Onlays.
An onlay covers one or more cusps while leaving sound axial walls intact. For a tooth that needs cuspal protection but not a crown, it is the more conservative restoration.
When to cover a cusp
The indications are a cusp that is undermined by caries or an existing restoration, a cusp that is thin enough to flex, a visible crack line, or a tooth that has already fractured once.
An intracoronal restoration in any of those situations wedges the cusps apart under load rather than holding them together, which is why large inlays in compromised teeth are followed by cuspal fracture.
Covering the cusp converts the restoration from a wedge into a cap, and the difference in the tooth's long-term survival is substantial.
Which cusps, and how much
Decide deliberately which cusps are being covered and reduce them properly. A cusp partially covered, or covered with inadequate material thickness, is the commonest cause of failure in onlay work — of the restoration or of the remaining tooth.
Functional cusps generally need more reduction than non-functional ones, and the reduction should follow the cuspal inclines rather than flattening the surface, which preserves structure and maintains resistance form.
Say on the prescription which cusps are covered. It is the single most useful piece of information on a partial coverage case.
Margins
Keep margins on sound structure and clear of occlusal contacts where the anatomy permits. A margin sitting directly under a contact is loaded where it is least able to tolerate it.
Where the restoration is to be bonded, enamel-bounded margins produce a considerably more reliable and durable bond than dentine ones, which is an argument for extending to sound enamel rather than stopping short.
Rounded internal line angles throughout, for the same stress-concentration reasons that apply to inlays.
Material and retention
Bonded lithium disilicate is the common choice, and the adhesive interface carries genuine structural load — which means isolation is not optional.
Zirconia is possible but its principal advantage, strength without bonding, matters less in a design that depends on adhesion anyway. Cast gold performs excellently in thin section and protects the antagonist better than any ceramic.
Where the preparation has limited retention form, that pushes toward a bonded material and a field you can control. Where it cannot be controlled, it pushes toward a full crown.
Occlusion on covered cusps
Once a cusp is covered, the restoration is carrying occlusal contact that the tooth previously carried. That contact should be comparable to the adjacent teeth rather than heavier, and directed as close to the long axis as the anatomy allows.
Check excursions specifically. A covered cusp providing working-side guidance puts a sliding lateral load on a bonded restoration, which is the vector it tolerates least.
Where the tooth was previously providing guidance and you want that preserved, say so — otherwise the laboratory will design the restoration free of it.
What to put on the prescription
State that the restoration is an onlay and list the cusps being covered. Name the material and mark the tooth and surfaces on the design chart.
Give the shade with the guide named, the antagonist, any evidence of parafunction, and the interproximal contact form and tightness.
If you have cut additional retention features, note them; if the preparation is short on retention and you have accepted that, note it too.
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