Thin white connective tissue layer covering bone graft underneath d-PTFE membrane?


I have been utilizing d-PTFE membranes for years now for my bone grafts. Sometimes when I remove the d-PTFE membrane, after 4 weeks, to uncover the bone graft, instead of the highly vascular osteoid matrix, I observe a white hard thin layer of connective tissue covering the graft. If I poke a hole with the explorer, it’s like poking a thin sheet of collagen membrane and it would bleed. I’m assuming maybe the thin layer is periosteum? But, I’m not quite sure. Maybe someone has a better idea than me. Thoughts?