## 2 Materials and Methods  
This study was structured as a multicenter randomized controlled trial with parallel groups. Patients, in the present study, presented with a completely edentulous mandible, required overdenture prosthetic rehabilitation with implants. They were enrolled and treated in seven public and private centers in Europe between December 2017 and November 2018. This study was conducted in accordance with the principles outlined in the 1964 Declaration of Helsinki for Biomedical Research Involving Human Subjects, as amended in 2013, and received ethical approval from the coordinating center located in Albania (protocol number 4/2018). Patients were duly informed about the nature of the study. A written informed consent form for surgical and prosthetic procedures, as well as for the use of clinical and radiological data, was obtained for each patient. This research was registered on Clinical trial.gov (NCT03640910), and the manuscript was written according to CONSORT guidelines. At the time of enrollment, healthy individuals, aged 24 years or older, with a completely edentulous mandible or deficient dentition in the mandible, scheduled to receive an implant-supported overdenture again were considered. Exclusion criteria were general contraindications to oral surgery, pregnancy, or lactation, intravenous bisphosphonate therapy, alcohol or drug abuse, heavy smoking (\(\geq\)20 cigarettes/day), radiation therapy to the head or neck region within the past five years, parafunctional activity, untreated periodontitis, psychiatric therapy, or unrealistic expectations, immunosuppression or immunocompromised, lack of opposing teeth/occluding dentition in the area intended for implant placement, acute infection in the area intended for bone augmentation and implant placement, poor oral hygiene and motivation, patients participating in other studies, and allergy or adverse reaction to restorative materials. For initial screening and evaluation, preoperative photographs, orthopanoramic radiographs, and periodontal status were obtained. Severely compromised tooth elements were extracted three months prior to implant placement and finalization of the new temporary complete removable denture. On the day of surgery, a single dose of an antibiotic (2 g amoxicillinin or 600 mg clindamycin or 500 mg azithromycin or clarithromycin or clarithromycin if allergic to penicillin) was administered 1 h before implant placement. Immediately before surgery, participants rinsed with 0.2% chlorohexidine mouthwash for 1 min. Local anesthesia prescribed by the surgeon was administered. The flapless procedure or a minimally invasive mucoperiosteal flap was lifted. Then, patients were randomly assigned to receive two (control group) or three (test group) non-splinted implants. Implants were placed in the interforminal region of the mandible according to a one-stage approach. Any brand of implants that provide OT-Equator OT attachments (Rhein83, Bologna, Italy) was placed according to the manufacturer suggestions, in order to achieve an insertion torque of at least 35 N cm. The implant lengths were dictated by the preoperative radiographs. Jaw bone quality was rated during the dental implant surgery, by the tactile resistance during drilling. After surgery, the patients were instructed to avoid brushing and trauma at the surgical site. A post-surgical cold and soft diet was recommended. Smokers were recommended to avoid smoking for 2 weeks postoperatively, and oral hygiene instructions were given (Chlorineidine 0.12% rinses 3 times/day). Analgesics (600 mg of ibuprofen or other) were prescribed as needed. Sutures (if present) were removed after 10 days. The prosthetic procedures began eight weeks after implant placement. All the patients received reliable, fully extended impressions of both jaws taken using a replica of the patient's removable denture, rebased with polysulfide-based or polyether material. The master cast was poured with low expansion, class IV gypsum.