Inclusion Criteria:  at least one edentulous maxillary premolar or molar site treated successfully by sinus floor elevation with a xenogenic bone substitute (cerabone, botiss biomaterials, Zossen, Germany) confirmed by preoperative cone-beam computed tomography full- mouth plaque and bleeding scores (FMPS and FMBS) <20% good patient compliance (including willingness to participate in the follow-up procedures) signed informed consent  Exclusion Criteria:  clinically relevant diseases (e.g.: diabetes, rheumatism, cancer) systemic steroid or bisphosphonate use acute or chronic inflammatory processes