1. Department of Ceramics and Refractory Materials, RWTH Aachen University, Mauerstrasse 5, 52064 Aachen, Germany;
2. ESK, Max-Schaidhauff-Strasse 25, 87437 Kempten, Germany;
3. Heraeus Kulzer, Quarzstrasse 8, 63450 Hanau, Germany;
4. W.C. Heraeus, Heraeusstrasse 12–14, 63450 Hanau, Germany; and
5. Dental Materials and Biomaterials Research, University Hospital Aachen, Pauwelsstrasse 30, D-52074 Aachen, Germany