Understanding Onlays in Dentistry: Definitions and Comparisons

onlay definition

partial coverage restoration for 1+ cusps and adjoining occlusal surfaces or entire occlusal surface and is retained by mechanical or adhesive means

inlay vs onlay

inlay = no cusps

onlay = covers 1+ cusps

when do you cover a cusp?

- remaining cusp structure is compromised

- cusp is missing

- opening the vertical dimension of occlusion

intercuspal dimension + remaining cusp size that means remaining cusp structure is compromised

- faciolingual width > 1/2-2/3 of intercuspal dimension

- remaining cusp thickness is less than 1.5-2mm

direct vs indirect + degree of coverage for:

crown

onlay

complex amalgam restoration

crown = indirect, full coverage

onlay = indirect, partial coverage

complex amalgam restoration = direct, partial coverage

onlay vs. crown amount of tooth removed

onlay = 3-30% removed

crown = 60-75% removed

onlay vs. crown cusps and axial wall

onlay = 1+ cusp and axial wall can be saved

crown = no cusps or axial wall can be saved

onlay vs. crown complexity

onlay = complex and versatile

crown = standardized design (less complex)

complexity and versatility of onlays depend on

- extension*

- occlusion*

- material*

crown design is standardized and only varies between ___ such as ___

materials*

gold, PFM, ceramics

onlay internal and external wall angulation type and º

internal walls = 5-15º divergent

external walls = 10-20º convergent

onlays should have ___ internal line angles, no external ___, and a ___ field for cementation

rounded

bevels

dry

onlay interproximal clearance

0.5-1mm (same as inlay)

onlay remaining cusp thickness in general, for a premolar, and for a molar

general = > 1.5-2mm

premolar = 1.5mm

molar = 2mm

how much do you reduce a cusp for an onlay? what does this depend on?

> 1-1.5mm

material

if lithium disilicate onlay, how much do you reduce cusp? what about for a weaker material?

lithium disilicate = 1.5mm

weak material = at least 2mm

the weaker the onlay material, the ___ the cusp reduction needs to be

greater

onlay dimension for rounded margin

1mm

occlusal contacts and onlays

occlusal contacts are NOT on the margin*

overlay definition

onlay that covers ALL cusps

is there one standardized prep for onlays? what is the prep based on?

no

it is standardized based on extent (how many cusps are you missing), material (ceramic vs gold), and occlusion

onlays: amount of reduction required/ how much tooth structure are you removing?

3-30%

are internal walls convergent/ divergent?

internal = divergent

external = convergent

onlay prep requirements summary:

margin =

cusp reduction =

internal walls =

external walls =

line angles =

interproximal clearance =

cusp thickness for remaining cusp =

margin = 1mm rounded margin

cusp reduction = >1-1.5mm

internal walls = 5-15º divergent

external walls = 10-20º convergent

line angles = rounded internal

interproximal clearance = 0.5-1 mm

cusp thickness for remaining cusp = > 1.5-2mm

when would you do an onlay to open vertical dimension of occlusion?

pt w/ bruxism

what happens if onlay internal line angles are not rounded?

it creates a stress area that can cause onlay to fracture

remaining cusp thickness references what part of prepped tooth?

the cusp your are not restoring with onlay

most common onlay material?

lithium disilicate

do you want bevels with onlays?

no

overlay is an onlay that covers ___ but does not cover ___

all cusps

axial walls

other names for onlay (4)

- crownlay

- vonlay

- table top

- occlusal veneer