Quantitative analysis of substance removal during tooth preparation for full ceramic restorations using digitally generated preparation designs

Abstract

Objectives: This study aimed to determine which factors most strongly affect volumetric tooth substance removal during preparation for full ceramic restorations and how these parameters interact. A novel digital method was used to design preparation geometries using three-dimensional (3D) graphic software.

Materials and methods: A digital workflow involving Boolean operations was applied to an STL dataset of a maxillary first molar to generate 720 preparation designs. Each design varied by preparation angle, chamfer depth, finish line distance from the cementoenamel junction (CEJ), and occlusal reduction. Volumetric tooth removal was evaluated for each parameter, as well as for their combined effects.

Results: All preparation parameters influenced tooth substance removal. The finish line distance from the CEJ showed the greatest effect, followed by chamfer depth. Preparation angle and occlusal reduction had less pronounced effects.

Conclusions: Increasing the vertical finish line distance and minimizing chamfer depth substantially reduce tooth substance removal. While adequate occlusal clearance is essential, its effect on total volume loss is relatively minor. The influence of the preparation angle was more relevant for full crowns than for partial restorations.

Clinical relevance: Clinicians are encouraged to favor partial restorations whenever possible, as they result in less invasive preparations, even when compared to full crowns made from high-strength materials with reduced thickness requirements such as monolithic zirconia. To preserve tooth structure, both the material's minimum thickness and the vertical position of the finish line should be carefully considered. These findings support a conservative preparation approach tailored to material properties and clinical requirements.

Keywords: All-ceramic restoration; Convergence angle; Minimally invasive dentistry; Partial-coverage restoration; Tooth Preparation.

Fig. 1

STL dataset of the scanned maxillary left first molar plastic resin tooth (Frasaco GmbH, Tettnang, Germany)

Fig. 2

Digital workflow for generating and applying preparation geometries: (a) Construction of the chamfer profile by merging a vertical and a horizontal line into a single, angled curve with a smooth transition.(b) Alignment of the chamfer profile along the defined preparation margin. (c) Generation and positioning of the 3D preparation geometry by sweeping the profile along the margin and aligning it with the tooth model. (d) Final prepared tooth surface resulting from the Boolean subtraction of the parametric geometry. (e) Visualization of the occlusal reduction with gradual smoothing toward the outer cusp areas. (f) Final prepared tooth surface showing the offset between the occlusal reduction mesh and the preparation due to postprocessing

Fig. 3

Volumetric tooth substance removal (vol%) in maxillary first molar preparations, illustrating the influence of chamfer depth, occlusal reduction, preparation angle, and finish line distance to the CEJ

Fig. 4

Comparison of tooth substance removal at varying finish line distances from the CEJ for a ceramic restoration with 1 mm layer thickness and an 8° preparation angle

Fig. 5

Tooth substance removal at various chamfer depths for a ceramic restoration with 1 mm layer thickness and an 8° preparation angle. Data include occlusal onlays (finish line distance of 6 mm to the CEJ), partial crowns (4 mm), and full crowns (1 mm)

Fig. 6

Tooth substance removal at various preparation angles for a ceramic restoration with 1 mm layer thickness. Data include occlusal onlays (finish line distance of 6 mm to the CEJ), partial crowns (4 mm), and full crowns (1 mm)

Fig. 7

Tooth substance removal at various occlusal reductions for a ceramic restoration with a 1 mm circumferential layer thickness and an 8° preparation angle. Data include occlusal onlays (finish line distance of 6 mm to the CEJ), partial crowns (4 mm), and full crowns (1 mm)

Fig. 8

Tooth substance removal during preparation for partial crowns made from lithium disilicate (adhesively cemented) compared with full crowns made from monolithic zirconia