Investigating the use of glass joins for dental restoration systems

Objectives: Our project aims to use glasses to join individually-optimized, solid freeform-fabricated veneer and core sections in dental restorations. Characterization of lab-developed lead-free glasses and their comparison to new commercial bismuth glasses is presented. Residual stresses in model joins and basic glass properties are used to asses the suitability of the glasses as join layers.

Methods: Lab-formulated glasses were tailored to achieve appropriate thermal expansion, melting range and chemical durability. Commercial bismuth glasses were chosen according to coefficient of thermal expansion (CTE), glass transition temperature (Tg) and working temperature. Two types of flat layered model crowns (veneer/ glass/core) were made. Alumina core restorations were modeled using Nobel-Rhondo veneer and Procera cores. Zirconia systems were modeled using Ivoclar Vivadent zirconia and veneer and Cyrtina zirconia and Elephant veneer. Vickers hardness tests were used to evaluate residual stress at and near the joins. The CTE of lab-developed glasses was measured using dilatometry, while Tg was determined by differential thermal analysis (DTA). The CTE and Tg of commercial glasses were measured by manufacturers.

Results: Residual stress tests on our glasses indicate the CTE was well-matched to both cores and veneers. Small amounts of porosity were apparent in some joins, but countering methods are under development. For the alumina system our lab-developed glass had a CTE of 6.60x10^-6/K at 50–250ºC, compared to calculated value of 6.00x10^-6/K used to formulate it. Its Tg was 556ºC. CTE and Tg measurements for the other systems are ongoing.

Conclusions: Property-matched glass joins for laminate dental ceramic restorations are a high-modulus alternative to resin-based composites for joining solid-freeform, individually-optimized veneer and core layers. They provide strong chemical bonding in the interface region, and an enhanced modulus to support load transfer from veneer to core. This project was funded by NIHF31DE017297-02.