Comparison of Extra-short (4 mm) Implants Used as Distal Support of a Maxillary Full-arch Fixed Dental Prosthesis vs. 10 mm Implants Installed After Sinus Floor Elevation.

Inclusion Criteria: * Edentulous maxilla * Willing to receive a full arch fixed restoration in the maxilla. * Latest extraction at least 8 weeks before implant insertion * Sinus floor height included between 4 to 6 mm * Bone width in the distal segments sufficient to allow the insertion of a 4 mm long implant of standard diameter. * In the anterior maxilla (from first premolar to first premolar) bone width sufficient to allow the insertion of 10 mm long implants of standard diameter. Minor horizontal augmentations with guided bone regeneration procedures was allowed in the anterior maxilla. the opposing arch have to present one of the following conditions: * natural dentition (at least 10 elements from 3.5 to 4.5) * Fixed dental prosthesis of at least 10 elements (from 3.5 to 4.5) supported by teeth or implants * Implant supported or teeth supported overdentures * Adequate partial removable prostheses. Exclusion Criteria: * Presence of conditions requiring prophylactic use of antibiotics (e.g., history of rheumatic heart disease, bacterial endocarditis, cardiac valvular anomalies, prosthetic joint replacements). * Major systemic diseases, or medical conditions requiring prolonged use of steroids, or alcoholism or chronically drug abuse. * Current pregnancy or breastfeeding women * Smokers \> 10 cigarettes per day * Physical handicaps that would interfere with the ability to perform adequate oral hygiene * Immunocompromised patients including patients infected with HIV * Conditions or circumstances, in the opinion of the investigator, which would prevent completion of study participation or interfere with analysis of study results, such as history of non-compliance, or unreliability. * Patients with an ongoing or previous treatment with bisphosphonates (for at least 2 months for oral therapy or 6 months for IV injection) Local exclusion criteria * Local inflammation, including untreated periodontitis * Pre-cancerous oral lesions * History of local irradiation therapy * Severe bruxism or clenching habits * Patients with inadequate oral hygiene or unmotivated for adequate oral home care * Previous Guided bone regeneration or Guided tissue regeneration treatment at the implant site * Total removable prosthesis in the lower arch