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Introduction

Developments in intra-oral three-dimensional (3D) imaging and ‘chair-side’ computer aided design – computer aided manufacturing (CAD-CAM) have resulted in efficient dental digital workflows for prosthesis fabrication, which reduce patient treatment times compared with traditional methodologies. However, a rate-limiting step for the manufacturing of all-ceramic restorations is the necessity to perform post-machining treatments to improve the mechanical properties including toughness and flexural strength prior to clinical use [ ]. Machining of the ceramic substrate in a precursor form – which lacks the final microstructure, is typically required to balance machining efficiency and machining tool longevity with the accumulation of strength-limiting damage in the machined prosthesis [ ]. Heat treatments are subsequently used to develop the microstructure by sintering of zirconia ceramics and crystallization of lithium silicate glass-ceramics. However, heat treatments are often undesirable, increasing patient treatment times between tooth preparation and restoration placement. As a consequence, material developments have been targeted at eliminating or reducing lengthy post-machining heat treatment processes.

Currently, the most common substrates used for the ‘chair-side’ manufacture of all-ceramic restorations are lithium silicate based glass-ceramics, which can accurately mimic the shade, translucency and lustre of the natural dentition [ ]. Much of the evidence demonstrating the long-term clinical success of CAD-CAM lithium silicate glass-ceramics has been generated for one commercial material, namely, IPS e.max® CAD (Ivoclar Vivadent Inc) [ , ]. IPS e.max® CAD contains a lithium metasilicate crystalline phase and requires a post-machining heat treatment to crystallise the lithium disilicate which largely replaces the lithium metasilicate [ ]. A recent development of lithium silicate glass-ceramics is the incorporation of zirconia dissolved in the parent glass. The zirconia is not present in a crystalline form, but rather acts alongside phosphorus pentoxide as a nucleating agent and results in a final ceramic microstructure comprised of lithium metasilicate crystals surrounded by nanometric lithium orthophosphate crystals and a minor lithium disilicate crystalline fraction within the residual glass [ ]. Zirconia-reinforced lithium silicate (ZLS) glass-ceramics are advocated to be machined in either a partially crystallized form (Vita Suprinity®, Vita Zahnfabrik) or alternatively with a ‘fully developed’ microstructure (Celtra® Duo, Dentsply Sirona). The manufacturer of Celtra® Duo advocates that considerable clinical efficiencies can be gained by machining ZLS with a ‘fully developed’ microstructure [ ], such that dental prostheses can be milled and manually polished prior to insertion in as little as ∼15 min., or with the addition of a glazing firing cycle – which reportedly acts to strengthen the ceramic, in ∼30 min [ ].

The multipoint contact grinding processes used in dentistry to shape the prosthesis in chair-side CAD-CAM are known to introduce complex patterns of surface and subsurface damage which limit the strength of the prosthesis [ , ]. Material removal during machining occurs when diamond particles embedded in the surfaces of rotating burs contact the ceramic thereby generating high surface strains which initiate microcracking and chip fragment formation [ , ]. The contact between the abrasive and the surface also results in residual cracks, both normal to and parallel with the cut surface, which interact with a subsurface residual stress field created through both plastic deformation and heat generated by friction [ , ]. Machining damage has been shown to extend to at least 50 μm from the surface and is microstructure dependent, with the hardness and toughness of the substrate determining the magnitude of the machining contact forces [ , ]. Therefore, the resultant surface defect population and the residual stresses dictate the effective strength of the restoration, but both may be modified by post-machining heat treatments [ ].

The primary aim of the study was to determine the impact of surface damage generated by a clinically relevant CAD-CAM process on the strength of a ZLS glass-ceramic machined in a fully crystallized form and to relate the findings to the nature of surface damage accumulation during machining. The secondary aim was to assess how the order of post-machining procedures that result in a modification of surface defects and residual stress states, including heat treatment, influence the mitigation of machining associated surface damage.

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Materials and methods