labial/ incisal surface of a retainer or pontic it is often possible to repair that replace the entire unit. The porcelain facing is removed with some of the underlying metal from the labial surface. Porcelain as well as metal are removed from incisal third of the palatal surface. This is a simple procedure when damaged unit is pontic, but when the damaged unit is a retainer and underlying pulp has to be considered. Common mistake is removal of too little porcelain and metal.
An impression is made of this and the two adjacent units. The technician is then asked to make metal ceramic crown that will have 2 surfaces instead of usual four. This sleeve crown is then cemented in usual way.
If too little porcelain is removed from original unit, the new sleeve crown will fill slightly bulky.
b) Porcelain Jacket Crown
Failures:-Since porcelain jacket crown have been in use for rarely a century, considerable clinical experience related to their failure is available.
With good preparation considerable success has been achieved on incisors, whereas fracture are more frequently observed when restorations are placed on posterior teeth and on canines because of occlusal force on these teeth.
Cause:- Quality of tooth preparation and magnitude of occlusal load are the main factors that determine clinical success or failure. They are more likely to fail in presence of heavy occlusal forces clenching/ bruxism.
Prevention:- Tooth preparation should be adequate but not excessive. Tooth reduction must be designed to support the restoration since no
metal is present to provide support.
Management:-- Short term repair can be done with GIC, resin and light cure composites.
- Severely chipped all porcelain crowns must therefore be replaced by a new crown.
- If an early failure occurs without any clinical/lab defects heavy occlusal forces are likely to be present that exceed strength of restoration. Metal ceramics should be seriously considered for the new restoration.
- If failure occurs after many years of service and optimal esthetics is still required a new all-ceramic should be considered.
- If fracture is due to trauma it should be replaced by another all ceramic restoration particularly when old restoration has served successfully for sometime.
Types of Ceramic a) Vertical
Fracture:-- Marginal area of jacket crown is often more closely adapted to prepared tooth than other areas. If tapered finish line is used, restoration contacts the tooth on a sloping surface resulting in forces that attempt to expand the restoration which are not well resisted by porcelain, leading to vertical fracture.
- Sharp areas on tooth such as line angles and incisal angles produce areas of high stress in restoration, leading to vertical fractures.
- A round preparation form that does not provides adequate resistance to rotational forces can also cause vertical fracture.
b) Facial Cervical Fracture:
Often assumes a semilunar form (Half moon fracture), generally occurs with a short tooth preparation. Inciso cervical length of the preparation should be 2/3rd to 3/4th that of the final restoration. When
opposing tooth contact is located incisally to prepared tooth, tipping forces are more frequently developed, with the restoration having a fulcrum on the cervically located incisal edges, leading to facial cervical fracture.
c) Lingual Fracture:
Cause:-- When occlusion is located cervically to the cingulum of the preparation, when forces on the porcelain are more shear in nature and not as well resisted.
- Inadequate lingual tooth reduction, in which <1mm of porcelain is present.
- Exceptionally heavy occlusal forces.
6) Cementation Failure:
Causes:-- Loosening of retainer due to inadequate mechanical retention as strength of chemical adhesion, and cohesive strength of cement are limited.
- Poor cementation technique:- Wrong choice of material, failure to observe the manufacturers mixing instructions, use of old or contaminated stock, inadequate P/L ratio. Insertion of prosthesis when cement has set. Inadequate isolation weakens the bond. Where full crowns are being employed, venting is usually inadequate.
- Resinous cements are considered to be the most retentive. But the main drawback of resin cements being H2O percolation which leads to increased pressure in the interface acting as an hydraulic chamber, which leads to failure.
8) Design Failure:
a) Abutment preparation design: i) Factors affecting
dislodgement:- Taper of preparation: Increased taper reduces ability of restoration to resist occlusally directed forces and also lessens its ability to interfere with arc of rotation as tipping forces act to unseat the restoration.
Taper/ angle between opposing walls determine the degree of retention against axial unseating forces. A parallel preparation is impractical as cement cannot extrude from the crown during cementation leaving excessive thickness of cement occlusally and at margins.
Once taper exceeds 30° or so failure through loss of retention
becomes common. Ideal taper for good retention is 7° with minimum
cement in between. However, it is not possible to achieve this taper clinically without producing some undercuts/ damaging the adjacent teeth.
Average taper for post preparation that have been shown to be clinically in successful in a large number of cases is 10-20°approx.
