If primary closure is not possible, the use of a membrane is highly recommended.
In general, the use of a dental membrane is to prevent the soft tissue cell from entering the graft site before bone forms and to help contain granular grafting materials in place (if they are used). This is the natural role for periosteum—the separation of hard and soft tissue.
As such, if there is primary closure of an intact periosteum, this is sufficient and there should be no need for a membrane when using PentOS OI™ Max.
However, without primary closure, and if the periosteum is damaged, torn or cut for vertical release, it becomes less effective as a barrier and a membrane should be used.