Hey everyone, Ryan here, and welcome back to our orthodontic series. So these past few videos on biomechanics, we've kind of worked backwards. We started with how teeth biologically respond to applied force. Then we talked about how force is applied to teeth and the different tooth movements that can result. And finally, in this video, we'll talk about how orthodontic treatment actually applies those forces. So in our discussion of orthodontic materials, we're really going to be focusing on the wire and the bracket. And my professors in residency explain this really well. The wire does all the work of moving the teeth, and the bracket is just a tooth handle that the wire can use to grab the tooth and move it to where it needs to go. So keep that in the back of your mind as we go through this video. Now to understand how orthodontic wires work, which is pretty complex, my all-time favorite example is the paper clip. So if you have one nearby, go ahead and grab it. I think this will really, really help. So a paper clip is really just a loop of metal wire. It's usually galvanized steel. Now go ahead and grab this part of the paper clip, this little end part there, And grab it with your fingernail and bend it out so that it has a It permanently sticks out like this So you have to use a decent amount of force to get it to stick out like that so your finger is no longer on it So to get from step one to step two, you've just overpowered the wire and now it is permanently deformed. Of course you could bend it back to approximately where it was at the start, but that's another story. So next with the pad of your finger, push gently against the part of the paper clip that's sticking out. So that the pad of your finger is just gently pushing in like that And you'll notice that as soon as you push on it You may feel a tiny little bit of resistance and the more you push on it the more resistance you feel and then finally Once you remove your finger it should spring back into place So that part of the paperclip sticks out once more Since it wants to return back to its shape So step three into step four is an example of elastic deformation because it wants to spring back to its original shape. So these four stages all together represent every stage of an orthodontic wire. Number one is out of the package, untouched. Number two is if we place a permanent bend in the wire in order to get a certain tooth or teeth to move in a certain way. Number three is when we engage the wire into the bracket slot. And number four is after the wire expresses itself after enough time and wants to return back to its original shape, which in this case is step two, because we put a permanent bend in the wire. And as it does so, as it returns back to its original shape, it carries the teeth with it. And that's how an orthodontic wire works. We may or may not place bends in the wire, But either way, the wire is then activated into the bracket slot of each tooth that is not perfectly straight. And then the wire, as it's deflected into those teeth that aren't aligned in the arch, the wire will gradually deactivate until it's back to its original straight arch form, and it will move those teeth to the best of its ability. So there are really two phases of the orthodontic wire, and that's activation, which is also known as loading, and that refers to the amount of force applied to engage the wire into the bracket slot. So it's engaging the wire into the bracket slot and then tying it into place. Deactivation, or unloading, is letting the wire return back to its original shape and it's the amount of force that the wire applies to the tooth to get back to its original shape. So based on how the wire is deflected when it's tied into these brackets it will apply a force to them because the wire wants to return back to its original shape because of its inherent elasticity. So in this example The wire originally started out as a straight line, a straight horizontal line. And so when we deflect it up here, we can't quite get it into the bracket slot, so they tie it around like this. And the wire wants to return back, so it's going to pull on this bracket and apply an extrusive force to the canine. But because of Newton's third law, we're going to get equal and opposite intrusive forces to the adjacent teeth, which also experience a moment that wants to tip those teeth towards that canine. Again, don't worry if you don't understand all of the detailed biomechanics here. For the board exam, we really just have to nail down the big picture basics. So speaking of which, let's talk about some mechanical terms that might pop up on the board exam. And I like to think of each of these having two separate definitions depending on if we're talking about the wire loading or unloading. Engaging the wire into the bracket slot, and then the wire applying force to the tooth as it's trying to get back to its original shape. So in terms of loading, strength refers to how easily the wire will break. Now this usually isn't an issue. We're usually not putting that much force on a wire to the point where it's going to snap in half, but a wire's strength is measured in megapascals, which is a unit of stress. Now in terms of unloading, strength refers to how much force a wire can deliver. So the stronger a wire is, the higher potential it has to deliver more force. And if we're looking at this image here, strength is related to three points on a stress-strain curve. And that is proportional limit, yield strength, and ultimate tensile strength. So, proportional limit... refers to the point at which this linear relationship of the graph ends. So this linear portion is essentially when the wire is purely elastic and will return back to its original shape and any amount of stress beyond this point means that the graph will start to behave a little bit differently. Yield strength