Crowns Flashcards
What is the functional cusp on an upper tooth?
Palatal cusp
What is the non functional cusp on an upper tooth?
Buccal cusp
What is the functional cusp on a lower tooth?
Buccal cusp
What is the non functional cusp on a lower tooth?
Lingual cusp
What is the non functional cusp reduction of a FGC?
1mm
What is the functional cusp reduction for a FGC?
1.5mm
Which burr would you use for occlusal reduction of an FGC?
554 burr
Which cusp bevel reduction is larger?
Larger functional cusp bevel and smaller non functional cusp bevel
When would you consider adding retentive features to a prep?
On teeth with short clinic crowns
What would you use to polish a crown prep?
White or green stones
What is a crown?
A restoration that encompasses coronal tooth tissue to cover any remaining tooth substance and any remaining restorations
What are the indications for a crown?
- Protection of weakened tooth tissue, e.g. RCT
- Fractured cusps
- Cracked tooth syndrome
- Maintenance of occlusion (prevention of overeruption of missing tooth)
- Replacement of failing crown
- Aesthetics
What must be considered when deciding to place a crown?
The provision of a crown must provide strength and protection that outweighs any weakening of the tooth caused by its preparation
What are contraindications of the crown?
- Inadequate coronal tooth tissue
- Poor OH
- Untreated primary disease, e.g. caries, per, periapical disease.
- Inadequate periodontal support
- Patient choice.
What must you do before the provision of a crown?
- PA to check for:
- Caries
- PA path (infection/occlusal trauma)
- Previous RCTs
- Size of pulp chamber
- Alveolar bone levels - periodontal
- Crown-root ratio
- Presence of ++ or any other path
- Sensibility testing
When would you use diagnostic casts?
When you intend to place multiple crowns to diagnose occlusal issues and for the formulation of a definitive treatment plan
What would you use to mount diagnostic casts?
Semi-adjustable articulator
What are the advantages of a diagnostic cast?
- Unobstructed view of the mouth from all aspects
- Occlusion can be viewed from the lingual aspect and discrepancies can be identified
- The length of teeth can be accurately assessed to determine what preps designs will improve adequate resistance and retention form.
- Incline, drifting of teeth, rotation and buccolingual displacement of teeth are clearly visible.
What are the biomechanics principles of crown preps?
- Preservation of tooth structure
- Retention and resistance form
- Structural durability of the restoration (material thickness)
- Marginal integrity
- Preservation of the periodontium
What can excessive removal of tooth structure cause?
- Pulpal inflammation (pain)
- Pulpal necrosis (RCT - 15% of teeth prepped for crowns)
- Acceleration of the restorative cycle (when a crown fails, a replacement crown is no longer an option)
- Reduced retention/resistance - reduced longevity
What is retention?
Retention is the ability of the preparation to prevent the removal of the restoration along its path of insertion
What is resistance?
Resistance is the ability of the preparation to prevent dislodging of the restoration by forces directed in an apical oblique or functional direction.
Why are resistance and retention important?
So the restoration can withstand the dislodging forces that it will encounter during function.
Which operator factors influence retention?
- Degree of taper (more parallel walls = greater retention)
- Length of preparation
- Type of cement selected
- Total surface area of the cement film
- Area of cement under shear
- Roughness of tooth surface
What is the optimal degree of taper?
A taper of 5-8 degrees is considered optimal
Taper of up to 16 degrees is clinically achievable whilst also affording adequate retention
Does a shorter prep have more or less resistance?
Less
How does surface area of the prep affect the retention?
Greater SA of prep = more cement coverage, greater retention of restoration casting
How can the total surface area and retention be increased?
- The size of the tooth
- The extent of coverage of the restoration
- Grooves and boxes
- Roughening of the fit surface of the restoration (sandblasting)
What will improve both resistance and retention?
Minimising the number of possible paths of insertion
Where should the finish line of the prep be?
On sound tooth - not underlying restorations
What is the supracrestal attachment (biological width)
Distance from the crest of the alveolar bone to the base of the sulcus
OR
The combined length of the junctional epithelium and connective tissue fibres which attach the periodontal tissue to the tooth root
What is the average biological width?
2.04mm
What happens if the biological width is breached?
- Inflammation of the periodontal tissues
- Apical migration of the connective tissues + bone = recession.
Advantages of full metal crowns?
- Relatively little tooth preparation (metal is thin in cross section)
- Reduce opposing toothwear
- Softer in the bite
What are the disadvantages of metal crowns?
Aesthetics
What can all ceramic crowns be made of?
- Feldspathic porcelain
- Glass ceramic with filler (leucite reinforced, lithium disilicate)
-Crystaline systems with glass fillers (alumina)
- Polycrystalline oxide ceramics (aluminium oxide, zirconium dioxide)
How are porcelain crowns made?
Built up in layer on a die allowing individual characterisation of crowns
How are ceramic crowns made?
Pressable ceramic (lithium dislocate) can be used with CAD/CAM technology
What are the advantages of all ceramic crowns?
Aesthetics
What are the disadvantages of ceramic crowns?
- Substantial tooth prep needed, ceramic breaks in thin cross section
- Increased opposing toothwear - porcelain is harder than enamel
- More prone to fracture
What are the advantages of PFMs?
- Aesthetic
- Reasonable tooth reduction
What are the disadvantages of PFMs?
- Increased opposing toothwear if ceramic is in contact with functional surface
- If recession occurs, metal collars may become visible §
What are the advantages of Zirconia crowns?
- Aesthetic
- Machined from a monolith block (CAD/CAM), no layers to chip
- Very strong/tough
- Tooth reduction is less than all ceramic crowns
What are the disadvantages of Zirconia crowns?
Increased opposing toothwear
Why is it important to have sufficient occlusal reduction?
- Prevent wear or distortion of crown in function
- Inadequate occlusal reduction results in thin material on occlusal surface - flexure and breakage
- High crown - will require adjustment, will become thin and distort
What finish lines can be used for any crown?
- Shoulder
- Deep chamfer
Which finish lines can be used for a margin in metal?
- Shoulder plus chamfer
- Knife edge
- Chamfer plus metal margin
What is the ferrule effect?
Minimum height of 1.5-2mm of intact tooth required apical to the crown margin for 360 degrees around the circumference of the tooth
