In the recent times, with advancement of dental cements, the popularity of onlays or table tops and partial coverage crowns have increased drastically.
The Glossary of Prosthodontic terms defines an onlay as a partial-coverage restoration that restores one or more cusps and adjoining occlusal surfaces or the entire occlusal surface and is retained by mechanical or adhesive means. Onlays have the advantage of better re-establishment of tooth contours, providing more strength and protection to tooth than average restorations and inlays. Full crowns provide the same but more tooth structure is lost during tooth preparation. Posterior crowns require a larger amount of tooth preparation, however, an onlay requires between 20% to 45% of coronal tooth preparation of that required for a full crown giving us the added advantage of preserved tooth structure. Also, minimal axial reduction is done when required, cervical enamel is also preserved. Thus, the longevity of a vital tooth increases.
Many clinical trials have shown how ceramic onlays have lasted successfully over years, making them the perfect alternative to full coverage crowns. Preservation of tooth structure is key in preventive fractures by allowing more tooth structure for distribution of load.
There is no clear guide for when to go ahead with onlay and when with crowns, onlays can be a better choice when there is significant loss of tooth structure due to cavities, as onlays gain retention from inside of the tooth, also when limited extra coronal height is present, onlays reduce the need of pre prosthetic endodontic treatment.
Onlays are also considered to be long lasting as they are bonded restorations. Benefits of bonded restorations also apply here, provided complete isolation and bonding protocols are followed by the clinician.
Some systematic studies have shown onlays to have better longevity than crowns and some have shown crowns to last longer. it has been reported, however, that, failures of tooth after onlay are more favorable as the fractures generally end in the coronal part of tooth, for crowns, the fracture line mostly persists till the root, making it impossible to save the tooth.
The literature comparing onlays and crowns is limited, making it difficult to draw meaningful conclusions.
Crowns and onlays are equally popular and maybe just depend on the choice of the clinician. It is however better to provide onlays when possible as they preserve more tooth structure.
References:
Ferro KJ, Morgano SM, Driscoll CF, Freilich MA, Guckes AD, Knoernschild KL, McGarry TJ, Twain M. The glossary of prosthodontic terms.
Wang B, Fan J, Wang L, Xu B, Wang L, Chai L. Onlays/partial crowns versus full crowns in restoring posterior teeth: a systematic review and meta-analysis. Head Face Med. 2022 Nov 21;18(1):36. doi: 10.1186/s13005-022-00337-y. PMID: 36411462; PMCID: PMC9677648.
Michaud PL, Dort H. Do onlays and crowns offer similar outcomes to posterior teeth with mesial‐occlusal‐distal preparations? A systematic review. Journal of Esthetic and Restorative Dentistry. 2024 Feb;36(2):295-302.
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