Inclusion Criteria:  General criteria:  Age: 18 to 70 Edentulous mandible with reduced bone level in the posterior region (> 5 mm and < 8 mm above nerve conduit) Physical and mental condition that allow surgery and a 5-year follow-up period to be carried out without foreseeable problems Obtained informed consent from the patient  Specific criteria:  Limited vertical space for implant placement, where conventional implants with lengths of more than 4 mm cannot be used Tooth loss or extraction at least 8 weeks before implant surgery Healthy implantation site Favorable and stable occlusal relationship Complete denture in the mandible. If no denture available, an interims prosthesis will be fitted Natural full dentition or sufficient complete or partial dentures in the maxilla  Exclusion Criteria:  General criteria:  Any conditions or circumstances which would interfere with the requirements for oral surgery Allergy to any metallic implant component Acute, untreated periodontitis Previous oro-maxillo-facial radiotherapy Any disorders in the planned implant area such as previous tumors or chronic bone disease (e.g., rheumatoid disease) Any ongoing application of interfering medication (steroid therapy, bisphosphonate, etc.) Alcohol or drug abuse Heavy smoking (>10 cigarettes/day) Uncontrolled diabetes Severe bruxism or other destructive oral habits Pregnant or lactating women  Specific criteria:  Need for a major bone regeneration technique Infections in adjacent tissue of the planned implantation site. Bleeding on probing (BOP) and plaque index (PI) higher 25% Insufficient bone volume in the posterior region for placing a Straumann Roxolid® / SLActive® 4mm short implant and in the interforaminal region for placing a Straumann Roxolid® / SLActive® implant with a minimum length of 10 mm determined by radiographic analysis (OPG)