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Introduction

Computer-aided design (CAD) and computer-aided manufacturing (CAM) monolithic zirconia restorations are becoming more popular in restoring posterior dentition due to their high mechanical characteristics and improvement in their optical properties. To minimise the formation of microscopic flaws during conventional manufacturing which could lead to premature failure, CAD/CAM-based materials were developed in the last two decades to enhance the load-carrying capability and fracture resistance. CAD/CAM technique enabled the development of better structures wherein all the production steps of manufacturing are electronically stored with a higher degree of precision, accuracy and reproducibility [ ]. Monolithic restorations which were introduced to replace multilayer restorations that had experienced challenges such as cracking, fragmenting (chipping) or veneer splitting (delamination). This type of failures could be explained by poor interfacial adhesion between the layers. It was concluded that zirconia-based monolithic crowns offered the best performance (split resistance) in comparison with lithium disilicate and resin-based composite crowns [ ]. Nevertheless, sufficient care needs to be exercised during the manufacturing process using the CAD/CAM technique as the material is highly sensitive to fabrication defects (imperfection/impurity/stress concentration) [ ]. Zirconia is one of the highly preferred ceramics due to its excellent mechanical properties, superior biocompatibility, translucency and minimal adhesion to bacteria [ ]. However, they also come with challenges such as high cost, very high toughness (could cause high wear on opposing occlusal surface, reducing the vertical distance), limited clinical performance data, low-temperature degradation and difficulty to assess any decay under the crown [ , ]. Zirconia is generally used as veneers, posts, fixed partial dentures, implants, abutments, and full or partial crowns. Zirconia crowns are found to have a higher survival rate of up to 97.6% after 7 years [ ].

Cusp angle is the angle made by the slopes of the cusp in buccolingual or mesiodistal directions. In a dental crown, the cusp angle is one of the primary influencing factors for the intricate stress distribution at crown fissure due to complex repetitive external occlusal loads [ ], especially on the crown surface of a monolithic structure [ ]. Several studies have shown that the cusp angle influences the local stress patterns and fracture of the tooth and the root [ ]. Based on the various failure modes of dentin supported zirconia structures, Zhang and Lawn [ ] concluded that the fracture could occur due to intaglio radial cracks initiated below the contact zone or from the occlusal surface.

Acoustic emission (AE) is a proven structural health monitoring (SHM) technique which analyses the transient elastic waves generated during rapid strain energy release when the material structure is changed (failure) [ ]. When a structure is loaded, elastic energy is stored in the material and when the magnitude of stresses exceeds the limiting value, cracks initiate, and the elastic energy is suddenly released and transmitted within the material in the form of stress waves. Piezo-electric based sensing is one of the reliable techniques over several decades for the evaluation of structural failure using AE. In this technique, the mechanical (elastic) energy from the material is converted into an electrical pulse by a piezo-electric surface mounted sensor and suitable for in-situ SHM [ ]. Many studies have been conducted in the past on both metals and non-metals to assess the location of the failure, the intensity of failure, type of failure and residual strength of the structure [ , ] using piezoelectric sensors. Time-domain analysis of individual AE signal yields several parameters such as amplitude, the energy released, signal duration, signal rise time and the number of counts (threshold crossings). Time-frequency analysis provides enhanced noise discrimination and superior source characterisation. AE technique has been used to assess the interfacial debonding and failure behaviour of various dental materials such as ceramics [ , ], resin-based composites [ , ], and fibre reinforced composites [ , ]. Many studies are focussed on the signal amplitude and the number of counts at failure. However, for high strength materials, the amplitude is always around 99 dB (peak value) which deters failure characterisation (initiation, propagation, and failure). Hence, other AE parameters such as energy released, signal duration and counts provide the required scientific insight into the failure analysis [ ].

Though many in-vitro investigations have been performed in the past to assess the failure strength of ceramic crowns [ , , ] and the location of failure initiation [ , ], no study has been conducted covering broad scatter of 60 to 120-degree cusp angles and using acoustic emission to analyse the failure process. The aims of this study are (a) to find an optimised cusp angle with maximum load-carrying capacity subjected to quasi-static compression load till failure of zirconia crowns (b) to understand the state of stress distribution at failure using finite element analysis and (c) to understand the failure behaviour of zirconia crowns using acoustic emission analysis.

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Materials and methods