The main objective of this study is to evaluate the efficacy of an acellular dermal matrix membrane to increase the peri-implant soft tissue thickness and to reduce marginal bone loss during non-submerged implant placement, as compared with the standard protocol for implant placement. The test hypothesis is that placing an acellular dermal matrix membrane simultaneous to implant placement in the posterior mandible, will increase the soft tissue thickness and consequently reduce the marginal bone level changes.

The study is designed as a doubled-blind, parallel groups, randomized clinical trial with a 1-year follow-up

The study is designed as a doubled-blind, parallel groups, randomized clinical trial with a 1-year follow-up. The randomization sequence will be created using a computer-generated list by an independent researcher. Allocation concealment will be kept by means of opaque sealed envelopes that will be open by one of the researchers during surgery, and revealed to the operator immediately after implant insertion.

All patients will receive standardized oral hygiene instructions consisting on the use of a manual or power-driven toothbrush and specific interdental brushes. Then, patients will be randomized to one of the following groups:

• Test Group :Mucoperiosteal flaps will be raised and the conventional drilling sequence for implants will be implemented (Camlog Conelog Screw-Line Promote Plus implants). Implants will be placed 1 mm below the bone crest. The diameter of the implant will be in the range of 3.8 and 4.3 mm. Consecutively, 4 to 6 mm height healing abutments and the allogenic membrane (NovoMatrix®, BioHorizon) folded in two on top of the implant (secured with the healing abutment), will be placed. Finally, flaps will be sutured with 6/0 polypropylene monofilament. Patients will be instructed to rinse with 15 ml of 0.12% chlorhexidine (Perio-Aid tratamiento, Dentaid SL, Barcelona, Spain) 60 seconds twice per day until suture removal, that will take place 14 days later. Anti-inflammatory drugs will also be prescribed (Ibuprofen 600 mg every 8 hours upon patient´s needs)
• Control Group: All the procedures will be the same as in the Test Group, with the only difference that no attempt to thicken the mucosa with the acellular dermal matrix will be performed.

Inclusion Criteria:

• Systemically healthy
• Periodontally healthy or with stable treated periodontitis and good oral hygiene (Full Mouth Plaque Score (FMPS) and Full Mouth Bleeding Score (FMBS) ≤ 15%, measured at six sites per tooth)
• Need for one or two implants in the posterior mandible (maximum of three missing teeth).
• Enough bone availability to place an implant with a minimum diameter of 3.8 mm and at least 7 mm length.
• Non-smoker or smokers < 10 cig/day (self-reported).
• Ability to understand the study procedures and to comply with them to the entire length of the study.

Exclusion Criteria:

• Subjects with uncontrolled systemic diseases (ASA type III).
• Subjects taking medications with immunosuppressors, bisphosphonates or high doses of corticosteroids; current drug or alcohol use or dependence that could interfere with adherence to study requirements.
• Pregnant or lactating women.
• Allergy to collagen or analgesics/anti-inflammatory non-steroid drugs.
• History of cancer requiring radiotherapy or chemotherapy during the last 5 years.
• Local inflammation (including untreated periodontitis)
• Severe bruxism or clenching habits.
• Any kind of bone augmentation performed on the implant site, with a healing period <6 months
• Less than 4 mm of keratinized mucosa
• Post-extraction sites with <12 weeks of healing
• Lack of primary implant stability assessed intrasurgically.

Clinical examiners evaluating and recording the outcome measurements will be masked to the treatment allocation and the patients will be asked not to reveal their treatment assignment to the examiners.