Length of Preparation: Minimum cervico-incisal height is that which allows the tooth structure to interfere with arc of rotation as tipping forces attempts to cause rotation around a fulcrum located at the finish line on the opposite side of the tooth.
In case of short teeth adequate height is achieved by extending margin subgingivally or only alternative is to prepare tooth with less taper.
Greater the length the more retentive. Minimum acceptable length will depend on nature of occlusal forces, number of teeth and whether the crown will be subjected to withdrawing forces from a FPD.
“Relationship between length and taper is important:”
Shorter clinical crowns require more parallel walls. If clinical crown is assessed to be too short for adequate retention it must be built up with a core (if there is sufficient occlusal clearance), or surgical crown lengthening or retention achieved by pins/ grooves. Both have the potential not only to resist loss of the crown in a direction other than long axis but also reduces the angle of the path of insertion.
Circumferential Irregularities:
Circumference of teeth is usually irregular in form and when tooth is uniformly reduced an irregular shape is formed which enhances ability of restoration to resist both tipping and twisting forces.
When tooth encountered is round/ short/ over tapered intentionally formed irregularities such as boxes, grooves may be used to produce areas that interfere with dislodgement of restoration.
Boxes are more effective than grooves and should be used when sufficient tooth structure is present. Best location being the proximal areas, where it adds resistance to faciolingual dislodging forces.
All partial veneers crown require use of boxes and grooves.
irregularities:-Aids in resistance to dislodging forces; flat reduction provides little interference and unnecessarily reduces the length of preparation. Irregular reduction according to occlusal plane produces an corrugated sheet effect which enhances the rigidity of the retainer than one plane reduction.
ii) Finish line
requirements:-Supragingival margins are more acceptable than subgingival as they aid in proper oral hygiene maintenance. They also reduces pulpal sensitivity as they are usually in enamel.
Margins should be smooth and even. Rough or irregular margins reduces adaptation and increase plaque formation and gingival inflammation.
iii) Path of
Insertion:-Considered in 2 dimensions – Faciolingual and mesiodistal
Faciolingual direction:- Faciolingual orientation can affect the esthetics of metal ceramic or PV crowns. For metal ceramics the path of insertion should be roughly parallel to the long axis of the tooth. A facially inclined path of insertion on a preparation for metal ceramic crown will leave the faciolingual line angle too prominent, resulting in overcontouring of restoration, opaque showing through or both.
For 3/4th crown on anterior teeth the path of insertion should be
parallel to the incisal half of the labial surface. If inclined more facially short grooves and unnecessary display of gold will result.
up at the proximal contact areas and may be locked out. This is a particular problem when restoring tilted teeth.
iv) Structural Durability:-a) Occlusal Reduction:
- Minimum of 1.5mm for functional cusps and 1.0 mm for non-functional cusps is needed
- Inadequate reduction leads to perforation and fracture of metal. - One plane reuction may reduce the incisocervical length and
jeopardize the pup.
- Rigidity of metal is increased by following the contours of the crown
b) Functional cusp bevel:
Bevel should be given on the maxillary lingual cusp and mandibular buccal cusp at an angle of 45° to provide space for
adequate bulk of metal in an area of heavy occlusal contact.
If crown is waxed and contoured to normal contour without a bevel, casting will be extremely thin in area overlying the junction between occlusal and axial reduction. To prevent thin casting from fracture an attempt is made to wax the crown to optimal thickness in this area. An overcontoured restoration will result leading to deflective occlusal contacts which can only be eliminated by reducing the opposing teeth.
b) Inadequate bridge
design:-Designing bridges is difficult. It is neither a precise science nor a creative form of art. It needs knowledge, experience and judgement,
which takes years to accumulate. Simple classification of failure
underprescribed and overprescribed bridges.
• Underprescribed Bridges :- These include designs that are
unstable or have few abutment teeth e.g. cantilever bridge carrying pontics that cover too long a span or a fixed movable bridge where again span is too long or abutment teeth with too little support have been selected. Another under design fault is too conservative in selecting retainer e.g. Class II inlays for fixed-fixed bridges. Little can be done other than removing and fabricating new prosthesis.
