All-ceramic dental bridges are desirable substitutes for todays metal-porcelain composite bridges.

F. Filser, H. Lüthy, P. Schärer, L. Gauckler

All-ceramic dental bridges show improved aesthetics, avoid allergenic reactions, offer high biocompatibility, and radiotransparency. High performance ceramic materials, eg. zirconia in its dense sintered form exhibit excellent mechanical properties and therefore are suitable for dental application.
To overcome the existing shaping difficulties of ceramics a new process Direct Ceramic Machining DCM was developed. DCM allows the fabrication of complex three dimensional shaped objects, like zirconia bridges, by machining a soft presintered blank which is sintered to full density afterwards. Surface shape data are recorded by mechanically digitising a wax model of the bridge and are linearly enlarged to compensate the final sintering shrinkage. All these data serve for generating the tool paths for the milling step. No further machining is necessary to achieve the desired accurary.
The absolute dimensional accurary of DCM was determined to be 19 ym plusminus 12ym using homogeneous presintered blanks of 30 mm lenght. These blanks showed isotropic predictable shrinkage. For blank fabrication four parameter sets were evaluated in terms of maximum relative density and minimal density scattering. One appropriate parameter set was chosen for dental bridge manufacturing. For testing mechanical properties of pure and veneered frameworks made of zirconia by the DCM process a bridge test setup was developed simulating clinical failure. Zirconia frameworks exceed 3 times the reliability of commercially availlable Vita-Celay glass-infiltrated porous alumina. Load bearing capacity of zirconia frameworks is more than 2.5 times of In-Ceram frameworks. Increasing the cross sectional area of the interdental connector results in a decrease of reliability and an increase of load bearing capacity in case of zirconia frameworks. Veneer porcelain fdifferent thicknesses has no effect on load bearing capacity in case of zirconia and In-Ceram. The strength of the framework's material determines the total strength of the dental bridge. Veneered zirconia frameworks showed a visible crack in the porcelaine coinciding with load peaks on load-strain diagrams in contrast to veneered In-Ceram frameworks. They do not show either load peaks nor visible cracks in porcelaine before total failure. The FEA analysis of the bridge test set-up revealed a qualitative conformance of the high tensile stresses at the gingival surface of the interdental connector with the failure initiation. Maximum tensile stress of 340 MPa account for only 50 % of the strength measured on polished specimen in 3-point bend test. A clinical study with patients will be performed where the all-ceramic bridges made with zirconia framework can prove their clinical feasibility. The study will give a feedback for improvements of the DCM process, of the tools, and of the ceramic materials. Pre-clincal studies may be extended to longterm behaviour of the all-ceramic dental bridges. FE analysis and simulation will be refined and performed with various clinical relevant load situations and different designs of frameworks.