1. Department of Periodontology and Oral Mucosal Diseases, Faculty of Dental Medicine, Medical University, 4002 Plovdiv, Bulgaria
2. Center for Laser Dentistry, Research Institute, Medical University of Plovdiv, 4002 Plovdiv, Bulgaria
3. Biomedical Statistics, 4004 Plovdiv, Bulgaria
4. Department of Periodontics and Endodontics, School of Dental Medicine, Stony Brook University, Stony Brook, NY 11794, USA