The purpose of this study is to compare the regenerative results of using separately two different xenografts after two-step sinus floor elevation.

Inclusion Criteria:

1. Patients who will present with a moderately or severely atrophic posterior maxilla with 1-6 mm residual alveolar bone .
2. Men and women over the age of 18 years.
3. Patient has signed the Informed Consent.

Exclusion Criteria:

1. Chronic steroid therapy.
2. Uncontrolled diabetes.
3. Cardiovascular disease.
4. Past irradiation of head and neck.
5. Maxillary sinus cysts.
6. Active chronic sinusitis.
7. Smoking more than ten cigarettes per day during the 3 months preceding this study .
8. Malignant disease in the 5 years preceding this study.
9. Uncontrolled bleeding disorders such as: hemophilia, thrombocytopenia, granulocytopenia
10. Disease that compromise the immune system
11. Psychiatric disorder
12. Hypersensitivity to titanium, collagen or bovine bone.
13. Women who are pregnant or nursing.
14. Patients with non-treated periodontal disease.
15. Medical and/or general contraindications for intraoral surgical procedures.