Abstract
Ceramic onlays are frequently used to restore teeth with loss of tooth structure due to decay. However, the conventional onlay preparation design is over-tapered and non-retentive, which provides minimal guidance during cementation. This can result in poor marginal and internal fit and subsequent restoration failure. The aim of this study is to investigate whether a modified preparation design could provide increased guidance for seating the onlay and improve the marginal and internal fit. This was assessed by measuring absolute marginal discrepancy (AMD) and internal cement thickness (ICT).
The conventional preparation design was prepared on a typodont (artificial) tooth, then scanned and duplicated (n = 20). To create the modified preparation design, a cross-shaped groove was added to the typodont tooth, which was then scanned and duplicated (n = 20). An onlay restoration was designed for each of the conventional and modified preparations. The onlays were then 3D-printed using computer-aided design and computer-aided manufacturing. Each onlay was assessed for fit and then cemented onto the duplicates using a custom-made device. For each group, 10 specimens were sectioned horizontally, and 10 specimens were sectioned vertically. The sections were stained with toluidine blue, followed by measurement of AMD and ICT using a stereomicroscope. Descriptive data analysis and hypothesis testing were conducted using the nonparametric Mann-Whitney U test (α = .05).
For the vertical sections, the modified preparation had a reduced mean AMD compared to the conventional preparation at palatal (P = 0.035), buccal (P = 0.006) and mesial (P = 0.193) sites. The conventional preparation had a reduced mean ICT compared to the modified preparation in the vertical (P = 0.009) and horizontal sections (P = 0.121).
The modified preparation presented improved AMD compared to the conventional preparation but produced increased ICT. However, AMD and ICT were within the clinically acceptable range for both preparation designs. These findings imply that the modified preparation has the potential to improve the marginal fit of onlay restorations and, therefore, their clinical longevity.