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The term “biologic width” has been replaced by “supracrestal tissue attachment” in 2017.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
Describe the relationship between contact angle, wettability and hydrophobicity
Higher contact angle = less wettability = greater hydrophobicity
Lower contact angle = greater wettability = lower hydrophobicity
44.Which dental cement appeared to affect the cell lines significantly less than others according to the study? (Mohammad)
Zinc oxide non-eugenol dental cements had lesser impact on cell lines compared to other cements tested
What are the guidelines for preparing teeth for complete crowns, suggested by Goodacre et al.? (Anna)
- TOC 10-20 degrees
- Anterior teeth + premolars - 3mm minimal height
- Molars - 4mm minimal height
- Ratio of height/width >0.4
- Presrve bucco-proximal and linguo-proximal corners to enhance resistance form
- Grooves/boxes may be utilised when resistance form is lacking
- Supragingival finish line where possible
- Rounded line angles
- Smooth preparation
How can you avoid coronal microleakage of an endodontically treated tooth?
- Restore endodontically treated teeth as eary as possible
- Use fiber posts
- Use resin based cement
Describe different types of posts
- Active vs passive
- Active = threaded and engage walls of canal
- Passive = retained by luting agent
- Active = more retentive
- Active = impart greater stress into the root - Parallel vs tapered
- Parallel = greater retention than tapered
- Parallel = greater removal of dentine required
- Tapered cast post = concentrate stress by wedging effect - Prefabricated vs custom
- Prefabricated - greater removal of dentine
- Custom - require interim restoration, potential for bacterial leakage into root canal system - Material: Gold alloy, cobaly-chrome. stainless steel, nickel chrome, glass fiber, zirconia
Compare and contrast the studies by Parker an Zuckerman regarding resistance form. What minimum axial wall height-to-base ratio do they recommend? (Zidu)
Parker
- >0.4 H/N ratio
Zuckerman
- >0.5 H/B ratio
Describe 5 types of pontic desings - their adv, disadv, indications and contraindictions
Ridge lap
- Surround the ridge
- Signifcant concavity
- Plaque retention -> inflammation
- Not recomended
Hygenic
- No contact with mucosa
- Poor esthetics
- Food trapping
- Difficult to clean as no guiding surfaces
- Posterior mandible
Conical
- Convex however only contacting and not appearing to ‘emerge’ from the soft tissue
- Molars w/o esthetic requirements
Modified ridge lap
- Overlap ridge on buccal aspect but not lingually
- Avoids deep concavity
- - High esthetic requirement
Ovate
- Egg shaped
- Convex
- Easy to clean
- Muscoas has to be shaped
- Very high esthetic requirement
- Describe a classification system for all-ceramic and ceramic-like restorative materials suggested by Silva et al. (2015). (Selina)
- Glass matrix ceramics
- Feldspathic
- Synthetic -> leucite, lithium disilicare or fluorapatite
- Glass-infiltrated -> alummina, alumina and magnesium, alumina and zirconia - Polycrystalline ceramics
- Alumina
- Stabilised zirconia
- Zirconia-toughened alumina
- Alumina-toughened zirconia - Resin matrix ceramics
- Resin nanoceramic
- Glass ceramic in resin interpenetrating matrix
- Zirconia-silica ceramic in a resin interpenetrating matrix
Define elasticity, flexural strength, fracture toughness and hardness
Elastic modulus
- Describes the stiffness of a material
- Relationship between stress and strain within the elastic range
- Lower = increased deformation under load - material absorbs more stress
- Higher = brittle - poorer for absorbing impact
Fracture strength
- Stress required for material failure
Fracture toughness
- Resistance and amount of energy required for fracture of a cracked material
- Estimate of the stress requried to cause a crack alraedy present to propagation
Hardness
- Resistance to permanent deformation by a constant localized load from a sharp object
Compare fracture strength of different ceramics
Feldspathic - 120MPa
Leucite - 200 MPa
Lithium silicate - 620 MPa
Lithium disilciate - 560 MPa
5Y - 800MPa
4Y - 1000 MPa
3Y - 1280 MPa
What is transformation toughening
- Property of zirconia
- Ceramic-strengthening mechanism occuring due to change in crystalline structure
- Pure zirconia has 3 allotripic forms dependent on temperature:
1. Monoclinic - <1170
2. Tetragonal - 1170-2370
3. Cubic - >2370 - Transformation toughnening rquires substantial portion of tetragonal phase to be stable at room temperature. To obtain sufficient tetragonal phase, the zirconia is partially stabilised with yttria.