or the yield point is a little bit further along in the curve. And this is where measurable permanent deformation starts. So if the wire is stressed beyond this point, it will experience some amount of permanent deformation. So when engaging the wire into the brackets, we really want to keep it in this linear elastic zone, preferably below proportional limit. But if we can help it, below the yield strength is also beneficial. that's when it will be most effective in returning back to its original shape. And finally, ultimate tensile strength is the maximum amount of stress the material can handle for loading. And it's also the maximum amount of force a wire can deliver for unloading. And then if we go a little bit further, this is the failure point where the wire breaks. Again, it's usually not experiencing that much strain to get to that point where it will fail completely. So that's all about strength. Now we can look at the same graph and look at the second term which is stiffness. In terms of loading, stiffness has to do with how flexible a wire is. The more flexible it is, the easier it will be to push and engage that wire into a bracket slot. In terms of unloading, stiffness refers to how much force a wire will deliver as it returns back to its original shape. So strength was how much force it can deliver, it was talking about the maximum amounts here, whereas stiffness is how much force it will deliver. So there's definitely an intimate relationship between these two concepts. And stiffness is represented by the slope of the elastic portion of the stress-strain curve. And so that's the part we were talking about is the ideal part where we want that wire to be to have the most clinical effect without permanently deforming when it gets up to this point. Now the more vertical this curve is, I should draw it as a straight line, the more vertical that straight line slope is, the stiffer the wire, whereas the more horizontal that line is the more flexible the wire will be and so the higher the slope the stiffer lower the slope the more springy that wire is or the more flexible that wire is so stiffness and springiness or flexibility or the inverse of each other Our third term is range and range is defined as the distance the wire will deflect elastically before plastic or permanent deformation occurs. So it's represented by this horizontal distance on the force deflection curve up until the yield point. So the force deflection curve is similar to, but a little bit different from the stress-strain curve, but that's outside the scope of the board exam. All you need to know is when you're outside of this range of action, a wire will no longer return back to its original shape. So in terms of loading, range of action is how far you can deflect the wire while maintaining its elasticity. In terms of unloading, range of action refers to how far and by extension for how long, how much duration of time a wire will remain active. So if it has a long range of action, the chances are it will remain active for more weeks of time and we can let the wire cook, so to speak. But if it has a short range of action, it will probably not remain active as long and we'll have to see the patient back a little bit sooner to be as clinically efficient as possible. Range is measured in millimeters. And I want to address this term springback. It's similar, but it means that even if a wire is deflected beyond its yield point, The wire will still try to go back somewhat to its original shape, but it will fail to get there completely. So this is important clinically because orthodontic wires, unfortunately, are typically deflected beyond their yield point. That's just a limitation to the material. So these three major properties of elastic beams like orthodontic wires are critical in defining their clinical usefulness and can be related to each other by this helpful equation that strength equals stiffness times range. There are two more terms I want to go over quickly. Resilience refers to the area under this stress-strain curve up to the proportional limit. So it's really, again, in that elastic zone, and it represents the energy storage capacity of the wire. So this is a combination of springiness and range and also a little bit of strength. Formability refers to the area under the stress-strain curve, but this time from the yield point or yield strength to the failure point. And this area represents the amount of permanent deformation, we're in the plastic or permanent deformation zone here, that the wire can tolerate before it breaks. So if we're bending the wire, we're putting a lot of permanent bends in there, how much can the wire tolerate before it will fail completely? So putting all of this together, let's consider how wire material and geometry can impact strength, stiffness, and range. So from least strong and most springy to strongest and stiffest, our order here is nickel titanium Titanium Molybdenum Alloy or TMA and then stainless steel. So nickel titanium is the weakest and the most flexible. Stainless steel is the strongest and the stiffest. And all three of these are routinely used and they all have different applications. Sometimes we want a wire that will give a bit more and is a lot more flexible when we start treatment. And then when we're towards the finishing stages of treatment, we want a wire that's stronger and stiffer. Increasing the diameter of a wire increases its strength and stiffness, but decreases its range. So how big a wire is in terms of its dimensions will certainly impact those three characteristics. Increasing the length of the wire, and this refers to the length of the wire between brackets, increases the range of action, but decreases its strength and stiffness. So if you have a long span of wire between two brackets, that's going to be a lot more flexible. If we have a long loop, for