The objectives of the present investigation are; to compare the dimensional changes and the histological composition after the use of an allograft or xenograft and a resorbable membrane in ridge preservation in molar sites, to evaluate the influence of bone plates thickness on dimensional changes and the effectiveness of ridge preservation in limiting the need of sinus elevation.

Twenty-four patients in need of maxillary or mandibular molar extraction and subsequent implant placement were included and randomly assigned to a group; allograft or xenograft, plus a collagen membrane. Cone Beam Computed Tomographies were obtained after molar extraction and after five months. A bone sample was harvested at the time of implant placement and analysed by histomorphometry.

Inclusion Criteria:

• Adult patients, over 18 years old.
• In need of removal of a maxillary or mandibular molar first or second molar.
• The molars to be extracted had to be bordered by at least one tooth.
• The molars had to have a presence of > 2 mm of keratinized gingiva and three intact bony walls.
• A dehiscence on the fourth wall, if any, had to be equal or less than 2 mm in height.
• Only one extraction per patient was accepted.

Exclusion Criteria:

• Acute periodontal or periapical infection.
• Pregnancy or lactancy.
• Smokers of more than 10 cigarettes per day.
• Previous adverse reactions to the biomaterials used in the study.
• Metabolic diseases affecting the mechanism of bone remodelling.
• Medications or treatments taken in the last twelve months and known to affect bone "turnover".
• Patients with poor plaque control -more than 20% (O'Leary, Drake, Naylor, 1972).
• Patients with absence of periodontal health maintenance or postoperative recommendations compliance.