Fixed prosthodontics - literature review exam Flashcards
- What are the differences in the philosophies of dowel diameter between the conservationist, the proportionist, and the preservationist groups? (Diana)
Conservationalist
- Narrowest diameter of dowel
Proportionist
- Dowel space with the apical diameter equal to 1/3rd of the narrowest diameter of the root at the terminus of the dowel.
Preservationist
- ≥1mm of sound dentin circumferentially
(Lloyd, 1993)
- Describe the clinical indications and contraindications for metal-ceramic, feldspathic, lithium disilicate, 5Y-TZP, 4Y-TZP and 3Y-TZP (Harshiv)
Metal-ceramic
- Indications: Single crown, FDPs
- Contraindications: Partial coverage
Feldspathic
- Indications: Partial coverage (anterior veneer), single crown (anterior)
- Contraindications: Posterior restorations, FDP
Lithium disilicate
- Indications: Partial coverage (anterior or posterior), single crown (anterior or posterior), 3 unit FDP (anteriorly in exceptional and limited circumstances)
- Contraindications: FDPs
5Y-TZP
- Indications: Single crowns, 3 unit FDP (anteriorly in exceptional and limited circumstances)
- Contraindications: Partial coverage, FDPs
4Y-TZP
- Indications: Single crowns, 3 unit FDPs
- Contraindications: Partial coverage, >3 unit FDPs
3Y-TZP
- Indications: Single crowns, FDPs
- Contraindications: Partial coverage
The term “biologic width” has been replaced by “supracrestal tissue attachment” in 2017. Describe the components of the supracrestal tissue attachment, its
average dimensions and the consequences of restorative margins violating this attachment. (Harshiv)
Supracrestal tissue attachment = junctional epithelium + supracrestal connective tissue
Current consensus is that biologic width is not constant and can vary between different patients and different teeth from 0.2-6.73mm
Infringment of restorative margins within the supracrestal tissue attachment is associated with inflammation +/- loss of periodontal supporting tissue.
The response may vary based on the below:
1. Low crest patient
- >3mm biologic width
- Found in 13%
- Violation = high risk of recession
2. Normal crest patient
- ~3mm biologic width
- Found in 86%
3. High crest patient
- <3mm biologic width
- Found in 2%
- Violation = high risk of recession
What are the ideal properties of provisional materials?
- Convenient handling: adequate working time, rapid setting time
- Biocompatibile: non toxic, non allergenic, non-exothermic
- Dimensional stability during solification
- Ease of contouring and polishing
- Adequate strength and abrasion resistance
- Easy to repair
What are the 5 factors for choosing a subgingival margin location on a crown?
- Improve retention and resistance form
- Esthetics
- Due to existing restorations
- Reduce caries risk (controversial)
- Excess removal of tooth structure
Amalgam and composite cores exhibited higher failure loads compared to other materials like glass ionomer in some studies. Amalgam cores also had the lowest
failure rate in a cyclic loading test. However, composite core is more favourable then amalgam core in the clinical setting. Why is that? (Diana)
- Amalgam cores had slightly higher but not sigificantly different failure loads than composite cores
- Only composite cores had repairable fractures, therefore the mode of failure was more favourable compared to amalgam cores
- In-vitro situations differ from intraoral as there is no PDL, there is a constant ferrule height and continuing loading to failure. Fatigue testing over time may better analyse the importance of core stiffness, since most clinical failures occur after years due to fatigue and not acute overload. Therefore, we can’t assume an amalgam core is better than a composite core based on a cyclic loading test.
Describe two types of provisional restoration materials according to their chemical composition and compare their properties. (Selina)
- Monomethacrylate/acrylic - PMMA/PEMA
- Dimethacrylate or bis-acryl composites - Bis-GMA
Flexural strength
- Dimethacrylate > monomethacrylate
Hardness
- Dimethacrylate > monomethacrylate
Fracture toughness
- No difference
Exothermic reaction
- PMMA > PEMA > Dimethacrylate
What is metamerism and how can it be avoided
Metamerism is a phenomenon where the color of an object appears different, depending upon the light source.
It occurs when the shade selection environement is not controlled
To reduce/avoid metamerism, need ot have controlled and ideal color corrected light of 5500K
- What is the purpose of placement grooves and boxes on crown preps? When would bilateral grooves be indicated? (Zidu)
- Grooves / boxes aim to improve resitance form
- Bilateral grooves and boxes may help reduce mesio-distal rocking
Methacryloyloxydecyl dihydrogen phosphate (10-MDP) is one of the mostcommonly used functional monomers, what is its mechanism of action? (Harshiv)
- Hydrophilic phosphate monomer
- Increases resin diffusion and adhesion by causing chemical decalcification and binding to calcium ions or amino groups in the tooth structure
- Describe the adhesive cementation protocol (using a resin based cement) of zirconia restorations, including both tooth and restoration preparation (Harshiv)
Zirconia restoration
- Try in
- Air particle abrasion with 50um Al2O3 at 2 bar pressure for 15s
- Clean in ultrasonic bath with 70% ethanol for 5 minutes
- Apply 10-MDP containing primer for 20sec
Tooth preparation
- Moisture control and isolation
- Remove residual temporary cement
- Remove biofilm using intra-oral air particle abrasion or pumice
Cementation/bonding protocol
- Etch enamel for 20-30 sec
- Wash and dry
- Apply adhesive and airdry without light curing
- Seat restoration
- Tack cure ~2 sec
- Remove excess cement
Describe the adhesive cementation protocol (using a resin based cement) of lithium disilicate veneers, including both tooth and restoration preparation
(Harshiv)
Lithium disilciate veneers
- Try in
- Hydrofluoric acid etching with 4.6% HF for 20 sec, wash and dry
- Clean in ultrasonic bath with 70% ethanol for 5 minutes
- Application of silane primer for 20 sec
Tooth preparation
- Moisture control and isolation
- Remove residual temporary cement
- Remove biofilm using intra-oral air particle abrasion or pumice
Cementation/bonding protocol
- Etch enamel for 20-30 sec
- Wash and dry
- Apply adhesive and airdry without light curing
- Seat restoration
- Tack cure ~2 sec
- Remove excess cement
- What type of coronal wall prep design increases the success rate of post? (Diana)
> 1mm of residual dentine control to shoulder of margin. Parallel as possible to maximise resistance form.
Base metal alloys form a thick oxide layer, what disadvantages may this lead to?
- Base metal alloys contain elements that readily form oxides in air
- These form a thicker oxide layer
Disadvantages are that: - Restoration can take on alloy color, greening at margins
- Greater risk of cohesive failure unless oxide layer is remvoed and re-formed more thinly
In base metals, the naturally formed oxide layer is thick which poses many disadvantages, how may this be managed?
Degassing procedure to remove the thick oxide layer
- Air abrasion with 50-100um Al2O3
- Steam cleaning of casting
- Low temperature firing (650-950) for 1 minute without vacuum
- What are some advantages and disadvantages of base metal alloys? (Harshiv)
Advantages
- Stronger
- Harder
- Lower ductility
- Can be used in thinner sections
- Better for FDPs
Disadvantages
- Harder to adjust, finish and polish due to hardness and lack of ductility
- Thicker oxide layer requires greater management to minimize greening and cohesive failure
- How can finish line bevels improve seating and sealing of crowns? What bevel angle provides the best seal? (Zidu)
- Bevel at 45 degrees improves seating and sealing
- ## Steeper bevel - 75 degrees = better sealing
