Inlays And Onlays Flashcards
What is inlay
Indirect intra-coronal restoration, places in a prepared cavity space
What to consider with inlays and be cautious about
They are held by wedge retention
Stress must be proportional to cusp height and width of isthmus (if wide isthmus but narrow cusp- prone to fracture)
There is no reinforcement of remaining tooth structure - can lead to marginal leakage, cusp fracture, loss of restoration
Indications and contraindications for inlays
Indications - same as for direct (you would go for inlay if you think direct restoration material is not sufficient), good OH
Contraindications - caries, MOD on posterior teeth, wide isthmus MOD on molars, root filled teeth
Wedge retention
Inlays are retained by wedge effect, they exert lateral forces on the cusps. If the resilience of the cusp is exceeded during the function, they will fracture.
The stress on the cusps is proportional to two factors:
1. Effective cusp height- the toller the cusp is (measured from the base.of the cavity ), greater the leverage
2. The width of the isthmus -if isthmus is wide, cusps will be narrow and that has negative impaction resilience of the cusps-more prone to fracture
If cusp is prone to fracture - NOT for inlay
What is an onlay?
Indirect extra-coronal restoration, usually placed after the tooth surface reduction
They cover the cusps and provide cuspal protection.
- Intra-coronal portion (with or without core) is replacing lost tooth tissue
*Extra-coronal portion is protection/restoring one or more cusps
Materials suitable for inlays/onlays
Metal- gold, cobalt chrome, nickel chrome
Composite
Porcelain/ceramic
Precious metal alloys-properties
Low corrosion
Low wear to opposing teeth
Relatively easy to cast and adjust
Expensive
Difficult to bond directly onto surface (surface treatment is required)
Non precious metals-properites
Very hard- chairsite adjustment is difficult
Greater wear of opposing teeth
Some have low corrosion resistance
Can be expensive as precious
Usually silver in colour- poor aeshetics
Can bond well to composite
Indirect composite
Good aesthetics
Less expensive
Easy to repair
Poor wear resistance
Ways of retention of inlays/onlays
4 ways:
1.luting cement-filling the gap between cavity and restoration
2. Mechanical restoration- parallel axial walls, resistance form/geometry
3. Bonding- chemical adhesion between restoration and cavity
4. Micro-mexhanical retention - retention due to adhesive material locking into surface irregularities
Retention and resistance
Retention is the ability of restoration to resist dislodgement along its path of insertion;direct function of the surface area of the prep
Resistance is ability to resist dislodgement though application of non axial forces, forces act in any direction than path of insertion -depends on the geometry of the prep and how well prep is adapted to restoration
Bonding materials depending on restoration material
If metal- MDP
If gold- surface treatment is needed by templating with high copper or sandblasting and then use of MDP cement
If ceramic- silane coupling agent
*Surface treatment
1. Sandblasting of the metal
2. Hydrofluoric acid to etch ceramic
Enamel bonding
Etching the surface of enamel to provide micromechanical retention
Isolation is critical- Sensitive to contamination as bond strength will be severely compromised if contamination occurs
Dentine bonding
Dentine surface must be conditioned to allow formation of adhesion to hybrid layer
Interface is vulnerable to contamination -isolation required
Do not dehydrate dentine- if not bond strength is compromised
Preparation and cementation techniques used for inlays/onlays
It depends on physical properties of the restoration material
Compared to metal restorations, ceramic/composite indirect restorations require:
1. Require bulk for strength
2. Greater extension on proximal/finishing -beyond contact area
3. Rounded internal angles needed to reduce stress concentration
4. Greater taper of the axial walls (less retention required because bonded)
5. Need to be Bonded to tooth tissue
