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The decision between placing a filling and recommending a cuspal restoration is fundamentally about structural engineering. A filling replaces lost tooth material within the confines of the prepared cavity. A cuspal restoration — whether an onlay, overlay, or cusp-replacement ceramic restoration — covers and protects the remaining tooth structure, distributing occlusal forces across a ceramic surface rather than concentrating them on the remaining unsupported cusps.
Understanding when each approach is appropriate prevents both under-treatment (filling a tooth that needs more protection) and over-treatment (preparing for a crown when a well-placed filling would suffice). At Dazzle, composite cavity fillings and ceramic onlay/crown restorations are both offered and matched to the structural requirements of each case.
What Determines the Choice?
Remaining tooth structure: When the cavity or prior restoration occupies more than 50% of the tooth's crown volume, the remaining walls are at high risk of fracture under normal biting forces. A filling in this situation restores the shape but does not protect the cusps from splitting. A cuspal restoration covers the tooth from the outside, reducing this risk substantially. Cusp involvement: Decay or fracture that involves or undermines a cusp indicates a cuspal restoration. A single undermined cusp in an otherwise healthy tooth: onlay. Multiple cusps involved or the marginal ridges compromised: overlay or crown. Tooth vitality: Root canal-treated teeth have had their pulp removed. The dentine becomes progressively more brittle without the moisture supplied by the pulp. Posterior root canal-treated teeth require cuspal coverage. A filling in a root canal-treated molar is a temporary measure, not a definitive restoration. Existing restoration size: A tooth that already has a large amalgam or composite filling that is now failing, with one or more cusps exhibiting cracks, is a candidate for a cuspal restoration rather than replacement of the filling alone.
Types of Cuspal Restorations
Onlay: Covers one or more cusps and the occlusal surface. Appropriate when two or three cusps require coverage. In-house fabrication at Dream Dental Studio: single appointment in most cases. Material: monolithic zirconia for posterior teeth, e.max for premolars requiring higher aesthetics. See our biomimetic dentistry guide for the tissue-preserving approach to onlay preparation.
Overlay/Full-coverage crown: When all cusps are involved or the tooth structure remaining is minimal, a full crown provides circumferential coverage. At Dazzle, full crown preparation is avoided when a less invasive option will provide equivalent protection.
When a Filling Is Appropriate
A filling remains appropriate when: the cavity is small to medium, located within the central groove or proximal box without extending to or undermining cusps; the remaining tooth structure is sound and adequate; the tooth is vital; and there is no history of cusp fracture at that tooth.
FAQs
Q1: My dentist recommended an onlay but I had hoped for a simple filling. Is the onlay necessary?
The recommendation depends on how much of the tooth has been lost. A filling placed in a tooth with undermined cusps will function temporarily but substantially increases the risk of catastrophic cusp fracture. If a cusp fractures below the gumline, the tooth typically cannot be restored and requires extraction. An onlay that costs more upfront is substantially less expensive than the combination of tooth loss + implant or bridge.
Q2: How long does an onlay last compared to a filling?
Ceramic onlays typically last 10–20 years with proper maintenance. Direct composite fillings last 5–10 years in posterior teeth. The additional longevity of the onlay often makes it the more cost-effective option over a 10–20-year horizon, particularly in posterior teeth under high occlusal load.
Q3: Is an onlay more painful than a filling?
The preparation is performed under local anaesthesia. Sensitivity after onlay preparation is similar to after a large filling preparation and typically resolves within 2–4 weeks. There is no meaningful difference in post-operative discomfort between the two procedures when both are performed correctly.
Q4: My dentist mentioned a cuspal restoration because of my root canal treatment. Is this standard?
Yes. Posterior root canal-treated teeth (molars and premolars) without cuspal coverage have significantly higher rates of tooth loss due to cusp fracture compared to root canal-treated teeth with crowns or onlays. This is an evidence-based recommendation, not a upsell. Devitalised teeth are significantly more susceptible to cusp fracture than vital teeth. If your root canal-treated molar has been protected by a filling rather than a cuspal restoration, an assessment of the remaining tooth structure is warranted.
