failure in fixed prostheses4... · 2020. 3. 22. · prosthesis wear. in case of metal castings this...

FAILURE in Fixed Prostheses

Mechanical failures 2

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

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.

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.

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

Loose retainer

• Bad taste or smell at the site of prosthesis .

• Sensitivity to temperature or sweet.

The patient complains from:

How to detect

looseness

?

Causes of looseness

Improper case selection

• Mobility of abutment .

• Torque .

Improper tooth preparation

• Over reduction .

• Over convergency .

• Short preparation .

• Insufficient retentive grooves or pins in partial

coverage restoration .

Improper bridge design

• Selection of partial coverage when complete

coverage is indicated .

• Insufficient number of abutment in relation to span

length .

Improper construction

• Ill fitting casting .

• Poorly adapted margins .

• Improper alloy selection .

Improper cementation

• Improper isolation or dryness .

• Improper manipulation .

• Unsteady positioning and loading during setting .

• 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 .

• A study found that, there is no difference in

marginal discrepancy between all ceramic

crowns and porcelain fused to metal crowns.

• 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 .

Prosthetic fracture

Joint fracture

Due to;

• Internal porosity.

• Improper solder alloy.

• Improper soldering technique.

• Strain hardening.

Veneering fracture

Due to;

• Inadequate bond between facing and metal.

• Metal and porcelain incompatibility.

• Excessive occlusal function..

• Improper laboratory procedures.

All ceramic crown fracture

Due to;

• Improper case selection.

• Improper preparation.

• Improper construction.

• Improper cementation.

Esthetic failure

3

Maintenance

failure

4

Does failed prosthetic appliance may

cause psychological disturbance ?