hey everybody this is Ryan and in this video we're gonna talk about the feral effects and biologic wits and how these two relatively complicated dental topics play a role in determining the restore ability of teeth so what is a feral well it's defined as a ringer cap typically a metal one that strengthens something and prevents it from splitting or wearing so this word comes from a combination of these Latin words Ferrum for iron and variola for bracelet and so it's technically an iron band that encircles something to give it strength so an example would be like at the end of this paintbrush there's this little metal piece that reinforces the bristles at the end or the iron rings around a barrel that help give it strength and support so how does this relate to dentistry so a dental ferrule is defined as a band that encircles the external dimension of two structure so what the heck does that mean basically the dental ferrule includes both the axial walls and the margin which is right at the edge here this crown which engage the axial walls of the prepared tooth so here we have a tooth that has been prepared to receive a crown and the coronal structure has been reduced in order that a certain thickness of crown material can fit on top without being overly bulky in this case there isn't any decay there is no root canal treatment no post and pore so it's a relatively straightforward case technically the ferrule is there for the entire axial wall of the crown which contacts and braces up against this axial tooth structure so this provides a lot of fracture resistance and lateral force resistance but let's get to a case where ferrule is a more important consideration so from this paper here's a drawing and we have two examples of teeth that were prepared to receive crowns however these teeth have already undergone a root canal treatment and they required a core filling material shown by this rectangle here to restore the missing and decayed coronal to structure and they had a post placed in the root canal system in order to retain the core so root canal treatment post and core pretty classic setup for tooth that's been through a lot so one of these is good and one of these is a bad design and I want you to figure out which one looks stronger right off the bat well I guess the picture sort of gives it away because this one has a ferrule setup which makes it inherently stronger but let's explain why the flat top tooth here offers no resistance to lateral forces and let me get a pen here to show you that if we were to say have lateral leverage forces on the crown of course here's a crown it would be fitting on top of this core any lateral leverage on the crown during function which transfer solely to the core material and then would translate to lateral forces from the post and so there's nothing stopping all those forces from translating to the core but this one has a little staircase of natural tooth structure here that this one does not have and just like our normal prep from the last slide it's similar but there's a lot of coronal to structure missing because of decay or something like that so the wall of natural tooth structure right here is much shorter than the previous example but the even this little staircase of tooth structure provides a place for the crown margin and the axial wall to brace against natural tooth structure so that it can better resist lateral forces from post and leverage from the crown and function so it's sharing the application of force against this this wall so that the tooth is able to absorb this and then the PDL is able to share the the blame not just the court and the post so according to some clinical research 1.5 millimeters should be the minimum ferrule length when restoring a route filled maxillary central incisor with a post and core retained crown 1.5 is a pretty widely accepted minimum for all teeth and I know this gets confusing but the 1.5 millimeters of ferrule refers to the crown the aspect of the crown which contacts the natural tooth which in turn needs to be 1.5 millimeters high so that's depicted by this bracket here so now we got ferrule so let's talk about biologic width next so biologic width refers to the attachment of periodontium to the tooth above the bone so between the bone and the tooth here is the periodontal ligament but above this and technically below in this picture because it's depicting an upper tooth is first a connective tissue layer a connective tissue attachment to the tooth and above that is an epithelial attachment to the tooth which is the weakest of the attachment layers and both the connective tissue and epithelial attachment layers together constitute the biologic attachment aka biologic width now on average as shown in this picture from spear education the average is about one millimeter for each of these layers for a total of two millimeters of biologic width so if we were placing a veneer or crown it must not impinge on this biologic width especially on the facial margin if it does you'll cause inflammation the tissue will get red and recede and you can even get bone loss as the tissue tries to literally recreate this two millimeters of natural attachment that you invaded with some foreign crown material so this is the danger zone we want to stay away from impinging on this attachment at all costs now in order to avoid getting anywhere close to this we usually try to stay super gingival which means above the gingiva but if we have to we can position a crown margin slightly sub gingival or below the gingiva since a sulcus as shown here is usually not less than one millimeters deep if you go no more than 0.5 millimeters below the gingiva you're generally pretty safe so we'll say if you came like somewhere around here with your crown margin and this is again these are all averages every patient is different which is important say on average we go-oh point five millimeters below the gingiva again since the sulcus is usually not less than 1 millimeters you're generally pretty safe so this allows at the very least a different distance of 2.5 millimeters from the bone level to the restorative margin of our crown here so that's another important number that 2.5 millimeters okay so with this information let's look at the second image we have to have at least two point five millimeters from bone to crown margin for the biologic with that we just talked about and if we add in our ferrule of 1.5 millimeters from the last slide we get a grand total of 4 millimeters from the height of the ferrule to the height of the bone all right so now let's apply the to a clinical case to help illustrate all these concepts so let's say on this molar number 30 we need to replace this restoration with a crown because there is extensive decay underneath for example this would be a situation when we need to consider both ferrell and biologic width so say we remove this old restoration and we effectively have this much natural tooth structure remaining after we do so hypothetically notice we don't have any stair steps here we have a really long one here but of course we can't make the crown margin this thick so we would need to make up our stair step further down in order to achieve that ferrule effect so we'll say maybe here we will have to drop our margin in order to get again that 1.5 millimeters bracing up against natural tooth but then we can't stop there we also need to go another 2.5 millimeters and we have to say 2.5 millimeters away from the height of bone so that we're sure that we're not impinging on that natural attachment that does not want to be interrupted and notice by this point we are way into bone so what can we do about that is this tooth actually restorable well simply put we can do clinical crown lengthening and move the bone down or orthodontic extrusion and move the tooth up now this is an oversimplification but if these methods are valid then we can restore this tooth but otherwise if we cannot obtain ferrule and we cannot stay away from biologic with without interrupting or disrupting the adjacent teeth and this toots would no longer be considered restorable again this is an oversimplification but hopefully it makes sense in terms of treatment planning determining if a tooth is restorable or not and ultimately doing what's best for your patients so I hope you found this video helpful please leave a like and subscribe to my channel if you haven't already thanks so much for watching guys and I'll see you all in the next video