• Overprescribed bridges :- Cautions dentist will sometimes
include more abutment teeth than are necessary and fate usually dictates that it is the unnecessary retainer which fails. E.g. 1st and
2nd premolar and 2nd molar included to replace 1st molar or use 3, 4,
5 on either side to replace incisors in upper arch. If the large bridge unit fails it is sometimes possible to section the bridge in the mouth and remove the failed unit leaving the remainder of the bridge to continue in function. The failed unit is remade as an individual restoration.
The retainers themselves may be overprescribed with complete crowns being used where partial crowns or intracoronal retainers would have been quite adequate or metal ceramic used where all metal crown would have been sufficient.
ii) Marginal Deficiencies:-Positive ledge
(overhang):-It is an excess of crown material protruding beyond the margin of preparation. These are more common with porcelain than any other margins. However, it is often possible to correct them without otherwise disturbing the restoration by grinding and polishing in situ. Negative
ledges:- This is a deficiency of crown material that leaves the margin of the preparation exposed but with no major gaps between the crown and the tooth. Again it is a fairly common fault, particularly with metal margins, but one that is difficult or impossible to correct at the try-in stage. It often arises because the impression did not give a clear enough indication of the margin of the preparation. The die was overtrimmed, resulting in under extension of the retainer.
Provided the crown margin is supragingival or just at the gingival margin, it is sometimes possible to adjust the tooth surface of the crown. When the ledge is subgingival, and particularly there is localized gingival inflammation associated with it, it may still be possible to adjust the ledge with pointed stone or bur, although this will cause gingival damage.
Sometimes it is necessary to remove the bridge and adjust the tooth surface with/ without surgically raising the flap.
iii) Dowel
design:-If a dowel is used its extension into root must at least be equal the length of the crown for optimum stress distribution and maximum retention or the dowel should be 2/3rd the length of the root whichever
A minimum of 4mm of gutta percha and more if possible should remain to prevent dislodgement and subsequent leakage.
Failures of Dowel and Possible Explanations:
Type of failure Explanation Loss of post from
root canal
Post does not fit the walls of the canal or has too much taper.
Post too short
Post not in canal but has been placed through a perforation into the alveolar bone. This should be suspected if there is any bleeding.
Radiographs taken from either side of the root will show the post to be apparently in different positions.
Longitudinal or oblique fracture of root. This can be confirmed by placing a probe in the post hole and gently forcing the walls of the tooth apart. In this situation blood can usually be seen in the crack. It may be due to excessive force, as would be imparted by a bridge abutment on a wedge-shaped post.
Fracture of post This usually occurs at the gingival margin.
In a cast post it may be due to the diameter being too small, or the alloy too soft, or porosity in the casting, or exceptional occlusal forces.
In a pre-formed post it may be due to corrosion, or selection of a post which was too thin. Posts which do not have sufficient resistance to stress will develop metal fatigue.
Pain on
cementation of post
Root has been split due to cement being partially set, or mixed too thick. Screw posts which contact the end of the post hole may also split the root.
A poorly sealed root filling, lateral canal or perforation may allow cement into the periodontal
membrane.
Loss of crown Core preparation too short or too conical in shape. Fracture of
crown
Core too thick especially palatally.
A bonded crown should have been made instead of a porcelain jacket crown. This would be indicated by the presence of wear facets, very short clinical height to the crown, or lack of space between the lingual surface of the core and the occluding surface of the opposing tooth.
III) Esthetic Failure:
Ceramic restoration more often fail esthetically than mechanically or biologically.
a) Colour Mismatch:
Main reason reported by dental laboratories is poor colour match. This could be the result
of:-- Inability to match the patient’s natural teeth with available porcelain colour
- Inadequate shade selection - Due to metamerism
- Insufficient tooth reduction or failure to properly apply and fire the porcelain may have created a restoration that does not match the shade guide or surrounding teeth.
- Because incorrect form or framework design that displays metal - In addition, natural teeth undergo colour changes that do not
occur in porcelain, so that an unacceptable colour match is caused over the years.
- The marginal fit or cervical form of a prosthesis can promote plaque accumulation, causing gingival inflammation, which produces an unnatural soft tissue colour or form that is esthetically unacceptable.
Removing of Crown and Bridges:
• Before removal possible need for temporary crown must be
remembered and provision for this should be made.
• Important is to protect the airway as chances of inspiration
is more due to the small size of crown, bridge particularly inlays. Alter chair position and make patient sit upright.
• Attempt to remove the prosthesis intact, if not possible
prosthesis is cut until the prepared tooth is exposed and then removed.
