Effect of incisal preparation design on stress distribution and load-to-failure of ceramic veneers

Abstract

Background: The incisal preparation designs for ceramic veneers have been widely discussed in the dental literature. Incisal preparation can be divided into two broad categories: overlap and non-overlap. The overlap group comprises the palatal chamfer (PC) and the butt joint (BJ) preparation designs, while the non-overlap group includes the window and the feathered edge (FE) preparation designs. A review of the literature shows there is no consensus on whether incisal reduction is necessary and how much of an incisal overlap should be provided when an increase in incisal length is not required. The two most common incisal preparation designs provided are the BJ and the FE. Although ceramic veneers are highly successful, the majority of the clinical failures were related to veneer fracture. The most frequently reported complications include fracture or chipping, followed by debonding and marginal discoloration. The lack of randomised longitudinal controlled trials with large study samples has proven to be a challenge in most prosthodontic studies. The difficulty of standardising patients (age, health, diet), occlusion, the dynamicity of intraoral environment, quality of bonding, bonded tooth substrate (enamel versus dentine), tooth vitality, and conducting a long-term clinical trial are what made this area of research a challenge. A meta-analysis of in vitro studies showed that the BJ incisal preparation design may be more favourable compared with the PC design in terms of ceramic fracture and frequency of tooth failure. One of the major difficulties of in vitro studies include standardisation of tooth specimens and obtaining a large enough sample size for appropriate statistical power analysis. Other variables in the studies also include veneer preparation designs, amount of tooth preparation, veneer fabrication techniques, loading angulations and loading points. To date there are no studies that have investigated the stress distribution and the load-to-failure of ceramic veneers with the BJ and the FE preparation designs under various loading conditions. Objectives: To evaluate the stress distribution and load-to-failure of ceramic veneers with BJ and FE incisal preparation designs under various loading conditions. Methods: This thesis contains two parts: Part I - Photoelastic analysis. Stress distribution of ceramic veneers with BJ and FE incisal preparation designs under different loading conditions was evaluated under a polariscope. Photoelastic models (control, BJ and FE) were loaded at i) 45°/at incisal edge, ii) 45°/1.5mm from incisal edge, iii) 20°/at incisal edge, and iv) 0°/at incisal edge. The development of the stress patterns was recorded using a digital camera. Part II - Static load-to-failure test. One hundred and forty-eight typodont teeth (customised Nissin A25A-UL19B) (n=37) were divided into four groups: i) BJ 20°, ii) BJ 0°, iii) FE 20°, and iv) FE 0°. Lithium disilicate ceramic veneers (IPS e.max CAD, Ivoclar Vivadent) were milled using the computer-aided- design-and-computer-aided-manufacturing (CAD/CAM) techniques. Veneers were bonded to typodont teeth with resin cement (IPS Variolink Esthetic, Ivoclar Vivadent). Each group was loaded at 20° and 0° angulations at the incisal edge using an Instron Universal Testing Machine at a cross-head speed of 0.01 mm/s till failure. Results: BJ photoelastic preparations had better stress distribution and lower stress concentration compared to FE photoelastic preparations under both axial and inclined loads. Stress concentration occurred at the incisal edge region and along the cement interface for all loading parameters. Pairwise comparison showed veneers from the BJ group had significantly higher load-to-failure value compared to the FE group. Veneers with FE preparation design loaded at 20° angulation (FE 20°) had the lowest load-to-failure value, with the failure mode being predominantly cohesive. None of the veneers in the four groups de-bonded during testing. Conclusion: Within the limitations of the present study, the following conclusions can be made: 1. Both incisal preparation designs and loading angulations have significant effects on the stress distribution and the load-to-failure values of ceramic veneers. 2. The BJ group exhibits a significantly higher load-to-failure value compared to the FE group. 3. Of all the groups analysed, the FE group loaded at 20° angulation has the lowest load- to-failure value and the least favourable stress pattern. Clinical significance: The BJ incisal preparation design is preferred to FE preparation design due to more favourable stress distribution and a higher load-to-failure value.