Three-quarter crowns and onlays.
Partial coverage restores what has failed and leaves what has not. Where a tooth needs cuspal protection but not full circumferential reduction, it is the more conservative answer. Tick 3/4 Crown Onlay on the Rx.
The conservation argument
A full crown removes a substantial volume of sound tooth structure, including intact enamel that was contributing to the tooth's strength and to the bond available for anything adhesive.
Partial coverage preserves that structure. For a tooth with an intact facial surface, sound margins on one or more aspects, and a failure confined to the occlusal and proximal, it restores what needs restoring without sacrificing the rest.
It also preserves enamel at the margin, which matters if the restoration is to be bonded — enamel-bounded margins produce a considerably more reliable and durable bond than dentine ones.
Retention and resistance form
This is where partial coverage is more demanding than a full crown. Without circumferential wrap, retention and resistance have to come from grooves, boxes, isthmus form and the path of insertion, all of which have to be cut deliberately.
In cast metal work, proximal grooves and an occlusal offset provide the geometry. In bonded ceramic work, the adhesive interface carries much of the load and the preparation can be less retentive — but only if the field can be isolated properly.
Choose the material and the retention strategy together rather than separately.
Material choice
Cast gold is the traditional and still excellent answer for a posterior three-quarter crown: it performs in thin section, adapts at the margin, and is kind to the antagonist.
Lithium disilicate is the usual ceramic choice for a bonded onlay, where its bondability is doing real structural work. Zirconia is possible but its advantage — strength without bonding — matters less in a design that depends on adhesion anyway.
Indirect composite is a reasonable option where the antagonist needs protecting or a definitive solution is being deferred.
Margins and cusp coverage
Decide which cusps are being covered and reduce them properly. A cusp left partially covered, or covered with inadequate thickness, is the commonest cause of fracture in partial coverage work — either of the restoration or of the remaining tooth.
Keep margins on sound structure and clear of occlusal contact where possible. A margin sitting directly under a contact point is loaded exactly where it is most vulnerable.
Rounded internal line angles matter here as much as anywhere. See crown preparation design.
When a full crown is the better answer
Partial coverage is more conservative and more demanding. Where the tooth has extensive existing restoration, where multiple surfaces have failed, where retention is already marginal, or where isolation for bonding is not achievable, a full crown is the more predictable choice.
The conservation argument only holds if there is genuinely sound structure being conserved. Preparing a partial coverage restoration on a tooth that is mostly composite achieves very little.
Assess how much sound enamel actually remains before committing to the design.
What to put on the prescription
Tick 3/4 Crown Onlay and state the material. Mark the tooth on the design chart and say clearly which cusps are being covered — that is the single most useful piece of information on a partial coverage case.
Give the shade with the guide named for anything tooth-coloured, the antagonist, and the occlusal relief you want. If you have cut additional retention features, note them; if you have not and the preparation is short, say that too.
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