METHODS/PROCEDURES A randomized controlled multi-center clinical study is designed to determine whether a difference exists between the outcome of treatment when Astra tech implants are placed according to a six or a four implants protocol. The investigation will be performed in four centers: three in Italy affiliated to Institute Franci and one affiliated to the Department of Prosthodontics, Faculty of Odontology, Malmö University. In each center the clinician and the examiner will be the same person. The study protocol will be approved by the Regional Human Review Board of Padova General Hospital.