The aim of the investigators study is to compare, in a prospective controlled clinical trial, the Locator® System with two other types of stress-breaking retention (Retentive Anchors and Magnets) for implant supported overdenture in atrophic edentulous mandible, with the use of Straumann Dental Implant System.

The study was divided in two parts - in the first part of the study 46 fully mandibular edentulous patients will be enrolled. The patients will be recruited from the University Hospital of Dentistry and 9 other private practices in Bucharest and surrounding areas.

Selected patients will be informed about the two different treatment options, about the benefit of treatment with an overdenture retained by two endosseous implants and a written informed consent will be obtained from all participants.

Initial examination

At the beginning of the treatment for each patient will be recorded:

• The medical status.
• The dental history, oral and radiographic examination.
• The hygiene status.
• An evaluation of the existing dentures.
• The estimation of dysfunctional problems.

Treatment procedure Each patient will receive 2 screw-type Straumann (Institut Straumann AG, Basel, Switzerland) standard implants Ø4.1 mm, with SLA surface in the canine region of the mandible with an interconnecting line approaching parallelism with the terminal mandibular hinge axis. The implant lengths will be 10mm and 12mm. The choice of implant length will be dictated by the preoperative radiographic assessment of bone height in the canine region and drilling distance with the principal concern achieving primary stability.

The implants will be inserted under local anaesthesia using a surgical template derived from a tooth wax-up. The implants will be placed in a 1-stage non-submerged procedure according to a strict protocol (Weingart D and ten Bruggenkate 2000). Postoperatively, antibiotics for 5 days treatment (Amoxicillin and Clavulanate Potassium or Erytromycin - for patients alergic to Penicillin), analgesics and Clorhexidine 0,12% mouth rinse will be prescribe for 14 days. Patients will not be allowed to wear the mandibular denture during the first two weeks post-operatively.

One, two and four weeks after the surgical procedure, the patients will be recalled for follow-up visits. At the first recall visit, sutures will be removed. At the second visit the lower denture will be adjusted by selective grinding at the implant location, and Protefix® (Queisser Pharma Germany) Adhesive Cushions will be provided for patients. At all recall visits, patients will receive oral hygiene instructions.

At the third follow-up visit the manufacturing of a new maxillary denture (for the full maxillary edentulous patients) and a mandibular over-denture will be initiated.

After 6 weeks healing period implants will be loaded and the patients will be randomly assigne to one of two main groups:

• Group B of 23 patients will receive retentive anchors (Institut Straumann AG, Basel, Switzerland).
• Group M of 23 patients will receive magnets (Titanmagnetics® Institut Straumann AG, Basel, Switzerland ).

Group B will be randomly divided in two subgroups: full lower denture with two gold matrices with variable retention (four lamellae functioning like a spring - 12 patients and full lower denture with two titanium matrices with defined retention (spring with a defined extraction force of 700-1100g - B.2) - 11 patients. A new mandibular overdenture with metal reinforcement will be made.

The two groups of patients will be compared in the second part of the study with 23 patients receiving Locator system abutments (Group L) following same research protocol.

A new mandibular overdenture with metal reinforcement will be made.

The prosthetic procedure will be performed according the recommendations of the producer (Straumann Dental Implant System) for retentive anchors, magnets and Locator System by experienced prosthodontists. The occlusion will be assessed on the articulator and intra-orally to secure a balanced occlusion in centric relation without anterior tooth contact.

Inclusion Criteria:

• Complains about the stability of the existing mandibular denture satisfactory from a technical point of view.
• Acceptance of a mandibular over-denture retained by two endosseous implants.
• Patients agree to a 5-year follow-up period.

Exclusion Criteria:

• Insufficient bone volume (height and with) for inserting at least a 10 mm implant (Ø 4,1).
• Angle class II relationship.
• Physical severe consideration that will affect the minimal invasive surgical procedure or constitute a hindrance for a 5-year follow-up.
• History of radiotherapy in the head and neck region.
• History of pre-prosthetic surgery (including bone graft procedures) or previous oral implants.