• Preparing the slot lingually is advantageous as material
bulk on the lingual side is comparatively less and therefore easily removed. As slot is on the lingual side the same prosthesis can be used as a temporary crown.
• Usually facial slots work best for maxillary and mandibular
molars.
Removing Old Bridges:
Methods:-1. Inertia forces 2. Reciprocal forces
Some bridges are easy to remove and some can be most difficult. Begin in securing the bridge by tying a long piece of dental floss so that it cannot be swallowed.
The causes of failure have been discussed. If an old bridge has come out, well and good, but some fail on one abutment and are held firmly on the other. The cement may have softened if there has been leakage but in other cases the cement can be remarkably strong after 20 to 30 years.
One retainer loose: If one retainer is loose it is important to support that end of the bridge with firm finger pressure, otherwise the dislodging force will slew the bridge and cause it to lock even tighter on the firm retainer. It is even possible that damage could occur if the force is misused in this way.
1. Inertia Forces:
a. Wire :
A loop of soft stainless steel wire or brass about 0.6 mm in diameter may be threaded under the connectors and formed into a loop. A bar or instrument handle can be passed through the loop and held
taught. A sharp tap with a light mallet such as a surgical hammer (or a fire door hammer for glass windows) may dislodge the bridge. If one retainer only should come loose, move the wire to the other connector and support that loose end as just described.
b. Inertia bridge remover
:- The inertia bridge remover has a captive weight on the end of a rod with an end stop. The other end has a hook or claw, often interchangeable, but this is usually engaged under the connector. In some bridges the claw may be engaged under a positive edge of a retainer. The rod is held as near to the axial direction as possible and a short blow is struck with a sliding action on the weight. When removing a bridge from the upper jaw the patient’s head should be steadied against the headrest by the DSA applying gentle pressure with the palm of the hand against the forehead of the patient. Support of the jaw when removing a lower bridge is more reassuring for the patient as well as cushioning the action.
Chisel:-A straight chisel may be used to remove an inlay and also a full veneer retainer by applying a gentle tap in the axial or near-axial direction with a light hammer or weight. The chisel should not be tungsten carbide tipped or it may become damaged. At first the chisel should be so angled that, with a tap or two, it is made to cut into the metal and then the angle is altered to be as near axial as possible. Not only is this a useful method, as every surgery will have a chisel, but it sometimes works where the inertia bridge remover is difficult to apply, although the inertia bridge remover is usually the most useful method.
2. Reciprocal forces:
a) Band-removing pliers : To use orthodontic band-removing pliers, a hole is made in the top of the full veneer crown, larger than the prong on the pliers, which rests on the surface of the abutment. The claw or beak engages the cervical margin and the handles are brought together.
b) Inlay remover : An inlay remover is made of hard tool steel similar to an engineer’s stud remover. A hole of suitable size is drilled in the full veneer crown so that the threads of the inlay remover engage in the metal. The end of the plug is brought to bear on the occlusal surface of the abutment. Both of these devices act reciprocally and no force is brought to bear on the periodontium.
A burr cut can be made on the buccal or lingual surface of a full veneer retainer. A chisel may be placed in the slot and twisted so as to separate each part and break the cement lute, or the chisel may be put in the groove so as to prize each part away from the tooth. This method usually works well even with a stubborn bridge but the problem is that the bridge will be spoilt for subsequent replacement or even as a temporary bridge.
Thought should be given as to whether to divide on the buccal or lingual. Gold is easier to bend than alloy with bonded porcelain and such bulky retainers have even had to be divided on both buccal and lingual to effect loosening.
Division of a connector: Sometimes it is indicated to divide a bridge connector. In the illustration it may be desirable to extract the first molar. The bridge could be divided at the conector between the pontic and the molar. This leaves the first premolar covered and the pontic continues to give aesthetics until the emergency is over.
A diamond disc in a disc guard with water will do this rapidly. The diamond should be confined to the peripheral part of the disc and then it will not jam in the metal so easily.
Direct temporary bridge replacement: It may be worth taking an alginate impression before starting to remove an old bridge that may disintegrate and then a direct temporary bridge replacement in autopolymerizing acrylic or epimine plastic may be readily produced. Richwill crown removal: It is a green sticky tube which is softened in hot water and placed over the crown. Patient is asked to bite and hold for a few seconds. Then opening the mouth quickly leads to removal of the crown.