- When exposed to external stress, for example a crack tip, the tetragonal phase undergoes transformation to the monoclinic phase.
- This is accompanied by a localised 4% increase in volume
- The increase in volume accompanies compressive stresses which can block or close the crack tip
What is the purpose of yttria in Zirconia?
- To partialyl stabilise the zirconia microstructure in the tetragonal phase at room temperature instead of monoclinic
- What factors are critical during cementation and curing of Porcelain laminateveneers (PLVs)?
- Cement thickness - too thick = decreased bond stregnth
- Color stability
Describe 4 bonding mechanisms between ceramic veneer and metal substructure
- Compression fit
- TEC of metal slighter higher than veneering porcelain
- Compressive forces develop in veneering ceramic - Micromechanical retention
- Irregularities allow micromechanical retention as ceramic flows during firing.
- Sandblasting can be used to increase the wettability of veneering ceramic - Chemical union
- Chemical bonds form via the oxide layer on the metal connecting the ceramic and metal alloy.
- Can be enhanced by adding tin, gallium, indium, iridium
- Base metals dont require trace elements to be added as they readily form oxide layer
- Bonding agents - Van der Waals forces
- Minor contribution
How to ceramics often fail?
- Ceramics almost always have flaws on surface
- These can act as starting points for microcracks
- Under tensile stress -> microcrack opens and stress develops at tip of the crack.
- Crack propagation
- Cyclic loading (mastication), crack growth occurs
- Catastriphic failure occurs when cross section is too small to withstand the load
- What are the surface modification protocols of zirconia-based ceramics for adhesive cementation with greater evidence in the literature? (Dina)
- Sandblast with silica coating particles
- Alumina blasting + 10-MDP primer
List 3 metal-ceramic system requirements
- High melting temperature alloy (higher than firing temp. of porcelain)
- Low fusing temperature of veneering procelain
- Veneering porcelain should be raedily wettable
- Strong bond between metal and porcelain
- Coefficent of thermal expansion of metal slightly higher than porcelain
- Metal is tiff - less tensile stress in porcelain
- High resistance to deformation at high temp
- Adequate thickness
Most common biolgoical and technical complications with conventional tooth supported FDPs
Biologic
- Loss of pulp vitality
- Caires
- Reccurent periodontitis
Technican
- Loss of retention
- Material fracture
- Abutment fracture
Most common biolgoical and technical complications with implant supported FDPs
- Ceramic fracture - 13.5%
- Abutment/screw loosening - 5.3%
- Loss of retention - 4.3%
Lithium disilciate vs zirconia manufacturing
Lithium disilciate
- Blue phase
- - Crystallization
- Wet grinding
Zirconia
- Green phase
- Dry milling
Survival rates for metal-ceramic and zirconia ceramic implant supported crowns
Metal ceramic implant supported - 98.7%
Zirconia-ceramic implant supported - 93.0%
Benefits of subtractive and additive manufacturing methods
Subtractive
- Better accruacy esp. at margin and orcclusal areas.
- Higher flexurcal stregnth
- Facourable hardness, fracture otughness, porosity, fatigue and volumetric strinkage.
Additive
- More favourable elastic modulous and wettability
. What steps can be taken to reduce the presence of pores in metal-ceramic restorations? (Briar)
- Sinter under vacuum
- Reduce pressure
- Gradually raise sintering temperature
- CTE of veneering porcelain slightely lower than alloy to allow comprsesion of ceramic when cooling
What is the definition of each reference line in the profile view of smile design?
Frankfurts plane = Highest point of external auditory meatus - lowest point of orbit. Parallel to horizon at neutral head position.
Camper’s plane/ala-tragal line - Superior border of tragus to inferior border of ala of ose. Determiens mx occlusal plane.
Nasiolabial angle - angle between sub-nasale by intersection of tangent from base of nose with the tangent to outer edge of the upper lip.
Ricketts E-plane - tip of nose to chin
What is the most common site for fracture for FDP? (Aditi)
- Fracture initiates from gingival connector to occlusal loading point on pontic.
- Gingival embrasure is important for FDP design (lingual embrasure should be larger than buccal)
- For long span FDP (4 unit) – fracture point is connector between pontics
What are the three theories for crown contour?
- Gingival protection theory - Wheeler
- Muscle action theory - Morris
- Access to oral hygiene