example, that's bent between two brackets, that's going to increase the length of that wire. It's going to make it easier to engage that wire. It will stay active longer, but it's also going to lose some strength and stiffness. A rectangular wire is stronger and stiffer than a round wire of the same dimension. So two wires that are similar in size and length but one's rectangular and one's round, that rectangular one is going to be stronger and stiffer. And lastly, a beam is stronger and stiffer than a cantilever. So a beam refers to a span of wire that's supported between two brackets. So you could pick any two brackets in an orthodontic patient's mouth and the wire between those two is considered beamed, whereas a cantilever would be a span of wire that connects to only one bracket. So if you have, let's say, braces from first molar to first molar, the wire at that terminal first molar would be considered a cantilever. So it's weaker and has less influence on the teeth. So that's about as summarized as I can make that, and there's some really good high-yield facts in there for how wire material and geometry can impact the mechanical properties. So that was all about wires. Now we'll finally finish this video by talking about brackets. So before we had brackets, orthodontists would actually put metal bands around every single tooth, and those bands had little openings toward the occlusal that the wire would drop into. Now, our brackets have slots that run horizontally along the incisal edges of these teeth, hence why they're called edgewise brackets. And when the system was first invented, the same bracket was placed on every single tooth. So every bracket looked identical to each other and a whole bunch of bends, in and out tip bends, were placed in the wire in order to position every single tooth individually for every single patient. And you can imagine that that was a ton of wire bending. So first order bends were necessary to position teeth. in a proper horizontal buccolingual position. Second order bends were necessary to provide proper mesiodistal angulation and third order bends were necessary to torque teeth in the buccolingual direction in order to improve their inclination. So remember that the wire has to be rectangular in order to provide a couple in this third order dimension. A round wire cannot express torque. So hopefully these sound a little bit familiar because these correlate to the couples that we discussed in the previous video. First order from the occlusal view, second order from the facial view, and then third order from the proximal or side view. And then we have pre-adjusted edgewise brackets, which are what we use today. And pre-adjusted refers to each bracket having its own prescription for each tooth. So each bracket has a prescribed base thickness, tip, and torque built into it for each tooth. So this was the invention of the straight wire appliance where no longer where individualized bends were required for every single tooth because the bends were already built into the bracket. So we have certain brackets for premolars, a certain one for the upper right canine, upper left canine, and so every one is individually prescribed for certain tips and torques depending on the tooth you're working on. Now they work really well as long as each bracket is positioned properly in the center of the facial surface of the clinical crown of each of these teeth. Now, there are other appliances out there like lingual braces that are placed on the lingual surfaces of teeth and don't worry that again is outside the scope of the National Board Dental Exam. So for a brief overview of the different types of brackets out there, we have metal brackets, which are of the ones we'll talk about. It's un-aesthetic because we see the metal showing. Some people like it, some people don't. The wire is placed into the bracket slot and it's ligated or held in place by an elastic ring as seen here, or a chain that connects from one bracket to the next. or a stainless steel tie to really fix the wire in place. The metal brackets are made primarily of stainless steel. Ceramic brackets are more aesthetic. They match the tooth shade, usually preferred by some adult patients. They are prone to fracture, however, and they have increased friction, which means that it's harder for the wire to slide through these bracket slots. There's some resistance to sliding. If we were trying to close or open space, the wire rubs up against that ceramic material a bit more than it would for the metal. And finally we have self ligating brackets that have a built-in door which is for the daemon appliance right here that locks the arch wire into the slot. So it eliminates the need for any ligature placement. We don't need to place a elastic band or a stainless steel tie to hold the wire in place. That's actually already built into the bracket for us. So this decreases the amount of friction and because we don't have extra friction that's caused by an elastic ring holding that wire in place and it's purported that they shorten treatment time by reducing this friction. They're also more expensive because they have some added components here that are more expensive to manufacture. And that's it for this video guys. Thank you so much for watching. I hope you enjoyed this video about orthodontic materials and appliances and how they work. So if you liked this video please subscribe to this channel for much more on dentistry and if you're interested in supporting this channel and what I do please consider checking out my patreon page. Thank you to all of my patrons here for all of their support. You can unlock extras like access to my video slides to take notes on and practice questions for the board exams, so go check that out, the link is in the description. Thanks again for watching everyone, and I'll see you in the next video.