Period of service before failure is traced in months for each lost implant. Lost implants will be categorized in three groups: 1) "Early Failure" which refers to implants lost before the insertion of the definite prosthesis. 2) "One-year Failure" addressing implants failed within the first year of loading. 3) "Late Failure" including implants lost more than one year following the insertion of the final prosthesis (Roos-Jansåker et al 2006).