FAILURE in Fixed Prostheses4... · 2020. 3. 22. · prosthesis wear. In case of metal castings this...
Transcript of FAILURE in Fixed Prostheses4... · 2020. 3. 22. · prosthesis wear. In case of metal castings this...
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FAILURE in Fixed Prostheses
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Mechanical failures 2
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er)Loss of Retention: (loose retain-A Loose retainers if not detected early may result in extensive caries. Loose retainer may be diagnosed by 1. Presence of an acidic smell or foul odor in the mouth.
2. The loose retainer may induce sensitivity to thermal changes and /or sweets.
3. An attempt to move the restoration occlusocervically will cause fluids to
be drawn under the casting which are expressed on seating producing bubbles.
:Cause of loose retainers
1. Improper cementation ex: moisture contamination.
2. Inadequate retention
3. Excessive span length or heavy occlusal forces.
Treatment:
If restoration is removed intact it maybe recemented.
However if the cause is lack of adequate retention due to inadequate retention
the preparation needs to be modified and a new prosthesis made
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Connector Failure: -B
Connectors may fracture due to metal weakening caused
metal weakening caused by casting porosity.
Connector fracture will cause excessive forces on the
surviving abutment as the prosthesis will act as a
cantilever bridge. Therefore immediate treatment should
be done.
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Occlusal wear -C Heavy forces, clenching and bruxism lead to accelerated
prosthesis wear. In case of metal castings
this eventually develops into a perforation which allows
leakage resulting in caries and prosthesis failure. Early detection maybe sealed by gold or amalgam prolonging
the service of the restorati In case of ceramic restorations
opposing natural teeth, enamel wear occurs that may even
reach dentine Ceramic restorations opposing metal Ceramic
restorations also cause their wear. In case of heavy bite it is
better to make castings with metal occlusal surfaces to
preserve the integrity of the opposing surfaces.
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Porcelain Fracture-D .IMetal ceramic failure: maybe caused by: 1.
Incorrect framework design.
Thin metal copings (less than 0.2 mm) do not support porcelain and allow metal flexure.
Frameworks that allow centric contacts on or close to metal ceramic junctions.
Improper metal ceramic cutback angle ex too close to occlusal or proximal cutback.
Sharp angles or irregular rough areas over the veneering areas cause stress concentration.
.II 2. Occlusal interferences caused by heavy forces, eccentric contacts or Para functional
habits.
.III 3. Trauma.
4. Debonding: a.
Separation of the metal and ceramic caused by improper metal handling such as
contamination and excessive oxide formation which may interfere with bonding. b.
Metal and porcelain incompatibility.
5. Undercut preparation, distorted impression and extended cervical feather edge margins may
cause cracks during forceful prosthesis insertion. If the
prosthesis is otherwise satisfactory, an attempt may be made to repair the fractured part using a
silane coupling agent or 4-meta to promote bonding. This solution is considered a temporary one
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Loose retainer
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• Bad taste or smell at the site of prosthesis .
• Sensitivity to temperature or sweet.
The patient complains from:
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How to detect
looseness
?
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Causes of looseness
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Improper case selection
• Mobility of abutment .
• Torque .
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Improper tooth preparation
• Over reduction .
• Over convergency .
• Short preparation .
• Insufficient retentive grooves or pins in partial
coverage restoration .
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Improper bridge design
• Selection of partial coverage when complete
coverage is indicated .
• Insufficient number of abutment in relation to span
length .
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Improper construction
• Ill fitting casting .
• Poorly adapted margins .
• Improper alloy selection .
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Improper cementation
• Improper isolation or dryness .
• Improper manipulation .
• Unsteady positioning and loading during setting .
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• A study showed that using a chamfer finish line
produced significantly smaller marginal gap than
shoulder finish line .
• Another study , found smaller marginal gap with
shoulder finish line .
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• A study found that, there is no difference in
marginal discrepancy between all ceramic
crowns and porcelain fused to metal crowns.
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• A study showed that marginal discrepancy
increased after cementation .
• The crowns luted with resin cements were found to
have 2-5 times smaller marginal discrepancy
than crowns which luted with zinc phosphate
cement .
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Prosthetic fracture
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Joint fracture
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Due to;
• Internal porosity.
• Improper solder alloy.
• Improper soldering technique.
• Strain hardening.
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Veneering fracture
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Due to;
• Inadequate bond between facing and metal.
• Metal and porcelain incompatibility.
• Excessive occlusal function..
• Improper laboratory procedures.
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All ceramic crown fracture
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Due to;
• Improper case selection.
• Improper preparation.
• Improper construction.
• Improper cementation.
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Esthetic failure
3
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Maintenance
failure
4
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Does failed prosthetic appliance may
cause psychological disturbance ?
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