MAINTENANCE:
Importance of high standard of maintenance i.e. by patient and dentist cannot be overemphasized. It is to be hoped that any crown/ bridge placed will have a life expectancy of at least a decade and with high level of maintenance, restoration are often seen surviving for 2-3 decades.
Following cementation patient should be instructed in particular oral hygiene procedures necessary because of the restoration. For e.g. A crown needs burnishing and flossing just as a sound tooth, but the position of margin and particular need for care in cleaning should be demonstrated to the patient. For a bridge, particular care has to be taken of the proximal area between retainer and pontic. The patient will not be able to use a dental floss; the use of floss threader or superfloss should be demonstrated.
In cases of high decay rate/ decreased salivary flow, dietary advice should be given and use of fluoride rinses encouraged. Athletes and patients with a tendency to brux should be provided with a suitable guard appliance.
The patient should be asked to return for review if any symptoms develop, mobility is felt or for some reason the restoration feels difficult from when cemented.
REVIEW APPOINTMENTS:
• Should be regularly made depending on caries rate and the
standard of oral hygiene.
• Should be done every 6 months.
• Restoration is examined with a sharp probe to detect if any
deficiency is present, mobility of tooth determined.
• Check occlusion
• Periodontal evaluation, bleeding on probing, gingival
recession, loss of attachment indicate active disease and patient will need to be encouraged in better cleaning.
• Periodic radiography is essential for patients with high
REFERENCES FROM JOURNAL:
1. Effect of connector design on the fracture resistance of all ceramic FPD. JDP 2002; 87; 536.
Conclusion:-1. The fracture resistance of 3 unit all ceramic FPDs was affected by modification of the radii of curvature within the embrasure space.
2. For the connector design tested, the radii of curvature at the gingival embrasure strongly affected the fracture resistance of all ceramic FPDs. As the radius at the gingival embrasure increased from 0.25 – 0.90 mm, the mean failure load increased by 140%.
3. The results of this study suggested that the occlusal embrasure can be designed as sharp as is practical for the esthetics of an all ceramic 3 unit FPD, provided that the gingival embrasure has a increase radius of curvature.
2. Success rate and failures for FPD after 20 yrs of service. IJP 1999; 11(2):133.
Knowledge of the background factors and conditions that cause FPD and crowns to become unserviceable should help dentists in their prosthetic treatment planning. Furthermore, a more reliable prognosis might be possible.
This study reports the cumulative success rate of 140 FPD (at least 5 units after 20 yrs in service).
Conclusion:-1. The cumulative success rate after 20 years in service was 65%.
2. The most frequent reason for the removal of a FPD were esthetic and periodontal problem, as well as loss of retention.
3. There was no difference in failure rate between FPD with / without a cantilever for the last 8 years of the 20 yr follow up period, even though such a difference had been discovered for the preceding follow-up.
4. The majority of the removed FPD had been replaced by a new fixed restoration.
3. A survey of crown and FPD failures: length of service and reasons for replacement: Length of service and reasons for replacement. JDP 1986; 56(4):416.
1) Mean length of service – 8.3 yrs
2) Caries was the most common cause of failure, affecting 22% of units failed
Mechanical problem – 69.5% Oral problem 28.5%
Resin veneer metal crowns provided the longest service and failed most frequently because of worn/ lost veneers. Complete veneer life span of 6.1 yrs fail because of caries or defective margins.
Ceramic metal life span 6.5 yrs. Failure because of porcelain fracture of poor esthetics
Resin veneer metal crown – longest service PV crown
ceramic metal
No relationship between span of prosthesis and its length of service.
4. Clinically significant factors in dowel design. JDP 1984; 52:28.
- Tapered cast dowel and core displayed a higher failure rate than teeth treated without intracoronal reinforcement.
- Parallel sided serrated dowel did not have failures caused by tooth fracture, whereas failures of the tapered cast dowel and core required extraction in approximately 1/3rd of the fractured teeth.
- Teeth that had a dowel length equal to or greater than crown length had a success rate that exceeded 97%.
- The cast parallel sided serrated dowel and core and the parallel sided serrated dowel with an amalgam or composite resin core recorded the highest success rate.
Conclusion:
Well organized and efficient postoperative care is the chief mechanism of success of FPD.
A restoration that is cemented forgotten and ignored is likely to fail regardless of how skillfully it was designed and executed.
