Comparative Evaluation Between One-piece and Two-piece Implants Supporting Mandibular Screw-retained All-On-Four Full-arch Prosthesis on Peri-implant Bone Changes.

Inclusion Criteria: * Completely edentulous patients, who can sign an informed consent will be considered eligible for this trial. * Residual bone height should have sufficient volume and quality for implant placement with a minimum of 12-14 mm in height and 6 mm in width. * Patients who already have existing maxillary and mandibular complete dentures, and after examination of the mandibular dentures, technical problems concerning denture design and/or occlusion are found, or who are dissatisfied with the retention and stability of their technically satisfactory mandibular dentures. Exclusion Criteria: * \- Patients with conditions that may contraindicate implant placement (e.g., Immunosuppressed or immunocompromised patients, patients under treatment of intravenous amino bisphosphonates, patients undergoing radiotherapy to the head and neck region). * Inadequate mouth opening * Heavy smokers, more than 10 cigarettes/day * Patients with poor oral hygiene and motivation. * Pregnancy or nursing. * Drug abusers. * Psychiatric problems or unrealistic expectations. * Patients with infection and or inflammation in the area intended for implant placement.