Post-operation peri-apical radiographs, taken at the day of "stage-one" surgery, will be retrieved from patient's charts and used as the reference point of supporting bone level for each implant. Reference bone levels on mesial and distal sides are determined by measuring the distance between implant platform and the most apical point of the alveolar crestal bone surrounding implant. A mean value between the mesial and distal sides is used for each implant as the reference marginal bone level.