ORALMED Research Group, Department of Dental Clinical Specialties, School of Dentistry, Complutense University of Madrid, Plaza Ramón y Cajal s/n, 28040 Madrid, Spain. IDIBO Research Group, Stomatology Department, School of Health Sciences, Rey Juan Carlos University, Av. de Atenas, S/N, Alcorcón, 28922 Madrid, Spain. Division of Restorative Sciences, Norris Dental Science Center, Herman Ostrow School of Dentistry, University of Southern California, 925 W 34th Street, DEN 311, Los Angeles, CA 90089-0641, USA.