Four stages are used to successfully regenerate bone and other tissues, abbreviated with the acronym PASS:
– Primary closure of the wound to promote undisturbed and uninterrupted healing
– Angiogenesis to provide necessary blood supply and undifferentiated mesenchymal cells
– Space creation and maintenance to facilitate space for bone in-growth
– Stability of the wound to induce blood clot formation and allow uneventful healing

The first application of barrier membranes in the mouth occurred in 1982 in the context of regeneration of periodontal tissues via GTR, as an alternative to resective surgical procedures to reduce pocket depths.
Several surgical techniques have been proposed regarding the tri-dimensional bone reconstruction of the severely resorbed maxilla, using different types of bone substitutes that have regenerative, osseoinductive or osseoconductive properties. In cases where augmentation materials used are autografts or allografts the bone density is quite low and resorption of the grafted site in these cases can reach up to 30% of original volume. For higher predictability, nonresorbable titanium-reinforced d-polytetrafluoroethylene (d-PTFE) membranes—as a barrier against the migration of epithelial cells within the grafted site—are recommended. In patients with systemic problems interdisciplinary collaboration is indicated to adjust therapy background so that it does not adversely affect implanto-prosthetic treatment.

Source= https://en.wikipedia.org/wiki/Guided_bone_and_tissue_regeneration