Welcome to this discussion on regeneration. Today we are going to discuss periodontal healing. And of course, we would like to regenerate the tissues that are lost as a result of the disease process. Here you have two cartoons. The one on the left is our wishful thinking. We would like to build up the lost bone in the apical part and then have new fibers inserting into new cementum on the root surface that has previously been contaminated. And the whole wound will be closed with a junctional epithelium that has the dimension as it used to have before the disease process started. This is, as I indicated, wishful thinking because the truth is that every periodontal wound, irrespective of the technique applied to instrument the root surface, will end up with a long junctional epithelium in green that is covering the wound surface from tip to toe. We would have to do tricks in order to overcome this proliferation of the epithelial attachment onto the wound surface and in connection with the root surface. We ask the questions, what is the contribution of various tissue components in the healing process? You can see from the cartoon on the left that various tissues contribute to the population of the wound space. We have of course, the epithelium with a high proliferation rate that covers the wound surface indicated here by the long green arrow. Then you have the influence of connective tissue of the gingiva that may contribute, and then you have the influence of the bone tissue. And finally and most importantly, the influence of the connective tissue from the periodontal ligament. Now if we let this heal, we have a situation where the epithelium is always the winner as indicated on the right hand side. Needless to say that the other three tissues do not really contribute to the attachment of the soft tissue to the root surface again. If we want a new attachment, this would mean that we need new cementum on the root surface. So far we know epithelium, connective tissue and bone cannot form cementoblasts and cannot form new attachment therefore. Why do we not get ankylosis regularly? Well, we do not get ankylosis because the good Lord gave the epithelial cells a very fast proliferation rate. The epithelial cells are the sprinters and cover the wound surface so the bone that is the marathon runner that cannot reach the root surface and form ankylosis. The question remains, we still have one tissue that we haven't looked at and that is the periodontal ligament. Can the periodontal ligament generate really new attachment, meaning new cementum? And for that, another experiment was done where you see now that in monkeys, one of the canines have been denuded. The bone, the covering bone had been removed. The root surface had been completely planed, meaning also the periodontal ligament fibers are gone. And now this is closed again but on one side it is closed with a membrane that has been put on top. Now this membrane is supposed to exclude the undesired cells from repopulating the wound space. That is the connective tissue and also the epithelium. While the bone has such a slow proliferation rate, we don't need to do anything. Here you have a cross section where basically on the right hand side it is before the membrane is placed, and on the left hand side you have the membrane placed and you see the two arrows indicate that only the cells of the periodontal ligament cells of the periodontal ligament can repopulate the space, and also the bone cells. But as I indicated, the bone cells are very slow. What we expect now is that this is going to heal and indeed when we look at the histology, you have there the notch that marks where the scaling had been performed and it is completely closed. While you see on the outside, you see the membrane still there in situ. When we go for a higher magnification in there, you see a beautiful new cementum layer with inserting fibers. This is new attachment. And with this, the new concept, biological concept of guided tissue regeneration was generated. This was later on tested in humans actually in the first human where a bulk biopsy was taken of that lateral incisor in the lower arch. Now this is a very, very extensive periodontal lesion and you see, there is a granulation tissue in that lesion, and that was treated by scaling and root planing alone. But needless to say, if we would close this wound it would heal by a long junctional epithelium. Here, one was attaching a membrane to the surface, and this membrane was glued with cyanoacrylate, leaving a wound space into which the periodontal ligament cells and the bone cells could proliferate. And if we now go to the histology where the arrow is the level of the bony crest, that is already indicating to us 3 mm of new attachment. You see that this is indeed new cementum with inserting fibers. This is location 6 mm coronal to the bottom of the defect, where you can see in higher magnification that indeed we have also cementum on the surface with inserting fibers. That means, 6 mm new attachment. Now I go 2 mm higher up after this and 2mm higher up at 8mm, we still have inserting fibers into new cementum. This is the demonstration that it is possible through the membrane to guide the periodontal ligament cells coronal in order to establish new attachment mediated by new cementum. Now for the finishing I will show you just one case where these principles could be applied. And here you certainly have a situation of a canine that is substantial for maintaining this bridge. Now this canine has attachment lost close to the apex. If we are going to save this particular tooth, we need to have coronal new attachment which you could never get with a conventional periodontal treatment. Now in this instance you see the lesion very big and very deep, deep lesion that had been debridged and root instrumented. Now on top of that, one was placing a membrane to kind of exclude the undesired cells which are the cells from the flap, that means of the soft tissue from the gingiva and the epithelium. And which give rise to the popular elation of cells that are desired, namely periodontal ligament cells and later on bones. As you can see this heals nicely and now we go and look at the results in radiographs. Now this is truly a new attachment and histology would confirm that but bear in mind that we wanted to save these tooth so we cannot show you the histology. But there are histologies that would prove that to you. This is the bone fill of that lesion after one year. Now I show you the same three years later where this abutment has been used for a new bridge and you can see this was a lifesaver for that particular tooth. And this is four years later when we have that bridge in place. So you see at this time in 1984, a new biological principle was born. It's actually the only revolution in periodontal therapy in the last century. And this guided tissue regeneration can also be applied for only bone tissue by using membranes to exclude soft tissue from proliferating into a wound space. I thank you very much for your attention.